Online Automotive Magazine
Adjusting Tire Pressure on a Track Day
January 16, 2007
Let's face it, the average sports car enthusiast doesn't have a ton of money to spend and, therefore, is not driving around with the fanciest 32-way adjustable shock absorbers. The average enthusiast also probably doesn't have high-quality adjustable sway bars (and if he did, he would probably lose his mind after adjusting them a few times at a track). My point is that most driving enthusiasts, myself included, either don't have the high-tech adjustable suspensions or simply don't have the patience to make drastic suspension adjustments, especially at a track day.
The only way to tinker with the handling of your car without having to lift it off the ground each time you want to make an adjustment or buy a $5000 adjustable shock package is to use tire pressure. Tire pressure is a great place to start tuning your car for several reasons. First of all, my biggest reason for using tire pressure to adjust the handling of my car is the quickness and easiness of the adjustment. When I am at a track day event, I can pull into the pits, let some tire pressure out of the appropriate tires and be back on the track in 5 minutes. I don't have to reach into the wheel wells blindly searching for the adjustment knob on my shocks or unbolt my sway bars to change it to a different stiffness level. This lets you spend more time on the track improving your driving instead of trying to improve your car's handling.
The other reason why tire pressure is a perfect adjustment technique for track days and especially for beginner drivers is because it is simple, intuitive, and relatively foolproof. All you have to remember about adjusting air pressure is that lower pressure gives more grip and higher pressure provides less grip on the end of the car where you make the change. For example, lowering the front tire pressure increases the front grip, which makes the car oversteer more.
Without boring everyone to death, let me give a short explanation for why this is. When a tire has lower pressure, it tends to "sag" more which allows more of the tire to come in contact with the ground. This, in turn, provides higher grip. In addition, the tire is less "bouncy" and comes up to temperature faster because there is more rolling resistance and heat generation with a slightly sagging tire. When the tire does heat up, the air inside also heats up and expands. If there was more air pressure in the tire to begin with, the tire would expand too much and cause loss of grip upon heating.
When you increase the pressure in a tire, the profile of the tire tends to curve so that more of the center part of the tire is touching the ground than the sides. This reduces grip. Also, the air pressure grows too high when the tire heats up, which leads to a loss in grip on long track runs.
So let's say that you put 35 psi in each tire when you first go out on the track. If the car is understeering, let some air out of the front tires depending on how bad the problem is. I would recommend letting out one psi at a time. If the car is oversteering, let out air from the rear tires. Of course, do not go below the minimum recommended tire pressure for the tire that you're using. Check with the manufacturer to find the minimum pressure. Very low tire pressure will destroy your tires in no time.
Even though I'm advocating air pressure adjustments on a track day, this does not mean that the car enthusiast shouldn't take notes about other possible suspension changes. I prefer to drive my car as much as possible on a track day and take note of how it's handling on long runs. Then I make suspension adjustments such as shocks, sway bars, camber, and toe when I get back from the event. This is time consuming work that you should do carefully when you prepare to go to the track. But while you're there, take advantage and drive as much as possible! Work with air pressure to adjust the car, and leave the bigger adjustments for your shop. This way, you maximize your driving time and avoid making hasty suspension changes while at the track.