This article is about the vehicle component. It is used for slowing or stopping a moving vehicle, wheel, axle, or to power brake system pdf its motion, most often accomplished by means of friction.
Unsourced material may be challenged and removed. Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or airplanes with both wheel brakes and drag flaps raised into the air during landing. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises quickly with speed. These allow the aircraft to maintain a safe speed in a steep descent.
Brakes may be broadly described as using friction, pumping, or electromagnetics. Frictional brakes are most common and can be divided broadly into “shoe” or “pad” brakes, using an explicit wear surface, and hydrodynamic brakes, such as parachutes, which use friction in a working fluid and do not explicitly wear. Other brake configurations are used, but less often. The drum is connected to the rotating roadwheel hub. Drum brakes generally can be found on older car and truck models. However, because of their low production cost, drum brake setups are also installed on the rear of some low-cost newer vehicles.
Compared to modern disc brakes, drum brakes wear out faster due to their tendency to overheat. Friction causes the disc and attached wheel to slow or stop. Pumping brakes are often used where a pump is already part of the machinery. For example, an internal-combustion piston motor can have the fuel supply stopped, and then internal pumping losses of the engine create some braking. Electromagnetic brakes are likewise often used where an electric motor is already part of the machinery. Some vehicles, such as some transit buses, do not already have an electric motor but use a secondary “retarder” brake that is effectively a generator with an internal short-circuit.