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