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DISC BRAKE OPERATION 1 The DISC brake differs from drum brakes in the use of a disc or rotor The disc is bolted to and revolves with the wheel hub The disc may be solid or slotted for improved cooling 2 The brake CALIPER generally houses one or two pistons Others have up to four pistons The three piston setup uses two small pistons on one side of the disc and a larger one on the opposite side Most four piston arrangements incorporate two pistons of equal size on each side of the disc 3 The caliper is bolted to the spindle on some models see Fig 1 Brake friction pads are so arranged that when hydraulic pressure is built up behind the pistons the pads will be forced against the disc providing braking effort see Fig 2 4 Another type of caliper uses a single piston on one side only This caliper is free to slide sideways to allow brake pads to align with disc This is also known as a floating caliper see Fig 3 5 Pistons which can be constructed from cast iron aluminum or plastic are fitted to the caliper cylinders with the outer ends resting against friction pads Rubber boots exclude the entry of dirt and moisture 7 Chapter Two Caliper Training Fig 1 Typical front wheel disc brake Disc brake has excellent cooling characteristics making it highly resistant to brake fade Fig 2 worn pads cause lower reservoir level GUIDE PIN GUIDE PIN SPLASH SHIELD ANCHOR PLATE CALIPER ASSEMBLY ROTOR BRAKE PAD MASTER CYLINDER RESERVOIR SMALL VOLUME BEHIND PISTON THICK PAD THIN PAD INCREASED VOLUME BEHIND PISTON 6 One design allows the brake pads to drag VERYLIGHTLYagainst the rotating disc at all times Another type operates with only a minimal pad to disc clearance about 005 in or 0 13 mm 7 Ahydraulic line from the master cylinder leads to one side of the caliper The other side will receive pressure through a crossover external line or bypassing the fluid internally through the casting see Fig 4 5 8 The caliper piston operates against a snug seal ring snapped into a groove in the cylinder wall When the brake is applied the piston moves outward In so doing it stretches the seal to one side When brake pressure is released the seal returns to its normal position This seal roll action pulls the piston back around 005 in 0 13 mm providing a small amount of pad lining to disc clearance As the lining pads wear the piston moves out through the seal automatically keeping the proper pad to disc clearance 8 Fig 3 Cross section of disc brake showing caliper construction This is a single piston sliding floating caliper setup Dodge Fig 4 Two piece disc brake caliper using four pistons Bendix WHEEL CALIPER ADAPTOR MOUNTING BOLT BOOT SEAL INNER BEARING SPINDLE OUTER BEARING BRAKING DISC HUB ASSEMBLY SPLASH SHIELD STEERING KNUCKLE SHOE LINING WHEEL STUD SINGLE PISTON BRAKE FLUID SEAL OUTER CALIPER HALF SEAL RING SEAL RING PISTON DUST BOOT PISTON SEAL PISTON SPRINGS PISTON SPRINGS CALIPER BOLTS INNER CALIPER HALF PISTON ASSEMBLIES PISTON ASSEMBLIES SHOE LINING ASSEMBLIES BLEED SCREW 9 Fig 5 Various disc brake assemblies A Three piston B Single piston caliper C Cross section showing relationship of caliper to disc or rotor D Four piston caliper E Ventilated disc and inboard and outboard brake plate and lining assembly Note pad lining wear indicator When pad is worn indicator strikes disc making noise to warn driver F Sliding and fixed caliper braking action G Dual two piston caliper H Solid or nonventilated disc Delco EIS Girling 1 What are phenolics Aphenol formaldehyde resin combined with 80 glass fiber to provide a high performance low cost material Phenolic materials are light weight heat resistant dimensionally stable at elevated temperatures and will not corrode or rust when exposed to harsh environments 2 What otherautomotive applications use phenolic materials The most popular use is in distributor caps and rotors Phenolic materials are also widely used in torque converters and their most recent automotive application is for pulleys on the new Serpentine belt system 3 I ve always used metal caliperpistons Why should I use phenolic pistons Phenolic brake pistons offer many advantages over their aluminum and steel counterparts Foremost among these is the fact that 1 phenolic pistons thermally insulate the brake fluid from the heat generated by the friction of pad against rotor This thermal insulation greatly reduces the possibility of brake failure resulting from fluid boil In addition phenolic pistons 2 resist corrosion due to moisture and road salt 3 improve fuel economy because of their light weight and low drag and 4 cost less than their metal counterparts 4 Who uses phenolic caliperpistons 90 of all Ford cars and light trucks now come with phenolic brake pistons and 100 of Chrysler production is equipped with phenolics GM is starting to use phenolics also 5 What about brake problems with mid 70s Chryslercars The phenolic piston has long been blamed for brake problems with mid 1970 Chrysler cars Brake experts now know that these problems were caused by poor dust boost design and incorrect piston diameters 6 Can I mix a phenolic and steel piston on opposing front calipers Yes however you should follow OE procedures whenever possible Of greater concern is piston quality All phenolic pistons are produced by OE approved sources and meet new car standards for safe performance service life and quality assurance 7 Sometimes when I work with a phenolic piston it chips Special caution should be used when these front disc brake calipers require service to avoid any unnecessary piston replacement Do not use a screwdriver or any similar tool to pry piston into or out of the bore to prevent piston chipping or scuffing Do not replace pistons for cosmetic irregularities or small chips between the piston boot grooves and shoe face 10 PHENOLICS WHY 1 Because of today s demands on braking a quality product is not only needed it is expected 2 Freeway traffic mountain roads bad weather conditions heavy stop and go traffic panic stops and down sized vehicles create demands on today s braking systems 3 Key friction material terms to be familiar with a Coefficient of friction also called friction effectiveness is a unit of measurement that can be expressed in many ways It is the distance it takes to stop a vehicle It is also expressed as required application pressure or developed torques under certain conditions The coefficient of friction involves several variables Various car weights and speeds alter stopping distances b Road surface conditions alter stopping distances c Different linings give different stopping results Superior or quality linings stop a vehicle in a shorter distance In addition the friction coefficient should be constant and it should be sufficiently high to ensure comfortable braking Comfortable braking means the slowing of a vehicle safely smoothly and quietly d Heat fadeis theability of the friction material to maintain its effectiveness at higher temperatures e Fade resistance is the ability of the friction material to maintain its effectiveness at elevated temperatures f Moistureor water recovery isthe ability of the lining to regain its original stopping power as it dries 4 The three types of lining a Semi metallicis a type of friction material that utilizes steel fibers and sponge ore for structural strength and reinforcement b Asbestos organicis a type of friction material that has asbestos fibers as its primary reinforcement ingredient c Asbestos free organiccan be made up of glass fibers steel fibers and other composites as its primary ingredients 5 Three basic ingredients of brake pads a Asbestos a foundation of organic linings This is used to attain required friction levels while adding strength and wear resistance to the lining Asbestos free friction material uses various combinations of natural and synthetic fibers as principal ingredients This type of lining performs well under most conditions However asbestos could create a health risk and is becoming obsolete b Resin binders are extremely important and must be high quality to assure proper binding of the formulation to keep the lining in tact c Friction modifiers are used to raise or decrease the friction levels of the lining as required to keep braking performance 6 Friction material is fastened to the metal shoe or disc plate in three ways a Bondingis when the friction material is actually glued to the metal b Riveted segmentsare actually riveted to the metal 11 FRICTION FACTS Brake Linings c Integrally molding is a process where holes are punched into the metal plate the plate is placed in a mold then filled with friction material Then under pressure the assembly is formed High temperature adhesives hold the friction material securely on the metal 7 Important points about brake noise a High friction linings tend to be noisier b Bonded linings tend to be noisier than riveted linings c Noise can be evident under special conditions such as extreme heat or cold d Most noises are not caused by friction materials most likely it comes from loose hardware disc or shoe vibration wear sensors or metal to metal contact 8 Lining integrity is the ability of the friction material to resist cracking checking and even chucking off the shoe or plate 9 There are three categories of friction material a Competitive or low priced Usually flexible and bonded to a brake shoe May contain large amounts of rubber filler and briefly cured in rolls Probably provide 10 000 to 20 000 miles before replacement is required b Standard or medium priced Semi rigid often with wire backing Tend to have better fade and recovery characteristics than competitive linings May be bonded or riveted c Premium linings the best Rigid and fully cured Contain best raw materials May be bonded or riveted Under normal use can expect 35 000 or more miles 10 Balanced braking is the equal distribution of vehicle weight on the brake system To achieve balance braking a Always replace brake parts according to O E specifications i e semi metallic pads should replace semi metallic pads b Always use the same brand linings on both the front and rear wheels c Unbalanced braking effects wear life fade and recovery characteristics d Always replace brake linings as a set Never separate to ensure equal braking 11 Benefits of premium linings a Better fit to drum b Longer wear life c Better recovery d Less break in time e Structural integrity f Smooth and balanced braking g Minimum noise factors h More economical in long term 12 Organic materials are manufactured with disc pads drum brakes and brake blocks and are used on heavy equipment for most automotive applications Some of the metallics were developed in the late 1960 s for use on police cars and taxis Today they are used on front brake systems of all automobiles offering better resistance and reduced noise 13 Fig 6 Exploded view of a rear caliper using an integral parking brake arrangement BRAKE FLUID INFORMATION Dot 3 95 of all vehicles mineral based Dot 3 4 Brake fluids are hygroscopic absorbs moisture Dot 5 5 of all vehicles silicone based Mixing NO first sign is brake failure and silicone lumps in reservoir
