The tire you use is one of the most important suspension tuning choices you will make in building and racing your car. Even a car with a professionally-tuned suspension will not be able to clock fast laps or handle well on a poor quality or inappropriate set of tires. The major performance factors to consider when choosing tires are compound, contact patch, and sidewall height.
A softer compound tire will have more grip but will wear out quickly while a harder compound tire will have less grip but will last for a relatively long time. Your choice of compound is mainly affected by the type of racing you plan to do and your budget. For serious racing, you will need a soft compound tire to be competitive. If you plan to do mostly street driving and/or do not have the money to frequently replace tires, go with a harder compound.
The second factor to consider is tire contact patch, or the area where the tire makes contact with the ground. You want the biggest contact patch that you can get. When more of the tire is in contact with the ground, the tire can exert more force and allow your car to achieve higher cornering speeds.
The contact patch is determined by the width of the tire and the tread pattern that is on the tire. (Usually the width provided in the tire specifications is the overall width, or section width, of the tire, not the width of the actual part of the tire that touches the ground. However, some manufacturers will provide the actual width of the contact patch, which should be slightly smaller than the overall width of the tire.) If the tire has very little tread, the contact patch will be larger (since there are fewer gaps on the tire surface for the tread), and the tire will have more grip. A tire with no tread, used only for racing, is called a slick. Slicks will maximize your car’s cornering ability but are illegal and dangerous on the street. As a rule of thumb, if you plan to drive on the street, use a tire with at least three vertical treads. These are often referred to as semi-slick tires.
You can use your choice of tire width to change the handling characteristics of your car. For instance, if your car tends to understeer, you can increase the width of the front tires to improve front grip. Also, if your engine produces substantial power, you should try to maximize the width of the drive wheels to make sure you can get the power to the ground.
The third factor to consider when choosing a tire is sidewall height. A tall sidewall flexes more than a shorter one. Therefore, a short sidewall tire will provide a more precise steering feel than a tire with a tall sidewall, but using a taller sidewall makes the car feel more forgiving and less twitchy. Also, a short sidewall tire makes the ride harsh on the street. For a dedicated race car, this is obviously insignificant.
Tire pressure can be adjusted to change the handling characteristics of your car. By increasing tire pressure, you cause the tire profile to become more round, which decreases the size of the contact patch. You also cause the spring rate at that corner of the car to increase since you are making the tire harder and, therefore, “bouncier” by filling it with more air. Both of these results take away grip from the tire. If you increase front tire pressure, the car will become tighter (added understeer), and if you increase rear tire pressure, the car will be looser (added oversteer).
Lowering tire pressure causes the tire to sag, which increases the size of the contact patch. In addition, it decreases the effective spring rate at that corner of the car and makes the tire rise in temperature. The additional rolling drag (or friction) produced by the sagging of the tire is responsible for the temperature change. These factors provide the tire with more grip. If you decrease pressure in the front tires, the car becomes looser, and if you decrease pressure in the rear tires, the car becomes tighter. The temperature increase may be desired if it will allow the tire to heat up to recommended operating temperature. However, overheating the tire can lead to loss of grip, “chunking” (large pieces of rubber tearing off the tire), and generally faster tire wear. Before making any tire pressure adjustments, find out the recommended minimum and maximum pressures for your tire from the manufacturer. Also, experiment with tire pressures by changing them in small increments until you find the ideal setup.
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I have been an automotive enthusiast throughout my life and have participated successfully in various amateur racing series. I specialize in tuning the Nissan S platform cars, particularly the US domestic market Nissan 240SX. Visit my website http://www.240edge.com to get information about suspension setup, quality upgrades, and general 240SX tuning.
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