A bicycle brake is used to slow down or stop a bicycle. At the most basic level, slowing is accomplished by using one surface to apply friction to another surface. Bicycle brakes themselves, however, can be anything but simple. This brief overview will describe the three basic types of bicycle brakes commonly in use today: rim, disc and drum.
The traditional brake used on modern bicycles is the rim brake. It works to slow a bike by applying force to the rim of a wheel. Most often, the braking force is actuated by pulling a brake lever on the handlebars. The lever, in turn, pulls on a brake cable which causes the brake pads to squeeze against the rim, applying the friction which impedes the wheel’s spinning.
This system is a good one as rim brakes are cheap, light, mechanically simple, easy to maintain and powerful. However, they are not perfect. Rim brake pads wear down and need to be periodically replaced (an easy process you can do yourself). They do not perform well if the rims are wet (water decreases the frictional force which can be applied to the rims).
Rim brakes tend to clog with mud and debris, and may not be appropriate for mountain bikes. If a small rock gets embedded in the brake pad it can scratch and damage the rim. If a rim is damaged, particularly if it is no longer straight, the brake will not be able to apply even force to the rim as it turns and braking power may be reduced.
The frictional force of rim brakes creates heat on the rim. Since most braking is short-term and limited in force, the heat produced usually dissipates into the surrounding air. However, on a heavily laden bike on a long descent when brakes may be being used more constantly, heat can build up faster than it can dissipate. This heat can cause the air pressure in the tire’s tube to increase and potentially lead to a blow-out.
Disc brakes work much like rim brakes in that when you pull the brake lever a brake pad applies friction to slow your bike. The difference is in where the friction is applied, and that is all the difference in the world.
A disc brake consists of a metal (usually steel) disc attached to a wheel hub that rotates with the wheel. A hydraulic- or cable-actuated caliper is bolted to the lower end of the fork or rear portion of the frame. When you pull the brake lever, powerful pistons squeeze the rotor to apply frictional force and slow or stop the wheel’s turning.
Several features of disc brakes have made them very popular on mountain bikes, and they are starting to show up on hybrid and touring bikes as well. Because the braking surface is farther from the ground (as opposed to a rim which is very near the ground at the bottom of the wheel), it is less prone to contamination from mud and other debris. Disc brake pads when fully retracted ride much closer to the braking surface than rim brake pads; this also helps prevent debris build-up. Additionally, the rotor is perforated to allow water and debris to get out from under the pads. Brake discs and pads can be made from heavier material than are most rims, allowing more braking force to be applied to them. Additionally, the brake’s function is not affected by tire width, a plus for knobby-tired mountain bikes.
As with rim brakes, there are potential disadvantages to disc brakes, as well. Disc brake assemblies are generally more expensive than rim brakes and require expertise in installation and maintenance (although they generally require less routine maintenance than rim brakes). Installation and adjustment of the brakes must be completed with care to avoid lateral play in the system which could diminish your braking ability. The assemblies require a hub built to accept the disc; they are not installable on any wheel. A wheel with a disc brake will be heavier than a standard wheel due to the weight of the hub assembly and the heavier components required to withstand the torque transmitted by the brake. If you use panniers, you may need to find a rack specifically designed to mount with disc brakes.
Bicycle drum brakes operate like those of a car, although the bicycle variety is mechanically rather than hydraulically actuated. Two pads are pressed outward against the braking surface on the inside of the hub shell. Drum brakes have been used on front hubs and hubs with both internal and external freewheels. Both cable- and rod-operated drum brake systems have been widely produced.
Much less common that the other types of brakes, drum brakes provide the advantage of consistent braking in wet or dirty conditions because the mechanism is fully enclosed. They are also less susceptible to wear than rims (which are made of lighter-weight material) and require less maintenance. However, drum brakes are heavier and more complicated than rim or disc brakes, and may be weaker than rim brakes. Many bikes are not constructed to accommodate anchoring a drum brake to the frame or fork, nor are many able to tolerate the forces applied by the brake.
The coaster brake (or back pedal brake) is a type of drum brake invented in the 1890s and still found on many kids’ and some single-speed adult bikes. While this is a low maintenance and easy-to-use brake, it has the disadvantage of failing suddenly and completely when it does fail. Additionally, too many bikes are equipped with only a coaster brake; there’s no back-up system in the event of brake failure (bikes with rim, disc or drum brakes usually have a brake on both the front and rear wheels; if one fails, the other should work).
This overview was intended to be just that: a brief summary of the main types of bicycle brakes. If you are buying a new bike, be sure to talk with the bike shop to find out what type of brakes your bike has and what maintenance needs to be done on them. Your safety could depend on it.
Ride safe and have fun!