Rick's Free Auto Repair Advice

Disc thickness variation

What is disc thickness variation and how to eliminate it

Disc thickness variation is a brake rotor condition where disc thickness variationopposite faces of the brake rotor accumulate an extra thickness of brake friction material or thinness due to increased wear. A brake rotor that has developed disc thickness variation causes brake pedal pulsation during braking. Commonly called rotor warp, the condition is NOT caused by warping of the rotor. Brake rotors rarely, if ever, actually warp. A rotor with thickness variation causes a floating brake caliper piston to extend or compress into the caliper bore as the thicker or thinner portion of the rotor passes past the brake pads. Since the piston is moving either out or in, it causes the brake fluid pressure to rise or fall, causing the driver to feel a rising or falling brake pedal during braking. Brake pedal pulsation caused by disc thickness variation tends to get worse as the rotor heats up from extended braking

What causes disc thickness variation?

Lateral runout or uneven clamping forces are the two most lateral runout brake rotorcommon causes of disc thickness variation.  Lateral runout is a condition where the rotor isn’t mounted perfectly parallel to the wheel hub, causing the rotor to rotate in a wobble pattern. As the inner face of the rotor hits the inner brake pad it accumulates an extra layer of brake friction material if the brake pad is formulated with an adherent formula. If the brake pad is made with an abrasive formula, the brake pad will wear the inner face of the rotor and reduce its thickness every time it hits the inner pad. The opposite condition occurs on the opposite face.

Lateral runout can be caused by uneven deposits of rust or dirt on the wheel hub. If the rust or dirt isn’t removed, the brake rotor will cock slightly when installed. In addition, uneven lug nut torque can also cause the rotor to mount in non-parallel orientation to the wheel hub. Lug nuts should always be tightened to manufacturer’s specifications using a torque. Uneven lug nut torque is most often caused when lug nuts or bolts are tightened using an impact wrench.

Some shop rely of torque sticks that achieve a pre-determined amount of torque and spring backwards between impact blows. Torque sticks are relatively accurate when new, torque stick and rotor warpbut their accuracy wanes over use. Brake manufacturers recommend replacing torque sticks annually if they’re used in a shop environment.

How to detect disc thickness variation

Measure the brake rotor thickness in at least 6 locations around the rotor using a micrometer. Brake manufacturers prefer zero DTV. Rotor thickness should be measured in at least six areas of the rotor’s face. DTV should not vary more than 0.001” from one spot to another. Measurements should be taken a quarter inch from the edge of the rotor.

Lateral runout greater than 0.006” is a sign that the wheel hub, rotor and/or bearing should be replaced. The needle of the dial indicator should be perpendicular to the rotor. Measurements should be taken a quarter inch from the edge

Prevent or correct DTV

The best way to prevent lateral runout and DTV is to thoroughly rust on wheel hubclean the wheel hub and inside rotor hat prior to installing the clean wheel hub with wire brushrotor. Assemble the wheel to the hub using an accurate tested torque wrench to assure even lug nut torque.

If the brake rotor has developed DTV, correct the condition by machining with an on-vehicle brake lathe or by installing lateral runout shims by BrakeAlign. In severe situations, replace the rotors.

For more information on correcting brake pedal pulsation, see this post.

rotor lateral runout correction shim

©, 2017 Rick Muscoplat

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