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Brake Tech Series: A True Story

Saturday, March 1, 2008 - 00:00

WHEN AS LITTLE AS .001 INCH of lateral rotor runout can be enough to lead to disc thickness variation (DTV), it's crucial that rotors run true when they leave the shop. After all, once DTV has developed, drivers feel it as pedal pulsation, a leading cause of customer comebacks. Several authorities say it's the leading cause.

Most of us understand that careless lug-nut installation can induce runout, so we take care to tighten nuts correctly. But runout prevention starts before you reach for your torque wrench or torque sticks. Your weapons of choice in rubbing out runout are a dial indicator, an on-car lathe and, in many cases, tapered shims.

Tapered shims? Yes. According to General Motors, they're often the preferred mechanism for truing-up hubless rotors. Accu Industries produces a line of runout correction plates under the Brake Align name, and they're referenced in GM publications.

TO CUT OR NOT TO CUT?

When and why do rotors need resurfacing? Here are three good reasons:

Non-parallel working surfaces.

Rough working surfaces that may wear pads erratically or excessively and/or prevent proper pad break-in.

Excessive runout that can't be cured by other means.

Most authorities preach the necessity of measuring rotors for thickness, parallelism and runout every time you service brake pads. But General Motors says not every measurement is needed every time.

According to The General, if a car or truck has smooth pedal feel and no pull, your best course of action may be to skip resurfacing the rotors and use them as is. GM's not alone in saying this, as smooth, used rotor surfaces help with new-pad break-in when using the same type of friction material (e.g., ceramic, semi-metallic) as before. But some GM bulletins also say there are times when it's unnecessary to measure for DTV.

This doesn't mean they're saying to throw away your micrometer. Rotors must always be checked for minimum thickness. It's also always necessary to check working surfaces for cracks, scores or other surface imperfections that may not make themselves obvious in pedal action.

But if there's no hint of pedal knock-back as the old pads are nearly worn, GM says the vehicle is telling you the rotors' surfaces are parallel. Measure, if you want. It should verify the conclusion they're parallel.

It's still vital to check for runout, though, since you don't feel runout in the pedal, just the DTV that results after extended driving with runout. And, of course, if the rotor hat has come off the hub, there's always a chance that foreign material (a rust flake, for example) has gotten between the two. So lack of DTV doesn't necessarily mean a lack of runout. And if runout is present, it'll lead to eventual DTV and a customer comeback.

USING RUNOUT CORRECTION PLATES

How do you true-up a rotor with parallel working surfaces, but which displays a measurable, beyond-spec wobble when checked for runout? If it's a hubless rotor, "indexing" is your first step. Mark the rotor's position relative to the studs, make sure rotor/hub mating surfaces are clean and rotate the rotor one stud position relative to the hub. Hand-snug the lug nuts and check for runout again. You have the same number of possible positions as there are lug studs.

But indexing won't always true things up. When that's the case and the rotors are hubless, GM has been calling since 2001 for the use of Brake Align's tapered shims rather than resurfacing whenever feasible. One argument in favor of not turning rotors strictly for truing is that cutting them removes metal, shortening service life. And if the rotors are either new or used smoothies, turning could remove a more-desirable surface finish than you may be able to achieve with a lathe.

Brake Align's process is simple. When excessive runout is found, note its amount and mark the high spot.

Place the appropriately tapered runout-correction plate from the assortment for the vehicle family being serviced between the hub and rotor, with the V-notch in the plate lined up with the high spot. Assemble. You probably want to verify your success by taking another dial-indicator reading.

Thomas "Tommy" Saunders, president of Accu Industries, makers of AccuTurn bench and on-car lathes, developed the Brake Align procedure.

DON'T TOSS YOUR ON-CAR LATHE YET

Even if every vehicle manufacturer eventually endorses the use of Brake Align shims, on-car lathes will still be necessary. Of course, as of right now, nearly every automaker requires on-car machining for warranty maintenance when resurfacing is needed. Besides, David Scribner of Hunter Engineering points out that not all rotors slip off and on as easily as hubless rotors do (or theoretically should).

"Removing and replacing a trapped rotor for bench-resurfacing can take up to an hour-and-a-half per side," he says. That kind of disassembly requirement would also apply if shimming between rotor and hub.

On-car resurfacing technology has been evolving, too. Scribner notes that Hunter's current series of lathes incorporates variable-speed machining to combat chatter, as well as an intermittent tool feed.

What does all this mean? When servicing rotors, you have two options: shim or cut the rotors. Shim the rotors with runout-correction plates when rotor design and condition make it feasible, though right now only GM is saying to go ahead and do it.

But most vehicle makers call for the use of on-car lathes when resurfacing is needed, and on-car lathes are the tools of choice when rotors are trapped or hubbed.

CAPTIONS:

I may have sent over more pics, and if you would like to use one I haven't captioned, let me know and I'll provide its background and caption. But I've picked out the ones I think work well, and here's the info.

DTV Views New[1].PDF is courtesy of Brake Align and it's a good illustration of how runout leads to DTV. Pretty self-explanatory, too. I didn't send it before because it includes a small mistake. But it's probably easy to fix, so I'm sending it. The error is in the caption under the left rotor. It says "Lateral runout caused by this rotor..." Can you black out the word "by?"

The PDF "Exploded view of caliper," etc. is also courtesy of Brake Align. It's a graphic explanation of how to use Brake Align.

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