Lower tire pressures - higher tire temperatures. Is it always like that ?

I have been talking a bit with some serious simracers today, and one of them is titled kart racer IRL. He told me that sometimes to rise temps for tire they used to inflate them more. I actually don't believe that, but it is hard not to believe someone who was a champion in Europe.

I wonder if it can be a unique case with kart tires, their small size ? Or something related with lower temps (perhaps below 10C) ? Or the guy is mixing something up. Because I only understand that less inflated tire flex more, and get hotter because of that.

 

The only reason I can think of why you'd get hotter tyres with more pressure is that you'd lose grip, slide more, and generate more heat that way but all else being equal you'd expect that to make you slower.
You would indeed generate less heat through tyre flexure with higher pressure.
Tempting to think he was confused, or at least misguided about why he was faster if he raised the pressures, cos it's not very likely that he had real-time temperature measurements.

 

It may be the type of tyre they're using, but my understanding has always been that a rise in tyre pressure doesn't necessarily raise the tyre peak temp, rather controls how quickly the tyre reaches it's peak temp. Lower pressure allows the tyre to heat faster, but it degrades somewhat quicker...higher pressures help stabilize the tyre, allowing it to reach peak temp slower, thus minimizing things like graining before the tyre reaches its optimum range. I spoke with a friend who knew a few crew members in the Porsche garage at Le Mans this year...what they said really impressed me, how high a temps the GT crew were putting in the tyres, way more than I thought, upwards of 25 psi. There's also a video somewhere on YouTube, watched it during June 2017...shows the cockpit of an LMP1 car during the night hours at Le Mans...at one point the driver scrolls through data on the wheel while on the Mulsanne...shows tyre psi, haha, at 24.

 

More pressure can reduce the contact patch thus allowing it to heat up more. Useful in the rain for instance.

 

If you fill nitrogen in the tires, the tires can warm but the pressure do not increase.
Therefore you can make more pressure.
Or so...

 

Feature Request!!!

The ability to choose filling our tires with A) Air, B) Nitrogen or C) Nacho Cheese.

 

Nitrogen is still subject to boyles law, it isn't a (totally) mystical element! It still has to obey the laws of thermodynamics

Pure nitrogen may expand LESS THAN compressed air, but it still expands, physics is physics...

 

Yep. Prolly half as much, so say 4psi instead of 8. Also less volatile, doesnt fluctuate as much (sliding and such).

But meh, I just put air in my track car.

 

Yes, thats what he was telling me. Also added example about driving on dusty tracks, that they heat up more because of slipping. But it is weird, sliding really isn't the way to get first. But perhaps thats some weird kind of driving technique.... One more thing about reduced contact patch, was his other point. But doesn't contact patch stay the same, just change shape ?

But I think I might have had confused it with wide vs narrow tire thing, when narrow tire has long contact patch, and wide tire has wide contact patch.

This one graph indeed makes better sense. So higher pressure might heat the contact patch more, but I suppose it won't mean that it will heat more the rest of the tire.



The other factor is probably that higher inflated tire would also cool less, because I suppose due to smaller contact patch less heat would be transfered back to the surface of the road.

Yes, but thats mostly because thread get more open, allowing more water to escape, I guess.

 

Agreed, though I'm not sure what you mean by "may expand less than compressed air".

Why do you think it expands only half as much? I see no reason for this - AFAIK both gases are pretty close to ideal gas behaviour at the kind of temperatures and pressures we're talking about here and I'd have expected any variations to be really small (a percent or so, not a factor of two). Links to definitive info would be cool.

I seem to recall reading that the lack of moisture in dry nitrogen was one reason for considering it for track use. But even that doesn't seem like a very big deal in terms of the impact on tyre pressure until you get really hot - if you heat water from freezing point to 100 degrees C then the vapour pressure rises from something very small (a few millibar) to about 1 bar; however at 150 degrees C it's closer to 5 bar which maybe starts to matter.
Since damp air only has ballpark 2% water content, if your tyre temps stay below about 100 degrees C then the water should cause at most a few percent increase on the final tyre pressure (depending on starting pressure), which of course will already be much higher than the starting pressure from the air/nitrogen expansion over the same temperature range - from 20 to 100 degrees C, you'd get a > 25% increase if the tyre didn't expand at all (clearly it will, but I don't know how much). I imagine dry air would work just as well as dry nitrogen...

Also, nitrogen weighs *slightly* less than air. Not really worth worrying about though

 

I tried finding actual numbers from tests but couldn't. Do remember hearing someting along these numbers tho... Either way, a quick google search tells plenty. For me it's unpractical so I dont do it for my time attack events, but if I was doing endurance races I'd strongly consider it, it does make a pretty big difference as far as having stable pressures is concerned, in any conditions really. Tho of course it's even more noticeable the more humid and warm it is. In such conditions it doesnt take much for pressures to spike up after a bit of "unwanted" hooning, then your tyres are junk for pretty much the rest of the session tbh, crawling around for 1-2 laps wont do much.

 

I agree with your friend. After having tested it in AC, particularly with a GT that didn't want to raise front tyres temps. I made a setup that will raise temps without lowering the pressure.

 

But you don't mention raising the pressure either. Ofcourse there are ways how to make tires hotter without touching the pressures at all. You can actually make them quite a bit hotter without even touching the setup, just by adjusting driving style.

Obviously you'll get them hotter in front with less ABS interference, bigger toes, stiffer suspension, more downforce at the front.

 

Yes, we speak assuming the same speed and same driving inputs. Actually I raised the pressure of the default setup which is 16 in the parameters on the left, to 18.

 

Example: you do a corner at x speed with pressure 20 psi, raise it to 22 and the car will slide more, with the same steering angle, you will need more room thus you will be slower. But temps will raise and the following corners and following laps you will have a more optimal temperature, thus you could be faster than before - with more pressure.

 

On the contrary. Same corner, same speed, lower the pressure, you will do it more smoothly, temps could gradually go down and you could lose grip. Resulting in both low temps and low pressure.

 

Are you referring now to real life experience, or experience in AC ?

For me it is unclear anyway, way more difficult than dampers

Speaking of doing it more smoothly with lower pressures... you can always push it harder, and it won't be smooth anymore. At the end it could also slip, and thus also generate the heat.

 

I speak of AC. There are different parameters involved: increasing pressure you'd wish to modify other parameters. For example if you increase pressure much, then you shall decrease your camber, as low pressure is fitted for high camber.