Correct tire pressure settings are the final step of adjustment to getting your race car to handle properly. It is the fine tuning done after you have already sorted out your spring rates, ride heights, shock valving, anti-roll bar adjustments, and alignment settings. For purposes of this discussion we will assume you are in the right ballpark with all of the above, and you are now finishing up your handling adjustments. If your not done getting these other variables correct yet, tire pressure tuning could be a waste of time.
Correct tire pressure is required to maximize the size of the tire contact patch with the racetrack, allowing maximum cornering traction. It is also required to keep constant tire temperatures across the entire contact patch. To properly tune for correct tire pressure, you must test for tire temperature as well. The best way to see if your tire pressure is correct for your car is to compare the center tire tread temperature against the outer tread temps. If the center temperature is higher, the tire pressure is too high; if the center temp is cooler, the pressure is too low. You can also test your camber settings this way. If the outer tire tread temps are higher than the inner tread, then your tires are rolling over too much. This can be corrected with more negative camber (assuming you already have correct spring rate and anti-roll bar settings). This testing procedure requires a properly trained assistant and a tire pyrometer, and about three laps at race pace to get the tires up to operating temperature and pressure. It also takes several test sessions and consistent note taking to get it right. (See Using a Tire Pyrometer on my ‘READING’ page.)
But if you’re like many budget racers, and all you have is yourself and a tire pressure gauge, there is still a technique you can use to get yourself in the ballpark. First you need a target hot tire pressure setting for your given tire. This is where the relationship you have built with your trackside tire vendor or your fellow competitors comes in. You are “borrowing” some data from them that took time and money to develop, so treat it properly. Most DOT-legal high performance radial tires designed for the track have an optimum hot pressure range around 35 psi. Tire pressure will rise about 5-10 psi when the tires heat up depending on the tire itself, track friction, and driver skill level. Tire pressure rise will also vary from right to left, and front to rear. More left hand corners and the right side tires will heat up more. Front wheel drive cars will heat up the front tires considerably more than rear wheel drives.
Lets say your initial goal is to get all four tires at the same hot pressure of 35 psi. You will need to discover your ‘tire pressure rise’ with some basic testing you can do by yourself. While tire temperature readings require immediate data collection fresh off a hot lap, tire pressures will take several minutes before they drop, so you have time to do this test. Step One – Before you go out for your first session, set all four tires at a chosen cold pressure, lets say 30 psi. Now go out for your practice session, get some good fast laps in, come back to your pits and quickly check your hot tire pressures, and save the readings in your notebook. For example you find LF (left front) 34, RF (right front) 37, RR (right rear) 35, and LR (left rear) 38. Now while the tires are still hot, adjust them all to 35 psi. Go do whatever you do between sessions now (eat, torque your lugnuts, tell your friends how fast you are). Step Two – Just before you go out for the next practice session probably an hour later, check your cold (at least cooler) tire pressures. They are no longer equal; keep taking notes. You are trying to determine what cold pressures are required at each corner to come in with them all at 35 psi hot. Go out and drive. Come back in from this session and check your hot pressures again. Are they at 35 psi all around? Adjust as necessary. After a few sessions you will have enough data to know what cold pressure for each tire will get you the desired hot pressure.
Now you are in a position to start a little more fine tuning. Is the car feeling a little loose (oversteering, rear end breaking loose first)? Try raising the rear pressures to 37 psi hot. Did this make things better? If not, now try lowering your rear pressures to 33 psi hot. How does that feel? Or is the the car a little tight (understeering, front end losing traction first)? Try the same adjustments with the front tires. Now you are collecting data you can “lend” to your fellow racers! (See Understeer/Oversteer Corrections on my ‘READING’ page.)
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