Disc brakes are one of the main types of bicycle braking systems. Disc brakes are made of a metal disc attached to the wheel hub. The disc braking system includes a caliper that squeezes the brake pads against the wheel, causing the tire to stop rotating. Disc brakes can be hydraulic or cable driven. They are heavier than rim brakes, but perform better in all weather conditions, including rain, snow and mud. Disc brakes are available in various sizes for different types of riding styles and biking needs.
Disc brakes come in various sizes measured in millimeters. The more common sizes include 150 mm, about 6 inches, which is one of the smaller versions; 160 mm; 185 mm; and 203 mm. The most common large size is 203 mm, which is 8 inches. Disc brake dimensions refer to the size of the rotor, which is the disc in the disc-braking system. The bike frame and rotor diameter must be compatible but can have some variance in size as long as the disc-specific hub is compatible with the rotor.
Disc brakes are often used on mountain bikes because they are not affected by water or mud. They perform better on rugged terrain and other extreme road conditions because they are located in the middle of the wheel and not on the rim. The downside to disc brakes is they are heavier than rim or V-brakes. Disc brakes are also more expensive. Most mountain bikers prefer larger disc brake sizes because they offer more stopping power. This is important for steep downhill runs and the extra workout braking systems get on mountain terrain. Mountain bike disc brake sizes typically run about 203 mm. Heat is dispersed easier in the large mountain bike disc rotors because of the higher heat capacity, which helps prevent brake failure.
Touring, or cross-country, bike disc brakes are smaller than mountain bike disc brakes. They still need the performance and extra stopping power but use the intermediate size for the lighter weight. Cross-country disc brake size is in the 160 mm range on the front wheel and is smaller on the rear wheel. As a rule, front disc brake sizes are larger on the front tire because it does most of the work.
Road bikes, also called racing bikes, do not use disc brakes. The biggest reason is because hub disc brakes are not legal for UCI-sanctioned road events or cyclocross, according to "Road Bike Action Magazine." The other primary reason is because disc brakes are still heavier, as of 2011, than rim brakes and extra weight slows the racer down.