|
Bicycle brake systems are used to stop a bicycle. There have been various
types through history, and several types still in use today.
History
Early bicycles such as the high wheeled penny-farthing bikes had no brakes as we would recognize them. As the machines were fixed gear bicycles a rider could reduce speed by reversing the thrust on the pedals. Otherwise a rider who wanted to stop quickly had to jump off the bike as it was moving. Unsurprisingly there were many accidents, many of them fatal, which limited the appeal of cycling, mostly to young and adventurous men.
The 1870s saw the development of the "safety bicycle" which was roughly the bicycle we would recognize today, with two wheels of equal size, initally with solid rubber tires. The braking system these bicycles used was often a simple leather pad which pressed against the top of the tire, which was driven by a rod attached to a lever on the handlebar, and there was no rear brake, although with no free wheel back-pedalling was an option. This was undoubtedly a big improvement on having no brakes at all, but it was not very powerful and had the big drawback that it was almost useless in wet weather.
With the introduction of the pneumatic tire also came the next advance in bicycle braking, which came around the 1890s, was the invention of the rim brake. This is the type of brake most commonly used on bicycles today.
Rim brakes
There are several types of rim brakes. Early types, operated by rods, pulled up the friction pads (usually made of leather or rubber) onto the inside surface of the wheel rim, they were often called "stirrup brakes" due to their shape. Rod brakes use a series of rods and pivots rather than Bowden cables to transmit the force generated at the hand-operated brake lever to the brake assembly. Such brakes could fit both front and rear wheels. Although rod brakes are heavy and in some ways complex, they can be repaired with simple hand tools, when replacement Bowden cables are not available or too expensive.
More modern designs use similar friction pads to squeeze the sides of the wheel rims, designs include the scissor-action "side pull", and "centre pull" brakes, and the lever action "cantilever" and "V" brakes. In all of these designs, the braking force is applied by the rider squeezing a lever mounted on the handlebar. This force is transferred to the brake by means of a Bowden cable.
Sidepull caliper brakes are widely used on inexpensive bikes, as well as on high-quality road bikes. These consist of two arms that pivot in the middle. These arms have extensions on one side. The cable housing is attached to one extension, and the inner cable to the other so that when the brake lever is squeezed, the calipers move together. These brakes are simple and effective when designed for relatively narrow tires, however they have serious disadvantages if made big enough to fit wide tires. Most of these disadvantages are answered with some form of cantilever brake.
The wider tires on mountain bikes present a problem with standard calipers, because the long distance from the pivot to the pad allows the arms to flex, reducing braking effectiveness. In higher-quality fat tire bikes this was solved by using cantiliver or V brakes that use two separate pivots on the frame or fork just below the rim.
Cantilevers have pivot points mounted to each side of the frame or fork, usually just below the rim. The traditional cantilever has an L shaped arm on each side, with a straddle cable between the arms. The cable from the brake handle pulls upwards on the straddle cable, causing the brakes to rotate up and inward. These require a cable stop on the frame or fork to hold the cable housing. V brakes (sometimes called linear- or direct-pull) mount similarly, but the arms extend straight up, and the outer housing is attached to one arm, the inner housing to the other.
Closely related is the U brake. Unlike standard cantilevers, the pivots are above the rim. The arms cross over, and usually have the same type straddle cable as a cantilever. This was used for a short time on mountain bikes, and somewhat longer on BMX bikes. It's main advantage was that it did not protrude from the frame like the early cantilevers. This advantage was reduced by redesigned low-profile cantilevers and nearly eliminated with V brakes.
The advantages of rim brakes is that they are inexpensive, lightweight, mechanically simple, and easy to maintain. Also, modern designs of rim brakes are very powerful. The main disadvantages of rim brakes is that their performance deteriorates in wet weather, when the rims are wet. This problem is far less serious on bikes which use rims made of aluminum alloys. Also, they are prone to clogging with mud when mountain biking. They also need regular maintenance. Brake pads wear down quickly, and therefore they have to be replaced at regular intervals. Also, if used over long periods of time, rims also be worn away and have to be replaced. Additionally the Bowden cables can jam up if not regularly lubricated and wear, requiring adjustment.penis
Disc brakes
Disc brakes consist of a metal disc attached to the wheel hub that rotates with the wheel. Attached to the frame or fork will be calipers and pads that squeeze together on the disc. Although this type of brake has been used on motorbikes for decades, only recently have they been added to bicycles. They are most suitable for and used mainly on mountain bikes ridden off road. They also are used on hybrid bicycles and some road bicycles, although this is less common.
The main advantage of disc brakes is that their performance is equally good in all conditions including water and mud. They also avoid the problem that rim brakes have of wearing out the wheel rims, especially in muddy conditions.
On some expensive disk brake models, a hydraulic system is used to push the pad instead of a cable.
The disadvantages are that they are usually heavier and more expensive than rim brakes, and in most cases require a special hub.
Hub brakes
Hub brakes are drum brakes that have their mechanism enclosed within the hub of the wheel, and are usually fitted to the back wheel. Because they are enclosed, hub brakes are completely unaffected by the weather. Some types of hub brake are operated by cables and levers, in the same way as rim and disc brakes. Other types are operated by the rider turning the pedals backwards. These are known as "back pedaling brakes", or "coaster brakes".
Hub brakes are used mainly on utility bicycles, and also on some tandems used in mountains. In the tandem use, the drum is not intended to stop the bike. Instead they are used to keep the speed down on long downhill sections where extended use of rim brakes can cause overheating, which can in turn cause tire damage.
In addition to being impervious to changes in the weather, hub brakes have the advantage of needing very little regular maintenance, especially the back-pedaling type.
When hub brakes do require maintenance, it is far more complicated than other braking systems. They occasionally need to be dismantled and re-greased, usually by a professional. Also hub brakes are heavier than all the other types of bicycle brake. Coaster brakes are not compatible with derailleur gears.
Braking technique
Effective use of a bicycle brake is highly counter-intuitive. The casual rider will at first avoid using the front brake, due to the unsettling feeling of "toppling up", or fear of being sent flying over the handlebars.
However, the most effective technique is to use the front brake almost exclusively. There are several exceptions where the rear brake is preferred; these are listed below. In any stop, the rider should shift their weight toward the rear and use their arms to brace against the deceleration.
During braking (either with the front or rear brake), the bike deceleration causes a transfer of weight to the front wheel. This means that the front wheel has a greater normal force pressing it against the ground, and the back wheel nearly none. Therefore, the front wheel can generate more frictional braking force than the back wheel before locking up and skidding. It is nearly impossible to lock the front wheel while braking. In any conditions and especially in wet conditions or going downhill, the rear brake can exert relatively little braking force before the wheel locks and starts skidding. A skidding rear wheel can lead to dangerous, uncontrollable bicycle movements eventually resulting in the cyclist falling on the ground.
In an emergency stop, it is important to grab the front brake and press it hard to stop in the minimum possible distance. The rider should shift their weight as far to the rear as possible. Maximum deceleration is accomplished by maintaining enough pressure on the front brake such that that the rear wheel is barely touching the ground, just before flipping over the handlebars. In the real world this is not practical. Instead, use light pressure on the back wheel, hard pressure on the front. The back wheel is primarily useful as an indicator—when it starts to skid, reduce the pressure to both brakes to prevent flipping over the front wheel, then increase both again.
There are a few special situations where limited use of the front brake, and heavier involvement of the rear brake is advisable:
- Slippery surfaces. It is nearly impossible to recover from a front-wheel skid, so on surfaces like wet pavement, mud, snow, or ice, when skidding is likely, reduced speed and use of the rear brake is preferred.
- Bumpy surfaces. If the front wheel comes off of the ground, braking will stop it completely. Coming down on a stopped front wheel is very dangerous.
- Front flat tire. Braking the front wheel when the tire is flat could cause the the tire to come off of the rim, which is more likely to cause a crash.
- Broken cable, other mechanical failure. When the front brake fails, the only braking force can come from the rear brake.
External links
| 
