Brakes work by converting kinetic energy (forward motion) into thermal energy (heat). The friction between the stationary brake pad and rotating disk or drum as it slides past the pad convert the motion of the wheel and tire into heat, much the way rubbing your hands together on a cold day will warm them up. Bringing your car to a stop generates enough heat at each wheel to boil a liter of water in about 7 seconds. Brake temperatures can reach around 500°F during normal everyday use, and can reach up to 1000°F under heavy or repetitive braking.
The brake disk or drum is designed to work like a heat sink, and absorbs as much as 80% of the heat generated during stopping. Fortunately, it also makes a good radiator, cooling as it spins through the air on the way to the next stop.
The front brakes do most of the work as the vehicle's weight pushes forward while stopping. Therefore, many vehicles are equipped with disc brakes on the front axle and drum brakes on the rear. A disc brake's superior performance is largely due to its ability to generate friction as the brake calipers force the pads to clamp against the rotors. The brake rotors are cleaned and dried by the brake pads dragging across them and the entire brake system is exposed to the air for efficient cooling. The advantages of rear drum brakes are lower cost and the ability to easily integrate a mechanical emergency/parking brake system.
Most Original Equipment brake pads are made of many different compounds, and try to achieve a compromise between stopping power, low noise, dusting and long wear. Gone is asbestos, now replaced with modern friction materials.
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