Nail the brakes and your car decelerates from 60 mph to zero quickly and with control. Your brake system does this well because it's properly engineered, you're applying the brakes skillfully, and the brake's frictional force transforms into heat energy and overcomes your vehicle's dynamic energy. Just what makes your brake system operate properly and safely is the subject of this month's How it Works. In recent years there has been a tremendous amount of braking advancement, from the OEs as well as the aftermarket, based on computer-controlled technology and improvements to ongoing designs. Bigger brake rotors and calipers, improved heat dissipation, brake pad technology, anti-lock brake systems, and an array of other improvements allow your car to stop quicker and safer than before.
In this month's edition of How it Works we delve into many of these new technologies, as well as the fundamental brake principles. Today's high-perfor-mance cars accelerate and turn faster than before. Consequently, your vehicle's brake system should be improved and tuned as well as your engine to provide optimal performance under demanding situations.
Pressure on the brake pedal hydrau-lically activates the brakes at all four wheels to slow or stop the car. This is largely a function of Pascal's principle that states: "when pressure is applied to a liquid, the pressure will be transmitted equally in all directions." One of the benefits of a hydraulically operated brake system is that the brake force can generally be regulated by changes in the applied (foot) pressure to the brake pedal or by the varied diameters inside the hydraulic components. Included in your vehicle's hydraulic system are the master cylinder, metal and rubber brake lines, wheel cylinders, and/or calipers and a combination valve.
The Master Cylinder
The modern master cylinder utilizes a single-bore, dual-hydraulic design that allows one channel to operate should the other channel fail. This dual-reservoir design has been used in the United States on all new cars sold since the '67 model year. But unlike the cast-iron master cylinders of the late '60s, most modern master cylinders are made of aluminum and have plastic fluid reservoirs.
The brake fluid that flows through the steel brake lines and flexible reinforced rubber hoses is a very important component. It must remain clean, prevent corrosion, and not boil during hard braking. As a general rule, brake fluid should be flushed and replaced every two years to prevent corrosion and to maintain performance.
The Big Squeeze
Disc-brake systems use brake calipers that act as large hydraulic clamps to squeeze the rotor (or disc) as the brake pedal is applied. Inside the calipers are one or more pistons that move outward (activated by pedal effort) via increased brake fluid pressure that forces the brake pads against the rotor surface. As pedal pressure is released, the caliper-piston pressure retracts and the brake-pad pressure diminishes, allowing the rotor to easily maintain or regain speed.
We're ExpandingIn a drum-brake system, wheel cylinders expand the brake shoes outward as brake pressure is applied and contact against the inside diameter of the brake drum to slow or stop the vehicle. As with disc-brake calipers, brake fluid is delivered to the wheel cylinders and provides the hydraulic pressure to the two movable pistons within each cylinder. This pressure forces both brake shoes (primary and secondary) onto the drum. As pedal pressure is released, a series of large return springs secured to the brake shoes and attached to the backing plate (brake hardware), force brake shoes and the wheel cylinder pistons back to their static positions.
Not all brake pads or shoes are created equal. Brake friction materials are designed in a variety of formulations designed to meet varying demands and driving conditions. High-performance brake materials are typically made from premium materials or ceramic compounds that are often teamed up with specially designed disc-brake rotors that allow increased air circulation for cooling. In addition, high-performance rotors may have milled slots to quickly release brake-pad gases for improved braking.
An anti-lock braking system (ABS) generally allows a vehicle to stop safely without having the wheels lock up on a varying road surface, allowing the car to stop quicker to maintain steering control as it stops. The major components of the anti-lock brake system are speed sensors, valves, hydraulic assembly (pump), and a computer module (controller).
Speed sensors are located individually at each wheel or axle, and they tell the computer when a wheel is about to lock up by generating a signal that changes with wheel speed. If one wheel is on dry pavement and another is on a slippery surface, a speed sensor will signal the computer that one or more wheels are approaching lock-up while the vehicle is attempting to stop. This information, in turn, signals the hydraulic modulator to adjust line pressure at the affected wheels.
The hydraulic assembly is a series of electro-hydraulic valves that adjust individual brake-line pressure at the wheels or axles to maintain vehicle control, as often as 15 times per second. Typically in an ABS system, each brake has a valve, and these valves have three positions. In position one, the valve is open and allows brake pressure sourced from the master cylinder to pass right through to the respective brake. In position two, the valve isolates the brake pressure from the master cylinder by blocking the valve. This stops the pressure from continuing to rise if the driver continues to apply more pedal pressure. In position three, the valve serves to bleed off some of the pressure. As the pressure is bled off, it requires some way to regain the pressure. To this end, the system employs a pump to build the pressure back up.Finally, a controller (electronic brake-control module) oversees the sensors at each wheel and controls the valves to provide stopping, as well as monitoring the ABS system during start up and vehicle operation. This first after-start-up test is a "self test." On some vehicles, you may hear the system checking itself (cycling) as the vehicle travels past 8-10 mph for the first time after an initial start up. During the self-test, the ABS dash light may remain on for 3 to 5 seconds. If the test fails, an ABS light will remain illuminated on the instrument panel and disable the antilock function.
Like all aspects of a modern car, the brake system continues to evolve to provide quicker and better performance. If you own a musclecar, there are many kits designed to bolt-on to your car and provide improved braking performance. Or, you can choose to simply add high-performance brake pads designed for high-demand driving. Newer cars can benefit from aftermarket slotted rotors or bigger brake systems. Just as your engine converts heat energy into dynamic energy, your brake system is a device that performs the opposite action. Demand high performance from both.