ASE Brakes Certification Test Prep: Disc Brake Diagnosis and Repair

[object Object],[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 76, the reader should be able to: Continued

[object Object],[object Object],OBJECTIVES: After studying Chapter 76, the reader should be able to:

[object Object],KEY TERMS: Continued

[object Object],[object Object],KEY TERMS:

DISC BRAKE ADVANTAGES ,[object Object],[object Object],Continued

Figure 76–1 A typical disc brake assembly. Continued

[object Object],Continued ,[object Object],[object Object],[object Object],Fade Resistance When a disc brake is compared with a drum brake of similar diameter, its biggest advantage is a much greater ability to resist fade. Disc brakes are more resistant to all kinds of fade, including the following: ,[object Object],[object Object],[object Object],[object Object]

[object Object],Figure 76–2 Braking force is applied equally to both sides of the brake rotor. Continued The primary symptom of lining fade is a hard brake pedal that requires greater force to maintain stopping power. Unlike a drum brake, however, increased application force will not distort the brake rotor because the caliper applies equal force to both sides.

Figure 76–3 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 1500°F (800°C) on a race vehicle. ,[object Object],Continued

[object Object],Continued Though disc brakes operate at higher temperatures than drum brakes, they have fewer gas fade problems for these reasons: ,[object Object],[object Object],[object Object],The symptoms of gas fade are the same as those for lining fade.

[object Object],Figure 76–4 Slots and holes in the brake linings help prevent gas and water fade. Continued These slots allow gasses and dust particles to escape. The holes required in riveted linings also perform this function. For even greater protection against gas fade, motorcycles and high-performance vehicles sometimes have holes or slots in the rotor. These allow gasses and water to escape, and the sharp edges wipe loose particles off the linings.

[object Object],Continued Wear Compensation The piston moves more than the caliper seal can distort. The piston moves through the seal until the pad contacts the rotor. The caliper piston returns to the released position by the seal distortion, the same as during normal operation, except now in a different, more applied position. As the wear occurs and the piston moves, additional brake fluid is needed behind the piston. This additional brake fluid comes from the master cylinder and the brake fluid level drops as the disc brake pads wear. See Figure 76–5.

Figure 76–5 The square-cut O-ring not only seals hydraulic brake fluid, but also retracts the caliper piston when the brake pedal is released. Continued

[object Object],Check the Tire Size for a Pulling Problem

DISC BRAKE DISADVANTAGES ,[object Object],Continued ,[object Object],[object Object],[object Object],[object Object],Brake Dust The lining is exposed on a disc brake, rather than being enclosed on a drum brake. Some brake dust can accumulate on the wheels. This brake dust is often dark brown or black and can stain wheels if not cleaned often or protected from the dust.

[object Object],Wax the Wheels No Self-Energizing or Servo Action The disc brake’s lack of self-energizing or servo action is a disadvantage for two reasons. It contributes to poor parking brake performance and requires the driver to push harder on the brake pedal for a given stop. The problem of high pedal pressures has been virtually eliminated through the use of brake power boosters, since the disc brake responds more directly to pressure on the brake pedal. Continued

Figure 76–6 Antirattle clips reduce brake pad movement and vibration. ,[object Object],Continued Shims may be metal or fiber. Antinoise sprays and brush-on liquids provide a cushion layer between the pad and the caliper piston. The bond lowers the natural frequency of the pad, and the cushion layer damps any vibration that may still occur.

Figure 76–7 Antivibration shims are used behind the pads on many disc brake caliper designs. Continued

DISC BRAKE CONSTRUCTION ,[object Object],Continued ,[object Object],[object Object],[object Object],[object Object]

Figure 76–8 This brake caliper attaches to the front spindle. ,[object Object],The brake caliper uses hydraulic pressure to create mechanical force required to move the pads into contact with the brake rotor. At the front axle, the caliper mounts to the spindle or steering knuckle, as shown here. Rear disc brake calipers mount to a support bracket on the axle flange or suspension. Continued See Figure 76–9.

Figure 76–9 This brake caliper attaches to a mounting bracket on the rear axle housing. Continued

DISC BRAKE PADS ,[object Object],Continued Compared to a brake shoe, a brake pad is a relatively simple part that consists of a block of friction material attached to a stamped steel backing plate. Figure 76–10 A typical disc brake pad. Some pad backing plates have tabs that bend over the caliper to hold the pad tightly in place and help prevent brake noise. See Figure 76–11.

Figure 76–11 To prevent noise, bent tabs on the backing plate hold some brake pads to the caliper housing. Continued

Figure 76–12 Holes in the backing plate are a common method of locating a pad in the caliper. ,[object Object],Continued A pin slips through the holes and fastens to the caliper body to hold the pads in position. Other pad backing plates have a retainer spring attached that locates the pad in the caliper by locking it to the caliper piston. See Figure 76–13.

Figure 76–13 Retainer springs lock the pad to the caliper piston to prevent brake noise. Continued

Figure 76–14 The lining edges of some brake pads are tapered to help prevent vibration. Continued

Figure 76–15 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. Continued

Figure 76–16 Electrical wear indicators ground a warning light circuit when the pads need replacement. ,[object Object],The electrode is wired to a warning light in the instrument panel and when the lining wears sufficiently, the electrode grounds against the rotor to complete the circuit and turn on the warning light. Continued

[object Object],Continued ,[object Object],[object Object],[object Object],Riveting In riveted linings , the brake block is attached to the backing plate with copper or aluminum rivets, which allows a small amount of flex between the brake block backing plate. Bonding High-temperature adhesive is used in bonded linings to glue the brake block directly to the shoe pad backing plate. Heat and pressure are then applied to cure the assembly. Bonding is a common form of shoe and pad assembly, and is most often used to mount organic friction materials.

[object Object],Continued Continued

Figure 76–17 Mold-bonded linings are commonly used in many applications. ,[object Object],Continued

See the chart on Page 916 of your textbook.

SEMIMETALLIC FRICTION MATERIAL ,[object Object],[object Object],Continued

Figure 76–18 Poor-quality semimetallic disc brake pad. The screwdriver is pointing to large chunks of steel embedded in the lining. Continued

[object Object],See the chart on Page 916 of your textbook.

[object Object],What Does D 3 EA Mean?

[object Object],What is a Brake Pad Recipe? Replacement linings are usually a compromise “generic” recipe that will give acceptable service. The brake lining recipe is just one of many factors that results in the fact that new brakes always seem to last longer than any replacement lining. Replacement lining should have the same friction code as the original. Although this will not guarantee the same braking performance, this edge code rating does help assure the service tech that the replacement brakes will give “as-new” performance.

LINING EDGE CODES ,[object Object],Continued Lining edge codes identify the coefficient of friction, a pure number indicating amount of friction between two surfaces. These codes were established by the SAE Code C 0.00 to 0.15 Code D 0.15 to 0.25 Code E 0.25 to 0.35 Code F 0.35 to 0.45 Code G 0.45 to 0.55 Code H 0.55 and above Code Z ungraded FF indicates that the brake lining material has a coefficient of friction between 0.35 and 0.45 when both cold and hot.)

[object Object],Competitively Priced Brakes 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. “ Competitive” asbestos linings should never be substituted for semimetallic or NAO original linings or pads. For best results, always purchase high-quality brake parts from a known brand-name manufacturer.

BRAKE ROTORS ,[object Object],Figure 76–19 Disc brake rotors can be either solid or vented. Largest and heaviest part of a disc brake, made of cast iron for excellent friction and wear. Vented rotors have cooling passages between friction surfaces Solid rotors are used on the rear of vehicles equipped with four-wheel disc brakes.

DISC BRAKE DESIGN ,[object Object],Continued ,[object Object],[object Object],[object Object],Fixed calipers have several unique features, but sliding and floating calipers share similar features.

[object Object],[object Object],Continued

Figure 76–20 Four-piston fixed caliper assembly on a race vehicle. Continued

Figure 76–21 (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. (Courtesy of Allied Signal Automotive Aftermarket). Continued (a) (b)

Figure 76–21 (c) Notice the cross-over hydraulic passage that connects both sides of the caliper. (Courtesy of Allied Signal Automotive Aftermarket). (c) Continued

Figure 76–22 Fixed brake calipers must be centered over the rotor with their pistons parallel to the rotor friction surfaces. ,[object Object],If the caliper is not properly aligned, pistons will contact pads at an angle and cause tapered wear of the brake linings. If misalignment is bad enough, the pistons will cock into their bores, suffer increased wear, and possibly crack.

Figure 76–23 This floating caliper mounts on a separate anchor plate that bolts to the vehicle suspension. Continued

[object Object],Figure 76–24 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. Continued

[object Object],Figure 76–25 Caliper flex can cause tapered wear of the brake lining. Caliper flex allows the caliper body to twist slightly when the brakes are applied, causing tapered wear of the brake pad lining material. Floating and sliding calipers do not have the mass of fixed calipers, and their flexible mounting systems slow transfer of heat from the caliper body to the anchor plate and other components that aid in cooling. Continued

[object Object],Figure 76–26 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. Continued The caliper body is supported by bushings and/or O-rings that allow it to “float” or slide on guide pins or locating sleeves attached to the anchor plate. For this reason, some automakers call the floating caliper a pin - slider caliper .

Figure 76–27 Floating calipers are supported by rubber O-rings or plastic bushings. ,[object Object],O-rings are generally made of high-temperature synthetic rubber. The guide pins and sleeves are made of steel and come in a variety of shapes and sizes for different caliper designs. See Figure 76–28. Continued

Figure 76–28 Metal guide pins and sleeves are used to retain and locate floating calipers. Continued

[object Object],What is a Low - Drag Caliper? - Part 1 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 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.

… Low - Drag Caliper? - Part 2 Figure 76–29 In a standard disc brake caliper, the square-cut 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, resulting in less friction between the disc brake pads and the rotor when the brakes are released.

Figure 76–30 Exploded view of a typical sliding brake caliper. ,[object Object],Continued

Figure 76–31 Sliding calipers move on machined ways. ,[object Object],Retaining clips and design of the caliper prevent the body from coming out of the ways once the caliper is assembled. On some, the ways may have to be filed for proper clearance between caliper body and anchor plate if the caliper is replaced. Sliding calipers depend on good lubrication for proper operation. If not properly coated with high-temperature brake grease, the ways can corrode, causing the caliper to drag or seize.

REAR DISC BRAKES ,[object Object],Continued

Figure 76–32 Exploded view of a typical rear disc brake with an integral parking brake. The parking brake lever mechanically pushes the calliper piston against the rotor. ,[object Object],[object Object],This is done by installing a series of cables, levers, and internal parts to mechanically actuate the brake caliper. See Figures 76–33 and 76-34. Continued

Figure 76–33 This single-piston brake caliper is mechanically actuated to serve as a parking brake. Continued

Figure 76–34 Drum parking brakes are fitted inside the rotors on this vehicle equipped with rear disc brakes. ,[object Object]

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued ( cont. )

SUMMARY ,[object Object],[object Object],[object Object],( cont. )

ASE Brakes Certification Test Prep: Disc Brake Diagnosis and Repair

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ASE Brakes Certification Test Prep: Disc Brake Diagnosis and Repair