madmoog

I've tried to search but only got a blank page, so firstly I apologise for asking what has already been covered (many times?) before.

964 C2 cup 17" rims with Conti 205 ands 255. Should be doing Silverstone GP on Tuesday evening and was wondering what pressures people would advise running initially cold and also hot.

thanks

madmoog

p.s. any uk listers going??

garrett376

I would personally start at 28/30 cold front/rear, mark the sidewalls with something (shoe polish or similar), then after the first session see how far over your tires are rolling on the sidewalls and adjust from there. You want to use the entire tread, but not roll over too far on the sidewall to wear them out prematurely. I don't use shoe polish since I can see the extent of the wear without the aid of a marker - if you're new at it, the marker helps.

SimonExtreme

Your aim with that set up is to get to 36 psi hot. As stated, this means starting with less when cold and 28/30 seems a good starting point. Go out and do 4 laps. Come in and immediately check the tyre pressure. Inflate/deflate until you achieve 36 all round. Then go out for 6 laps and repeat. Don't be suprised if some of the tyres heat more than others, due to the direction of the track and corners. Just do what is needed to get them consistant on all tyres.

Once you have achieved 36 psi all round, leave it there and remember to heat the tyres up at the beginning of each session before really pushing it.

From there, you can adjust to personal tastes. On road tyres on my RS, I prefer to stay at 36 all round but you can fine tune handling through the tyres.

Finally, don't forget at the end of the day to return the tyres to road pressure!

tonytaylor

As Simon says target tyre pressures of 36psi hot would seem sensible.

How you get to 36psi hot is a matter of opinion and to some extent depends how much track time you have and how hard you drive the car. If you're a relative novice you are better starting off with 36psi cold and bleeding the pressure off after each session until you reach a stable 36psi.

Quite a few experts think you shouldn't run on low pressure at anytime and should always bleed off pressure in any case.

SimonExtreme

Tony

I have heard these stories about not running on low pressure but I cannot see how that works. It certainly doesn't work when racing. Or the supersprints I do. My fastest times are when the tyres are hot but the pressure is right. If I start at the right pressure, it will be too high when up to temperature.

Or lets look at trackdays. How are you meant to drive? Go out at normal pressure - come in and reduce pressure after 2-3 laps - go out again for rest of session - inflate tyres when they cool between sessions and repeat! I have never seen anybody do that!

I am sure that theory says never run under inflated but I haven't heard of any significant problems from running the way that has been suggested.

TR6

I seem to have good luck with starting cold at 28 front/31 rear. A 3 pound differential front to rear seems to keep the understeer more manageable. However, I'm running 18" wheels (225/40 front, 265/35 rear). I usually find that the tires gain approx 6 pounds of pressure as they heat up on a dry, warm day.

I ran a week ago on a rainy day and found that lowering the air pressure a couple of pounds below where I usually start out resulted in a little better grip on a cool, wet track. More mechanical grip I suppose since the tires aren't going to heat up in the rain anyway.

boxsey911

Interesting subject. Let me throw this one in the ring.

If the recommended tyre pressures for normal driving are set cold (e.g. 36 psi all round) then presumably the operating pressure is a little higher as they heat up (say they get to 38 psi after half an hour of driving).

Therefore, shouldn't the 'hot track tyre pressures' be optimised to 38 psi rather than 36 psi which in theory is the cold pressure?

p.s. I haven't measured the 'hot pressure' after road driving so my 38 psi is a guess.

garrett376

On track, you aren't shooting for an actual number, per se. You are trying to maximize your tire's traction given your car's weight, track demands, setup, etc. That's why marking the tires is the best way to do it. Target pressures are nice to shoot for, but it's more accurate to "see" how the tires wear and adjust from there.

madmoog

Thanks for the info so far, interesting.

I went to silverstone last night, my first track event and really enjoyed it. Rather nervous at first and never having pushed a 911 before "interesting" !!! Had a session with an instructer and things improved considerably

back to tyres: They were set as 36 cold and drspite the good advice above, as I didn't think I'd be going that fast or hard I thought I'd go aot like that and see how it went. After a while I came in and checked them and found them to be between 41 and 45, so I dropped them to 38, which the maintained consistantly for the rest of the evening, probably should have dropped them a little further. having said that they weren't rolling and the wear was consistant accross the whole contact patch.

Ambient last night was about 22 degrees and I was doing sessions of about 20 mins or so as after that it felt likme the tyres might have been overheating as it was getting rather loose (interesting for a novice) and the breaks were going too.

I'm going to do this again, addicted

TR6

If your pressures are too high once it heats up, the tire will have a curved or crown surface in contact with the track rather than a flat patch of tread. In other words, too much pressure causes you to have less tread surface in contact with the road. This will cause the car to slide more. If that is happening, lowering the pressure by two or three pounds will help the car stick to the track better.

dfinnegan

On the other hand, too little rear tire pressure will cause oversteer, as well. Because the tire rolls over sideways, I expect.

TR6

Very true, but if he is anywhere in the mid 30's, that's not happening most likely. I think you'd have to get into the low-mid 20's cold to worry about rollover. Of course, my experience is only with 18's. Smaller diameter tires may be different.

boxsey911

If you're interested, I'm orgnanising a group of Porsches to go on a day at Bruntingthorpe aerodrome (Leicestershire), first saturday in August. Not a proper race track but a great place to explore the limits

madmoog

Very possibly

Would you mind pm ing me some more details. Event type/layout, cost etc......

Little nervous though, the owner before last did some top speed runs there and did a set of ends, I think (though possibly due to a faulty stat ).

boxsey911

Don't worry, they don't use the whole track so you can't get to Vmax anywhere. I'll pm you the details.

Oh and if any other UK rennlisters want to join in, just let me know