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, 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 Photo courtesy Hunter Engineering 20 June 2004
June 2004 21
with depth in excess of 1.5mm or 0.060 inch, pulsation from excessive lateral runout 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 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? Clearly, you ll see vehicles that fall outside the normal description above. Every vehicle is different, which is why each rotor must be 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 Rotors may develop a healthy accumulation of rust near the edges. If the rust extends into the live area where the pads will make contact, be sure to figure this into your rotor thickness calculation. By the time the rust is removed, the rotor may be close to its discard dimension. Many rotors have the minimum thickness dimension stamped or cast in a prominent location. A little work in the blast cabinet revealed the specs on this import rotor. Unvented rear rotors are often replaced rather than resurfaced due to their relatively thin construction. This one was swapped for a new one. 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 com- Photos: Karl Seyfert 22 June 2004
plaints 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 cause 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. Another potential cause of excessive runout (one that s common with thinhat hubless rotors) is poor wheel installation practice. If the wheel fasteners were improperly tightened during wheel installation, it s very likely that the rotor hat may warp due to uneven or excessive fastener torque. Measuring Runout Use a dial indicator to check lateral runout. Mount it securely to a stationary fixture. For on-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 average allowable specification for latemodel vehicles is between.001 and.003 in. This is the total indicated runout specification, as measured on the vehicle. Total indicated runout can be affected by several factors hub runout, cleanliness of the hub-to-rotor All lathe hardware should be clean and damage-free. Dirt or rust on the machined surfaces of the mounting cups will prevent proper rotor mounting and will introduce unwanted rotor runout into your setup. Always use the largest possible mounting cup, and make sure both cups are the same size. The inside and outside mating surfaces must be clean and flat before you resurface a hubless rotor. The bearing races on conventional hubbed rotors should also be cleaned before the rotor is installed on the lathe. This rotor s mating surface was heavily rusted, and required several minutes in a blast cabinet before it was ready to be resurfaced and reinstalled. mating surface and wheel lug torque. Runout problems on hubless rotors occasionally can be corrected by indexing the rotor. Place a chalk mark on a stud and at the adjacent area on the rotor hat. Now relocate the rotor clockwise to 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 it from the vehicle and mounting it on an offcar 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. On-Car vs. Off-Car Rotor Resurfacing When used properly, both on-car and off-car brake lathes can provide excellent results. On-car lathes were originally developed for vehicles with captive rotors, and others known to be sensitive to runout problems. In fact, some vehicle manufacturers specify 24 June 2004
on-car brake lathes only for resurfacing the rotors on their vehicles. On captive rotor systems, the rotor is mounted behind the hub. Removing the rotor means removing the hub, which may then be difficult to mount on an off-car lathe for accurate machining. Unless you compensate for the rotor runout that exists on the vehicle, you can t guarantee a true-running rotor when it s reinstalled. When possible, rotors on these vehicles should be machined in place. Shims are also available to compensate for runout on rotors that have been machined on an off-car lathe. The shims are available in a variety of sizes, and fit between the hub flange and the rotor hat. Shifting the position of the shim zeroes out the assembled runout. On-car lathes are designed to compensate for runout problems, and do not require major disassembly of the hub and rotor on captive rotor brake systems. Some even compensate for runout automatically, although setup still requires a properly trained operator. Rotor Finish Rotor finish (smoothness) is affected by the crossfeed rate of the lathe, as well as the depth of the cut. Turning Attach a vibration damping ring to all vented rotors to dampen vibration and assure a smooth, chatter-free resurfacing job. Other lathe attachments are available for quelling vibrations on thinner, unvented rotors. a rotor too quickly leaves grooves in its surface. The grooves are not parallel, but form a spiral like the grooves in an old LP. When the pads make contact with these grooves, they jerk up and down, 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 (at least initially). As the brakes wear in, the pads eventually wear down the peaks and increase the area of contact. This accelerates rotor and pad wear. The type and condition of the lathe Once the rotor is installed on the lathe, check it for runout with a dial indicator before you begin to refinish it. Something as simple as the way you tighten the lathe arbor nut can introduce lathe runout. For the record, tighten the nut slowly and evenly; don t jerk the wrench to get the nut extra tight. A very light scratch cut provides a visual indication of rotor runout. If the cut covers less than 50% of the rotor s circumference, there are two possible causes: Either the rotor really is in need of major resurfacing or your lathe setup is introducing runout into the process. 26 June 2004
This Hunter lathe with digital readout capability takes a lot of the guesswork out of rotor and drum resurfacing. As a first step, the lathe measures the rotor thickness to determine whether it should be discarded before machining. Total indicated runout can also be easily calculated. tool bits have a direct effect on rotor surface finish. Round carbide bits produce a smoother finish than angular bits. They also allow a faster crossfeed rate, which reduces resurfacing time. Top shelf titanium bits have parallel, peripheral ground edges. Lathe Setup Error We ve discussed on-car runout. It s also possible to add runout to a rotor during off-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. Here s how: Position the bits approximately 1 2 in. away from the outside edge of the rotor. 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. If the scratch cut covers at least 50% of the rotor, proceed with the Photo courtesy Hunter Engineering machining process. The scratch cut may be less than 50% because the rotor has runout and needs to be machined. It may also be because the lathe setup is incorrect. Adapter cleanliness, arbor runout and tightening of the arbor nut may be responsible for setup problems. To verify the setup, loosen the arbor nut. While holding the inner and outer lathe cup, 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 back 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 be in phase with 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. On bench lathes with adjustable spindle speeds, a spindle speed of 100 to 150 rpm with a crossfeed rate of.002 to.005 in. per revolution should produce a smooth rotor finish. Spindle speed should be adjusted to match rotor size. Smaller rotors should be turned at a faster spindle speed than larger, heavier rotors. Composite Rotors Resurfacing a composite rotor without the proper adapters can quickly turn the rotor into scrap. The thin, stamped steel hat on these rotors will wobble and flex if machined with ordinary adapters. This can create runout and surface finish problems (extreme chatter). Adapters designed for turning composite rotors are large and heavy, clamping a large area of the hat and adding sufficient mass to the rotating assembly to allow a smooth cut. Both sides must also be cut simultaneously, and both sides of the center hat must be clean and rust-free for the rotor to turn true. These adapters also work well with standard hubless drums and rotors. If you do not have the required composite rotor adapters, resurface the rotor on the vehicle with an on-car lathe. Sanding Rotors As a finishing touch, a nondirectional finish may be applied to the newly refinished rotor. There s some disagreement as to whether this step is really necessary, especially if the lathe has produced an ultra-smooth rotor finish. But if it s done properly, it s hard 28 June 2004
to believe that finish sanding could cause any real harm. 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 lathe. This technique may be Most shops don t have an electronic surface profilometer to measure the surface finish. Alternatively, a surface comparator provides samples of various surface finishes for visual comparison. If you don t have either, use your eyes and fingers. If you can feel the grooves with your fingernail, the rotor is too rough. Try writing your name on the rotor with a ballpoint pen. If the ink breaks up into dots instead of forming a continuous line, the rotor is still too rough. Use sandpaper and a block or a dedicated lathe attachment to apply a nondirectional finish to the rotor. When you re finished, wash the rotor with soap and water to remove any leftover metal particles. If this step is overlooked, the metal particles will become embedded in the new pads. difficult and possibly dangerous. Some lathes feature attachments that quickly (and safely) apply a nondirectional finish after resurfacing is complete. No matter which method you use, sand evenly and don t remove any more metal than necessary. Otherwise, you may end up creating the same problems you were trying to eliminate. Rotor Cleaning Rotors should always 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. This could keep the new brake pads from transferring a layer of friction material onto the rotor. This can lead to noise, longer stops and increased pad or rotor wear. The most effective method is a mild soap solution, with the rotor wiped dry with a clean rag or paper towels. Special soaps and cleaners will achieve a clean surface. Do not rely on brake cleaner and compressed air to do the job. Those who do wash rotors say it reduces noise comebacks. New Rotors New rotors should be installed as-is, with no additional machining required. Most new OE and aftermarket rotors arrive with a surface finish that s somewhere between 30 and 60 in. RA (roughness average). Runout on new rotors should also be within accepted industry standards. You won t be able to improve on this by applying a cleanup cut before installing them. Removing as little as.0015 in. reduces rotor longevity unnecessarily. 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. As with freshly resurfaced rotors, new rotors must be cleaned before installation. Most are treated with an anticorrosion coating that will contaminate the new brake pads. Remove it with brake cleaner or other solvent. If you drop the rotor in your parts cleaning tank, follow up with a soap and water bath to remove any solvent residue. Once the new rotor is mounted on a vehicle, verify total installed runout 30 June 2004
with a dial indicator. Try indexing the rotor to several locations to achieve the lowest possible total installed runout. If you follow this procedure and you still encounter a large number of new rotors with more than.003 in. of runout, it may be time to shop for another rotor supplier. Hub Cleanliness Cleaning rotor and hub mating surfaces is a critical part of the brake job. The hub-to-rotor mating surface must be free of rust, or runout-induced disc 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 method used to clean the hub depends on the severity of the rust buildup. The hub s mating surface can be a difficult surface to clean due to the wheel studs. The area between the wheel stud and hub-centering flange is the most difficult area to gain access to. 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. 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. The tool uses a drawstring boot that closes around the work area. Once the hub has been cleaned, the rotor can be installed. As a final step, total installed runout should be rechecked at this point. A two-minute investment per axle takes the guesswork out of the equation. If runout is above the.002-in. range, try indexing the rotor to reduce it as much as possible. Your customers will thank you. Wheel Installation Many perfectly machined, cleaned and indexed rotors have been ruined by the last step in the procedure tightening the wheel lugs. Ideally, this procedure should be accomplished using a properly calibrated torque wrench. An impact wrench used at full throttle will quickly distort a hubless rotor, and may actually damage the wheel lugs. If you re not willing to use a torque wrench, at least invest in a set of torque-limiting sockets. Never use lubricants or penetrating fluids on wheel studs, nuts or mounting surfaces, which must be clean and dry. A thin layer of lube may be used between the inner surface of the rotor and the hub to slow down corrosion. Visit www.motor.com to download a free copy of this article. Circle #17 June 2004 31