Originally Posted by
cyccommute
Thermoplastic polyurethane (TPU) tubes shouldn’t have issues with increases in pressure due to heat as the pressure increase is much smaller than people think…on the order of 1 psi increase for each 10°F increase in temperature. TPU does start to soften at 80-85°C (185°F). That temperature would be difficult to achieve on a rim brake wheel due to the thermal mass of the rim, tire, and tube as well as due to the radiant cooling provided by the surface area of the same. If you drug the brakes down a very long hill, you might be able to get to that temperature but it would be tough.
Thanks, That is helpful. When you say "start to soften" do you think that is actually close to failure? Or is there yet more room for error?
It is possible to get a rim above 185 F but I am pretty sure it isn't the norm. I have seen a rim hot enough to make spit sizzle when a wetted finger touched it, but that was an extreme case that could and should have been avoided. That wasn't on my bike. I am thinking that if I do decide to run TPU tubes I'd have the heat issue in the back of my mind and never go that far without stopping to let the rims cool if there was ever a need to. Not that I ever expect to heat the rims anywhere close to that hot by braking. I have gone down some pretty long and steep grades with a load and checked the rims halfway down and they were sometimes pretty warm, but never all that hot that I'd be worried they were close to 185 F.
I see that some brands say they are for rim and disc brakes and some say disc only. I doubt the difference is the tubes and figure it has more to do with how worried they are with avoiding possible lawsuits.