BUMPY Braking REVISITED

BUMPY Braking REVISITED

Bob Freudenberger

Pesky pedal pulsation problems predominate

Probably half the articles you read in your professional journals deal with the subtleties of driveability work, but we who actually work out there in the real world know the news in service volume dollars is in brakes.

Current common complaints include premature pad wear, low pedal, squeal, and, our topic here, pedal pulsation, a condition that seems to have grown every time we investigate it. It tends to repeat, too, Sure, the car goes out of your shop with nice, smooth braking action, but in a distressingly high percentage of cases, it shows up at your door again within a couple of months, and the customer will be disappointed at best, homicidal at worst. There’s a good chance he’ll have lost his faith in your work, even though the reason bumpy braking reappeared usually has more to do with the design of the car than with anything you can control (see sidebar).

According to the experts we’ve interviewed, Honda and Acura are the worst offenders, followed by expensive European cars. But this condition is appearing in just about all makes. Whether mild or violent, it can take the fun out of driving an otherwise fine vehicle.

GENESIS

The whole thing started back in the ’70s with the energy crises, which pushed automotive engineers into ambitious (some would say reckless) weight-reduction programs. Eliminating drag, both mechanical and aerodynamic, was another hotly-pursued quest.

So, we went from heavily-built, boxy RWD cars that slowed down when you took your foot off the gas to lithe, streamlined FWDs with overdrive that not only coasted forever, but also blocked air flow to the brakes.

Compare the brakes on a twenty-year-old Caprice, say, to those of a recent Lumina. The old model used asbestos pads against massive 22-lb., well-ventilated discs with a large friction area and average surface temperatures of maybe 350 deg. F., while the newer one has abrasive semi-mets against skinny 10lb. composite rotors that reach 800 deg. F. at the slightest provocation. As a tech services rep for one of the major brake parts makers tells MS, “Take the GM-10 platform as an example — with four panic stops we were over 1,100 deg. F., and it’s very easy to hit 900.” All that heat in a thin little rotor can obviously cause warpage.

Greg McConiga, former NAPA/ASE Tech of the year and our “Talking Shop” columnist, has a theory about why pulsation problems increased when semi-mets supplanted asbestos. He believes it’s related to what happens when you stop at a light. By their very nature, semi-mets conduct heat away from the rotor, whereas asbestos is nothing if not a good insulator. This was intended, but it has an unintended consequence: The area of the disc that’s gripped by the pads cools faster than the metal around it, causing warpage.

Others see it differently. The marketing manager for a rotor and drum manufacturer, for instance, says, “A fundamental fact that many technicians don’t realize is that semi-mets are designed primarily to get the heat out of the rotor. If you put on a set of organic pads, you’ll guarantee that the rotor will be ruined.”

Regardless, heat is the enemy.

CHAIN OF EVENTS

Right about now you might be asking, “But shouldn’t floating and sliding single piston calipers just ride with the runout? What’s making those pulses?”

Good question. The answer: When it comes right down to basics, the direct cause of pulsation is DTV (Disc Thickness Variation), which can also be seen as a lack of parallelism between the two sides of a rotor. The wobble we’ve been talking about causes the rotor to wear unevenly as it hits those abrasive pads in one spot on each side every revolution of the wheel. In other words, the contact areas will end up thinner than the rest of the disc. One on-car lathe manufacturer claims that, typically, .002 in. of runout with zero-clearance bearings will cause about .0004 in. of DTV in 3,000 to 5,000 miles.

Some people want thickness variation to be held to .0002 in., while others say it’ll take .0004 to generate a complaint. Either way, you’re going to have to take your time checking for discrepancies this tiny.

There’s yet another factor that aggravates the development of DTV on late models. To reduce rolling drag, in many vehicles the engineers opted for pre-loaded, zero-clearance front wheel bearings, which require a less scuff-producing toe-in setting. But with no end play to absorb hub and rotor imperfections, any runout at all causes contact between the pads and rotor, increasing the wear that results in thickness variation.

CUTTING ‘EM

A tech guy with a major brake parts manufacturer mentions a related complaint he often hears: Right out of the box, composite rotors show wobble/runout when chucked up on the lathe. The mistake here is not using the proper lathe adapters for composites (they’re solidly clamped down during machining at the factory). Under the pressure of all that heavy metal, or when mounted on the car and squeezed down by the lug nuts, the rotor will run true. So don’t even think about turning new ones unless you’ve got the adapters.

A brake maker tech services supervisor reminded us of one more basic point: “On pulsation, many guys don’t maintain their lathes, and they can cut runout into a rotor,” he says. To corroborate, a hotline expert tells us he saw a tech actually standing on a lathe arbor in order to stack parts on a high shelf. Does that sound like a good practice with a device that’s supposed to be able to hold tolerances to the tenth?

ZERO IN

Now to nail down the offending components. As a preliminary, you have to make sure the condition is indeed due to the friction components and not the result of a malfunctioning ABS. The module could misinterpret a bad signal from a damaged or contaminated wheel speed sensor to be a skidding tire, so may pulse the hydraulics. If you take the time to carefully experience the condition, however, your mechanic’s instincts should allow you to differentiate between the two.

Next, you’ve got to determine whether the pulses are coming from the front or the rear (or both). Commonly, DTV in the front will cause the steering wheel to rock or shudder on light brake applications at low speeds. You’ll feel the side-to-side movement if you hold the wheel with just a few fingers. To find out if the rears are at fault, find an uncrowded street, coast in neutral at about 20-25 mph, and apply the parking brake gradually. Out-of-round drums will typically produce a heavy bumping and hitching during this test.

Get the car safely up in the air, but don’t pull the wheels yet. Instead, measure runout on the inside of the rotor with the wheel installed and the lugs torqued, if possible. This will tip you off to real-world runout.

POTATO CHIPS AND MICS

Besides ordinary runout, there’s disc flatness, the kind of warpage that makes the rotor resemble a potato chip. With your dial indicator, check for this at points 90 deg. apart and close to the outer edge of the wear surface. Find the high point and mark it “1,” rotate the disc one-quarter turn, mark it “2” and record the reading. Do the same thing twice more to establish points “3” and “4.” The difference between “1” and “3” will be max runout, whereas the biggest deviation from flatness will be between “1” and “2,” “2” and “3,” “3” and “4,” or “1” and “4.”

In the past we were told we could check for flatness with a straightedge and feeler gauges, but most authorities today say that’s apt to be inaccurate and misleading.

Uncovering DTV requires that you pay attention. Measure thickness at eight evenly-spaced points around the disc. By the way, you’ll need both a 0-1 in. and a 1-2 in. micrometer to cover cars and light trucks.

Everything we’ve mentioned so far applies to both front and rear discs. Since rears only account for maybe 20-30% of stopping power, however, they won’t be the cause of as many complaints.

New brand-name rotors are usually pretty close to perfect when you purchase them, but many brake specialists like to either check them with a dial indicator or chuck them up in the lathe for a light cut just to be sure.

DRUM DEAL

What about drums? Of course, they’re frequently out of round, and that’ll pump fluid back to the master every time the smaller diameter squeezes those shoes. This “ovalarity” can be the result of several things. We’ve been told that rough handling, as in dropping a drum on its edge, can put it out over .030 in. If a drum wasn’t particularly robust when new, it’ll be pretty weak after it’s been turned a couple of times, so just putting on the parking brake when everything’s hot can make it cool down into an oval.

Another weakening factor we’ve seen reach scary proportions is rusty drums with huge chunks of iron oxide falling off. How strong do you suppose a drum is then? Replacement is the answer.

One hotline expert tells us, “If there are hard spots left in the drum after turning, and you’re close to the max diameter, you’d better replace it.”

Pulsation Prevention & Cures

One of the primary statements of the modern version of the Hippocratic Oath that physicians take is, “Never do harm.” The same should be applied to what you do when you’re performing brake service.

Cranking down those lugs with your thermo-nuclear impact is a case in point. Sure, we all know that’s how just about everybody out there in the trenches does it, but it can, and does, cause harm. As a veteran brake expert with a major brake manufacturer explains it, “What happens is you tighten the first lug to 90 ft. lbs, and that cocks the wheel. Then, you may tighten the second to 90 also, but the wedge effect makes the other one actually closer to 130, which puts an uneven strain on the rotor. After a couple of months, or 1,500 to 4,000 miles, the iron relaxes to match the stress, and you’ve got a pulsation problem.

“You can avoid this by installing the wheel and tightening the lugs while the car’s on the lift, lowering it, backing them each off half a turn, then torquing them.”

Good point, and we’ll add that you should use the proper torquing pattern, either star or criss-cross, depending on the number of lugs.

There’s more, though. An experienced tech trainer with another brake company gives us what he calls “the ultimate tip.” He says, “Tolerance stack-up is a big reason for the wobble that wears the rotor unevenly. Make sure you do a good job cleaning the axle flange and the rotor’s mounting surface, then mount the rotor and torque the lugs. Check runout — if it’s less than .004 in., send the car out. If it’s anything more, remove the rotor, rotate it one lug, remount it, and check again. Also, if there’s no pulsation present when the car comes in, mark the rotor mounting position before you remove it so you can put it back where it was.”

Those are the things you can control, but pulsation is often simply due to the weaknesses of the design. It’s still your job to eliminate it, however. One great (if expensive) method is to use an on-car lathe, which will make up for mounting runout problems. It’s the best way to hold the minute tolerances needed to achieve the ideal of wobble-free discs with perfectly parallel friction planes and can make you the undisputed brake trouble terminator in your area.

Of course, springing for new rotors is often the best approach, and it’s usually not all that expensive — we commonly get domestics for $90-$100 a pair. They’ll be more resistant to a recurrence of the complaint simply because they haven’t been cut, so are thicker.

If you’ve got a chronic situation, however, there’s an even better choice: primo drilled and slotted discs. You’ll pay a premium, but their superior heat-dissipation characteristics amount to great insurance against a comeback.

COPYRIGHT 2000 Adams Business Media

COPYRIGHT 2001 Gale Group