“What tire pressure should I run?”
This is the most common question heard at the track. Unfortunately, the answer is different for every car due to suspension, driving style and tire differences.
The best tool for determining ideal tire pressure is the pyrometer. A pyrometer measures tire temperature.
It’s also a great tool for evaluating alignment settings, tire sizes and sway bar settings. Would you benefit from more negative camber? Are your tires too small? Too big? A pyrometer can tell you.
Tires get hot when they are loaded; acceleration, braking, cornering and steady state driving all deform tires and cause them to heat up. Tire temperature can tell much about how the load is carried and distributed over the tires.
Temperature readings should be taken when tires are fully warmed up, typically after 5 to 10 hot laps on the track. Readings should be taken immediately after the last hot lap, no cool down allowed.
Measurements are taken with a tire pyrometer. The best type of pyrometer has a probe that is inserted into the tire tread. By measuring temperature below the tread surface, the probe provides improved readings since the tread surface cools relatively rapidly.
Alternatively infrared (IR) thermometers may be used. However IR thermometers read surface temperature and therefore provide less reliable readings.
Three readings are taken on each tire; inner tread, center tread and outer tread. Inner and out readings are taken 1 inch from tread shoulder and in the center of a tread block. Write down the readings for evaluation.
Evaluating the readings
We can draw useful conclusions by evaluating each tire individually and by comparing readings of the tires.
Tires have an ideal temperature range in which they produce maximum grip. When cooler than that range, tires loose grip. When hotter than that range the tires become “greasy”, even hotter and they begin to break apart. Check with your tire manufacturer to find the recommended operating temperature for your tires. A typical operating range for a DOT-R tire is 180F to 200F. Street tires tend to run a bit cooler and racing tires a bit hotter.
Use the following table as a general guideline to evaluate readings:
|Center hotter than edges||Tire pressure too high. Reduce 1 psi for each 5′ F delta.|
|Edges hotter than center||Tire pressure too low. Add 1 psi for each 5′ F delta.|
|Inner edge hotter than outer||Too much negative camber.|
|Out edge hotter than inner||Not enough negative camber or too much toe-in.|
|Tire below ideal temperature range||Tire pressure too high, tire too wide, or springs/sway bars too stiff at that axle.|
|Tire above ideal temperature range||Tire pressure too low, tire too narrow, or springs/sway bars too soft at that axle.|
|Front tires hotter than rear||Car is under steering (pushing). Too much front spring/sway bar, not enough rear spring/sway bar, front pressure too low, rear pressure too high, front tires too narrow, rear tires too wide.|
|Rear tires hotter than front||Car is over steering (loose). Too much rear spring/sway bar, not enough front spring/sway bar, rear pressure too low, front pressure too high, rear tires too narrow, front tires too wide.|