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For any car and tires, if the weight of the car is constant and the inflation pressure of the tires does not change, there essentially no change in the contact patch area. This is because the weight of the car is supported by the air pressure in the tires. This weight is transfered to the ground through the contact patch. Thus, the pressure times the contact patch area must equal the weight of the car. For example, a car weighting 3500 lbs. supported on tires inflated to 35 psi will require a total contact area of 100 square inches. If the weight is distributed evenly, then each contact patch will be 25 square inches. If the weight is 60/40 front to rear but even from side to side, the front tires will each have a 30 square inch contact patch and the rear tire will each have a 20 square inch contact patch. Note that tire size does not enter into this calculation. Thus, all else being equal, a 245 tire will have the same contact area as a 275 tire. There may be slight differences due to differences in the stiffness of the side walls, but these will be minor. The shape of the contact patch will, however be different. The wider the tire, the wider the contact patch. Since area must remain the same, this means that wider tires will have a shorter (front to back) contact patch. This change in shape will have effects on the tires performance vis a vis handling and tire spin.
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10:07:44 08/23/99
