Automotive Brake Systems Chapter 3 Braking System Components and Performance Standards Learning Objectives • Describe the fundamentals of brake systems. • Describe brake design requirements. • List the six brake system categories. • State the purpose of an antilock brake system. • Discuss federal brake standards. BRAKE SYSTEM OVERVIEW •How many different types of braking systems you can come up with? •What do you think was the first brake design? EARLY BRAKE DESIGNS • Like every other part of the modern automobile, brake systems evolved from simple beginnings. • Shortly after the invention of the wheel, a new problem emerged. Carts and wagons were reluctant to slow or stop once set in motion. • The locked wheel was one of the first known brake systems. • It prevented driving down a hill to fast for the animal, and served as a parking brake. • A person would have to walk along side the cart to insert the stick through the hole in the wheel. EARLY BRAKE DESIGNS • What are some of the problems with this design of brakes? • Would this type of brake system work on today's vehicles? EARLY BRAKE DESIGNS • Brakes should stop the vehicle without damage to the brakes or anything else. • Brakes should stop the vehicle smoothly and consistently. • Brakes should stop the vehicle over thousands of times before service is required. EARLY BRAKE DESIGNS A MAJOR ADVANCE OF THE BRAKE • Will this stop the vehicle “Smoothly”? • What is the current name for the “Stopping brake”? • What are some problems with this type of brake? • What happens if the brake is only on one side of the vehicle? EARLY BRAKE DESIGNS • The next advance in vehicle brakes saw widespread use in the 1800’s. • The farm wagon of the period used a wooden brake shoe that was pressed against the wheel rim by a lever-operated linkage. • Notice the position of the fulcrum, pivot points, and levers in the drawing. • How much force would be exerted by the block on the wheel if the operator pushed the lever with 100 pounds of force. The rim-contact farm wagon brake used levers to increase the brake application force. EARLY BRAKE DESIGNS The rim-contact farm wagon brake used levers to increase the brake application force. • Would this stop today's vehicles? • What about friction? • What is friction? • How much power does the brake system need? • What speeds are the vehicle capable of? • Now how fast can the vehicle stop? AUTOMOTIVE BRAKE DESIGNS • The first automobiles were little more than farm wagons or carriages fitted with engines and drivetrains. • The early cars had brake systems much like those of farm wagons, including lever-operated brake shoes that contacted iron rims of spoked wooden wheels. Notice the length of the upper lever compared to the distance from the fulcrum to lower end. External Contracting-Band Brakes • Wagon brakes would no longer stop the faster vehicles and more braking power was required. The external band brake was introduced to solve the problem. Internal Expanding-Band Brakes • The failure of the external brakes in wet weather and quickly wearing out from grit and sand led to the introduction of the internal band brake. • The internal band was unable to expand enough when the drums heated up to make solid contact and braking power was lost • “Brake Fade”, “Mechanical Fade” • A few cars did use internal expanding-band brakes as their primary system, but the drawbacks made the design less than ideal. • Because of these inherent problems, most of these were used as parking brakes. Timeline of Brakes • • • • • • • • • • • • • • • Model T, external contracting-band brake systems 1908 Asbestos brake linings 1921 Duesenberg, hydraulic brakes 1924 Chrysler, first mass produced 4 wheel hydraulic brakes 1927, Vacuum booster 1929, Vacuum power brakes 1956 Citroen DS, front disc brakes 1959, Self adjusting brakes 1960, Dual circuit hydraulic brakes 1967 DOT formed, dual master cylinder mandatory 1969, First ABS on the Chrysler Imperial 1970, Front disc brakes as standard equipment 1980s, ABS 1990s, Traction control 2000s, ESP Internal Expanding-Shoe Brakes • Equipped with 2 or more rigid brake shoes, this design was highly reliable and was the standard of the industry well into the 60’s. It is still in use today as rear brakes. Disc Brakes • Disc Brakes have been around since early automotive years but were not very practical in mechanically actuated form. • Disc Brakes require higher application forces to achieve the same amount of stopping power as a drum brake. Fundamentals of Brake Systems • Brakes are by far the most important mechanism on any vehicle. • The safety and lives of those riding in the vehicle depend on proper operation of the braking system. • Brakes are an energy-absorbing mechanism that converts vehicle movement into heat while stopping the rotation of the wheels. • All braking systems are designed to reduce the speed and stop a moving vehicle. • And to keep the vehicle from moving if it is stationary. • Service brakes are the main driver-operated brakes of the vehicle. FIGURE 3–1 Typical vehicle brake system showing all typical components. Fundamentals of Brake Systems: Brake System Parts • Most vehicles built since the late 1920s use a brake on each wheel. • To stop a wheel, the driver exerts a force on a brake pedal. • The force on the brake pedal pressurizes brake fluid in a master cylinder. • This hydraulic force (liquid under pressure) is transferred through steel lines and flexible brake lines to a wheel cylinder or caliper at each wheel. Fundamentals of Brake Systems: Brake System Parts • Hydraulic pressure to each wheel cylinder or caliper is used to force friction materials against the brake drum or rotor. • The friction between the stationary friction material and the rotating drum or rotor (disc) causes the rotating part to slow and eventually stop. • Since the wheels are attached to the drums or rotors, the wheels of the vehicles also stop. Fundamentals of Brake Systems: Brake System Parts • Drum Brakes • Used on the rear of many modern RWD, FWD, and 4WD vehicles. • Also used as a parking brake within some rear disc systems. • Actuated by shoes being moved outward to make contact with inner surface of the drum. • Economical to manufacture, service and repair. • Disc Brakes • Used on the front of most vehicles built since the early 1970s and also on the rear of many vehicles. • Actuated by the caliper squeezing brake pads on both sides of a rotor/disc. Type of Brake Rotating Part Friction Part Drum brakes Brake drum Brake shoes Disc brakes Rotor or disc Brake pads FIGURE 3–2 Typical drum brake assembly. FIGURE 3–3 Typical disc brake assembly. Brake Design Requirements • All braking forces must provide for the following: • Equal forces must be applied to both the left and right sides of the vehicle to assure straight stops. • Hydraulic systems must be properly engineered and serviced to provide for changes as vehicle weight shifts forward during braking. • Antilock braking systems (ABS) are specifically designed to prevent wheel lockup under all driving conditions, including wet or icy road conditions. Brake Design Requirements • All braking forces must provide for the following: • The hydraulic system must use a fluid that will not evaporate or freeze. • The friction material (brake lining or brake pads) must be designed to provide adequate friction between the stationary axles and the rotating drum or rotor. • The design of the braking system should secure the brake lining solidly to prevent the movement of the friction material during braking. • Most braking systems incorporate a power assist unit that reduces the driver’s effort but does not reduce stopping distance. Brake System Categories • Brake system components can be classified into six subsystems: 1. Apply system: The apply system consists of all the pedals, levers, or linkage needed to activate a braking force. 2. Boost system: The boost (power brake) system is used to reduce the force that the driver must exert on the brake pedal. 3. Hydraulic system: Brake pedal force is transferred to the hydraulic system, where the force is directed through pipes and hoses to the wheel brakes. 4. Wheel brakes: Hydraulic pressure moves a piston(s) to press the friction material against a rotating disc/rotor or drum. 5. Brake balance control system: Metering valves, proportioning valves, and ABS systems are the components used to ensure that brakes are applied quickly and with balanced pressure. 6. Brake warning lights: There are two brake system related warning lights. 1. 2. Red brake warning lamp (RBWL) signifies a hydraulic system failure. Amber ABS warning lamp or dim RBWL indicates a possible ABS concern. FIGURE 3–4 Typical brake system components. FIGURE 3–5 The red brake warning light will remain on after a bulb test if there is a fault with the hydraulic part of the brake system. FIGURE 3–6 A typical adjustable pedal assembly. Both the accelerator and the brake pedal can be moved forward and rearward by using the adjustable pedal position switch. FIGURE 3–7 Typical components of an antilock braking system (ABS) used on a rear-wheel-drive vehicle. Federal Brake Standards • The statutes pertaining to automotive brake systems are part of the Federal Motor Vehicle Safety Standards (FMVSS) • Established by the United States Department of Transportation (DOT). • Several standards apply to specific components within the brake system. • The overall service and parking brake systems are dealt with in standard 135. Federal Brake Standards: FMVSS 135 Brake Test • The overall FMVSS 135 brake test procedure consists of up to 24 steps, depending on the vehicle’s configuration and braking system. • The actual performance tests are made with the vehicle loaded to both the manufacturer’s specified gross vehicle weight rating (GVWR) • And the lightly loaded vehicle weight (LLVW), with certain applied brake forces. • http://www.nhtsa.gov/cars/rules/import/FMVSS/index.html • There are precise instructions for every step of the test, including • The number of times the tests must be repeated, the sequence of the testing, and the allowable stopping distance for the particular type of vehicle. Summary • Drum brakes are used on the rear of many vehicles. • Disc brakes are used on the front of most vehicles. • An antilock braking system (ABS) pulses the hydraulic force to the wheels to prevent the tires from locking up. • The six brake subsystems include • Apply system, boost system, hydraulic system, wheel brakes, brake balance control system (which includes ABS), and brake panel warning lights. • The federal brake standards regulate specific brake performance requirements. For Next Class (Homework) •Take chapter 3 quiz on CANVAS by Sunday at 11:59pm. •Start reading chapter 4 in the text.
