Chapter 32

Chapter 32

Disc Brake System

Introduction (1 of 2)

• Disc brakes use friction to create braking power.

• Disc brakes create braking power by forcing flat friction pads against sides of rotating disc

Introduction (2 of 2)

• Higher applied forces can be used in disc brakes than in drum brakes, because the design of the rotor is stronger than the design of the drum.

Disc versus drum brakes.

Disc Brake System (1 of 6)

• Modern vehicles always equipped with disc brakes on at least the front two wheels.– Rotor

– Caliper

– Brake pads


• Pushrods transfer force through brake booster.

• Master cylinder converts pedal force to hydraulic pressure.


• Hydraulic pressure transmitted via brake lines and hoses to piston(s) at each brake caliper.• Pistons operate on friction

pads to provide clamping force

• Rotors are free to rotate due to wheel bearings and hubs that contain them

• Hub can be part of brake rotor or separate assembly that the rotor slips over and is bolted to by the lug nuts

The hub and hubless rotors.


• The brake caliper assembly is normally bolted to the vehicle axle housing or suspension

Caliper mounting methods.


• Advantages– Greater amounts of heat to atmosphere

– Cooling more rapid

– Rotors scrape off water more efficiently

– Self-adjusting

– Don’t need periodic maintenance

– Easier to service


• Disadvantages– Prone to noise (squeals and squeaks)

– Rotors warp easier

– Not self-energizing

– Hard to use as parking brakes

Disc Brake Calipers (1 of 11)

• Bolted to vehicle axle housing (steering knuckle)

• Two types of calipers: fixed and sliding/floating


Fixed calipers with multiple pistons.

Fixed caliper being applied.


• When the brakes are applied, hydraulic pressure forces the piston toward the rotor.– Takes up any clearance

– Pushes pad into rotor

• Once all clearance is taken up on outer brake pad, clamping force will increase equally on both brake pads, applying brakes.

Sliding/floating caliper application.


O-rings. A. Square cut O-ring and O-ring cut to show square section. B. Square cut O-ring groove in caliper.


• Square cut O-ring seals piston in disc brake calipers.– Compressed between piston and caliper housing

– Keeps high-pressure brake fluid from leaking

– Prevents air from being drawn into system


Square cut O-ring. A. Square cut O-ring during brake application. B. Square cut O-ring during brake release.


• Low-drag calipers designed to maintain larger brake pad-to-rotor clearance.


• Although the phenolic pistons themselves do not corrode, the cast iron bore of the caliper does corrode and rust– can cause a phenolic

piston to seize in the bore


• Phenolic pistons transfer heat slower than steel pistons– Helps prevent

boiling of the brake fluid

Heat transfer. A. Phenolic piston (slow heat transfer). B. Steel piston (fast heat transfer).


• Bushings must be lubricated with high-temperature, waterproof disc brake caliper grease.

• Floating calipers are mounted in place by guide pins and bushings


• Sliding calipers slide in the caliper mount and are held in place by a spring steel clip.

Disc Brake Pads and Friction Materials (1 of 11)

• Disc brake pads consist of friction material bonded or riveted onto steel backing plates.


• Backing plate has lugs that correctly position the pad in the caliper assembly and help the backing plate maintain the proper position to the rotor

Brake pad locating lugs.


• Amount of friction expressed as ratio– Coefficient of friction

• Kinetic energy (motion) of sliding surfaces converts to thermal energy (heat).


• Composition of friction material affects brake operation– Materials that provide good

braking with low pedal pressures tend to lose efficiency when hot

• Wear out quicker– Materials that maintain stable

friction coefficient over a wide temperature range

• Generally require higher pedal pressures

• Tend to put added wear on disc brake rotor


• Disc brake pads and drum brake linings are made from materials that have a moderate coefficient of friction.


• Brake friction materials:– NAO materials

– Low-metallic non-asbestos organic (NAO)

– Semimetallic materials

– Ceramic materials


• Combination of weighted qualities:– Stopping power

– Heat absorption and dispersion

– Resistance to fade

– Recovery speed from fade

– Wear rate

– Performance when wet

– Operating noise

– Price


• Coefficients of friction:– C: ≤0.15– D: 0.15–0.25– E: 0.25–0.35– F: 0.35–0.45– G: 0.45–0.55– H: >0.55– Z: Unclassified


• Disc brakes more prone to squealing– Due to vibrations between brake pad and rotor

– Shims and spring-loaded clips help reduce squealing.


• Anti-noise measures:– Softer linings

– Brake pad shims

– Springs to hold in place

Example of brake pad retainers.


• Anti-noise measures:– Contour and groove

linings

– Bendable tangs

– Noise-reducing compounds

Wear Indicators

• Inspect brakes at regular intervals.

• Wear Indicators– Spring steel

scratchers

– Warning lamps

– Messages on dash

Disc Brake Rotors (1 of 7)

• Brake disc or rotor is main rotating component of disc brake unit. – Withstand high temperatures

– Made of cast iron

– Two-part rotor

• Composite rotor


• Rotors can fail in two ways:– Parallelism

– Lateral runout

• Dust shields help to shield the rotor from dust, water, and debris.


• Types of rotors– Solid

– Ventilated


• Some ventilated rotors are directional, meaning they are designed to force air through the rotor in one direction only.


• Some rotors are slotted and drilled– Better dissipation of heat– Better removal of water from the surface of the pads


• Most rotors have the minimum thickness stamped or cast on them.


Worn rotors cannot absorb as much heat and therefore are subject to brake fade much sooner.

Parking Brakes (1 of 3)

• Parking brakes are designed to hold vehicles stationary when parked. – Holds vehicle on specified grade in both directions

– Separately active from service brake

– Mechanically latches into applied position.

• Foot or hand operated


• Two types of parking brakes used in standard disc brakes:– Integrated

– Top hat drum


• Electric parking brakes:• Pull on a conventional parking brake cable

• Mounted on caliper and directly drive caliper piston

– Electric motor to apply disc brake assemblies

– Automatically released by electronic control module (ECM)

Diagnosis (1 of 12)

• Diagnosis starts with understanding customer’s concern.– Communicate directly.

– Diagnosis should identify any and all issues.

Diagnosis (2 of 12)

• Tools used for diagnosis:– Brake lining thickness gauge

– Brake wash station

– Caliper piston pliers

Diagnosis (3 of 12)

• Tools used for diagnosis:– Disc brake rotor micrometer

– Dial indicator

– Parking brake cable pliers

Diagnosis (4 of 12)

• Tools used for diagnosis:– Caliper piston retracting tool

– C-clamp

– Off-car brake lathe

Diagnosis (5 of 12)

• Tools used for diagnosis:– On-car brake lathe

– Caliper dust boot seal driver set

Diagnosis (6 of 12)

Disc brake tools. A. Brake lining thickness gauges. B. Brake wash station. C. Caliper piston pliers. D. Disc brake rotor micrometer. E. Dial indicator.

Diagnosis (7 of 12)

F. Parking brake cable tool. G. Caliper piston retracting tool. H. Off-car brake lathe. I. On-car brake lathe. J. Dust boot seal/bushing driver set.

Diagnosis (8 of 12)

Diagnosis (9 of 12)

• To diagnose stopping, noise, vibration, pulling, grabbing, dragging, or pulsation:– Verify the customer concern by operating the vehicle

if safe to do so.

– Remove and inspect calipers.

– Inspect caliper mountings, slides, and pins.

– Inspect brake pads and wear indicators.

Diagnosis (10 of 12)

• To diagnose stopping, noise, vibration, pulling, grabbing, dragging, or pulsation:– Check brake pads.

– Disassemble caliper.

– Reassemble calipers.


• To diagnose stopping, noise, vibration, pulling, grabbing, dragging, or pulsation:– Retract and readjust pistons.

– Inspect and measure disc brake rotors.

– Remove and reinstall rotors.


• To diagnose stopping, noise, vibration, pulling, grabbing, dragging, or pulsation:– Refinish rotors.

– Inspect and replace wheel studs.

– Install wheels and torque lug nuts, and make final checks.

Summary (1 of 7)

• Disc brakes create braking power by forcing flat friction pads against the outer faces of a rotor.

• The vehicle’s kinetic energy is transformed into heat energy by the disc brake components, which slow the vehicle when applied.

• Disc brake assemblies consist of a caliper, brake pads, and a rotor.

Summary (2 of 7)

• Caliper pistons use hydraulic pressure to create a clamping force of the brake pads to the faces of the rotor.

• Disc brake pads require much higher application pressures to operate than drum brake shoes because they are not self-energizing.

Summary (3 of 7)

• Advantages of disc brakes over drum brakes: more effective at transferring heat to atmosphere, self-adjusting, resistant to water fade, and easier to service.

• Disadvantages of disc brakes compared to drum brakes: more prone to noise, more prone to pedal pulsations due to warpage, and more difficult to use as an emergency brake.

Summary (4 of 7)

• Disc brake calipers come in two main styles: fixed and floating/sliding.

• In disc brake calipers, the piston is sealed by a square cut O-ring.

• Floating/sliding calipers require clean and lubricated pins, bushings, or guides for proper operation.

Summary (5 of 7)

• Brake pad lining is either riveted or bonded to the pad backing plate.

• Brake pad lining is available in a variety of materials with varying amounts of coefficient of friction.

• Brake pads may use shims, spacers, guides, and bendable tangs to help minimize squealing.

Summary (6 of 7)

• Brake pad wear indicators, if used, can be of the mechanical or electronic type.

• Rotors rotate with the wheels and are usually made of durable cast iron with friction surfaces that run true and parallel.

• Brake rotors can be solid or ventilated.

Summary (7 of 7)

• Disc brake parking brakes can be of the integrated caliper style, top hat drum style, electric pull-cable style, and integrated electric motor caliper style.

• Diagnosing brake faults requires good information from the customer, an adequate test-drive when possible, and a good understanding of brake theory.

Chapter 32

Documents

brake rotor

disc brake systemintroduction

brake application

brake booster

brake lines

brake release

bushingsdisc brake calipers

systemdisc brake calipers