|MadSci Network: Engineering|
Brakes do not stop cars--not directly. That is why they are called wheel brakes. The wheel brakes stop the wheel. Tires stop the car. Antilock brake systems (ABS) stop cars faster (most of the time) because they prevent wheel locking. A locked wheel typically has a lower tire-road friction than when rolling with some slippage. Tire-road friction normally reaches a maximum when the slip rate is around 20%. The reason for this slippage is that the tire is elastic, and must deform to transmit the tire-road tread force to the wheel. When the tire- road friction is greatest, this deformation (rolling 'slip') is about 4 centimeters (1.6 inches) for a typical passenger car tire on a good road surface. Present antilock brake systems do not optimize the tire-road friction. However, they do prevent wheel locking. This does two things; it permits vehicle steering control (very important) and usually increases the tire- road friction, compared with locked-wheel braking. If you want to stop a car in the shortest possible distance, you first should apply the brakes very rapidly. The rotating components of a car, mostly the wheel-tire assemblies, provide about 5 percent of the vehicle's kinetic energy. Rapidly braking the wheels helps reduce this energy and also starts the tire tread deformation needed to maximize tire-road forces. Since the wheels are directly bolted to the brake drum or disc, almost any torque can be used. More is better in this operation. When the tires reach maximum tire-road friction, you then should reduce the brake pedal force to maintain maximum friction. Experienced performance test drivers can do this surprisingly well. Some use the tire sound to help indicate the degree of slippage. Now you have enough knowledge to answer your own question. Brakes that are large enough to just lock the wheels (but no more) will complete a stop as fast as larger brakes. However, larger brakes may reduce the time to initiate braking of the tires. Since this is at the beginning of braking (when the vehicle is moving fast), a small savings of time can shorten stopping distance by a significance amount. If your goal is to stop even faster, you might wish to consider increasing the load on the tires. Racecars use airfoils (small wings) to increase the tire loading. If this added down force is equal to the initial car weight, the stopping distance can be cut in half. With this scenario, you definitely will need bigger brakes to generate the additional torque. Some racecars have used air 'dams' that are employed during braking to increase aerodynamic drag. Such devices have little value, however, at legal highway speeds.
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