Tire Pressure

Before I tell you how I go about determining the optimum pressure, let me just say if anyone is just using their car for regular commutes back and forth to the office, use whatever pressure is stated in the owner's manual. If they do that and remember to check it every month they'll probably be fine. Also, if someone's car handles poorly because it's got 10-year-old blown struts, the sway bar mounts have rusted off, or the tires are bald, they've got other things to consider.

I know of two ways to "read" a tire and determine the optimum tire pressure. The best method that I know of requires the use of a pyrometer. The first reaction of some people may be, ‘Now I have to buy a pyrometer, just to air up my tires?!?!?’. I can understand that response. Buying a temperature sensing tool just to put air in a tire does seem like a bit much, but it’s the best way I know to insure that I’m getting the most traction that I can out of a tire.

If anyone spends six hundred dollars every eighteen months on street tires, but still feels that a $100 or so for a pyrometer is too much, here is the two-dollar chalk method.

At the track, or on your favorite curvy stretch of road, take some chalk and mark all four of your tires. Make the mark across the tread portion of the tire, around the edge, and about an inch down the sidewall (about where the lettering starts on the side of the tire).

After making a few passes through both left and right hand curves, pull over and check the tires. What you hope to see is all the chalk rubbed off the tread on the tires, and almost all of it still on the sidewalls. How much is almost? If you've only rubbed off one sixteenth of an inch of the chalk on the sidewalls, that's good. If no chalk remains on the outer edges of the tread, you have too much air pressure. If the chalk is rubbed off down to the lettering on your tires, you've got way too little.

That's as close as the chalk method gets. It lets you know that you're not at the extreme end of either spectrum.

Pyrometers work a little differently. Accelerating, braking, cornering and even driving down a straight road will cause the tires to heat up. The tread heats up based on how the weight of the vehicle is spread out across the tires. You will find that the tread typically will not heat up evenly. If you know how the temperatures are spread across the tread, you can come to conclusions about the alignment settings, whether you are running the proper tire sizes, your sway bar settings, and whether you’re running the proper amount of air pressure.

To read the air pressure with a pyrometer, I would take the car out on that same track or curvy road and do a little spirited driving. As soon as I made that last turn, I would bring the car to a stop and immediately take some tire temperature readings. Cool down laps or continuing to drive after the last curve will throw the readings off. I record the temperature of the tread in three places; the outside (about an inch from the edge), the center, and the inside (also, about an inch from the edge).

Now that I have recorded all the numbers, what do they mean?

If the rear tires are hotter than front, the car is oversteering. That can be from rear springs that are too stiff, a rear sway bar that is too thick, front springs that are not stiff enough, a front sway bar that’s not thick enough, the rear tire pressures being too low, front pressures too high, the rear tires too narrow, or the front tires too wide.

What are the goals? The ultimate setup would result in all the tires having the same temperature readings in all three spots. Rest assured, that's not going to happen. What I hope to settle for is having the center of the tread average between the inner and outer portions of the tread. Secondly, I want all four tires as close as possible to the same temperature. Thirdly, I want the temperature range between the three areas as close as possible. Achieving these three goals usually results in all four tires working as equally as possible to achieve the maximum amount of grip as I negotiate the turns.

Once the tire pressures are such that the temperatures are where you want them, there will be some leeway for tuning. For example, if the car pushes (understeers) in most corners, you will be able to reduce the front tire pressures a little without throwing the temperatures off. If you can’t lower the front’s enough, you can increase the rear tire pressure. The reverse is true if the car oversteers. I like to adjust the pressures on the axle that is having the problem before making adjustments on the other axle.

I really recommend that you buy or borrow a pyrometer. The first thing that you’ll have to decide when buying a pyrometer is which type to get. There are probe type pyrometers and infrared (IR) pyrometers. If exacting tire temperatures for a track car are the sole purpose for this purchase, a probe type pyrometer is the best choice. IR pyrometers only measure the surface temperature, and don’t reveal the temperatures of the rubber at the cord. The temperatures at the cord can be as high as 40 degrees F higher than the surface temperature. When a tire manufacturer supplies the optimum temperature range, that temperature is typically taken with a probe type pyrometer. Engine heat, brake heat, and air temperatures can also influence IR temperatures. That doesn’t mean IR pyrometers are useless, it only means that probe types are better for this specific application. For simply determining air pressures on a street car, using an IR pyrometer is fine. If you’re going to buy a pyrometer for a wider range of tasks, an IR pyrometer is usually the better purchase. IR pyrometers can be used to determine the following: