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Raybestos Brake Tech School, Part One: Rotors Don't Warp

Written by PFM Staff

Brake rotors do not warp from heat, even when driven by the most aggressive traf­fic officer. Instead, they wear unevenly. This uneven wear is caused by the brake pads themselves as they intermittently touch an out-of-true rotor. The root cause of the uneven wear is one of two things: either the rotor was installed out-of-true with the hub, or the tire was improperly torqued to the hub during the last tire change.

All of this is important for fleet managers because their vehicles have the tires removed frequently and the pads and rotors replaced frequently. You won’t solve a problem caused by wear if you treat it as a problem caused by heat. Instead, fix the rotor installation or wheel lug nut torquing.

The problem of pedal vibration, incorrectly called rotor warp, occurs 3,000 to 5,000 miles after the brake or tire change. Because it is caused by uneven rotor wear, not the rotor warping like a potato chip from heat, you can’t solve this wear problem by better control of the heat, i.e., by the use of specially processed rotors or drilled and slotted rotors. Instead, you solve the wear problem by fixing the shortcuts in rotor installation or the improper lug nut tightening.

By taking a few steps, the fleet manager can easily, quickly and permanently fix the pedal pulsation problem. First, for each rotor change, verify the runout of the rotor, and then do one or two quick things to have it less than 0.002 inch, which is the OE spec for most vehicles. Two, for each tire and wheel change or rotation, torque the lugs in a star pattern using either a torque wrench or torque stick. Those two steps will virtually eliminate premature rotor wear, period.

Think about it this way: What are your conservative officers doing to their cars to “warp” the brake rotors in less than 5,000 miles? They don’t get the brakes warm (350 F), let alone patrol-hot (600 F), and never pursuit-hot (850 F). Yet their rotors are warping? No. They are not warping. They are unevenly wearing during the times of zero brake pedal pressure, and your officers are not doing anything to either prevent it or cause it.

Foundation Brakes

While a lot more was covered, that is the essence of the four-day Affinia-Raybestos Brake System Diagnosis & Repair Course, i.e., Foundation Brakes. This course is half classroom-based and half hands-on. It is held at Affinia-Raybestos headquarters in McHenry, IL and is also conducted at locations across the nation. Class sizes run a maximum of 12 because of all the hands-on work.

The Foundation Brake class covers hydraulic braking theory, brake system dynamics, diagnostics, troubleshooting, repair techniques, preventative maintenance practices, rotor resurfacing techniques, bench and on-car lathe operation, brake (friction) pad materials and noise and dust solutions.

The Foundation Brake class is not one bit product-oriented. It is not a veiled sales job. It is tech training, and you will get your hands dirty. You will install Raybestos pads, rotors and calipers in this class and Raybestos master cylinders, hoses, lines and valves in other classes. However, brand to brand comparisons are never made, and brand names are seldom mentioned.

Instead, the brake tech school is all about brake jobs, accurate diagnosis, fixing it right the first time and preventing comebacks—not about what brand of pad or rotor is better than another. Police Fleet Manager attended the course and recommends it, without reservation, to all new police fleet managers, new maintenance techs, and as a refresher for veteran maintenance techs and new shop superintendents or maintenance supervisors.

Just as important as the Foundation Brakes class is for many maintenance techs, so is the three-day Affinia-Raybestos ASE Brake Test Preparation Course. This course provides information, training and sample testing to help techs prepare for ASE Brake Test A5. Affinia-Raybestos also has shorter, stand-alone, hands-on brake lathe (both bench type and on-car), brake system diagnostics and installation, ABS advanced diagnosis and service, and a variety of chassis-suspension and steering courses.

The brake foundations course started the way that all “real” training classes start: with a pre-test. What is the most effective way to pinpoint a spongy brake pedal? Perform an isolation test by clamping all of the brake line hoses. What type of seal is used around the piston in the caliper? A Square Cut Seal.

A metering valve restricts initial pressure to where? The front brakes. A proportioning valve limits pressure under hard braking to where? The rear brakes. What is the probable cause for a car with a spongy pedal? Air in the system. Improper lug nut torque or tightening sequence causes rotors to “warp.” True.

Tightening Sequence

That last question is where the class instruction started. In the old days, star-pattern tightening using torque wrenches was the only way lug nuts were tightened. Not any more. Today, it is throw the tire and wheel over the lugs, hand start the five lug nuts, then use an air wrench in a once-around circle pattern, and you are done.

Doing this, the first few wheel lugs lock the rotor into location while the last few wheel lugs pre-load the rotor, which is like slightly bending a spring. Even worse, even if perfectly tightened to 100 ft-lbs, the last few lugs are false readings. Some of the torque is absorbed in flexing the rotor, not tightening the lug. This is almost guaranteed to cause uneven rotor wear (incorrectly called warped rotors), which results in the tell-tale pedal pulsations after just a few thousand miles.

The unevenly torqued rotor, even with the correct amount of torque, will not be bent when the rotor is cold. However, as the rotor heats up in normal use, it will expand unevenly. The most uneven area will, of course, be near the first tightened and last tightened lugs. As the rotor heats up and expands, a runout will be caused, i.e., a high spot on one side and a high spot on the other side. These high spots will come into intermittent contact with the retracted pads during normal driving, i.e., without brake pedal pressure.

As this happens, the semi-metallic pad used with police cars will grind away the high spots on either side. On the other hand, the ceramic pad used with retail cars will transfer material to the high spots. In just a few thousand miles, the rotor will have a significant thickness variation, either from worn away rotor or material transfer from the pad. Rotor thickness variation causes brake pedal pulsation and steering wheel vibration!

Air Wrench Blues

The wrong tightening sequence, even with the right amount of torque, can cause a rotor problem. So can the right tightening sequence with the wrong amount of torque. (No, your air impact wrenches are not calibrated!) Here is what happens.

The air wrench increases or decreases in torque as it is being used. The line pressure falls, the air compressor kicks in, the line pressure increases. The result is that an air wrench nominally set at 100 ft-lbs may actually torque the lug nuts to 85, 110, 100, 90, 110, 90. Any variation above 10 percent is bad. The result is exactly the same in the situation where warming causes uneven expansion, as in the situation with the incorrect circular tightening sequence situation.

These two causes of premature rotor wear can be easily fixed. First, use a star pattern instead of a circle pattern to tighten the lugs. Second, for those using a torque wrench, first pre-tighten the lugs with the socket wrench, then tighten them afterwards with the torque wrench. Don’t overdo it with the socket wrench. The torque wrench must be allowed to tighten the lug at least a little. If it clicks-out without tightening the nut at all, the nut could be way over-torqued. That is what happens if you use a breaker bar for the initial tightening.

Third, for those using an air wrench, use torque sticks. Or set the air wrenches under the necessary torque and finish them off with a torque wrench. Rule one: star pattern. Rule two: torque sticks or torque wrench. Follow these rules every time you replace a tire.

The “Proper” Brake Job

Properly tightening the lug nuts prevents one major cause of premature rotor wear. The other major cause is “improper” installation of the rotor. And here is where you may get some resistance from your veteran techs who have been doing brake jobs for 10 years. You cannot do a proper brake job without a dial indicator to measure rotor runout.

Those who have turned a wrench before may think they already know how to properly change brake pads and rotors. This may be. However, if the brake job is performed on a modern two-piece hub and rotor the same way it was on the old one-piece hub and rotor, the job is probably not being done properly. That means the brake job is not being done per OE factory spec and how the OE dealers do it.

This habit of “continuing to do it the old way” may explain why rotors produce a pedal pulsation, i.e., “warped” rotors in just a few thousand miles. That alone may explain premature pad wear and premature rotor wear, even in premature for police operations.

“A brake repair shop without a dial indicator is like an engine rebuild shop without a torque wrench,” said Dann Ingebritson, technical instructor, Affinia Under Vehicle Group.

The rotor lateral runout is a slight wobble or slight wave in the surface of the rotor as it is being rotated. This out-of-true condition can be caused by an out-of-parallel condition of the rotor—even a brand new one or one that just came off the bench lathe. This can also be caused by an out-of-parallel hub mounting surface or by a stack up of out-of-true conditions on both the rotor and hub.

This runout or wobble will cause exactly the same intermittent contact between the rotor and the pad as the improperly torqued lug nuts. Again, material will be removed from the rotor by semi-met pads, or added to the rotor by ceramic pads in just the high spots. Again, the result is rotor thickness variation which causes brake pedal pulsation and steering wheel vibration. Again, the rotors need replacement in less than 5,000 miles. Again, the officer driving the car does nothing to cause the problem.

This was not a problem 10 years ago when the runout spec was 0.010 inch or so and the brake pads used softer friction materials. Today, this is a major problem, when the runout is 0.002 inch max and the pads are rock hard.

Dial Indicator

When the new or cut rotor is put back on, torque it in place. Clamp a flex-arm dial indicator on a solid part of the suspension, place the indicator tip on the rotor, zero the dial and slowly rotate the rotor. A rotor runout gauge set costs about $50 and is available at any auto parts store or online. Google the term “rotor runout dial indicator” and you will find many. Check the OE spec, but most police sedans call for a maximum runout of 0.002 inch. If the runout is less than that, put the caliper and tire on!

If you have cleaned the hub with a wire brush or roto-brush, including the surface very close to the lugs, and you are using a new high quality rotor, the majority of the time the runout will be in-spec. If the hub surface is not perfectly clean, if you bought a cheap rotor or if the rotor has been cut on a lathe with an excessive runout, the rotor runout will be out-of-spec. Mark the high spot. Something must be done or this rotor will cause pedal pulsation in just a few thousand miles.

If the rotor is indicating an excessive amount of lateral runout, the tech has three options. One is a quick and easy option which works about 80 percent of the time, one is a fairly easy but more expensive option that works 95 percent of the time, and one is a slower and harder option that absolutely works every time.

Rotor Indexing

The easy solution is to take the rotor off the hub, rotate it about half way around and bolt it back in place. (On the 5-lug police cars, rotate it not quite 180 degrees, one way or the other.) Now indicate the rotor. Most of the time it will now be in-spec.

Why? The rotor surface has a slight runout and, separately, the hub surface has a slight runout. When you bolt the rotor to the hub, you stack these tolerances. Let’s assume the rotor is out by 0.003 inch and the hub is out by 0.002 inch. Mounted with the high spot of the rotor on top of the higher spot on the hub, the assembly will have a runout of 0.005 inch, which is terrible. Now rotate the rotor on the lug pattern, i.e., “index” the high spot of the rotor over a low spot on the hub, and the runout is just 0.001 inch, which is excellent.

Indexing the rotor works most of the time. It only takes a few minutes to remove the rotor, rotate it, replace it and indicate it. You may think, “Hey, that’s not in my flat rate.” Actually, it probably is.

Brake Align Shims

The fairly easy option that works almost all the time involves the use of Brake Align™ correction plates, AKA shims or spacers.

Again, indicate the mounted rotor and mark the high spot on the rotor. Also, mark the lug, or the two closest lugs, nearest the high spot. Remove the rotor and put the correct shim over the wheel lugs. Put the notch in the correction plate closest to the marked lugs. The notch marks the thinnest part of the shim. Put the rotor back on, just how it came off, and indicate it. It will be in-spec.

The Brake Align shims come in a wide assortment of thicknesses, in 0.001-inch increments or so to make correction for a variety of runout conditions. If you need a 0.009-inch shim, start replacing parts rather than shimming them.

The Brake Align shims are a bit expensive at about $20 each. That is a pretty costly solution to an out-of-spec rotor. Cut the rotor on a lathe or send it back to Cheap ‘R’ Us auto parts store where it came from. However, what if the high quality rotor is perfect? What if the excessive runout is in the hub? The Brake Align shim is certainly less expensive than a new hub.

The final solution, and one that works every time, is the on-car rotor lathe. Of course, that assumes your shop has one of these $3,500 to $5,000 tools. Hunter Engineering, Kwik-Way and Pro-Cut all make this equipment. We’ll have more on rotor cutting in the next issue. For more information on classes and schedules, contact Affinia-Raybestos Technical Services at (815) 363-9000 or (800) 274-4631.

Published in Police Fleet Manager, Mar/Apr 2010

Rating : 8.4


Comments

Comment on This Article

Hmmm?

By Paul

This sounds perfectly reasonable in a class setting. However, in real world applications I find reality differs. If indeed the issue is one of alignment between rotor and hub...then why the pulsation after 3-5k? Why not immediately? It would seem that the time it takes for the rotors to prematurely wear also conincide with the time it takes our drivers to "warp" the rotors to the point of pulsation felt at the pedal...I'm not quite convinced. I will however try my own experiment... using the instructions posted in this article and decide for myself.

Submitted Feb 18 at 12:06 AM

Great!

By Len

I'm a DIYer who does a lot of brake work and this article is very helpful! Harbor freight sells the dial indicator you need for cheap, and using the knowledge in the article I always index my rotors now! The star pattern makes a big difference, many engine procedures specify a precise bolt tighten sequence. I always do two star tightenings, to be sure lugs are up to spec.

Submitted Aug 15 at 10:23 AM

Rotor Shim Question Answered

By Andy

So glad I found this. Was looking for a good answer to why one would use a rotor shim. Most people, even most "mechanics" don't know what this article explains; that is scary. Thank you!

Submitted Jun 18 at 11:35 AM

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