# 19mm vs 23mm Rims?



## giosblue (Aug 2, 2009)

I've recently changed my drive train to DA9000. The only wheels I have that are 11sp are Mavic Ksyriums I'm looking to buy a set of hand built 11spwheels with 23mm wide rims. Is the a noticeable difference between the two in ride quality. I have a couple of spare Open Pro rims I was going to use, but I fancy trying
23mm. Would the 23mm ride any better than these? BTW I run 25mm tyres.


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## looigi (Nov 24, 2010)

The big diff will come from being able to run ~10 psi lower pressure in the wide rims for a given tire width for the same pinch resistance. 23s become like 25s and 25s like 28s in this regard.

The wider rims provide a bit more lateral tire stiffness so handling/corning precision is a slightly improved. Whether it's perceptible or not depends on how/where you ride and how perceptive you are.


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## bikerjulio (Jan 19, 2010)

I used OP's for a long time and still have some, but recently started switching to wider rims and 25mm tires. Tried Stans, Pacenti and H+Son.

Of those 3 the H+Son are equal in quality and by far the best value.

They are a definite improvement but as looigi says you must run lower pressures to get the benefits. I'm 200# and run 75# front and 90# rear.

Obviously your frame has to have the clearance needed. Not all do.


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## changingleaf (Aug 20, 2009)

No only will you be able to ride lower pressure, but you will have to ride lower air pressure with wider rims. The wider rims increases the air volume inside so a 23mm tire will act like a 25mm or bigger tire. Depending on the rim there are also claims that the wider rims are more aero. The tire will more closely match the width of the rim.


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## giosblue (Aug 2, 2009)

Am I right in thinking that the tyre width stays the same, but the profile changers?
I can just about get a 25mm on the rear of my Icon. If these rims make the tyres any fatter they're a non starter.


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## changingleaf (Aug 20, 2009)

The tire will be wider and the profile changes from a lightbulb shape to more of a U-shape. The tire will not get taller, but only wider in the middle so it depends what part of your frame is close to the tire whether it will touch or not.


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## Blue CheeseHead (Jul 14, 2008)

Going from a 19mm to 23mm wide rim you can expect your tire to get approximately (23-19)/Pi = 1.28 mm wider in diameter so you will see a ~.64 mm reduction in clearance between the side of the tire and the frame. Yes, that is just an approximation, not exact math.

Going to a 25mm wide rim I saw my tires grow by ~2mm in width, but only .2mm in height. I am running tubeless and decided to run tires sized at 23 that actually measure 25.6mm installed. They work great at 80 psi even for a 205# Clydesdale.

If you can fit a wider rim/tire combo, DO IT!

If you are going to the trouble to have a set of wheels built, do not skimp on the rims. Sell the Open Pro's to fund a more aero/wider set of rims.


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## coachboyd (Jan 13, 2008)

changingleaf said:


> The tire will be wider and the profile changes from a lightbulb shape to more of a U-shape. The tire will not get taller, but only wider in the middle so it depends what part of your frame is close to the tire whether it will touch or not.


Actually, the tire will get both wider and taller. Hed did a great study where modeled the tire shapes using a 23 and 25mm tire on varying widths of rims. The wider rims cause the tire to be both wider and taller.


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## bikerjulio (Jan 19, 2010)

giosblue said:


> Am I right in thinking that the tyre width stays the same, but the profile changers?
> I can just about get a 25mm on the rear of my Icon. If these rims make the tyres any fatter they're a non starter.


Usually there is a critical point of interference. What is it in this case?

You could still try 25mm tires on the wider rims, but if that does not work, install 23mm. They will end up a similar width and comfort to a 25mm on a standard rim.


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## Blue CheeseHead (Jul 14, 2008)

coachboyd said:


> Actually, the tire will get both wider and taller. Hed did a great study where modeled the tire shapes using a 23 and 25mm tire on varying widths of rims. The wider rims cause the tire to be both wider and taller.


From actual measurements taking a Hutchinson Fusion 3, 23mm tire from a DA 7801 rim (~20mm) to a HED Ardennes Plus (25mm) rim saw the width go from 23.7 to 25.6mm and the height go from 22.2mm to 22.4mm as measured from outside edge of brake surface to top of tire. Yes, the height of the tire grew, so a vague statement that they get taller is technically true, but it is very little compared to the width. 

If you think about the construction of a tire, they are made to resist expansion in the large diameter (as we have all fought to put tires on certain rims), but the sidewalls are free to move in and out.


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## Blue CheeseHead (Jul 14, 2008)

bikerjulio said:


> Usually there is a critical point of interference. What is it in this case?
> 
> You could still try 25mm tires on the wider rims, but if that does not work, install 23mm. They will end up a similar width and comfort to a 25mm on a standard rim.



Exactly :thumbsup:


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## giosblue (Aug 2, 2009)

Thanks for all your input chaps much appreciated It's looking like I'll be sticking to my 25mm tyres on 19mm rims. on the the Litespeed.The only bike I have which will take 23mm rims with 25mm tyres' is my Bianchi Sempre.


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## K Dub Cycle (Oct 22, 2013)

Most 23mm rims have a 17c rim bead seat diameter. According to the ERTRO chart they recommend a minimum of 25 mm width clincher. 







It seems that going smaller than their recommendation can still be done?


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## bikerjulio (Jan 19, 2010)

Don't stress about it. Nothing will explode.


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## hernluis (May 1, 2014)

do the difference in rim width have anything to do with strength of rim. is the 23mm rim stronger than the same 19mm rim? im sure there is a weight difference.


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## robt57 (Jul 23, 2011)

hernluis said:


> do the difference in rim width have anything to do with strength of rim. is the 23mm rim stronger than the same 19mm rim? im sure there is a weight difference.


yes, yes, and maybe.

Is a 3x6 stiffer than a 2x6? In the case of a rim, the base of the triangle being wider has a lot to do with stiffness in my mind's eye.

To what extent it matters is another question. I mean when you factor in what design aspects might be implemented to make the wider rim weigh less. Not to mention spoke count when built and all the other aspects that come into play I'd postulate...


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## Maglore (Dec 24, 2012)

The biggest improvement I've found with the switch to wider rims, is how much more confidence I have in the handling of my bike. The front especially feels so much more planted and I can commit to decents with more corner speed.


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## dracula (Mar 9, 2010)

coachboyd said:


> Actually, the tire will get both wider and taller. Hed did a great study where modeled the tire shapes using a 23 and 25mm tire on varying widths of rims. The wider rims cause the tire to be both wider and taller.


Common sense tells me this Figure illustration is simply wrong. 

The red 'string' (25mm tyre on the Ardenes) is much longer than the 'white' (25 mm tyre on 19mm rims) one. Where does this additional tyre material - for the same tyre - come from?

He should have chosen either one: e.g. the red one on the smaller and wider rim.


Edit: 'Blue CheeseHead' did some real world experiments to back the Figure up. But I cannot believe it. Where does the additional tyre 'material' come from? The volume as a function if inner rim width can increase for the same tyre, but you cannot produce tyre material (increasing the physical tyre circumference from bead-to-bead) out of thin air.

Edit2: I haven't thought it through but the 'light pulpe' shape may explain it. You wouldn't get more tyre material but due to the shape the height may remain the same although the tyre spreads out a bit and becomes wider on the wider rim. Thus will work at least at small increments (e.g. going from a 15mm inner width to 17mm). However, if you follow that logic and you mount the tyre on a very wide rim (e.g. 35mm inner width) it will definitely become less tall but much wider.
The Figure should be re-drawn to show a light pulp shaped tyre on a rim.


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## cnardone (Jun 28, 2014)

A couple of question.

When doing any calculations, we should be referring to the internal diameter instead of the external. correct? It seems like some threads like the flow cycling article often referenced should be using the internal diameter to me.

For the general rule of -10 lbs of pressure, is this due the increase in the actual tire size? Meaning when referencing the 15% deformation chart we are all familiar with, we can accurately look at 26cc tire instead of 25 (assuming the tires actually measured 25 / 26 on the 2 different rims). Or is that 15% char only relevant for traditional 15c internal width rims? 

I hope that question is clear.


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## ibericb (Oct 28, 2014)

dracula said:


> Common sense tells me this Figure illustration is simply wrong. ...


This struck me as awkward as well because of the way I was thinking about the relationship between tire width and height. But, it's kind of hard to argue against real data from a reliable source (HED), even when it conflicts with our common sense. The real challenge is explaining why. 

My SWAG is it is related to a wrapping effect where, with the narrower rim, more tire material is pulled together more closely into a tube from a flat sheet. Think of rolling a flat sheet of paper into a tube, and then beginning to overlap the edges into a spiral. As you move past a round half circle the paper will pull together inwardly in all directions as it approaches a tube. So as it progresses both the width and height both decrease.


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## Mike T. (Feb 3, 2004)

ibericb said:


> This struck me as awkward as well because of the way I was thinking about the relationship between tire width and height. But, it's kind of hard to argue against real data from a reliable source (HED), even when it conflicts with our common sense. The real challenge is explaining why.


Tires don't break the rules of science. Increase the diameter of a circle sideways and the height will drop to create an oval. My tires went 1mm wider and lost 1mm in height. Anything else is magic.


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## dracula (Mar 9, 2010)

ibericb said:


> This struck me as awkward as well because of the way I was thinking about the relationship between tire width and height. But, it's kind of hard to argue against real data from a reliable source (HED), even when it conflicts with our common sense. The real challenge is explaining why.
> 
> My SWAG is it is related to a wrapping effect where, with the narrower rim, more tire material is pulled together more closely into a tube from a flat sheet. Think of rolling a flat sheet of paper into a tube, and then beginning to overlap the edges into a spiral. As you move past a round half circle the paper will pull together inwardly in all directions as it approaches a tube. So as it progresses both the width and height both decrease.


As I added in my post it may work with a 'light pulp shape' if the increment is small compared to standard rim with.


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## ibericb (Oct 28, 2014)

Mike T. said:


> Tires don't break the rules of science. Increase the diameter of a circle sideways and the height will drop to create an oval. My tires went 1mm wider and lost 1mm in height. Anything else is magic.


There's no violation of science in what I suggested. The issue is explaining the science. I'll try to do better, with a further explanation why different folks can get different results, and both be right.

With wrapping a formed and possibly reinforced flat sheet (either paper or a flat formed reinforced rubber tire) into a cylinder, is at the halfway point into the circle the sides aren't truly radial. They tend toward parallel and flat rather than conforming to the half circle of the cylinder. How flat vs. radial is reflection of the nature of the elastic effect and how the tire was formed. Basically the material being forced into a cylinder is trying to return to it's flatter shape due to elastic memory. The more flat sheet-like the forming was, the more parallel and less radial the sidewalls will be as it is wrapped into a cylinder. There is a point as the wrapping continues when both height and width draw in as the shape becomes more radial. It is basically a result of enough force to overcome the elastic effect and deform the flat sheet into the cylinder. 

The amount of height vs. width change should vary depending on the original formed shape, the elastic strength of the material being deformed, the thickness of the material being deformed, and the extent of the deformation. That means that different tires will change shape, and dimensions, differently as they are forced into a more cylindrical shape. Further, different tires will reach the point where they begin to draw in both dimensions at different points of closure toward a cylinder owing to differences in their original, unstrained and unstressed shapes and thickness.


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## Mike T. (Feb 3, 2004)

ibericb said:


> There's no violation of science in what I suggested. The issue is explaining the science. I'll try to do better, with a further explanation why different folks can get different results, and both be right.
> 
> With wrapping a formed and possibly reinforced flat sheet (either paper or a flat formed reinforced rubber tire) into a cylinder, is at the halfway point into the circle the sides aren't truly radial. They tend toward parallel and flat rather than conforming to the half circle of the cylinder. How flat vs. radial is reflection of the nature of the elastic effect and how the tire was formed. Basically the material being forced into a cylinder is trying to return to it's flatter shape due to elastic memory. The more flat sheet-like the forming was, the more parallel and less radial the sidewalls will be as it is wrapped into a cylinder. There is a point as the wrapping continues when both height and width draw in as the shape becomes more radial. It is basically a result of enough force to overcome the elastic effect and deform the flat sheet into the cylinder.
> 
> The amount of height vs. width change should vary depending on the original formed shape, the elastic strength of the material being deformed, the thickness of the material being deformed, and the extent of the deformation. That means that different tires will change shape, and dimensions, differently as they are forced into a more cylindrical shape. Further, different tires will reach the point where they begin to draw in both dimensions at different points of closure toward a cylinder owing to differences in their original, unstrained and unstressed shapes and thickness.


I think we all need to get outside and ride. Now that PR has just finished, I'm gonna do just that.


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## ibericb (Oct 28, 2014)

Mike T. said:


> I think we all need to get outside and ride. Now that PR has just finished, I'm gonna do just that.


Maybe the fact that I'm on day 3 of unrelenting Gulf coast stormy weather (rain, lots and lots of rain) with another five days to come will help you understand. :crazy:


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## Mike T. (Feb 3, 2004)

ibericb said:


> Maybe the fact that I'm on day 3 of unrelenting Gulf coast stormy weather (rain, lots and lots of rain) with another five days to come will help you understand. :crazy:


It's a balmy (50!) sunny, almost windless day here in Ont Canada. The best so far this year. Byeeeee!


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## November Dave (Dec 7, 2011)

Mike T. said:


> Tires don't break the rules of science. Increase the diameter of a circle sideways and the height will drop to create an oval. My tires went 1mm wider and lost 1mm in height. Anything else is magic.


 We've measured this exhaustively. Keep bead hook shape exactly the same but increase bead seat width from 14mm to 17 and a given tire will increase width appx 1.75mm and increase in height ~ half that much. This is easily attributable to the reduction in lightbulb shape or the 'wrapping effect' (good description btw). 

Bead hook shape is not constant from rim to rim. The Stan's expo display has a keychain showing their rims. Even among those there are significant variations in bead hook shape. 

Did a marathon mtb race yesterday so this nice day will pass without me riding. I'm shattered.


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## dracula (Mar 9, 2010)

November Dave said:


> We've measured this exhaustively. Keep bead hook shape exactly the same but increase bead seat width from 14mm to 17 and a given tire will increase width appx 1.75mm and increase in height ~ half that much. This is easily attributable to the reduction in lightbulb shape or the 'wrapping effect' (good description btw).
> 
> Bead hook shape is not constant from rim to rim. The Stan's expo display has a keychain showing their rims. Even among those there are significant variations in bead hook shape.
> 
> Did a marathon mtb race yesterday so this nice day will pass without me riding. I'm shattered.


But only up to a point from small increments in relation to a standard rim width. I don't think you will see an equal increase of height and width when mounting said tyre on a very wide rim, say 35 mm. In the latter case the tyre width will increase by a large margin but the height of the trye will be being slashed.


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## robt57 (Jul 23, 2011)

I would like to know if that graph was published before wider rims where made for road use. And if the tubeless rim bead design which may not have existed before chart was published makes for a difference.

Considering for examole HED and Zipp 17-19C rims are designed for 23C tires really, the question(s) get begged from where I sit...



K Dub Cycle said:


> Most 23mm rims have a 17c rim bead seat diameter. According to the ERTRO chart they recommend a minimum of 25 mm width clincher.
> View attachment 305039
> 
> It seems that going smaller than their recommendation can still be done?


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## K Dub Cycle (Oct 22, 2013)

Looks like this ETRTO chart was last updated in 2007. This was before the wider rims became popular. I have been unable to find anything more recent.


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## robt57 (Jul 23, 2011)

K Dub Cycle said:


> Looks like this ETRTO chart was last updated in 2007. This was before the wider rims became popular. I have been unable to find anything more recent.


And equally an issue the bead types being different as was part of my point. I can say for all the PITA using tubeless [mounting and repairing on road] tubes and tubed tires are compared to older rims, including some non tubeless A23 wider velocity rims... When I have flatted at speed the tire sure stays on the bead until I get hauled down to a stop. I have had older rims and flats at speed get all kinds screwy including damage to rim form hitting the pave. Last two flats moving the tire stayed hooked tight and the tire stayed symmetrically in profile, rim riding on equal amount of tire across both sides, good! I find this a very desirable aspect of the rim design personally.


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## November Dave (Dec 7, 2011)

dracula said:


> But only up to a point from small increments in relation to a standard rim width. I don't think you will see an equal increase of height and width when mounting said tyre on a very wide rim, say 35 mm. In the latter case the tyre width will increase by a large margin but the height of the trye will be being slashed.


Absolutely correct, yes. All of our studies were done with road tired between 22 and 25mm on rims with bsw between 14 and 18mm. The 23mm Maxxis Padrone tubeless that I now have on a Grail rim (20.3mm bsw) measures very nearly exactly the same as when mounted on a Rail or Pacenti SL23 both of which have 18mm bsw. That showss a point of diminishing returns with a point of reversing returns predictably following. 

With respect to ERTRO chart, masses of empirical data show that23mm tires work wonderfully on 18mm bsw rims. I have a set up in current use that is 23mm tire on 20.3mm bsw and its a great fit.


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## Blue CheeseHead (Jul 14, 2008)

This is really quite simple the cross section of a tire is roughly a circle. Adding width to the rim is very much like adding circumference to the cross section. Add circumference and you add diameter. 

Why does the tire grow more in width than height? Two things 1.) the thickest part of the tire is the wearing surface, so it would want to deform less than the flexible sidewalls and 2.) So far we have been thinking of the tire in 2 dimensions (cross section), now introduce the 3rd dimension and look at it from the side view of the rim perspective. In this perspective the circumference of the tire (as you would measure for your speedometer or odometer) is constrained by the length of the cord and rubber. Changing the rim as little impact on overall rolling circumference of the tire.

Totally predictable.


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## ibericb (Oct 28, 2014)

Blue CheeseHead said:


> This is really quite simple the cross section of a tire is roughly a circle. Adding width to the rim is very much like adding circumference to the cross section. Add circumference and you add diameter.
> 
> Why does the tire grow more in width than height? Two things 1.) the thickest part of the tire is the wearing surface, so it would want to deform less than the flexible sidewalls and 2.) So far we have been thinking of the tire in 2 dimensions (cross section), now introduce the 3rd dimension and look at it from the side view of the rim perspective. In this perspective the circumference of the tire (as you would measure for your speedometer or odometer) is constrained by the length of the cord and rubber. Changing the rim as little impact on overall rolling circumference of the tire.
> 
> Totally predictable.


The circle model is a reasonable first approximation. There's even a tool for estimating inflated tire width by combining inner rim width and the bead-to-bead distance of a tire. The circle model works reasonably well, until it doesn't.

A possibly broader model is that of viewing the tire as a supported arch. The rolling surface makes up the arch and the sidewalls make up the supports. The problem is it's near impossible to anticipate the shapes of the sidewalls. So the circle model remains the best practically useful tool.

The side view part, however, doesn't work for me. There's no way you can increase the distance or the tire height from the rim and not have that extend to tire circumference. There's only one tire-to-rim distance, no matter which way you turn the rim or how viewed. I believe this is where understanding the supported arch can be insightful, even if not practically useful. 

By moving the arch supports from near touching at the bottom to a more vertical and parallel configuration, the arch is raised by the increase in wall length normal to the rim. The tires rolling surface gets pushed away from the rim by the increase in sidewall height. How much will depend upon both tire design and construction - there's a limit of how far out the rolling surface can be held from the rim, dictated in part by length and elasticity of the tire cords/belts. Under load inflation gets added into the picture.


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## Blue CheeseHead (Jul 14, 2008)

I get the pure circle is but an approximation. One could get a bit closer by considering the distance between the inner rim lips as a chord of a circle. It would be more precise but does not change the point.

My point on the side view is exactly that the tire height grows very little compared to the width. Width can grow because the structure in that direction is only constrained by the rim. From the side view the tire structure is complete. The only thing that allows it to grow in that direction is the elasticity of the structure and some release of tension on it by the allowing the sidewalls to move out. The side view of a clincher tire is not unlike the cross section of a tubular tire, it forms a complete structure in and of itself in those respective planes. It is influenced mainly by the air pressure and very little by the rim.

Sorry, not buying the supported arch model. For a fixed length arch moving the side supports out would lead to a flattened, (ie, shorter) total height, with a tire moving the supports out nominally increases height. I won't bother getting into a tire being a tensile structure vs an arch being a compressive structure.

I am not buying HED's "model" as I am not sure they considered the influence of the 3rd dimension. A "model" is not necessarily a real life experiment, but more likely a computer simulation. The simulation is only as good as the variables and mechanical properties entered. I am also surprised that the tire acted as circular as it appears given the varied material makeup and thickness as you go from bead to sidewall to wearing surface. (I do love their wheels) 

My real world measurements have shown me that for Fusion 3's the height of the tire only grows about 1/10th as much as the width. I would encourage people to do their own measurements vs relying on a model that may or may not be constructed properly.


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## ibericb (Oct 28, 2014)

Blue CheeseHead said:


> Sorry, not buying the supported arch model. For a fixed length arch moving the side supports out would lead to a flattened, (ie, shorter) total height, with a tire moving the supports out nominally increases height. I won't bother getting into a tire being a tensile structure vs an arch being a compressive structure.


An arch will have it greatest height above it's support base when the two supports are normal to that base. When those supports move either inward or outward from that position, the top of the arch will lower toward the base. A simpler model would be to replace the curve of the arch with a straight line. At the extremes with the two supports (sidewalls) pulled all the way together you have a triangle with a vertex support point on the base. Now, move the two side that make that vertex apart and towards vertical. The flat top surface will increase in height away from the support base until the supports are normal, then it will begin to drop again towards the base as they pass the normal angle.

A real tire is somewhere between the supported arch and a circle.

It was my understanding that HED report was real and measured, not a model. Is that incorrect?

edit added - BTW, the arch is a reference to shape and form. Obviously the arch form in a flexible material isn't a structural, rigid arch.


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## ibericb (Oct 28, 2014)

Added food for thought. There was a very good (and lengthy) discussion on the subject of tire width and height a bit over 3 years ago in the mtbr forums. In the initial post is an illustration (copied below) that shows what happens when you begin opening a circle, and spread the ends apart laterally, representing what happens with both width and height of a tire as rim width would increase. All of the arc lengths are the same as the original circle's circumference. I think this is essentially the circle with a chord variation Blue CheeseHead suggested when we debated the limitations of the circle model.

It very clearly shows that as the circle is opened and the chord length between the two ends increases, both width and height initially increase. But at some point as width continues to increase height reaches a maximum, and further spreading continues to increase width but height decreases.

To the extent that the circle model represents the reality of a road bike tire (which I believe is questionable, but useful to a point), this illustration shows what will happen as you move from a narrow rim to a wide rim. In reality we don't start with a circle but are already well spread. It's now a matter of how much further a wider rim spreads the two bead edges.


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## Blue CheeseHead (Jul 14, 2008)

ibericb said:


> View attachment 305177


This illustration is consistent with what I have seen. I agree that with a tire we start with it well spread. When you get to that point, the diagram shows where the width grows, but the height not so much. (4th or 5th segment is probably is close for a standard 19mm rim.

BTW, the poster of the HED diagram described it as being a result of HED's modelling, not measurements.


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## ibericb (Oct 28, 2014)

Yeah, I saw the comment about HED modeled, but as specific tires and shapes appeared it looked to me like those were actual profiled cross sections. If it was a modeling-only study I don't understand why they attached specific tire ID's with the profiles. Maybe Boyd will see this born-again segment and enlighten us some more on the study.

I'm completely speculating here, but it seems to me that for a given tire you would want want a rim width that holds the bead spread no greater than the width where the maximum height is achieved, and not wide enough to enter into the decreasing height domain. Once the maximum is passed, and it would appear that the effect of further bead spread would be to effectively "push" thinner sidewall material into the contact patch zone, and more so when cornering.

Since tires are designed and molded with a non-circular shape, I would think that the designers had nominal and maximum rim internal widths in mind. It sure would be nice to now what the range was.


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## bigjohnla (Mar 29, 2010)

ibericb said:


> Maybe the fact that I'm on day 3 of unrelenting Gulf coast stormy weather (rain, lots and lots of rain) with another five days to come will help you understand. :crazy:


I can relate. I live in Baton Rouge and I hate riding on wet roads in driving rain storms. Pretty much shot the whole weekend. I spent the whole weekend in my shop watching the Masters on my Iphone while rebuilding an old Raleigh Wyoming. The last of my parts for my 23mm Velocity A23 wheel build arrived this weekend. I have both 23 and 25 mm tires of the same brand. When I get them built I will try both sizes and see what I get. I bought 23 for the new wheels based on several posts recommending this setup with this rim. Supposed to give a "totally tubular, Dude!!" kind of ride.


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## ibericb (Oct 28, 2014)

bigjohnla said:


> I can relate. I live in Baton Rouge and I hate riding on wet roads in driving rain storms. Pretty much shot the whole weekend. I spent the whole weekend in my shop watching the Masters on my Iphone while rebuilding an old Raleigh Wyoming. The last of my parts for my 23mm Velocity A23 wheel build arrived this weekend. I have both 23 and 25 mm tires of the same brand. When I get them built I will try both sizes and see what I get. I bought 23 for the new wheels based on several posts recommending this setup with this rim. Supposed to give a "totally tubular, Dude!!" kind of ride.


I'm right there with ya, bigjohnla. Get ready, more is coming your way. Look to your SW - that band is the latest of the "series on impulses" that ran over us about 04:00 this morning. Our forecast is 40-50% chance of T-storms right on through Friday. this really sucks!


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## Pirx (Aug 9, 2009)

ibericb said:


> To the extent that the circle model represents the reality of a road bike tire (which I believe is questionable, but useful to a point),


I don't have the time to weigh in on this in a substantial way, but I will note that indeed the circle model is fundamentally flawed, since, to a good approximation, the height of the tire is essentially fixed, at least if we model the tire as a membrane with infinite tensile stiffness (no stretching, in other words, the same assumption that underlies the constant-circumference idea in the 2-D model you are presenting). With this assumption, since the circumference of the tire along the third dimension is then constant, you are forced to drop the assumption of a circular cross section. Long story short, since the tire height is fixed, in almost all of the cases in your diagram (except the most extreme width case) the tire width will increase less than shown by this 2-D model, and the tire shape will be more oval.


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## ibericb (Oct 28, 2014)

Pirx said:


> I don't have the time to weigh in on this in a substantial way, but I will note that indeed the circle model is fundamentally flawed, since, to a good approximation, the height of the tire is essentially fixed, at least if we model the tire as a membrane with infinite tensile stiffness (no stretching, in other words, the same assumption that underlies the constant-circumference idea in the 2-D model you are presenting). With this assumption, since the circumference of the tire along the third dimension is then constant, you are forced to drop the assumption of a circular cross section. Long story short, since the tire height is fixed, in almost all of the cases in your diagram (except the most extreme width case) the tire width will increase less than shown by this 2-D model, and the tire shape will be more oval.


True, once you reach that tensile limit. But before that height increases are indeed possible, just as loading decreases the tension and results in a drop of tire height. The tires height and attendant circumference isn't constrained to be a fixed length, but it is limited in how high and long, respectively, it can be. Below whatever inflation pressure it takes to achieve that tensile limit, height increases with increasing rim width up to the max height can and probably will occur. But the real practical difference's in rim widths are relatively small compared to the baseline minimum rim width typically considered, so the height increases possible are also small.


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## Blue CheeseHead (Jul 14, 2008)

This whole conversation would be better with beer.


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## ibericb (Oct 28, 2014)

Blue CheeseHead said:


> This whole conversation would be better with beer.


:thumbsup:


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## ibericb (Oct 28, 2014)

This occurred to me in a moment of later thought, but I believe there may be a flaw in the argument for fixed circumference of a _bicycle_ tire based on the inability of the underlying material to elongate (i.e., the infinite tensile modulus premise). 

Unlike contemporary automotive, truck and heavy machinery tires that are built with very high modulus belts, bicycle tires (at least the road versions we are discussing here) are typically built with much lower modulus fiber reinforced casings. As a result they should be appreciably more extensible.

If I did the calculation about right (no guarantees there), considering a 700 x 25 tire with a 2100 mm circumference as a base, a 1mm increase in tire height, which leads to a 2 mm increase in mounted tire diameter, amounts to a whopping 0.3% elongation in tire circumference. It doesn't take much elongation to account for a 1mm increase in tire height, and that small elongation seems well within the realm of reason for a corded casing construction.

A caveat here would be a possible reduction in extensibility with the inclusion of high modulus breaker belts (e.g., Kevlar, Vectran, etc.), depending on how they are incorporated into the tire.


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## HermesSport (Oct 3, 2014)

hernluis said:


> do the difference in rim width have anything to do with strength of rim. is the 23mm rim stronger than the same 19mm rim? im sure there is a weight difference.


At the very least, it does increase lateral stiffness. The further away from center you put the material, the less likely the actual thing is liable to flex about that axis. There's some weight penalty, but most people have been willing to tolerate it.


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## November Dave (Dec 7, 2011)

Great point ibericb. I remember one time we were bored driving on the DC Beltway and the question of "how much longer is the outer loop than the inner loop" was posed. It was like 300' over 80 miles. A little extra diameter doesn't add too much to circumference at all.

Also bear in mind that this discussion is implying rollout circumference and overall tire diameter, but I bet it's all being measured as distance outside of/above the brake track. The volume outside the brake track is very key, of course, but a rim with a slightly shy diameter or with a shallow bead hook could produce a change equal to or potentially much greater than the change wrought by widening bead seat.


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## Pirx (Aug 9, 2009)

ibericb said:


> If I did the calculation about right (no guarantees there), considering a 700 x 25 tire with a 2100 mm circumference as a base, a 1mm increase in tire height, which leads to a 2 mm increase in mounted tire diameter, amounts to a whopping 0.3% elongation in tire circumference.


Your point is well taken. However, keep in mind that you are postulating that the circumference changes _without_ any changes in load on the tire, at constant pressure. So, yes, 0.3% elongation is not that much, but it's not that small, either, and stretching the material by that amount would take quite a bit of force. Given that the pressure does not change at all, it's not easy to see where a change in tension should come from. Mind you, I do understand that the slightly different geometry will have an effect, put I'm just going to postulate that it's negligible, and be done with it...


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## ibericb (Oct 28, 2014)

Pirx said:


> Your point is well taken. However, keep in mind that you are postulating that the circumference changes _without_ any changes in load on the tire, at constant pressure. So, yes, 0.3% elongation is not that much, but it's not that small, either, and stretching the material by that amount would take quite a bit of force. Given that the pressure does not change at all, it's not easy to see where a change in tension should come from. Mind you, I do understand that the slightly different geometry will have an effect, put I'm just going to postulate that it's negligible, and be done with it...


The dimensional changes are only realized with the tire normally tensioned via inflation, and as we've all stated are indeed quite small. 

Just for grins, a 700x25 tire presents ~180 sq. in of area and @ 100 psi that's 18,000 lbs of force. That strikes me as not insignificant for a reinforced elastomer.

edit added- I went back and looked at the prior posts, and all those that note increases in measured height are all fractious of a mm - really small. Using my 1mm = 0.3% elongation, we're talking like 0.1-0.2 % elongation with something on the order of 100 psi pressure to push things out. What the chorded circle model suggests is that, yes, further extending the chord, as a wider rims will do, can lead to either an increase or decrease in tire height depending on the tire's dimensions, the original rims inner bead width, and the amount of change. It can go either way in height as width is increased. The final point was that bicycle tire circumference increases are reasonable given the very small extensions some have noted and how road bike tires are constructed.

Now, where's that beer?


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