Dear Tom and Ray:

I did a front-end brake job on my '89 Dodge Ram 150 truck. I took the front rotors off and had them turned at a local shop. I put in new pads and put everything back together correctly. I drove away the next day and experienced a slight vibration that had the sound of a low hum, like big, off-road tires on pavement. It only occurs when applying the brakes firmly. I checked to be sure everything is tightened correctly, but I still have the hum. What could be causing this? -- Phil

Ray: What's causing this? The guy at the local shop that turned your rotors, Phil.

Tom: He did a lousy job. He did, what we call in the trade, a "record cut," because the rotor looks like an old 33 rpm record, but with deeper grooves in it.

Ray: Ideally, the rotors should be perfectly smooth. You put the rotor on the lathe, and as the rotor spins, you move the blade from the inside to the outside. But if you move the blade too quickly across the spinning rotor, cut too deeply, or use a dull cutting tip, you'll end up with grooves. And when the brake pads press up against the grooved rotor, you get that hum.

Tom: So you can either wait until the metallic particles in your brake pads wear down the grooves and smooth them out, or you can go back and ask them to redo the rotors -- assuming you have enough thickness left in the rotor to cut it again. Good luck, Phil.

In their pamphlet "Should I Buy, Lease, or Steal My Next Car?" Tom and Ray break down the strategies for buying a car, so you can make the most of your money. To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1998 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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