Brakes - An Inside Look

Every Porsche owner lives and drives in the complete comfort of knowing that his or her Porsche is equipped with the very best automotive braking system available anywhere on this planet. Everything we have ever heard or read about Porsche brakes is always most complementary and positive. The one I like best is—the brakes on a Porsche work as if you had driven into a lane of wet concrete—I can mentally feel that level of effectiveness. I really appreciate the braking performance of my 993 after a week or two of driving our 20,000 pound motor home around the countryside and jumping on the brakes on that beast to regain control of the laws of momentum and my composure as well. The motor home does have a contemporary, power-boosted, 4-wheel disc brake system with ABS, but it certainly is no Porsche.

I think we all pretty much take the braking system for granted on all of our vehicles, Porsche or otherwise, until a system component fails or until we mentally fail in operating our brakes. Then it gets our attention immediately—like when it costs $950.00 for new brake pads and rotors, or when we suddenly see the yellow light as we approach a busy intersection. For a moment or two, our vehicle’s brakes become number one! To avoid these kinds of surprises, some basic understanding of your automobile’s braking system might prove beneficial to your health as well as your wallet.

To understand the theory of braking, ask any of your physicist pals, and they will tell you that the brakes convert your vehicle’s kinetic energy of motion into heat. Translation: Brakes stop the car—or more accurately, brakes stop the wheels. There is a big difference, because the most powerful brakes in the world will not stop your vehicle effectively if the road surface has little or no traction. Mash the brake pedal and the wheels will stop turning sure enough, but the vehicle will skid along happily down the road. You, on the other hand, will be a lot less happy. Many drivers tend to think of a skid as “brake failure�, when in fact the situation is really a failure of the driver to understand the driving conditions and drive accordingly.

A typical brake system is relatively simple. When you step on the brake pedal, the force your leg exerts is applied to a device called a master cylinder. The master cylinder contains a piston that pressurizes a network of hydraulic lines that lead to each of the vehicle’s wheels. At each wheel the brake fluid pressure operates the brakes by driving pistons that force the linings or pads against a rotating drum or disc creating frictional forces and heat. The friction force slows the wheel, and in turn, the entire vehicle. The heat generated by the friction force slowing the wheel is dissipated to the surrounding air aided by wheel spoke design and the design and materials of the rotating drum or disc hardware. All modern braking systems are many times more powerful than the vehicle’s engine, so even at full throttle a very powerful vehicle can be easily stopped with the brakes.

Many engineering refinements over the history of the automobile have specifically improved the capability and reliability of braking systems. Power brakes are now standard on virtually all modern automobiles, using energy supplied by the engine to help power the brakes so your right or left leg doesn’t have to do all the work. To eliminate the possibility of a sudden or complete brake failure, modern vehicles actually have two parallel brake systems with each system controlling two of the vehicle’s wheels.

A significant advancement came in the form of all-wheel disc brakes, which are found on all Porsches since the Model 356C. Disc brake systems have a metal (or now an available composite) rotor that spins along with the wheel, and a stationary caliper that squeezes the disc with a replaceable pad when the brakes are applied. Additionally, the discs are usually internally vented to allow greater air flow for more rapid dissipation of the heat generated by the frictional forces of stopping.

Your vehicle’s tires generate the maximum deceleration when braking forces are brought right up to the brink of wheel lock-up, but not beyond that point. Once the brakes lock and the wheels skid, the actual rate of deceleration is reduced and any directional control from steering is lost. Computer controlled anti-lock braking systems (ABS) have provided great advances in vehicle controllability and reduced stopping distances in most situations, particularly in bad weather or when cornering. ABS uses a combination of electronics and hydraulic controls to allow normal braking rates right up to the point of wheel lock-up, then the system steps in to reduce fluid pressures to the brakes to keep the vehicle deceleration rate at its maximum depending upon the road conditions.

ABS systems have speed sensors at each wheel that continuously feed back information to the ABS system computer. The computer uses this data to determine the overall vehicle speed and to detect when a wheel begins to lock-up. Each wheel is independently controlled (in a four-channel ABS on all Porsches) and the pressure is automatically limited or reduced to only the wheel that is locking up. ABS brakes have a significant advantage over non-ABS brakes. When one wheel locks up on a non-ABS car, the only way to allow it to spin again and regain full directional control is by the driver reducing the brake pressure, which reduces the braking force at all four wheels at once. ABS is capable of providing shorter stopping distances in difficult situations than non-ABS system even with an expert doing the driving.

Driving with ABS requires no special training, though you might need to “un-learn� a technique that makes some sense with non-ABS brakes. With the old style, non-ABS, drivers were taught to “pump� the brakes when they were approaching lock up. This rule was meant to help the average driver avoid fully locking the brakes and skidding straight ahead without any steering control. With ABS, you very simply push on the brake pedal as hard and steady as possible to make the stop. If traction is marginal, you may feel a pulsing sensation through the brake pedal and/or hear a chattering noise, which is completely normal. Throughout the stop you have complete steering control, so you can swerve or turn if required to avoid an obstacle. The superb braking performance of our everyday and our trackday Porsches can be maintained at their peak levels by a few simple maintenance practices and routine inspections.

1. Drain and refill the complete braking system with fresh brake fluid every two years on your everyday car. Do this every year on your track cars. Bleed the brake system at each caliper port before and after each track day. After each track day is the most important.
2. Inspect the brake pad thickness at least twice yearly. Know the operational minimum pad thickness for your model. When replacing your brake pads, replace all the pads on a common axle.
3. Use the recommended brake fluid for your model application—don’t skimp here.
4. Inspect all hoses, lines and connections for evidence of brake fluid leakage when you inspect your brake pads. Any leaking components must be repaired/replaced immediately.
5. Inspect pedal mechanism for smooth operation, free travel and cleanliness of the area.6. Check stop lights for proper operation at least twice yearly.

Enjoy the superb stopping power generated by the braking system of your Porsche. Your brakes will stop the car faster than you can think ahead in a tight autocross situation—to use your brakes well takes practice and finesse. Happy braking!