Bicycle brake systems
From Free net encyclopedia
Bicycle brake systems are used to brake a bicycle. There have been various types through history, and several types still in use today.
Contents |
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, initially with solid rubber tires. These were generally equipped with a front spoon brake and 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 almost useless in wet weather.
With the introduction of the pneumatic tire also came the next advance in bicycle braking around the 1890s, the invention of the rim brake. This is the type of brake most commonly used on bicycles today. Track bicycles, however, which are ridden at top speed continuously, continue to be built with no brakes; since they are fixed gear bicycles, braking on these bikes is still achieved only by reversing the force on the pedals. This is not a disadvantage for the very specialized manner in which these bicycles are ridden.
Types of bicycle brakes
Coaster brakes
Image:Rücktrittbremse geschnitten.jpg A coaster brake, also known as a back pedal brake or foot brake, is an off-shoot of the drum brake integrated into hubs with an internal freewheel. Freewheeling functions as with other systems, but, when back pedalled, the brake engages after a fraction of a revolution. It can frequently be found in both single-speed and geared hubs.
When such a hub is pedalled forwards, a clutch which moves along the axle drives the hub shell or gear assembly. When pedalling is reversed, the clutch is either forced between two brake pads, pressing them against the shell, or into a split collar, expanding it against the shell. The braking surface is often steel, and the braking element brass or phosphor-bronze as in the UK Birmingham made Perry Coaster Hub.
Coaster brake bicycles are generally equipped with a single cog and chainwheel and use a ½" chain. However, there have been several models of coaster brake hubs with derailleurs in the past, most notably the Sachs 2x3. These use special extra-short derailleurs which both can stand up to the rigors of being straighted out frequently and don't require an excessive amount of reverse pedal rotation before the brake engages.
Because coaster brakes are, by definition, located in the rear wheel, their full power cannot be exploited before the bicycle begins to skid. This is due to the placement of the rider's weight ahead of the rear tyre's contact with the ground. Additionally, although coaster brakes generally go years without needing maintenance, they are more complicated than rim brakes to repair if it becomes necessary.
Disc brakes
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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. Although these brake types have 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. Many tandem bicycles have a disc brake fitted 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.Template:Fact
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, as well as the requirement that the rim be true. Also, disc brakes offer better modulation of braking power and generally require less finger effort to achieve the same braking power. The disadvantages are that they are usually heavier and more expensive than rim brakes, and in most cases require a special hub.
Cheaper disc brakes, called mechanical disc brakes, use regular brake cables to actuate the pistons. On most other disc brake models, hydraulic lines are used to transmit the force instead of a cable, eliminating the problems of cable maintenance and breakage. However, hydraulic disc brakes require bleeding of the brake lines to remove air bubbles. Some people are more comfortable dealing with cables than hydraulic fluid. There are two types of brake fluid used in disc brakes today: mineral oil and DOT fluid. Mineral oil is generally inert and while DOT has a higher boiling point, it is known to be corrosive to frame paint.
Most disc brakes have two pistons that are actuated from both sides of the caliper. Hydraulic disc brakes have a self-adjusting mechanism so as the brake pad wears, the pistons will keep the distance from the pad to the disc consistent in order to maintain the same brake lever throw. Calipers are now generally made in one piece to increase stiffness and reduce the threat of leaks.
There are two standards for mounting disc brake calipers - IS (Internation Standard) and post mount. IS is the more common of the two. Manitou is the inventor of post mount, although when they started using it most of their products were confined to the bottom end of the market. Nowadays the post mount is frowned upon as 'bad engineering' and almost all bikers distrust it. With post mount, installation and alignment of the caliper is no easier than IS mounting, and because the caliper bolts directly into the fork lowers, which are made out of magnesium, there is a high risk of stripping these threads. IS standard overcomes the issue of stripped threads by having the bolts thread into the caliper of the brake. The caliper is typically made out of aluminum, therefore it is harder to strip. Installation and alignment of the caliper is more precise and the mount is inherently stronger. Frame manufacturers have standardized the IS mount for the rear disc brake mount.
There are two standards for disc hubs - IS and centerlock. IS is a six-bolt mount and is the industry standard. Centerlock is patented by Shimano and uses a splined interface along with a lockring to secure the disc. The advantages of centerlock are that the splined interface is stiffer, removing the disc is easier (no stripped screws), and the hubs and discs are lighter than the 6 bolt ones.
Drum brakes
Image:Front end of a Swedish m42 military bicycle.jpg Drum brakes are used mainly on bicycles which are operated in wet or dirty conditions. Though heavier, more complicated, and frequently weaker than rim brakes, they require much less maintenance and are less affected by road conditions. Both cable- and rod-operated drum brake systems have been widely produced. While most common on utility bicycles in certain countries, especially the Netherlands, they are also frequently found on freight bicycles.
A bicycle drum brake's operation is identical to that of a car's with the exception of the absence of a ratching adjustment mechanism and hydraulic actuation. Two pads are pressed outward against the braking surface on the inside of the hub's shell, which is packed with grease. Shell diameters on a bicycle drum brake are typically 70 - 120 mm. Drum brakes have been used on front hubs and hubs with both internal and external freewheels.
Image:Rollenbremse01.jpg A common design of drum brake is the Roller Brake, manufactured by Shimano. This is a modular cable-operated drum brake for use on specially splined front and rear hubs. Unlike a normal drum brake, the Roller Brake can be removed entirely from a hub, allowing it to function as a regular freewheel. It also contains a torque limiting device which reduces its effectiveness on bicycles with adult-sized wheels.
Drag brake
In addition, drum brakes are also installed on some tandems used in mountainous areas as a so called drag brake. In this application, the drum is not intended to stop the bike but, rather, to keep the speed down on long downhill sections where extended use of rim brakes can cause the tire to become hot enough to explode.
The largest manufacturer of this type is Arai, whose brakes are screwed onto hubs with conventional freewheel threading on the left side of the rear hub and operated via Bowden cables.
Rim brakes
There are several types of rim brakes. In all of these designs, the braking force is applied by the rider squeezing a lever mounted on the handlebar; this causes friction pads (usually made of leather or rubber) to contact the rim of the rotating wheel, thus slowing it and the bicycle.
Rod brakes use a series of rods and pivots (rather than Bowden cables) to pull the friction pads upwards onto the inner surface (facing the hub) of the wheel rim. They were often called "stirrup brakes" due to their shape. In order to fit the rear wheel as well as the more directly actuated front wheel, these brakes need a mechanism to transmit the force but still allow rotation where the fork attaches to the frame. Although rod brakes are heavy and the linkage is complex, they are reliable and durable and can be repaired or adjusted with simple hand tools in areas where replacement Bowden cables are not available or are too expensive. They are still used on some bicycles, particularly in the Far East.
More modern designs use similar friction pads to squeeze the sides of the wheel rims, with the force transmitted to the brake from the lever by means of a Bowden cable. Designs include the scissor-action "side pull" and "centre pull" brakes, and the lever action "cantilever" and "V" brakes.
Image:Sidepull caliper brakes.jpg
Side-pull caliper brakes are widely used on the most inexpensive bikes. They 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; the cable housing is attached to one extension and the inner cable to the other, so that when the brake lever is squeezed, the arms move together and the brake pads squeeze the rim. These brakes are simple and effective when designed for relatively narrow tires, but have serious disadvantages if made big enough to fit wide tires. The cheaper varieties also have a tendency 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.
Center-pull caliper brakes were developed to fill the price niche between the cheaper side-pull brakes and the more expensive ones, being reasonably priced but without the tendency of the cheaper sidepull brakes to have one brake pad continually rub the rim. In this type of brake the actuation of the two brake arms is designed to be symmetrical. The cable housing is attached to a fixed cable stop attached to the frame, and the inner cable attaches to a sliding piece or sometimes a small pulley, over which runs a straddle cable which connects the two brake arms. Thus tension on the cable is evenly distributed to the two arms, preventing the brake from taking a "set" to one side or the other.
Dual-pivot caliper brakes are used on most modern racing bicycles. The cable housing is attached like that of a side-pull brake. Centering of side-pull brakes was simplified with the mass-market adoption of dual-pivot side-pulls (an old design re-discovered by Shimano in the early 1990s) and these brakes also offer a higher mechanical advantage. Dual-pivot brakes have slightly higher weight than conventional side-pull calipers and cannot accurately track an out-of-true rim. One arm pivots at the center, like a side-pull; and the other pivots at the side, like a center-pull.
Cantilever, direct-pull, and V-brakes have each arm attached to a separate pivot point on one side of the seat stay or fork just below the rim. 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. Cantilever brakes are one solution to this problem.
The traditional cantilever has an L-shaped arm protruding outwards on each side, with a cable stop on the frame or fork to hold the cable housing and a straddle cable between the arms similar to the centerpull brake. The cable from the brake handle pulls upwards on the straddle cable, causing the brake arms to rotate up and inward and squeezing the rim between the brake pads.
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 and the inner housing to the other, similar to the cable attachment for sidepull brakes. They are generally more powerful and easier to adjust than cantilever brakes, and have the additional advantage of functioning well with the suspension systems found on many mountain bikes.
Closely related is the U-brake; this type has the pivots for the arms mounted to the frame or fork on each side above the rim. The arms cross over similarly to sidepull or centerpull brakes, and BMX bikes; its main advantage was that it did not protrude sideways from the frame like the early cantilevers. This advantage was reduced by redesigned low-profile cantilevers, and nearly eliminated with V brakes.
One of the least common rim brakes is the hydraulic rim brake. These brakes are generally able to be mounted on the same pivot points used for cantilever and linear-pull brakes. They were available on some high-end mountain bikes in the early 1990's, but declined in popularity with the rise of disk and linear-pull brakes. The moderate performance advantage (greater power and control) they offer over the latter is offset by their greater weight and complexity. The only significant current use of these brakes is on bicycles used for trials riding.
Another design is the delta brake. In this design, the pivot points for the arms are above the rim but the arms do not cross over, and instead of a straddle cable, the inner brake cable attaches to a wedge shaped piece between the brake arms; when the brakes are applied the wedge forces the arms apart at the top, squeezing the rim between the pads. This has an advantage in that the shape of the wedge can be varied other than straight-sided, to allow for a very high mechanical advantage at the point where the pads contact the rim to give high braking power, but a lower mechanical advantage when the pads are not contacting the rim so that the pads move well away from the rim when the brake is not applied, preventing any rubbing.
The advantages of rim brakes are that they are inexpensive, lightweight, mechanically simple, and easy to maintain, as well as very powerful. The main disadvantage of rim brakes is that their performance deteriorates in wet weather when the rims are wet. This problem is less serious on more expensive bikes which use rims made of aluminum alloys than on those with steel rims. They are also prone to clogging with mud, particularly when mountain biking. Rim brakes also need regular maintenance. Brake pads wear down quickly, and have to be replaced. Over long periods of time and use, rims also become worn. Bowden cables can jam if not regularly lubricated or if water gets into the housing, causing corrosion. The cables also wear, requiring frequent checking and replacement. If the inner cables are not replaced when they fray, they will suddenly break when brakes are applied strongly, causing brakes to be lost precisely when they are most needed. Rim brakes also require that the rim be relatively true; if the rim has a pronounced wobble, either side to side or inwards and outwards, it will be impossible to adjust the brake pads so that they can apply sufficient pressure to the rim but do not rub when released and do not contact the tire, which would lead to a rapid blowout.
Rim brakes also suffer from the disadvantage of rim heating, because the brake converts kinetic energy into heat, which increases the temperature of the rim (the brake caliper and frame do not become hot because brake pads are excellent thermal insulators). In normal use and with lightweight bicycles this does not present a problem as the brakes are only applied with a limited force and for a short time, with the heat being quickly disspated to the surrounding air. However, on heavily-laden touring bikes and tandems in mountainous regions the heat build-up can be sufficient to increase tire pressure to the extent that the tire blows off the rim. If this happens on the front wheel a serious accident is almost inevitable. The problem is worse when descending cautiously at slow speeds because the brakes are "always on" and the cooling airflow over the rim is insufficient. The risk can be minimised by not over-inflating tires and the adoption of an aggressive riding style, only braking for the corners, but the real solution is a hub brake or a disc brake which allows the rim to remain cool.
Brake pads are of many designs. Most consist of a replaceable rubber pad held in a metal channel, with a post or bolt protruding from the back to allow attachment to the brake, but some are made as one piece with the attachment directly moulded in the pad for greater mechanical stability. The rubber can be softer for more braking force with less lever effort, or harder for longer life. The rubber can contain abrasives for better braking, at the expense of rim wear. Compounds vie for better wet braking efficiency. Typically pads are relatively short, but longer varieties are also manufactured to provide more surface area for braking; these often must be curved to match the rim. It should be noted that a larger pad does not give more friction, but it does wear more slowly and therefore a new pad can be made thinner, simplifying wheel removal with V-brakes in particular. In general, a brake can be fitted with any of these many varieties of pads, so long as the pad mounting method is compatible. Carbon rims, as on some disc wheels, generally have to use non-abrasive cork pads.
Spoon brakes
Image:Marian038.jpg The spoon brake was one of the first types of bicycle brakes and precedes the pneumatic tire. They were first used on penny farthings with solid rubber tires in the late 1800's and continued to be used after the introduction of the pneumatic tired safety bicycle. It consists of a pad (often leather) which is pressed onto the top of the front tire. These were almost always rod-operated by a right-hand lever. In developing countries, a foot-operated form of the spoon brake is sometimes 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 tire by the rider's foot.
Perhaps more so than any other form of bicycle brake, the spoon brake is very sensitive to road conditions and increases tire wear dramatically.
Though made obsolete by the introduction of the coaster brake and rod brake, they continued to be used supplementally on adult bicycles in the until the 1930's and childrens' bicycles until the 1950's, in the West. In the developing world, they were manufactured until much more recently.
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 for powerful stopping 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. 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 his or her weight as far to the rear as possible to avoid flipping over the handlebars. 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. In reality this is not practical for most cyclists. Instead, use light pressure on the back wheel and 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, then increase pressure to both again. Incidentally, on tandems, with their greater weight and long wheelbase, it is virtually impossible for heavy front braking to cause the machine to flip.
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 during braking, it will stop completely. Landing on a stopped front wheel with the brakes still applied is likely to cause the rider to flip over.
- Flat front tire: Braking the front wheel when the tire is flat could cause the tire to come off of the rim, which is more likely to cause a crash.
Sources
- Braking and Turning your Bicycle by Sheldon Brown
- Sheldon Brown's Bicycle Glossary
- Marian Rosenberg, The Bikes of China (2005-2006)
- Ekström, Gert; Husberg, Ola (2001). Älskade cykel (1st ed.). Bokförlaget Prisma. ISBN 91-518-3906-7. Template:Sv icon
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