30 | February 2016
eight locations. Refer to the minimum ser-
vice limit (this should be visible on the rear
cavity of the rotor hat). Even if one mea-
surement location is within the allowable
thickness, measuring for thickness variation
at several spots may locate a thickness that
is too close to minimum. This is sort of a
double-check of fndings that result from
checking runout.
Generally speaking, allowable thickness
variation should be no more than 0.0002-
inch (some OEs may spec a tolerance range
of 0.001-inch to as little as 0.0004-inch).
It's best to monitor both runout and
thickness variation at the same time.
Checking lateral runout and thickness
variation applies to all vehicle applications,
and should not be limited only to vehicles
that exhibit a brake pulsation issue.
Hot spots
In the days of old (not that long ago),
brake pads were made of relatively soft
materials that contained asbestos and
other material mixes. They tended to wear
the rotor evenly. Today, many pad materi-
als contain ceramic, designed to transfer a
The Chassis
Seen here is a specialty micrometer designed for rotor thickness measurement. The combina-
tion of one pointed and one fat anvil provides a more accurate measurement as opposed to a
traditional micrometer that has two opposing fat anvils.
Photo
courtesy
of
Raybestos
Brakes
All rotors should indicate minimum accept-
able thickness, as this example that displays
a 16 mm minimum thickness.