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A coaster brake is a brake built into a rear hub. By turning the pedals a quarter-turn or so backward, the brake is applied.
On the plus side, this is a dirt-resistant, fairly lightweight, reliable, and simple way to add a brake to a bicycle.
On the downside:
Because the brake is applied when pedaling backward, one cannot turn the pedals backward when wanted, such as to negotiate a tight turn.
Sometimes, in some situations as the brake is applied, it is difficult to maintain sufficient control, such as when the pedals are at the 12 o'clock and 6 o'clock position.
The coaster brake only works on the back wheel, which is much less effective in stopping a bike than the front. Of course, a front brake of some other type can be added.
The coaster brake concentrates a lot of heat in the rear hub area when braking down a long incline. This would seem to be a problem, but when old brakes are disassembled, they almost always seem to be unaffected by heating. That's because the attachment of spokes is an almost-perfect way to radiate heat.
Coaster brakes have been incorporated into multi-speed planetary gearset hubs, and for a while many years ago New Departure made a coaster-like brake for the front wheel that was cable-actuated.
Coaster brakes work in one of two ways:
The more common form, used by Bendix among others, is two or four half-round or quarter-round metal brake shoes, or an expanding metal cylinder that is squeezed with a wedge outward against the inside of the hub.
The other form is an alternating pack of disks. Every other disk is attached to, and spins with the hub. The alternating disks are attached around the axle and do not spin. These are squeezed together, bringing the bike to a stop.
Coaster brakes are lubricated with grease, which can last for years, even decades if the bike isn't ridden in harsh weather.
You'll see that coaster brakes have an arm sticking out of the left side. This is called a "reaction arm." It is tied or clipped to the left chainstay. When the brake is applied, this reaction arm transfers the power to the bike's frame. People have been known to leave the reaction arm detached. The braking power is sufficient to spin the axle within the frame, causing axle nuts to stick or even strip their threads.
A particularly bad idea is something boys in the early 20th century used to try from time to time: They'd pour kerosene into their coaster brakes, ride their bikes fast, bring them to a quick stop, and enjoy seeing the rear hub burst into flames. Don't try this at home unless you have flameproof pajamas.