Disc brakes consist of a metal disc attached to the wheel hub that rotates with the wheel. Attached to the frame or fork will be calipers and pads that squeeze together on the disc. Although this type of brake has been used on motorbikes for decades, only recently have they been added to bicycles. They are most suitable for and used mainly on mountain bikes ridden off road. They also are used on hybrid bicycles and some road bicycles, although this is less common. Many tandem bicycles have a disc brake fitted on the rear wheel in addition to rim brakes; the disc brake can be set to provide a constant drag, so that during long descents, the rim brakes are not overworked by this heavier machine.
The main advantage of disc brakes is that their performance is equally good in all conditions including water and mud. They also avoid the problem that rim brakes have of wearing out the wheel rims, especially in muddy conditions, as well as the requirement that the rim be true.
On some expensive disk brake models, a hydraulic system is used to push the pad instead of a cable, eliminating the problems of cable maintenance and breakage.
The disadvantages are that they are usually heavier and more expensive than rim brakes, and in most cases require a special hub.