Rick F Guyon - In most cases, a vehicle's braking system is able to provide more braking force than can be transmitted to the pavement. Therefore, the maximum deceleration is limited by the coefficient of friction between the tires and pavement.
The coefficient of friction, f, between a vehicle and the supporting roadway is the frictional force divided by the normal force. The normal force is essentially the total weight of the vehicle, w , on all but the most extreme grades. The coefficient of friction is dependent on the condition of the vehicle's tires, the type and condition of the pavement, and the weather conditions.
There are two coefficients of friction: static and dynamic(kinetic). The coefficients of friction may also be referred to as coefficients of road adhesion. The coefficients of static friction is larger than the coefficient of dynamic friction. While a vehicle's tires are rotating, the relative velocity between a point of contact on the tire and roadway are zero and the coefficient of static friction controls.
Once a vehicle enters a skid, however the coefficient of dynamic friction controls. Therefore a vehicle is held to its maximum braking deceleration without entering a skid will take less distance to come to a complete stop than if the vehicle locks up its tires and skids to a stop.
The coefficient of friction is not constant throughout the braking manuever, but varies inversely with speed. This level of sophistication is not normally considered, and an average value that is representative of the speed and conditions are used.
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