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Performance Tire Setup
By Steve Grosekemper
We have all read article after article on the suspension upgrades available for our cars: torsion bars, anti-sway bars, springs, shocks, and bushings. While it is true that all of these things do help
when properly installed with a good alignment, we should not forget that the last link to the asphalt is our tires.
As a rule of thumb, it is best to mount these tires to the widest wheels that will fit your car. The tire choice is not an easy one. Rule #1 states that it is best to talk to others who run a similar
car setup as you and get their feedback. The only truth in the tire world is that there is no best tire. There are only tires that are best for you and your car. If you have $600 to
spend on race tires for the year and you buy the world's stickiest tire, it may not be the best tire for you if it lasts only two events.
You now have your wheels and tires mounted on your car and are ready to go racing. The first question you are going to ask is "How much air do I put in these things?" Back to Rule #1.
You can check with the tire manufacturer or supplier for specific information (Tirerack.com has some great information on brand setup). A good starting point is to set them to stock specification or a few pounds higher. There will be a trial period with a new brand of tires until you have the setup down.
The best indicator of correct tire pressure is tire temperature. The air pressure acts like three springs inside the tire holding the car up. Overinflation causes the center to push too hard, creating
excessive heat. Too little pressure causes insufficient push and reduced heat in the center. Our goal is to have the entire width of the tire share the workload evenly. If one section of the tire is
overworked it will overheat and wear prematurely. Tire temperatures will never be perfectly even across the tire. But if a linear reading within 10 degrees can be achieved, far greater tire wear and
performance will result.
Tire temperatures are affected by many things: pressure, alignment settings (camber and toe), corner balance, track layout (more right turns=more heat on the left side), and ambient and track surface
temperature.
As tire temperature increases, so does pressure (about 1 psi for every 10 degrees of temperature). This will explain why on a warm day of autocrossing the rear of the car will seem to lose traction on
the last practice laps. The temperature goes up, raising the pressure and pushing on the center of the tire, effectively decreasing the size of the tire patch on the ground. The cure is to start your
run group a pound or two low, so you will not exceed the tires pressure range by the end of the practice.
The advantage to manipulating tire pressure on a stock car is just as important as it is on a racecar. The stock car will not have adjustable sway bars to change under/over steer. The rear tire
pressure can be raised, for instance, to increase oversteer on a stock car or the front pressures can be increased to induce understeer. This is tuning that could normally be done only with
adjustable anti-sway bars.
If tire pressures have been optimized for the best linear reading and the 10 degree window cannot be reached, an alignment problem may exist (see figure 1). In this example the inside tire temperatures are high due to excessive negative camber. This
condition is causing the tire to be pushed harder on the inside edge. The result will be excessive heat, tire wear and poor handling.
One of the first suspension tuning tools any of us ever got our hands on was probably a tire pressure gauge. If you can breathe, chances are you can use one, either analog or digital. A tire
temperature gauge will take a little more practice to master. There are two basic types of temperature gauges. The first (and most common) is the infrared type which is aimed at the tire and shot
like a gun (see Figure 2). These are very quick and easy to use, but do not give as accurate a reading as
the probe type (see Figure 3). This is due to the fact that the surface temperature changes quickly and the
core tire temperature does not.
Whichever type you use, it is important to test the tire in the proper spot. That spot is in the center tread and about 1 inch in from the outside and inside edges (see Figure 3 again). After you have acquired this information, it is imperative to keep a record of it for
comparison. I have acquired (see stolen) a great tire temperature record sheet from our Chief Driving Instructor, Vince Knauf. It is in PDF format and can be downloaded from the San Diego Region Web site: www.pcasdr.org.
After a few rounds with your new tire temperature gauge, you will be a true suspension set-up guru (or at least you will look like one!).
Good Luck!
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