<ul><li>Prepare for ASE Brakes (A5) certification test content area “C” (Disc Brake Diagnosis and Repair). </li></ul><ul><li>Describe how disc brakes function. </li></ul><ul><li>Name the parts of a typical disc brake system. </li></ul><ul><li>Describe the construction of disc brake pads. </li></ul>OBJECTIVES: After studying Chapter 76, the reader should be able to: Continued
<ul><li>Describe the difference between fixed caliper and floating or sliding caliper. </li></ul><ul><li>Explain the difference between a standard caliper and a low-drag caliper. </li></ul>OBJECTIVES: After studying Chapter 76, the reader should be able to:
<ul><li>mechanical fade • moldbonded lining </li></ul><ul><li>NAO • NAS • natural frequency • nonasbestos pad wear indicators • pin-slider caliper riveted linings semimets • sintered metal • sintering • sliding caliper • swept area water fade • ways </li></ul>KEY TERMS:
DISC BRAKES <ul><li>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 term and commonly used spelling. The rotor is attached to and stops the wheel. Disc brakes are used on the front wheels of late-model vehicles, and on the rear wheels of an increasing number of automobiles. Disc brakes were adopted primarily because they can supply greater stopping power than drum brakes with less fade. Disc brakes are well suited for use as front brakes, which must provide 60% to 80% of the vehicle’s total stopping power. </li></ul>
DISC BRAKE ADVANTAGES <ul><li>Although increased Federal brake performance standards hastened the switch to disc brakes, the front drum brakes would eventually have been eliminated anyway because disc brakes are superior in almost every respect. The disc brake friction assembly has several significant strong points, and only a few relatively minor weak points. </li></ul><ul><li>See Figure 76-1. </li></ul>Continued
Figure 76–1 A typical disc brake assembly. Continued
<ul><li>The main advantages of the disc brake include the following: </li></ul>Continued <ul><li>Fade resistance </li></ul><ul><li>Self-adjustment </li></ul><ul><li>Freedom from pull </li></ul>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: <ul><li>Mechanical fade </li></ul><ul><li>Lining fade </li></ul><ul><li>Gas fade </li></ul><ul><li>Water fade </li></ul>
<ul><li>Disc brakes avoid heat-induced fade because all of the major parts of a disc brake are exposed to the air flowing over the friction assembly. They also have greater swept area , the amount of brake drum or rotor friction surface that moves past the brake linings every time the drum or rotor completes a rotation. A larger swept area allows the heat generated in braking to be transferred more rapidly into the rotor for better cooling. A disc brake has swept area on both sides of the rotor. A drum brake has swept area only on the inside of the drum. Large diameter rotors, however, require larger diameter wheels to provide the necessary clearance. This is why high-performance vehicles use 17-inch diameter and larger wheels. </li></ul>Continued
<ul><li>Mechanical Fade Unlike a brake drum mechanical fade is not a problem, as the disc brake rotor expands toward the brake linings as it heats up rather than away from them. This fundamental design difference makes it impossible for heat to cause the rotor to expand out of contact with the brake linings. There is never the need to move the brake linings out to keep them in contact with the rotor, so brake pedal travel does not increase. If the brake pedal on a vehicle with disc brakes drops toward the floor, it is almost always a sign of vapor lock, a fluid leak, fluid bypassing the seals in the master cylinder, or mechanical fade of the rear drum brakes. </li></ul>Continued
<ul><li>Lining Fade If the brakes become overheated lining fade can occur . A warm brake performs better than a cold brake. When too much heat is generated by braking, the lining material overheats, friction coefficient drops, and lining fade occurs. </li></ul>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. <ul><li>If the pads are overheated to the where the lining material is physically damaged, the brakes will not recover their full stopping power until the pads are replaced. </li></ul>Continued
<ul><li>Gas Fade A problem only under severe braking conditions when hot gasses and dust particles from the linings are trapped between the brake linings and rotor, where they act as lubricants. </li></ul>Continued Though disc brakes operate at higher temperatures than drum brakes, they have fewer gas fade problems for these reasons: <ul><li>Disc brakes do not have a drum to contain gasses and particles in the area around the brake linings. </li></ul><ul><li>The constant flow of air over the brake carries away contaminants that might otherwise build up. </li></ul><ul><li>The surface area of the brake lining material in a disc brake is smaller than that of a comparable drum brake and this allows gasses and particles to escape more easily. </li></ul>The symptoms of gas fade are the same as those for lining fade.
<ul><li>To help prevent gas fade, many brake pads have slots cut in the lining material. </li></ul>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.
<ul><li>Water Fade Not a big problem with disc brakes because centrifugal force created by spinning rotor throws off moisture, and the brake pads positioned only a few thousandths of an inch away from the rotor continuously wipe it clean. When the brakes are applied, the leading edge of the brake pad lining material wipes the last bit of water from the disc. Water fade is prevented once good lining-to-rotor contact is established. Although far more resistant to water fade than drum brakes, disc brakes are not entirely free from its effects. Splash shields and the vehicle’s wheels help keep water off of the rotor, and the brake lining materials specified for most vehicles minimize the effects of water fade. </li></ul>Continued
<ul><li>Self-Adjusting Ability Disc brakes are self-adjusting because any wear of the linings is automatically compensated for by the action of the brake caliper. When brakes are applied the caliper pistons move out as far as needed to force the brake pads into contact with the rotor. When the brakes are released, the piston retracts only the small distance dictated by rotor runout and piston seal flex. The surface finish on the piston must be clean to allow the piston to slide past this seal. Moisture accumulation inside the caliper often causes the piston to rust. If the force of the caliper seal is not strong enough, the piston stays in the applied position. Because the brake pads are still in contact with the rotor, one or both pads will show excessive wear. </li></ul>Continued
<ul><li>Normal Operation The piston moves just enough to distort the caliper seal and returns to the original position when the brake pedal is released. </li></ul>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
<ul><li>Freedom from Pull A disc brake will stop straighter under a wider range of conditions than will a drum brake. A disc brake is self-cleaning, will throw off most water, and is less likely to pull. Disc brakes do not have self-energizing or servo action. These actions increase the power of drum brakes, but depend on friction between the linings and drum for their effect. This means that even a small loss of lining-to-drum friction causes a large loss of braking power and a significant side-to-side variation in the amount of braking force. Since disc brakes do not use friction between the linings and rotor to increase their braking power, the effects of a loss of friction on one side of the vehicle are less pronounced than drum brakes. </li></ul>Continued
<ul><li>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. </li></ul>Check the Tire Size for a Pulling Problem
DISC BRAKE DISADVANTAGES <ul><li>The most notable fact about the disadvantages of disc brakes is that there are so few. The weaknesses of disc brakes include: </li></ul>Continued <ul><li>Brake dust </li></ul><ul><li>No self-energizing or servo action </li></ul><ul><li>Brake noise </li></ul><ul><li>Poor parking brake performance </li></ul>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.
<ul><li>Brake dust from semimetallic brake pads often discolors the front wheels. Customers often complain to service techs 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 brake dust. </li></ul>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
<ul><li>Brake Noise The biggest complaint about disc brakes is various squeaks and squeals during a brake application. These noises are caused by high-frequency rattling or vibration of brake pads. Several methods are used to quiet noisy disc brakes. Manufacturers use specific lining materials that damp vibrations, and most calipers have antirattle clips or springs that hold the pads in the caliper under tension to help prevent vibration. See Figure 76–6. </li></ul>Continued
Figure 76–6 Antirattle clips reduce brake pad movement and vibration. <ul><li>Some calipers use shims between brake pad backing plate and caliper piston to damp vibrations. </li></ul>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
<ul><li>Poor Parking Brake Performance The lack of self-energizing and servo action plays a large part in poor disc brake parking brake performance. The lining-to-rotor contact area of a disc brake is somewhat smaller than the lining-to-drum contact area of a drum brake. This causes the disc brake to have a lower static coefficient of friction, and less holding power when the vehicle is stopped. </li></ul>Continued
DISC BRAKE CONSTRUCTION <ul><li>A disc brake is relatively simple compared with a drum brake. The major disc brake friction assembly components include: </li></ul>Continued <ul><li>Caliper </li></ul><ul><li>Splash shield </li></ul><ul><li>Brake pads </li></ul><ul><li>Brake rotor </li></ul>
Figure 76–8 This brake caliper attaches to the front spindle. <ul><li>Caliper Except the rotor, the caliper is the largest part of a disc brake friction assembly. </li></ul>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
<ul><li>Splash Shield The splash shield bolts to the front spindle or steering knuckle, or in rear disc brake applications, to the axle flange or a suspension adapter plate. The job of the splash shield is to protect the inner side of the brake rotor from water and other contaminants, where the outer side of the rotor is protected by the wheel. Most splash shields are made of stamped steel or plastic. </li></ul>
DISC BRAKE PADS <ul><li>The lining of a disc brake is part of the brake pad assembly. </li></ul>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. <ul><li>Other pad backing plates have tabs with holes in them as shown here. </li></ul>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
<ul><li>As with brake shoes, the lining material of a disc pad can be any one of a number of products that can be fastened to the backing plate in several ways. The edges of the lining material on a brake pad are usually perpendicular to the rotor surface, although a few larger pads do have tapered edges to help combat vibration and noise. See Figure 76–14. </li></ul>Continued
Figure 76–14 The lining edges of some brake pads are tapered to help prevent vibration. Continued
<ul><li>Pad Wear Indicators Although not required by law, a growing number of vehicle manufacturers are fitting pad wear indicators to their brakes for safety reasons. 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. See Figure 76–15. </li></ul>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. <ul><li>Electrical wear indicators use a coated electrode imbedded in the lining material to generate the warning signal. </li></ul>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
<ul><li>Pad Assembly Methods As mentioned in Chapter 74, there are several methods used to mount brake linings, including: </li></ul>Continued <ul><li>Riveting </li></ul><ul><li>Bonding </li></ul><ul><li>Mold bonding </li></ul>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.
<ul><li>Mold Bonding Manufacturing process that combines the advantages of bonding with some of the mechanical strength of riveting makes mold - bonded linings . Friction material in a mold-bonded pad is cured on the backing plate during manufacture. This process is also called integrally molded . High-performance disc brake pads are made in this way. One or more holes are punched in the pad backing plate, and a high-temperature adhesive is applied to it. The backing plate is then installed in a molding machine where uncured friction material is formed onto the plate and forced into the holes. See Figure 76–17. </li></ul>Continued Continued
Figure 76–17 Mold-bonded linings are commonly used in many applications. <ul><li>Once the pad is cured under heat and pressure, the bonding adhesive combines with the portions of the lining that extend into the backing plate holes to solidly lock the brake block in place. </li></ul>Continued
<ul><li>Brake Lining Composition Shoes and pads operate under the most extreme conditions in the entire brake system and are subject to a great deal of wear. The replacement of worn brake shoes and pads is a common part of brake service. Friction materials such as disc brake pads or drum brake shoes contain a mixture of ingredients. These materials include a binder such as a thermosetting resin, fibers for reinforcement, and friction modifiers to obtain a desired coefficient of friction. The various ingredients in brake lining are mixed and molded into the shape of the finished product. The fibers in the material are the only thing holding this mixture together. A large press is used to force the ingredients together to form a brake block , which eventually becomes the brake lining. </li></ul>Continued
SEMIMETALLIC FRICTION MATERIAL <ul><li>The term semimetallic refers to brake lining material that uses metal rather than asbestos in its formulation. It still uses resins and binders and is, therefore, not 100% metal, but rather, semimetallic. Semimetallics are commonly called semimets . </li></ul><ul><li>The metal in most metallic linings is made from metal particles that have been fused together without melting. This process is called sintering and the result is called sintered metal linings. See Figure 76–18. </li></ul>Continued
Figure 76–18 Poor-quality semimetallic disc brake pad. The screwdriver is pointing to large chunks of steel embedded in the lining. Continued
<ul><li>Most semimetallic linings do not contain asbestos . Semimetallic linings require a very smooth finish on the rotor because the metal in the lining does not conform to the surface of the rotor, as does asbestos lining. </li></ul>See the chart on Page 916 of your textbook.
<ul><li>Nonasbestos Friction Material Brake pads and linings that use synthetic material such as aramid fibers instead of steel are usually referred to as nonasbestos , nonasbestos organic ( NAO ), or nonasbestos synthetic ( NAS ). Linings are called “synthetic” because synthetic (man-made) fibers are used. These linings use aramid fiber instead of metal as the base material. Aramid is the generic name for aromatic polyamide fibers. Kevlar is the Dupont brand name of aramid and a registered trademark of E.I. Dupont de Nemours and Company. Nonasbestos linings are often quieter than semimetallics and do not cause as much wear to brake rotors as do semimetallic pads. </li></ul>Continued
<ul><li>Carbon Fiber Friction Material Newest and most expensive of the lining materials. Carbon fiber material is often called CFRC ( carbon fiber - reinforced carbon ). It is composed of a carbon mix into which reinforcing carbon fibers are embedded. CFRC is commonly used in the brakes of jet aircraft and racing cars. CFRC brakes provide constant friction coefficient whether cold or hot, low wear rates, and low noise development. </li></ul>Continued
<ul><li>Ceramic Friction Material Some vehicle manufacturers use friction materials that contain ceramic fibers. These ceramic fibers are usually potassium titanite. Some vehicle manufacturers do not recommend the use of ceramic friction material because they tend to wear the rotors more than NAO or semimetallic friction materials. </li></ul>Continued
<ul><li>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 D 3 EA. This test is currently voluntary and linings that pass the test can have a “D 3 EA certified” seal placed on the product package. </li></ul>What Does D 3 EA Mean?
<ul><li>The actual amount of each ingredient in a typical brake lining is varied for each application. Each vehicle has its own “recipe” based on vehicle weight and options. For example, a Chevrolet with a light four-cylinder engine and no air conditioning may use a different brake lining recipe than the same vehicle, but with the heavier V-6 engine, air conditioning, and other options that increase the vehicle weight. Both of these brake linings (shoes or pads) may physically fit other similar vehicles, yet their brake lining recipe is different. </li></ul>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 <ul><li>The first letter indicates coefficient of friction when brakes are cold; the second letter the coefficient of friction of the brake lining when the brakes are hot. </li></ul>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.)
<ul><li>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. </li></ul>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 <ul><li>The brake rotor provides the friction surfaces for the brake pads to rub against. </li></ul>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 <ul><li>While the hydraulic operation of all brake calipers is similar, calipers differ in two important areas: how they attach to the vehicle, and how they apply the brake pads to the rotor. The manners in which these tasks are performed determine the design of a disc brake friction assembly. There are three types of calipers: </li></ul>Continued <ul><li>Fixed </li></ul><ul><li>Floating </li></ul><ul><li>Sliding </li></ul>Fixed calipers have several unique features, but sliding and floating calipers share similar features.
<ul><li>Fixed Caliper Design The fixed brake caliper is the earliest design. The fixed caliper has a body manufactured in two halves, and uses two, three, or four pistons to apply the brake pads. The fixed caliper gets its name from the fact that the caliper is rigidly mounted to the suspension. When the brakes are applied, the pistons extend from the caliper bores and apply the brake pads with equal force from both sides of the rotor. No part of the caliper body moves when the brakes are applied. </li></ul><ul><li>See Figure 76-20. </li></ul>Continued
Figure 76–20 Four-piston fixed caliper assembly on a race vehicle. Continued
<ul><li>Fixed Caliper Advantages They are large and heavy, which enables them to absorb and dissipate great amounts of heat. This allows the brake rotor and pads to run cooler, and reduce the amount of heat transferred to the brake fluid. Compared with other caliper designs, a fixed caliper is able to withstand a greater number of repeated hard stops without heat-induced fade or vapor lock of the hydraulic system. Size and rigid mounting of a fixed caliper means it does not flex as much as other designs, usually felt as a spongy brake pedal. Fixed calipers are strong and provide a firm and linear brake pedal feel. The strength and heat-dissipating abilities of fixed calipers make them best suited for heavy-duty use such as in most race vehicles. </li></ul>Continued
<ul><li>Fixed Caliper Disadvantages They add weight to the vehicle. To obtain better fuel economy, manufacturers want to eliminate as much weight as possible from new vehicles. With multiple pistons and split bodies, service is more difficult and allows greater opportunity for leaks. The drilled passages that route fluid through the inside of the caliper body contribute to cracking as miles accumulate and the caliper goes through hundreds of thousands of heating and cooling cycles. See Figure 76–21. </li></ul>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. <ul><li>Fixed Caliper Alignment The caliper body is locked in position, so a fixed caliper must be centered over the rotor and aligned so the pistons contact the brake pad backing plates parallel to the friction surface of the rotor. </li></ul>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.
<ul><li>Floating and Sliding Caliper Design The front brakes of most vehicles are fitted with either floating or sliding calipers, which are not rigidly mounted. The caliper is free to move within a limited range on an anchor plate that is solidly mounted to the vehicle suspension. The anchor plate may be cast into a suspension member (often the front spindle) or it can be a separate piece that bolts to the suspension. See Figure 76–23. </li></ul>Continued
Figure 76–23 This floating caliper mounts on a separate anchor plate that bolts to the vehicle suspension. Continued
<ul><li>When brakes are applied the caliper piston moves out of its bore and applies the inner brake pad. The caliper body moves in the opposite direction on the anchor plate and applies the outer brake pad. </li></ul>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
<ul><li>Floating and Sliding Caliper Advantages Advantages of floating and sliding calipers are lower cost, simple construction, and compact size. Because they have fewer pieces, floating and sliding calipers are cheaper to build and service, and have fewer places where leaks can develop. The smaller size allows better packaging of the caliper on the vehicle. A single-piston caliper fits easily within the diameter of a small wheel. The inboard position also contributes to better cooling because the bulk of the caliper body is exposed to the passing airflow. Floating and sliding calipers have poor parking brake performance. Unlike a fixed caliper, a floating or sliding caliper can be actuated by applying the single piston with a cable and lever mechanism. </li></ul>Continued
<ul><li>Floating and Sliding Caliper Disadvantages The movable caliper body allows a certain degree of flex, which can contribute to a spongy brake pedal. </li></ul>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
<ul><li>Floating Calipers The body of a floating caliper does not make direct metal-to-metal contact with the anchor plate. </li></ul>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. <ul><li>Bushings that support floating calipers are made from a number of materials including rubber, Teflon, and nylon. </li></ul>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
<ul><li>Floating calipers depend on proper lubrication of their pins, sleeves, bushings, and O-rings for smooth operation. If these parts become rusted or corroded, the caliper will bind and stick, causing loss of braking power that is usually accompanied by rapid and unusual wear of brake pads. Special high-temperature brake grease must be used to lubricate these parts any time the caliper is disassembled. Many manufacturers recommend that floating caliper pins, sleeves, bushings, and O-rings be replaced whenever the caliper is serviced. These parts come in a “small parts kit” available from brake part suppliers. </li></ul>Continued
<ul><li>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. </li></ul>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. <ul><li>Sliding Calipers The body of a sliding caliper mounts in direct metal-to-metal contact with the anchor plate. </li></ul>Continued
Figure 76–31 Sliding calipers move on machined ways. <ul><li>Instead of pins and bushings, sliding calipers move on ways cast and machined into the caliper body and anchor plate. </li></ul>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 <ul><li>In recent years, four-wheel disc brake systems have become more common. In most 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. </li></ul>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. <ul><li>Rear Disc Parking Brakes There are two methods of providing parking brakes when rear discs are installed on a vehicle: </li></ul><ul><li>Adapt the disc brake to also function as the parking brake. </li></ul>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. <ul><li>Use mechanically actuated drum brakes inside the rear rotors. </li></ul>
SUMMARY <ul><li>Disc brakes are superior to drum brakes because they are fade resistant, self-adjusting, and are less likely to pull during braking. </li></ul><ul><li>Disc brakes, however, lack self-energization requiring greater force be applied to the brake pedal compared with the drum brakes. </li></ul><ul><li>Disc brakes are more prone to noise than drum brakes. </li></ul><ul><li>A typical disc brake assembly includes the caliper assembly, splash shield, brake pads, and brake rotor </li></ul>Continued
SUMMARY <ul><li>The three basic types of disc brake calipers include fixed, floating, and sliding designs. </li></ul><ul><li>A low-drag caliper requires the use of a quick-take-up master cylinder. </li></ul><ul><li>Some disc brakes are equipped with integral parking brakes. </li></ul><ul><li>Brake pads can be attached using rivets, bonding, or integrally molded. </li></ul>Continued ( cont. )
SUMMARY <ul><li>Typical semimetallic brake linings contain phenolic resin, graphite or carbon particles, steel fibers, ceramic and metal powders plus other modifiers such as rubber scraps. </li></ul><ul><li>Other types of friction material include nonasbestos organic (NAO), nonasbestos synthetic (NAS), and carbon fiber-reinforced carbon (CFRC). </li></ul><ul><li>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. </li></ul>( cont. )