Like many automotive ser-vice procedures, muchhas changed when itcomes to rotor service.When cars were heavierand mostly rear-wheel-

drive, it was a matter of routine to turnthe rotors each time the pads were re-placed. The rotors were heavy andthick, and could be turned at least acouple of times before they required re-placement. Cars today are lighter andmany are front-wheel-drive. Savingweight saves fuel, but it’s also true that alighter car can get by with a lighterbraking system. It takes less to stop

3000 pounds than it does to stop 5000pounds.

The rotors on these lighter brakingsystems start out thinner, with less“meat” available for turning. The firsttime a rotor is turned may be the onlytime before it’s ready to be replaced. Yetmany shops are still resurfacing rotorsevery time the pads are replaced. Thismay no longer be necessary. Some vehi-cle manufacturers, including GM, argueagainst automatic rotor resurfacing.

In TSB No. 00-05-22-002, GMstates: “Brake rotors should only beturned when one of the following rotorsurface conditions exist: severe scoring P

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20 June 2004

BY KARL SEYFERT

With safety (and your shop’s reputation)at stake, doing it right the first time isessential when servicing rotors. When

used properly, both on-car and off-carbrake lathes will get the job done.

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

21June 2004

with depth in excess of 1.5mm or 0.060inch, pulsation from excessive lateralrunout of more than 0.080mm or 0.003inch, thickness variation in excess of0.025mm or 0.001 inch or excessivecorrosion on rotor braking surfaces.Rotors are not to be resurfaced in anattempt to correct the following condi-tions: noise/squeal, cosmetic corrosion,routine pad replacement or discol-oration/hard spots.”

GM says resurfacing is “ineffectiveat correcting brake squeal and/or pre-mature lining wear and should not beused to address these conditions—un-less specifically directed to do so in aservice bulletin.” If GM discouragesrotor resurfacing during pad replace-ment, when is this procedure appropri-ate? Clearly, you’ll see vehicles that falloutside the “normal” descriptionabove. Every vehicle is different,which is why each rotor must be care-fully inspected to determine the appro-priate course of action.

Rotor ThicknessEvery rotor inspection should beginwith a rotor thickness check. If the ro-tor is too thin, has previously beenresurfaced or has been damaged byworn-out pads, there’s no point inwasting any more time on it.

Check the rotor thickness at eightpoints around the perimeter of the ro-tor. Never base your rotor thicknesscalculation on a single measurement

taken at one spot on the rotor. Use amicrometer to measure rotor thick-ness, preferably one that’s designedspecifically for the job. A rotor mic fea-tures one flat anvil and one pointedanvil. The pointed anvil allows mea-surement of the real minimum thick-ness of a scored rotor. A mic with twoflat anvils will contact only the shoul-ders of any grooves or scoring lines.

Nominal thickness is the rotor thick-ness when new. This specification canusually be found in the vehicle servicemanual. The machine-to thickness isthe thinnest a rotor can be machined toand returned to service. The machine-

to or minimum thickness specificationprovides enough rotor material for safeand reliable brake performance.

The discard thickness is the dimen-sion at which the rotor must be re-placed. If a rotor is at or below its dis-card thickness, brake performance willbe compromised. In a worst-case sce-nario, a rotor that’s worn below its dis-card dimension, combined with worn-out brake pads, could allow the caliperpiston to fall out of the caliper housing.Rotors worn below the discard dimen-sion are also prone to warping and otherdamage. A rotor that’s below the discardthickness should never be reinstalled.

The rotor friction surfaces must alsobe parallel to one another. The allow-able tolerance is known as parallelism,or thickness variation. Variation in ro-tor thickness will cause pedal pulsa-tion. The variation might be the resultof excess heat buildup that has warpedthe rotor, or the rotor may be contami-nated by isolated thick spots caused byrust or corrosion buildup.

RunoutIf a rotor appears to wobble from sideto side as it rotates when viewed fromits front edge, this off-center deviationis called lateral runout. Long before it’svisible to the naked eye, runout cancause brake pedal pulsation, steeringwheel wobble and vibration duringbraking. Many customer brake com-

22 June 2004

LET THE CHIPS FLY: ON-CAR & OFF-CAR BRAKE ROTOR SERVICE

Many rotors have the minimum thickness dimension stamped or cast in aprominent location. A little work in the blast cabinet revealed the specs on thisimport rotor. Unvented rear rotors are often replaced rather than resurfaceddue to their relatively thin construction. This one was swapped for a new one.

Rotors may develop a healthy accumulation of rust near the edges. If the rustextends into the “live area” where the pads will make contact, be sure to fig-ure this into your rotor thickness calculation. By the time the rust is re-moved, the rotor may be close to its discard dimension.

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plaints can be traced to runout prob-lems.

Rotor runout may or may not becaused by rotor thickness variation. If arotor thickness check shows no evi-dence of a variable dimension, the rotormay be rotating off its true axis as a re-sult of other damage. The wheel bear-ing may be badly worn and excessivelyloose, which would cause rotor wobbleas the vehicle rolls down the road.

In the case of a hubless rotor, thehub itself may also have excessiverunout, which is then transferred tothe rotor. The contact area betweenthe hub and hubless rotor hat may alsobe to blame. Contamination (rust orgrit buildup) may create an unevenmounting surface that will cause therotor to rotate off its intended axis.

Another potential cause of excessiverunout (one that’s common with thin-hat hubless rotors) is poor wheel instal-lation practice. If the wheel fastenerswere improperly tightened duringwheel installation, it’s very likely thatthe rotor hat may warp due to unevenor excessive fastener torque.

Measuring RunoutUse a dial indicator to check lateralrunout. Mount it securely to a station-ary fixture. For on-car measuring, at-tach the fixture to the spindle or con-trol arm. Locate the dial indicator’s

plunge tip about an inch inboard of therotor edge, and zero the indicator.

Runout is defined as the amount oflateral (side-to-side) movement of therotor as it rotates through 360°. Theaverage allowable specification for late-model vehicles is between .001 and.003 in. This is the total indicatedrunout specification, as measured onthe vehicle. Total indicated runout canbe affected by several factors—hubrunout, cleanliness of the hub-to-rotor

mating surface and wheel lug torque.Runout problems on hubless rotors

occasionally can be corrected by index-ing the rotor. Place a chalk mark on astud and at the adjacent area on the ro-tor hat. Now relocate the rotor clock-wise to the next stud position beforerepeating your runout measurement.Repositioning the rotor may minimizethe total assembled runout caused byminor deviations on the machined sur-faces of both the underside of the rotorhat and the hub flange. It may take afew tries to find the best match-up.

Rotor runout can be measured inde-pendent of the hub by removing it fromthe vehicle and mounting it on an off-car brake lathe. Likewise, hub runoutcan be measured without the rotor inplace, to remove as many variables aspossible during a complicated runoutdiagnosis. If the hub flange or rotor arecausing the runout problem, you’ll beable to isolate them in this way.

On-Car vs. Off-CarRotor ResurfacingWhen used properly, both on-car andoff-car brake lathes can provide excel-lent results. On-car lathes were origi-nally developed for vehicles with “cap-tive” rotors, and others known to besensitive to runout problems. In fact,some vehicle manufacturers specify

24 June 2004

LET THE CHIPS FLY: ON-CAR & OFF-CAR BRAKE ROTOR SERVICE

The inside and outside mating surfaces must be clean and flat before youresurface a hubless rotor. The bearing races on conventional hubbed rotorsshould also be cleaned before the rotor is installed on the lathe. This rotor’smating surface was heavily rusted, and required several minutes in a blastcabinet before it was ready to be resurfaced and reinstalled.

All lathe hardware should be clean and damage-free. Dirt or rust on the ma-chined surfaces of the mounting cups will prevent proper rotor mounting andwill introduce unwanted rotor runout into your setup. Always use the largestpossible mounting cup, and make sure both cups are the same size.

on-car brake lathes only for resurfacingthe rotors on their vehicles.

On captive rotor systems, the rotoris mounted behind the hub. Removingthe rotor means removing the hub,which may then be difficult to mounton an off-car lathe for accurate ma-chining. Unless you compensate forthe rotor runout that exists on the vehi-cle, you can’t guarantee a true-runningrotor when it’s reinstalled. When possi-ble, rotors on these vehicles should bemachined in place.

Shims are also available to compen-sate for runout on rotors that havebeen machined on an off-car lathe.The shims are available in a variety ofsizes, and fit between the hub flangeand the rotor hat. Shifting the posi-tion of the shim “zeroes out” the as-sembled runout.

On-car lathes are designed to com-pensate for runout problems, and do notrequire major disassembly of the huband rotor on captive rotor brake sys-tems. Some even compensate for runoutautomatically, although setup still re-quires a properly trained operator.

Rotor FinishRotor finish (smoothness) is affectedby the crossfeed rate of the lathe, aswell as the depth of the cut. Turning

a rotor too quickly leaves grooves inits surface. The grooves are not paral-lel, but form a spiral like the groovesin an old LP. When the pads makecontact with these grooves, they jerkup and down, setting up vibrationsand noise. Always remove the small-est amount of material possible, whileachieving a smooth rotor finish. Thisextends rotor life and reduces the

amount of folded and torn metal onthe surface of the rotor.

A rough rotor finish can increasepedal effort because the pads canmake contact only with the peaks (atleast initially). As the brakes wear in,the pads eventually wear down thepeaks and increase the area of contact.This accelerates rotor and pad wear.

The type and condition of the lathe

26 June 2004

LET THE CHIPS FLY: ON-CAR & OFF-CAR BRAKE ROTOR SERVICE

Attach a vibration damping ring to all vented rotors to dampen vibration andassure a smooth, chatter-free resurfacing job. Other lathe attachments areavailable for quelling vibrations on thinner, unvented rotors.

Once the rotor is installed on the lathe, check it for runout with a dial indica-tor before you begin to refinish it. Something as simple as the way you tightenthe lathe arbor nut can introduce lathe runout. For the record, tighten thenut slowly and evenly; don’t jerk the wrench to get the nut extra tight.

A very light scratch cut provides avisual indication of rotor runout. Ifthe cut covers less than 50% of therotor’s circumference, there aretwo possible causes: Either the ro-tor really is in need of major resur-facing or your lathe setup is intro-ducing runout into the process.

tool bits have a direct effect on rotorsurface finish. Round carbide bitsproduce a smoother finish than angu-lar bits. They also allow a faster cross-feed rate, which reduces resurfacingtime. Top shelf titanium bits haveparallel, peripheral ground edges.

Lathe Setup ErrorWe’ve discussed on-car runout. It’s al-so possible to add runout to a rotorduring off-car rotor resurfacing.Lathe-induced runout can be causedby runout in the lathe arbor, latheadapters that are dirty or in poor con-dition or a sloppy initial setup.

To verify your lathe setup beforeresurfacing a rotor, always conduct ascratch 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 ofthe rotor inward until it barely touch-es the rotor. While holding the outerknob, zero the inner dial. If the dialcan’t be zeroed, take note of the actu-al dial reading.

•Back the cutting tip away fromthe rotor a small distance and turnthe lathe off.

•If the scratch cut covers at least50% of the rotor, proceed with the

machining process. The scratch cutmay be less than 50% because the ro-tor has runout and needs to be ma-chined. It may also be because thelathe setup is incorrect.

Adapter cleanliness, arbor runoutand tightening of the arbor nut maybe responsible for setup problems. Toverify the setup, loosen the arbor nut.While holding the inner and outerlathe cup, rotate the rotor 180°, thenretighten the arbor nut. This changesthe relationship between the rotorand the lathe adapters.

Turn on the lathe, then move thetwin cutter forward or back a smalldistance. Make a second scratch cutby turning the dial to zero. This stepassures that each scratch cut is thesame depth, which makes comparingthe cuts easier.

Back off the cutting tip slightly,turn off the lathe, then compare thetwo scratch cuts. If the problem is inthe rotor, the cuts should be in phasewith one another. A setup problemwill cause the second cut to be in adifferent position from the first. Ifthis is the case, disassemble every-thing and check for cleanliness, cor-rect adapters and nicks on the matingsurfaces.

28 June 2004

LET THE CHIPS FLY: ON-CAR & OFF-CAR BRAKE ROTOR SERVICE

This Hunter lathe with digital readout capability takes alot of the guesswork out of rotor and drum resurfacing.As a first step, the lathe measures the rotor thickness todetermine whether it should be discarded before machin-ing. Total indicated runout can also be easily calculated.

On bench lathes with adjustable spindle speeds, a spindlespeed of 100 to 150 rpm with a crossfeed rate of .002 to.005 in. per revolution should produce a smooth rotorfinish. Spindle speed should be adjusted to match rotorsize. Smaller rotors should be turned at a faster spindlespeed than larger, heavier rotors.

Composite RotorsResurfacing a composite rotor with-out the proper adapters can quicklyturn the rotor into scrap. The thin,stamped steel hat on these rotors willwobble and flex if machined with or-dinary adapters. This can createrunout and surface finish problems(extreme chatter).

Adapters designed for turningcomposite rotors are large and heavy,clamping a large area of the hat andadding sufficient mass to the rotatingassembly to allow a smooth cut. Bothsides must also be cut simultaneously,and both sides of the center hat mustbe clean and rust-free for the rotor toturn true. These adapters also workwell with standard hubless drums androtors. If you do not have the re-quired composite rotor adapters,resurface the rotor on the vehiclewith an on-car lathe.

Sanding RotorsAs a finishing touch, a nondirectionalfinish may be applied to the newly re-finished rotor. There’s some disagree-ment as to whether this step is reallynecessary, especially if the lathe hasproduced an ultra-smooth rotor fin-ish. But if it’s done properly, it’s hard

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to believe that finish sanding couldcause any real harm.

A final sanding knocks off thesharp peaks, as well as the torn andfolded metal left behind by the lathebits, and may improve the surface fin-ish by 2 to 5 microinches. This im-

proves pedal feel, pad seating andoverall brake performance.

A nondirectional finish can be ap-plied with #120 to #150 sandpaper anda flat sanding block. Some techs movethe block in and out as the rotor turnson the lathe. This technique may be

difficult and possibly dangerous. Somelathes feature attachments that quickly(and safely) apply a nondirectional fin-ish after resurfacing is complete. Nomatter which method you use, sandevenly and don’t remove any moremetal than necessary. Otherwise, youmay end up creating the same prob-lems you were trying to eliminate.

Rotor CleaningRotors should always be cleaned withsoap and water after machining. Afreshly resurfaced rotor is covered withmetal particles that can become em-bedded in the brake pads. This couldkeep the new brake pads from trans-ferring a layer of friction material ontothe rotor. This can lead to noise, longerstops and increased pad or rotor wear.

The most effective method is a mildsoap solution, with the rotor wiped drywith a clean rag or paper towels. Spe-cial soaps and cleaners will achieve aclean surface. Do not rely on brakecleaner and compressed air to do thejob. Those who do wash rotors say itreduces noise comebacks.

New RotorsNew rotors should be installed as-is,with no additional machining re-quired. Most new OE and aftermar-ket rotors arrive with a surface finishthat’s somewhere between 30 and 60in. RA (roughness average). Runouton new rotors should also be withinaccepted industry standards. Youwon’t be able to improve on this byapplying a cleanup cut before in-stalling them. Removing as little as.0015 in. reduces rotor longevity un-necessarily. And if your lathe setup isincorrect, you may make the finishrougher or introduce a runout prob-lem that wasn’t there to begin with.

As with freshly resurfaced rotors,new rotors must be cleaned before in-stallation. Most are treated with an an-ticorrosion coating that will contami-nate the new brake pads. Remove itwith brake cleaner or other solvent. Ifyou drop the rotor in your parts clean-ing tank, follow up with a soap and wa-ter bath to remove any solvent residue.

Once the new rotor is mounted ona vehicle, verify total installed runout

30 June 2004

LET THE CHIPS FLY: ON-CAR & OFF-CAR BRAKE ROTOR SERVICE

Most shops don’t have an electronic surface profilometer to measure the sur-face finish. Alternatively, a surface comparator provides samples of varioussurface finishes for visual comparison. If you don’t have either, use your eyesand fingers. If you can feel the grooves with your fingernail, the rotor is toorough. Try writing your name on the rotor with a ballpoint pen. If the ink breaksup 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 nondi-rectional finish to the rotor. When you’re finished, wash the rotor with soapand water to remove any leftover metal particles. If this step is overlooked,the metal particles will become embedded in the new pads.

with a dial indicator. Try indexing therotor to several locations to achievethe lowest possible total installedrunout. If you follow this procedureand you still encounter a large num-ber of new rotors with more than.003 in. of runout, it may be time toshop for another rotor supplier.

Hub CleanlinessCleaning rotor and hub mating sur-faces is a critical part of the brake job.The hub-to-rotor mating surface mustbe free of rust, or runout-induced discthickness variation can occur shortlyafter the brake job is completed. As lit-tle as .001 in. of rust at the outsideedge of the hub will translate into .002to .004 in. of runout when measured atthe outside edge of the rotor.

The method used to clean the hubdepends on the severity of the rustbuildup. The hub’s mating surface canbe a difficult surface to clean due tothe wheel studs. The area between thewheel stud and hub-centering flange isthe most difficult area to gain access to.

If there’s mild rust buildup on thehub, use an angle grinder equippedwith an abrasive disc to clean the ma-jority of the surface area. Abrasiveblasting can be used for heavy rust re-moval. Short of removing the hubfrom the vehicle, specialized equip-ment is available to blast the hubwhile it’s still on the vehicle. The tooluses a drawstring boot that closesaround the work area.

Once the hub has been cleaned, therotor can be installed. As a final step,total installed runout should berechecked at this point. A two-minuteinvestment per axle takes the guess-work out of the equation. If runout isabove the .002-in. range, try indexingthe rotor to reduce it as much as possi-ble. Your customers will thank you.

Wheel InstallationMany perfectly machined, cleaned andindexed rotors have been ruined by thelast step in the procedure—tighteningthe wheel lugs. Ideally, this procedureshould be accomplished using a prop-erly calibrated torque wrench. An im-pact wrench used at full throttle willquickly distort a hubless rotor, and may

actually damage the wheel lugs. Ifyou’re not willing to use a torquewrench, at least invest in a set oftorque-limiting sockets.

Never use lubricants or penetratingfluids on wheel studs, nuts or mountingsurfaces, which must be clean and dry.

A thin layer of lube may be used be-tween the inner surface of the rotor andthe hub to slow down corrosion.

31June 2004

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Circle #17