553 Brake Systems Technician Handbook Energy Conversion Kinetic energy is the energy of a mass in motion. When it ’ s time to stop the vehicle the brakes, which are connected to the vehicle, press against a rotating surface which is connected to the wheels. The friction between these two surfaces converts the moving vehicle’s kinetic energy into heat. • Kinetic energy increases in direct proportion to weight. If you double the vehicle’s weight, its kinetic energy doubles. • Kinetic energy increases in proportion to speed squared. If you double the vehicle’s speed, its kinetic energy quadruples; the brakes must absorb four times as much heat to stop the vehicle.
The Coefficient of Friction, or ‘ COF ’ is a measurement of the friction between surfaces. The ideal COF for braking is about 0.33. At this level the brakes can be applied smoothly and stop the vehicle. When the COF is too low, the vehicle will stop too slowly. When the COF is too high, the brakes will lock up too easily. The amount of friction between two surfaces – and thus the amount of heat generated during braking – is determined by the three main factors: • The composition of the surfaces – some materials are ‘stickier’ than others.
• The condition of the surfaces – rust, corrosion or scoring has a significant effect on friction • The pressure applied to the two surfaces – with greater pressure, more friction and heat are generated 4 Technical Training