Bicycle brake systems
|
|
Bicycle brake systems are used to stop a bicycle. There have been various types through history, and several types still in use today.
| Contents |
History
Early bicycles such as the high wheeled penny-farthing bikes had no brakes as we would recognize them. As the machines were fixed gear bicycles a rider could reduce speed by reversing the thrust on the pedals. Otherwise a rider who wanted to stop quickly had to jump off the bike as it was moving. Unsurprisingly there were many accidents, many of them fatal, which limited the appeal of cycling, mostly to young and adventurous men.
The 1870s saw the development of the "safety bicycle" which was roughly the bicycle we would recognize today, with two wheels of equal size, initally with solid rubber tires. The braking system these bicycles used was often a simple leather pad which pressed against the top of the tire, which was driven by a rod attached to a lever on the handlebar, and there was no rear brake, although with no free wheel back-pedalling was an option. This was undoubtedly a big improvement on having no brakes at all, but it was not very powerful and had the big drawback that it was almost useless in wet weather.
With the introduction of the pneumatic tire also came the next advance in bicycle braking around the 1890s, the invention of the rim brake. This is the type of brake most commonly used on bicycles today. Track bicycles, however, which are ridden at top speed continuously, continue to be built with no brakes; since they are fixed gear bicycles, braking on these bikes is still achieved only by reversing the force on the pedals. This is not a disadvantage for the very specialized manner in which these bicycles are ridden, and the reduction in weight is advantageous for performance.
Rim brakes
There are several types of rim brakes. In all of these designs, the braking force is applied by the rider squeezing a lever mounted on the handlebar; this causes friction pads (usually made of leather or rubber) to contact the rim of the rotating wheel, thus slowing it and the bicycle.
Rod brakes use a series of rods and pivots (rather than Bowden cables) to pull the friction pads upwards onto the inner surface (facing the hub) of the wheel rim. They were often called "stirrup brakes" due to their shape. In order to fit the rear wheel as well as the more directly actuated front wheel, these brakes need a mechanism to transmit the force but still allow rotation where the fork attaches to the frame. Although rod brakes are heavy and the linkage is complex, they are reliable and durable and can be repaired or adjusted with simple hand tools in areas where replacement Bowden cables are not available or are too expensive. They are still used on some bicycles, particularly in the Far East.
More modern designs use similar friction pads to squeeze the sides of the wheel rims, with the force transmitted to the brake from the lever by means of a Bowden cable. Designs include the scissor-action "side pull" and "centre pull" brakes, and the lever action "cantilever" and "V" brakes.
Sidepull caliper brakes are widely used on the most inexpensive bikes, as well as on the most expensive high-quality road bikes. They consist of two curved arms that cross at a pivot above the wheel and hold the brake pads on opposite sides of the rim. These arms have extensions on one side; the cable housing is attached to one extension and the inner cable to the other, so that when the brake lever is squeezed, the arms move together and the brake pads squeeze the rim. These brakes are simple and effective when designed for relatively narrow tires, but have serious disadvantages if made big enough to fit wide tires. The cheaper varieties also have a tendency to rotate to one side during actuation and to stay there, so that one brake pad continually rubs the rim even when the brake is released. Centering of sidepull brakes was greatly simplified with the mass-market adoption of dual-pivot sidepulls (an old design re-discovered by Shimano in the early 1990s) and these brakes also offer a higher mechanical advantage. Dual-pivot brakes have slightly higher weight than conventional sidepull calipers and cannot accurately track an out-of-true rim.
Centerpull caliper brakes were developed to fill the price niche between the cheaper sidepull brakes and the more expensive ones, being reasonably priced but without the tendency of the cheaper sidepull brakes to have one brake pad continually rub the rim. In this type of brake the actuation of the two brake arms is designed to be symmetrical. The cable housing is attached to a fixed cable stop attached to the frame, and the inner cable attaches to a sliding piece or sometimes a small pulley, over which runs a straddle cable which connects the two brake arms. Thus tension on the cable is evenly distributed to the two arms, preventing the brake from taking a "set" to one side or the other.
The wider tires on mountain bikes present a problem with standard calipers because the long distance from the pivot to the pad allows the arms to flex, reducing braking effectiveness. In higher-quality fat tire bikes this was solved by using cantilever or V brakes mounted to two separate pivots on the frame or fork just below the rim. Cantilever brakes have each arm attached to a separate pivot point on one side of the frame or fork, usually just below the rim. The traditional cantilever has an L shaped arm protruding outwards on each side, with a cable stop on the frame or fork to hold the cable housing and a straddle cable between the arms similar to the centerpull brake. The cable from the brake handle pulls upwards on the straddle cable, causing the brake arms to rotate up and inward and squeezing the rim between the brake pads. V brakes (sometimes called linear- or direct-pull) mount similarly, but the arms extend straight up, and the outer housing is attached to one arm and the inner housing to the other, similar to the cable attachment for sidepull brakes.
Closely related is the U brake; this type has the pivots for the arms mounted to the frame or fork on each side above the rim. The arms cross over similarly to sidepull or centerpull brakes, and BMX bikes; its main advantage was that it did not protrude sideways from the frame like the early cantilevers. This advantage was reduced by redesigned low-profile cantilevers, and nearly eliminated with V brakes.
Another design is the delta brake. In this design, the pivot points for the arms are above the rim but the arms do not cross over, and instead of a straddle cable, the inner brake cable attaches to a wedge shaped piece between the brake arms; when the brakes are applied the wedge forces the arms apart at the top, squeezing the rim between the pads. This has an advantage in that the shape of the wedge can be varied other than straight-sided, to allow for a very high mechanical advantage at the point where the pads contact the rim to give high braking power, but a lower mechanical advantage when the pads are not contacting the rim so that the pads move well away from the rim when the brake is not applied, preventing any rubbing.
The advantages of rim brakes are that they are inexpensive, lightweight, mechanically simple, and easy to maintain, as well as very powerful. The main disadvantage of rim brakes is that their performance deteriorates in wet weather when the rims are wet. This problem is less serious on more expensive bikes which use rims made of aluminum alloys than on those with steel rims. They are also prone to clogging with mud, particularly when mountain biking. Rim brakes also need regular maintenance. Brake pads wear down quickly, and have to be replaced. Over long periods of time and use, rims also become worn. Bowden cables can jam if not regularly lubricated or if water gets into the housing, causing corrosion. The cables also wear, requiring frequent checking and replacement. If the inner cables are not replaced when they fray, they will suddenly break when brakes are applied strongly, causing brakes to be lost precisely when they are most needed. Rim brakes also require that the rim be relatively true; if the rim has a pronounced wobble, either side to side or inwards and outwards, it will be impossible to adjust the brake pads so that they can apply sufficient pressure to the rim but do not rub when released and do not contact the tire, which would lead to a rapid blowout.
Rim brakes also suffer from the disadvantage of rim heating, because the brake converts kinetic energy into heat, which increases the temperature of the rim (the brake caliper and frame do not become hot because brake pads are excellent thermal insulators). In normal use and with lightweight bicycles this does not present a problem as the brakes are only applied with a limited force and for a short time, with the heat being quickly disspated to the surrounding air. However, on heavily-laden touring bikes and tandems in mountainous regions the heat build-up can be sufficient to increase tire pressure to the extent that the tire blows off the rim. If this happens on the front wheel a serious accident is almost inevitable. The problem is worse when descending cautiously at slow speeds because the brakes are "always on" and the cooling airflow over the rim is insufficient. The risk can be minimised by not over-inflating tires and the adoption of an aggressive riding style, only braking for the corners, but the real solution is a hub brake which allows the rim to remain cool.
Brake pads are of many designs. Most consist of a replaceable rubber pad held in a metal channel, with a post or bolt protruding from the back to allow attachment to the brake, but some are made as one piece with the attachment directly molded in the pad for greater mechanical stability. The rubber can be softer for more braking force with less lever effort, or harder for longer life. The rubber can contain abrasives for better braking, at the expense of rim wear. Compounds vie for better wet braking efficiency. Typically pads are relatively short, but longer varieties are also manufactured to provide more surface area for braking; these often must be curved to match the rim. Incidentally, a larger pad does not give more friction, but it does wear more slowly and therefore a new pad can be made thinner, simplifying wheel removal with V-brakes in particular. In general, a brake can be fitted with any of these many varieties of pads, so long as the pad mounting method is compatible. Carbon rims, as on some disc wheels, generally have to use non-abrasive cork pads.
Disc brakes
Disc brakes consist of a metal disc attached to the wheel hub that rotates with the wheel. Attached to the frame or fork will be calipers and pads that squeeze together on the disc. Although this type of brake has been used on motorbikes for decades, only recently have they been added to bicycles. They are most suitable for and used mainly on mountain bikes ridden off road. They also are used on hybrid bicycles and some road bicycles, although this is less common. Many tandem bicycles have a disc brake fitted on the rear wheel in addition to rim brakes; the disc brake can be set to provide a constant drag, so that during long descents, the rim brakes are not overworked by this heavier machine.
The main advantage of disc brakes is that their performance is equally good in all conditions including water and mud. They also avoid the problem that rim brakes have of wearing out the wheel rims, especially in muddy conditions, as well as the requirement that the rim be true.
On some expensive disk brake models, a hydraulic system is used to push the pad instead of a cable, eliminating the problems of cable maintenance and breakage.
The disadvantages are that they are usually heavier and more expensive than rim brakes, and in most cases require a special hub.
Hub brakes
Hub brakes are drum brakes that have their mechanism enclosed within the hub of the wheel, and are usually fitted to the back wheel. Because they are enclosed, hub brakes are completely unaffected by the weather. Some types of hub brake are operated by cables and levers, in the same way as rim and disc brakes. Other types are operated by the rider turning the pedals backwards. These are known as "back pedaling brakes", or "coaster brakes".
Hub brakes are used mainly on utility bicycles, and also on some tandems used in mountains. In the tandem use, the drum is not intended to stop the bike. Instead they are used to keep the speed down on long downhill sections where extended use of rim brakes can cause overheating, similar to the use of disc brakes on tandems, discussed above.
In addition to being impervious to changes in the weather, hub brakes have the advantage of needing very little regular maintenance, especially the back-pedaling type.
Although hub brakes go for extended periods without maintenance, often their entire lifetime, they will eventually need to be dismantled and re-greased, usually by a professional, and sometimes they break and need repair. When hub brakes do require maintenance, it is far more complicated than other braking systems. Hub brakes are also heavier than other types of bicycle brake. Coaster brakes are not compatible with derailleur gears, where the freewheel built into the gear cluster on the rear wheel prevents back pedaling. Coaster brakes are also, by their basic design, inherently much less powerful than other braking systems, for the simple reason that they can only be installed on the rear wheel, which can provide much less braking force before skidding than the front wheel can.
Braking technique
Effective use of a bicycle brake is highly counter-intuitive. The casual rider will at first avoid using the front brake, due to the unsettling feeling of "toppling up", or fear of being sent flying over the handlebars.
However, the most effective technique is to use the front brake almost exclusively. There are several exceptions where the rear brake is preferred; these are listed below. In any stop, the rider should shift their weight toward the rear and use their arms to brace against the deceleration.
During braking (either with the front or rear brake), the bike deceleration causes a transfer of weight to the front wheel. This means that the front wheel has a greater normal force pressing it against the ground, and the back wheel nearly none. Therefore, the front wheel can generate more frictional braking force than the back wheel before locking up and skidding. It is nearly impossible to lock the front wheel while braking. In any conditions and especially in wet conditions or going downhill, the rear brake can exert relatively little braking force before the wheel locks and starts skidding. A skidding rear wheel can lead to dangerous, uncontrollable bicycle movements eventually resulting in the cyclist falling on the ground.
In an emergency stop, it is important to grab the front brake and press it hard to stop in the minimum possible distance. The rider should shift their weight as far to the rear as possible. Maximum deceleration is accomplished by maintaining enough pressure on the front brake such that that the rear wheel is barely touching the ground, just before flipping over the handlebars. In the real world this is not practical. Instead, use light pressure on the back wheel, hard pressure on the front. The back wheel is primarily useful as an indicator—when it starts to skid, reduce the pressure to both brakes to prevent flipping over the front wheel, then increase both again. Incidentally, on tandems, with their greater weight and long wheelbase, it is virtually impossible for heavy front braking to cause the machine to flip.
There are a few special situations where limited use of the front brake, and heavier involvement of the rear brake is advisable:
- Slippery surfaces. It is nearly impossible to recover from a front-wheel skid, so on surfaces like wet pavement, mud, snow, or ice, when skidding is likely, reduced speed and use of the rear brake is preferred.
- Bumpy surfaces. If the front wheel comes off of the ground, braking will stop it completely. Coming down on a stopped front wheel is very dangerous.
- Front flat tire. Braking the front wheel when the tire is flat could cause the tire to come off of the rim, which is more likely to cause a crash.
- Broken cable, other mechanical failure. When the front brake fails, the only braking force can come from the rear brake.
External links
- Braking and Turning your Bicycle (http://www.sheldonbrown.com/brakturn.html) by Sheldon Brown