UNEVEN BRAKE PAD WEAR

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1/21/15
10:38 AM
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UNEVEN
BRAKE PAD WEAR
L
ast month in this space we
concentrated on factors
that can cause uneven vehicle braking, or brake
pull. A closely related topic concerns uneven brake
pad wear. This article will detail the possible causes of uneven brake pad wear
and discuss their remedies.
Ideally, we’d like to see all of the
brake pads that are removed during a
brake service evenly worn, with each
pad having delivered its full measure of
service. But this frequently is not the
case. One pad on one side of the vehicle may be worn much more than its
mate or the other two pads on the other side of the vehicle. Pads may be
worn at an odd angle or show evidence
of damage. To do a complete brake job
and to make certain that the new pads
do not experience a similar fate, the
cause of each of these conditions must
be identified and corrected. What follows are descriptions of uneven or otherwise abnormal brake pad wear symptoms, and their cures.
In the best case, all of the front brake
pads will have a nearly equal amount of
friction material wear. This indicates
that both brake calipers and all of the
related hydraulic components are operating properly, and new brake pads can
be installed with confidence. Be sure to
replace the related brake hardware, including shims, abutment clips and antirattle clips. Clean the caliper mounting
brackets and service the caliper guide
pins and slides.
One of the outboard brake pads may
show greater wear than the inboard
pad. This indicates that the outer pad
Brake Pad on Outer Side
Is More Heavily Worn
Illustration courtesy Bendix Brakes
Brake Pad Guide
Brake Pad on Piston Side
Is More Heavily Worn
Guide Sleeve
Piston
Sliding
Sleeves
On a floating brake caliper, if the more heavily worn brake pad is on the outside of
the caliper, this may be due to jammed or sluggish sliding caliper pins. After cleaning
the caliper pins and sleeves, apply the appropriate lubricant paste to the sleeves. If
the more heavily worn brake pad is on the piston side, this may be due to a jammed
piston or a piston that’s slow to retract when the brakes are released.
is continuing to press against the rotor
after the caliper has released. The
caliper guide pins and bushings may
be seized. It’s also possible that the pad
may be seized in its mounting bracket,
due to corrosion.
Before installing new pads, service
the caliper guide pins and bushings.
Replace any worn parts as necessary.
Clean and lubricate the caliper slides.
Inspect the caliper for guide pinhole
damage. Replace any parts that are
damaged or corroded.
If the inboard pad shows greater wear
than the outboard pad, this may indicate
a worn caliper piston seal that’s not allowing the piston to return to the rest
position. The caliper piston may also
have damage or corrosion. The caliper
guide pins and bushings may be seized
or the pad may be seized in the mounting bracket. Or there may be a problem
with the master cylinder that’s keeping
the caliper piston from fully releasing.
Before replacing the pads, inspect
the hydraulic brake system and check
for residual brake pressure. Service the
caliper guide pins and bushings or replace them if they’re worn or damaged.
Clean and lubricate the caliper slides.
Inspect the caliper for damage to the
guide pinholes and piston boot. Replace
the caliper if it’s damaged or corroded.
The friction material may be worn in
a wedge pattern. This is referred to as
tapered pad wear, and it can take place
across the width of the pad as well as
along its length (horizontally or vertically). Tapered pad wear may be caused by
pads that were originally installed incorrectly. Pads must be installed in their intended location for proper function.
Worn caliper guide pin bushings may
allow a caliper to cock to one side, exerting uneven pressure across the pad’s
friction material. One of the caliper’s
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A stuck caliper may cause significantly higher pad and rotor wear on one side of the
vehicle. These rotors include integral wear indicator grooves, which have been completely worn away on the inside surface of the rotor on the left. If any portion of the
groove is no longer visible, the rotor has reached the discard dimension.
for the application is selected and perform the recommended break-in procedure. Always inspect the friction surfaces of the rotors. If only a partial section of rotor has been polished by the
brake pad and the rest is rusty, this indicates incomplete contact between the
pad and the rotor. This may occur
when one end of a pad is binding in its
Photo: Karl Seyfert
guide pins or slides may be seized.
Once again, this allows the caliper to
move at an angle, allowing it to apply
pressure on the pad unevenly.
Before replacing the pads, service the
caliper guide pins and bushings as described above, or replace them if
they’re worn or damaged. Always replace calipers in pairs. Mixing and
matching old and new components can
result in a braking imbalance or pull.
The friction material on one or
more of the pads may be damaged and
show signs of thermal distress. Examples of this type of damage include
cracking, glazing or lifted pad edges. If
all front brake pads exhibit this kind of
damage, the problem is not confined
to one caliper, and may be caused by a
driver who has abused the brakes. Improper break-in procedures when the
pads were installed may also be responsible, or the brake pads simply
may be defective. Thermally damaged
pads must be replaced.
If the damage is confined to one
caliper, check for evidence that the hydraulic system is preventing pressure
release at that caliper. A damaged flexible brake hose may be responsible. The
caliper also may not be capable of releasing due to seized components.
Before replacing the brake pads,
make certain the correct pad material
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Photo courtesy ATE
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The rusted and pitted surface on the inner diameter of this rotor indicates that the
pad has not been making contact with a portion of the rotor for a significant period
of time. The diminished friction surface will reduce braking ability at that wheel and
may cause a brake pull. This type of uneven pad contact and rotor damage is most
likely caused by the pad binding in the mounting bracket.
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February 2015
mount at the abutment clip or a caliper
slide is binding or frozen. Rotors
should wear evenly.
In a worst case, one entire surface of
a rotor may be rough and rusty, indicating the corresponding pad has not been
making contact with the rotor for a considerable length of time. Braking ability
will be compromised in this instance
and uneven braking is very likely as
well. Before installing new pads and replacing the rotor, identify the factors
that are keeping the pads from making
full contact with the rotor.
If the edge of a pad overlaps the top
of a rotor, the wrong pads for the vehicle may have been installed. Alternately,
worn guide pins, a worn caliper bracket
or a worn caliper may have caused the
pad to wear unevenly and extend over
the edge of the rotor. Before replacing
the pads, compare the rotor diameter
with OE specifications. If pads or a rotor have been worn past recommended
levels, inspect the caliper piston boot
and the piston. If the piston has been allowed to extend too far, it may not retract properly.—Karl Seyfert
This article can be found online at
www.motormagazine.com.