chap76 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 76, the reader should
be able to:

•
•
•
•

Prepare for ASE Brakes (A5) certification test content area
“C” (Disc Brake Diagnosis and Repair).

Describe how disc brakes function.
Name the parts of a typical disc brake system.
Describe the construction of disc brake pads.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 76, the reader should
be able to:

•
•

Describe the difference between fixed caliper and floating or
sliding caliper.
Explain the difference between a standard caliper and a lowdrag caliper.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
anchor plate • antirattle clips • aramid fiber
bonded linings • brake block • brake pad
carbon fiber-reinforced carbon (CFRC)
fixed brake caliper • floating caliper • gas fade
integrally molded

Kevlar
lining fade • low-drag caliper
Continued
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KEY TERMS:
mechanical fade • moldbonded lining
NAO • NAS • natural frequency • nonasbestos
pad wear indicators • pin-slider caliper
riveted linings
semimets • sintered metal • sintering • sliding caliper • swept area
water fade • ways

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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DISC BRAKES
Disc brakes use a piston(s) to squeeze friction material (pads) on
both sides of a rotating disc (rotor). Disc may be spelled disk by
some manufacturers, but disc is the SAE term and commonly used
spelling. The rotor is attached to and stops the wheel.
Disc brakes are used on the front wheels of late-model vehicles,
and on the rear wheels of an increasing number of automobiles.
Disc brakes were adopted primarily because they can supply
greater stopping power than drum brakes with less fade.
Disc brakes are well suited for use as front brakes, which must
provide 60% to 80% of the vehicle’s total stopping power.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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DISC BRAKE ADVANTAGES
Although increased Federal brake performance standards hastened
the switch to disc brakes, the front drum brakes would eventually
have been eliminated anyway because disc brakes are superior in
almost every respect.
The disc brake friction assembly has several significant strong
points, and only a few relatively minor weak points.
See Figure 76-1.

Continued
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Figure 76–1 A typical disc brake assembly.

Continued
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The main advantages of the disc brake include the following:
Fade resistance
Self-adjustment
Freedom from pull
Fade Resistance When a disc brake is compared with a drum
brake of similar diameter, its biggest advantage is a much greater
ability to resist fade. Disc brakes are more resistant to all kinds of
fade, including the following:
Mechanical fade
Lining fade
Gas fade
Water fade
Continued
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Disc brakes avoid heat-induced fade because all of the major parts
of a disc brake are exposed to the air flowing over the friction
assembly. They also have greater swept area, the amount of brake
drum or rotor friction surface that moves past the brake linings
every time the drum or rotor completes a rotation.
A larger swept area allows the heat generated in braking to be
transferred more rapidly into the rotor for better cooling. A disc
brake has swept area on both sides of the rotor. A drum brake has
swept area only on the inside of the drum.
Large diameter rotors, however, require larger diameter wheels to
provide the necessary clearance. This is why high-performance
vehicles use 17-inch diameter and larger wheels.
Continued
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Mechanical Fade Unlike a brake drum mechanical fade is not a
problem, as the disc brake rotor expands toward the brake linings
as it heats up rather than away from them.
This fundamental design difference makes it impossible for heat to
cause the rotor to expand out of contact with the brake linings.
There is never the need to move the brake linings out to keep them
in contact with the rotor, so brake pedal travel does not increase.
If the brake pedal on a vehicle with disc brakes drops toward the
floor, it is almost always a sign of vapor lock, a fluid leak, fluid
bypassing the seals in the master cylinder, or mechanical fade of
the rear drum brakes.

Continued
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Lining Fade If the brakes become overheated lining fade can
occur . A warm brake performs better than a cold brake. When too
much heat is generated by braking, the lining material overheats,
friction coefficient drops, and lining fade occurs.
Figure 76–2 Braking force is applied equally to
both sides of the brake rotor.

The primary symptom of lining
fade is a hard brake pedal that
requires greater force to maintain
stopping power.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman

Unlike a drum brake, however,
increased application force will
not distort the brake rotor because
the caliper applies equal force to
both sides.
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Figure 76–3 Disc brakes can absorb and dissipate a great deal of heat. During this demonstration,
the brakes were gently applied as the engine drove the front wheels until the rotor became cherry
red. During normal braking, the rotor temperature can exceed 350°F (180°C), and about 1500°F
(800°C) on a race vehicle.

If the pads are overheated
to the where the lining

material is physically
damaged, the brakes will
not recover their full
stopping power until
the pads are replaced.

Continued
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Gas Fade A problem only under severe braking conditions when
hot gasses and dust particles from the linings are trapped between
the brake linings and rotor, where they act as lubricants.

Though disc brakes operate at higher temperatures than drum
brakes, they have fewer gas fade problems for these reasons:
1. Disc brakes do not have a drum to contain gasses and
particles in the area around the brake linings.
2. The constant flow of air over the brake carries away
contaminants that might otherwise build up.
3. The surface area of the brake lining material in a disc brake
is smaller than that of a comparable drum brake and this
allows gasses and particles to escape more easily.
The symptoms of gas fade are the same as those for lining fade.
Continued
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Figure 76–4 Slots and holes in the brake
linings help prevent gas and water fade.

To help prevent gas fade, many
brake pads have slots cut in the
lining material.

These slots allow gasses and dust
particles to escape.
The holes required in riveted
linings also perform this function.
For even greater protection against
gas fade, motorcycles and highperformance vehicles sometimes
have holes or slots in the rotor.

Continued

These allow gasses and water to
escape, and the sharp edges wipe
loose particles off the linings.

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Water Fade Not a big problem with disc brakes because
centrifugal force created by spinning rotor throws off moisture,
and the brake pads positioned only a few thousandths of an inch
away from the rotor continuously wipe it clean.
When the brakes are applied, the leading edge of the brake pad
lining material wipes the last bit of water from the disc. Water
fade is prevented once good lining-to-rotor contact is established.
Although far more resistant to water fade than drum brakes, disc
brakes are not entirely free from its effects. Splash shields and the
vehicle’s wheels help keep water off of the rotor, and the brake
lining materials specified for most vehicles minimize the effects of
water fade.
Continued
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Self-Adjusting Ability Disc brakes are self-adjusting because any
wear of the linings is automatically compensated for by the action of
the brake caliper.
When brakes are applied the caliper pistons move out as far as
needed to force the brake pads into contact with the rotor. When the
brakes are released, the piston retracts only the small distance
dictated by rotor runout and piston seal flex.
The surface finish on the piston must be clean to allow the piston to
slide past this seal. Moisture accumulation inside the caliper often
causes the piston to rust. If the force of the caliper seal is not strong
enough, the piston stays in the applied position.
Because the brake pads are still in contact with the rotor, one or both
pads will show excessive wear.
Continued
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Normal Operation The piston moves just enough to distort the
caliper seal and returns to the original position when the brake
pedal is released.
Wear Compensation The piston moves more than the caliper
seal can distort. The piston moves through the seal until the pad
contacts the rotor. The caliper piston returns to the released
position by the seal distortion, the same as during normal
operation, except now in a different, more applied position.
As the wear occurs and the piston moves, additional brake fluid is
needed behind the piston. This additional brake fluid comes from
the master cylinder and the brake fluid level drops as the disc
brake pads wear.
See Figure 76–5.
Continued
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Figure 76–5 The square-cut O-ring not only seals hydraulic brake fluid, but also retracts the
caliper piston when the brake pedal is released.

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Freedom from Pull A disc brake will stop straighter under a
wider range of conditions than will a drum brake. A disc brake is
self-cleaning, will throw off most water, and is less likely to pull.
Disc brakes do not have self-energizing or servo action. These
actions increase the power of drum brakes, but depend on friction
between the linings and drum for their effect.
This means that even a small loss of lining-to-drum friction causes
a large loss of braking power and a significant side-to-side
variation in the amount of braking force.
Since disc brakes do not use friction between the linings and rotor
to increase their braking power, the effects of a loss of friction on
one side of the vehicle are less pronounced than drum brakes.
Continued
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Check the Tire Size for a Pulling Problem
If an unequal braking problem is being diagnosed, check that the front tires match
and that the rear tires match. Brakes slow and stop wheels. Unequal diameter tires
create an unequal braking force. The result may be a pulling toward one side while
braking. Tire diameter can vary from one tire manufacturer to another even though
the size designation is the same. Even slight differences in the wear of tires can
cause a different tire diameter and, therefore, a different braking force.

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DISC BRAKE DISADVANTAGES
The most notable fact about the disadvantages of disc brakes is
that there are so few. The weaknesses of disc brakes include:
Brake dust
No self-energizing or servo action
Brake noise
Poor parking brake performance
Brake Dust The lining is exposed on a disc brake, rather than
being enclosed on a drum brake. Some brake dust can accumulate
on the wheels. This brake dust is often dark brown or black and
can stain wheels if not cleaned often or protected from the dust.
Continued
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Wax the Wheels
Brake dust from semimetallic brake pads often discolors the front wheels. Customers
often complain to service techs about this problem, but it is normal for the front wheels
to become dirty because the iron and other metallic and nonmetallic components wear
off the front disc brake pads and adhere to the wheel covers. A coat of wax on the
wheels or wheel covers helps prevent damage and makes it easier to wash off brake dust.

No Self-Energizing or Servo Action The disc brake’s lack of
self-energizing or servo action is a disadvantage for two reasons.
It contributes to poor parking brake performance and requires the
driver to push harder on the brake pedal for a given stop.
The problem of high pedal pressures has been virtually eliminated
through the use of brake power boosters, since the disc brake
responds more directly to pressure on the brake pedal.

Continued

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Brake Noise The biggest complaint about disc brakes is various
squeaks and squeals during a brake application. These noises are
caused by high-frequency rattling or vibration of brake pads.
Several methods are used to quiet noisy disc brakes.
Manufacturers use specific lining materials that damp vibrations,
and most calipers have antirattle clips or springs that hold the pads
in the caliper under tension to help prevent vibration.
See Figure 76–6.

Continued
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Figure 76–6 Antirattle clips reduce brake pad movement and vibration.

Some calipers use shims
between brake pad backing
plate and caliper piston to
damp vibrations.

Shims may be metal or fiber.
Antinoise sprays and brush-on
liquids provide a cushion layer
between the pad and the
caliper piston.
The bond lowers the natural
frequency of the pad, and the

cushion layer damps any
vibration that may still occur.
Continued
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