Even though the engineering science behind how brakes work can be complicated, the actual mechanics of how brakes work is simple. Brakes work on a principle known as friction. To stop a spinning wheel on a motor vehicle, whether it is an automobile, motorcycle, farm tractor, or semi truck, a brake lining comes into contact with a metal surface affixed to the wheels that stops them from spinning. If you have ever heard that terrible grinding sound of metal on metal as you hit the brake pedal, you may have waited too long to replace your brakes and the sound you are hearing means that you may be causing damage to the vehicle.
Linings on brakes are softer than the metal that it is pressed against. Brake linings deteriorate similar to the action of rubbing a pencil eraser on piece of paper. Just as little pieces of the eraser rub away and become dust on the paper, little bits of brake lining rub away and collect on metal structures of the brake assembly. Because asbestos was cheap and prevented brakes from squeaking, some manufactures ignored the known health hazards and continued to manufacture asbestos brake linings well into the 1990s, despite widely available substitutes.
Drum vs. Disc Brakes
The two most widely used brake systems are drum brakes and disc brakes. Before the 1970s, drum brakes were used almost exclusively on motor vehicles. Disc brakes gradually became more popular in passenger vehicles. At first they were used only on the front wheels, but by the 1980s, disc brakes were often used on all four wheels. Drum brakes are still used on the rear axles of some passenger vehicles and trucks. Drum brakes are predominant on buses and large commercial trucks used in construction and for hauling freight.
Likewise, disc brakes offer better stopping performance than drum brakes, and the braking force is proportional to the pressure placed on the braking pedal. When sports cars were first introduced, disc brakes were commonly used because these vehicles demanded better braking performance.
Here is an example of of a typical disc brake system: