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DISC BRAKES DISC BRAKE ADVANTAGES FIGURE 12–2 Braking force is applied equally to both sides of the brake rotor.
DISC BRAKES DISC BRAKE ADVANTAGES 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.
DISC BRAKES DISC BRAKE ADVANTAGES FIGURE 12–4 Slots and holes in the brake linings help prevent gas and water fade.
DISC BRAKES DISC BRAKE ADVANTAGES FIGURE 12–5 The square-cut O-ring not only seals hydraulic brake fluid, but also retracts the caliper piston when the brake pedal is released.
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.
DISC BRAKES DISC BRAKE DISADVANTAGES FIGURE 12–6 Antirattle clips reduce brake pad movement and vibration.
DISC BRAKES DISC BRAKE DISADVANTAGES FIGURE 12–7 Antivibration shims are used behind the pads on many disc brake caliper designs.
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.
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.
DISC BRAKE PADS PAD WEAR INDICATORS 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.
DISC BRAKE PADS PAD WEAR INDICATORS FIGURE 12–16 Electrical wear indicators ground a warning light circuit when the pads need replacement.
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.
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.
There are basically three types of calipers: fixed, floating, and sliding designs.
FIXED CALIPER DESIGN
FIXED CALIPER ADVANTAGES
FIXED CALIPER DISADVANTAGES
FLOATING AND SLIDING CALIPER DESIGN
NORMAL CALIPER OPERATION
FLOATING AND SLIDING CALIPER ADVANTAGES
FLOATING AND SLIDING CALIPER DISADVANTAGES
DISC BRAKE DESIGNS 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.
DISC BRAKE DESIGNS FIGURE 12–20 This floating caliper mounts on a separate anchor plate that bolts to the vehicle suspension.
DISC BRAKE DESIGNS 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.
DISC BRAKE DESIGNS FIGURE 12–22 Caliper flex can cause tapered wear of the brake lining.
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.
DISC BRAKE DESIGNS FLOATING CALIPER OPERATION FIGURE 12–24 Floating calipers are supported by rubber O-rings or plastic bushings.
DISC BRAKE DESIGNS FLOATING CALIPER OPERATION FIGURE 12–25 Metal guide pins and sleeves are used to retain and locate floating calipers.
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.
DISC BRAKE DESIGNS FLOATING CALIPER OPERATION 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.
There are two methods of providing parking brakes when rear discs are installed on a vehicle.
1. Adapt the disc brake to also function as the parking brake. This is done by installing a series of cables, levers, and internal parts to mechanically actuate the brake caliper.
2. Use mechanically actuated drum brakes inside the rear rotors.
REAR DISC BRAKES REAR DISC PARKING BRAKES 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.
REAR DISC BRAKES REAR DISC PARKING BRAKES FIGURE 12–30 This single-piston brake caliper is mechanically actuated to serve as a parking brake.
REAR DISC BRAKES REAR DISC PARKING BRAKES FIGURE 12–31 Drum parking brakes are fitted inside the rotors on this vehicle equipped with rear disc brakes.
Some disc brakes are equipped with integral parking brakes.
Brake pads can be attached using rivets, bonding, or integrally molded.
Typical semimetallic brake linings contain phenolic resin, graphite or carbon particles, steel fibers, ceramic and metal powders, plus other modifiers such as rubber scraps.
Other types of friction material include nonasbestos organic (NAO), nonasbestos synthetic (NAS), and carbon fiber-reinforced carbon (CFRC).
Lining edge codes identify the manufacturer and include two letters at the end, which identify the coefficient of friction of the material. The first letter indicates the coefficient when the lining is cold and the second indicates the coefficient when the lining is hot
2. Two technicians are discussing the reason that the brake fluid level in the master cylinder drops. Technician A says that it may be normal due to the wear of the disc brake pads. Technician B says that a low brake fluid level may indicate a hydraulic leak somewhere in the system. Which technician is correct?
3. Two technicians are discussing a floating-type disc brake caliper. Technician A says that if the caliper slides are corroded, one pad may wear more than the other pad on the same wheel brake. Technician B says that if a caliper slide is corroded, reduced braking may occur. Which technician is correct?
4. Technician A says that disc brakes are self-adjusting and the brake pedal height should not become lower as the disc brake pads wear. Technician B says that as the disc brake pads wear, the level of brake fluid in the master cylinder reservoir drops. Which technician is correct?
7. Technician A says that all vehicles equipped with rear disc brakes use a parking brake that is integrated in the caliper. Technician B says that all vehicles equipped with rear disc brakes use a small drum brake inside the rear rotor for a parking brake. Which technician is correct?
10. Technician A says that riveted disc brake pads are often quieter than bonded disc brake pads. Technician B says integrally molded disc brake pads are molded into holes in the steel backing of the pads. Which technician is correct?