Brakes use friction to convert kinetic energy into heat. Pads of one type or another press against the wheel rim, a drum or disc,which dissipates most of the heat, as the pads are smaller, sometimes enclosed, and often of a material which conducts heat poorly.. The surface of the pads is in contact with the rim or disc, and so it is at the same temperature.
A bicycle brake may work at the rim, or at the hub.
Rim brakes have the advantages of light weight, large heat-dissipating area, and low stress on the bicycle frame, fork and wheel.
Hub brakes are more weatherproof, and and are not affected by rim dents or wheel true. Because heat-dissipating area is smaller than with rim brakes, these brakes run hot -- but some are designed to.
Bicycle brakes are operated by hand levers by way of cables, or sometimes hydraulic lines -- except for one type of rear hub brake, called in British English a "backpedaling brake" or "foot brake". In American English, it is called a "coaster brake", an expression which dates back to the its introduction in the late 19th century -- also allowing coasting, unlike a fixed gear.
There is also a page about cables on this site. Poor brake performance often results from cable problems.
What Brakes Would Work Best for Me?
Different types of riding favor different brakes.
Rim brakes are lightest because rims also serve as brake discs. Wheels must be true for rim brakes to work smoothly -- and so, brake maintenance also involves wheel maintenance. If rims are wet, rim-brake performance suffers until the brake shoes wipe them dry -- a very serious problem with chromed steel rims, much less a problem with aluminum-alloy rims. Aluminum conducts heat very well, and so the area of an aluminum rim's surface outside the brake track contributes significantly to heat dissipation. Nonetheless, tires on tandem bicycles are known to overheat and blow out on long downhill run. Even on a solo bike, inner-tube patches may come unglued. Rim brakes wear rims, and an aluminum rim may last only a couple of thousand miles in muddy or sandy conditions. Carbon fiber-epoxy transmits heat poorly, so tires don’t get so hot, but the rim surface gets hotter, and rim brakes and carbon-fiber rims are a poor combination due to brake wear. Also see the article about rim brakes-- which links in turn to articles about the several different kinds.
Though drum brakes all are very weather-resistant, their performance varies widely. They are common on utility bicycles, because of their weatherproofness, but only a few special drum brakes can avoid overheating on long downgrades. Generally, the larger the drum, the stronger the braking, and the better heat dissipation. A drum brake which is integrated into a hub can suffer contamination from lubricants. If the drum wears out, the hub, and usually the wheel -- must be replaced. These problems do not occur if the drum brake is external to the hub. Also see the article about drum brakes.
The Shimano Rollerbrake is a variety of drum brake that attaches to the hub with a special fitting which is found only on Shimano Nexus and Nexave hubs. As an external brake, a Rollerbrake can be replaced separately from the hub. Several sizes are available. Also see the article about Rollerbrakes.
Disc brakes are designed so they can run hot, and dissipate heat better than most drum brakes. You may smell hot metal if you are using disc brakes to hold down your speed on a long, steep downgrade. Disc-brake rotors are of steel, and most of the radiating area is in the brake track. Steel transmits heat quite well, but relatively little heat is transferred along the narrow spokes of the disc toward the hub.
Disc brakes have become popular on mountain bikes and are gaining popularity for other bicycles. Disc brakes can only be installed on frames with special fittings, and have unique problems, worst among which is that a front disc brake can pull the wheel out of the fork. Also see the article about disc brakes.
A coaster brake is operated by backpedaling, and so can be installed only on the rear wheel. Coaster brakes are the most maintenance-free, and are suitable for children with limited hand strength. On a folding or take-apart bicycle, a coaster brake avoids the need for a cable connection. A coaster brake complicates starting and stopping, and prevents the use of toe clips and straps, clipless pedals or derailer gearing. A coaster brake may, however. be incorporated into an internal-gear hub --- but then, except with a two-speed kickback hub, a shifter cable is needed. A coaster brake is OK for utility cycling but unsuitable for speed control on long downhill runs. Also see the article about coaster brakes. A coaster brake is often installed as the only brake on a bicycle -- but every bicycle should have two brakes because every brake fails sooner or later.
Band brakes have a flexible band which wraps around the outside of a brake drum and is tightened to apply the brake. These brakes are light in weight but have many disadvantages. Also see the article about band brakes.
Some e-bikes have regenerative braking. In addition to other brakes: the motor also can work as a generator, slowing the bicycle and charging the battery. If an e-bike is throttle-controlled, regenerative braking can be actuated by releasing the throttle all the way, or through a brake lever or pushbutton. If pedal controlled, spinning the pedals backwards can send a signal to actuate regenerative braking. This is different from the way a coaster brake works because the pedals turn continuously backward -- and so is possible with derailer gearing or a sprung chain tensioner.
Gimme Two Brakes!
With only a few exceptions, every bicycle should be equipped with two brakes -- front, and rear. Again: any brake will fail sooner or later, and then you really need the other one!
Notable exceptions are:
fixed-gear road bicycles. These can get by with only a front brake because the rear wheel can be slowed by pushing back against the turning pedals;
track racing bicycles, brakeless fixed-gear bicycles, because these are the only traffic on the track, and abrupt stops would be hazardous;
tandems and other bicycles which carry a heavy load. These generally should have three brakes -- powerful front and rear rim brakes for stops, and a drum or disc brake capable of heavy heat dissipation to control speed on descents. See the page on tandem brakes.
e-bikes with regenerative braking. These need the usual brakes as well, because regenerative braking is too weak to substitute for either of them.
adult tricycles. (OK, not exactly bicycles.) Recumbent "tadpole" tricycles, with two wheels in the front, generally have dual handbrakes -- one on each front wheel, controlled by the hand lever on its side, allowing the brakes to help with cornering. Racing "delta" tricycles, with the two wheels in the back, often have two handbrakes, both operating on the front wheel. Tricycles may have only one rear wheel driven, and only that wheel may have a brake. For safety's sake, there should always be a brake on the front wheel.