Tyre pressure for track event

[Patronus]

My '85 Carrera has 245x45x16 Bridgestones at the rear and 205x55x16 Bridgestones at the front. These are road tyres and I just check the pressure for any irregularities before a track event. However, I've been wondering whether tyre pressures should be dropped for a track event since the tyres get so hot.

[NZ9Iwon]

Sorry, I cant answer your question but i have the same concern. I found my tyre pressures were up around 41-2psi after a few laps. OK quite a few laps. Should you start lower?

[Jack Olsen]

My advice: start 31F/33R. Immediately check when you come in from each session, and bleed off pressure to about 40psi hot. If the first session only brings them up to 37 psi or so, don't worry. The pressure will move up as the wheels heat up.

Different tires do best at different pressures, and R compound tires are a different ball game entirely. But 40 hot is the number I've heard most often for street tires on the track.

[Jack Olsen]

And speaking of 'tyres,' does the whole world use psi for tire pressure, or do some gauges read kilograms per square centimeter?

[targa88]

Although I am not an expert, there are a number of components to consider: ambient temperature outside (colder temps require less tire pressure), age and condition of the tires (older tires with less tread tend to heat up faster), brand ("c" compound , "r" compound).
Ideally, if you want to get technical you would need to measure tire temperature (I think a "pyrometer is required) to obtain the optimum tire pressure.

[Joe Bob]

Get a pyrometer as well. Jack gave good starting numbers.....after a hot session, check the temps on the inside middle and outside....

High in the middle means a little too much pressure, low temp not enough.....uneven sides.....probably time for some alignment.

[David Ceruti]

Hi Martin

I would support the advice about the pyrometer - its the only way to really know the best pressure for your car and setup. It will alsogive you some feedback about the setup, but you will always foin that the inside of the tyre will run hotter than the outside (camber and cornering dynamics). What you re aiming for is an even spread from outside to inside.

About the starting pressures - I would go higher. More initial pressure = less sidewall flex = less heat buildup (and better handling). The group N saloon car guys used to start with 3 bar plus but they generally used skimmed tyres so the bulge in the middle was flattened. Remember that road car specs give a pressure spec that takes comfort as well as performance into consideration.

When is the track day? - I may come along to see your new baby

David

[Jack Olsen]

A pyrometer for someone's first track day? That seems like overkill, to me. I fine-tune pressures with temperature readings, now. But when I started out I was lucky to remember where I'd set my helmet. The first couple of days out there can be pretty overwhelming.

[David Ceruti]

Good point Jack - I got a bit carried away!

Maybe a starting point waould be to start with a harder than normal pressure and keep an eye on warm pressure and the wear patterns on the tyres. I don't have first hand experience with p-cars on the track but with my own I would start with 2,3 and 2,8 bars (for what my thumb suck is worth)

David

[Wil Ferch]

Thos who follow Todd Serota's ( of Trackquest fame ) advice on Rennlist....
typical is about 30-32 front...and 34 or so rear.....
you usually pick up 6 psi after a hard run...don't go too high cold...
It appears this even applies to the 17 and 18 inch tires that usually are pumped to 44 or so cold per Porsche recommendations...many say way too high..
---Wil Ferch

[jluetjen]

It's virtually impossible to suggest the correct tire pressures for someone elses car since in addition to factors mentioned above, they also depend on the track temperature, construction of the tire, the compounds, the alignment settings, the weight of the car and the size of the wheels. Some people even argue that the amount of moisture inside the tire makes a difference.

The right way to do it is with a pyrometer. But I agree that it's not something that most people need invest in for their first DE. My suggestions would be to try to get as much feedback as you can from the information that you have available. Keep in mind that your tires are the ONLY contact between your car and the road - so the affect of the tire set-up will trump just about everything else like springs, bars, etc. When I used to race in ITB, I found that .5 pound difference in pressure might be the difference between an evil handling car and a personal best lap.

My suggestion:
1) Start out with the stock (as recommended by the tire manufacturer) tire pressures. (I tend to agree that +1 or +2 pounds may make the tires run cooler, but let's KISS to start. The engineers at the factory do know something about tires.) Chaulk some broad stripes on the sidewalls of all 4 tires all the way across the shoulder of the tire. Take a good look at the tread of the tire and remember how it looks across the tread.

2) Do your first session.

3) As soon as you come in look at and feel each tire and try to figure out the following:
A) (By feel) - are the tires hot to the touch? Are they hotter on the inside? Middle or Outside? Does any tire feel noticeably hotter then the others?
B) Look at each tire -- do they look like they are worn evenly all the way across?
C) Look at the chaulk marks -- how far is the tire rolling over onto the shoulder?

Then you want to adjust your tire pressure to achieve the following "ideal" tire profile.

1) The tires should be getting hot. The manufacturers will tell you the specifics, but slicks like to work at up to about 200 degrees. R rated tires I believe are happiest a little cooler then that. (Can anyone confirm this for me? I remember that my ITB car used to race at about 160 to 180 when it was happy.) That should give you an idea of the magnitude of tire temperatures. We're talking "hot to the touch" as soon as the car comes off the track.
2) For Radial tires, the inside of the tread should be about 10 to 20 degrees hotter then the middle and outside.
3) If the tire is rolling over the outside shoulder (the chaulk is buffed off), then you need some more air and/or some more camber.
4) The tires should be roughly the same temperature. If one end (front or back) of the car is hotter then they other, then that is the end which is working the hardest. So if the rear tires are hotter -- your car is loose, if the front tires are hotter -- your car is pushing or understeering.

[Joe Bob]

Nobody said anything about buying one.....

Our club had one that was a loaner.....I liked it so much, I eventually DID buy one.

[cstreit]

I agree with these guys. Try to end up in the high 30's as a starting point. A pyrometer is a decent investment as it can also help ID camber issues.