Bicycle brake
A bicycle brake reduces the speed of a bicycle or prevents the wheels from moving. The two main types are: [|rim brakes] and [|disc brakes]. Drum brakes are less common on bicycles.
Most bicycle brake systems consist of three main components: a mechanism for the rider to apply the brakes, such as brake levers or pedals; a mechanism for transmitting that signal, such as Bowden cables, hydraulic hoses, [|rods], or the bicycle chain; and the brake mechanism itself, a [|caliper] or [|drum], to press two or more surfaces together in order to convert, via friction, kinetic energy of the bike and rider into thermal energy to be dissipated.
History
included a pivoting brake shoe that could be pressed against the rear iron tyre of his 1817 Laufmaschine. This was continued on the earliest bicycles with pedals, such as the boneshaker, which were fitted with a spoon brake to press onto the rear wheel. The brake was operated by a [|lever] or by a cord connecting to the handlebars. The rider could also slow down by resisting the pedals of the fixed-wheel drive.The next development of bicycles, the penny-farthings, were similarly braked with a spoon brake or by back pedalling. During its development from 1870 to 1878, there were various designs for brakes, most of them operating on the rear wheel. However, as the rear wheel became smaller and smaller, with more of the rider's weight over the front wheel, braking on the rear wheel became less effective. The front brake, introduced by John Kean in 1873, had been generally adopted by 1880 because of its greater stopping power.
Some penny-farthing riders used only back pedalling and got off and walked down steep hills, but most also used a brake. Having a brake meant that riders could coast down hill by taking their feet off the pedals and placing the legs over the handlebars, although most riders preferred to dismount and walk down steep hills. Putting the legs under the handlebars with the feet off the pedals placed on foot-rests on the forks had resulted in serious accidents caused by the feet getting caught in the spokes.
An alternative to the spoon brake for penny-farthings was the caliper brake patented by Browett and Harrison in 1887. This early version of caliper braking used a rubber block to contact the outside of the penny-farthing's small rear tyre.
The 1870s and 1880s saw the development of the safety bicycle which roughly resembles bicycles today, with two wheels of equal size, initially with solid rubber tyres. These were typically equipped with a front spoon brake and no rear brake mechanism, but like penny-farthings they used fixed gears, allowing rear wheel braking by resisting the motion of the pedals. The relative fragility of the wooden rims used on most bicycles still precluded the use of rim brakes. Rim brakes and the freewheel were introduced in the late 1890s.
With the introduction of mass-produced pneumatic tyres by the Dunlop Tyre Company, the use of spoon brakes began to decline, as they tended to quickly wear through the thin casing of the new tyres. This problem led to demands for alternative braking systems. On November 23, 1897, Abram W. Duck of Duck's Cyclery in Oakland, California, was granted a patent for his Duck Roller Brake. The [|Duck brake] used a rod operated by a lever on the handlebar to pull twin rubber rollers against the front tyre, braking the front wheel.
In 1898, after the advent of freewheel coasting mechanisms, the first internal coaster brakes were introduced for the rear wheel. The coaster brake was contained in the rear wheel hub, and was engaged and controlled by backpedaling, thus eliminating the issue of tyre wear. In the United States, the coaster brake was the most commonly fitted brake throughout the first half of the 20th century, often comprising the only braking system on the bicycle.
Brake types
Spoon brakes
The spoon brake or plunger brake was probably the first type of bicycle brake and precedes the pneumatic tyre. Spoon brakes were used on penny farthings with solid rubber tyres in the 1800s and continued to be used after the introduction of the pneumatic-tyred safety bicycle. The spoon brake consists of a pad or metal shoe, which is pressed onto the top of the front tyre. These were almost always rod-operated by a right-hand lever. In developing countries, a foot-operated form of the spoon brake sometimes is retrofitted to old [|rod brake] roadsters. It consists of a spring-loaded flap attached to the back of the fork crown. This is depressed against the front tyre by the rider's foot.Perhaps more so than any other form of bicycle brake, the spoon brake is sensitive to road conditions and increases tyre wear dramatically.
Though made obsolete by the introduction of the Duck brake, coaster brake, and rod brake, spoon brakes continued to be used in the West supplementally on adult bicycles until the 1930s, and on children's bicycles until the 1950s. In the developing world, they were manufactured until much more recently.
Duck brake
Invented in 1897, the Duck brake or Duck roller brake used a rod operated by a lever on the handlebar to pull twin friction rollers against the front tyre. Mounted on axles secured by friction washers and set at an angle to conform to the shape of the tyre, the rollers were forced against their friction washers upon contacting the tyre, thus braking the front wheel. A tension spring held the rollers away from the tyre except when braking. Braking power was enhanced by an extra-long brake lever mounted in parallel with and behind the handlebar, which provided additional leverage when braking. Used in combination with a rear coaster brake, a cyclist of the day could stop much more quickly and with better modulation of braking effort than was possible using only a spoon brake or rear coaster brake. Known colloquially as the duck brake, the design was used by many notable riders of the day, and was widely exported to England, Australia, and other countries. In 1902, Louis H. Bill was granted a patent for an improved version of the Duck Roller Brake for use on motorized bicycles.Rim brakes
Rim brakes are so called because braking force is applied by friction pads to the rim of the rotating wheel, thus slowing it and the bicycle. Brake pads can be made of leather, rubber, or cork and are often mounted in metal "shoes". Rim brakes are typically actuated by a lever mounted on the handlebar.Advantages and disadvantages
Rim brakes are inexpensive, light, mechanically simple, easy to maintain, and powerful. However, they perform relatively poorly when the rims are wet, and will brake unevenly if the rims are even slightly warped. Because rims can carry debris from the ground to the brake pads, rim brakes are more prone to clogging with mud or snow than disc brakes, particularly when riding on unpaved surfaces. The low price and ease of maintenance of rim brakes makes them popular in low- to mid-price commuter bikes, where the disadvantages are alleviated by the unchallenging conditions. The light weight of rim brakes also makes them desirable in road racing bicycles.Rim brakes require regular maintenance. Brake pads wear down and have to be replaced. As they wear down, their position may need to be adjusted as the material wears away. Because the motion of most brakes is not perfectly horizontal, the pads may lose their centering as they wear, causing the pads to wear unevenly. Over longer time and use, rims can become worn. Rims should be checked for wear periodically as they can fail catastrophically if the braking surface becomes too worn. Wear is accelerated by wet and muddy conditions. Rim brakes require that the rims be straight. If a rim has a pronounced wobble, then the braking force may be intermittent or uneven, and the pads may rub the rims even when the brake is not applied.
During braking, the friction surfaces will experience thermal heating. In normal use this is not a problem, as the brakes are applied with limited force and for a short time, so the heat quickly dissipates to the surrounding air. However, on a heavily laden bike on a long descent, heat energy may be added more quickly than it can dissipate causing heat build-up, which may damage components and cause brake failure.
A ceramic coating for the rims is available which may reduce wear and can also improve both wet and dry braking. It may also slightly reduce heat transfer to the inside of the rims because it is a thermal insulator.
Brake pads
Brake pads are available with numerous shapes and materials. Many consist of a replaceable rubber pad held on a mounting, or brake shoe, with a post or bolt on the back to attach to the brake. Some are made as one piece with the attachment directly molded in the pad for lower production costs; brake pads of the cartridge type are held in place by a metal split pin or threaded grub screw and can be replaced without moving the brake shoe from its alignment to the rim. The rubber can be softer for more braking force with less lever effort, or harder for longer life. Many pad designs have a rectangular shape; others are longer and curved to match the radius of the rim. Larger pads do not necessarily provide more braking force, but will wear more slowly, so can usually be thinner. In general, a brake can be fitted with a variety of pads, as long as the mounting is compatible. Carbon fiber rims may be more sensitive to damage by incorrectly matched brake pads, and generally must use non-abrasive cork pads.Ceramic-coated rims should be used with special pads because of heat build-up at the pad-rim interface; standard pads can leave a "glaze" on the ceramic braking surface, reducing its inherent roughness and leading to a severe drop in wet-weather braking performance. Ceramic pads usually contain chromium compounds to resist heat.
For wet-weather use, brake pads containing iron oxide are sometimes used as these have higher friction on a wet aluminum rim than the usual rubber. These salmon-colored pads were first made by Scott-Mathauser and are now produced by Kool-Stop.
To minimise excessive rim wear, a brake pad should be hard enough that it does not embed road grit or chips of rim metal in the face of the pad, since these act as grinding/gouging agents and markedly reduce rim life.