Contact patch

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A contact patch is the term applied to the portion of a vehicle's tire that is in actual contact with the road surface.

The shape of a tire's contact patch can have a great effect on the handling of the vehicle to which it is fitted. Specifically, for the type of wide tire fitted to many modern performance cars, a contact patch that is wider than it is long will increase the tendency for the vehicle to 'tramline' or follow uneven road contours. Furthermore, in front wheel drive cars, the offset between the centroid of the contact patch, and the point about which the wheel steers, can lead to a condition known as torque steer.

[edit] Proper inflation

With normal street tires on an automobile the contact patch will remain uniform across the tread of the tire. If the tire is over-inflated the tire will tend to bulge in the center of the tread which will lift the edges off the road surface. This can decrease the handling performance of the vehicle and also decrease the life of the tire. Prolonged use of a tire which is over-inflated will cause the tread in the center to wear away faster than the tread on the edges.

An under-inflated tire can have negative effects as well. In this case the center of the tread will not make as much contact with the road surface and the edges of the tread will wear down faster because the sidewalls of the tire will push the edges into the pavement.

One method of checking for proper inflation is to find a long stretch of pavement such as an empty parking lot and then draw a line across the tread with chalk. Then simply drive straight across the parking lot. If the entire line of chalk has rubbed off, the tire is properly inflated. If the center of the line is rubbed off but the ends are still present, the tire is over-inflated. On the other hand, if the line is rubbed off at the ends but is still present in the center of the tread, the tire is under-inflated^{[citation needed]}.

[edit] Tire pressure

The size of the contact patch is inversly proportional to the tire pressure (up to a limit).

The size of the contact patch in square inches multiplied by the pressure in the tire in PSI is equal to the weight of the car on that tire (1/4 of the total car weight usually).

This means that increasing the pressure in the tire will reduce the size of the contact patch and vice versa.

The upper limit applies when you put so much air in the tire, that it acts as a rigid object and does not deform under the weight of the car, this is not good for traction since it means any tiny bounce will lift the tire off of the road surface.

The lower limit is when there is so little pressure in the tire that the rubber itself is providing the stiffness holding up the car.

In between those limits the formula applies. The means you can calculate the size of the contact patch from the weight of the vehicle, and the pressure in the tire. The correct size however is debatable, except that it is clear that the middle of the contact patch and the edges should hold the same weight.

A larger contact patch does not mean more traction on a normal road, but might on snow or mud.^{[citation needed]}

Conversely, a larger contact patch may generate more rolling resistance in snowy conditions, wheras a narrower tire can slice through snow and grip better.^{[citation needed]}