48
ConMet
10. Hub and Rotor Inspection (continued)
51
106835a
Acceptable Crack
FIGURE 109
Deep Grooves or Scoring
Grooves or scoring may be
caused by contaminants trapped
between the lining and the rotors,
worn out linings, or the lining plate
contacting the rotor. Grooves or
scoring on the brake rotor are
acceptable if they are less than
0.060" deep. If the rotor thickness,
when measured across a groove,
is less than the minimum allowable
thickness for the rotor, the rotor
should be replaced. See figure
110 for example of grooves on the
braking surface of the rotor.
52
106838a
Grooves on Brake Surface
FIGURE 110
Blue Marks or Bands
If the rotor has become extremely
hot, the braking surface may exhibit
blue marks or bands. See figure
111 for examples of these marks.
This condition may be caused by
continued hard stops, improper
brake balance, improper brake
caliper clearance, operation,
or adjustment. The cause of
overheating should be determined
and corrected. It is not necessary
to replace the rotor as long as
it meets the proper dimensional
specifications for runout and
thickness.
53
106839a
Blue Marks
FIGURE 111
Polished Rotors
A polished rotor will have a mirror-
like finish on the braking surface
(see figure 112). This condition
may be caused by improper lining
friction rating, lightly dragging
brakes, or a rotor resurfaced to
too fine of a microfinish. To correct
this condition, sand the braking
surface on the rotor and the brake
pads with 80 grit emery cloth. The
microfinish should be 120 to 150
RMS. If the condition returns, verify
that the linings on the brake pads
have the correct friction rating for
the application.
54
106840a
Polished Rotor
FIGURE 112
Martensite Spotted Rotors
Rotors subjected to extremely high
heat followed by rapid cooling
can exhibit a Martensite Spotted
condition. This condition appears
as black spots on the rotor that are
slightly raised and are hard and
brittle (see figure 113). The high
temperatures cause a structural
change to the rotor material. This
condition can make the rotor more
susceptible to cracking.
Rotors with this condition should
be replaced. Check the brake
linings for uneven wear and replace
as necessary. After the rotor has
been replaced, and the hub and
rotor reinstalled onto the axle, the
brake system should be checked
for brake drag and proper brake
balance.
55
106857a
Martensite Spotted Rotor
FIGURE 113
Grease-Stained Rotors
Brake rotors that have
discolorations due to grease or
oil on the brake surface should
be removed from the vehicle and
cleaned to remove the grease or oil
(see figure 114). The brake pads
should be inspected and replaced
if they are found to be soaked with
grease or oil. The source of the
grease or oil should be identified
and repaired.