Brake systems

Motor, Sep 1999 by Layne, Ken

The International Automotive Technicians' Network (iATN) is an organization of about 23,000 professional technicians, worldwide. One of its principal functions is to provide mutual assistance and problem-solving advice for its members through Internet e-mail forums. One of the most popular topics-right up there with driveability and OBD II diagnosis-is brake problems. It may seem strange that technology as wellestablished as hydraulic brake systems could generate so many service headaches for technicians and shop owners, but it does. And often the solution to a nightmare problem is found in the application of basic brake principles. So let's review "Brake Systems lA" with an eye toward gaining some troubleshooting insight.

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. Energy Conversion

and Friction

The power to stop a vehicle comes from converting kinetic energy (motion) to thermal energy (heat). 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.

This is not going to be a review of a physics textbook; but if you understand the simple science at work in a brake system, you'll find it easier to keep your eye on the goal of all brake service-to maintain the same coefficient of friction and braking force at all four wheels that has been designed into the vehicle.

Engineers measure friction 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.

More Is Not Always Better

It's not a simple proposition that the highest coefficient of friction will produce the greatest stopping power. In fact, the coefficient of friction between brake linings and rotors or drums is always less than 1.0. A coefficient of friction greater than 1.0 means that material actually is being transferred from one surface to another. Although brake friction surfaces wear, material is not transferred from pads to rotors or from shoe linings to drums. Additionally, excessive friction leads to brake grabbing and lockup, which, as you know, reduces braking effectiveness.

The coefficient of friction between a tire and the road can exceed 1.0, however. When this happens, material transfers from the tire to the road-a skid mark. That means that the coefficient of friction momentarily exceeded 1.0. These two examples illustrate that more friction does not always mean better braking. When the coefficient of friction equals or exceeds 1.0-either at the brake or at the tire-you get lockup. And that means you've just lost effective heat transfer and braking power.

Engineers determine the required coefficient of friction for the best braking performance on a particular vehicle. The friction materials selected for brake pads and rotors or brake shoes and drums are designed to give the best cold-temperature and hot-temperature performance. The best materials have a coefficient that stays within narrow limits over a wide range of temperatures. If the selected material increases or decreases its coefficient of friction as temperature changes, the brakes can fade or grab. This illustrates an important reason to use replacement brake parts that are equivalent to OE specifications: so braking performance will be up to the system design requirements.

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.

Finish In Style

Carmakers 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 carmakers 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.