Trojan Man

I have stock wheels/tires and I am curious about the proper tire pressure.

I was always taught that the faster a car is going, the higher the tire pressure should be. For example, mercedes manuals say that if you are travelling over 100MPH, you should add around +4 psi to the tires (cold).

This is the confusing part: last time I was at the track, one of the people who was putting on the event told me to drop around 4-6 psi because the lower pressure will let me get better grip around the twisties.

Both lines of thought have good arguments to be made for them - so, which is the "right way" to do it?

What kinds of pressures do you guys run street vs. track (please state if you mean hot or cold).

TIA

docjackson1

there was a long thread in june or july, confirming what kim says. most knowledgeable people thought that tire pressures should be significantly decreased for track use-they all thought that there was a chance of tire failure if the tire pressures were not decreased. it seemed that pressures could increase 15-20 ppsi during aggressive track use. i recommend you search for the thread-it was long and informative, with various points of view.

1AS

Do what the good doctor says and fight the demon of ancient teachings.
I think the best explanation of Porsche's ridiculously high rec's are to keep the tires on the wheels if somebody tries a bonzai start on cold tires. Start low and start slow to let the temps and pressures rise. AS

Johninrsf

Just remember when experimenting with tire pressure at the track, that you you want to err on the high side, becuase you lose more grip with a small amount of underinflation than a small amount of overinflation. For example, in the case of a tire which has optimum grip at 32 lbs. of pressure, 3 lbs. of underinflation will produce about double the loss of grip when compared to 3 lbs. of overinflation.
You can experiment with a pyrometer. For correct tire pressure, the temperature of the center of the tire should be equal to the average of the edges.
JP

JayM

We just had a major debate on this issue last weekend at the Silver State open road race. This is a 90-mile timed event on a closed (for the day) section of two-land road near the end of the earth in Nevada. Highest average speed for the 90 miles was in excess of 180 mph, highest trap speed was in excess of 200mph, but most cars were running significantly slower. Some experienced guys at these events were recommending the same sort of approach as MB, namely that the tires should be inflated a couple of pounds HIGHER (cold) than you would normally run on the street. The people with a lot of track experience had the exact opposite recommendation, namely that you should be trying for the same pressure hot as you would have on the street, but since the temperature will be higher than on street driving, you need to start out with a lower cold pressure than you would on the street. Going into the whys and wherefores, I decided that for this event, I would go with the guys recommending higher pressures, but for track I would go with the guys recommending lower pressures. Here is the logic:

At this event, the amount the tires heat up is a function of the inflation pressure (among other things), and the higher the starting pressure the less the tire will heat up. This course has long sections of straighaway separating the curves, which in general are not that challenging, so the loss in traction from higher inflation pressures is not a big issue, especially at my speed class (we were "only" in the 140mph average with a 165 mph max). So by starting a couple of pounds high (we ran 38/46), we reduce the heat build-up, and in fact we never saw temps higher than about 135 degrees. Several guys run nitrogen in the tires to further limit the heat build-up. Obviously, you want to make sure that you don't come close to the maximum safe pressure, but we had plenty of margin.

At the track, I want the extra traction that comes from the lower pressures and the tires are definitely going to heat up because of the frequent braking and sidewall rolls, so I would lower my "cold" pressures to achieve something like 42/47 hot.

Does this make sense?

Greg Fishman

I won't comment on the rest of your post but this is 100% inaccurate. Nitrogen does not prevent heat build up but does prevent (if all the water vapor is removed from the tire) some of the pressure increase you see in tires that have "normal" air that is not dried. If you use regular air that has been through a drier you will see a smaller pressure change as well. The benefit of nitrogen is that it is 100% dry.

Ruf-Racer

Greg is correct about nitrogen benefits because of being dry! In Nevada a small scuba size tank is easy to handle on the trailer.
I will comment on Ruf experience at high speed Nevada events. We start with 36/38 cold w/nitrogen for most Nevada events. Our best average at the race you mentioned is 187.xx and tire temps and pressure are never a concern. Two years past Mike and I ran Battle Mountain twice (red flagged when pizza rolled) or about 160 miles at speed. our trap best was 208.5 and shortly before and after the trap run Mike carefully checked the Smartire sensor. Never a worry. We run a Smartire set up but only as a trend indicator. Forget what you hear at the races from the "boys" since everybody has a different opinion and we have big car,small car,fast car etc. For your purposes and speeds I have to think pressures somewhere close to what we run would work well! A good Hunter dynamic tire balance and close to neutral suspension settings also help. Actually we run exactly what the Ruf factory recommended (suspension) for our purposes.
BTW, congratulations on your last run. Ya did good!

R

Sun Ra

yo Jay,

Look forward to meeting you in the Spring in Nevada and congratulations on your 318 run!!!! and your Ruf!!!!

Ruf racer's esteemed 200++ mph compatriot, Mr Carl Young, has me running 34/41 [factory gt2 is 32/39] for speeds to 165 in the 100 mile Nevada races. One concern that he and Ruf Racer focus on is heat build up due to side wall flex, which can be reduced somewhat, he has concluded after years of experience, with higher tyre pressures.

cornering loads are usually less than 1/2 G, so heat, debris and impact tyre failures are the concern here.

trackwise, i experiment from 32/37 to 34/39,

mr greg fishman has much experience to offer on this subject

what's it like at 208.5 miles per hour???

Johninrsf

It seems as if Trojan Man's question was directed to tire pressure on a road course/track with "twisties", wasn't it?

Ruf-Racer

>>>
I was always taught that the faster a car is going, the higher the tire pressure should be. For example, mercedes manuals say that if you are travelling over 100MPH, you should add around +4 psi to the tires (cold). <<<
Perhaps Jay responded to the above. We all chimed in! Who's the idiot high fiving at speed?





R

Sun Ra

luckily the children he teaches do like he say not like he do

JayM

Hey, Roland, we missed you and Mike last weekend (and Carl & Ellen). Watt, what a great picture. Look forward to meeting you as well. Tyson promised he was gonna show last weekend in the M3, but we didn't see him either. Next year...?