• Static friction coefficient,which describes how well a clutch disc will hold onto a flywheel under acceleration. If the coefficient is low, the clutch disc will slip against the flywheel, get hot, and wear away.
• Dynamic friction coefficient,which describes how smoothly (or abruptly) a clutch disc will ‘grab’ a flywheel during engagement. If the coefficient is too high, the clutch will grab immediately, leading to uncomfortable shifts that make low-speed manuevering very difficult (a key concern for truck clutch discs).
• 夹紧力,which is the amount of force or “weight” that must be applied to a given clutch disc to make sure it doesn’t slip against the flywheel. The more force applied, the more pedal effort for the driver, the greater the load on the hydraulic system, etc.
• Fade temperature,which is the temperature at which the clutch material begins to lose cohesion. If the temperature is too low, an afternoon spent towing the family boat (or at the local drag strip) can ruin the clutch disc.

There are five different materials utilized in modern clutch design:

Ceramic and feramic clutch disc materials are used in racing or on vehicles with large amounts of torque (commercial trucks, farm vehicles, even 3/4 and 1-ton diesel trucks, if they’ve been sufficiently modified). A new material called FeramAlloy will likely replace ceramic and feramic clutch materials, as it offers the same key benefits (high static friction coefficient, high fade temp), but with a much lower dynamic friction coefficient that makes shifts much smoother.