drum v. disc brakes

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Monday, April 19, 2010

Brakes work by applying friction to a moving surface, slowing it down by converting its kinetic energy into heat.

In a drum brake, a pair of curved shoes is pressed radially outward against the inside wall of a cylindrical drum that rotates with the wheel. When the brake pedal is depressed, hydraulic cylinders move the brake shoes against the drum.
In modern vehicles, if drum brakes are used at all, they are mounted on the rear wheels.
The shoes can also be engaged mechanically by pulling on the hand brake. Because drum brakes can double as a hand brake, they are cheaper than disc brakes, which require a separate hand brake.
Drum brakes exhibit a self-applying tendency - when the brakes begin to grip, friction forces one or both of the shoes to press harder against the drum. This increases stopping power, but makes drum brakes more difficult to use smoothly.

To slow or stop a wheel, disc brakes apply pressure to the sides of a rotating disc. Friction comes from brake pads mounted on a caliper, which squeezes the disc.
Disc brakes have better stopping performance than drum brakes, but they are more expensive to manufacture.
When a vehicle is moving forward, most of the stopping effort is done by the front brakes. To get the best balance of price and performance, modern cars often have front disc brakes and rear drum brakes. Disc brakes are sometimes used on all 4 wheels as well.
Because the mechanism in disc brakes is exposed to the air, they dissipate heat more effectively. This makes them less susceptible to brake fade, reduced stopping power caused by heat buildup after sustained use.
Because they are enclosed, drum brakes drain slowly after immersion in water; disc brakes tend to fling water off, and dry quickly.
Unlike self-applying drum brakes, disc brake pressure can be easily and smoothly modulated by varying pressure on the pedal.