There are several types of rim
brakes on mountain bikes,
but the most common types are cantilever and linear pull brakes. Braking force is applied by the rider squeezing a lever mounted on the handlebar,
which causes friction pads (usually made of leather or rubber) to contact the rim
of the rotating wheel,
thus slowing it and the bicycle.
Cantilever, direct pull, and linear pull brakes have each arm attached to a separate pivot point on one side of the seat stay
or fork just below the rim.
This solves the problem for standard calipers on wide tires
(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 center-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
Linear pull brakes (sometimes referred to by the trademarked name "V-brakes") mount similarly to cantilever brakes; however, the arms extend straight up, and the housing is attached to one arm and the cable to the other (similar to the cable attachment for side pull brakes). They are generally more powerful and easier to adjust than cantilever brakes, but require a smaller gap between the brake pad and the rim
surface. They function well with the suspension
systems found on many mountain bikes
because they do not require a separate cable stop on the frame
or fork. Due to their higher mechanical advantage, linear pull brakes require levers with longer cable travel
than levers intended for caliper brakes or traditional cantilever brakes.
Disc brakes consist of a metal disc attached to the wheel hub
that rotates with the wheel.
Calipers are attached to the frame
or fork along with pads that squeeze together on the disc. Although these brake types have been successfully used on, and have been the principal choice for motorbikes for decades, numerous (and partly successful) attempts at introducing disc brakes for bicycles over the past few decades may have finally reached fruition. Recent advances in lower weights, less costs, and more reliable material have led
to the development and implementation of disc brake systems by several firms to the extent that they are becoming a standard feature on many bicycles (and are almost certainly here to stay). Disc brakes are most suitable for and used mainly on mountain bikes
The main advantage of disc brakes is that their performance is equally good in all conditions, including water, mud and snow. This is due to their position closer to the hub
and away from the ground (including possible contaminants like water, which can coat and freeze on the rim
in colder temperatures). They also avoid the problem that rim
brakes have of wearing out the wheel
rims, especially in muddy conditions, because they don't build up heat on the rim
of the tire.
Another advantage that is very useful to the average rider is that disk brake performance isn't effected by the trueness of the rim,
which eliminates unwanted rubbing between a rim
brake and the rim.
Also, disc brakes offer better modulation of braking power and generally require less finger effort to achieve the same braking power. The advantages of discs make them well-suited to steep, extended downhills
through wet and muddy off-road terrain, which falls under the category of downhill
and freeride bicycle riding. The use of tires
as large as 3.0 inches wide also makes disc brakes a necessity, as rim
brakes simply cannot straddle a tire
The disadvantages of disc brakes are that they are usually slightly heavier and more expensive than rim
brakes, they require a hub
built to accept the disc, and a bicycle frame
or fork specially built to accept the caliper. Also, rigid forks on road bikes that are made to handle the forces of a front disc brake are heavier and may not have the ride quality of a regular fork.
Furthermore, a disc brake puts more stress on a wheel's spokes
than a rim
brake, since the torque of braking occurs between the hub
and the rim
with disc brakes, unlike with rim
brakes. For this reason, cross-lacing of spokes
is usually employed with disc brakes, while rim
brakes sometimes allow the option of slightly lighter radial lacing.
Hydraulic vs Mechanical
Two main disc brake systems exist: hydraulic and mechanical (cable-actuated).
Mechanical disc brakes (which are almost always less expensive than hydraulic) have less modulation than hydraulic disc brake systems. And since the cable is usually open to the outside, mechanical disc brakes tend to pick up small bits of dirt in the cable lines when ridden in harsh terrain.
Hydraulic disc brakes use fluid from a reservoir
pushed through a hose to activate the pistons in the disc caliper, that in turn actuate the pads. Hydraulic disc brake systems generally keep contaminants out better than mechanical. However, since hydraulic disc brakes usually require relatively specialized tools to bleed the brake systems, repairs on the trail are difficult to perform, whereas mechanical disc brakes rarely fail completely. Hydraulic disc brakes occasionally require bleeding of the brake lines to remove air bubbles.
There are two types of brake fluid used in disc brakes today: mineral oil and DOT fluid. Mineral oil is generally inert and while DOT has a higher boiling point, it is known to be corrosive to frame
paint. The two are generally not interchangeable as the different fluids may cause seals to swell or be corroded. Also, the hydraulic fluid may boil on steep, continuous downhills.
This is due to heat building up in the disc and pads and can cause the brake to lose its ability to transmit force through incompressible fluids, since some of it has become a gas, which is compressible. To avoid this problem, 203 mm (8 inch) diameter disc rotors have become common on downhill
bikes. Larger rotors dissipate heat more quickly and have a larger amount of mass to absorb heat. For these reasons, one must weigh the advantages and disadvantages of using a hydraulic system versus a mechanical system.
Disc Mounting Standards
There are many standards for disc rotor mounting: International Standard (IS), centerlock, Cannondale's 4 bolt pattern, Hope's 5 bolt pattern and Rohloff's 4 bolt pattern, and many others.
IS is a six-bolt mount and is the industry standard. An advantage of IS six-bolt mounts is 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. 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, and a Shimano cassette
lockring tool is needed to remove the rotor and it is more expensive and less common than a Torx key.
Disc brake rotors come in many different sizes (generally 150mm, 177mm, or 203mm in diameter) as all brake manufacturers make discs specific to their calipers and the dimensions often vary by a few millimeters. SRAM (Avid) has even introduced a 140mm rotor intended for rear use on cross country
and road bikes.
Larger rotors provide greater stopping power by virtue of a longer moving arm for the caliper to act on. 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.
Smaller rotors provide less stopping power, but also less weight. Cross country
racers will typically run smaller rotors, which can easily handle the much smaller braking loads and offer 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 dynamics
of braking, which shifts most of the rider weight to the front wheel
during braking. This provides greater traction at the front wheel
and allows for greater braking force. Conversely the weight shift off of the rear wheel
does not allow for much braking force. Using a smaller rear rotor will save weight and allow for better modulation of the rear brake while more efficiently using the brakes' capacity.
Effective use of a bicycle brake is highly counter-intuitive. The casual rider will at first avoid using the front brake, due to the unsettling feeling of "toppling up", or fear of being sent flying over the handlebars.
However, the most effective technique for powerful stopping is to use the front brake almost exclusively. There are several exceptions where the rear brake is preferred, these are listed below. In any stop, the rider should shift their weight toward the rear and use their arms to brace against the deceleration.
During braking (either with the front or rear brake), the bike deceleration causes a transfer of weight to the front wheel.
This means that there is more force pressing the front wheel
to the ground and nearly none on the back wheel.
Therefore, the front wheel
can generate more frictional braking force than the back wheel
before locking up and skidding. In any conditions, and especially in wet conditions or going downhill,
the rear brake can exert relatively little braking force before the wheel
locks and starts skidding.
A skidding rear wheel
can lead to dangerous, uncontrollable bicycle movements eventually resulting in the cyclist falling to the ground. A key scenario for this is "light" braking on rapid alpine-type hairpin (serpentine) descents. Further attenuation of speed to negotiate a decrease in curve radius (tightening of the bend in the road) can be executed successfully while pulling through a tight corner regulating with the front brake only.
In an emergency stop, it is important to grab the front brake and press it hard to stop in the minimum possible distance. The rider should shift his or her weight as far to the rear as possible to avoid flipping over the handlebars.
Maximum deceleration is accomplished by maintaining enough pressure on the front brake such that the rear wheel
is barely touching the ground, just before lifting up. In reality this is not practical for most cyclists. Instead, use light pressure on the back wheel
and hard pressure on the front. The back wheel
is primarily useful as an indicator; i.e. when it starts to skid, reduce the pressure to both brakes to prevent the rear wheel
from lifting, then increase pressure to both again.
There are a few special situations where limited use of the front brake and heavier involvement of the rear brake is advisable:
1) Slippery surfaces: It is difficult to recover from a front-wheel skid on a slippery surface, especially when leaning over, so on surfaces when skidding is likely (e.g. wet pavement, mud, snow, or ice), reduced speed and use of the rear brake may be preferred.
2) Bumpy surfaces: If the front wheel
comes off of the ground during braking, it will stop completely. Landing on a stopped front wheel
with the brakes still applied is likely to cause the front wheel
to skid and, possibly, for the rider to flip over the front bars.