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 TM 9-8000
maintain a clearance of approximately 0.005 Inch (fig.
rub against the rotor as the wheel turns. Because of its
small frictional area, and lack of self- energizing and
Drum Brakes. Both the disk and drum brake assemblies
servo effect, the disk brake assembly requires the use of
used on modern vehicles are well-designed systems.
an auxiliary power booster to develop enough hydraulic.
Each system exhibits certain Inherent advantages and
pressure for satisfactory braking.
disadvantages. The most Important points of Interest are
discussed below. One major factor that must be
discussed In automotive brakes, as well as all other
is designed to move laterally on its mount. This
brake systems, is the system's ability to dissipate heat.
movement allows the caliper to maintain a centered
As discussed In paragraph 34-1, the byproduct of friction
position with respect to the rotor. This design also
is heat. Because most brake systems use this concept to
permits the braking force to be applied equally to both
develop braking force, It Is highly desirable for brake
sides of the rotor. The floating caliper usually Is a one-
systems to dissipate heat as rapidly and efficiently as
piece solid construction and uses a single piston to
possible. The disk brake assembly, because of Its open
develop the braking force. This type of caliper operates
design, has the ability to dissipate heat faster than the
by pressurized hydraulic fluid like all other hydraulic
drum brake. This feature makes the disk brake assembly
calipers. The fluid under pressure first enters the piston
less prone to brake fade due to a buildup of excess heat.
cavity and begins to force the piston outward. As this
The disk assembly also may have additional heat
happens, the brake pad meets the rotor. Additional
transfer qualities due to the use of a ventilated rotor. This
pressure then forces the caliper assembly to move In the
type of rotor (fig. 34-11) has built-in air passages
opposite direction of the piston, thereby forcing the brake
between friction surfaces to aid in cooling.
pad on the opposite side of the piston to engage the
rotor. As pressure Is built up behind the piston, it then
While the drum brake assembly requires an Initial shoe-
forces the brake pads tighter against the rotor to develop
to-drum clearance adjustment and perodic checks, the
additional braking force.
disk brake assembly Is self- adjusting and maintains
proper adjustment at all times. The disk assembly
automatically compensates for lining wear by allowing
mounted rigidly to the spindle or splash shield. In this
the piston In the caliper to move outward, thereby taking
design, the caliper usually is made In two pieces and has
up excess clearance between pads and rotor (fig. 34-11).
either two, three, or four pistons In use. The pistons,
The disk system is fairly simplistic In comparison to the
which may be made of cast iron, aluminum, or plastic,
drum system. Due to this design, and its lack of moving
are provided with seals and dust boots and fit snugly In
parts and springs, the disk assembly Is less likely to
bores machined In the caliper. The centering action of
malfunction. Overhauling the disk brake assembly Is
the fixed caliper Is accomplished by the pistons as they
faster because of Its simplistic design.. It also Is safer
move In their bores. If the lining should wear unevenly on
due to the fact that the disk brake assembly Is open and
one side of the caliper, the excess clearance would be
asbestos dust from linings Is less apt to be caught In the
taken up by the piston simply by moving further out In its
brake assembly. Like brakedrums, rotors may be
bore. As the brakes are applied, the fluid pressure enters
machined if excessive scoring Is present. Rotors also
the caliper on one side and Is routed to the other through
are stamped with a minimum thickness dimension (fig.
an Internal passageway or an external tube con- nected
to the opposite half of the caliper. As pressure Is
assembly requires the drum be removed for lining
Increased, the pistons force the brake pads against the
Inspection, while some disk pads have a built-in lining
rotors evenly and therefore maintain an equal amount of
wear Indicator (fig. 34-11) that produces an audible high-
pressure on both sides of the rotor. As discussed above,
pitch squeal when linings are worn excessively. This
the fixed calipers use a multlplston design (fig. 34-13) to
harsh squeal Is a result of the linings wearing to a point,
provide the braking force. The fixed calipers may be
allowing
a
metal
Indicator
to
designed
34-13
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