# Should I go up to 25c from my 23c?



## LowriderS10 (Aug 22, 2011)

Hey guys, I have 700x23c tires on my Specialized, but I'm flatting a lot (thanks to a lot of garbage on the roads and very crappy tires on my bike)...so, while I have been searching for a new tire (thinking Gatorskins or Armadillos if I can get them here), I have seen people suggest going up to 25c tires. 

What are the advantages of a wider tire? Disadvantages? I'm guessing it'll be slower, but by how much? 

I'm a recreational rider, I ride for fun and to stay in shape...I have never raced and, while I do want to give that a shot, I doubt I'll race within the next season or two...

Thanks,
T


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## brewster (Jun 15, 2004)

You probably don't know the can of worms you've just opened. Many opinions on this topic. My experience comes from riding 23's for decades, then switching to 25s. What you gain is a more comfortable ride and lower pressure needed so reduced chance of pinch flats. Punctures have more to do with the quality of tire and not the width. I don't notice any difference in speed. 

brewster


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## beston (Jul 4, 2008)

My perspective is that if you're racing, tires / tubes matter. For training, it doesn't mater. 

Given your goals over the next couple of years, go with the tires that are going to keep you rolling on the road. I don't really feel much of a difference between 23 and 25c tires, so I tend to stick with 23c. Adjust the tire pressure for comfort and be done with it.

How much slower will you be. Given that you're asking about tires like gatorskins, you are going to definitely be slower than you would with many other options out there. For example, Tufo tires (the puncture resistant ones) are going to cost you nearly 10 watts per tire over the fastest tires you can get. That's about 1-2 seconds per mile slower.


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## superjesus (Jul 26, 2010)

A new route is cheaper and better than new tires.


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## LowriderS10 (Aug 22, 2011)

Thanks for all the replies except for the last one  Changing routes is not an option...everywhere I go (currently living in South Korea) there is tons of crap on the road, usually shrapnel/small sharp pieces of metal (I literally pick out 3-4 of these after every 40-50km ride). Changing my aging tires out is my last-ditch option..if I still flat all the time I'm gonna have to switch to a mountain bike  

A couple of seconds a mile doesn't concern me in the least bit...if that's really all we're talking about, then it sounds like a good trade...aside from a better ride quality, what else is a plus of a wider tire? Better handling? More stable? But NOT more resistant to punctures? (I was thinking it would be more resistant, since weight is distributed over a larger area, therefore reducing the pressure on any given point of the tire).


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## cxwrench (Nov 9, 2004)

25*mm* tires, not 25*c*, that's the part that describes in detail which 700 rim you're using...like '700c'...
won't be any more puncture resistant than 23mm tires. the difference in pressure isn't large enough, and the 25mm tire actually has a slightly larger contact patch...ymmv


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## mimason (Oct 7, 2006)

If you have that much debris on the roads then try installing a tire liner or use rim strip between tube and tire.


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## LowriderS10 (Aug 22, 2011)

Rim strip? Do tell me more about this! Brands? Cost? How do I get one?


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## jjmurch1 (Nov 16, 2002)

Check this link out on tire liners.

Mr. Tuffy Ultra Lite Tire Liners 700c x 20 - Bike Tire Accessories


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## LowriderS10 (Aug 22, 2011)

ooooh awesome!!! I'm gonna see if my locals have anything like that...any brand suggestions on these? 

Also...how good are these? Can I just buy an average road tire (Maxxis Detonators) and slap these on and have a fairly flat-free experience?


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## Kerry Irons (Feb 25, 2002)

*Contact patch*



cxwrench said:


> the 25mm tire actually has a slightly larger contact patch.


To at least two significant figures, the size of the contact patch is the weight born by the tire divided by the pressure in the tire (comparable units). If a tire is carrying 100 lbs. and is inflated to 100 psi, then the contact patch will be one square inch, regardless of tire size. Contact patch shape will change with tire width, but contact patch size will not.


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## cxwrench (Nov 9, 2004)

Kerry Irons said:


> To at least two significant figures, the size of the contact patch is the weight born by the tire divided by the pressure in the tire (comparable units). If a tire is carrying 100 lbs. and is inflated to 100 psi, then the contact patch will be one square inch, regardless of tire size. Contact patch shape will change with tire width, but contact patch size will not.


yeah...you're right on that one, it's pure physics...dohhhhhhhh. but most of the time you're running lower pressure in the larger tire, does that mean the contact patch gets bigger or smaller? long day...can't think.


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## dookie (Oct 1, 2007)

how hard is it to try? check it out yourself.

i went from 23 to 25 (both open corsa) on everything i ride...fixed steel, carbon, ti. much more comfortable, better grip, better wear and no less fast in my opinion. i also use latex tubes (and did also with the 23s).

i would not have considered clearance issues and have had none myself, but a friend did. a michelin 25 just barely rubbed the underside of the crown on a cannondale slice fork. if your frame is ultra-tight, you might do a test fit first.

175lbs
23 pressures ~105/115
25 ~95/105


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## superjesus (Jul 26, 2010)

cxwrench said:


> yeah...you're right on that one, it's pure physics...dohhhhhhhh. but most of the time you're running lower pressure in the larger tire, does that mean the contact patch gets bigger or smaller? long day...can't think.


Bigger.


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## Mersault (Jan 3, 2005)

dookie said:


> i would not have considered clearance issues and have had none myself, but a friend did. a michelin 25 just barely rubbed the underside of the crown on a cannondale slice fork. if your frame is ultra-tight, you might do a test fit first.


I ran michelin 23 for many years. I bought another brand of tire in 23. hmm this new brand looks small to me I thought on mounting. I took out my calipers and measured the width of the inflated tires. The michelin 23 measured 25mm wide. The other brand (Maxis) were 23. I don't feel a difference in the two while riding, but I may get some Maxis 25 in the future.

I don't have many points to compare, but do Michelin run a bit larger than stated?


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## ukscotty2hotty (Aug 15, 2011)

I've got a couple questions in relations to this topic. I bought the bike a few weeks ago as I am just getting into the sport and this is my first road bike - used mainly for commuting/joyriding/fitness. I figured I would buy a relatively cheap one (Fuji Ace) to start. 

I am 200 pounds and am currently on 25s. I am curious should I stay with 25s or even go up to 26s or 27s because my commuting route is on not so good roads (many bumps and cracks)? Not sure how the affect of my weight would be on the tires.

Also being that it is used, the tires are two different tires (one: Specialized All Condition, two: Forte GT2). They say they are the same size 700x25C, but from the looks of them the Forte is much, much smaller (width & height). Why is it that they look different sizes, even though they say they are the same?


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## dharrison (Oct 27, 2009)

In the past year I've ridden:

Vittoria Open Corsa Evo 23mm
Vittoria Open Corsa Evo 25mm
Vitoria Diamante 25mm
Conti GP 4000s 23mm
Conti GP 4000 25mm
Conti Gatorskin 25mm

My favorite in terms of ride quality are the Open Corsa 25mm. But I got too many flats and too much tire damage. At 70 bucks a tire thats too expensive to deal with. I switched to Gatorskins and haven't had a single problem yet. They're a bit slower feeling but thats just my perception. The reliability is well worth it.


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## Kerry Irons (Feb 25, 2002)

*Sizes*



ukscotty2hotty said:


> I am 200 pounds and am currently on 25s. I am curious should I stay with 25s or even go up to 26s or 27s because my commuting route is on not so good roads (many bumps and cracks)? Not sure how the affect of my weight would be on the tires.
> 
> Also being that it is used, the tires are two different tires (one: Specialized All Condition, two: Forte GT2). They say they are the same size 700x25C, but from the looks of them the Forte is much, much smaller (width & height). Why is it that they look different sizes, even though they say they are the same?


Typically, you find tires in 23, 25, 28, 32, 35 etc. and not in 24, 26, 27, etc. Whether you need wider tires depends on your riding style as much as anything. 25mm should be OK at your weight, but if you are a "sit like a brick" rider then your tires will take a lot more beating than if you spot the obstacles and avoid them or rise off the saddle a bit as you go over them. The short answer is that if you're not getting pinch flats, then your combination of tire size and tire pressure are working for you. If you want to experiment with different sized tires, just get a different size when your current rear tire wears out.

Tire sizes are nominal - think women's clothing sizes. The rim width also influences mounted tire width a bit. It is not clear why, but some manufacturers cannot be bothered to produce a tire that, when mounted, actually matches the marked size. Sometimes it's due to wanting their tire to be lighter than the next guy's tire, and so they make it smaller and mark it larger. Sometimes, I suspect, it is just incompetence somewhere along the design/manufacturing chain.


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## ukscotty2hotty (Aug 15, 2011)

Thanks Kerry... A lot of good info!


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## sneakyracer (Dec 1, 2007)

Hi, If your issue is a lot of flat tires get a set of Specialized All-Condition Armadillo Elite tires. You wont flat again. The ride will be a tad more uncomfortable but the flat protection is totally worth it.


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## terbennett (Apr 1, 2006)

Kerry Irons said:


> Typically, you find tires in 23, 25, 28, 32, 35 etc. and not in 24, 26, 27, etc. Whether you need wider tires depends on your riding style as much as anything. 25mm should be OK at your weight, but if you are a "sit like a brick" rider then your tires will take a lot more beating than if you spot the obstacles and avoid them or rise off the saddle a bit as you go over them. The short answer is that if you're not getting pinch flats, then your combination of tire size and tire pressure are working for you. If you want to experiment with different sized tires, just get a different size when your current rear tire wears out.
> 
> Tire sizes are nominal - think women's clothing sizes. The rim width also influences mounted tire width a bit. It is not clear why, but some manufacturers cannot be bothered to produce a tire that, when mounted, actually matches the marked size. Sometimes it's due to wanting their tire to be lighter than the next guy's tire, and so they make it smaller and mark it larger. Sometimes, I suspect, it is just incompetence somewhere along the design/manufacturing chain.


Well said....


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## nhluhr (Sep 9, 2010)

cxwrench said:


> 25*mm* tires, not 25*c*, that's the part that describes in detail which 700 rim you're using...like '700c'...
> won't be any more puncture resistant than 23mm tires. the difference in pressure isn't large enough, and the 25mm tire actually has a slightly larger contact patch...ymmv





Kerry Irons said:


> To at least two significant figures, the size of the contact patch is the weight born by the tire divided by the pressure in the tire (comparable units). If a tire is carrying 100 lbs. and is inflated to 100 psi, then the contact patch will be one square inch, regardless of tire size. Contact patch shape will change with tire width, but contact patch size will not.


I'd love to read your testing methodology and results on this!

Nonetheless, cxwrench's claim is still 100% accurate since you don't run the same pressure in a 25 as you do in a 23 in any real world situation.


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## dmong2 (Sep 14, 2011)

In AZ,for training I use a Gatorskin (25 or 23, doesnt matter) and an Conti Ultra (25 or 28) on the front. Switching to race wheels/tires GP4000s 23, feels zippier at first, bu then exactly the same after an hour. I don't even bother to try to figure if one is actually faster or more efficient than another. 

We had tons of these goatheads in NM when I lived there: Tribulus terrestris - Wikipedia, the free encyclopedia 

which is Hell to ride with. The riders there have a "system" where you take an old tire, cut off the bead then use the old tread as a tire liner. Some even add slime if using a tube where the valve core comes out.


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## Kerry Irons (Feb 25, 2002)

*Methodology*



nhluhr said:


> I'd love to read your testing methodology and results on this!


Since you are the one (apparently) claiming that a fundamental physical principle is not correct, it seems that you are the one who should be providing the testing methodology and results showing my statement to be false.


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## nhluhr (Sep 9, 2010)

Kerry Irons said:


> Since you are the one (apparently) claiming that a fundamental physical principle is not correct, it seems that you are the one who should be providing the testing methodology and results showing my statement to be false.


The "fundamental physical principle" being that Area = Pressure / Force ? I frankly don't see how that applies in a situation where tire casings have non-zero stiffness and especially when you consider the wide variance in sidewall stiffness between say, a Suprsonic and an Armadillo.

Since you cited a number of significant digits, it sounds like you have data to the contrary. Are you just gonna sit on it and make unsupported claims or are you going to ante up?


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## nhluhr (Sep 9, 2010)

Not to mention "2 significant figures" could be a very big difference. For instance, 10.5 adjusted to two figures is 11. 11.4 adjusted to two figures is also 11. So there you have a difference of as much as 8.6% for that example. That's a pretty BIG difference.

Which is why I again, would really like to see the data out of my own curiousity.


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## JPorter (Sep 29, 2011)

Kerry Irons said:


> To at least two significant figures, the size of the contact patch is the weight born by the tire divided by the pressure in the tire (comparable units). If a tire is carrying 100 lbs. and is inflated to 100 psi, then the contact patch will be one square inch, regardless of tire size. Contact patch shape will change with tire width, but contact patch size will not.





Kerry Irons said:


> Since you are the one (apparently) claiming that a fundamental physical principle is not correct, it seems that you are the one who should be providing the testing methodology and results showing my statement to be false.


I think you may have misunderstood what the P = F / A equation refers to. The "pressure" figure is not the air pressure in the tire, it is the pressure between the tire and the ground based on the amount of weight being applied to it from above. 

To break the equation down further... the "force" is the normal force upward from the ground to cancel the downward weight applied, and the "area" is the size of the contact surface.

It's just an equation to figure out the applied PSI figure for a given weight applied through a known contact size... it has nothing to do with internal pressure in a tire. Tire dynamics are very complex, vary significantly based on carcass structure and elasticity, and generally can't be reduced to simplistic equations like this.

I hope I'm being helpful?


P.S. - Your math was right, your context was wrong.... for any object (tire or otherwise), if the weight/force is 100lb and the PSI across the contact point is measured at 100psi, then the contact patch must have been 1 inch. That doesn't mean the contact patch changes size when the weight is increased, it means the contact pressure goes up along with the change in force. Internal pressure in the tire isn't directly relevant, at all.


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## cleon (Jul 19, 2011)

OP, read this Learn About Bikes with Rivendell Bicycle Works, Truths About Tires.

Let the racers argue all they want about how 23s are still comfortable, but try size and you'll see for yourself.


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## brmike22 (Sep 12, 2011)

25mm won't make you slower, but you'll enjoy the ride quality a bit more. i'd make the switch, it won't cause MORE flats.


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## Kerry Irons (Feb 25, 2002)

*Sig figs*



nhluhr said:


> The "fundamental physical principle" being that Area = Pressure / Force ? I frankly don't see how that applies in a situation where tire casings have non-zero stiffness and especially when you consider the wide variance in sidewall stiffness between say, a Suprsonic and an Armadillo.
> 
> Since you cited a number of significant digits, it sounds like you have data to the contrary. Are you just gonna sit on it and make unsupported claims or are you going to ante up?


If you have data showing that normal road tire casings have significant stiffness compared to roughly 100 psi in a tire, it would be very interesting to see.

You have made a reputation on this forum of insulting people based on irrelevant claims. You took some heat for your behavior and your claims then. Do you want to start that up again? It's not the path that Dale Carnegie would recommend.

I responded to cxwrench's statement that a 25 mm tire would have a larger contact patch than a 23 mm tire. The implication of his post, which he acknowledged, was that a larger cross section tire would have a large contact patch at the same pressure. I stand by my statement that 100 psi and a 100 lb. load on the tire would have a 1.0 square inch contact patch whether it was 25 or 23 mm in cross section.

If you want to get into semantics and start talking about 25 mm cyclocross tires vs 23 mm silk track tires at the same pressure, knock yourself out.


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## Kerry Irons (Feb 25, 2002)

*Misunderstanding*



JPorter said:


> I think you may have misunderstood what the P = F / A equation refers to. The "pressure" figure is not the air pressure in the tire, it is the pressure between the tire and the ground based on the amount of weight being applied to it from above.
> 
> To break the equation down further... the "force" is the normal force upward from the ground to cancel the downward weight applied, and the "area" is the size of the contact surface.
> 
> ...


So can we extrapolate from your argument that if the pressure in the tire drops from 100 psi to 50 psi, then the size of the contact patch will not change at all since "Internal pressure in the tire isn't directly relevant, at all." Perhaps you should rethink your argument. Are you suggesting that somehow the internal pressure in the tire changes due to a load being applied. Please explain how that would happen. This would require the volume of the tire to be reduced. What is the mechanism for this?

What makes you think that the pressure between the tire and the ground is signfiicantly different from the internal pressure in the tire? If we take the limit case where the tire pressure is dropped to the point where there is just enough resistance to prevent the tire sidewall from touching the rim, then perhaps sidewall stiffness would have some contribution. When you consider how little force it takes to flex the sidewall of a tire, and then consider how little the tire flexes when pumped to 100 psi with a 100 lb. load applied, the "tire casing is a significant factor" argument is hard to accept.


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## JPorter (Sep 29, 2011)

Wow, Kerry. I don't know if you took the time to actually read what I wrote, but none of the things you've claimed in this post could possibly be attributed to my previous statements. Build straw men and knock them down all you want, but it doesn't change the fact that your description of the physics involved is based on a series of unfortunate misconceptions. Since you're being so defensive, it's probably worth saying that harboring a misconception doesn't make you a bad person, it just makes you a person that's been victimized by bad information.



Kerry Irons said:


> So can we extrapolate from your argument that if the pressure in the tire drops from 100 psi to 50 psi, then the size of the contact patch will not change at all since "Internal pressure in the tire isn't directly relevant, at all." Perhaps you should rethink your argument.


You can't extrapolate anything of the sort, and I didn't claim anything like that. The contact patch would almost certainly change size, but not by 50%, and not in any way based on the formula that you quoted. It's simply not applicable.



Kerry Irons said:


> Are you suggesting that somehow the internal pressure in the tire changes due to a load being applied. Please explain how that would happen. This would require the volume of the tire to be reduced. What is the mechanism for this?


I didn't claim anything about internal pressure in the tire. In fact I barely mentioned it. The "pressure" that I referred to, as I explained before, is the actual _P_ in the equation, which is the pressure between the contact patch and the road, not the air pressure in the tire.



Kerry Irons said:


> What makes you think that the pressure between the tire and the ground is signfiicantly different from the internal pressure in the tire? If we take the limit case where the tire pressure is dropped to the point where there is just enough resistance to prevent the tire sidewall from touching the rim, then perhaps sidewall stiffness would have some contribution. When you consider how little force it takes to flex the sidewall of a tire, and then consider how little the tire flexes when pumped to 100 psi with a 100 lb. load applied, the "tire casing is a significant factor" argument is hard to accept.


The pressure in the tire and the pressure between the contact patch and the road have almost nothing to do with each other. Why is it that you think they are related? Once there is sufficient air in the tire to fully inflate and pressurize the carcass to its designed operating range, the tire no longer acts in an elastic fashion as many people seem to infer from the fact that the rubber is "floppy" when uninflated. The pressure between the tire and the ground changes with weight applied, and while the contact patch size _does_ change as the weight changes, it does not do in a way that is easily mathematically predictable, and certainly not in a linear fashion as you have suggested. A tire at 100psi internal pressure with a 100lb weight applied _does not necessarily_ have a 1 sq inch contact patch. To figure out the contact patch area, we would need to know the contact pressure, not the internal pressure.

I can provide some links for further reading, if you're interested in the subject and you're not too irrationally angry at me for daring to contradict you.


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## Kerry Irons (Feb 25, 2002)

*Further reading*



JPorter said:


> I can provide some links for further reading, if you're interested in the subject and you're not too irrationally angry at me for daring to contradict you.


What you are saying contradicts everything I have ever read about bicycle tires, tire pressure, and contact patch size. To say that the contact patch size does not vary directly with tire pressure suggests that tire pressure changes due to compression or that the sidewall stiffness contributes significantly to the pressure applied to the ground.

If the pressure inside the tire is 100 psi, and the tire deforms such that 1 square inch of the tire is in contact with the ground, then the internal pressure of the tire against the inside of that 1 square inch contact patch (it seems to me and to every reference I have ever read on the subject) must be essentially equal to the pressure of the tire on the ground (minus a tiny amount of force applied by the casing/tread). Unless the internal pressure of the has changed, then the internal pressure determines the contact patch size.

I would be VERY interested in further reading that contradicts these principles.


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## Foma2 (Oct 1, 2011)

but will it less ?


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## NJBiker72 (Jul 9, 2011)

I went the other direction. Prefer the 23's. I had no problem with flats on my Mavic Refuse 23's but the Specialized Turbo's? Verdict is still out.


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## Salsa_Lover (Jul 6, 2008)

no, 23 is all you need


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## JPorter (Sep 29, 2011)

Kerry Irons said:


> What you are saying contradicts everything I have ever read about bicycle tires, tire pressure, and contact patch size. To say that the contact patch size does not vary directly with tire pressure suggests that tire pressure changes due to compression or that the sidewall stiffness contributes significantly to the pressure applied to the ground.
> 
> If the pressure inside the tire is 100 psi, and the tire deforms such that 1 square inch of the tire is in contact with the ground, then the internal pressure of the tire against the inside of that 1 square inch contact patch (it seems to me and to every reference I have ever read on the subject) must be essentially equal to the pressure of the tire on the ground (minus a tiny amount of force applied by the casing/tread). Unless the internal pressure of the has changed, then the internal pressure determines the contact patch size.
> 
> I would be VERY interested in further reading that contradicts these principles.


The misconception that is sticky here is that the "tire pressure" at 100psi results in deformation of 1 inch when 100lb is applied, in all cases... and that's not the case. The behavior is much more complex than that, a tire is a semi-elastic toroid. It's exceptionally difficult to predict exactly what the contact patch size will be with a given internal pressure and applied weight, because it depends on the size of the tire, the shape of the tire, the construction of the carcass (including both the elasticity of the rubber and the relative inelasticity of the non-rubber components), the affect that temperature has on the rubber's behavior, etc.

In short, the pressure formula P = F / A is not designed for what you're using it to represent.

On the inside of the tire, if P is used to represent air pressure, then the F is the normal force holding the pressure in place (the strength of the carcass itself), and A is the area of the inside of the wheel/tire.

On the outside of the tire, the internal tire pressure isn't a direct factor, remember it's being held in place by the normal force (tire carcass). The only P value that makes sense on the outside of the tire is the contact pressure between the tire and the road, the F is the normal force of the road pushing up (to counteract the weight pushing down), and the A is the size of the contact patch. The only known value is the weight pushing down, though... the P value and the A value change together, so it's almost impossible to predict _exactly_ what they are without a direct measurement of one or the other.

The only way to figure out the interaction between both pressure systems is to figure out the amount that the normal force on the inside of the tire is changed by the application of the point pressure on the outside of the tire, combined with the functional elasticity of the tire at a given pressure and temperature based on its design, and figuring that out is _very_ difficult.

If you want to put together an equation that ties together the pressure dynamics on the inside of the tire, the semi-elastic behavior of a given (specific) tire, and the pressure dynamics on the outside of the tire, all as a combined physical system, I'd love to see it. I can tell you this, though: it's a heck of a lot more complicated than declaring "tire pressure and weight are all that determine contact patch size" because that's not demonstrably true.


Here's a link to the same argument played out in long form (with a lot of direct measurement) on automotive tires, a topic where the exact same claim that you've made in this thread is often seen: http: //performancesimulations.com/fact-or-fiction-tires-1.htm

(sorry, I can't post a link yet, you'll have to copy/paste)


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## dracula (Mar 9, 2010)

dookie said:


> how hard is it to try? check it out yourself.
> 
> i went from 23 to 25 (both open corsa) on everything i ride...fixed steel, carbon, ti. much more comfortable, better grip, better wear and no less fast in my opinion. i also use latex tubes (and did also with the 23s).
> 
> ...


When I assembeld my new Argon 18 frame last October I fitted Michelin Krylion 700x25 tyres. It was the most comfortable ride I ever had on any of my road bikes.

I run 700x25 Michelin Optimum Pro on my former bike which was a full aluminium frame. The ride was always harsh and unforgiving (I am doing a century nearly every Saturday on bad roads in Scotland).

I am still undecided yet why the following combo is such a ride: the Argon 18 alu frame + carbon seatstays + carbon seatpost. Or is it the Michelin 700x25 Krylion.

The problem I have the Krylion in size 25 is huge (70mm from bead-to-bead) and the rear tyre does not safely fit into the frame (top clearance only 1mm). I "lifted" the rear Fulcrum Racing 5 wheel out of the read drop outs to gain an additional 1.5 mm. I always felt this is a very unsafe option though the quick relase held up well.

I then bought a pair of Vittoria Open Paves in 700x24 because everyone is raving about their good hand sewed related ride qualities. What the ****: the ride was/is harsh and unforgiving. I then swapped tyres between the 700x24 Vittoria and 700x25 Krylion to confirm my observation. Okay because the Vittoria is smaller (64mm from bead-to-bead) I run it only with 7 bar. I run the Michelin often with only 6 bar. I tried to use the Vittoria with 6 bar but got a snake bite in the rear on one of my rides.

However, I have now fitted a new pair of Michelin Krylion 700x23. They run true in size (60mm from bead-to-bead) and the combination: 6 bar front and 7 bar rear gives me nearly a s much a plush ride on my Argon 18 as the one I get when using the Krylion 700x25. Michelin tyres can be run with much lower inflated pressures

I have no idea why the Open Pave Vittoria is such an unforging hard to ride tyre and the Krylyin is so mucu better. It must be the carcass. The 320 tpi did not do it for me.


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## Kerry Irons (Feb 25, 2002)

*Misconceptions*



JPorter said:


> The misconception that is sticky here is that the "tire pressure" at 100psi results in deformation of 1 inch when 100lb is applied, in all cases... and that's not the case. The behavior is much more complex than that, a tire is a semi-elastic toroid. It's exceptionally difficult to predict exactly what the contact patch size will be with a given internal pressure and applied weight, because it depends on the size of the tire, the shape of the tire, the construction of the carcass (including both the elasticity of the rubber and the relative inelasticity of the non-rubber components), the affect that temperature has on the rubber's behavior, etc.
> 
> In short, the pressure formula P = F / A is not designed for what you're using it to represent.
> 
> ...


I'm sorry, but we are talking about bicycle tires here, not car tires. Relative to their internal pressure, bicycle tire sidewall and tread stiffness is negligible. In addition, the point of contact bicycle tires are circular in cross section as opposed to car tires which have a very thick/stiff tread and a wide flat contact area. Comparing car tires to bicycle tires is not relevant and article you provided is therefore not relevant. 

Unless you can make some kind of argument that tire volume changes significantly when loaded, then you cannot argue that tire pressure changes significantly. In a loaded bicycle tire the force "applied" to the road by the tread is balanced by the internal tire pressure. If the road is seeing 100 lbs. force over a 1 square inch contact patch, then the only place you can get the 100 psi balancing force is from the air pressure in the tire. If the road sees a higher load from the tire, then that is balanced when the contact patch or the tire pressure increases proportionately to the increased load.

We're not talking a comparison between MTB, CX, hybrid, touring, and road tires. We're talking about the comparison between a 23 mm and a 25 mm road tire.


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## bwbishop (Sep 17, 2011)

{horrible math redacted out of embarrassment for it's rustiness}


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## Kerry Irons (Feb 25, 2002)

*Offensive*



bwbishop said:


> As an engineer, I'm offended by the poor assumptions and logic happening here
> 
> First off, every tire has a spring constant associated with it based off the material, construction, as well as tire pressure, and it does deform when loaded based off the applied weight and the spring constant, which causes a (albeit very slight) deformation which increases the tire pressure. Roll your empty bike over some water and then a piece of paper, and then ride your bike through some water and over another piece of paper. You'll notice a much larger contact area. This deformation causes a decrease in volume and a corresponding increase in pressure. The laws of physics can not be broken just because you don't comprehend them or because you can't differentiate the difference with your eyes. Even at 110psi I can jump on my bike and see the front tire deform and flatten out which means that the pressure in the tire increases.
> 
> ...


You can be offended all you like, but you have made a "fatal error" in your assumption. You assumed a 2 square inch contact patch regardless of the tire pressure. If you are applying 90 lb. force to a wheel and have 120 psi in the tire, your contact patch is 90/120 = 0.75 square inches. Simple unit analysis tells us this. The contact patch size varies with tire pressure and applied load. Yes, the tire casing does have its own spring constant, but compared to the roughly 100 psi tire pressure, it is insignificant. Your analysis has failed completely.

It is true that the tire volume changes (very) slightly when load is applied, and therefore so does tire pressure, but the change is tiny. The only way the pressure can change in the tire is if the volume of the tire changes and the volume change from being compressed due to rider weight is insignificant. 

The external circumference of a 700 x 23 road tire is 82.5 inches, the contact patch is only about 2 inches long, and the tire (at the contact point) is widening as it is compressing. The tire casing size remains unchanged so you are just striking a chord across a small part of the circular cross section of the tire and over a very small portion of the tire.


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## bwbishop (Sep 17, 2011)

You see, that's what happens when you move an engineer up into management. All that extra money makes my brain go soft.  Let's have an argument about EVM. I'm much more practiced there. I bet I could Budgeted Cost of Work Scheduled your A$$ off!


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## bwbishop (Sep 17, 2011)

So after reading this post, I thought to myself, "I should give 25's a try." Then as I was washing my Felt, I read the side of the tire and realized I have 25's. So to the OP, yeah, go get some 25s. They're awesome 

FWIW, all of the Felts I saw at the LBS today had 25s. Not sure if it's just a Felt thing or the industry is headed that way...


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## JPorter (Sep 29, 2011)

Kerry Irons said:


> Yes, the tire casing does have its own spring constant, but compared to the roughly 100 psi tire pressure, it is insignificant. Your analysis has failed completely.


If what you are claiming were true, there would be no perceptible difference whatsoever in ride quality or rider experience between different brands and models of tire, given the same size.

Since that is not the case, your assertion that tire construction has no impact on the dynamics is a fairly thin, in my opinion.


On the previous posts... The same mathematical assertion that you've made is very often made by people about tire pressure and contact patch regarding automotive tires. If you now suddenly claim that a bicycle tire and an automotive tire are not bound by the same physics, then where exactly does the applicability change? What about motorcycle tires, are they somewhere in the middle? What magical threshold must be passed in order for the tire carcass to suddenly have an impact on the dynamics? Come on, you have to be honest with yourself about it: the tire carcass always has an impact on the outcome, though in varying amounts based on the design of the particular tire. To claim that on bicycle tires the tire carcass is always "inconsequential" is a cop-out, in my opinion.


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## JPorter (Sep 29, 2011)

Kerry Irons said:


> If you are applying 90 lb. force to a wheel and have 120 psi in the tire, your contact patch is 90/120 = 0.75 square inches. Simple unit analysis tells us this.


Again, I disagree with your math. This is the same mathematical equation you posited originally, and I still disagree that it was ever intended to refer to internal tire pressure vs. contact patch size. The equation only works with contact area pressure and applied weight over a given contact area, it just doesn't work mathematically for internal pressure in a vessel with a spot force applied to the outside of the vessel.


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## bwbishop (Sep 17, 2011)

JPorter said:


> If what you are claiming were true, there would be no perceptible difference whatsoever in ride quality or rider experience between different brands and models of tire, given the same size.
> 
> Since that is not the case, your assertion that tire construction has no impact on the dynamics is a fairly thin, in my opinion.


Ask anyone who's ridden on Armadillos or Gatorskins or whatever they are called vs other high end, softer tires. There is a difference in ride quality based off the stiffness of the sidewalls. It matters. You can't just dismiss it.


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## Blue CheeseHead (Jul 14, 2008)

Kerry, your point really should be that for a tire of the same model the contact area remains the same (or very close) for a given pressure regardless of cross section. The 100 psi - 1 sq inch example is illustrative, but not exact. It may be knit picking to call you out on it, but they are correct.

In this article Schwalbe talks about tire construction making a difference and claims the contact area to be a proportion. 

http://www.schwalbetires.com/tech_info/rolling_resistance

JPorter is spot on with his argument that different tires offer different ride quality, thus would not deform the same. Heck, people even say latex tubes offer impoved ride quality within the same tire, so tubes may impact deformation as well.

Its all good.


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## Blue CheeseHead (Jul 14, 2008)

To the original poster, get 25mm folding Gatorskins and be done with it. You will get a good rolling, reasonably comfortable tire that resists flats.


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## Kerry Irons (Feb 25, 2002)

*Beliefs*



JPorter said:


> If what you are claiming were true, there would be no perceptible difference whatsoever in ride quality or rider experience between different brands and models of tire, given the same size.
> 
> Since that is not the case, your assertion that tire construction has no impact on the dynamics is a fairly thin, in my opinion.
> 
> On the previous posts... The same mathematical assertion that you've made is very often made by people about tire pressure and contact patch regarding automotive tires. If you now suddenly claim that a bicycle tire and an automotive tire are not bound by the same physics, then where exactly does the applicability change? What about motorcycle tires, are they somewhere in the middle? What magical threshold must be passed in order for the tire carcass to suddenly have an impact on the dynamics? Come on, you have to be honest with yourself about it: the tire carcass always has an impact on the outcome, though in varying amounts based on the design of the particular tire. To claim that on bicycle tires the tire carcass is always "inconsequential" is a cop-out, in my opinion.


The information you provided on car tires is illustrative - their "starting point" pressure was 18 psi, where it is "intuitively obvious" that the stiffness of the tire casing and tread will have a significant impact on the total system stiffness. In order for your claims to be true, bicycle tire casing stiffness would have to be significant relative to the roughly 100 psi "spring stiffness" of the air in the tire. I have NEVER stated that car and bicycle tires "are not bound by the same physics." Misrepresenting my argument is not the way to "win" your point. What I have said is that in car tires casing stiffness contributes significantly to the overall vertical stiffness of the tire. It does not contribute SIGNIFICANTLY in a normal road bicycle tire.

There is a huge difference between being able to feel ride differences in tires and saying that those ride differences significantly affect the area of the contact patch. The feel of a tire combines tread thickness, rubber hardness, and casing characteristics. 

When you compress an inflated bicycle tire with your fingers, you can barely make a dent in the tire. You can easily flex an uninflated bicycle tire - not so with car tires. The bicycle tire casing stiffness is INSIGNIFICANT relative to the stiffness of the inflated tire. This should be instructive to you, but it apparently is not. I have never said there was zero influence of bicycle tire casing on vertical spring stiffness but it certainly is negligible. Contact patch on a road bicycle is determined (to any reasonble extent) by the pressure in the tire and the load applied, not by the tire construction. 

People who say that a 25 mm tire has a larger contact patch than a 23 mm tire at the same tire pressure are not talking about some difference in tire construction. They are mistakenly assuming that because the 25mm tire is wider, it must have a bigger contact patch.



JPorter said:


> Again, I disagree with your math. This is the same mathematical equation you posited originally, and I still disagree that it was ever intended to refer to internal tire pressure vs. contact patch size. The equation only works with contact area pressure and applied weight over a given contact area, it just doesn't work mathematically for internal pressure in a vessel with a spot force applied to the outside of the vessel.


Disagree all you want. If you can posit a condition in which road bicycle tire casing stiffness is significant relative to the rougly 100 psi pressure in the tire, please make that case. You have not come close to doing that.

You made the point that the tire presses on the road with the force applied to the wheel. The only SIGNIFICANT force that can balance that force is the tire pressure. Therefore if the tire is transmitting 90 lb. of vertical force to the road, then the contact patch size is determined by the internal pressure in the tire.


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## kreyszig666 (Jan 6, 2010)

Irons, you are either not understanding the physics, or arguing for the sake of arguing. 
The internal pressure of the tyre can be completely unrelated to the external contact area. A tyre made of steel, for example, can have an internal pressure of 100psi or 1 psi and have a miniscule contact area. 
For you to unilaterally decide that the material properties of the tyre are insignificant in calculating the contact area via the internal tyre pressure is taking a huge liberty, and probably very insulting to the engineers who design high performance bicycle tyres.


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## nhluhr (Sep 9, 2010)

kreyszig666 said:


> Irons, you are either not understanding the physics, or arguing for the sake of arguing.
> The internal pressure of the tyre can be completely unrelated to the external contact area. A tyre made of steel, for example, can have an internal pressure of 100psi or 1 psi and have a miniscule contact area.
> For you to unilaterally decide that the material properties of the tyre are insignificant in calculating the contact area via the internal tyre pressure is taking a huge liberty, and probably very insulting to the engineers who design high performance bicycle tyres.


I still can't get over his extremely specific implication of data supporting his claim but outright refusal to display that data. Arguing with him is like arguing with an autistic. He just keeps shouting the same thing over and over again, whether it's right or not. Therefore, I've given up.


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## Kerry Irons (Feb 25, 2002)

*Stiffness*



kreyszig666 said:


> Irons, you are either not understanding the physics, or arguing for the sake of arguing.
> The internal pressure of the tyre can be completely unrelated to the external contact area. A tyre made of steel, for example, can have an internal pressure of 100psi or 1 psi and have a miniscule contact area.
> For you to unilaterally decide that the material properties of the tyre are insignificant in calculating the contact area via the internal tyre pressure is taking a huge liberty, and probably very insulting to the engineers who design high performance bicycle tyres.


You are correct that a tire made of steel would disassociate the internal pressure from the external contact area. The question is: what on earth does this have to do with the discussion about road bicycle tires? Reductio ad absurdum arguments can be useful in philosophical discussions but don’t offer much value where mechanical parameters are measureable and not the subject of conjecture.

There are two sources of vertical stiffness in a tire - the internal air pressure and the sidewall stiffness. A tire for a road bicycle has very flexible sidewalls and are designed specifically to be that way. Therefore bicycle tire sidewalls supply no significant vertical stiffness compared to the roughly 100 psi in a tire. 

You can easily flex a road bicycle tire sidewall with your pinky finger. Please explain how that amount of force is significant in comparison to the 100 psi pressure in the tire.

The question here is not how there could be tires built with enough sidewall stiffness to affect the contact area separate from the internal pressure. The question is where there ARE road bicycle tires built that way. I have not seen any.


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## Kerry Irons (Feb 25, 2002)

*Specific implication*



nhluhr said:


> I still can't get over his extremely specific implication of data supporting his claim but outright refusal to display that data. Arguing with him is like arguing with an autistic. He just keeps shouting the same thing over and over again, whether it's right or not. Therefore, I've given up.


I have NEVER suggested that I have taken measurements of contact area. You have set that up as a straw man argument to "prove" that I can't support what I say. I repeat: there are two sources of vertical stiffness in a tire - the internal air pressure and the sidewall stiffness. A tire for a road bicycle has very flexible sidewalls and are designed specifically to be that way. Therefore bicycle tire sidewalls supply no significant vertical stiffness compared to the roughly 100 psi in a tire. If you have data to the contrary, please enlighten us.

This reminds me of your insults about my being a high school dropout when discussing the "value" of nitrogen instead of air in tires. You made all kinds of noise about how "only you" realized that N2 and O2 were not ideal gasses. 

FYI, when you plug in the Van der Walls constants for O2 and N2, you find that at 100 psi they deviate 0.67 and 0.5% from ideality. As far as water condensing, you would see less than 2 psi deviation (if the tire was totally filled with air at 80 F dewpoint air) from cooling to 50F. Again meaningless at 100 psi total pressure. Yet you wanted to make everyone into fools for not considering these issues. You're doing the same thing here.


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## kreyszig666 (Jan 6, 2010)

Kerry Irons said:


> You are correct that a tire made of steel would disassociate the internal pressure from the external contact area. The question is: what on earth does this have to do with the discussion about road bicycle tires? Reductio ad absurdum arguments can be useful in philosophical discussions but don’t offer much value where mechanical parameters are measureable and not the subject of conjecture.
> 
> There are two sources of vertical stiffness in a tire - the internal air pressure and the sidewall stiffness. A tire for a road bicycle has very flexible sidewalls and are designed specifically to be that way. Therefore bicycle tire sidewalls supply no significant vertical stiffness compared to the roughly 100 psi in a tire.
> 
> ...


ugh. my example was, I thought, useful as an example of how the material property affects the relationship between internal pressure and contact area when the tyre is loaded externally. 

"You can easily flex a road bicycle tire sidewall with your pinky finger. Please explain how that amount of force is significant in comparison to the 100 psi pressure in the tire"

the way the force of the ground is absorbed by the tyre, i.e the stress state around the contact patch (or variation of pressure; it's the same thing) is *very* dependant on the material properties of the tyre. 

The tyre will deform more in one principle direction (loosely speaking, that means tangentially vs. radially) than another, for example. The differences in stiffness of the tyre along with its geometry in the principle directions will dictate the shape of the contact point and importantly the way the internal pressure balances the external force (i.e the *area* of the contact patch). 

Mathematically, for your argument about the direct relationship between internal pressure and contact area to hold true, the tyre has to be infinitely thin, perfectly elastic, and anisotropic. 
None of these hold (assuming the tyre is not made of rubber only, in which case you could argue it was damn near perfectly elastic, and anisotropic), and as others have stated, the difference you feel in ride quality over different tyres is in no small part to differences in the material properties of the tyres. 
If this *weren't* true, the only thing affecting ride quality would be the radius of and pressure within the tyre.

As I tried to explain in my first post, there are some complicated reactions going on inside a modern road tyre, and it is unrealistic to decide on the basis of your pinky finger that the tyre is behaving according to an incredibly simplistic internal force/external area relationship.


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## Dancer (Dec 8, 2004)

I actually have measured the contact patch for my bike. I coated an area on the tires with railroad chalk and the carefully set the bike on a couple of sheets of paper. I sat on the bike and carefully got of and removed the bike from the sheets of paper. The tire contact patch is what you expect from taking the product of the guage pressure and the area and equating it to the total weight of the bike and rider. I would give you the numbers, but it's been a few years and I don't remember them. 

Dancer



Kerry Irons said:


> I have NEVER suggested that I have taken measurements of contact area. You have set that up as a straw man argument to "prove" that I can't support what I say. I repeat: there are two sources of vertical stiffness in a tire - the internal air pressure and the sidewall stiffness. A tire for a road bicycle has very flexible sidewalls and are designed specifically to be that way. Therefore bicycle tire sidewalls supply no significant vertical stiffness compared to the roughly 100 psi in a tire. If you have data to the contrary, please enlighten us.
> 
> This reminds me of your insults about my being a high school dropout when discussing the "value" of nitrogen instead of air in tires. You made all kinds of noise about how "only you" realized that N2 and O2 were not ideal gasses.
> 
> FYI, when you plug in the Van der Walls constants for O2 and N2, you find that at 100 psi they deviate 0.67 and 0.5% from ideality. As far as water condensing, you would see less than 2 psi deviation (if the tire was totally filled with air at 80 F dewpoint air) from cooling to 50F. Again meaningless at 100 psi total pressure. Yet you wanted to make everyone into fools for not considering these issues. You're doing the same thing here.


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## Kerry Irons (Feb 25, 2002)

*Vertical stiffness*



kreyszig666 said:


> ugh. my example was, I thought, useful as an example of how the material property affects the relationship between internal pressure and contact area when the tyre is loaded externally.
> 
> "You can easily flex a road bicycle tire sidewall with your pinky finger. Please explain how that amount of force is significant in comparison to the 100 psi pressure in the tire"
> 
> ...


What needs to be demonstrated is whether/how the tire casing provides SIGNIFICANT additional vertical stiffness. When a tire deforms due to a load being applied, it deforms (as you describe) tangentially, flexing the side wall. How does that flex provide significant vertical stiffness? Clearly, the piece of tread that contacts the road does carry the load on the wheel, so the question is what forces in the tire balance that load. As you said, a perfectly flexible tire would follow the (load force)/(tire pressure) = contact patch size relationship. I am waiting for an explanation of how a SIGNIFICANT amount of vertical force (5%?) is carried by the tire casing to the rim. If the tire is deflated to the point where it flexes enough to almost touch the rim edge on both sides, is that highly distorted casing carrying significant forces to the rim?

As Dancer has noted, many people have done similar tests to his and found that the contact patch is pretty much determined by the load and the tire pressure. I've not found what I would call a rigorous set of measurements, but it is reported often. IMO it is not enough to say "there could be other resolution of forces in a complex system" but rather some sort of rational explanation or actual measurement is required to show that all these other reports are false.

The “pinky finger” test simply shows that the tire sidewall has virtually no vertical stiffness. It is your contention that when the tire is inflated, that sidewall in effect gains vertical stiffness. I’m waiting to hear an explanation of how that could happen.


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## asgelle (Apr 21, 2003)

Kerry Irons said:


> What needs to be demonstrated is whether/how the tire casing provides SIGNIFICANT additional vertical stiffness.


We do have additional data beyond the "pinkie test." I've never seen a flat tire that could support the weight of even the lightest wheel without completely collapsing. Even at 4 lbs, the support from the sidewalls would be less than 4/140 ~3% for a very light bike+rider. For typical wheel and rider weights, the percentage would be even less.


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## letitsnow (Jul 9, 2011)

I switched from the stock 23c 60 TPI Mondo tires (Specialized Allez) to 25c 120 TPI All Season S-Works. I had over 200 miles on the 25c's before taking them off in favor of the 23c's. No matter what I did, my average speeds (when going faster than 17mph) were/are about 1 mph slower with the bigger tires. This surprised me, especially being as how the 25c tires were about 60 grams lighter (each) than the 23c tires. 

I tried...


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## Pierre (Jan 29, 2004)

Let's have a bit of fun with Kerry's argument - let's take the human body. The casing (skin) is flexible, just like a bicycle tire. Actually probably as flexible, and provides as little 'vertical stiffness' as a bicycle tire. We're not talking about a steel body here, I'm not Ironman, just a normal dude - we're talking about casing that's as flexible as bike tires. The body is full of blood, with a pressure somewhere between 1 and 2 atmospheres. 

Let's stand on my feet, say the contact area is 20square inches, and say I am 200 lbs. Weight = 200lbs. Area = 20sq. in. Let's use Kerry's formula. I've been a rocket scientist (not joking), so I can do some math. The internal pressure, at least in my foot, is 200 / 20 = 10psi? mmm, my doctor is going to be worried, this is pretty low, I must be quite sick. Now, let's stand on my toes, say this reduces the contact area to only 10square inches. Wow, my internal pressure just jumped to 20psi? impressive, but now my doctor is going to start telling me I have hypertension. arghhh, help, I feel like I'm about to have a stroke, can somebody call 911....


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