# Does a 25mm wide tire have less contact area than a 23?



## dasho (Apr 8, 2002)

I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?


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## johnny99 (Apr 2, 2004)

dasho said:


> I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?


Depends on the tire pressure. Wider tires should generally be run at a lower tire pressure, which increases the contact area.


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## Spoke Wrench (Aug 20, 2001)

dasho said:


> I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?


Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.

At equal pressure, the shape of the contact patch on a 23mm tire will be longer from front to back and the 23's contact patch will be shorter and wider. A long, narrow contact patch will generate more rolling resistance because the tire has to deform more from it's natural round shape to make the flat patch where it contacts the road.


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## Fredrico (Jun 15, 2002)

*Bigger tires are better.*



Spoke Wrench said:


> Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.
> 
> At equal pressure, the shape of the contact patch on a 23mm tire will be longer from front to back and the 23's contact patch will be shorter and wider. A long, narrow contact patch will generate more rolling resistance because the tire has to deform more from it's natural round shape to make the flat patch where it contacts the road.


You mean the 25C's contact path will be shorter and wider, right?

I've heard that a long, narrrow contact patch provides more rolling resistance from the tire deforming, too. Its quite noticeable at 95-110 psi. So you have to air up the 23Cs really hard, and then they ride harshly.

Besides better pinch flat protection and providing a more comfortable ride, that's another good reason to run 25Cs. Too bad LBSs don't stock more of them. Heck, the 28Cs on my commuter bike are about as fast as the 25s, same tire, Conti Ultra 2000. Nobody stocks those. Tyranny of the weight weenies.


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## Cory (Jan 29, 2004)

*A vote for bigger tires for nearly all riding*

In my speed-obsessed youth, I did the traditional experimenting with all kinds of tires, down to 19mm. I weighed about 220 then, and skinny tires were just unpleasant. I moved up to 25s despite the then-common belief that skinnier was always faster, and I was a lot more comfortable and didn't go any slower. Eventually I settled on 32s for most riding, 35s for rides when I know the pavement will be bad. Last summer I put on a set of 23s (apparently what most people on this board use), and I HATED 'em. 
As one of the other posts mentioned, its hard to find anything bigger than 25mm that isn't a big, clunky touring tire. Two I've used a lot, and really like, are Panaracer Paselas (come in 28, 32, 35 and 37mm) and Rivendell's Ruffy Tuffy, which I think is 27.
The labels, incidentally, aren't worth much. I just measured the Paselas on my bike and my wife's, and the 35s are actually 30.5 mm, the 32s about 25mm.
All right, I've got too much time on my hands. I've been shoveling snow for six hours, and there's another storm due tonight.


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## Fredrico (Jun 15, 2002)

*Thanks for the heads up.*



Cory said:


> In my speed-obsessed youth, I did the traditional experimenting with all kinds of tires, down to 19mm. I weighed about 220 then, and skinny tires were just unpleasant. I moved up to 25s despite the then-common belief that skinnier was always faster, and I was a lot more comfortable and didn't go any slower. Eventually I settled on 32s for most riding, 35s for rides when I know the pavement will be bad. Last summer I put on a set of 23s (apparently what most people on this board use), and I HATED 'em.
> As one of the other posts mentioned, its hard to find anything bigger than 25mm that isn't a big, clunky touring tire. Two I've used a lot, and really like, are Panaracer Paselas (come in 28, 32, 35 and 37mm) and Rivendell's Ruffy Tuffy, which I think is 27.
> The labels, incidentally, aren't worth much. I just measured the Paselas on my bike and my wife's, and the 35s are actually 30.5 mm, the 32s about 25mm.
> All right, I've got too much time on my hands. I've been shoveling snow for six hours, and there's another storm due tonight.


My search is over. I'll check out those Panaracer Paselas down at the shop. 30.5 cross section would be a great commuter/winter tire.

Watch your back with all that shoveling. Can you believe that? Snow! And more coming? Outside Reno, Nevada? Heck, put some more logs on the fire, make a hot chocolate, and read that book you've been putting off.


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## cxwrench (Nov 9, 2004)

*what are you talking about...?*



Spoke Wrench said:


> Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.


huh? how can pressure determine contact patch. enlighten me. what if it's a 26" tire. or a 650c. or a 27". what if i don't go by psi, but use bar instead...that means my contact patch would be in sq/cm, right? what about the difference between 20mm tires and 25mm tires. or what about difference in brands. they all have different shapes. this sounds like a crock to me, but i'm open to any science that'll prove it.


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## curlybike (Jan 23, 2002)

cxwrench said:


> huh? how can pressure determine contact patch. enlighten me. what if it's a 26" tire. or a 650c. or a 27". what if i don't go by psi, but use bar instead...that means my contact patch would be in sq/cm, right? what about the difference between 20mm tires and 25mm tires. or what about difference in brands. they all have different shapes. this sounds like a crock to me, but i'm open to any science that'll prove it.



For example, if you weighed 200 lb and the weight on your wheels was distributed 50%/front/50%rear. That would give you 100 lb per wheel, the bike weighs 20 lb, for a total of 220 and 110 per wheel. A tire that has 110 psi inflation pressure will have a contact patch of 1 sq. in. Becuse the down pressure is matched by the tire pressure you get a round # as the combined bike weight goes up you divide the individual loaded weight of each tire by the inflation pressure to derive the area of the contact patch. Works for any size tire and any tire pressure. You will have to do the metric conversion. 1 bar =14.7psi, 1 square inch =6.4516 sq. cm. I hope that this is not too muddy a concept, but that is the way it works.


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## Spoke Wrench (Aug 20, 2001)

cxwrench said:


> huh? how can pressure determine contact patch. enlighten me. what if it's a 26" tire. or a 650c. or a 27".


OK, take a look at a friend's bike just standing by itself. Rim diameter and tire width don't matter. The normally inflated tires will look virtually round even at the bottom. Now have your friend sit on the bike and check out the contact patch. The tire will be squished down somewhat and have a flat contact patch area from front to back. With your friend still sitting on his bike, let some air out of his tire. The contact patch will elongate and become bigger because he will have less air pressure to hold him up.

Now I'm curious. What did you think determined the size of your tire's contact patch?


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## AFS (Sep 15, 2004)

The narrower tires may buy you something aerodynamically, if you go fast enough. The aerodynamic drag reduction would have to out weigh the increase in rolling resistance. The necessary average speeds maybe out of reach for most riders on a given course . I run 28-mm tires.


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## cxwrench (Nov 9, 2004)

*oh c'mon...*



curlybike said:


> For example, if you weighed 200 lb and the weight on your wheels was distributed 50%/front/50%rear. That would give you 100 lb per wheel, the bike weighs 20 lb, for a total of 220 and 110 per wheel. A tire that has 110 psi inflation pressure will have a contact patch of 1 sq. in. Becuse the down pressure is matched by the tire pressure you get a round # as the combined bike weight goes up you divide the individual loaded weight of each tire by the inflation pressure to derive the area of the contact patch. Works for any size tire and any tire pressure. You will have to do the metric conversion. 1 bar =14.7psi, 1 square inch =6.4516 sq. cm. I hope that this is not too muddy a concept, but that is the way it works.


this is sounding ridiculous. the tire has 110psi in it. that 110 pounds per sq inch pressure is exerting itself outwards against the tire casing. i understand that. but who came up w/ the formula that says this pressure equals this size tire patch? the amount of air pressure inside a tire versus the amount of weight riding on it *can't* strictly determine the size of the contact patch.at this point you're not even looking at the size of the tire, you're basing this solely on pressure vs. weight supported. obviously it is going to have an effect on the size, but make that work for me w/ a car tire. c'mon, i dare ya...what effect does speed have on this? what about flat profiled car tires vs round profile motorcycle tires. there are just too many variables that you aren't taking into account. 
why are we even arguing about this anyway? this is why the term "bike geek" exists.


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## cxwrench (Nov 9, 2004)

*to tell you the truth...*



Spoke Wrench said:


> OK, take a look at a friend's bike just standing by itself. Rim diameter and tire width don't matter. The normally inflated tires will look virtually round even at the bottom. Now have your friend sit on the bike and check out the contact patch. The tire will be squished down somewhat and have a flat contact patch area from front to back. With your friend still sitting on his bike, let some air out of his tire. The contact patch will elongate and become bigger because he will have less air pressure to hold him up.
> 
> Now I'm curious. What did you think determined the size of your tire's contact patch?


i had never bothered wasting time thinking about it. you think i don't realize that a tire w/ no rider weight on it will have a smaller contact patch that one w/ the weight of a rider on it? c'mon, i did go to school some of the time, and i'm not blind. i just don't understand how you can "formulate" this w/o taking the size of the tire into consideration. get somebody w/ "dr." after his name or an engineer to say that this is the way things are, and i'll believe it, but 'til then, i don't.


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## Lifelover (Jul 8, 2004)

*Razor's edge*



Spoke Wrench said:


> Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in.


WOW there must not be much contact area when the 20 lb bikes does not have anyone sitting on it.

Theoreticallly, any statement that starts with the word "Theoreticallly" is wrong.


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## asgelle (Apr 21, 2003)

cxwrench said:


> i had never bothered wasting time thinking about it. you think i don't realize that a tire w/ no rider weight on it will have a smaller contact patch that one w/ the weight of a rider on it? c'mon, i did go to school some of the time, and i'm not blind. i just don't understand how you can "formulate" this w/o taking the size of the tire into consideration. get somebody w/ "dr." after his name or an engineer to say that this is the way things are, and i'll believe it, but 'til then, i don't.


O.K. I'm an engineer and have Dr. before my name, is that good enough? Gravity provides a force based on the mass of an object. On earth, that force is equal to the object's weight directed downward. For an object to be at rest, the forces on it must be in balance. So for a bicycle standing on it's tires (rather than the rims) there must be an upward force equal to the weight of the bicycle. That force is provided by the pressure in the tire acting over the contact area. Pressure is force per unit area directed perpendicular to the surface of the object; so the force from the tires is inflation pressure times contact area directed perpendicular to the ground. Setting the downward force equal to the upward force gives: weight of bicycle (and rider) = pressure in tires X tire contact area. Think of lowering the bicycle to the ground. At first the tire will be undeformed and there will be no upward force. The bicycle will move down and the tire will deform untill the up and down forces are equal. At that point, the contact area, A, will be A=weight on the tire/tire pressure. Size and shape of the tire will determine the shape of the contact patch, but the contact area is set only by the weight on the tire and the pressure in the tire.


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## Spoke Wrench (Aug 20, 2001)

AFS said:


> The narrower tires may buy you something aerodynamically, if you go fast enough. The aerodynamic drag reduction would have to out weigh the increase in rolling resistance. The necessary average speeds maybe out of reach for most riders on a given course . I run 28-mm tires.


Yeah, there are some other factors that impact tire performance also. 

Generally you have to use higher air pressure with a narrower tire or you'll get pinch flats. That pretty much negates the whole tire patch rolling resistance thing that we were just talking about. 
A tire with a thin tread layer and nice supple sidewalls will deform more easily and consequently have less rolling resistance.

All of those things have an effect and, when added together, can make a difference, but everything combined is still small potatoes when compared with the amount energy that has to be used to push your torso through the air.


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## RCA (Nov 18, 2002)

*I'm with Cory*

I am big and I ride a bike (those are my qualifications for my expertise) I have tried 23s,25,and 32s I now put 95lbs of air in Conti 2000sx32 and ride. If they are slowing me down then I will work harder and lose more weight. If as some of you allege they are faster then I will enjoy the speed. They allow me to head out as a roadie then scurry home ahead of a storm on an unpaved rail trail. 
Don't ever stop arguing boys and girls here in the land of ice and snow you are my entertainment. I'll go out and shovel now so the forum can build up again 
Rick


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## AFS (Sep 15, 2004)

Spoke Wrench said:


> Yeah, there are some other factors that impact tire performance also.
> 
> Generally you have to use higher air pressure with a narrower tire or you'll get pinch flats. That pretty much negates the whole tire patch rolling resistance thing that we were just talking about.
> A tire with a thin tread layer and nice supple sidewalls will deform more easily and consequently have less rolling resistance.
> ...



It's funny how we tend to worry about the small potatoes.


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## MisterMo (May 31, 2004)

*Ok*



asgelle said:


> Size and shape of the tire will determine the shape of the contact patch, but the contact area is set only by the weight on the tire and the pressure in the tire.


I should know better to stick my nose in this but... You are correct in the 'textbook' sense, one where the tire materials and resistance to shape deformation are assumed to have little enough effect to be ignored. The stiffer the tire materials involved, the less effect the pneumatic properties have...the ride of course goes to hell as well.


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## asgelle (Apr 21, 2003)

MisterMo said:


> I should know better to stick my nose in this but... You are correct in the 'textbook' sense, one where the tire materials and resistance to shape deformation are assumed to have little enough effect to be ignored. The stiffer the tire materials involved, the less effect the pneumatic properties have...the ride of course goes to hell as well.


And which model bicycle tire would that be where the sidewall stiffness is significant? We're not talking runflat car tires here. The bike tires I use aren't stiff enough to support their own weight but I'll grant you, if you use total weight to calculate contact area your error may be as great as 0.03% (20 gm sidewall support for a bike+rider weight of 150 lbs). Now tell me on what other aspects of your life do you impose that rigor? Do you measure your ride distance to 3 parts in 10,000? What computer do you use and how often do you correct roll out to account for changes in weight and tire pressure (don't forget your weight and the air temp in the tire changes during a ride).

Oh my God, I just realized I also forgot to include the van der Waals attraction of the tire surface to the roadway. That would pull the tire to the road and increase contact area. It get's even worse since the area will increase with time the longer the tire rests on the road.


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## dasho (Apr 8, 2002)

AFS said:


> It's funny how we tend to worry about the small potatoes.


I wasn't really worrying about it but I found it hard to believe.


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## C-40 (Feb 4, 2004)

*huh??*

"Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in."

I don't think so, Spoke Wrench. Your statement might sound logical, but the contact area would NOT be one square inch, just because the air pressure and load are the same.


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## asgelle (Apr 21, 2003)

C-40 said:


> I don't think so, Spoke Wrench. Your statement might sound logical, but the contact area would NOT be one square inch, just because the air pressure and load are the same.


What would the area be then, and how would you determine it?


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## bigrider (Jun 27, 2002)

*Sounds like the simple answer is the right answer*



C-40 said:


> "Theoretically, air pressure determines the size of your tire's contact patch. If the loading on your tire is 125 pounds and your tire is inflated to 125psi, your contact patch will be 1 sq/in."
> 
> I don't think so, Spoke Wrench. Your statement might sound logical, but the contact area would NOT be one square inch, just because the air pressure and load are the same.


Copied and pasted from the internet on HOW A TIRE WORKS

The next time you get in your car, take a close look at the tires. You will notice that they are not really round. They are squished at the bottom. The flat spot on the bottom where the tire meets the road is called the contact patch. 
If you were looking up at a car through a glass road, you could measure the size of the contact patch. You would multiply the length of the contact patch by its width to get the area, then add up area for all four tires to get the total area of the contact patch. 



A tire showing the side and bottom view of the contact patch. 
For your 2-ton (4,000 lb) car, you will find that the area of the contact patch is about equal to the weight of the car divided by the tire pressure. In this case 4,000 pounds divided by 30 pounds per square inch equals 133 square inches. That may seem like a lot, but your car's tires are probably about 7 inches wide. That means that the contact patch for each tire will be about 4.75 inches long.


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## Bill70J (Sep 23, 2004)

*The Doc's Right*



asgelle said:


> What would the area be then, and how would you determine it?


It's very difficult to refute what asgelle is saying - it's proven theory. Infinitesimal as they would be, he has even offered to take into consideration differences between ideality and a physical measurement.


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## Reynolds531 (Nov 8, 2002)

*Another omission*



asgelle said:


> And which model bicycle tire would that be where the sidewall stiffness is significant? We're not talking runflat car tires here. The bike tires I use aren't stiff enough to support their own weight but I'll grant you, if you use total weight to calculate contact area your error may be as great as 0.03% (20 gm sidewall support for a bike+rider weight of 150 lbs). Now tell me on what other aspects of your life do you impose that rigor? Do you measure your ride distance to 3 parts in 10,000? What computer do you use and how often do you correct roll out to account for changes in weight and tire pressure (don't forget your weight and the air temp in the tire changes during a ride).
> 
> Oh my God, I just realized I also forgot to include the van der Waals attraction of the tire surface to the roadway. That would pull the tire to the road and increase contact area. It get's even worse since the area will increase with time the longer the tire rests on the road.


When the weight is applied to the tire the tire deforms and reduces the volume in the tube, thereby increasing the pressure and reducing the contact area. This might reduce the contact area by another 0.1%or so, however, this error can be eliminated if you inflate the tires while sitting on your bike.

And what about the angular acceleration which effectively pushes outward on the tire, tube, and the air molecules within the tube? Doesn't this also reduce the contact area? All these errors reduce the contact area for 125 psig tires loaded with 125 lbs of weight to 0.99873 square inches. That's my number, prove me wrong.


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## cxwrench (Nov 9, 2004)

*but wait!*



bigrider said:


> Copied and pasted from the internet on HOW A TIRE WORKS
> 
> 
> 
> ...


we all know that a bicycles weight distribution isn't 50/50. it's more like 30/70 or maybe 40/60 (fr to rr), so i WAS RIGHT! ha!


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## Kerry Irons (Feb 25, 2002)

*Wow!*

I couldn't believe it when I saw 25 posts in this thread. I looked at it yesterday, and it was all sewn up with the posts from johnny99 Spoke Wrench and Frederico. What could there possibly be to talk about that would add 20 more posts? Then I thought, it couldn't be someone not believing that contact patch area = weight/pressure, could it? But sure enough, it was. Amazing. Try this, for those who just can believe it: divide pounds by pounds per square inch. What you get is square inches. Funny thing about engineering and physics but the units tell the story. Always have, always will.


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## asgelle (Apr 21, 2003)

Kerry Irons said:


> Funny thing about engineering and physics but the units tell the story. Always have, always will.


Unless there's a dimensionless proportionality constant such as drag coefficient in the relation F_d=1/2 C_d A rho v^2 . The question here was whether there was a similar term, C_a depending on tire size, in the relation of contact area to weight and tire pressure. Of course there isn't (or more accurately, C_a=1).

Actually, that's not correct. The coefficient C_a does depend on the size of the tire for the reason previously mentioned, i.e., as the tire deforms, the volume changes and the pressure in the tube rises lowering the contact area relative to that calculated for an undeformed tire. However, as also mentioned before, the magnitude of this change is so small as to be negligible as are the other effects mentioned before.


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## Reynolds531 (Nov 8, 2002)

*Engineers just don't realize...*



Kerry Irons said:


> I couldn't believe it when I saw 25 posts in this thread. I looked at it yesterday, and it was all sewn up with the posts from johnny99 Spoke Wrench and Frederico. What could there possibly be to talk about that would add 20 more posts? Then I thought, it couldn't be someone not believing that contact patch area = weight/pressure, could it? But sure enough, it was. Amazing. Try this, for those who just can believe it: divide pounds by pounds per square inch. What you get is square inches. Funny thing about engineering and physics but the units tell the story. Always have, always will.


How ignorant most people are about the most basic concepts of physics and engineering.


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## bigrider (Jun 27, 2002)

*Now your'e grasping*



cxwrench said:


> we all know that a bicycles weight distribution isn't 50/50. it's more like 30/70 or maybe 40/60 (fr to rr), so i WAS RIGHT! ha!



The poster stated how much weight there was on the rear tire only and conveniently made that example the exact weight as the psi in the tire to get a nice round number, a 1. 

The post I made just used a four tire example and assumed equal distribution of the weight among the four tires.


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## MisterMo (May 31, 2004)

*Touchy?*



asgelle said:


> And which model bicycle tire would that be where the sidewall stiffness is significant? We're not talking runflat car tires here. The bike tires I use aren't stiff enough to support their own weight but I'll grant you, if you use total weight to calculate contact area your error may be as great as 0.03% (20 gm sidewall support for a bike+rider weight of 150 lbs). Now tell me on what other aspects of your life do you impose that rigor? Do you measure your ride distance to 3 parts in 10,000? What computer do you use and how often do you correct roll out to account for changes in weight and tire pressure (don't forget your weight and the air temp in the tire changes during a ride).
> 
> Oh my God, I just realized I also forgot to include the van der Waals attraction of the tire surface to the roadway. That would pull the tire to the road and increase contact area. It get's even worse since the area will increase with time the longer the tire rests on the road.


I SHOULD have known better than to dive in to this. But since I did....when I posted that I was thinking (just a little) off-topic about a totally rigid wheel and an equally rigid rolling surface. In that instance the contact area is theoretically zero in area and the ground pressure in psi is therefore theoretically infinite, the point here being that the math says, I think, that these two things are true, but the reality is that other factors come into play and so they aren't quite so. 
So it is with pneumatic tires, your math says (correctly) that if the pressures are such and such then the contact area will be so and so. The fact that other modifiying considerations are most likely minimal does not mean that they don't exist.
I realize all of this may seem childlishly trivial, but, the world here is really snowy at the moment & I have nothing better to do. No offense intended & I hope none taken...


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## cxwrench (Nov 9, 2004)

*no...*



bigrider said:


> The poster stated how much weight there was on the rear tire only and conveniently made that example the exact weight as the psi in the tire to get a nice round number, a 1.
> 
> The post I made just used a four tire example and assumed equal distribution of the weight among the four tires.


the original post stated "a 200lb rider on a 20lb bike w/ 110psi in the tires" that's 220lb/2= 110lb sitting on 2 tires ea w/ 110psi. so...110/110=1sq" 

also, virtually no cars have 50/50 wt distribution either, front to back or side to side, so they would be off just i little bit as well...the average would be right, i guess.


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## AJS (Aug 7, 2003)

MisterMo said:


> I SHOULD have known better than to dive in to this.


That's right, Mo, you _should have_ known better. LOL! There are too many propeller-heads and quasi-geeks on this site for a topic like this to go past lightly!

It's all good! 
.
.


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## C-40 (Feb 4, 2004)

*no formula..*

There is no formula that can be used to determine the contact area. If you knew what the area was, then the load on the tire would be the weight on the tire divided by the area. It could just as well be .5 square inches with a pressure of 250 lbs per square inch. Makes just as much sense.

You might get a crude estimation by wetting the tire and sitting on the bike, then trying to measure the contact area. Trying to accurately measure the area of an odd shaped spot isn't easy to do either.

Comparing car tires which are quite flat to a very round bike tire is also pretty lame.


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## asgelle (Apr 21, 2003)

*Huh?*



C-40 said:


> If you knew what the area was, then the load on the tire would be the weight on the tire divided by the area.


What is the load on the tire if not the weight? In which case how can weight = weight divided by area. However, if by load on the tire, you mean pressure on the tire (which must equal the pressure in the tube), then it would seem that you are saying pressure(load) = weight / area. If that is so, could you please explain why area does not equal weight / pressure?


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## Reynolds531 (Nov 8, 2002)

*This is the funniest thread in months*



asgelle said:


> What is the load on the tire if not the weight? In which case how can weight = weight divided by area. However, if by load on the tire, you mean pressureon the tire (which must equal the pressure in the tube), then it would seem that you are saying pressure(load) = weight / area. If that is so, could you please explain why area does not equal weight / pressure?


What is so obvious and simple to us engineers is baffling to some of the others. I almost think that some of the posters are trolling. Is the concept that force equals pressue times area really that difficult?


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## Spoke Wrench (Aug 20, 2001)

C-40 said:


> There is no formula that can be used to determine the contact area. /QUOTE]
> 
> asgelle just gave you a formula in message #10 along with the logic behind it. What part of that doesn't make sense to you?


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## Spoke Wrench (Aug 20, 2001)

Reynolds531 said:


> What is so obvious and simple to us engineers is baffling to some of the others. I almost think that some of the posters are trolling. Is the concept that force equals pressue times area really that difficult?


There is a common misconception among bicyclists that a wider tire will necessarily have a larger contact patch and greater rolling resistance. That misconception is repeated so often that many riders accept it on it's face as being true. In fact, the original post referenced a contributor to Bicycling Magazine that said (incorrectly) that a 25mm tire will have less contact area with the road than a 23mm tire. 

Trust me, C-40 and cxwrench aren't the only ones reading all of this that still think that way, they're just the only ones that have the courage and the self confidence to post.

Mark Twain, one of my favorite Missouri natives, said something like: "It ain't what we don't know that gits us. It's the things that we know for sure that ain't so."


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## curlybike (Jan 23, 2002)

How ignorant most people are about the most basic concepts of physics and engineering.


Not only that, their minds are made up and they refuse to be confused by the facts!!!

Heh Heh


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## C-40 (Feb 4, 2004)

*weight/pressure does NOT equal contact area !!!*

Asgelle, I'm an engineer too, (mechanical) but no doctorate or masters for that matter. A fancy title doen't make everything you say correct, particuarly if you have nothing of substance to back up your contention.

I think all can agree that a load is unit of weight. In the example given by spoke wrench, it was a load of 125 pounds on the rear wheel, which could be imposed by a heavy rider.

The contension is that due to the 125 psi air pressure in the tire, that the contact area of the tire with the ground must be 1 inch, since 125lbs/1inch = 125 psi. I contend that this is not necessarily true, since it assumes that the tire is merely a free-moving membrane like a rubber balloon, which it is NOT. A tire is much more highly constrained by the fabric that dictates it's shape. This (faulty) analysis also assumes that the tire size and shape make no difference in the contact area. It also assumes that the entire contact area is under the same pressure, when in reality, some portions of the contact area are more heavily loaded than others.

A tire has essentially no strength until air pressure is applied, much like a wheel spoke with no tension that cannot carry a compressive load. With 125 psi of air pressure inside the tire, the tire becomes tensioned by the air pressure. The formula for hoop stress on a cylinder would provide a rough estimate of this tension.

http://physics.uwstout.edu/StatStr/Strength/Columns/cols75.htm

Plug in 125 psi, a .43 inch internal radius and a wall thickness of .025 inch (I measured one of my tires) and you get a hoop stress of 2150 psi. Other formulas can be found that apply to a torroid (donut), giving the circumferential stress.

I did my own real-world test using a no-tread tire inflated to 100 psi with an applied load of 137 lbs (me sitting on top of it). I wet the tire and carefully placed it on a piece of very smooth fiberboard, where the water would make an imprint of the contact patch. I found the contact patch to be around the predicted 1.37 inch (and yes I repeated the test several times). Seems like I must be wrong? Well, it depends on the tire. This first test was done with a front tire that still had it's well rounded shape. Perform the same test with a rear tire (same model) that was pretty well worn and I got a contact area of almost 2 inches, when pumped to the same 100 psi. Right there the theory is blown.

Increase the tire pressure to 125 psi and the contact area does NOT become 25% smaller, although it was (predictably) smaller. Drop the air pressure to a mere 60 psi and the contact area does NOT double, although it did increase about 70%. Interestingly, the front tire with 60 psi didn't have a much larger contact area than the rear tire with 100 psi. Roughly, I could get about the same contact area from a slightly worn tire at 70 psi that I got from the well worn (flat profile) tire at 100 psi.


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## asgelle (Apr 21, 2003)

C-40 said:


> The formula for hoop stress on a cylinder would provide a rough estimate of this tension.


Please relate hoop stress in the tire casing to the force exerted by the tire on the ground since it is the force of the tire on the ground that supports the load on the tire. Thank you.
(hint: remember force is a vector quantity)


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## C-40 (Feb 4, 2004)

*why don't you???*

I'm just a old mechanical engineer who's engaged in practical application but not much high powered math in the last 24 years. All I do is keep up with my trig and geometry. You're the one with the fancy title.

I think my test shows quite well that there's a lot more to the subject than something as simple a weight/pressure = contact area. The area is actually larger than this simplistic formula would predict, since all of the contact area is not applying the same pressure to the ground.


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## asgelle (Apr 21, 2003)

C-40 said:


> I think my test shows quite well that there's a lot more to the subject than something as simple a weight/pressure = contact area.


Except you told us nothing about the methodology of your test, it's accuracy, repeatability, random or systemic errors. For most examples you gave no actual data but just used vague terms such as "area does NOT double" So lets see it; give us data of load vs contact area including errors bars and how the data were measured and then we might have something to talk about.


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## Reynolds531 (Nov 8, 2002)

*Very poor measurement system*

The water can wet the surface even if the tire is not in contact with the surface. As the tire squishes down against the surface the water will make a bridge between the tire above the surface and the surface to minimize free energy. Your area numbers derived by this technique do not reflect the true contact area. Defining and measuring contact area is extremely difficult. You'll know you have a good measurement system when the results confirm that force equals area times pressure.


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## Len J (Jan 28, 2004)

*Maybe we should have an engineers only board..........*

This really cracks me up. Every couple of years we have a thread like this where a group of engineers argue, seemingly endlessly, about some point of esoterica, the difference which makes absolutly no difference to anyone but them.

It's like hearing those old college arguments about angels on the head of a pin.

Very entertaining.

Len


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## haiku d'etat (Apr 28, 2001)

this is precisely the reason snoop douggy sloan trims the nubs off new tires.


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## C-40 (Feb 4, 2004)

*too funny...*

Typical baloney from a PHD. What's your PHD in? 

The results from the the two tire shapes were so obvious and repeatable, that no amount of experimental error would change the conclusion, only fine tune the percentages. The fact that the tire shape due to wear affected the contact area so much surprised me. The pattern was nearly as long but much wider than the newer tire contact pattern. Totally blows the theory that weight/pressure = contact area. Your wonderful formula would predict an area of 1.37 inches and the actual area was a little less 2 inches (.5 x 4) repeated several times.


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## Bill70J (Sep 23, 2004)

*Straightforward Principles*



C-40 said:


> I'm just a old mechanical engineer who's engaged in practical application but not much high powered math in the last 24 years. All I do is keep up with my trig and geometry. You're the one with the fancy title.
> 
> I think my test shows quite well that there's a lot more to the subject than something as simple a weight/pressure = contact area. The area is actually larger than this simplistic formula would predict, since all of the contact area is not applying the same pressure to the ground.


The whole issue here involves very straight forward principles, which have been explained in early posts by asgelle and others. I thought the post describing automobile tire contact patch is another good practical example of what we're talking about. This has nothing to do whatsoever with fancy titles or esoteric concepts. If you must, take it on faith. The Doc's right.


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## Reynolds531 (Nov 8, 2002)

*Didn't you get spanked by E-richie on head tube angle and trail, too?*

You contribute a lot to this board, but when you're flat out wrong you fight like a bulldog.


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## asgelle (Apr 21, 2003)

C-40 said:


> Typical baloney from a PHD. What's your PHD in?


Why? Do you think the laws of physics change based on an individual's degree?
And why do you think it is O.K. to disparage someone based on their education, maybe what I wrote is garbage, but would anyone here find it acceptable to write, "Typical baloney from a janitor, farmworker, or schoolteacher" I know I wouldn't. I'll let a person's words speak for themselves. I don't care what their professon or educational background is.


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## wasfast (Feb 3, 2004)

Perhaps doing a better, real world test, is in order. I imagine a piece of plate glass, mounted to a rigid substrate, like a piece of plywood. The plywood has a window cutout in it where you could look from underneath with a digital camera. The plywood would need to be spaced off the floor, say with a pair of 2 x 4 blocks. You should also place a machinist 6" scale parallel to the rim, next to the tire. 

You place the bike on the glass and take photos of the contact patch, which will appear blacker than the tire tread where it's sitting on the glass. The machinist ruler gives you a relative scale for the photo.


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## Reynolds531 (Nov 8, 2002)

*Will you believe Boeing?*



C-40 said:


> Typical baloney from a PHD. What's your PHD in?
> 
> The results from the the two tire shapes were so obvious and repeatable, that no amount of experimental error would change the conclusion, only fine tune the percentages. The fact that the tire shape due to wear affected the contact area so much surprised me. The pattern was nearly as long but much wider than the newer tire contact pattern. Totally blows the theory that weight/pressure = contact area. Your wonderful formula would predict an area of 1.37 inches and the actual area was a little less 2 inches (.5 x 4) repeated several times.


Calculating Tire Contact Area 
The tire contact area for any aircraft tire is calculated by dividing the single wheel load by the tire inflation pressure. If the load is expressed in pounds, and the tire pressure in pounds per square inch, then the area is in inches squared. The same thing works with kilograms and kg/cm2 - the result will then be in square centimeters. 
The shape of the footprint area is usually understood to be a 1.6 ellipse (as referenced in the US Corps of Engineer's S-77-1 Report), wherein the major axis is 1.6 times the minor axis. The calculation to solve for the minor axis is .894 times the square root of the contact area. Note that the major axis runs parallel to the normal direction of motion of the aircraft, and the minor axis is perpendicular to the major axis 
Information concerning the landing gear footprint, tire inflation pressures, center of gravity location and load on the nose or main gear are provided in Section 7 of the "Airplane Characteristics for Airport Planning" manual for each airplane model at the following website: www.boeing.com/airports. 
Example: 777-300 Main Gear Tire Contact Area 
For this case, use the maximum taxi weight of 662,000 lbs configuration of the 777-300 as shown in Figure 7.2 “Landing Gear Footprint - 777-200/300” and Figure 7.3 “Maximum Pavement Loads - 777-200/300.” Figure 7.2 provides the main gear tire pressure of 215 PSI. Figure 7.3 shows the V(mg) per strut / maximum load at the static aft center of gravity for this airplane configuration of 313,900 pounds. Given that the 777-300 has six wheels per main gear as shown in Figure 7.2, to calculate the contact area first determine the load per tire (313,900 / 6 = 52,317) then to calculate the contact area, divide the load per tire by the PSI (52,317 / 215 = 243.3 in2 contact area). 
The footprint area is a 1.6 ellipse determined as follows: 
Minor axis is .894 x square root of the contact area (0.894 x sq root of 243.3 = 13.94 inches minor axis) 
Major axis is 1.6 x minor axis (1.6 x 13.94 = 22.30 inches major axis) 
Additional questions concerning this issue can be directed to Boeing’s Airport Technology group as follows: 
Boeing Airport Technology 
P.O. Box 3707, MC 67-KR 
Seattle, WA 98124 
425-237-0126 
[email protected] 
12/7/2004 Airport Technology Boeing Commercial Airplanes


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## haiku d'etat (Apr 28, 2001)

LOL, you guys certainly need something better to do. and some prozac.

check out the "heat molding a saddle" thread.


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## C-40 (Feb 4, 2004)

*theory busted...*

Hey, you're the guy who started his response with "O.K. I'm an engineer and have Dr. before my name, is that good enough?", then procceded to dish out the crap about the error analysis. Anyone can pull a forumla out of a book or off the internet, but it doesn't necessarily mean it's the right one.

Just because I've got a BSME doesn't make me right either, but at least I perfomed a series of tests that casts serious doubt on the idea that weight/tire pressure = contact area. 

I did another series of tests with both my well worn rear tire and the matching, hardly worn front. This time I put my foot between the widely spaced spokes and carefully placed all my weight on each wheel. I got about the same results; larger contact areas than the formula would predict ( I thought they might be smaller) and nowhere near a doubling of the contact area by reducing the prssure from 100 to 50 psi or from 120 and 60 psi. About the most I could get was a 60% increase from reducing the pressure in half. Interestingly, the contact area came closer to the formula's prediction at the lower pressure. Theory busted as far as I'm concerned.

As an example of a typical contact pattern for my rear tire, the average width was .55 inches. The contact area by the formula predict a length of only 2.15 inches, but the actual length was over 3 inches at full width, before tapering off at both ends to a total length of 3.8 inches. .55 inches times 3 is 1.65 sq. in. and I conservatively estimated the total to be 1.85 sq. inches against a formula prediction of 1.18 sq. inches. Not even close at the normal rear tire pressure.

At 60 psi I got around a 2.8 sq.in. pattern against a predicted 2.37, which isn't that far off, but certainly not twice the area of the 120 psi contact pattern.


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## asgelle (Apr 21, 2003)

C-40 said:


> Hey, you're the guy who started his response with "O.K. I'm an engineer and have Dr. before my name, is that good enough?"


Maybe you should read the thread more carefully. I was replying to cxwrench who wrote, " i just don't understand how you can "formulate" this w/o taking the size of the tire into consideration. get somebody w/ "dr." after his name or an engineer to say that this is the way things are, and i'll believe it, but 'til then, i don't." Which is what I replied to. The "is that good enough" referred to the Dr before my name rather than after as cxwrench wrote. I found his requirement amusing since the laws of physics don't depend on anyone's level of education. Either an argument stands on its own merits or it doesn't; it doesn't matter what letters anyone has before or after his (her) name.


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## C-40 (Feb 4, 2004)

*Ok*

"Except you told us nothing about the methodology of your test, it's accuracy, repeatability, random or systemic errors. For most examples you gave no actual data but just used vague terms such as "area does NOT double" So lets see it; give us data of load vs contact area including errors bars and how the data were measured and then we might have something to talk about."

Now I've given some relevant data and I'm convinced it's accurate enough to show that this simplistic formula doesn't govern the contact area of bicycle tires. My test is only relevant for the tire model being tested. I'll bet every brand would produce a slightly different result. 

Sorry I didn't include "error bars" and "systemic errors".


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## Reynolds531 (Nov 8, 2002)

*You are hemorrhaging credibility*



C-40 said:


> "Except you told us nothing about the methodology of your test, it's accuracy, repeatability, random or systemic errors. For most examples you gave no actual data but just used vague terms such as "area does NOT double" So lets see it; give us data of load vs contact area including errors bars and how the data were measured and then we might have something to talk about."
> 
> Now I've given some relevant data and I'm convinced it's accurate enough to show that this simplistic formula doesn't govern the contact area of bicycle tires. My test is only relevant for the tire model being tested. I'll bet every brand would produce a slightly different result.
> 
> Sorry I didn't include "error bars" and "systemic errors".


The more you argue about this, the more credibility you lose. You are making arguments that contradict yourself. In one post you say "A tire has essentially no strength until air pressure is applied" which logically infers that only air pressure supports the weight of the load, yet now you insist that the tire model has a significant affect on the contact area. You brought irrelevent calculations of hoop stresses into the argument. You argue that your crude and faulty measurement system for tire contact area disproves the calculation that Boeing uses for contact area of airplane tires. I'll never look at your posts in the same way again. The signature line from another member applies to you perfectly. Always certain. Frequently wrong.


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## Lucille (Oct 30, 2004)

Reynolds531 said:


> The more you argue about this, the more credibility you lose.... You argue that your crude and faulty measurement system for tire contact area disproves the calculation that Boeing uses for contact area of airplane tires. I'll never look at your posts in the same way again. The signature line from another member applies to you perfectly. Always certain. Frequently wrong.



Roger that. This thread has really blown my confidence in at least one of the more prolific contributors here. Simple physics is just that. I'm really startled that this debate never got any closer to resolution on a topic that seemed adequately addressed from the first half dozen posts.

With the identification of degrees, it would be helpful if the college from which the degree was taken was also identified. Not so that I can know how much pressure to use with my tires, but so that I can know where NOT to send my sons to school for an engineering education.


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## AJS (Aug 7, 2003)

*In defense of geeks!*



Len J said:


> This really cracks me up. Every couple of years we have a thread like this where a group of engineers argue, seemingly endlessly, about some point of esoterica, the difference which makes absolutly no difference to anyone but them.
> 
> It's like hearing those old college arguments about angels on the head of a pin.
> 
> ...


Don't be so hard on us gearhead types, Len. Arguments about minutiae is (to me at least) often times fun and can be quite informative. It can expose bad theory or processes, as well as cause new ideas to pop out of nowhere. It can also end up like a dog chasing its tail, or mash egos. But a lot of people's brains don't work that way and they can get bored to tears. Understandable!

When you consider the amount of engineering that can go into the complex system of a bicycle, it's not any wonder that these discussions are going to crop up.


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## C-40 (Feb 4, 2004)

*airplane tires = bicycle tire???*

I googled that site too. Doesn't mean squat unless it says that they've also tested bicycle tires and know that they have the same characteristics. Wouldn't you think that airplane tire have very thick tread that compresses and extremely stiff belts? Doesn't seem like a staightforward comparsion to me.

For all those complaining about faulty testing and lack of credibilty, please elaborate on what was faulty about the technique. I'd be perfectly willing to test more if you experts could help me refine my technique. 

My tire pump is relatively well calibrated. I have an expensive, liquid filled pressure gauge to check it against. I know how much I weigh, so the applied load shouldn't be far off. The load was equivalent to the rear wheel of a bike with a hefty 250lb rider. The technique I used to leave the imprint was very reapeatable. I didn't get any large variations in the size of the imprints taken at the same pressure, with the same tire. If the technique had errors, then one imprint would vary significantly from the next. Such an error could easily occur if the weight was applied too quickly, creating a momentary dynamic force. If this did occur, at least it was consistent and the proposed formula would still apply. Rolling the tire forward or back or side to side would alter the size of the imprint, but his too, would show up as a discrepant (much larger) imprint than the others. I had this problem a time or to, but threw out those imprints.

It was extremely easy to see the consistant difference between the contact patterns left by tire at the 120 and 60 psi pressures. Unfortunately, the patterns at 120 psi were no where near 1/2 the size of those at 60 psi. Even if there were errors, how would you explain that over 20 comparisons failed to show a single one that even remotely followed the formula's prediction? 

Perhaps one of you skeptics should try this yourself and see if you get different results.


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## Reynolds531 (Nov 8, 2002)

*yes airplane tire = bicycle tire and this is getting tired*



C-40 said:


> I googled that site too. Doesn't mean squat unless it says that they've also tested bicycle tires and know that they have the same characteristics. Wouldn't you think that airplane tire have very thick tread that compresses and extremely stiff belts? Doesn't seem like a staightforward comparsion to me.
> 
> For all those complaining about faulty testing and lack of credibilty, please elaborate on what was faulty about the technique. I'd be perfectly willing to test more if you experts could help me refine my technique.
> 
> ...


If the tire tread or the belts had a significant effect on the imprint of the airplane tire, then the simple equation presented by Boeing would not work. Neither the airplane tire nor the bicycle tire has sufficient strength in the sidewall and tread to support the load, and the same simple pneumatic equation applies to both. Are you actually spending hours inflating your tires to different pressures, wetting your tires, pressing them against a board, and measuring the area? If so, it is a desperate ands futile attempt to prove yourself right! Can you post an MPEG movie of your procedure so we can all analyze your technique? Maybe we can get this thread up to 100 posts and 2000 views. Things are slow at work this week.


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## symbo (Dec 7, 2004)

What a thread. forgive me if this has been brought up already, but I started to scroll pretty fast once we got into the Boeing stuff (and I'm an aerospace engineer to boot).

The pressure equation guys have apoint, but so do the shape/other factor guys. I can tell you my mountain bike with a hard compound knobby and ice screws does not have the same contact patch as a gummy hardpack tire - even if it was the same size and pressure. plus where it really matters, the ground surface and your force vector will change that contact patch as you ride. Static comparisons may be fun, but extrapolating them to dynamic performance is a bit more complicated. Ride the tire, then you'll know how it feels.

As the gun nuts will tell you, sectional density matters.


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## bigrider (Jun 27, 2002)

*Force Vector question*

[I can tell you my mountain bike with a hard compound knobby and ice screws does not have the same contact patch as a gummy hardpack tire - even if it was the same size and pressure. plus where it really matters, the ground surface and your force vector will change that contact patch as you ride. .[/QUOTE]



Is the Force Vector a hardtail or a full suspension?


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## AJS (Aug 7, 2003)

Reynolds531 said:


> Are you actually spending hours inflating your tires to different pressures, wetting your tires, pressing them against a board, and measuring the area?


I don't think that's the point. C-40's attempting at least some sort of empirical testing to sort out the equations, rather that just relying on the theory of the numbers. Even a simple or crude hands-on test is better than none.

As any good engineer knows, everything is subject to change when you take it from the slide rule and apply it to the actual problem. That's when you really start to get answers. - good ones or bad.

Then you go back to the drawing board - lather, rinse, repeat - until you get something that works.


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## Reynolds531 (Nov 8, 2002)

*Now don't be reasonable!*



symbo said:


> What a thread. forgive me if this has been brought up already, but I started to scroll pretty fast once we got into the Boeing stuff (and I'm an aerospace engineer to boot).
> 
> The pressure equation guys have apoint, but so do the shape/other factor guys. I can tell you my mountain bike with a hard compound knobby and ice screws does not have the same contact patch as a gummy hardpack tire - even if it was the same size and pressure. plus where it really matters, the ground surface and your force vector will change that contact patch as you ride. Static comparisons may be fun, but extrapolating them to dynamic performance is a bit more complicated. Ride the tire, then you'll know how it feels.
> 
> As the gun nuts will tell you, sectional density matters.


If we are reasonable, this thread would have ended long ago. You wouldn't want that, would you?

Dynamic loads open up an entire new area for debate. I'll start by contending that even under dynamic loads, there is not a significant difference in contact area for wide and narrow bicycle tires with a flexible tread and sidewall, inflated to equal pressures unless the dynamic load is great enought to cause the tire tread to compress against the rim.


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## haiku d'etat (Apr 28, 2001)

like, gag me with a spoon


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## Kaboom (Jul 18, 2003)

cxwrench said:


> we all know that a bicycles weight distribution isn't 50/50. it's more like 30/70 or maybe 40/60 (fr to rr), so i WAS RIGHT! ha!


 the only couple of cars i know with a 50/50 distribution are the Mazda mr2 and the Lotus elise, both of which are very, VERY uncommon machines (well the mazda perhaps not so much). not even ferraris are 50/50.
Also, while in bikes we pump both tires to the same pressure (usually) cars get different front and rear pressures


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## Kaboom (Jul 18, 2003)

ok, so after ALL of this...
Do fatter tires roll faster, or do they not?


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## elviento (Mar 24, 2002)

*A little illustration here.*

My dad went to a top engineering school and was an engineer for many years so I am hoping the genes tricked down a bit: 

The principle is fairly simple. The argument is how much the other factors such as structural strength of tire casing and shape of the tire contribute to the patch size and whether that's so small as to be neglected. 

C-40's worn out tire example illustrates an extreme scenario but he does have a point. Much of the contact area in that scenario (near the sides) are not really supporting the weight of the bike, it's just that the shape of the tire dictates that the sides come into contact with the ground. These sides are technically part of the contact area but not the "contact area" the engineers have in mind. But surely such contact area will wet the floor. 

In the graph (if it uploads), you can see support is only provided along A, but the tire touches the ground along B. 

So if by "contact area" you mean the area that supports the bike and rider, then the principle is generally right. If you mean size of rubber actually touching the ground, then these other factors could be significant. 

BTW, I think plane tires and bike tires are similar on this question.


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## asgelle (Apr 21, 2003)

Kaboom said:


> ok, so after ALL of this...
> Do fatter tires roll faster, or do they not?


That's not the question.


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## haiku d'etat (Apr 28, 2001)

yeah, don't make them start another post-doctoral thesis on the general board.


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## AJS (Aug 7, 2003)

Kaboom said:


> ok, so after ALL of this...
> Do fatter tires roll faster, or do they not?


We don't know yet, because there seems to be some  here.

BUT, we DO know that fatter _people_ roll better than skinnier ones.


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## Reynolds531 (Nov 8, 2002)

*I think that you are completely right. I'll pay penitence*



elviento said:


> My dad went to a top engineering school and was an engineer for many years so I am hoping the genes tricked down a bit:
> 
> The principle is fairly simple. The argument is how much the other factors such as structural strength of tire casing and shape of the tire contribute to the patch size and whether that's so small as to be neglected.
> 
> ...


You've finally settled this debate. I'll quote myself "It is very difficult to define and measure contact area." We argued ad nauseum because we never really defined what we were arguing about. I'm guilty and for penitence I'll ride some $7 Cheng Shin tires inflated to 140 psi down a cobblestone road until the tire pops or I get hemorrhoids. By the way, will a wide tire or a narrow tire bounce more down the cobblestone road?


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## MikeBiker (Mar 9, 2003)

Why is a triangular tire better than a square tire?


One less bump per revolution!


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## duckstrap (Mar 8, 2002)

*This only looks bad until...*

you get a bunch of statisticians arguing.


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## Spoke Wrench (Aug 20, 2001)

asgelle said:


> That's not the question.


Actually, I think "Do fatter tires roll faster?" IS the question. After all of this talk about contact patch, nobody really cares about that anyway. The only reason it came up is that people assume it has something to do with traction and rolling resistance. What people do care about is anything that will make their bike faster by even the tiniest amount. I gave the mechanic's answer to this rolling resistance question about three days ago. Now I'm ready to read how the engineers and the scientists thrash it out. So who here is brave enough to fire the first shot?

I have another question to ask too just as soon as you answer this one.


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## divve (May 3, 2002)

I'll scan in a test tomorrow which shows (at least in this test sample) that tires of the same model but in a fatter version, have a slightly higher rolling resistance. However, there's a far greater variation in rolling resistance between different models and makes, than in tire diameter. The difference can be greater than the effect of a good aero wheel set in pretty ideal circumstances.


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## AJS (Aug 7, 2003)

I can't remember the site now, but I think it was some cycling mag or test site out of Germany that one of the guys here on RBR turned me on to (maybe divve or Spindawg?); it had several popular brands/models of 25c tires and their rolling resistances and other data. I recall seeing a Schwalbe, one or two each of Conti's & Mich's, a Vred, a Panaracer, a Specialized, and a few others. 

They also measured for actual width of each tire with a given PSI, (forgot what that figure was now). Many models were less than 25mm wide - anywhere from 23 to 24.5mm. Weights were taken too, but that's another story.

Not knowing German very well, I _think_ they were saying overall, a 25c has less resistance than a "standard" 23c. I'm not sure if they spoke of _contact patch area_, but it's possible they mentioned it in the article.

I've Googled it but haven't ran across it yet. I'll post it if I do.


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## C-40 (Feb 4, 2004)

*Fwiw...*

I came up with a more controllable test that verified the applied load and still got results that don't support support this formula, EXCEPT at very low pressures.

I made another setup with my bike mounted in a trainer, so it was stable. My trainer is bolted to a large piece of 3/4" fiberboard that keeps it from moving. I placed a scale under the tire and took more imprints at 100 psi tire pressure with a 100 lb load. I created the 100 pound load by hanging enough weight from the handlebars to register a starting load of 30 lbs, and then carefully leaned on the handlebars until the scale registered 100 lbs. With this method I knew for sure that the tire wasn't moving side to side or rolling when the imprint was made. I still got contact areas that were consistently 1.4 square inches or 40% more than the formula would predict. I tried another series of test at 130 psi and finally got a 1 square inch average contact pattern. 

I also tried a test at 50 psi with the same 100 lb load. This time I got imprints that were only 5% less than the predicted 2 square inch value. My previous results were also much closer to the formula's prediction at this lower pressure.

The results still show that the contact are is not doubled when the air pressure is reduced by 50%. 

If I get bored enough during this snowy weather, maybe I'll try the test with a big fat Tioga "City Slicker" tire that's on my wife bike. Max pressure on this tire is only 90 psi, so all the testing will be done at lower pressures.


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## AJS (Aug 7, 2003)

Could the PSI portion of the formula be some logarithm instead of a fixed value?


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## BigFatSal (Jul 10, 2003)

*Whew, My Head's a Spinnin'*

This is quite the thread! The only problem is that it's so far over my head I don't know what buttons to push to pi$$ you guys off.... Darn, it was so much easier on the "heat molding a saddle" thread.....

Next time, how bout we play Rock 'Em, Sock 'Em Robots. Winner of the fight wins the debate! 

I guess I just like to ride my bike. When the pedals are hard to push, I go to an easier gear. If they're too easy to push then I go to a tougher gear.... Can you tell I'm not an engineer?

But I'm glad you guys exist so you can figure out this stuff so I don't have to!

Rock on.


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## xcandrew (May 30, 2002)

Wow, I have the same reaction as Kerry Irons. This stuff is the kind of stuff I learned in jr. high school science class. I'm sorry, but I really had to laugh out loud at the old mechanical engineer...


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## divve (May 3, 2002)

I was wrong regarding the rolling resistance numbers. After looking at the test again the results are inconclusive and in a few cases the fatter tire has slightly less rolling resistance at the same pressure. One should however take into account that normally thinner tires take a higher pressure and have a small aero advantage as well. Ultimately the surface and conditions you ride in will determine the most optimal combination.


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## Reynolds531 (Nov 8, 2002)

*very interesting--indicates tire construction is major factor*



divve said:


> I was wrong regarding the rolling resistance numbers. After looking at the test again the results are inconclusive and in a few cases the fatter tire has slightly less rolling resistance at the same pressure. One should however take into account that normally thinner tires take a higher pressure and have a small aero advantage as well. Ultimately the surface and conditions you ride in will determine the most optimal combination.


Since this is in German, the data must be credible. 

The rolling resistance at 30 km/hr with an 85 kg total system weight depends more on the tire construction than it does on changing the tire pressure by 1 bar (about 14.7 psi). (I think that the two pressue comparisons are high pressure Rear/Front 8.5 bar/7 bar (123 psig/101psig) and low pressure 7.5 bar/6 bar (109 psig/87 psig).) Reducing the tire pressure only increases rolling resistance by about 1-2 watts, but there is about a 20 watt difference in between the highest and lowest tire models. This is good evidence that you can lower your tire pressure, get a more comfortable ride, and have a very minimal impact on efficiency. A good tire at lower pressure will ride smoother and have lower rolling resistance than a bad tire at high pressure. 

Also note that tire weight does not correlate to rolling resistance. If you pick the wrong lightweight tire (like the Tufo Elite jet) you can lose a lot more energy in rolling resistance than you gain by weight reduction.

Personally, I hate flats and will sacrifice a lot of efficiency to avoid them, especially in the winter.

I don't see any case where the same tire model with different widths was tested and there is no data on contact area, so we can keep arguing.


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## divve (May 3, 2002)

It's in German, but here's the link to the 25mm clincher test:

http://www.dk-content.de/tour/pdf-archiv/tests/breitreifen_0703.pdf

It touches a bit on the subject of narrow vs wider tires as well. In short they state that the contact patch of a comparative narrower tire is the identical to a wider tire at equal pressure and of <b>equal construction</b>. On a wider tire the contact patch is wider and shorter compared to a narrower tire. The result is that the narrower tire will deform more than a wider tire during compression, which in turn causes more energy loss due to internal friction.


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## AJS (Aug 7, 2003)

Yep, that's the one.  Sorry if I was incorrect in any of what I had recollected earlier.

Now we just need someone to translate it from the German.


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## elviento (Mar 24, 2002)

*I will take a shot at that.*

In a perfect word (ideal road surface, ideally strong tire material), the narrower tire rolls faster at high enough pressure. 

In the real world, the RIGHT size tire rolls faster. I am not trying to be a wise guy here. The fact of the matter is people have not gone to 10mm tires or 100mm tires, but mostly use 20-23-25 ones. 

What's RIGHT for a particular rider and terrain will depend on many variables: rider weight, road surface, tire pressure, etc. 

If you are 145lbs and ride on smooth roads all the time, 25mm will be overkill, but if you are a 200-pounder bombing through NYC, 25mm will be great. 



Spoke Wrench said:


> Actually, I think "Do fatter tires roll faster?" IS the question. ... Now I'm ready to read how the engineers and the scientists thrash it out. So who here is brave enough to fire the first shot?


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## Reynolds531 (Nov 8, 2002)

*Summary translation*



AJS said:


> Yep, that's the one.  Sorry if I was incorrect in any of what I had recollected earlier.
> 
> Now we just need someone to translate it from the German.


Those are thick, man! Wider tires are used on rough roads because the large air volume and greater height give a more comfortable ride and helps prevent flats. But also a wider tire has less rolling resistance than a narrower tire if both tires have the same construction and have equal pressure. The wider tire and the narrower tire have the same contact area, but the narrow tire has to flex more to achieve this area, and the large flex results in more energy loss due to internal friction. The lower rolling resistance of the wider tower is offset by the higher weight of the wider tire, which makes it harder to accelerate. The wider tire also has a higher wind resistance, but this is probably not a factor unless you are going 40 km/hr (25 mph). Attempts to measure these performance differences in the real world have failed because other factors are so much larger. Studies on 23 mm tires have shown that the reducing tire pressure from 8 bar to 6 bar (116 psig to 87 psig) increased comfort by 22%. Wider tires will allow lower pressure and may be expected to give similar increase in comfort. Tire clearance on some frames might be an issue.


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## GirchyGirchy (Feb 12, 2004)

AJS said:


> I don't think that's the point. C-40's attempting at least some sort of empirical testing to sort out the equations, rather that just relying on the theory of the numbers. Even a simple or crude hands-on test is better than none.
> 
> As any good engineer knows, everything is subject to change when you take it from the slide rule and apply it to the actual problem. That's when you really start to get answers. - good ones or bad.
> 
> Then you go back to the drawing board - lather, rinse, repeat - until you get something that works.


This is exactly what I was thinking. From my work this summer on FM transmission equipment (antennas and transmission lines), I'm well aware that what's on paper means diddly squat in the real world. What looks good on a design might go to pot once you find how the elements couple and how screens affect the patterns. The joke holds true...FM really _does_ stand for F*cking Magic.

You can't treat equations as Jesus' own words, without tests to back them up. They're good for generalizations of ideal situations, and are helpful and time-saving. But the next step is to perform tests, and only one person went that far.

Kudos to C-40 for keeping an even keel, and not resorting to insulting language. Sheesh.




Lucille said:


> Not so that I can know how much pressure to use with my tires, but so that I can know where NOT to send my sons to school for an engineering education.


I hope you're kidding about that horrid comment. For one thing, you should be able to trust any ABET-accredited engineering program at a university. Plus, it's not what school you go to, it's how you go about learning the material...you'd be surprised how easy it is to coast through some classes while learning absolutely nothing. Finally, that was just plain rude.

I've also found that formal education has very little to do with practical knowledge - the smartest man I've ever worked with had an 8th grade education.


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## symbo (Dec 7, 2004)

Reynolds531 said:


> If we are reasonable, this thread would have ended long ago. You wouldn't want that, would you?
> 
> Dynamic loads open up an entire new area for debate. I'll start by contending that even under dynamic loads, there is not a significant difference in contact area for wide and narrow bicycle tires with a flexible tread and sidewall, inflated to equal pressures unless the dynamic load is great enought to cause the tire tread to compress against the rim.


Yes, lets go dynamic, then simple formulas won't apply. Get out your favorite Finite Element Code and let's get busy.

I agree with the basic force/pressure = area equation - how could you not? I think this debate is if different shaped tires produce more drag as they move. If you look at each instant in time, they must have the same contact area, but if you look at how they ride, one shape could have more drag than another. (although I don't think there is appreciable difference between road tires. knobbies, even at road pressure, could be different - hey lets get someone to try and air up their 26x2.25's to 125psi.. anyone up for it? 

As far as adynamic example, what about when you hit a patch of gravel?

The tire is the same, pressure the same (initially), your weight the same (assuming no fear induced losses), but the contact patch gets smaller as the force concentrates along the crest of the rocks, ...you slide and are down. So what of the force/pressure = area equation? I would contend that some was tranfered to kinetic energy, while there were also spikes in tire pressure. It you stopped the bike on the sharp edges, and let everything equalize, you would have to have an increase in tire pressure to account for the drop in area.

Were you the Boeing guy?, here's a photo of the Airbus Beluga swallowing the wing and fuselage an A320 at the factory in Finkenwerder.


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## Spoke Wrench (Aug 20, 2001)

OK, so here's the follow up question:

The Continental tire people, for example, say that above some level, higher tire pressures have little, if any, effect on rolling resistance. I took their advice, experimented with using lower air pressures, and tend to agree based entirely on subjective experiences. That seems counter-intuitive to me based on the discussion above. 

1. Is the air pressure/rolling resistance curve linear?
2. If not, why not? Are the extraneous caseing construction factors etc. that C-40 seems to be citeing what makes the difference?
3. Is it possible that road bicycle tire rolling resistance is so insignificant that the decrease associated with air pressures above 100psi or so aren't likely to be felt subjectively?


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## symbo (Dec 7, 2004)

Spoke Wrench said:


> OK, so here's the follow up question:
> 
> The Continental tire people, for example, say that above some level, higher tire pressures have little, if any, effect on rolling resistance. I took their advice, experimented with using lower air pressures, and tend to agree based entirely on subjective experiences. That seems counter-intuitive to me based on the discussion above.
> 
> ...


I don't know about the tire casing stuff C-40 was quoting, I let my subscription to Cycling Science end years ago. But,
1. Diffinitely not linear. Imaging the difference between 1 and 10 psi vs 100 and 110.
2. maybe, but probably not significant.
3. I think it's a matter of ratio's. once you get high enough, you don't see much benefit, but you certainly see the negatives mounting. (handeling, traction)

Take weight for example, which, without thinking about it much, is linear. saving 100g on 20lb bike is the same benefit as saving 100g on a 17lb bike. But if that light bike becomes too flexy without the 100g, i'd rather do have it there.


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## AJS (Aug 7, 2003)

Here's the text as translated by Babelfish. It's kind of goofy in some parts, but you can get some sense out of it:

"Peter Van Petegem hat's before-made. The Belgian drove in the spring on broad tires to his victories with Flanders round travel and Paris Roubaix. Before both running the professional of the Domo Lotto team tire of the type "Continental Competition 25" let install: a schlauchreifen, which is broad nominal 25 millimeters. Now the use of thick tires on rauem plaster in Belgium and north France is clear quite. Finally they offer arguments, which count doubly on bad roads owing to the larger air volume and the overall height free of doubts more travelling comfort and are at the same time less susceptibly to copies - both. But still another third, hardly less weighty is added: Broader tires roll also more easily than narrow. Like that?, now many racing cycle drivers will ask - nevertheless up to 18 the millimeters suggested narrow Top tires of the manufacturers to center of the 90's that only the thin Dinger ran correctly fast. 

But already 1994 proved ROUTE that this equation is wrong (ROUTE 8/94, page 55). The physical connection: Two tires, which in the width (and concomitantly in the height) differs, otherwise however are identically developed, point with gleichem air pressure and equivalent load by the operator's weight equal large tire road-contact areas up. The surfaces differ in the form - with narrow tires them precipitate long and narrowly, with broader tires are shorter and broader them; the area is however in both cases identical. The narrow tire must deform when bouncing however on larger length and height more strongly than the broad - it mills more. This deformation can one as a kind internal friction in the tire understand oneself, which verzehrt much energy. It precipitates with broader tires smaller than with narrow; therefore broad tires - with same air pressure - have a smaller rolling friction. Naturally gibt's the rolling friction advantage not in vain. Broader tires are somewhat heavier - the Stelvio Rain of swallow for example weighs 25 millimeters of broadly approximately 25 gram more than with 23 millimeters. The acceleration values are somewhat worse therefore although in an order of magnitude takes place, which, related to the overall system from driver and wheel, in practice is negligible. 

On the other hand broader tire offers more attack region to the wind. Since air resistance in the square increases (double speed means quadruple air resistance), the rolling friction however only linear, the light run advantage of the thick rollers starting from a certain speed counterbalanced. Those usually lies beyond of 40 kilometers per hour. Besides it is not even completely clarified whether this objection applies against broad tire, which was confirmed several times on ideal conditions on the course also in the road enterprise. So could not prove Guido Mertens, project manager Cologne Vector Racing Liegerad teams, with driving attempts with the racing cycle on the road also assistance of the SRM Messkurbel aerodynamic differences between 23 and 25-Millimeter-Reifen. Too many factors affect the system: For example the outline of the impeller (for instance the transition from the tire to the rim), the footstep frequency of the driver and the incident flow of the rear wheel additionally changed thereby. Against it the higher comfort and the smaller breakdown susceptibility of broader tires are unquestionable. Larger volume, overall height and width make it possible to drive 25-Millimeter-Pneus on the average depending upon operator's weight with one without less air pressure than 23-Millimeter-Reifen, without a copy threatens. Result: More comfort with approximately same rolling friction. There are not exact numbers, which express the relationship of air pressure, section width, rolling friction and comfort, unfortunately. 

But ROUTE could prove 1999 that the reduction of the air pressure causes a comfort improvement of 22 per cent of eight on six bar with a 23-Millimeter-Reifen (ROUTE 2/99, page 26). Reduced pressure in broader tires might produce a similar effect. Every now and then also strong problems stand to the change on broad tires in the way many racing cycle frameworks and forks are so closely built that they take up only tires up to maximally 25 millimeters of width; sometimes conclusion even already is with 23 millimeters. The table gives therefore apart from the measured section width, which with many models from the actual width deviates, also the overall height of the tire on. The deviation is indicated in millimeters of one the usually-driven 23-Millimeter-Reifen: the Continental Grand Prix 3000. On the basis the deviation you can estimate whether the models within your frameworks, shown by us, fit. Which can speak in addition against the assembly of broader tires: The installation and removal of the Rades can become more difficult, because the easing levers of the brakes open the brake arms possibly not far enough. With precise attitude of the brake courses and the clearance, which the course adjusting screws grant, the problem can be repaired in most cases however. Discharging air when taking out and using the Rades might mean so only in few exceptional cases the last rescue."


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## jim hubbard (Mar 10, 2002)

*veloflex*

All I have to say is at least I use the right tires! Go veloflex! Thanks for the german article guys.


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## Fredrico (Jun 15, 2002)

*Illuminating!*

"...the higher comfort and the smaller breakdown susceptibility of broader tires are unquestionable. Larger volume, overall height and width...Result: More comfort with approximately same rolling friction."

I knew that. 25Cs rock!


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## MikeBiker (Mar 9, 2003)

Fredrico said:


> I knew that. 25Cs rock!


 You sure are easy to please. Try dating a woman who is 38C if you think 25Cs rock!


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## AJS (Aug 7, 2003)

MikeBiker said:


> You sure are easy to please. Try dating a woman who is 38C if you think 25Cs rock!


Hey, we're talking here about things made _without_ silicon!


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## Cat 3 boy (Aug 20, 2002)

jim hubbard said:


> All I have to say is at least I use the right tires! Go veloflex! Thanks for the german article guys.


I second the vote for Veloflex tubulars, cost me an arm & a leg but they run beautifully on my Reynolds stratus TT wheels, plus I now have proof "in German" too


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## Reynolds531 (Nov 8, 2002)

*Tires have silicon, boobs have silicone*



AJS said:


> Hey, we're talking here about things made _without_ silicon!


Actually, silicon is used in the rubber formulation of many tires. It is refered to as "white carbon" in the tire industry and is an elemental mineral. Silicone is a polymer chain with a backbone of silicon and was used for breast implants until a frivolous lawsuit based on erroneous science, misinformation, and lies forced them to be pulled off the market in the U.S. I think that bags of saline solution are now used in the U.S. for implants, but I think that you can get the much nicer silicone implants in other countries. Kerry Irons can probably tell us more about Silicone and the many medical and industrial uses than we ever would want to know.


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## CFBlue (Jun 28, 1999)

*common sense, too*



Len J said:


> This really cracks me up. Every couple of years we have a thread like this where a group of engineers argue, seemingly endlessly, about some point of esoterica, the difference which makes absolutly no difference to anyone but them.
> 
> It's like hearing those old college arguments about angels on the head of a pin.
> 
> ...


Despite all the engineering, I don't think we can neglect common sense, particularly with respect to the original question. There is no way, all else equal, that a wider tire can have a smaller contact patch. Extrapolate in both directions, narrower and wider, let's say 6 feet wide, and try to envision a scenario where wider has less contact. I don't think engineering or math is really necessary for this one.


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## Kaboom (Jul 18, 2003)

personally, i long arrived to the conclusion that if you like riding, the increase in riding pleasure you are going to get from riding a 21 pound 8-speed steel steed with hand laced 36 spoke 3x wheels and basic pedals is definately NOT proportionate to the 3000+ you have to spend to make it a sub16 carbon 10-speed chichi wheel sub-200 gram pedal bike. The performance gain is, also, quite negligeable (sp), no matter what people say. if you are able to go up a 15% gradient for half an hour with that bike, u will with the chichi, but you migth travel slightly further? Then, Why did i bust 500bux on my K elites? why did i upgrade to Ultegra 9? why did i get meself a 100 buck (well actually my father paid for it, so this dont count) titanium railed saddle, and why did i spend a couple 100 more repainting my beautiful aluminium steed? Becase not only is this a sport, its a HOBBY.
Part of the hobby is discussing whether a 50buck clincher has any technological advantage WHATSOEVER over the 10bux walmart steel-beaded tires. or whether u are wasting ur precious megabux on sth, when there is sth else for more or less the price which can offer you a 0.0002 increase in speed, which u will never take advantage of anyway, because the day you go out and ride, you are thinking about avoiding taxes instead of concentrating on the road.
Except those who cycle for a living, which dont get to choose their equipment anyways, ANY piece of equipment that doesnt enhance convenience (puncture resistance tires, for example) are WAY overkill for the recreational cyclist that owns it.
Armstrong said "its not about the bike". i severely disagree. Its not about the bike if you are a pro, in which case u already have the best steed money can buy. But if you are a recreational rider and/or the occasional racer like me (i intend to start this year, wish me luck!) having a cool, neat piece of kinetic art that you can actually go out and enjoy urself on IS part of what its about. At least for me it is.
Would i stop riding if someone took my wheels and handed me a huffy? no
will i change my michelin pro races 23c to 25c's after reading this thread? hell, i just might, after all, they'll be wearing out pretty soon when i hit the road again this year.
So that's my long, stupid, boring and irrelevant explanation (and possibly wrong, too) of why we discuss this kind of thing.
lets go ride.


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## Reynolds531 (Nov 8, 2002)

*100 posts! Ain't it great!*



dasho said:


> I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?


The beauty of the forum is that a simple question with a simple 1 paragraph answer can generate 100 posts of b.s. and vinegar. The simple definitive answer:

Theoretically, the contact area for the two tire widths are equal. There is also a theory that the wider 25 mm tire with the same construction and air pressure has a lower rolling resistance than a narrower 23 mm tire because the wider tire flexes less. The narrow tire has a lower weight and lower wind resistance. In real application, these differences are so small that real world measurement attempts have failed to discerrn a significant difference in efficiency. A wider tire does provide more comfort and protection from flats on rough roads.


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## AJS (Aug 7, 2003)

Reynolds531 said:


> The beauty of the forum is that a simple question with a simple 1 paragraph answer can generate 100 posts of b.s. and vinegar. The simple definitive answer:
> 
> Theoretically, the contact area for the two tire widths are equal. There is also a theory that the wider 25 mm tire with the same construction and air pressure has a lower rolling resistance than a narrower 23 mm tire because the wider tire flexes less. The narrow tire has a lower weight and lower wind resistance. In real application, these differences are so small that real world measurement attempts have failed to discerrn a significant difference in efficiency. A wider tire does provide more comfort and protection from flats on rough roads.



The fun part is wading through the BS and vinegar, iddnit?

I can say anecdotally that 25's are "better than" 23's, even though I'll sometimes buy 23's that I haven't ridden just to try them if they're not made in 25c.


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## Cloxxki (Feb 21, 2004)

Why does nobody demand that rim width is also adjusted for those "honest tests"? Or did I miss that, skimming through a 100 posts? Testing a 20mm vs an otherwise identical 25mm, the rims' inner widths should be proportional as well. Say, 16mm and 20mm inner width respectively. Even with 50+mm MTB knobbies, 4mm of rim width is not to be overlooked in riding performance (flatting, comfort, cornering, etc), so why not drag?


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## Super E (Jan 15, 2005)

dasho said:


> I remember in one of the older issues of Bicycling magazine, Garrett Lai wrote that a 25mm wide road bike tire has less contact area on the road than a 23mm. If that is true, wouldn't the 25 give you an edge speed wise? I know it sounds crazy but that is what the article said. Are there any engineers out there that can validate or dispute it?


I registered in the roadie forum just to comment on this weirdness. Obviously engineers can not agree on anything (my life experience). That being said are there any real scientific studies out there that provide some sort of real logical proof to all these engineer's arguments or are you all just spouting off your personal egos here? It looks like you are willing to argue to death, even it you are wrong. Leave your unproven personal opinions on your desktops and show us something true and worth reading.


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## dasho (Apr 8, 2002)

Super E said:


> I registered in the roadie forum just to comment on this weirdness. Obviously engineers can not agree on anything (my life experience). That being said are there any real scientific studies out there that provide some sort of real logical proof to all these engineer's arguments or are you all just spouting off your personal egos here? It looks like you are willing to argue to death, even it you are wrong. Leave your unproven personal opinions on your desktops and show us something true and worth reading.


I didn't think I would get so many replies or so much arguing over this post but don't be so hard on engineers. I've worked around them for many years and in almost all cases they are very decent people and worked their tail off to get a very difficult degree. That doesn't mean every one of them is brilliant and of course there are those with no degrees that are or would make excellent engineers. But sometimes, as in this case, there are so many variablesto a problem that a clear cut answer may not be possible.


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## AJS (Aug 7, 2003)

Cloxxki said:


> Why does nobody demand that rim width is also adjusted for those "honest tests"? Or did I miss that, skimming through a 100 posts? Testing a 20mm vs an otherwise identical 25mm, the rims' inner widths should be proportional as well. Say, 16mm and 20mm inner width respectively. Even with 50+mm MTB knobbies, 4mm of rim width is not to be overlooked in riding performance (flatting, comfort, cornering, etc), so why not drag?


Why would rim widths have to be 'adjusted' for either road or MTB tire tests, unless you are testing for a tire's aerodynamics specifically? As long as all models are tested on the same diameter and width rims, that variable is removed or at least equalized. The idea in scientific testing is to remove or control as many variables as possible that are directly relevant to the test, in order to isolate the variable that is under test.

If you want to test rim width as a function of tire contact patch area, you'd use a group of the same sized tires, say a dozen different models of 23c tires, on various width rims, (or a single model of tire on those several rims). Then test another group (or single) of 20c tires on the same group of various width rims, another group or single of 25c's, and so on.


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## AJS (Aug 7, 2003)

Reynolds531 said:


> Actually, silicon is used in the rubber formulation of many tires. It is refered to as "white carbon" in the tire industry and is an elemental mineral. Silicone is a polymer chain with a backbone of silicon and was used for breast implants until a frivolous lawsuit based on erroneous science, misinformation, and lies forced them to be pulled off the market in the U.S. I think that bags of saline solution are now used in the U.S. for implants, but I think that you can get the much nicer silicone implants in other countries. Kerry Irons can probably tell us more about Silicone and the many medical and industrial uses than we ever would want to know.


I know about the "silica" in some rubber compounds, but I thought you guys would get the joke. (Guess I forgot to add that last "e".)


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## Super E (Jan 15, 2005)

dasho said:


> I didn't think I would get so many replies or so much arguing over this post but don't be so hard on engineers. I've worked around them for many years and in almost all cases they are very decent people and worked their tail off to get a very difficult degree. That doesn't mean every one of them is brilliant and of course there are those with no degrees that are or would make excellent engineers. But sometimes, as in this case, there are so many variablesto a problem that a clear cut answer may not be possible.


I've worked with engineers for the last 15 years and swear they all argue like this. I'm just sure it's programmed into them is school. I don't discount their knowledge or ability I just want a real answer and some scientific proof (in laymans terms) to back their arguments. I really want to know the answer to this question, because I have asked it before and never got a straight answer then.


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## Lifelover (Jul 8, 2004)

*I Get It!*



Super E said:


> "real answer and some scientific proof (in laymans terms)


What an oxymoron




Super E said:


> I really want to know the answer to this question, because I have asked it before and never got a straight answer then.


There is NO straight answer.

However, some times for better and sometimes for worse, engineers won't accept that. 

Since they almost never get laid they argue.


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## Cloxxki (Feb 21, 2004)

I feel that to have both a narrow and wide tire have a fair chance, at least they should be used with not the same rim, but the same relative sized rim. A narrow and wide tire don't have the same shape, and the bigger volume of the wider rim is hampered by using the same narrow rim. Pressures will not be lowered as much as they might.


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## Axe (Sep 21, 2004)

C-40 said:


> I came up with a more controllable test that verified the applied load and still got results that don't support support this formula, EXCEPT at very low pressures.


Jebus. Just read this thread. What a stubborn mofo this C-40.

What is so hard to comprehend in that he is just measuring his experimental errors? Is he trying to disprove basic mechanics?


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## Axe (Sep 21, 2004)

Super E said:


> I've worked with engineers for the last 15 years and swear they all argue like this. I'm just sure it's programmed into them is school. I don't discount their knowledge or ability I just want a real answer and some scientific proof (in laymans terms) to back their arguments. I really want to know the answer to this question, because I have asked it before and never got a straight answer then.


The part that you do not get, is that any scientific discussion is about a strictly defined model with well understood applicability and accuracy. It IS the real answer - within those limitations. There ain't other one. Within those limitations.

In the current thread the model is that of a tire of zero thickness - perfect membrane. A good engineer does understand applicability of such model and does not need to go around "laymans" facts.

Obviously such model is not perfect. Nothing is. A good engineer knows what accuracy is about. In this case it is pretty darn good - it is used widely in quite critical applications.

Now, my tires have knobs and all that does not apply.


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## AJS (Aug 7, 2003)

Cloxxki said:


> I feel that to have both a narrow and wide tire have a fair chance, at least they should be used with not the same rim, but the same relative sized rim. A narrow and wide tire don't have the same shape, and the bigger volume of the wider rim is hampered by using the same narrow rim. Pressures will not be lowered as much as they might.


No matter what you "feel", that would still be a bad methodology in this case. 

I don't understand this statement: 

"...the bigger volume of the wider rim is hampered by using the same narrow rim. Pressures will not be lowered as much as they might."


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## blehargh (Mar 17, 2004)

*other 50/50 cars*



Kaboom said:


> the only couple of cars i know with a 50/50 distribution are the Mazda mr2 and the Lotus elise, both of which are very, VERY uncommon machines (well the mazda perhaps not so much). not even ferraris are 50/50.
> Also, while in bikes we pump both tires to the same pressure (usually) cars get different front and rear pressures


All BMW's to my knowledge have 50/50 weight distro. they even put the battery in the trunk to distribute weight better. The honda S2000 also is 50/50. 

-don


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## blehargh (Mar 17, 2004)

*This is the post that prove's C-40's point....*



elviento said:


> My dad went to a top engineering school and was an engineer for many years so I am hoping the genes tricked down a bit:
> 
> The principle is fairly simple. The argument is how much the other factors such as structural strength of tire casing and shape of the tire contribute to the patch size and whether that's so small as to be neglected.
> 
> ...



elviento has hit it on the head...

GL
-don


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## treebound (Oct 16, 2003)

*Long threads*

When threads get this long I tend to just skim down and look at the pictures.


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## spookyload (Jan 30, 2004)

Reynolds531 said:


> If the tire tread or the belts had a significant effect on the imprint of the airplane tire, then the simple equation presented by Boeing would not work. Neither the airplane tire nor the bicycle tire has sufficient strength in the sidewall and tread to support the load, and the same simple pneumatic equation applies to both. Are you actually spending hours inflating your tires to different pressures, wetting your tires, pressing them against a board, and measuring the area? If so, it is a desperate ands futile attempt to prove yourself right! Can you post an MPEG movie of your procedure so we can all analyze your technique? Maybe we can get this thread up to 100 posts and 2000 views. Things are slow at work this week.


When you said airplane tires=bike tires you were wrong. I am in the air force and my job is to compute surface area's of the tires you put on the aircraft so as not to exceed the max PSI of the aircraft cargo floor. Radial tires (which aircraft tires are) are computed differently form a bonded tires like a bike. To compute the surface area of a radial tire, you take the pad contact area (LxW) and multiply it by .785 to compensate for the rounded edges of the contact area. This is THE formula that Boeing gave us for computing it as they write our technical orders. For non radial tires, there is a chart that was developed by Boeing that gives us a set number based on the tire type that you simply multiply by the width of the tire.


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## HAL9000 (May 1, 2002)

*And risk rath for*

bringing old stuff to the top.

Oops, I did it again....Pile on.


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## AJS (Aug 7, 2003)

HAL9000 said:


> bringing old stuff to the top.
> 
> Oops, I did it again....Pile on.


Oh NOOO! Not the dreaded..._"Do 25c tires have less/more [insert favorite tire aspect here] than 23c?"_...thread again.



HAL! WHAT IS YER PROBLEMO, MAN?!?! Don't you have some leg shaving or something to do?

(Now I really DO feel more like I do now than when I first got here)


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## jaldridge (Feb 27, 2005)

> this is sounding ridiculous. the tire has 110psi in it. that 110 pounds per sq inch pressure is exerting itself outwards against the tire casing.



Actually, a flat tire on a standard day at sea level would have an interior pressure of about 15 PSI, so a tire filled to a pressure of 110 PSI would actually have air in it exerting an outward pressure of 125 PSI against an exterior pressure of 15 PSI giving a differential pressure of 110 PSI. My wife tells me this is more than anyone is likely wanting to hear... - Jim


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## AJS (Aug 7, 2003)

jaldridge said:


> My wife tells me this is more than anyone is likely wanting to hear... - Jim


Listen to your wife!


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## phil. (Aug 3, 2004)

In one of my physics labs in college we calculated the weight of 3 different automobiles using the contact area of the tires on the ground. We took 4 large sheets of white paper, raised the vehicle up, rubbed grafite on the tread, lowered the vehicle, raised the vehicle, cut out the graphite tread patterns, weighed those in relation to a full sheet of paper to determine the area in contact, and then used the simple force = pressure/area formula. We were able to get within 5% on all three vehicles of the gross weight according to the weight listed on the inside panel of the vehicle.

I tend to agree w/ the asgelle b/c of this. Yeah, I'm an engineer also.....an Imaginary Engineer


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## Nigeyy (Mar 30, 2003)

*Ha, Ha, Ha!!!!!!!!!!!!*

Cory:

am I glad I read your message; I thought I was going a bit crazy recently. A couple of weeks ago I bought some 700cx32 Paselas on sale from Nashbar -even when I opened the package, the first thing I thought was "surely these aren't 32s?". I checked the label -yep, 700c x 32.

Once on, I too measured them to be no more than 25 (I partially convinced myself that the reason they were 25 was because my rim was too narrow -though in the end I was convinced they were mislabeled). It was actually no problem for me because I'd ordered 32s by mistake, so it turned out well. I pondered calling Nashbar to let them know, but I've got better things to do with my time.




Cory said:


> In my speed-obsessed youth, I did the traditional experimenting with all kinds of tires, down to 19mm. I weighed about 220 then, and skinny tires were just unpleasant. I moved up to 25s despite the then-common belief that skinnier was always faster, and I was a lot more comfortable and didn't go any slower. Eventually I settled on 32s for most riding, 35s for rides when I know the pavement will be bad. Last summer I put on a set of 23s (apparently what most people on this board use), and I HATED 'em.
> As one of the other posts mentioned, its hard to find anything bigger than 25mm that isn't a big, clunky touring tire. Two I've used a lot, and really like, are Panaracer Paselas (come in 28, 32, 35 and 37mm) and Rivendell's Ruffy Tuffy, which I think is 27.
> The labels, incidentally, aren't worth much. I just measured the Paselas on my bike and my wife's, and the 35s are actually 30.5 mm, the 32s about 25mm.
> All right, I've got too much time on my hands. I've been shoveling snow for six hours, and there's another storm due tonight.


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