[ZDan]

Originally Posted by Icecreamtruk

@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.

Me too!

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.

For sure it is. If you're talking about heat generated from the "extra pressure" on a smaller contact patch running higher tire pressures, consider that you may be getting more heat onto the smaller area, but it *is* over a smaller area. Overall, shouldn't be much if any more heat into the structure of 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).

You're still measuring temps near the surface of the tread of the tire, and not near the internal structure of the tire. And what is heating the air inside the tire is the *inside* surface of the rubber, you're definitely a long way from there. I hope so anyway!

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.

Something else going on for sure here. It's been a long time since I used a pyrometer, partly because I didn't find it as useful as I expected it to be. As I recall it's best to come in HOT, not after a cool-down lap, and preferably have someone there to measure temps immediately right in the hot pit. And even then you get more info about the last corner than average tire temps around the whole track.

Disclaimer: its about to get a bit nerdy.

I'm IN

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,

For different initial pressures? I'd say volume does change a small amount. I mean, a flat tire has less volume than an inflated one! If you mean no volume change as tire heats up on the track, again there will be *some* volume change but in that case small enough to neglect.

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.

OK, now you're *definitely* talking about different initial pressures, more molecules pumped into tire to increase pressure, I don't think we can neglect that, I mean, that's how we are increasing P, by increasing n!

So if the product PV goes up (pressure increases), so must T (temperature) if the law is to be respected.

It's not clear what you're saying here, but based on your "n changes" comment above, I have to think you're talking about temperature added to the static tire by adding pressure? If so, P is going up with n as you add air. Any tiny amount you raise the temperature of the air already in the tire by increasing its pressure is going to be offset somewhat by the new air going into the tire, which is seeing a pressure DROP!
Even if there was one, it would equalize back to ambient over time. Assuming we both arrived at the track with our desired starting pressures, you at (say) 22psi and myself at 26psi, we are starting out at the exact same tire temps, I don't have higher temps in my tires just because the pressure is higher. that's not how PV = nRT works. (my college roomate's mnemonic for this: Per-Vert equals neRT)

*If* you're talking about pressure gained by being out on track, that is due to the temperature change of the air inside the tire. Pressure goes up because temp goes up, not the other way around! But the important temperature here is the temp of the *interior* of the tire, and the wheel.

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.

I'm gonna say this is down to where you're measuring temps: at tire tread, well outside the structure of the tire. Those temps might locally be higher. But the interior tire temps are going to be higher with lower pressures.

Going back a few years, but back when Garry McCoy was throwing his 2-stroke MotoGP bike around, sliding the rear all over the place, people were *amazed* that he wasn't quickly overheating the rear tire, as that was a huge problem for most bikes/riders at the time. A theory was that he was *working* the tire less by lighting up the rear and sliding it around, so while outer tread surface temps may have been locally higher, the internal tire temps were lower due to less deformation under loads! Like I say, this was a "theory"...

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.

Well clearly you're paying a lot more attention to this than I am, so I'm not going to say you're end results are wrong for your driving style and your car setup! 34-36 hot seems to work for me and I don't have enough open track time available or driver consistency to determine where the optimum is.
For sure if I don't want to go out on less than 26psi in general, 24 psi absolute minimum on the tire that sees most pressure change, I'm going to see hot pressures of 36psi by end of session, so just based on that I don't see myself trying to run less.