Bicycle Brake Systems
Four principal braking systems are used on bicycles: disc, coaster, caliper, and drum brakes.
Disc brakes consist of a metal disc attached to the wheel hub that rotates with the wheel. Calipers are attached to the frame or fork along with pads that squeeze together on the disc. Such brakes have been successfully used on motorcycles for decades, and are the principal choice there. They are finally becoming more popular on bicycles, after many (partly successful) attempts to introduce them over the last decades. Recent material advances in weight, costs and reliability have led several firms to develop and implement disc brake systems, and those are becoming a standard feature on many bicycles. They are used mainly on mountain bikes ridden off-road, but sometimes on hybrid bicycles and touring bicycles. Many tandem bicycles have a disc brake 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 the heavier machine.
Disc and drum brakes, which operate much like the disc and drum brakes on automobiles, are less common but are sometimes used on tandems, utility bicycles, mountain bicycles, and recumbents that must carry heavy loads.
Coaster brakes, which are mounted inside the hub of the rear wheel, are usually found only on single-speed utility bicycles or on children’s bicycles (most children do not have hands big or strong enough to squeeze brake levers). Coaster brakes are operated by rotating the foot pedals backward half a revolution until they lock; this expands a mechanism inside the hub that creates friction on an internal brake sleeve.
Single pivot side-pull caliper brakes consist of two curved arms that cross at a pivot above the wheel and hold the brake pads on opposite sides of the rim. These arms have extensions on one side, one attached to the cable, the other to the cable housing. When the brake lever is squeezed, the arms move together and the brake pads squeeze the rim. These brakes are simple and effective for relatively narrow tires, but have serious disadvantages if made big enough to fit wide tires. Low-quality varieties also tend to rotate to one side during actuation and to stay there, so that one brake pad continually rubs the rim even when the brake is released. These brakes are now used on inexpensive bikes; before the introduction of dual-pivot caliper brakes they were used on all types of road bikes.
Caliper brakes are almost universal on multigeared road, BMX, and utility bicycles. They consist of a pair of arms that are pivoted together and fitted with brake pads. These arms close onto the metal wheel rims in a scissor-like motion. They are controlled by hand levers mounted on the handlebars that connect to the arms by way of cables. Caliper brakes are available with either side-pull or center-pull cables. Side-pull brakes are frequently used in road racing because their braking force is easily feathered (adjusted minutely). Center-pull brakes are sometimes found on recreational and light touring bicycles; their braking leverage is greater and they can be designed for the larger clearances required for wider tires and rims.
Cantilever, cam, and U brakes—always of a center-pull design—are caliper brakes with exceptionally large brake pads and leveraging mechanisms to increase the amount of braking force delivered. These large, leveraged brakes are found most commonly on mountain, touring, commuter, hybrid, and BMX bicycles, where a premium is placed on retaining braking ability even when the bicycle is ridden through dirt, water, or mud.