Chapter 12 Disc Brake System Components and Operation 193
Note: In this and other chapters, we will
be referring to inboard and outboard
components. Inboard components are
those installed on the inside of the wheel assembly.
Outboard components are installed on the outside of
the wheel assembly.
Figure 12-2. Brake rotors are either solid or ventilated. Note that with the ventilated rotor, air can pass between the braking surfaces
to remove heat. This provides a more rapid cooling. (Saab)
Bleeder screw
protective cap
Bleeder screw
Brake
hose
Backing plate
Disc vents Heat being carried
away in airstream
Braking surface
Cold air traveling through disc
to help remove heat from braking
Ventilated braking
disc and hub assembly
Caliper
housing
Cold air
Direction
of vehicle travel
Rotor Construction
All disc brake rotors are made of cast iron and are similar
in appearance. Rotors are manufactured in many different
diameters and thicknesses. Since the linings clamp against the
rotor to stop it, both sides are machined to provide a smooth
braking surface. The rotor must also be the same thickness
throughout its diameter. Variations in thickness will cause
pulsation when the brakes are applied. As well as its diameter
and thickness, rotors can be either ventilated or solid.
Rotor Diameter
The rotor diameter directly affects the system’s brak-
ing ability. A larger diameter rotor provides more braking
area and exposes more area to the air, which aids in heat
dissipation. Rotor diameter is chosen depending on the
brake system and the vehicle’s intended use. Most disc
brake rotors are 9-11” (22.86-27.94 cm) in diameter. Larger
rotors are used on sports cars, trucks, and vans.
Rotor Thickness
To absorb heat, rotors must have sufficient metal. The
rotor is built with enough metal so the surface finish can
be restored by removing a layer of metal. This removal
process is called machining or turning. The minimum
thickness of the rotor is usually stamped on the hub or
the inner part of the rotor body, Figure 12-1. If the rotor is
below the minimum thickness before or after machining, it
must be replaced.
Solid Rotors
Solid rotors have no openings between the machined
surfaces. The rotor is cooled by air passing over the outside
surfaces of the rotor. Solid rotors are smaller than ventilated
rotors and are used on lighter vehicles.
Ventilated Rotors
Ventilated rotors have internal fins between the two
friction surfaces. See Figure 12-2. The fins are arranged to
create a centrifugal air pump inside the rotor. As the rotor
spins, the fins draw air into the center of the rotor and dis-
charge it from the edges. This extra air circulation causes
the rotor to give up frictional heat much more rapidly than
would otherwise be possible. This type of rotor is only
used on larger cars and light trucks and on many smaller
vehicles.