Also included is rubber compound where more than one tire choice is available.
Tires make traction through friction with the racing surface and mechanical grip, which actually causes rubber to shear off the tire contact patch on asphalt and dry, slick dirt tracks. (On tacky dirt surfaces, the dirt shears more than the rubber.) Factors affecting total possible tire traction include the softness of the rubber compound, the effective size of the tire contact patch, the coefficient of friction of the racing surface, the temperature of the rubber at the tire contact patch, and the vertical load at the tire contact patch. Our goal in creating a good setup for a race car is to maximize traction at all four tire contact patches. Everything we do to the car should have that basic goal in mind. While we cannot change the rubber compound (unless more than one compound is available for your class), we can control or at least influence every other factor previously mentioned.
Tire Contact Patch
A bigger tire contact patch, all else being equal, means more traction. For most classes, tire size is dictated by rules. For those of you who run in a class where you can select from several tire sizes, bigger is not always faster. Sometimes a wider tire is slower because it increases rolling resistance too much, and/or because the suspension cannot control the tire contact patch effectively (camber change, and so forth). But when the tire size is mandated, or you run on a spec tire, the size of the contact patch is very important. If you have not maximized the tire contact-patch loading with the track surface, you will not have as much traction as you could. This occurs when camber, stagger, and tire pressures are set to optimum, which is determined by tire temperatures across the surface of the tire tread. Nearly equal tire temperatures across the surface mean that the tire contact patch is equally loaded and doing the maximum amount of work possible. Anything less and you will lose performance (Illustration 1).
Tire Load
Of the aforementioned factors regarding tire traction, the most important concept to understand is that vertical load affects traction. Increase vertical load on the tire, and the traction increases. That is why aero-dynamic downforce increases traction--it adds vertical load to the tires without adding weight to the car. Keep in mind that adding weight to the car will increase traction, but it also will increase the work the tires and engine must do. Even though you have more traction, the car will be slower. Here's the big catch: The relationship between increased vertical load and increased tire traction is nonlinear. In other words, if you double the vertical load on a tire, you will not doubly increase the traction of that tire.
For example, on a given tire, the vertical load is 1,000 pounds and the amount of traction force is 1,100 pounds. If the relationship between load and traction was linear, increasing the vertical load to 2,000 pounds should increase the traction force to 2,200 pounds. But the relationship is not linear, so increasing the vertical load to 2,000 pounds would only increase the traction force to about 2,000 pounds. Understanding this relationship is crucial to understanding other elements of race car dynamics (Illustration 2).
An additional factor here is getting the load on each tire optimized in a variety of dynamic states, such as cornering, corner exit, and so on. Chassis tuning and driver-control inputs are big here.
The solid line on the graph shows typical tire traction force at several loads. The dashed line shows what a linear relationship would look like, in which tire traction force increases at the same rate as vertical load. The curve of the graph indicates that as vertical load increases, so does tire traction, but a rate that is less than equivalent (Illustration 3).