Abrasive Friction: The mechanical rubbing of the brake pad material directly onto the rotor disc, resulting in the mechanical wear of both pad and rotor.
Adherent Friction: The transfer of a thin layer of brake pad material that sticks (adheres) to the rotor face. The layer of pad material, once evenly established on the rotor, is what rubs on the brake pad. The bonds that are broken for the conversion of kinetic to thermal energy are formed instantaneously before being broken again.
Bite: The speed at which the friction material reaches its maximum coefficient of friction when braking is initiated. The amount of bite is a compromise. Too much bite makes initial modulation difficult. Too little causes a delay in braking. In racing, different drivers prefer pads with different degrees of bite.
Brake bias: The term used to indicate the ratio between the amount of brake torque exerted on the front brakes compared with the rear. Brake bias is normally expressed as a percentage of brake torque at one end of the car to the total brake torque, as in "60 percent front."
Brake line pressure: The instantaneous hydraulic pressure within the brake lines. Brake line pressure in pounds per square inch is the force applied to the brake pedal in pounds multiplied by the mechanical pedal ratio (plus any booster assist where applicable) divided by the area of the master cylinder piston in square inches. For the same amount of pedal force, the smaller the master cylinder bore and/or the greater the mechanical pedal ratio and/or booster contribution, the greater the brake line pressure and the longer the pedal travel.
Braking torque: Braking torque in pounds-feet on a single wheel is the effective rotor radius in inches times clamping force in pounds times the coefficient of friction of the pad against the rotor (a unit-less value) all divided by 12. Braking torque is the force that actually decelerates the wheel and tire. To increase the braking torque it is necessary to increase the line pressure, the piston area (clamping force), the coefficient of friction, or the effective rotor radius. Increasing the pad area will not increase the braking torque.
Coefficient of friction: A dimensionless indication of the friction qualities of one material vs. another. A coefficient of 1.0 would be equal to 1g. The higher the coefficient, the greater the friction. Typical passenger car pad coefficients are in the neighborhood of 0.3 to 0.4. Racing pads are in the 0.5 to 0.6 range. With most pads the coefficient is temperature-sensitive, so claims that do not specify a temperature range should be viewed with some suspicion. The optimum is to select a pad with a virtually constant but decreasing coefficient over the expected operating range of temperatures. As a result, the driver does not have to wait for the pad to heat up before it bites, and the pad fade will not be a factor so that modulation will be easy.
Clamping force: The clamping force of a caliper in pounds is the brake line pressure multiplied by the total piston area of the caliper in a fixed caliper and two times the total piston area in a floating design. To increase the clamping force it is necessary to either increase the line pressure or the piston area. Increasing the pad area or the coefficient of friction will not increase clamping force.
Differential bores: The leading edge of a brake pad wears faster then the trailing edge. This is due to the migration of particles of incandescent friction material carried from the leading to trailing edge of the pad. In effect, the trailing portion of the pad is riding on a layer of incandescent material. By providing an optimally designed larger caliper piston at the trailing edge of the pad, wear can be evened along the length of the pad.
Effective temperature range: The range of operating temperatures within which a pad material remains effective or consistent. As with coefficient of friction, this should be used for comparative purposes only as measurement procedures vary between manufacturers and pad temperatures are strongly affected by disc mass and rate of cooling.
Friction consistency: The Thickness Variation (TV): Variation in the transfer layer that initiates brake vibration. While the impact of an uneven transfer layer is almost imperceptible at first, as the pad starts riding the high and low spots, more and more TV will be naturally generated until the vibration is much more evident. With prolonged exposure, the high spots can become hot spots and can actually change the metallurgy of the rotor in those areas, creating "hard" spots in the rotor face that are virtually impossible to remove.
Thermal shock: Disc materials, particularly cast iron, are degraded not only by the magnitude of temperatures reached, but also by the "delta" temperatures--the speed at which the temperature increases and decreases. Cracks are largely caused by weakening of the bonds between the grains of the metal brought about by rapid change in temperature.
Threshold braking: Braking at maximum possible retardation in a straight line.
Vanes: The term given to the central webs that serve to separate the inboard and outboard friction surfaces of ventilated discs. Beneficial for cooling if designed right. There are three types:
Straight vanes: Straight vanes are the easiest to manufacture. They extend in straight lines radially outward from the inner surface to the outer surface of the disc. This design is often used in production automobiles and trucks because the same part can be used on both sides of the vehicle.
Curved vanes: The vanes are shaped as curves to act as more efficient pump impellers and increase mass airflow through the disc. They also act as barriers to the propagation of cracks and, as each vane overlaps the next, they dimensionally stabilize the disc. Curved vane discs are more expensive to produce than straight vanes and must be mounted directionally.
Differential vanes: Some discs are designed with alternating vanes of different length. This modern design feature has been dictated by flow studies. It was found that the volume of air that a disc can flow increases by alternating the length of the inlet without much of a sacrifice in surface area. The more air a vent flows, the more convective cooling can be realized.