The disc brake is a device for slowing or stopping the rotation of a wheel. A brake disc is connected to the wheel and/or the axle. To stop the wheel, friction material in the form of brake pads (mounted on a device called a brake caliper) is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Some newer mopeds come equipped with disc brakes, and older mopeds can be modified to use disc brakes.
Converting your moped to use a front disc brake
If you have the money, the easiest way to do this is purchase an EBR front fork w/ disc brake kit and Grimeca spoked wheel with disc from 1977 Mopeds, which provides a disc brake setup nearly identical to that used on the Tomos Streetmate and Revival TS (and similar to that used on the Arrow and MC50).
It is also possible to buy other small disc brake setups intended for scooters and small motorcycles, but procedures for adapting them for use on a moped vary.
See also: Drum brake