Modern Automotive Technology

1. Modern Automotive Technology by Russell Krick PublisherThe Goodheart-Willcox Co., Inc.Tinley Park, Illinois PowerPoint for

2. Chapter 71 Brake System Fundamentals

3. Contents • Basic brake system • Braking ratio • Brake system hydraulics • Brake system components • Parking brakes

4. Automotive Brakes • Provide a means of using friction to either slow, stop, or hold the wheels of a vehicle • When a car is moving, it has energy stored in the form of inertia (kinetic energy) • To stop the vehicle, the brakes convert kinetic (moving) energy into heat

5. Basic Brake System

6. Brake Operation • When the driver pushes on the brake pedal, lever action pushes a rod into the brake booster and master cylinder • Pressure developed in the master cylinder forces fluid through the brake lines to the wheel brake assemblies • Brake assemblies use this pressure to cause friction for braking

7. Parking Brake • Parking brake system uses cables or rods to mechanically apply the rear brakes • Provides a system for holding the wheels when the vehicle is parked, or stopping the vehicle during complete hydraulic brake system failure

8. Drum and Disc Brakes • Two common types of brake assemblies: • Disc brakes • often used on the front wheels • Drum brakes • often used on the rear wheels

9. Drum and Disc Brakes Disc brakes Drum brakes

10. Braking Ratio • Comparison of front wheel braking effort to rear wheel braking effort • When a vehicle stops, its weight tends to transfer onto the front wheels • Rear tires lose some of their grip • Front wheels must do more of the braking

11. Typical Braking Ratios • Rear wheel drive vehicles: • front brakes may handle 60% to 70% of the braking, rear brakes handle 30% to 40% • Front wheel drive vehicles: • more weight is concentrated on the front wheels • braking ratio is even higher at the front wheels

12. Brake System Hydraulics • Automotive brakes use a hydraulic system • Hydraulic brakes use a confined brake fluid to transfer brake pedal motion and pressure to each of the wheel brake assemblies

13. Hydraulic Principles • Liquids in a confined area will not compress • When pressure is applied to a closed system, pressure is exerted equally in all directions • Hydraulics can be used to increase or decrease force or motion

14. Hydraulic System Action • Pressure and motion can be transferred from one cylinder to another • Cylinders of the same size: • if one piston is moved, the other will move the same amount with the same force • Cylinders of different size: • if the smaller piston is moved, the larger piston will move with more force but will move a shorter distance

15. Hydraulic System Action In a hydraulic jack, a small piston acts on a large piston, resulting in great force, but a small amount of movement

16. Hydraulic System Action Hydraulic jack operation

17. Hydraulic Brake Action • Master cylinder acts on the pumping piston that supplies system pressure • Wheel cylinder acts as the power piston, moving the friction linings into contact with the rotating drums or discs

18. Hydraulic Brake Action

19. Brake System Components

20. Brake Pedal Assembly Acts as a lever to increase the force applied to the master cylinder piston

21. Master Cylinder • Foot operated pump that forces fluid to the brake lines and wheel cylinders • Develops pressure to apply the brakes • Equalizes pressure required for braking • Keeps the system full of fluid as the linings wear • May maintain a slight pressure to keep contaminants from entering the system

22. Master Cylinder Brakes applied Brakes released

23. Master Cylinder Components

24. Master Cylinder Components • Cup and piston • used to pressurize the system • when they are pushed forward, they trap fluid, building pressure • Intake port • allows fluid to enter the rear of the cylinder as the piston slides forward • fluid flows from the reservoir, into the area behind the piston and cup

25. Master Cylinder Components • Compensating port • releases pressure when the piston returns to the released position • fluid can flow back into the reservoir through the compensating port • Residual pressure valves • maintain residual fluid pressure of approximately 10 psi (69 kPa) to help keep contaminants out of the system

26. Master Cylinder Components • Rubber boot • prevents dust, dirt, and moisture from entering the back of the master cylinder • Reservoir • stores an extra supply of brake fluid • cast as part of the housing or a removable plastic part

27. Piston and Cup

28. Residual Pressure Valve Brakes applied, fluid flows freely Brakes released, valve closes to restrict the return of fluid to the master cylinder

29. Dual Master Cylinder

30. Dual Master Cylinder • Uses two separate hydraulic pistons and two fluid reservoirs • Each piston operates a hydraulic circuit that controls two wheel brake assemblies • If there is a leak in one of the hydraulic circuits, the other circuit can still provide braking action on two wheels

31. Dual Master Cylinder

32. Dual Master Cylinder(Normal Operation) Both pistons produce pressure to all four wheel brake assemblies

33. Dual Master Cylinder(Rear Brake Circuit Leak) Primary piston pushes on thesecondary piston, two wheel brakeassemblies still work

34. Dual Master Cylinder(Front Brake Circuit Leak) Secondary piston slides forward,primary piston operates twowheel brakes normally

35. Brake Fluid • Specially blended hydraulic fluid that transfers pressure to the wheel brake assemblies • Rated by the SAE and DOT • SAE (Society of Automotive Engineers) • DOT (Department of Transportation)

36. Brake Fluid • Brake fluid must have the following characteristics: • correct viscosity at all temperatures • high boiling point • noncorrosive • water tolerant • lubricates components • low freezing point

37. Brake Lines and Hoses • Transfer fluid pressure from the master cylinder to the wheel brake assemblies • Lines • made of double wall steel tubing • tubing ends use double flare or ISO flare • Hoses • made of reinforced rubber • used where flexing action is necessary

38. Brake Lines and Hoses

39. Brake Line Hardware Brackets and clips secure the hoses and lines to prevent damage

40. Brake Systems Often used on rear-wheel-drive vehicles Often used on front-wheel-drive vehicles, with high front to rear brake ratio

41. Disc Brake Assembly Assembly consists of a caliper, brake pads, a rotor, and hardware

42. Single Piston Caliper Caliper slides as the piston moves in to clamp the brake pads against the rotor

43. Fixed Caliper Caliper remains stationary as pistons on each side clamp the rotor

44. Brake Caliper Assembly

45. Caliper Components • Piston seal • prevents fluid leakage between the piston and the cylinder • helps pull the piston back into the cylinder when the brakes are not applied • Boot • prevents road dirt and water entry • Bleeder screw • allows air to be removed from the system

46. Caliper Operation • When the brakes are applied, brake fluid flows into the caliper cylinder • Fluid pressure pushes the piston outward, forcing the brake pads into the rotor • When the brakes are released, the stretched piston seal pulls the piston back into the bore, as fluid pressure drops

47. Caliper Operation Brakes applied Brakes released

48. Disc Brake Pads • Steel plates to which linings are riveted • Linings • made of heat-resistant organic or semimetallic friction material • semimetallic linings withstand higher temperatures without losing their frictional properties

49. Disc Brake Pads • Anti-rattle clips • keep the brake pads from vibrating and rattling • Pad wear sensor • metal tab on the brake pad • emits a loud squeal when it scrapes against the brake disc when the lining has worn too thin

50. Brake Disc (Rotor) • Uses friction from the brake pads to slow or stop wheel rotation • Normally made of cast iron • Constructed as part of the hub, or a separate unit • May be solid, or a ventilated rib construction