EFFECTIVE BRAKE SERVICE TECHNIQUES

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1 EFFECTIVE BRAKE SERVICE TECHNIQUES BY KARL SEYFERT From a safety perspective, brake repairs are the most important service you deliver. We ll explain how to do the job right, to assure that your customers brakes work properly, especially when it matters most. Rotors on today s vehicles are thinner and lighter than those on older vehicles. The first time a rotor is turned may be the last time before it must be replaced. Some vehicle manufacturers argue against automatic rotor resurfacing. In a service bulletin, GM says resurfacing is ineffective at correcting brake squeal and/or premature lining wear and should not be 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? Every vehicle is different, which is why each rotor must be carefully inspected to determine the appropriate course of action. The following information will help you decide if rotor resurfacing is indicated, and highlight the techniques necessary to do the job properly. Every brake service 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 time on it. Photo: Wieck Media 42 June 2007

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EFFECTIVE BRAKE SERVICE TECHNIQUES Check the rotor thickness at eight points around the perimeter of the rotor using a micrometer that s designed for the job.

Photos: Karl Seyfert If rotor rust extends into the area where the pads should make contact, it may be impossible to resurface the rotor without approaching the discard dimension.

3 EFFECTIVE BRAKE SERVICE TECHNIQUES Check the rotor thickness at eight points around the perimeter of the rotor using a micrometer that s designed for the job. This conventional mic has two flat anvils that contact only the shoulders of any grooves. A rotor mic s one flat anvil and one pointed anvil allow accurate measurement of a scored rotor s thickness. Photos: Karl Seyfert If rotor rust extends into the area where the pads should make contact, it may be impossible to resurface the rotor without approaching the discard dimension. This rotor has already received a first cut, and a large area of heavy rust remains near the inside edge. Never base your rotor thickness calculation on a single measurement taken at one spot on the rotor. Check the rotor thickness at eight points around the perimeter of the rotor, using a micrometer that s designed specifically for the job. A rotor mic has 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. A rotor s thickness when new is called nominal thickness. This specification can usually be found in the vehicle service manual. The machine-to or minimum thickness is the thinnest a rotor can be machined to and returned to service. The machine-to or minimum thickness specification provides enough rotor material for safe and reliable brake performance. The typical difference between nominal and machine-to thickness is.050 to.060 in. 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. Rotors worn below the discard dimension are prone to warping and other damage and should never be reinstalled. In a worst-case scenario, a rotor that s worn below its discard dimension, combined with worn-out brake pads, could allow the caliper piston to fall out of the caliper housing. The typical difference between the machine-to and discard dimensions is.015 in. The rotor friction surfaces also must be parallel to one another. The allowable tolerance is known as parallelism, or thickness variation. 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. Variation in rotor thickness will cause pedal pulsation. 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. Many customer brake complaints can be traced to runout problems. Runout can cause brake pedal pulsation, steering wheel wobble and vibration during braking long before it s visible to the naked eye. 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 cause rotor wobble as the vehicle rolls down the road. In the case of a hubless rotor, the vehicle 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. Use a dial indicator to check lateral runout. Mount it securely to a stationary fixture. For on-the-car measuring, attach the fixture to the spindle or control arm. Locate the dial indicator s plunge tip about an inch inboard of the rotor edge, and zero the indicator. Runout is defined as the amount of lateral (side-to-side) movement of the rotor as it rotates through 360. The allowable indicated runout specification, as measured on the vehicle, is between.001 and.003 in. Total indicated runout can be affected by hub runout, cleanliness of the hub-to-rotor mating surface and wheel lug torque. Runout problems on hubless rotors can occasionally be corrected by indexing the rotor. Place a chalk mark on a stud and at the adjacent area on the rotor hat. Relocate the rotor clockwise to 44 June 2007

EFFECTIVE BRAKE SERVICE TECHNIQUES This fixture attaches to the caliper bracket and provides a steady mounting point for a dial indicator.

Repositioning the rotor may minimize the total assembled runout caused by minor deviations on the machined surfaces of both the underside of the rotor hat and the hub flange.

Likewise, hub runout can be measured without the rotor in place, to remove as many variables as possible during a complicated runout diagnosis.

4 EFFECTIVE BRAKE SERVICE TECHNIQUES This fixture attaches to the caliper bracket and provides a steady mounting point for a dial indicator. This assures an accurate on-car rotor runout measurement, either before or after rotor resurfacing. the next stud position before repeating your runout measurement. Repositioning the rotor may minimize the total assembled runout caused by minor deviations on the machined surfaces of both the underside of the rotor hat and the hub flange. It may take a few tries to find the best match-up. Rotor runout can be measured independent of the hub by removing the rotor from the vehicle and mounting it on an off-car brake lathe. Likewise, hub runout can be measured without the rotor in place, to remove as many variables as possible during a complicated runout diagnosis. If the hub flange or rotor are causing the runout problem, you ll be able to isolate them in this way. It s also possible to add runout to a rotor during offthe-car rotor resurfacing. Lathe-induced runout can be caused by runout in the lathe arbor, lathe adapters that are dirty or in poor condition or a sloppy initial setup. To verify your lathe setup before resurfacing a rotor, always conduct a scratch cut test first. Position the bits approximately 1 2 in. away from the outside edge of the rotor. Photo courtesy Continental Teves Bring the bit facing the outside of the rotor inward until it barely touches the rotor. While holding the outer knob, zero the inner dial. If the dial can t be zeroed, take note of the actual dial reading. Back the cutting tip away from the rotor a small distance and turn the lathe off. Observe the scratch cut. If it covers at least 50% of the rotor, proceed with the machining process. The scratch cut may be less than 50% because the rotor has runout and needs to Dirt, rust or damage on the lathe mounting hardware s machined surfaces will prevent proper rotor mounting and will introduce unnecessary rotor runout into your setup. After you remove the rotor, its inside and outside mating surfaces must be clean and flat before resurfacing. Hubbed rotor bearing races also must be cleaned before installing the rotor on the lathe. be machined. It also may be because the lathe setup is incorrect. Adapter cleanliness, arbor runout and tightening of the arbor nut can cause setup problems. To verify the setup, loosen the arbor nut. While holding the inner and outer adapter, rotate the rotor 180, then retighten the arbor nut. This changes the relationship between the rotor and the lathe adapters. Turn on the lathe, then move the twin cutter forward or backward a small distance. Make a second scratch cut by turning the dial to zero. This step assures that each scratch cut is the same depth, which makes comparing the cuts easier. Back off the cutting tip slightly, turn off the lathe, then compare the two scratch cuts. If the problem is in the rotor, the cuts should fall on top of one another. A setup problem will cause the second cut to be in a different position from the first. If this is the case, disassemble everything and check for cleanliness, correct adapters and nicks on the mating surfaces. Do not proceed with the machining process until the scratch cuts are in the same position. 46 June 2007

Rotor finish (smoothness) is affected by the lathe s crossfeed speed, as well as the depth of the cut.

5 Photo courtesy Hunter Engineering With the rotor installed on the lathe, a light scratch cut provides a visual indication of rotor runout. If the cut covers less than 50% of the rotor s circumference, either the rotor is in need of major resurfacing or the rotor mount is introducing runout and needs further adjustment. Rotor finish (smoothness) is affected by the lathe s crossfeed speed, as well as the depth of the cut. Turning a rotor too quickly will leave grooves in its surface, which form a spiral like the grooves in a record. The pads will jerk up and down when they make contact with these grooves, setting up vibrations and noise. Always remove the smallest amount of material possible, while achieving a smooth rotor finish. This extends rotor life and reduces the amount of folded and torn metal on the surface of the rotor. A rough rotor finish can increase pedal effort because the pads can make contact only with the peaks. As the brakes wear in, the pads eventually wear down the peaks and increase the area of contact. This accelerates rotor and pad wear. Many techs apply a nondirectional finish to the newly refinished rotor. A final sanding knocks off the sharp peaks, as well as the torn and folded metal left behind by the lathe bits, and may improve the surface finish by 2 to 5 microinches. This improves pedal feel, pad seating and overall brake performance. A nondirectional finish can be applied with #120 to #150 sandpaper and a flat sanding block. Some techs move the block in and out as the rotor turns on the bench lathe. This technique may be difficult and possibly dangerous with on-car lathes. Some lathes feature an attachment that quickly (and safely) applies a nondirectional finish after resurfacing is complete. To avoid creating the same problems you re trying to eliminate, sand evenly and don t remove any more material than necessary. Rotors must be cleaned with soap and water after machining. A freshly resurfaced rotor is covered with metal particles that can become embedded in the brake pads, and could keep the new pads from transferring a layer of friction material onto the rotor, leading to longer stops and increased pad or rotor wear. Use a mild soap solution, then wipe the rotor dry with a clean rag or paper towels. Don t rely on brake cleaner and compressed air to do the job. If a rotor is worn close to the discard dimension, new rotors should be installed as-is, with no additional machining required. Most new rotors arrive with a surface finish that s somewhere between 30 and 60 in. RA (roughness average), and many are in the 40 to 50 RA range. Runout on new rotors should also be within accepted industry standards. You won t be able to improve on this with a clean-up cut. Removing as little as.0015 in. unnecessarily reduces rotor longevity. And if your lathe setup is incorrect, you may make the finish rougher or introduce a runout problem that wasn t there to begin with. New rotors also must be cleaned before installation. Most are treated with an anticorrosion coating that can be removed with brake cleaner or another solvent. If left in place, the coating will contaminate the new brake pads. If you drop the rotor in your parts cleaning tank, make sure you follow up with a soap and water bath to remove any solvent residue. The hub-to-rotor mating surface must be free of rust, or runout-induced Circle #22 June

EFFECTIVE BRAKE SERVICE TECHNIQUES If you don t have an electronic surface profilometer or surface comparator to measure or evaluate the final rotor finish, use your eyes and fingers.

If the line breaks up into dots instead of runs as a continuous line (as shown here), the rotor is too rough.

Skipping this step will cause metal particles to become embedded in the new pads. disk thickness variation can occur shortly after the brake job is completed. As little as.001 in.

6 EFFECTIVE BRAKE SERVICE TECHNIQUES If you don t have an electronic surface profilometer or surface comparator to measure or evaluate the final rotor finish, use your eyes and fingers. The rotor is too rough if you can feel the grooves with your fingernail. Draw a line on the rotor with a ballpoint pen. If the line breaks up into dots instead of runs as a continuous line (as shown here), the rotor is too rough. Use sandpaper with a sanding block or a dedicated lathe attachment to apply a nondirectional finish to the rotor. Wash the rotor with soap and water to remove any leftover metal particles. Skipping this step will cause metal particles to become embedded in the new pads. disk thickness variation can occur shortly after the brake job is completed. As little as.001 in. of rust at the outside edge of the hub will translate into.002 to.004 in. of runout when measured at the outside edge of the rotor. The hub s mating surface can be difficult to clean due to the wheel studs. The area between the wheel stud and hub-centering flange is the most difficult area to reach. If there s mild rust buildup on the hub, use an angle grinder equipped with an abrasive disc to clean the majority of the surface area. Get as close to the studs as possible and change the sanding disc when needed. Finish the process by cleaning the area around the studs. Use a tool that fits over the wheel stud to allow easy cleaning of the hard-to-reach areas of the hub. Abrasive blasting can be used for heavy rust removal. Short of removing the hub from the vehicle, specialized equipment is available to blast the hub while it s still on the vehicle. As a final step, total installed runout should be rechecked after the rotor has been reinstalled. If runout is above.002 in., index the rotor to several locations to achieve the lowest possible total installed runout. Look for another rotor supplier if you see a large number of new rotors with more than.003 in. of runout. A potential cause of rotor lateral runout is poor wheel installation practice, especially with hubless rotors. Many perfectly machined, cleaned and indexed rotors have been ruined by overtorquing. This procedure should be accomplished using a properly calibrated torque wrench. An impact wrench will quickly distort a hubless rotor, and may damage the wheel lugs. Never use lubricants or penetrating fluids on wheel studs, nuts or mounting surfaces. Wheel nuts, studs and mounting surfaces must be clean and dry. A thin layer of lube may be used between the inner surface of the rotor and the hub to reduce corrosion. Hand-tighten all five lugnuts in a star pattern. Tighten all five lugnuts to approximately half specification using the star pattern. Now tighten all five lugnuts to full specification in the star pattern, using a torque wrench. Alloy wheels should be retorqued after a short test drive. New rotors should be ready to install straight out of the box. Most have a finish somewhere between 30 and 60 microinches RA (roughness average). You ll be unable to improve upon that, and you ll shorten the rotor s useable life by turning it. Visit to download a free copy of this article. 48 June 2007

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

ROTOR BRAKE ON-CAR& OFF-CAR SERVICE. Let the Chips Fly: Let the Chips Fly: ON-CAR& OFF-CAR BRAKE ROTOR SERVICE BY KARL SEYFERT With safety (and your shop s reputation) at stake, doing it right the first time is essential when servicing rotors. When used properly,