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Bicycle brake systems
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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 tyres.
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Encyclopedia
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 tyres. 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.
With the invention of pneumatic tyres in the 1890s came the rim brake, the type of brake most commonly used on bicycles today. However, in American 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.
Types
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 and mounted in metal "shoes") to contact the rim of the rotating wheel, thus slowing it and the bicycle. The following are among the many sub-types of rim brakes:
Rod-actuated brakes
These use a series of rods and pivots (rather than Bowden cables) to transmit the braking force from the hand levers 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. To fit the rear wheel and the more directly actuate front wheel, they use a mechanism to transmit the force but still 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 in areas where replacement Bowden cables are not available or are too expensive. They are still used on some bicycles, typically roadsters, particularly in East and South Asia. 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 which apply the pads on opposite sides of the rim.
Single pivot side-pull calliper brakes
These 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 tyres, but have significant flex, and resulting poor performance, if made big enough to fit wide tyres. 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.
Centre-pull calliper brakes
These 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.
Dual-pivot calliper brakes
This type is 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, and resulting better braking. 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
These 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 (also known by the trademarked term "V-brakes")
These mount on the same frame bosses as cantilever, 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 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 calliper brakes or traditional cantilever brakes. This cable pull ratio was later adopted for disc brakes when they were introduced, making the "V-brake" levers standard for mountain bikes.
U-brakes
This type is also referred to as 990-style after the popular Dia-Compe model which set the standard for U-brake frame and fork pivot mounting locations. The U-brake has the pivots for the arms mounted to the frame or fork on each side above the rim. The arms cross over in the same manner as centre-pull brakes; in fact, U-brakes and centre-pull calliper brakes are essentially the same design; the difference is they are used on different types of bike, and U-brakes are usually mounted on pivots integral to the frame or fork, centre pull mount via a bridge to a single bolt like calliper. U-brakes are the current standard on Freestyle BMX frames and forks, and were also used on some mountain bikes through the early 1990s. The U-brake's main advantage over cantilever and linear-pull brakes is that sideways protrusion of the brake and cable system is less. This is especially valuable on freestyle BMX bikes, where any protruding parts are liable to get damaged, and preventing interference between the brake and the rider's body or the drivetrain is a difficult design task with any other type of rim brake.
Hydraulic rim brakes
These 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.
Delta brakes
Made most promently by Campagnolo in 1985, but also manufactured by Weinmann, and others, is a road bicycle brake, named due to its triangular shape. They are very uncommon, and are no longer manufactured. 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. It has been criticized for being heavy, giving mediocre stopping power, and suffering disadvantageous variable mechanical advantage.
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 aluminium, 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 unravelled. If the inner cables are not replaced when they fray, they can suddenly break when brakes are applied strongly, causing brakes to be lost 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 can increase tyre pressure so much that the tyre 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 tyres and adopting an aggressive riding style, only braking for the corners, but the real solution is a drum brake or a disc brake which avoids rim heating.
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.
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 . 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.
Advantages
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 disk 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. Disk 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 as such 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 also makes disc brakes necessary, as rim brakes are not designed to straddle such a wide tyre.
The advantages of discs 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.
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 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 precludes the use of many types of pannier racks; for this reason, disc brakes are rarely found on touring bikes, although several rack manufacturers are addressing this issue.
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.
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 be 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 calipers 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. IS (International Standard) is different for 6-inch and 8-inch rotor and differs between forks with a QR and 20mm thou 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 speciality 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. 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 callipers when using post mount . 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 .
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 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.
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.
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
Drum brakes are useful for wet or dirty conditions. They are heavier, more complicated, and often 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. They are most common on utility bicycles in some countries, especially the Netherlands, and are also often found on freight bicycles.
A bicycle drum brake operates like those of a car, but has no ratcheting 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 traditional drum brake, the Roller Brake can be easily removed from the hub. 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 tyre 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 ˝" 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.
Coaster brakes have the advantage of being protected from the elements and thus immune to ice or water. However, they provide limited braking force before the rear wheel begins to skid. This is because the rider's weight is ahead of the rear tyre's contact with the ground, and because the weight transfers forward proportional to braking force, which unloads the rear wheel. Also, although coaster brakes generally go years without needing maintenance, they are more complicated than rim brakes to repair if it becomes necessary. Also, coaster brakes cannot be used while 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.
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.
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 (cable)
Mechanical 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 calliper brakes or traditional centre-pull 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.
Hydraulic
Levers for hydraulic brakes push the hydraulic fluid down a hose toward the brakes.
Braking technique
There are several wide-spread techniques for efficient braking on a standard, two-brake bicycle. The most commonly taught and used one is the 25-75 technique. This method entails supplying 75% of the stopping power to the front brake, and about 25% of the power for the rear. This is one of the most effective means of slowing a bicycle. The reason is that during any braking, the bicycle's deceleration causes a transfer of weight to the front wheel. This means that there is much more traction on the front wheel than the back wheel. Therefore (especially in wet conditions), the rear brake can exert much less braking force before the wheel starts skidding on the ground. For a more-detailed analysis, see Bicycle and motorcycle dynamics.
But if too much power is applied to the front brake, then the momentum of the rider propels him/her over the handle bars, thereby flipping the bicycle. Some front brakes have a spring that limits the applied force; this is easier to use but limits the braking force. On tandem bicycles and other long-wheel-base bicycles (including recumbents and other specialized bicycles), their long wheelbase and lower relative centre of mass makes it virtually impossible for heavy front braking to flip the bicycle.
In some situations, it is advisable to slow down, and to use the rear brake more and the front brake less:
- 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 front bars.
- 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.
- 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.
Bikes without brakes
Track bicycles are built with no brakes, for safety, so as to avoid sudden changes in speed when racing on a velodrome. Since they have a fixed gear, braking can be done by reversing the force on the pedals.
Some modern BMX bikes do not have brakes. The usual method of stopping is for the rider to put one or more 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.
See also
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