Drum & Disc Brake Tech - Whoa, Whoa, Whoa!

Brake Basics

Bob Mehlhoff Sep 28, 2005 0 Comment(s)

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.

Causing Friction
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.

Keeping Control
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.

Conclusion
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.

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A disc-brake system uses calipers, which are hydraulically operated to squeeze the rotors (via brake pads) to stop the vehicle. The caliper shown here is being removed from an '87 El Camino (G-body) and is a single-piston floating design. With this style, a single piston is mounted inside the inboard side of the caliper, and as the driver applies pedal pressure, hydraulic fluid originating from the master cylinder forces the caliper piston outward against the inner brake pad toward the inner side of the rotor. At the same time, the caliper housing (sliding side-to-side across two large locating pins) moves in a counteracting direction from the caliper piston, forcing the outer brake pad toward the rotor. The simultaneous action of the pads slows or stops the vehicle.

This upper pin (arrow A) is one of two that allow a floating caliper to move perpendicular to the face of the rotor. The caliper is typically guided at upper (arrow B) and lower slide contact positions incorporated into a stationary caliper-mounting bracket.

These new Brutestop brake pads from Raybestos are a simple bolt-on and improve the vehicle's stopping performance over production pads. They're made from specially formulated material for performance driving and are post cured (requiring no break-in time).

With the front spindle nut adjusted properly, there should be a slight amount of endplay on a modern RWD vehicle, as felt by grasping the rotor and moving it laterally. It should move no more than 0.001-0.008 inch. To make this adjustment, spin the rotor several times (more than 8 or 10) while carefully tightening the adjusting nut to zero preload. Then back the nut off only enough to install a new cotter pin into the first available slot. Check the endplay. Because the adjusting nuts have six sides and the spindle is drilled both horizontally and vertically, very accurate bearing adjustment is possible.

New brake pads installed on a floating caliper must have the mounting tabs bent down to secure the pad tightly and prevent pad shudder. To do this, tap a medium-size chisel carefully between the bottom of the pad and the rotor hub, so that the brake pad's bottom clearance against the caliper housing is zero. Next, use large channel locks (grabbing only the lower metal portion of the brake pad and the upper tap with both jaws); bend each tap down securely against the caliper ears.

This late-model Camaro is equipped with an ABS brake system, which keeps the wheels from locking as the brakes are applied. The major components of the anti-lock brake system are a computer module, the hydraulic assembly (arrow), and speed sensors.

Hard use will cause brake temperatures to climb. High-performance brake products are designed to withstand very high temperatures. During competition, a circle track or NASCAR racecar will typically have glowing red brake rotors (much higher than the 334*F shown here). If your brakes encounter high temperatures during a driving episode, be sure to drive the car slowly for a mile or two prior to parking, to allow brake temperatures to cool and stabilize.

The rotor on the right is a low-cost, non-vented production Ford Explorer brake rotor. In contrast, all performance brake rotors and most medium-duty or higher-performance OE rotors are vented (arrow). A vented rotor is basically a rotor with two surfaces joined by vents cast between the rotor that provides a larger area to dissipate heat and a path to exhaust the heat. Vented rotors have been in use on most production Chevrolets since the '60s and earlier on some other makes. Typically, smaller cars use non-vented rotors.

This brake-backing plate serves to protect the brakes from road debris and also allow outside air to cool the rotor.

This high-performance rear-brake rotor from Bear is drilled and slotted to allow heat to escape and to exhaust brake gases.

Since '67, all domestically sold vehicles have been equipped with a tandem master cylinder that typically divides the front and rear brakes to provide two separate braking systems (redundancy). To balance braking pressure, most vehicles with front-disc and rear-drum or four-wheel discs use a combination valve and a proportioning valve to bias the front-to-rear pressure, because as a vehicle stops or slows, the center of gravity moves forward. Many performance-brake applications use an adjustable proportioning valve activated by a knob (arrow) to tune the front-to-rear brake pressure.

Brake hardware is critical to good performance. This complete kit from Raybestos is being added to a Strange Engineering S-60 and its parts are designed to assist in restoring the brake system's performance. New brake hardware, installed properly with the right tools, helps provide consistent and well-balanced braking.

When installing new caliper hoses, be certain to add both copper washers properly. This hose uses a copper gasket on both sides of the banjo fitting.

The master cylinder provides fluid pressure to each wheel. Disassembled from this master are the primary and secondary pistons. As you step on the brake pedal, the primary piston moves forward in the bore and fluid pressure builds. Next, the pressure between the primary and secondary pistons forces the secondary piston to compress the fluid in its circuit. Finally, all the pressure is transferred to each brake to slow or stop the vehicle.

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