What tire pressures should I run?

[Larry Herman]

Here is an advanced copy of an article I am going to put in Der Gasser

What tire pressures should I run?

Probably the most oft asked question in the pits is: what tire pressures should I run? The most accurate answer to that question is: it depends. And that is what makes it so difficult to determine. It depends on a variety of factors including wheel size, tire profile, tire composition, tire brand, car weight, suspension setup, spring stiffness, track temperature and how hard you drive! No wonder so many are confused. I would like to explain some of the parameters, both objective and subjective, to help you make a more informed decision.

Let’s briefly examine what happens to a tire under cornering loads, as that is usually what you are optimizing your tires for. Under braking, the weight shifts to the front of the car and the front tires compress the sidewalls, spreading out the contact patch length-wise. As you begin to turn, the front wheels turn, twisting the sidewalls, and coercing the tire tread to turn as well. As the cornering load builds, the sidewalls flex more under the lateral forces, and the tread patch becomes distorted. The wheel will actually move over the tread, and the outer edge of the outside tire will roll under, with the inside edge rolling up and possibly lifting off of the road. The more this happens, the less grip that you have. How much the tire patch distorts will depend on a variety of factors, not the least of which is tire pressure.

It is the internal pressure which stiffens the sidewalls and supports the tire. Too much pressure and the tire will be firm, but will have less contact patch. Too little pressure and the tire will distort too much, also reducing the contact patch. The right amount will allow for as much contact patch as possible, with minimal distortion. The way to understand where to start is to consider how the variables listed above affect the optimal pressure:

Less < pressure < More
Wheel width – Wide < pressure < Narrow
Tire sidewall profile – Tall < pressure < Short
Tire type – Race < pressure < Street
Car Weight – Light < pressure < Heavy
Suspension stiffness – Soft < pressure < Stiff
Track Temperature – Hot < pressure < Cool
Driver Aggressiveness – Ragged < pressure < Smooth

All of these factors combine to determine what your optimal hot pressure should be. And I do mean HOT pressure; that is where your tires are operating. Cold pressure is merely the starting point at which to set the tire pressures when you start in the morning. The best way to figure out what you want is after you get your pressure where you want them, check them the next morning after they have cooled back down, and that will be your starting point. And this leads us back to the original question, what tire pressures should I run?

The ideal way is to take tire temperature readings. This will tell you how effectively the tire is working across the width of its tread. The outside and middle of the tread should be approximately the same temperature, with the inside usually being cooler. If the center of the tread is cooler than the outside, you need more pressure, and vice versa. Tire temps can also show the effectiveness of your camber and toe settings.

Many of us will just use feel to get the tires to close, and for DEs that is fine. I find that if my pressures are too low, the tire will brake well, but feel mushy and unresponsive in the corners. If the pressures are too high, then the tires feel crisp, but will feel greasy and slide too easily under both braking and cornering. Your best pressure and maximum grip lie right in the middle.

But many of you still want to know what pressures to run.  I hope that you can see that there is no one right setting.  It is something that you will have to experiment with.  That being said, there are ranges which most tires seem to operate in.  Slicks have the stiffest sidewalls, are run with the least amount of pressure, and can start cold at as low as 24 lbs, and increase to between 30~34 lbs hot pressure.  R compound tires need a little more, starting between 26~30 lbs cold and settling between 34~38 lbs hot and street tires need the most pressure, starting cold between 30~34 lbs and getting up to 38~44 lbs hot.  As I mentioned above, you will need to experiment to fine that exact pressures that suit you and your car.

Hope that helps.  I'll see you at the track!