Brake Systems: Not an abstract theory

Brake operation is not abstract theory but a great example of applied science. We can look at brake troubleshooting from two engineering viewpoints-conversion of kinetic energy to thermal energy, and the application of hydraulic principles. Let's start with stopping power and then move to the hydraulic applications.


The power to stop a vehicle comes from converting kinetic energy to thermal energy. Kinetic and thermal energy are two sides of the same coin, which is heat transfer. The primary job of a brake system is to dissipate heat. In our rush to troubleshoot today's sophisticated braking systems, however, we often jump past basic heat transfer to the high-tech realm of electronic controllers and computer programs. In reality, the major factor that determines good or poor braking performance is simple friction.

The braking system exists to convert the energy of a vehicle in motion into thermal energy, more commonly referred to as heat. Friction is measured by its coefficient, which is calculated by dividing the force required to slide an object over a surface by the weight of the object. For example, if it takes 100 pounds of force to slide a 100-pound block of iron over a concrete floor, the coefficient of friction between the two materials is 1.0. If it takes only 2 pounds of force to slide a 100-pound block of ice over the same floor, the coefficient is only .02.

Friction exists at two points for each wheel during braking-between pad or shoe linings and rotors or drums, and between the tires and the road. These are the areas you want to think about when troubleshooting a braking problem.

Three factors affect the coefficient of friction in a brake system, and these involve some important service operations on your part:
• The surface finish of both friction surfaces.
• Temperature.
• The material-the metal of the rotors and drums and the friction material of the pad and shoe linings.

Manufacturers almost universally recommend against machining the surfaces of brand-new rotors and drums or refinishing rotors unless they're worn or scored beyond certain limits. All manufacturers do, however, specify definite surface finish requirements for rotors and drums.
Because contact between brake drums and shoe linings is linear, surface finish for a brake drum is not as critical as it is for rotors. The finish is not unimportant, however. That's why carmakers and parts suppliers tell you to make the final refinishing cut on a drum at a shallow depth and slow feed rate to ensure a uniform finish, free of grooves and spirals.