911Jetta

I had the wheels off of my car this weekend and took advantage of the opportunity to closely inspect the tread and wear pattern. Noticed the tires looked a little under-inflated...all four tires were at 29 psi. (I think 36 psi. is stock/preferred for road?). [note: car drives really well at this pressure]

Tires: Sumimoto HTZ III (17") Ft 205, Rr 255

Question is...I'll be doing my first DE this weekend, should I leave the tires at 29 psi. with the knowledge that the pressure will increase at the track after a couple laps? OR, should I arrive at the track with 36 psi. in all tires?

I'm worried that if I start lapping at 36 psi., the tires will heat up to 40+ psi. and the performance of the tire will really degrade...how much do tires heat up after a couple laps? (VIR - long course)

Thanks in advance!

elbeee964

I think you're about to be chastised for running around @ 29 psi.

Also, think you're about to be advised to arrive at the DE with 36 [cold] (or thereabouts) at all four corners. Maybe even higher in the back.

Re DE: Don't worry - be happy.

chopperzz

Thats a huge shot in the dark with many variables to consider. But here's the easiest.

- track temps
-ambient temp
-time of run sessions
-your driving style, speed and smoothness being 2 large parts. (heavy understeer stresses front tires and greatly increases tire temps)

These 3 issues can wildly change the pressure due to temperature in the tires.

Your best bet is to buy a portable cigarette lighter air pump and drive to the track with 36 psi, so you can let some out at track and keep an eye if you need more.

I'm sure there's a lot more suggestions out there........

911Jetta

I was surprised when I saw 29 psi., given the tire's side profile it was hard to detect that is was that low. Like Chopperzz suggested, it's time to test-out the cigarette lighter air pump and drive to the track with 36 psi.

Thanks for the advice! Looking forward to the DE, no stress.

boxsey911

I arrived at the track on Sunday with 36 all round. After 2 x 10 min sessions I had taken 8 psi out of the rears and 6 psi out of the front to get them to 36 all round hot. This is what I normally find when I go on track.

savowood

I would check with the manufacturer to see what the proper operating temperature (range) is for the tires. For example, most people run Toyo R888 tires at 38 hot. For me, that equates to about 29 front and 31 rear when cold on a 75 degree day with the track being about 80. Also, there's about a .75 lb. difference between the left and right when cold depending on the track direction. Clockwise, the left gets hotter. CCW, the right gets hotter. I start with them about the same, but fix it after about 5-6 laps. Hoosiers on my car run best about 35 when hot. Michelin PS2 tires run best for me at about 40 hot.

Given it's your first time, I honestly wouldn't be too concerned with exactly the right temperature. Talk to your instructor. (S)He'll be able to determine your ability and will be in a much better position to let you know if your tire pressures are affecting your driving at all.

When your instructor does say something, an inappropriate response would be, "Well, the gang on rennlist said I should run XX psi." We're not with you on the track.

I've had students who at first were so timid, there was little chance they were getting more heat in the tires than they would on a back road in the mountains. Others were pushing the limits and getting the pressures up to a dangerous level, usually because "somebody told me to run 42 psi cold all around." When that results in over 50 on track, that's a safety issue.

Your instructor will be glad to know you're concerned about tire pressure and should be able to give you some good advice trackside. Just have a good gauge, not the pen-type one they use at the gas station. One with a nice dial like the Longacre or similar.

Dwane

I run 36 or 37 front and 38 rear on hot tires.

chopperzz

don't forget to look at the sidewalls during your 1st break. There is an indicator line at the edge of the tread pattern that shows the flex line of the tread pattern.
On a high corner speed track you can easily find that a "38 front-40 rear" strategy isn't the best and in fact you might need to put 1-2lbs more on the outside tires of the direction of the track because the tread is flexing beyond the indicator lines...........comments anyone???

Nader Fotouhi

I am making three guesses here. I appologize if they are incorrect. You are running 17" wheels, are in the instructed group, and based on your location, going to VIR. If those are correct, adjust cold pressure before leaving for the track. If the air temperature is around 80F, decrease the pressure to 30 if you are shooting for 36 and check immediately after each run. The tire pressure is affected by outside temperature as well.

For the same Brand of tiires (225/17F and 255/17R) we try to have about 34-35F and 36-37R depending on how the car feels. She is in the instructed group. Do not worry too much about the optimum tire pressure and just keep them at a consistant level. BT, it is not unusual to have the temperature higher on one side than the other.

Have fun

911Jetta

Really great feedback guys...all of it.

I tested my mini pump and inflated all four to 36 psi. (cold). Good to know that the pump works, that alone was worth the effort. From 29 psi (all around) to 36 will be quite a difference in the car's feeling (even on the road)!

It's an all day event so I hope to get lots of drive time, that's the worry about tire temps (and VIR is a meaty course with lots of elevation and fast turns). Thanks for the tip about getting a good tire pressure guage, I'm tired of looking at those tiny numbers on the stick...

It's my first time on the track, but I have decades of experience with road and mountain bike racing...so I have a good feeling for traction at the limit (every mountain bike ride around here feels like rally stage, with controled sliding of both front and rear wheels). Can't wait to see if any of that transfers to the track?

The tire psi. question comes out of my respect for the whole track experience (and I realize this is just the start for me) that I'm just checking every and going over the day in my head to get ready. I know I'll feel overwhelmed when I get there!

Henry964

Here in Argentina "common knowledge" is to increase pressure for the track, we run about 10 psi more than for the street ( usually 42 - 46 or 44 - 48) to protect the side of the tire and also because a hard tire increases much less its level of pressure than a soft tire.(and wears less) This is only if you are using normal tires, if you use semi slick it is the other way around

Before you flame me, I know in EEUU you use a totally different approach and I think maybe you are right and we are wrong, but I simply state a different way of doing things

joey bagadonuts

Completely agree. I would add that starting street tires too low results in excessive sidewall flex, heat build up and subsequent traction loss:LOL

Matt Lane

Hmmm, I assume this a joke? All tires have an ideal pressure for best performance when hot. As previously posted, it can vary based on tire compound, 34, 36, 38. Maybe 40.

But 46/48 hot PSI on track? I am confused, but would love to hear more. Which tires have specs like that?

Best,

Matt

Henry964

No Matt it´s not a joke and it is not hot pressure either, we put around 8-10 more psi on cold tires (before entering the track) and we do it in all kinds of cars from Carrera 3.2 up to 997 turbos and 997 GT2s and with diffferent brands and sizes of tires.
The level of competition is very high, the GT2 won the last solo racing event with a total difference of 0.25 sec. in the full 5 laps and he won over a heavily modified 997 turbo.It was quite a sight to watch these 600 plus horsepower cars going flat out in the track trying to best its rival times.
Never saw a track incident related to tires in my 7 years in the Porsche Club

chopperzz

does that mean you are starting a track session at 47-48 lbs cold? If so, what is the tire pressure after a 20-30 minute session? I've seen fluctuation in tire pressure in excess of 10 lbs after a normal street tire is hot.
Is it possible that your hot tire pressure is 55-58? Wow thats high and a new one for me.