Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community - FT86CLUB - View Single Post - Why RE-71R's torn up after one event?

Icecreamtruk · 09-24-2019, 05:54 PM

@ZDan Yeah you are pretty much spot on on most things. I just wanna come back into the temperature and pressure relation topic, since I find it interesting.

Whether or not the extra movement on the carcass is generating heat is of course not a question, it is generating more heat. But what I wonder is, if that heat is actually more than the heat generated from the extra pressure on the tire. I measure temps with a needle pyro so it should not be surface temps that I get but rather the actual core temps (or closer to it at least). On the same day, with track temps not varying much, dropping about 4PSI on the tire made the temps drop by about 15F after a 4 +2 laps (out lap, 4 hotlaps, cooldown lap). Where is the extra temp coming from at higher pressures? I dont think it is from more tire slipping or extra slip angle, I wasnt getting much more of that anyways, I think its the interaction of pressure and temperature.

Disclaimer: its about to get a bit nerdy.
We arent in a lab, but the ideal gas law should still apply to air to some degree, and PV = nRT gives us a simple way to look at it. Tire volume (V) does not change, only the shape of the contact patch does, R is a constant, n changes (goes up) a bit, but a very minuscule amount, so small that its ok to assume it a constant. So if the product PV goes up (pressure increases), so must T (temperature) if the law is to be respected. My readings at the track seem to indicate, altho not linear, that this is what affects temps the most, as higher pressures have always yielded higher temperature, not once has it been the other way around since I started monitoring it.

Before recording this, I was convinced it was the other way around, and was setting my pressures accordingly (lower pressures for more temp and vice-versa) and I could not understand the results I was getting. Every time I made an adjustment, it got worse so at some point I stopped caring and started doing the "34-36psi hot and forget it" method. I then got the pyro to check my alignment settings, not for tyre pressure adjustment, and it was then that I started noting this on the recorded that. Im actually able to get better times after adjusting pressures now, most of the times at least.

09-24-2019, 05:54 PM	#27
Icecreamtruk Senior Member Join Date: Dec 2015 Drives: Track preped NA FRS Location: Montreal, Canada Posts: 987 Thanks: 1,056 Thanked 681 Times in 405 Posts Mentioned: 12 Post(s) Tagged: 0 Thread(s)	@ZDan Yeah you are pretty much spot on on most things. I just wanna come back into the temperature and pressure relation topic, since I find it interesting. Whether or not the extra movement on the carcass is generating heat is of course not a question, it is generating more heat. But what I wonder is, if that heat is actually more than the heat generated from the extra pressure on the tire. I measure temps with a needle pyro so it should not be surface temps that I get but rather the actual core temps (or closer to it at least). On the same day, with track temps not varying much, dropping about 4PSI on the tire made the temps drop by about 15F after a 4 +2 laps (out lap, 4 hotlaps, cooldown lap). Where is the extra temp coming from at higher pressures? I dont think it is from more tire slipping or extra slip angle, I wasnt getting much more of that anyways, I think its the interaction of pressure and temperature. Disclaimer: its about to get a bit nerdy. We arent in a lab, but the ideal gas law should still apply to air to some degree, and PV = nRT gives us a simple way to look at it. Tire volume (V) does not change, only the shape of the contact patch does, R is a constant, n changes (goes up) a bit, but a very minuscule amount, so small that its ok to assume it a constant. So if the product PV goes up (pressure increases), so must T (temperature) if the law is to be respected. My readings at the track seem to indicate, altho not linear, that this is what affects temps the most, as higher pressures have always yielded higher temperature, not once has it been the other way around since I started monitoring it. Before recording this, I was convinced it was the other way around, and was setting my pressures accordingly (lower pressures for more temp and vice-versa) and I could not understand the results I was getting. Every time I made an adjustment, it got worse so at some point I stopped caring and started doing the "34-36psi hot and forget it" method. I then got the pyro to check my alignment settings, not for tyre pressure adjustment, and it was then that I started noting this on the recorded that. Im actually able to get better times after adjusting pressures now, most of the times at least.