Vibrating Brakes

Why would the front disc brakes on a 1989 Chevy Caprice seriously vibrate the entire car? Car has new rotors and pads and the wheel bearings do not seem to be the problem (no noise when driving until the brakes are applied). Calipers were not replaced but the pistons retracted relatively freely when “C” clamp pressure was applied to install the pads and rotors. Note: Rear brakes are shoes/drums and were replaced in the last 6 months.

Make sure you have the wheelbearings adjusted right. Then, if it only happens when you are brakeing it has to be the rotors. You might have to have the rotors turned again. Are you sure the machine shop would know if the rotors are bad?

It is possible the new rotors were not properly torqued when installed. That could have led to subsequent warping. Remove the rotors, have a shop check for runout, then reinstall at proper torque.

Thanks. Since I posted this on 3/9/08, I have replaced and bled both front calipers and have replaced both front inner/outer wheel bearing and seals. I set the wheel bearings acording to the Haynes Manual (torqueing the bearing nut to 12 foot pounds as called for in the manual). The vibration/chatter (appears to feel like quick grabbing and releasing) can be noticed only when braking at speeds of 30-35mph or more. I do not notice it when braking at lower speeds. Since this has occurred with both the old and new rotors, I would suspect that it is not the rotors (would have to have real bad luck to get a 2nd set of bad rotors). Two questions: 1. Does 12 ft lbs of torque on the bearing nuts sound like enough? and 2. Could something in the front suspension (bad tie rod, bad ball joints, etc.) be causing this? Thanks.

Just from my experience, adjusting a wheel bearing with a torque wrench is really difficult. I’ve always used the “loose enough to spin freely, but tight enough that you can’t wiggle the wheel from side to side” method and I’ve never had any problems. Also, the bearings will loosen up with use. One way to minimize this is to, before you adjust them, to tighten the nut down very tight and then, with the tire on to give you leverage, turn it a few rotations in either direction. Retighten it and repeat the process. Then loosen the nut up and re-tighten to the spins free/doesn’t wiggle point and give it a few more spins in either direction and re-check it.

However, if the noise happens only when you’re braking, it’s probably not a wheel bearing issue. One other thought I had was that if your tires are cupping or wearing in a strange manner, sometimes you’ll get a weird vibration that only happens when braking. Rotating your tires might be worth a try.