Bicycle brake
Bicycle brake systems are used to slow down or brake a bicycle. There have been various types through history, and several are still in use today.
History
Early bicycles such as the high-wheeled penny-farthing bikes were fitted with spoon brakes. As they were fixed gear bicycles, a rider also could reduce speed by reversing the force on the pedals. 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, like the penny-farthings fixed gears, allowing control of speed by control of pedalling. Spoon brakes were not very powerful and potentially dangerous in wet weather. A calliper brake was patented by Browett and Harrison in 1887.[1]
With the invention of pneumatic tires in the 1890s came the rim brake, the type of brake most commonly used on bicycles today. However, in America throughout most of the 20th century, the most common type of brake was the coaster brake, engaged by pressing backwards on the pedals. The rim brake began to supersede the coaster brake in the 1970s.
Brake types
Spoon brakes
The spoon brake was probably the first type of bicycle brake and precedes the pneumatic tire. Spoon brakes were used on penny farthings with solid rubber tires in the 1800s and continued to be used after the introduction of the pneumatic tired safety bicycle. The spoon brake 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 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 tire 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 tire wear dramatically.
Though made obsolete by the introduction of the 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.
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 or rubber and are mounted in metal "shoes". Rim brakes are typically activated by the rider squeezing a lever mounted on the handlebar.
Advantages and disadvantages
Rim brakes are cheap, light, mechanically simple, easy to maintain, and powerful. However, they perform poorly when the rims are wet. This problem is less serious with rims made of aluminum, found on more expensive bikes, than on those with steel or chromed rims. Rim brakes are also prone to clogging with mud, particularly when mountain biking.
Rim brakes require 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 if the rim sidewalls become too worn. Depending on the brake pads and rim, this can happen after a few thousand miles if heavily used in wet and muddy conditions. Bowden cables can become sticky if not regularly lubricated or if water gets into the housing, causing corrosion, although modern lined and stainless steel cables are less prone to these problems. The cables also can wear through repeated use over a long time, however they are more likely to get damaged through getting kinked or the open end becoming unraveled. If the inner cables are not replaced when they fray, they can suddenly break when brakes are applied strongly, causing the brakes to fail 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 and uneven pressure to the rim when certain brakes e.g. dual pivot, are applied.
Rim brakes also heat the rim because the brake functions by converting kinetic energy into thermal energy. 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. However, on heavily-laden touring bikes and tandems in mountainous regions, the heat build-up over a long descent 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. For this reason, disc brakes are often fitted instead of rim brakes in this situation. Another common solution is to use a drum brake, activated constantly for long descents, in addition to the rim brake.
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 molded 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.
Although becoming superseded by disc brakes on off-road machines, rims with a hard, rough ceramic coating on the braking surface are available. This coating significantly reduces rim wear and can also improve both wet and dry braking provided appropriate pads are used. It also reduces heat transfer to the air in the tire because the ceramic coating, although thin, is a thermal insulator. The latter characteristic does mean that special pads should be used 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. Kool-Stop, Mavic and Swisstop make "ceramic" pads, which contain substances such as chromium to improve heat tolerance.
The following are among the many sub-types of rim brakes:[2]
Rod-actuated brakes
The rod-actuated brake, or simply rod brake, uses a series of rods and pivots (rather than Bowden cables) to transmit force applied to a hand lever to pull friction pads upwards against the inner surface (facing the hub) of the wheel rim. They were often called stirrup brakes due to their shape. Rod brakes are used with a rim profile known as the Westwood rim, which has a slightly concave area on the braking surface and lacks the flat outer surface required by brakes that apply the pads on opposite sides of the rim.
The back linkage mechanism is complicated by the need to allow rotation where the fork attaches to the frame. Although heavy and complex, the linkages are reliable and durable and can be repaired or adjusted with simple hand tools. The design is still in use, typically on roadsters, particularly in East, South Asia and Africa (boda-boda).
The Calliper Brake Design
The calliper brake is a class of cable-actuated brake in which the brake mounts to a single point above the wheel, theoretically allowing the arms to auto-centre on the rim. Arms extend around the tire and end in brake shoes that press against the rim. While some designs incorporate dual pivot points, the arms pivot on a sub-frame — the entire assembly still mounts to a single point.
Calliper brakes are generally considered unsuited to bicycle suspensions, though implementations do exist. They also tend to become less effective as tires get wider. Thus calliper brakes are rarely found on modern mountain bikes. But they are almost ubiquitous on road bikes, particularly the dual-pivot side-pull calliper brake.
Side-pull calliper brakes
Single-pivot side-pull calliper 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 significant flex and resulting poor performance 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. 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.
Dual-pivot side-pull 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 improved 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, and resulting better braking. Dual-pivot brakes are slightly heavier than conventional side-pull callipers and cannot accurately track an out-of-true rim.
Centre-pull calliper brakes
Centre-pull calliper brakes have symmetrical arms and as such centre more effectively. The cable housing attaches to a fixed cable stop attached to the frame, and the inner cable bolts 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.
U-brakes
U-brakes (also known by the trademarked term "990-style") are essentially the same design as the centre-pull calliper brake. The difference is that the two arm pivots attach directly to the frame or fork while those of the centre-pull calliper brake attach to an integral bridge frame that mounts to the frame or fork by a single bolt. Like roller cam brakes, this is a first-class cantilever design with pivots located above the rim. Thus U-brakes are often interchangeable with, and have the same maintenance issues as, roller cam brakes.
U-brakes were used on mountain bikes through the early 1990s, particularly in the then-popular under-the-chainstays rear location;[3]
They are the current standard on Freestyle BMX frames and forks. The U-brake's main advantage over cantilever and linear-pull brakes in this application is that sideways protrusion of the brake and cable system is minimal, and the exposed parts are smooth. This is especially valuable on freestyle BMX bikes where any protruding parts are liable to damage and may interfere with the rider's body.
The Cantilever Brake Design
The cantilever brake is a class of brake in which each arm is attached to a separate pivot point on one side of the seat stay or fork. Thus all cantilever brakes are dual-pivot. Both first and second-class lever designs exist; second-class is by far the most common. In the second class lever design, the arm pivots below the rim. The brake shoe is mounted above the pivot and is pressed against the rim as the two arms are drawn together. In the first class lever design, the arm pivots above the rim. The brake shoe is mounted below the pivot and is pressed against the rim as the two arms are forced apart.
Cantilever brakes are preferred for bicycles that use wide tires, such as those on mountain bikes. (Standard calliper brakes are problematic in these applications since the long distance from the pivot to the pad allows the arms to flex, reducing braking effectiveness.) Because the arms move only in their designed arcs, the brake shoe must be adjustable in several planes. Thus cantilever brake shoes are notoriously difficult to adjust.
There are several brake types based on the cantilever brake design: cantilever brakes and direct-pull brakes - both second class lever designs - and roller cam brakes and U-brakes - both first class lever designs.
Traditional Cantilever brakes
The traditional cantilever brake, or commonly cantilever brake, pre-dates the direct-pull brake. It is a centre-pull cantilever design with an outwardly-angled arm protruding on each side, a cable stop on the frame or fork to terminate the cable housing, and a straddle cable between the arms similar to centre-pull calliper brakes. The cable from the brake lever pulls upwards on the straddle cable, causing the brake arms to rotate up and inward thus squeezing the rim between the brake pads.
Traditional cantilever brakes are difficult to adapt to bicycle suspensions and protrude somewhat from the frame. Accordingly they are usually found only on bicycles without suspension.
V-brakes
Linear-pull brakes or direct-pull brakes, commonly referred to by Shimano's trademark V-brakes, are a side-pull version of cantilever brakes and mount on the same frame bosses . However, the arms are longer, with the cable housing attached to one arm and the cable to the other. As the cable pulls against the housing the arms are drawn together. Because the housing enters from vertically above one arm yet force must be transmitted laterally between arms, the flexible housing is extended by a rigid tube with a 90° bend known as the "noodle". The noodle sits in a stirrup attached to the arm. A flexible bellows often covers the exposed cable.
Since there is no intervening mechanism between the cable and the arms, the design is called "direct-pull". And since the arms move the same distance that the cable moves with regard to its housing, the design is also called "linear-pull". The term "V-brake" is trademarked by Shimano and represents the most popular implementation of this design.
V-brakes usually lack an adjustment mechanism for cable length (other than the cable clamp and a barrel adjuster on the brake lever). 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. Because of the higher mechanical advantage of V-brakes, they require levers with longer cable travel than levers intended for calliper brakes or traditional cantilever brakes. This cable pull ratio was later adopted for disc brakes, making linear-pull levers standard for modern mountain bikes. Standard road levers normally do not pull enough cable to function with V-brakes.[4] To solve this problem, devices that use an eccentric pulley to increase the amount of cable pull of road levers, such as the "QBP Travel Agent", may be used.
Roller cam brakes
Roller cam brakes are centre-pull cantilever brakes actuated by the cable pulling a single two-sided sliding cam. (First and second-class lever designs exist; first-class is most common and is described here.) Each arm has a cam follower. As the cam presses against the follower it forces the arms apart. As the top of each arm moves outward, the brake shoe below the pivot is forced inward against the rim.[5] There is much in favor of the roller cam brake design. Since the cam controls the rate of closure, the clamping force can be made non-linear with the pull. And since the design can provide positive mechanical advantage, maximum clamping force can be higher than that of other types of brakes. They are known for being strong and controllable. On the downside, they require some skill to set up and can complicate wheel changes. And they require maintenance: like U-brakes, as the pad wears it strikes the rim higher; unless re-adjusted it can eventually contact the tyre's sidewall.
The roller cam design was first developed by Charlie Cunningham of WTB around 1982 and licensed to Suntour.[6] Roller cam brakes were used on early mountain bikes in the 1980s and into the 1990s, mounted to the head tube and seat stays in the standard locations, and below the chain stays for improved stiffness as they do not protrude to interfere with the cranks. It was not unusual for a bicycle to have a single roller cam brake combined with another type. They are still used on some BMX and recumbent bicycles.[7] Note that the common first-class lever roller cam brake is generally not convertible to second-class lever cantilever brake types as the pivots are in different locations.
There are two rare variants that use the roller cam principle. For locations where centre-pull is inappropriate, the side-pull toggle cam brake was developed.[8] Also a first-class cantilever, it uses a single-sided sliding cam (the toggle)[9] against one arm that is attached by a link to the other arm. As the cam presses against the (single) follower, the force is also transmitted to the other arm via the link. And specifically for suspension forks where the housing must terminate at the brake frame, the side-pull sabre cam brake was developed.[10] In the sabre cam design, the cable end is fixed and the housing moves the single-sided cam.
Delta brakes
The delta brake is a road bicycle brake named due to its triangular shape. The cable enters at the centre, pulls a corner of a parallelogram linkage housed inside the brake, pushing out the brake arms above the pivots, resulting in the arms below the pivots, with the pads, pushing in against the rim. While considered attractive, the delta brake has been criticized for being heavy, giving mediocre stopping power, and suffering disadvantageous variable mechanical advantage.[11][12]
Made most prominently by Campagnolo in 1985, but also manufactured by Weinmann, and others,[13] they are no longer made and are now very uncommon.
Hydraulic rim brakes
Hydraulic rim brakes are one of the least common types. 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 brakes. The moderate performance advantage (greater power and control) they offer over cable actuated rim brakes is offset by their greater weight and complexity. The only significant current use of these brakes is on bicycles used for trials riding.
Disc brakes
Disc brakes consist of a metal disc attached to the wheel hub that rotates with the wheel. Callipers 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 are 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. They are used mainly on mountain bikes ridden off-road, but sometimes on hybrid bicycles and touring 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.[14]
Advantages and disadvantages
Disc brakes perform equally well in all conditions including water, mud and snow. This is due to a number of factors:
- 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 keeps the disc rotor clean and working well.
- The disc brake calliper operates with a higher mechanical advantage than rim brakes, and as such squeeze the disc harder than rim brakes do the rim. For this reason, and due to the holes in the rotor, disc brakes maintain their stopping power in wet conditions, by more effectively clearing the disc of water.[15]
- Disc brakes are able to operate at a higher mechanical advantage than rim brakes because disc rotors in good condition are more true than rims in good condition, and thus do not need to retract as far from the rim when released.
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. The pads are usually made from metal sinters or an organic compound instead of rubber, and as such usually last longer than rim pads. Disc brakes offer better modulation of braking power and generally require less effort at the lever to achieve the same braking power. The use of tires as wide as 3.0 inches (76 mm) also makes disc brakes necessary, as rim brakes are not designed to straddle such a wide tire.
The benefits of disc brakes make them of great advantage in mountain bike riding, especially the more aggressive forms, such as freeride and downhill. Disc brakes are also becoming increasingly popular on hybrids, as they perform well in all weather conditions, and usually require less maintenance than rim brakes.
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 calliper. Older designs for front disc hubs often move the left hub's flange 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 be stronger, this leads to slightly heavier and more expensive wheels.
The design and positioning of disc brakes can interfere with the pannier racks not designed for them. For this reason, many manufacturers produce "disc" and "non-disc" versions.
While a disc brake does not heat the rim, excessive heat build up can lead to disc failure. Bicycle discs are extremely lightweight, relative to the mass of bike and rider, compared to the large cast iron discs on an automobile, and therefore have little heat capacity. If brake friction exceeds convection and radiation losses, the temperature of the disc can very rapidly reach levels at which the metal begins to weaken and may warp or crack under braking stress. Therefore a disc is less suitable than a drum as a tandem drag brake,[citation needed] and heavy bikes (and riders) should choose the largest diameter discs available to increase heat capacity and cooling area.
Recently, a number of riders have experienced a dangerous problem with disc brakes. Under extreme braking conditions, the front wheel has come off 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 riding.[16]
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 calliper, 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 require less calliper pressure for equal stopping power, dissipate heat more quickly, and have a larger amount of mass to absorb heat. Two types of brake fluid are used today: mineral oil and DOT fluid. Mineral oil is generally inert, while DOT is corrosive to frame paint but has a higher boiling point. Using the wrong fluid may cause the seals to swell or become corroded.
Single vs dual actuation
Many disc brakes have their pads actuated from both sides of the calliper, 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. Most mechanical discs have a manual control to adjust the pad-to-rotor gap. Callipers are now generally made in one piece to increase stiffness and reduce the threat of leaks, but the two-piece design still reduces heat build-up more effectively, and most top-end models still have a two-piece calliper. Also many top end callipers have four or more pistons as lower end models usually only have one or two.
Calliper mounting standards
There are many standards for mounting disc brake callipers. I.S. (International Standard) is different for 6-inch (150 mm) and 8-inch (200 mm) rotor and differs between forks with a QR and 20 mm through axle. The post-mount standard also differs by disc size and axle type. Many incompatible variants were produced over the years, mostly by fork manufactures.[citation needed] 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 adapters to fit their brakes to various mounting patterns.[citation needed]
Advantages and disadvantages of various types of mounts
A disadvantage of post mounts 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 callipers when using post mount [citation needed]. A limitation of the mount is that the location of the rotor disc is more tightly constrained: it is possible to encounter incompatible hub/fork combinations, where the rotor is out of range. With an IS mount, the calliper can be moved closer to or further from the mount point using spacers; this can permit a wider range. [citation needed]
Disc mounting standards
There are many options 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 (typically M5x0.8x10mm with locking patch) with either a hex socket or Torx socket to secure them to the hub. This can make IS rotors more time consuming to remove. Torx screws are preferred for the superior torque: it is easy to strip the socket of a hex bolt by over tightening it, leaving a rotor that is hard to remove.[citation needed]
Standards
- International Standard (IS) (in widespread use)
- Centerlock (Shimano proprietary, in widespread use)
- Cannondale's 4-bolt pattern (obsolete)
- Rohloff's 4-bolt pattern (proprietary)
- Hope Technologies' 5-bolt pattern (Hope proprietary, obsolete)
- Hope Technologies' 3-bolt pattern (Hope proprietary)
- Rock Shox 3-bolt pattern (proprietary, obsolete)
Disc sizes
Disc brake rotors come in many different sizes, generally 160 millimeter, 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 millimeters. 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.[17] 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 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
Bicycle drum brakes operate like those of a car, although the bicycle variety use mechanical rather than hydraulic actuation. Two pads are pressed outward against the braking surface on the inside of the hub shell. Shell inside 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. Both cable- and rod-operated drum brake systems have been widely produced.
A Roller brake is a modular cable-operated drum brake for use on specially splined front and rear hubs. Unlike a traditional drum brake, the Roller Brake can be easily removed from the hub. It also contains a torque-limiting device called a power modulator designed to make it difficult to skid the wheel. In practice this can reduce its effectiveness on bicycles with adult-sized wheels.
Drag brakes are drum brakes intended to slow down a bicycle on long downhills rather than stop it. Drag brakes are employed on some tandem bicycles in mountainous areas where extended use of rim brakes can cause the rim to become hot enough to cause a blowout.[18] 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.
Advantages and Disadvantages
Drum brakes are useful for wet or dirty conditions since the braking mechanism is fully enclosed. They are heavier, more complicated, and often weaker than rim brakes, but require less maintenance. They are most common on utility bicycles in some countries, especially the Netherlands, and are also often found on freight bicycles.
Coaster brakes
The coaster brake, also known as a back pedal brake or foot brake (or torpedo in some countries), is a type of drum brake integrated into hubs with an internal freewheel. Freewheeling functions as with other systems, but when back pedaled, the brake engages after a fraction of a revolution. The coaster brake can be found in both single-speed and internally geared hubs.
When such a hub is pedaled forwards, the sprocket drives a screw which forces a clutch to move along the axle, driving the hub shell or gear assembly. When pedaling 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 chain wheel and often use 1/8" wide chain. However, there have been several models of coaster brake hubs with dérailleurs, most notably the Sachs 2x3. These use special extra-short dérailleurs which can stand up to the forces 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, and the Shimano Nexus 3-speed.
Advantages and Disadvantages
Coaster brakes have the advantage of being protected from the elements and thus perform well in rain or snow. Though coaster brakes generally go years without needing maintenance, they are more complicated than rim brakes to repair if it becomes necessary. Coaster brakes also do not have sufficient heat dissipation for use on long descents. A coaster brake can only be applied when the cranks are reasonably level, limiting how quickly it can be applied. As coaster brakes are only made for rear wheels, they have the disadvantage common to all rear brakes of skidding the wheel easily. This disadvantage may, however, be alleviated if the bicycle also has a hand-lever-operated front brake and the cyclist uses it. As backpedaling is not possible with a coaster brake, it is necessary to place both feet on the ground, and then place the non-braking foot on the forward pedal to restart efficiently. A coaster brake is therefore not compatible with toe clips and straps, or with a clip-in shoe-pedal system.
Brake Levers
Brake levers are usually mounted on the handlebars within easy reach of the rider's hands. They may be distinct or integrated into the shifting mechanism. Road bicycles with drop handlebars may have more than one brake lever for each brake to facilitate braking from multiple hand positions.
Mechanical
Mechanical (cable) brake levers come in two varieties based on the amount of brake cable that they pull for a given amount of lever movement:
- Standard pull levers work with most brake designs, including calliper brakes and traditional cantilever brakes.
- Long pull levers work with "direct-pull" cantilever brakes, such as Shimano "V-Brakes"
The mechanical advantage of the brake lever must be matched to the brake it is connected to in order for the rider to have sufficient leverage to actuate the brake.
For example brake levers designed for calliper brakes may work with centre-pull cantilevers, but not with direct-pull, and linear-pull brakes. Direct pull cantilevers have twice as much mechanical advantage as traditional brakes, so they require a lever with half as much mechanical advantage. Long pull levers pull the cable twice as far, but only half as hard.[19]
Some bikes, especially road bikes with drop handlebars are set up with multiple levers to actuate a single brake. Levers that allow the rider to work the brakes from the tops of the bars, introduced in the 70s, are called extension levers or safety levers.[20] The modern equivalent are called interrupt brake levers and are considered superior.[21]
Hydraulic
Hydraulic brake levers push the hydraulic fluid down a hose towards the brakes.
Braking technique
There are several techniques for efficient braking on a standard, two-brake bicycle. The one most commonly taught is the 25-75 technique. This method entails supplying 75% of the stopping power to the front brake, and about 25% of the power to the rear. Since the bicycle's deceleration causes a transfer of weight to the front wheel, there is much more traction on the front wheel. Therefore, the rear brake can exert less braking force than on the front before the rear wheel starts skidding. For a more-detailed analysis, see Bicycle and motorcycle dynamics.
If too much power is applied to the front brake, then the momentum of the rider propels him/her over the handlebars, thereby flipping the bicycle. The skidding of the rear wheel can serve as a signal to reduce force on the front brake; a skillful cyclist in effect becomes a human anti-lock braking controller.
Some front brakes have a spring that limits the applied force; this is easier to use but limits the braking force and cannot compensate for changes in brake effectiveness due, for example, to a wet rim or overheated brake disc. On tandem bicycles and other long-wheel-base bicycles (including recumbents and other specialized bicycles), the lower relative centre of mass makes it virtually impossible for heavy front braking to flip the bicycle; the front wheel would skid first. On these bicycles, the safest quick stop is achieved with a somewhat higher proportion of force at the rear brake.
A skillful bicyclist often will use the front brake alone for moderate braking when riding on a good, paved surface. As the front wheel does not skid, the front brake poses less risk of loss of control, and does not cause rapid tire wear.
In some situations, it is advisable to slow down, and to use the rear brake more and the front brake less:
- When unfamiliar with the braking characteristics of a bicycle. It is important to test the brakes and learn how much hand force is needed when first riding a bicycle.
- When leaning in a turn (or preferably, brake before turning).
- Slippery surfaces, such as wet pavement, mud, snow, ice, or loose stones/gravel. It is difficult to recover from a front-wheel skid on a slippery surface, especially when leaned over.
- Bumpy surfaces: If the front wheel comes off 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 may flip the rider over the handlebar.
- Very loose surfaces (such as gravel and loose dirt): In some loose-surface situations, it may be beneficial to completely lock up the rear wheel in order to slow down or maintain control. On very steep slopes with loose surfaces where any braking will cause the wheel to skid, it can be better to maintain control of the bicycle by the rear-brake more than one would normally. However neither wheel should stop rotating completely, as this will result in very little control.
- Wet weather conditions, when the road surfaces are generally more slippery.
- Long descents: alternating the front and back brake can help prevent hand fatigue and overheating of the wheel rims which can cause a disastrous tire blow-out.
- Flat front tire: braking a tire that has little air can cause the tire to come off the rim, which is likely to cause a crash.[22]
It is customary to place the front brake lever on the left in right-side-driving countries, and vice versa,[23] because the hand on the side nearer the centre of the road is more commonly used for hand signals, and the rear brake can not pitch the bicyclist forward. However, a skilful bicyclist does better with the front brake on the side that is less often used for hand signals. In an emergency situation, operation of the brake has to be second nature; an unskilled bicyclist could find reversed brake levers confusing. Fortunately, it is usually easy to switch brake cables. The standard motorcycle configuration has the right-hand lever operating the front brake, and the left-hand lever operating the clutch. The levers are generally not easily reversed, leading to difficulties for a person who has learned on a bicycle with the front brake lever on the left.
Bicycles without brakes
Track bicycles are built without brakes so as to avoid sudden changes in speed when racing on a velodrome. Since they have a fixed gear, braking can be accomplished by reversing the force on the pedals while the rider shifts their weight forward to accomplish a skid. [citation needed]
Fixed gear road bikes may also lack brakes. For safety reasons many fixed gear bikes are, however, fitted with a front brake, which is a legal requirement for a road bike in some jurisdictions.
Some modern BMX bikes do not have brakes. The usual method of stopping is for the rider to put one or both feet on the ground, or to wedge a foot between the seat and the rear tire, but this can be very dangerous and is not recommended for riding on public highways or where a rider may collide with bystanders.
See also
References
- ^ Hudson, William (2008). "Myths and Milestones in Bicycle Evolution". Jim Langley. Retrieved 2009-09-22.
- ^ Rim brakes have been the subject of countless "engineering innovations". Some of the more unusual results can be seen here.
- ^ Brown, Sheldon. "Adjusting Cantilever Brakes". Sheldon Brown. Retrieved 2009-07-31.
- ^ http://www.sheldonbrown.com/canti-direct.html
- ^ Pictures of a rear and a front roller cam brake.
- ^ Private communication by Jeff Archer of the Museum of Mountain Bike Art & Technology [1]
- ^ Brown, Sheldon. "Sheldon Brown's Bicycle Glossary". Sheldon Brown. Retrieved 2009-07-31.
- ^ "Retrobike Gallery and Archive". Retrieved 2009-08-03. Drawings and technical description.
- ^ An example of the toggle integral to the toggle cam brake appears here. The cam surface is the upper edge of the 'tail' on the large central piece. The cable attaches to one of the three holes. Note that the cam must flare at twice the rate of the two-sided roller cam design in order to move the arms the same amount.
- ^ A picture of the rare sabre cam brake may be found at MOMBAT - click on the 2nd thumb from the left.
- ^ Heine, Jan (2008). "Slow Down, The Story of Bicycle Brakes". Bicycle Quarterly (Winter 2008). Vintage Bicycle Press LLC: 36. ISSN 1941-8809.
{{cite journal}}: Cite has empty unknown parameter:|coauthors=(help) - ^
Brandt, Jobst (October 2005). "Brakes from Skid Pads to V-brakes". sheldonbrown.com. Retrieved 2009-01-22.
{{cite web}}: Check|authorlink=value (help); Cite has empty unknown parameter:|coauthors=(help); External link in|authorlink= - ^ Sutherland, Howard (1995). Sutherlands Handbook for Bicycle Mechanics (6th Edition). Berkley, CA, USA. pp. 13.27 to 13.28. ISBN 0-914578-09-X.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: location missing publisher (link) - ^ Brown, Sheldon. "Brakes for Tandem Bicycles". Sheldon Brown. Retrieved 2007-10-19.
- ^ Heine, Jan (2008). "Slow Down, The Story of Bicycle Brakes". Bicycle Quarterly (Winter 2008). Vintage Bicycle Press LLC: 39. ISSN 1941-8809.
{{cite journal}}: Cite has empty unknown parameter:|coauthors=(help) - ^ Annan, James. "Disk brakes and quick releases - what you need to know". Retrieved 2007-10-19.
- ^ "Disc Brake weight listing". Retrieved 2006-11-07.
- ^ "Sheldon Brown Glossary: Drag Brake". Retrieved 2008-05-20.
- ^ Brown, Sheldon. "Sheldon Brown Glossary: Brake". Sheldon Brown. Retrieved 2008-02-01.
- ^ "Sheldon Brown's Glossary: Extension levers". Retrieved 2009-09-23.
- ^ "Sheldon Brown's Glossary: Interrupter Brake Levers". Retrieved 2009-09-23.
- ^ Brown, Sheldon. "Braking and Turning your Bicycle". Sheldon Brown. Retrieved 2007-10-19.
- ^ Brown, Sheldon. "Cables: Right front or left front?". Sheldon Brown. Retrieved 2009-08-06.
- Bicycle Glossary from Sheldon Brown's website
- Template:Sv icon Ekström, Gert (2001). Älskade cykel (in Swedish) (1st ed.). Bokförlaget Prisma. ISBN 91-518-3906-7.
{{cite book}}: Cite has empty unknown parameters:|accessyear=,|origmonth=,|accessmonth=,|month=,|chapterurl=, and|origdate=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: unrecognized language (link)