Cycles of all sorts

A coaster brake, also known as a back pedal brake or foot brake (or torpedo in some countries), is a drum brake integrated into hubs with an internal freewheel. Freewheeling functions as with other systems, but, when back pedalled, the brake engages after a fraction of a revolution. It can frequently be found in both single-speed and geared hubs.

When such a hub is pedalled forwards, the sprocket drives a screw which forces a clutch to move along the axle, driving the hub shell or gear assembly. When pedalling is reversed, the screw drives the clutch in the opposite direction, forcing it either between two brake pads and pressing them against the shell, or into a split collar and expanding it against the shell. The braking surface is often steel, and the braking element brass or phosphor-bronze, as in the UK Birmingham made Perry Coaster Hub.

Coaster brake bicycles are generally equipped with a single cog and chainwheel and use a ½" chain. However, there have been several models of coaster brake hubs with derailleurs in the past, most notably the Sachs 2x3. These use special extra-short derailleurs which both can stand up to the rigors of being straighted out frequently and don't require an excessive amount of reverse pedal rotation before the brake engages.

Although coaster brakes have the advantage of being protected from the elements and thus immune to ice or water, because they are located in the rear wheel only limited braking force can be applied before the rear wheel locks up. This is due to the placement of the rider's weight ahead of the rear tyre's contact with the ground, as well as the weight transfer forward proportional to braking force, which further unloads the rear wheel. Additionally, although coaster brakes generally go years without needing maintenance, they are more complicated than rim brakes to repair if it becomes necessary.