Optimal Tire Pressure (255 on 8.5")

[KRyn]

Running a square set of 255/35/18 RE11s on 8.5" rims this season. For guys who have run this setup at what front and rear tire pressure did you find optimal performance while tracking or driving the car at auto-x events?

[RyanDavies]

High-20s for a starting point, based on my experience with optimum pressures for a 255 17 on a 9in wheel.

[2PUTT]

Depends on track and track temps. I would start at mid/high 20's and adjust from there.

[dcaron9999]

Ive been running squared 255/35R18's at the track for a couple of seasons, and measure my tire pressure after each 20 minute stints.

I keep them at or under 38PSI hot.

I start off my first lapping stint at around 30-31PSI warm, when I arrive at the track ...

When the car sits in the garage overnight, pressure sits at around 28 PSI cold, which I usually bump to 32-34PSI for street driving.

Use of the car is 70% track and 30 % street on same set of wheels/tires for now (style 261 18x8.5 ET52 with squared 255/35R18 Direzza ZII's StarSpecs). They are almost done.

Next set will be installed on my four 17x8.5 ET50 style 68's with Nitto NT01's or Hankook RS3 V2's, or R-compound with a few heat cycles in them bought from racers dirt cheap. Will get them in 245 or 255/40R17 format...

[lakefront]

Is it worth filling tires with nitrogen to minimize the pressure differences as the tires heat up/ cool down? Anyone do that?

[Revil]

I think it depend on the track, if the track has good grip with 25/28 would be ok to avoid the tyres get heavy at high speed, here in Argentina we some times reach less than 15 psi to get some grip..

[tuj]

When I have done autox, *usually* I have found that I need to go higher pressures, generally because the stock setup is camber-challenged. On the track, I run the Dunlop RFT's at over 40 PSI to keep them from rolling over. I knew plenty of Mini drivers in autox who were running over 50 PSI on V710's.

Nitrogen is not necessary. You should adjust your pressures after each run. The easiest way to do this is get some chalk and rub it on the tread and the shoulder and sidewall. See if you are getting wear on the shoulder, if so, you're about right on. If the edge of the shoulder or the side-wall are losing chalk, you should increase pressures. Likewise if you are not getting the shoulder to wear, you should decrease pressures.

Lastly, you can use the tire pressures to tune the behavior of the car. Our cars tend to understeer, so you can raise the rear pressure to get the car to trail-brake and rotate easier.

[joe86]

+1 on chalking tires- I used white shoe polish on 3 spots on the sidewalls to find my minimum tire psi at auto-x. Some tires have a little triangle indicating where the sidewall starts and the tread ends. I'd start at 42 psi front and 38 rear, check sidewall marks and lower psi's between runs. Note: if you spin out you may get more sidewall rollover than normal.