Tires 201: Contact Patch Management

June 2, 2010

In Auto Racing and driving in general, tires play a pivotal role. An experienced race driver and coach once told me that racing is really about contact patch management.

The contact patch of a tire is the part of the tire that makes contact with the road. You only have a few square inches of rubber touching the pavement at any given time, and the entire venture of keeping the car controlled rests on these 8”x3” (estimated) patches of rubber.

Under gravitational forces, these contact patches actually shrink and expand, depending on where the majority of the vehicular weight is shifting. Under acceleration, your two rear contact patches expand, and your two front contact patches shrink. Under deceleration, the opposite takes place; your front contact patches expand while your rears shrink. Turning left or right has a similar effect, transferring weight to one side more than the other and accordingly, expanding the contact patch on the heavy side and shrinking it on the light side.

The amount of contact patch is static--it doesn’t change. What does change is where it's being expanded and where it's being shrunk. You get to decide how these are allocated based on your inputs to the vehicle. Abruptly changing acceleration/deceleration/turn-in/turn-out causes sudden weight transfer and sudden changes in contact patch availability, not good.

Keeping the weight on the rear axle during a turn keeps that rear contact patch fat, and gives you increased grip. Suddenly tapping the brakes in a turn will upset this balance, and actually decrease your grip when you need it most. Conversely, when turning-in hard, you want the weight transferred to the front of the car…so that your front contact patches will be fat and afford you the most grip possible.

Being smooth with your inputs and aware of the relationship between your inputs and your contact patches allows you to maximize the capacity of your vehicle, minimize the lap time, and keep your car under control through the exercise.

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