Disc and drum brakes create friction to slow the wheels of a motor vehicle. When a driver presses on the brake pedal of a vehicle, brake lines filled with fluid transmit the force to the brakes. In a disc brake, the fluid pushes the brake pads in the caliper against the rotor, slowing the wheel. In a drum brake, the fluid pushes small pistons in the brake cylinder against the hinged brake shoes. The shoes pivot outward and press against a drum attached to the wheel to slow the wheel.
Brakes enable the driver to slow or stop the moving vehicle. The first automobile brakes were much like those on horse-drawn wagons. By pulling a lever, the driver pressed a block of wood, leather, or metal, known as the shoe, against the wheel rims. With sufficient pressure, friction between the wheel and the brake shoe caused the vehicle to slow down or stop. Another method was to use a lever to clamp a strap or brake shoes tightly around the driveshaft.
A brake system with shoes that pressed against the inside of a drum fitted to the wheel, called drum brakes, appeared in 1903. Since the drum and wheel rotate together, friction applied by the shoes inside the drum slowed or stopped the wheel. Cotton and leather shoe coverings, or linings, were replaced by asbestos after 1908, greatly extending the life of the brake mechanism. Hydraulically assisted braking was introduced in the 1920s. Disk brakes, in which friction pads clamp down on both sides of a disk attached to the axle, were in use by the 1950s.
An antilock braking system (ABS) uses a computer, sensors, and a hydraulic pump to stop the automobile’s forward motion without locking the wheels and putting the vehicle into a skid. Introduced in the 1980s, ABS helps the driver maintain better control over the car during emergency stops and while braking on slippery surfaces.
Automobiles are also equipped with a hand-operated brake used for emergencies and to securely park the car, especially on uneven terrain. Pulling on a lever or pushing down on a foot pedal sets the brake.