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
Disc Brake System (2 of 6)
• Pushrods transfer
force through brake
booster.
• Master cylinder
converts pedal force
to hydraulic pressure.
Disc Brake System (3 of 6)
• 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.
Disc Brake System (4 of 6)
• The brake caliper
assembly is normally
bolted to the vehicle
axle housing or
suspension
Caliper mounting methods.
Disc Brake System (5 of 6)
• 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
Disc Brake System (6 of 6)
• 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
Disc Brake Calipers (2 of 11)
Fixed calipers with multiple
pistons.
Fixed caliper being applied.
Disc Brake Calipers (3 of 11)
• 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.
Disc Brake Calipers (4 of 11)
O-rings. A. Square cut O-ring and O-ring cut to show square section.
B. Square cut O-ring groove in caliper.
Disc Brake Calipers (5 of 11)
• 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
Disc Brake Calipers (6 of 11)
Square cut O-ring. A. Square cut O-ring during brake application.
B. Square cut O-ring during brake release.
Disc Brake Calipers (7 of 11)
• Low-drag calipers
designed to maintain
larger brake pad-torotor clearance.
Disc Brake Calipers (8 of 11)
• 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
Disc Brake Calipers (9 of 11)
• 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).
Disc Brake Calipers (10 of 11)
• Bushings must be
lubricated with hightemperature,
waterproof disc brake
caliper grease.
• Floating calipers are
mounted in place by
guide pins and
bushings
Disc Brake Calipers (11 of 11)
• 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.
Disc Brake Pads and Friction
Materials (2 of 11)
• 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.
Disc Brake Pads and Friction
Materials (3 of 11)
• Amount of friction expressed as ratio
– Coefficient of friction
• Kinetic energy (motion) of sliding surfaces
converts to thermal energy (heat).
Disc Brake Pads and Friction
Materials (4 of 11)
• 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 Friction
Materials (5 of 11)
• Disc brake pads and drum brake linings are
made from materials that have a moderate
coefficient of friction.
Disc Brake Pads and Friction
Materials (6 of 11)
• Brake friction materials:
– NAO materials
– Low-metallic non-asbestos organic (NAO)
– Semimetallic materials
– Ceramic materials
Disc Brake Pads and Friction
Materials (7 of 11)
• 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
Disc Brake Pads and Friction
Materials (8 of 11)
• 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 Brake Pads and Friction
Materials (9 of 11)
• Disc brakes more prone to squealing
– Due to vibrations between brake pad and rotor
– Shims and spring-loaded clips help reduce
squealing.
Disc Brake Pads and Friction
Materials (10 of 11)
• Anti-noise measures:
– Softer linings
– Brake pad shims
– Springs to hold in
place
Example of brake pad retainers.
Disc Brake Pads and Friction
Materials (11 of 11)
• 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
Disc Brake Rotors (2 of 7)
• Rotors can fail in two ways:
– Parallelism
– Lateral runout
• Dust shields help to shield the rotor from dust,
water, and debris.
Disc Brake Rotors (3 of 7)
• Types of rotors
– Solid
– Ventilated
Disc Brake Rotors (4 of 7)
• Some ventilated
rotors are directional,
meaning they are
designed to force air
through the rotor in
one direction only.
Disc Brake Rotors (5 of 7)
• Some rotors are slotted and drilled
– Better dissipation of heat
– Better removal of water from the surface of the pads
Disc Brake Rotors (6 of 7)
• Most rotors have the minimum thickness
stamped or cast on them.
Disc Brake Rotors (7 of 7)
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
Parking Brakes (2 of 3)
• Two types of parking
brakes used in
standard disc brakes:
– Integrated
– Top hat drum
Parking Brakes (3 of 3)
• 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.
Diagnosis (11 of 12)
• To diagnose stopping, noise, vibration, pulling,
grabbing, dragging, or pulsation:
– Retract and readjust pistons.
– Inspect and measure disc brake rotors.
– Remove and reinstall rotors.
Diagnosis (12 of 12)
• 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 testdrive when possible, and a good understanding
of brake theory.