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Glossary of Disc Brake Terms

Rotor: The “disc” part of the brake that attaches to the hub, usually made of stainless steel

Rotor: The “disc” part of the brake that attaches to the hub, usually made of stainless steel.

Caliper: The housing that encapsulates the brake pads and generates the clamping force between the pads and rotors.

Brake pads: A pair of steel plates with friction material bonded to the sides that face the rotors. Pads are retained by magnets or clips so they may be replaced easily without special tools.

Cable-actuated caliper (Also, mechanically actuated): Any type of mechanism other than hydraulic fluid, used to generate clamping force within the caliper. Most use threads or ball-ramps (Avid and Hayes make good examples).

Hydraulic caliper: The most common type of disc brake uses pressurized fluid to force one or more pistons against the brake pads.

Master cylinder: Rock Bottom’s Boss; nemesis to the Professor in old Felix the Cat cartoons. Also refers to the brake-lever-mounted piston and cylinder that drive fluid to actuate the brake

Open hydraulic system: A master cylinder with a piston that retracts past a port that vents to a fluid reservoir. This allows heated fluid to expand into the reservoir when necessary. The self-adjusting open system pumps new fluid from the reservoir into the caliper as the brake pads wear (Hayes, Shimano, Magura, Formula, Grimecca and some Hope models).

Closed system: The simplest hydraulic system consists of a brake-lever-mounted piston that drives the caliper pistons through a connecting hose. The system is sealed, so fluid expansion due to heat and brake pad wear must be compensated for by external adjustments (Mountain Cycle, some Hope, and Cannondale brakes).

DOT type fluid: A caustic automotive brake fluid that eats some types of rubber and plastic seals (used on Hope and Hayes brakes).

Mineral oil: A non-reactive hydraulic fluid used for a wide variety of industrial and automotive applications (Shimano, Magura).

Bleed fitting: Hollow, hex-bolt screws with barbed heads that accept a plastic tube. Bleed fittings are used to vent air and fluid from the caliper.
Bleeding the system: A term for a method of pumping fluid through a hydraulic system to evacuate all traces of air. Air is compressible and thus causes hydraulic brakes to feel mushy and stop poorly.

Single-action caliper: A caliper with a piston or mechanical actuating system on only one side of the rotor (Magura Louise).

Double-action caliper: A caliper with pistons or actuating mechanisms on both sides of the rotors. May use a single pair or multiple pairs of pistons.

Four-piston calipers: Hydraulic calipers that use two pairs of small pistons that drive elongated brake pads (Shimano Deore XT, Magura Gustav and Grimeca). The purpose is to gain more braking force by moving the centerline of the braking surface further out on the rotor.