Cycles of all sorts

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 (partly successful) attempts at introducing disc brakes for bicycles over the last decades may now finally have reached fruition. Recent weight/costs/reliability material advances have led to 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 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 the heavier machine.

Advantages

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 and possible contaminants like water which can coat and freeze on the rim in colder temperatures. They also avoid the problem that rim brakes have of wearing out the wheel rims, especially in muddy conditions, as well as the requirement that the rim be true. 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 in width also makes disc brakes a necessity, as rim brakes simply cannot straddle a tire that wide.

Disadvantages

The disadvantages are that they are usually heavier and more expensive than rim brakes, and require a hub built to accept the disc and a bicycle frame or fork built to accept the caliper. Also, rigid forks on road bikes and tandems 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 (c. 15g per wheel).

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 and grit in the cable lines when ridden in harsh terrain. Hydraulic disc brakes use fluid from a reservoir, pushed through a hose, to actuate the pistons in the disc caliper, that then actuate the pads. Hydraulic disc brake systems generally keep contaminants out better. 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.

Single vs dual actuation

Many disc brakes have their two pads actuated from both sides of the caliper, while some (less expensive in both hydraulic and mechanical varieties) have only one pad that moves. Many hydraulic disc brakes have a self-adjusting mechanism so as the brake pad wears, the pistons will keep the distance from the pad to the disc consistent in order to maintain the same brake lever throw. Mechanical discs have a manual adjuster to dial in the pad to rotor distance. Calipers are now generally made in one piece to increase stiffness and reduce the threat of leaks, although the two piece design still reduces heat buildup more effectively than the one piece, and most top end models still feature the 2 piece caliper.

Caliper mounting standards

There are many standards for mounting disc brake calipers. IS (International Standard) is different for 6" and 8" rotor and differs between forks with a QR and 20mm thru axle. Post mount also differs depending on disc size and axle type. Additionally, there have been many incompatible variants produced over the years, mostly by fork manufactures. The mount used on the Rockshox Boxxer is the most typical of these specialty mounts, but most fork manufactures have or still do use a proprietary mounting pattern. As a point of reference, Hayes currently sells no less then 13 different adaptors to fit their brakes to various mounting patterns.

The post mount standard was develeoped by Manitou. The advantage of post mounting is that it's easier to align the caliper to the rotor, since there is some allowable side to side adjustment. You must use spacers to properly align IS calipers. The disadvantage to post mount is that the bolt is threaded directly into the fork lowers. If the threading was stripped or if the bolt was stuck, then it would require purchasing new fork lowers. Frame manufacturers have standardized the IS mount for the rear disc brake mount. In recent years post mount has grained ground and is becoming the dominate pattern for forks. This is mostly due to decreased manufacturing and part cost for the brake calipers when using post mount.

Disc mounting standards

There are many standards for disc rotor mounting - International Standard (IS), centerlock, Cannondale's 4 bolt pattern, Hope's 5 bolt pattern and Rohloff's 4 bolt pattern to name a few. IS is a six-bolt mount and is the industry standard. Centerlock is patented by Shimano and uses a splined interface along with a lockring to secure the disc. The advantages of centerlock are that the splined interface is stiffer and removing the disc is quicker because it only requires one lockring to be removed. Some of the disadvantages are that the design is patented requiring a licensing fee from Shimano. A Shimano cassette lockring tool is needed to remove the rotor and is more expensive and less common then 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 makes IS rotors more time consuming to remove.

Disc sizes

Disc brake rotors come in many different sizes, generally 150mm, 177mm, or 203mm in diameter, however there are many different sizes available as all brake manufacturers make discs specific to their calipers and the dimensions often vary by a few millimeters. SRAM this year has introduced a 140mm rotor intended for rear use on cross country and road bike. Larger rotors provide greater stopping power by virtue of a longer moment arm for the the caliper 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.

Disc Brake weight listing. Retrieved on 2006-11-07. 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.