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Automotive Brake Systems, 5/eBy James D. Halderman
PARTS AND OPERATION• Disc brakes use a piston(s) to squeeze friction
material (pads) on both sides of a rotating disc (rotor).
• Disc may be spelled disk by some manufacturers, but disc is the SAE (Society of Automotive Engineers) term and the most commonly used spelling in the industry.
• The rotor is attached to and stops the wheel.
CHAPTER 3: DISC BRAKES
Automotive Brake Systems, 5/eBy James D. Halderman
FIGURE 12–3 Disc brakes can absorb and dissipate a great deal of heat. During this demonstration, the brakes were gently applied as the engine drove the front wheels until the rotor became cherry red. During normal braking, the rotor temperature can exceed 350°F (180°C), and about 1,500°F (800°C) on a race vehicle.
Automotive Brake Systems, 5/eBy James D. Halderman
• If an unequal braking problem is being diagnosed, check that the front tires match and that the rear tires match. Brakes slow and stop wheels. Unequal diameter tires create an unequal braking force. The result may be a pulling toward one side while braking. Tire diameter can vary from one tire manufacturer to another even though the size designation is the same. Even slight differences in the wear of tires can cause a different tire diameter and, therefore, a different braking force.
Automotive Brake Systems, 5/eBy James D. Halderman
• Brake dust from semimetallic brake pads often discolors the front wheels. Customers often complain to service technicians about this problem, but it is normal for the front wheels to become dirty because the iron and other metallic and nonmetallic components wear off the front disc brake pads and adhere to the wheel covers. A coat of wax on the wheels or wheel covers helps prevent damage and makes it easier to wash off the brake dust.
Automotive Brake Systems, 5/eBy James D. Halderman
• Although not required by law, a growing number of vehicle manufacturers are fitting pad wear indicators to their brakes for safety reasons. • Pad wear indicators are either mechanical or electrical, and
signal the driver when the lining material has worn to the point where pad replacement is necessary.
• A mechanical wear indicator is a small spring-steel tab riveted to the pad backing plate. • When the friction material wears to a predetermined
thickness, the tab contacts the rotor and makes a squealing or chirping noise (when the brakes are not applied) that alerts the driver to the need for service.
Automotive Brake Systems, 5/eBy James D. Halderman
FIGURE 12–15 Typical pad wear sensor operation. It is very important that the disc brake pads are installed on the correct side of the vehicle to be assured that the wear sensor will make a noise when the pads are worn. If the pads with a sensor are installed on the opposite side of the vehicle, the sensor tab is turned so that the rotor touches it going the opposite direction. Usually the correct direction is where the rotor contacts the sensor before contacting the pads when the wheels are being rotated in the forward direction.
Automotive Brake Systems, 5/eBy James D. Halderman
• As mentioned previously, there are several methods that are used to mount brake linings, including:• Riveted linings• Bonded linings• Mold-bonded linings
Automotive Brake Systems, 5/eBy James D. Halderman
• The term competitively priced means lower cost. Most brake manufacturers offer “premium” as well as lower-price linings, to remain competitive with other manufacturers or with importers of brake lining material produced overseas by U.S. or foreign companies. Organic asbestos brake lining is inexpensive to manufacture. In fact, according to warehouse distributors and importers, the box often costs more than the brake lining inside.
• Professional brake service technicians should only install brake linings and pads that will give braking performance equal to that of the original factory brakes. For best results, always purchase high-quality brake parts from a known brand-name manufacturer.
Automotive Brake Systems, 5/eBy James D. Halderman
• Original equipment brake pads and shoes are required to comply with the Federal Motor Vehicle Safety Standard (FMVSS) 135, which specifies maximum stopping distances. There is also a requirement for fade resistance, but no standard for noise or wear. Aftermarket (replacement) brake pads and shoes are not required to meet the FMVSS standard. However, several manufacturers of replacement brake pads and shoes are using a standardized test that closely matches the FMVSS standard and is called the “Dual Dynamometer Differential Effectiveness Analysis” or D3EA. This test is currently voluntary and linings that pass the test can have a “D3EA certified” seal placed on the product package.
Automotive Brake Systems, 5/eBy James D. Halderman
• The brake rotor provides the friction surfaces for the brake pads to rub against.
• The rotor, the largest and heaviest part of a disc brake, is usually made of cast iron because that metal has excellent friction and wear properties.
• There are two basic types of rotors:• Solid—Solid rotors are most often used on the rear
of vehicles equipped with four-wheel disc brakes.• Vented—Vented rotors have radial cooling
passages cast between the friction surfaces.
Automotive Brake Systems, 5/eBy James D. Halderman
• FLOATING AND SLIDING CALIPER DESIGN• NORMAL CALIPER OPERATION• WEAR COMPENSATION• FLOATING AND SLIDING CALIPER ADVANTAGES• FLOATING AND SLIDING CALIPER
DISADVANTAGES
Automotive Brake Systems, 5/eBy James D. Halderman
FIGURE 12–19 (a) Many fixed caliper disc brakes use a simple retaining pin to hold the disc brake pads. (b) Removing the retainer pads allows the brake pads to be removed. (c) Notice the cross-over hydraulic passage that connects both sides of the caliper.
Automotive Brake Systems, 5/eBy James D. Halderman
FIGURE 12–21 Hydraulic force on the piston (left) is applied to the inboard pad and the caliper housing itself. The reaction of the piston pushing against the rotor causes the entire caliper to move toward the inside of the vehicle (large arrow). Since the outboard pad is retained by the caliper, the reaction of the moving caliper applies the force of the outboard pad against the outboard surface of the rotor.
Automotive Brake Systems, 5/eBy James D. Halderman
• The body of a floating caliper does not make direct metal-to-metal contact with the anchor plate.
FIGURE 12–23 A typical single-piston floating caliper. In this type of design, the entire caliper moves when the single piston is pushed out of the caliper during a brake application. When the caliper moves, the outboard pad is pushed against the rotor.
Automotive Brake Systems, 5/eBy James D. Halderman
• A low-drag caliper differs from a standard caliper in the area of the square-cut O-ring. A V-shaped cutout allows the O-ring to deflect more and, as a result, is able to pull the caliper piston back into the bore when the brakes are released. Because of this further movement, the brake pads are pulled further from the rotor and are less likely to drag. The negative aspect of this design is that greater volume of brake fluid is needed to achieve a brake application. To compensate for this need for greater brake fluid volume, a quick-take-up master cylinder was designed and is used whenever low-drag calipers are used.
Automotive Brake Systems, 5/eBy James D. Halderman
FIGURE 12–26 In a standard disc brake caliper, the squarecut O-ring deforms when the brakes are applied and returns the piston to its original (released) position due to the elastic properties of the rubber seal. In a low-drag caliper design, the groove for the square-cut O-ring is V-shaped, allowing for more retraction. When the brake pedal is released, the piston is moved away from the rotor, further resulting in less friction between the disc brake pads and the rotor when the brakes are released.
Automotive Brake Systems, 5/eBy James D. Halderman
4 components includes in disc brakes assembly and their function:
• Disc brake rotors: The brake disc rotates with the road wheel. It provides a smooth surface against which to force the brake pads, to slow or stop the vehicle.
• Disc brake pads: A disc brake pad has a rigid, molded, friction material bonded to a steel backing plate for support during brake application. It transforms the hydraulic force of the caliper into a frictional force against the disc.
• Disc brake calipers: Disc brake calipers provide a housing for the hydraulic piston or pistons that force the brake pads into contact with the disc.
• Proportioning valves: The proportioning valve divides up the braking effort applied to front and rear wheels under heavy braking, according to how load is distributed across a vehicle.
REAR DISC BRAKES• In recent years, four-wheel disc brake systems
have become more common. • In most rear-wheel applications, drum brakes are
adequate to provide the relatively small portion of a vehicle’s total braking power required of them.
• Because rear drum brakes are lightly loaded, fade is a problem only in extreme conditions when the front brakes fade and force the rear brakes to take on a larger part of the braking load.
• The automatic adjusting ability of disc brakes is also less of an advantage in slow-wearing rear brakes.
Automotive Brake Systems, 5/eBy James D. Halderman
FIGURE 12–29 Exploded view of a typical rear disc brake with an integral parking brake. The parking brake lever mechanically pushes the caliper piston against the rotor.
Automotive Brake Systems, 5/eBy James D. Halderman