Bicycle brake systems
Bicycle brake systems are used to slow down, or brake 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 today. As they were fixed gear bicycles, a rider could reduce speed by reversing the force 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, some fatal, which limited the appeal of cycling mostly to young and adventurous men.
The 1870s saw the development of the "safety bicycle" which roughly resembles bicycles 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 not an option. This was a big improvement on having no brakes at all, but was not very powerful and potentially dangerous in wet weather.
With the invention of pneumatic tires in the 1890s came the rim brake, the type of brake most commonly used on bicycles today.
Track bicycles are built with no brakes, for safety, so as to avoid sudden changes in speed. Since they have a fixed gear, braking is still done by reversing the force on the pedals. This is not a disadvantage for their very specialized use.
Types of bicycle brakes
Rim brakes
In rim brakes, 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. There are several types of rim brakes.
Rod brakes use a series of rods and pivots (rather than
In newer designs, friction pads squeeze the sides of the wheel rims with a force transmitted from a hand lever by a Bowden cable. Designs include the scissor-action "side pull" and "centre pull" brakes, and the lever action "cantilever" and "V" brakes.
Single pivot side-pull caliper brakes 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, one attached to the cable, the other to the cable housing. When the brake lever is squeezed, the arms move together and the brake pads squeeze the rim. These brakes are simple and effective for relatively narrow tires, but have serious disadvantages if made big enough to fit wide tires. Low-quality varieties also tend 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. These brakes are now used on inexpensive bikes; before the introduction of dual-pivot calliper brakes they were used on all types of road bikes.
Centre-pull calliper brakes have symmetrical arms and by design do not rub the rim when they are released, by actuating the brake arms symmetrically. The cable housing attaches to a fixed cable stop attached to the frame, and the inner cable attaches to a sliding piece or a small pulley, over which runs a straddle cable connecting the two brake arms. Tension on the cable is evenly distributed to the two arms, preventing the brake from taking a "set" to one side or the other. These brakes were reasonably priced, and in the past filled the price niche between the cheaper and the more expensive models of side-pull brakes.
Dual-pivot calliper brakes are used on most modern racing bicycles. One arm pivots at the centre, like a side-pull; and the other pivots at the side, like a centre-pull. The cable housing attaches like that of a side-pull brake; the centring 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). These brakes offer a higher mechanical advantage. Dual-pivot brakes are slightly heavier than conventional side-pull callipers and cannot accurately track an out-of-true rim.
Cantilever, direct-pull, and linear-pull brakes have each arm attached to a separate pivot point on one side of the seat stay or fork just below the rim. This solves the problem for standard callipers on wide tyres (such as those on mountain bikes) where the long distance from the pivot to the pad allows the arms to flex, reducing braking effectiveness.
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 centre-pull 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.
Linear-pull brakes (sometimes referred to by the trademarked term "V-brakes") mount similarly, but the arms extend straight up, and the housing is attached to one arm and the cable to the other, similar to the cable attachment for side-pull brakes. They are generally more powerful and easier to adjust than cantilever brakes but require a smaller gap between the brake pad and the rim surface. They function well with the suspension systems found on many mountain bikes because they do not require a separate cable stop on the frame or fork. Due to their higher mechanical advantage, linear-pull brakes require levers with longer cable travel than levers intended for caliper brakes or traditional cantilever brakes. See Linear-pull brake adjustment.
Closely related is the U-brake (sometimes referred to by the term "990's" after the Dia-Compe u-brake model); 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 centre-pull brakes; its main advantage is that it does not protrude sideways from the frame like the cantilevers. U-brake mounts are the current standard on Freestyle BMX frames and forks.
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 1990s, but declined in popularity with the rise of disc 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, where the arms pivot above the rim but do not cross, and the inner cable attaches to a wedge-shaped piece between the brake arms, instead of a straddle cable. 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.
Advantages and disadvantages
Rim brakes are cheap, light, mechanically simple, easy to maintain, and very powerful. But they perform poorly in wet weather when the rims are wet. This problem is less serious with rims made of aluminium, found on more expensive bikes, than on those with steel or chromed rims. They are also prone to clogging with mud, particularly when mountain biking.
Rim brakes also need regular maintenance. Brake pads can wear down quickly, and have to be replaced. Over longer time and use, rims become worn. Rims should be checked for wear periodically as they can fail catastrophically after a heavy wear from a few thousand miles (or much less if heavily used in wet and muddy conditions). 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 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 straight; if the rim has a pronounced wobble, then either the brake pads rub against it when the brakes are released, or apply insufficient pressure to the rim when the brakes are applied.
Rim brakes also heat the rim, because the brake converts kinetic energy into thermal energy, which increases the temperature of the rim (the brake calliper and frame do not become hot because brake pads are excellent thermal insulators). In normal use and with lightweight bicycles this is not a problem, as the brakes are only applied with a limited force and for a short time, so the heat quickly dissipates to the surrounding air. But on heavily-laden touring bikes and tandems in mountainous regions, the heat build-up can increase tire pressure so much 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 reduced by not over-inflating tires and adopting an aggressive riding style, only braking for the corners, but the real solution is a hub brake or a disc brake which does not heat the rim at all.
There are many designs of brake pads (brake blocks). Most consist of a replaceable rubber pad held in a metal channel (brake shoe), with a post or bolt protruding from the back to allow attachment to the brake. Some are made as one piece with the attachment directly moulded in the pad for lower production costs. The rubber can be softer for more braking force with less lever effort, or harder for longer life. The rubber can also 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. A larger pad does not give more friction but wears more slowly, so a new pad can be made thinner, simplifying wheel removal with linear-pull brakes in particular. In general, a brake can be fitted with any of these many varieties of pads, as long as the pad mounting method is compatible. Carbon rims, as on some disc wheels, generally have to use non-abrasive cork pads.
Disc brakes
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. Such brakes have been successfully used on motorcycles for decades, and been the principal choice there. They are finally becoming more popular on bicycles, after many (partly successful) attempts to introduce them over the last decades [citation needed]. Recent material advances in weight, costs and reliability have led several firms to develop and implement disc brake systems, and those are becoming a standard feature on many bicycles, and will probably stay popular. They are used mainly on mountain bikes ridden off-road, but sometimes on hybrid bicycles and some road bicycles. Many tandem bicycles have a disc brake 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.[1]
Advantages
Disc brakes perform equally well in all conditions including water, mud and snow. This is due to their position closer to the hub and away from the ground and possible contaminants like water which can coat and freeze on the rim in colder temperatures. 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 straight.
Disc brakes offer better modulation of braking power and generally require less finger effort to achieve the same braking power. The advantages of discs make them well-suited to steep, extended downhills through wet and muddy off-road terrain, common in freeride bicycle riding. The use of tires as large as 3.0 inches in width also makes disc brakes necessary, as rim brakes cannot straddle such a wide tire.
Disadvantages
Disc brakes are sometimes heavier and more expensive than rim brakes, and require a hub built to accept the disc and a bicycle frame or fork built to accept the caliper. Older designs for front disc hubs often move the left hub inward which causes the wheel to be dished, and therefore laterally weaker when forced to the non-disc side. Rigid forks on road bikes and tandems, made to handle the forces of a front disc brake, are heavier and may not have the ride quality of a regular fork.
A disc brake puts more stress on a wheel's spokes than a rim brake, since the torque of braking is between the hub and the rim. The spokes therefore must to be stronger, thus ruling out very light and thin double-butted spokes and aluminum nipples that are popular on more expensive bikes with rim brakes. Thus, spokes often feature a straight diameter and brass nipples and are cross-laced, and so slightly heavier (about 15 to 70g per wheel) than radially laced, double butted spokes possible with rim brakes.
The design and positioning of disc brakes precludes the use of most types of pannier-rack; for this reason, disc brakes are rarely found on touring bikes, although several rack manufacturers are addressing this issue. For example the Topeak Super Tourist DX, the Arkel Sherpa, and the Axiom Journey rear racks all are specially made to fit bikes with rear disc brakes.
Recently, a number of riders have experienced a dangerous problem with disc brakes. Under extreme braking conditions, the front wheel has come off of the dropouts. Certain front forks using quick release skewers have been shown to have this problem. Riders should make sure the skewers are properly tightened before serious riding.[2]
Hydraulic vs mechanical
There are two main types of disc brake: mechanical (cable-actuated) and hydraulic. Mechanical disc brakes are almost always cheaper, but have less modulation, and may accumulate dirt in the cable lines since the cable is usually open to the outside.
Hydraulic disc brakes use fluid from a reservoir, pushed through a hose, to actuate the pistons in the disc caliper, that actuate the pads. They are better at excluding contaminants, but are difficult to repair on the trail, since they require fairly specialized tools. The brake lines occasionally require bleeding to remove air bubbles, whereas mechanical disc brakes rarely fail completely.
Also, the hydraulic fluid may boil on steep, continuous downhills. This is due to heat build up in the disc and pads and can
cause the brake to lose its ability to transmit force ("brake fade") through incompressible
fluids, since some of it has become a gas, which is compressible. To avoid this problem, 203 mm (8 inch) diameter disc rotors
have become common on downhill bikes. Larger rotors dissipate heat more quickly and have a larger amount of mass to absorb heat. Two types of brake
fluid are used today:
Single vs dual actuation
Many disc brakes have their pads actuated from both sides of the caliper, while some cheaper kinds have only one pad that moves. Many hydraulic disc brakes have a self-adjusting mechanism so as the brake pad wears, the pistons keep the distance from the pad to the disc consistent to maintain the same brake lever throw. Mechanical discs have a manual control to adjust the pad-to-rotor gap. Calipers are now generally made in one piece to increase stiffness and reduce the threat of leaks, but the two-piece design still reduces heat buildup more effectively, and most top-end models still have a two-piece caliper.
Caliper mounting standards
There are many standards for mounting disc brake calipers. IS (International Standard) is different for 6-inch and 8-inch rotor and differs between forks with a QR and 20mm thru axle. The post-mount standard also differs by disc size and axle type. Also, many incompatible variants were produced over the years, mostly by fork manufactures. The mount used on the Rockshox Boxxer is the most typical of these specialty mounts, but most fork manufactures now use either the IS or post-mount standard for their current forks. As a point of reference, Hayes currently sells no less than 13 different adaptors to fit their brakes to various mounting patterns.
The post-mount standard was developed by Manitou; it makes it easier to align the caliper to the rotor, since it allows some side-to-side adjustment. Spacers must be used to properly align IS calipers. The disadvantage to post mount is that the bolt is threaded directly into the fork lowers. If the threading is stripped or if the bolt is stuck, then new fork lowers are required. Frame manufacturers have standardized the IS mount for the rear disc brake mount. In recent years post mount has gained ground and is becoming the most common. This is mostly due to decreased manufacturing and part cost for the brake calipers when using post mount.
Disc mounting standards
There are many standards for disc rotor mounting - International Standard (IS), centerlock, Cannondale's 4-bolt pattern, Hope's 5-bolt pattern and Rohloff's 4-bolt pattern, to name a few. 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 and removing the disc is quicker because it only requires one lockring to be removed. Some of the disadvantages are that the design is patented requiring a licensing fee from Shimano. A Shimano cassette lockring tool is needed to remove the rotor and is more expensive and less common than a Torx key. Advantages of IS six-bolt are that you have more choices when it comes to hubs and rotors. IS rotors use button head socket cap screws with either a hex socket or Torx socket to secure them to the hub. This can make IS rotors more time consuming to remove.
Disc sizes
Disc brake rotors come in many different sizes, generally 160 millimetre, 185 mm, or 203 mm in diameter, however there are many different sizes available as all brake manufacturers make discs specific to their callipers and the dimensions often vary by a few millimetres. Larger rotors provide greater stopping power by virtue of a longer moment arm for the calliper to act on. Smaller rotors provide less stopping power but also less weight. Larger rotors will also dissipate heat more quickly preventing brake fade or failure. Typically downhill racers will run larger brakes to handle the greater braking loads and extended braking duration. Cross country racers will typically run smaller rotors which can easily handle the much smaller braking loads and offer a considerable weight savings of over 100g per rotor.[3] It is also common to use a larger diameter rotor on the front wheel and a smaller rotor on the rear wheel. This is due to the braking dynamics which shifts most of the rider weight to the front wheel during braking. This provides greater traction at the front wheel and allows for greater braking force. Conversely the weight shift off of the rear wheel reduces its braking force. Using a smaller rear rotor will save weight and allow for better modulation of the rear brake while more efficiently using the wheel's braking capacity.
Drum brakes
Drum brakes are useful for wet or dirty conditions. They are heavier, more complicated, and frequently weaker than rim brakes, but 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 some countries, especially the Netherlands, they are also frequently found on freight bicycles.
A bicycle drum brake operates like a car's but has no ratching adjustment mechanism or 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.
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
Some bicycles have drag brake, drum brakes intended to slow down the bike on long downhills rather than stop it. Such brakes occur on some tandem bicycles used in mountainous areas, where extended use of rim brakes can cause the tire to become hot enough to explode. The largest manufacturer of this type of brake 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.
Coaster brakes
A coaster brake, also known as a back pedal brake or foot brake (or torpedo in some countries), is a 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, the sprocket drives a screw which forces a clutch to move along the axle, driving the hub shell or gear assembly. When pedalling is reversed, the screw drives the clutch in the opposite direction, forcing it either between two brake pads and pressing them against the shell, or into a split collar and expanding it against the shell. The braking surface is often steel, and the braking element brass or phosphor-bronze, as in the Birmingham made Perry Coaster Hub.
Coaster brake bicycles are generally equipped with a single cog and chainwheel and use a ½" pitch 1/8" wide chain. However, there have been several models of coaster brake hubs with dérailleurs in the past, most notably the Sachs 2x3. These use special extra-short dérailleurs which both can stand up to the rigours of being straightened out frequently and don't require an excessive amount of reverse pedal rotation before the brake engages. Coaster brakes have also been incorporated into hub gear designs - for example the AWC from Sturmey Archer.
Although coaster brakes have the advantage of being protected from the elements and thus immune to ice or water, because they are located in the rear wheel only limited braking force can be applied before the rear wheel locks up. This is due to the placement of the rider's weight ahead of the rear tyre's contact with the ground, as well as the weight transfer forward proportional to braking force, which further unloads the rear wheel. Additionally, although coaster brakes generally go years without needing maintenance, they are more complicated than rim brakes to repair if it becomes necessary.
Coaster brakes do have the disadvantage that they cannot be used whilst the bike is stationary unless the crank was horizontal beforehand. On a slope any other brake can be applied to stop the bike from moving, not so with a coaster.
Spoon brakes
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 1800s 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 until the 1930s and children's bicycles until the 1950s, 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 handle bars.
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 there is more force pressing the front wheel to 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. For a more-detailed analysis, see Bicycle and motorcycle dynamics.
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 the rear wheel is barely touching the ground, just before lifting up. 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 the rear wheel from lifting, then increase pressure to both again. Some front brakes have a spring leaving a stop and limiting the force in the event that a scared driver pulls the brake too hard. It is adjusted so that the back wheel lifts so slowly, that the driver can react and lose grip before the bike tips over totally. The bike has by far the most danger of tipping over during heavy braking at low speeds, so front braking should be reduced and the rear brake used for final deceleration.
Incidentally, on tandems, and long-wheel-base recumbents with their long wheelbase and centre of mass further from the front wheel, 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 difficult to recover from a front-wheel skid on a slippery surface, especially when leaned over, so on surfaces when skidding is likely (e.g. wet pavement, mud, snow, ice, or loose stones/gravel), reduced speed and use of the rear brake may be 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 front wheel to skid and, possibly, for the rider to flip over the front bars.
- 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.[4]
Caution should be used when braking on a cheap bicycle. The frame can flex and the brakes are spongy, even when set "correctly" from the shop. Frequently they have a spring in the front brake cable to reduce the braking force; this is opposite to what a good mechanic/rider would aspire.
See also
References
- ^ Brown, Sheldong. Brakes for Tandem Bicycles. Retrieved on 2007-10-19.
- ^ Annan, James. Disk brakes and quick releases - what you need to know. Retrieved on 2007-10-19.
- ^ Disc Brake weight listing. Retrieved on 2006-11-07.
- ^ Brown, Sheldon. Braking and Turning your Bicycle. Retrieved on 2007-10-19.
- Sheldon Brown's Bicycle Glossary
- (Swedish) Ekström, Gert; Husberg, Ola (2001). Älskade cykel, 1st (in Swedish), Bokförlaget Prisma. ISBN 91-518-3906-7.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
