You're not wrong. Most of the heat comes from the frictional deformation of the tire though, not the brakes. You can heat the tires and increase pressures easily enough by driving in circles and never touching the brakes. The brakes' contribution to pressure increase is so small it can be ignored in most cases.

 

Yes it does. Lightweight cars with wide tires run very low pressures. A friend's Ralt runs only 12 psi in the huge rear tires

 

Not true and a common myth. Any gas expands when it is heated as all gases obey the Gas Laws. Very easy to prove and a fundamental understanding in any physics class.

Compressed air has water vapor in it and if you are not using a water trap after your compressor some of it goes into the tire. This causes the tire pressures to climb at a rate that is not linear because at some point some of the heat energy will turn the water to steam and thus will increase pressure at an even greater rate.

Nitrogen (or any other "pure" gas) has no water vapor and so it will cause pressure to increase at a linear rate. You may also forget that the plain old air outside is like 85% nitrogen. The nitrogen itself has no improved qualities other than lacking water vapor if it is pure. Any completely dry source of inflation will give the same results.

Plain old clean, dry compressed air has the same performance as nitrogen with the advantage of being cheaper and easier.

 

This has been done for a long time, and is still a usefull tool for getting tire pressures in the right range. If you want to be exact, a tire pyrometer is required.

As soon as the car gets off the track, you take temp readings across the tires. If its hotter in the middle, you let some air out, and vice versa. Ideally, the tire temps will be even across the entire tire. That rarely happens unless you invest some serious time in your suspension tuning.

For most of us, chalk and white shoe polish is as scientific as it gets.

 

I've used shoe polish and a stopwatch. The stopwatch I always believe, but sometimes I like the car better "slower". Planning to add a tire pyro along with some simple track mapping and chassis data logging to the toolbox this season


www.extremegeez.com

 

Choosing the right tyre pressure I guess takes experience... like mentioned by the many other people before me... you need to determine that yourself as there are too many variables.

Although you can get a rough idea through calculations, there are really other variables to consider. Is the tyre wall stiffness linear? What kind of suspension setup are you running?

I'm currently running yoko es100's and for autocross I generally run around 42psi in the front and 32psi in the rear depending on conditions. The way I determine these pressures is by rollover. On performance tyres these days, there are like 6 rollover marks on the tyres. They are triangles on the sidewall of the tyre and you shouldn't be rolling your tyres lower than that mark. I chalk these marks to make reading easier. This method as stated above is becomming obsolete with modern tyres because sidewalls are getting stiffer and stiffer. So the best way is to get a pyrometer and adjust tyre temperatures to be even across it.

The reason I'm running such a high pressure differential between the fronts and the rears is because my suspension is shot and there is a large weight transfer whenever I brake or turn.

As far as track racing goes. I've only been to one track event but I got advice from a really experienced racer. He told me that as a rule of thumb you want your tyre pressures to increase by about 3-4 psi in-between sessions. The lower pressure you run, the larger the pressure differential will be and vice-versa. This should put you in the ballpark of optimum tyre pressures and you should also check it with a pyrometer. I ran with err 35psi cold and 38psi hot.

On the street,.... I just decided to go with 34psi because I noticed tyre rollover (over those triangles) when I ran 32psi.

 

Like rynberg, I've always used 32 psi cold. Tires I've used are 225/50-16 Bridegestone Expedia S-07, Potenza RE71, Pirelli P-Zeros, Yokohama A032Rs, Kumho V700s, Ecsta 711s all on 8 wides; 245/45-16 Hoosier Motorola Cups on 8.5 wides; 255/40-17 Yokohama AVS Is, Pirelli P6000s on 9 wides; 245/45-17, 275/40-17 Hoosier R3S03s on 8.5 and 9.5 wides, respectively. With the Hoosiers asymmetric setup, I did serious tire testing with a pyrometer, and the test resulted in the setup being 32 psi cold (which would heat up to the proper tire temps and pressures after a few laps), so...the Mazda engineers had it right all along LOL

Note that the side wall heights are 4.5 inches for 225/50-16s; 4.2 inches for 245/45-16s, 245/45- and 275/40-17 Hoosiers; and 4 inches for 255/40-17. I suspect with 30 to 35-series 18 tires, tire pressures would need to decrease; for example street tires in 285/30-18 would yield a 3.4-inch sidewall, while 285/30-18 Hoosiers would yield a 3.25-inch sidewall. Proper tire testing would need to be done to verify my belief that lower pressures are needed for shorter sidewall height 18-inch tire sizes.

Interesting sidenote. Hoosier recommends against using 285/30-18 because of the recent tire failures (specifically with 911 Porsches). They feel the tire sidewall is too short, and thus 275/35-18 is what they recommend.