Originally Posted by
Spoonrobot
That is a beautiful mess of a thread. Starts out with a bunch of math and theory, pivots to real world measurements that don't quite back up the postulations so moves back to math and theory but obfuscated as "real world models" with "observed measurements" and then fizzles out with no agreement. It's also 7 years old.
Tire height only increases at the narrow margins. Going from something like 13/15 inside diameter to 17/19 may cause a small increase less than 5% of the casing length. 1mm for a 23mm tire.
Anything larger than 13/15 and tire height decreases significantly as the rim gets wider. Going from 17/19 to 21/23 and tire height often decreases by 8-10%. This factor doesn't stay constant but tire height will continue to decrease as the rim gets wider.
99% of the time a 50mm tire is going to get shorter as the rim width goes from 17->19->21->23 and so forth.
One of the major errors the few people who measure height often make is that when moving a tire to a wider rim they inflate to the same pressure as the narrower rim. As a wider rim increases the overall volume inside the tire the same pressure as on a narrower rim actually increases casing strain and causes the tire to appear taller than it would be if the pressure was at the correct level. Flo Cycling has a couple interesting items here: Table 4 Page 6
https://www.flocycling.com/FLO_Cycli...sion_Study.pdf
And this article:
FLO Cycling - Casing Tension Study with Union University
OK, I am going to push back on a few things here:
The thread I linked to..
1- That thread is far from a "mess", unless you consider reading what a professional tire developer (bholwell) knows on the matter. There is actually a lot of very good info in there.
2- You have a few mathematical models (including from the professional tire developer) being thrown out there all showing the same thing: that until rim width starts approaching close to the tire width, the height INCREASES, though (and this is also a key point) this increase is
very small.
3- You have general agreement that there are a number of confounding factors that make these very small differences hard to measure in practice.
4- You have Derby (of Derby Rims) measuring tires on different width rims, and coming to the conclusion that the difference in height is negligible. Shiggy's mtb tire site (where he used to measure tires fanatically) does not seem to be up anymore, but as I remember, (I was in that thread, if you did not notice) he was not posting anything that significantly contradicted the mathematical claims. Certainly did not show the tires getting shorter (that clearly would have been addressed).
5- bholwell actaully addressed casing stretching in that thread.
6- You are incorrect that the thread fizzles with no agreement. Everyone in the thread agrees that tires will theoretically increase a very small amount with tire width, though just how much is not settled. NOBODY - not Derby (of Derby Rims), or bholwell (from Maxxis and CST) or Shiggy (who has neurotically put calipers to more tires than any human being alive) - indicate that tires will get narrower with increasing widths (within typical mtb widths). The areas of disagreement were very much nuanced points and did not question the underlying premise.
7- The fact that the thread is 7 years old is utterly irrelevant.
Casing strain
That is an interesting and well done article you linked to, but it does not really back up your point. When you look at Table 4 Page 6, it does show height increasing with pressure, but look at the actual numbers! It shows the height increases roughly ~0.01" (~0.25mm) per 20 psi increase. That is essentially meaningless. And consider that in reality going to a wider rim on a tire in the 50mm range will, in the real world, mean a drop of 10 psi MAX. So now we are talking what... 0.13mm difference? Even if you scale that up from a 25mm tire to a 50 mm tire, that's ~0.025mm. Basically nothing.
Anything larger than 13/15 and tire height decreases significantly as the rim gets wider. Going from 17/19 to 21/23 and tire height often decreases by 8-10%. This factor doesn't stay constant but tire height will continue to decrease as the rim gets wider.
99% of the time a 50mm tire is going to get shorter as the rim width goes from 17->19->21->23 and so forth.
Where are you getting these numbers from? They don't make theoretical sense for a 50mm tire, and the only actual measured data I have seen does not back this up for a set of tires 2.1" range. You got some other source on this?
I could believe this if we were talking about a ~19mm wide tire, but not a 50mm tire. I think the confusion here may be that you are assuming the 50mm tire will behave the same way as a 23mm tire. It won't, because it is the
width of the rim relative to the size of the tire, that determines if the tire will get taller or shorter.
Unless the OP is looking at a rim close to 50mm wide, I just can't see how a wider rim (like going from a 17mm to 25mm rim) could possible make the tire shorter..