Let the Chips Fly: ON-CAR & OFF-CAR BRAKE ROTOR SERVICE

Motor,  Jun 2004  by Seyfert, Karl

• E-mail
• Print
• Link

With safety (and your shop's reputation) at stake, doing it right the first time is essential when servicing rotors. When used properly, both on-car and off-car brake lathes will get the job done.

Like many automotive service procedures, much has changed when it comes to rotor service. When cars were heavier and mostly rear-wheeldrive, it was a matter of routine to turn the rotors each time the pads were replaced. The rotors were heavy and thick, and could be turned at least a couple of times before they required replacement. Cars today are lighter and many are front-wheel-drive. Saving weight saves fuel, but it's also true that a lighter car can get by with a lighter braking system. It takes less to stop 3000 pounds than it does to stop 5000 pounds.

The rotors on these lighter braking systems start out thinner, with less "meat" available for turning. The first time a rotor is turned may be the only time before it's ready to be replaced. Yet many shops are still resurfacing rotors every time the pads are replaced. This may no longer be necessary. Some vehicle manufacturers, including GM, argue against automatic rotor resurfacing.

In TSB No. 00-05-22-002, GM states: "Brake rotors should only be turned when one of the following rotor surface conditions exist: severe scoring with depth in excess of 1.5mm or 0.060 inch, pulsation from excessive lateral nmout of more than 0.080mm or 0.003 inch, thickness variation in excess of 0.025mm or 0.001 inch or excessive corrosion on rotor braking surfaces. Rotors are not to be resurfaced in an attempt to correct the following conditions: noise/squeal, cosmetic corrosion, routine pad replacement or discoloration/hard spots."

GM says resurfacing is "ineffective at correcting brake squeal and/or premature lining wear and should not he used to address these conditions-unless specifically directed to do so in a service bulletin." If GM discourages rotor resurfacing during pad replacement, when is this procedure appropriate? Clearly, you'll see vehicles that fall outside the "normal" description above. Every vehicle is different, which is why each rotor must 1% carefully inspected to determine the appropriate course of action.

Rotor Thickness

Every rotor inspection should begin with a rotor thickness check. If the rotor is too thin, has previously been resurfaced or has been damaged by worn-out pads, there's no point in wasting any more time on it.

Check the rotor thickness at eight points around the perimeter of the rotor. Never base your rotor thickness calculation on a single measurement taken at one spot on the rotor. Use a micrometer to measure rotor thickness, preferably one that's designed specifically for the job. A rotor mic features one flat anvil and one pointed anvil. The pointed anvil allows measurement of the real minimum thickness of a scored rotor. A mic with two flat anvils will contact only the shoulders of any grooves or scoring lines.

Nominal thickness is the rotor thickness when new. This specification can usually be found in the vehicle service manual. The machine-to thickness is the thinnest a rotor can be machined to and returned to service. The machineto or minimum thickness specification provides enough rotor material for safe and reliable brake performance.

The discard thickness is the dimension at which the rotor must be replaced. If a rotor is at or below its discard thickness, brake performance will be compromised. In a worst-case scenario, a rotor that's worn below its discard dimension, combined with wornout brake pads, could allow the caliper piston to fall out of the caliper housing. Rotors worn below the discard dimension are also prone to warping and other damage. A rotor that's below the discard thickness should never be reinstalled.

The rotor friction surfaces must also be parallel to one another. The allowable tolerance is known as parallelism, or thickness variation. Variation in rotor thickness will cause pedal pulsation. The variation might be the result of excess heat buildup that has warped the rotor, or the rotor may be contaminated by isolated thick spots caused by rust or corrosion buildup.

Runout

If a rotor appears to wobble from side to side as it rotates when viewed from its front edge, this off-center deviation is called lateral runout. Long before it's visible to the naked eye, runout can cause brake pedal pulsation, steering wheel wobble and vibration during braking. Many customer brake complaints can be traced to runout problems.

Rotor runout may or may not be caused by rotor thickness variation. If a rotor thickness check shows no evidence of a variable dimension, the rotor may be rotating off its true axis as a result of other damage. The wheel bearing may be badly worn and excessively loose, which would muse rotor wobble as the vehicle rolls down the road.

In the case of a hubless rotor, the hub itself may also have excessive runout, which is then transferred to the rotor. The contact area between the hub and hubless rotor hat may also be to blame. Contamination (rust or grit buildup) may create an uneven mounting surface that will cause the rotor to rotate off its intended axis.