Do you mean the front rotors or the rear drums?
If the rotors where machined, jack the front end up, remove the wheels, and run in it in gear. If the rotors are loose and can flop around, use washers and the lug nuts to hold them in place. If you can see any sideways runout in one of the rotors, (do you know what I mean by "runout"?), Take that one off and inspect the back mounting surface for round spots of rust buildup. Those form where water settles in the surface after it gets through the access holes in the hub flange. Most machine shops will scrape that rust buildup off, but check it anyway. Also look for any flakes of rust or dirt that got stuck between the rotor and hub. Look too at the rust buildup just where it starts next to the inside of the cooling fins, at the edge of hub contact area. If the machinist wasn't careful, the cone he used to mount it to his lathe could have been resting on that rust, so it would have machined a wobble into the rotor.
When watching for runout, you might also see the brake caliper moving back and forth sideways as the rotor turns. That would be the cause of the shimmy you're feeling. If you can't find any rust buildup or debris between the rotor and hub, look at the outer ring of rust on the mounting surface to see if there is evidence that the mounting cone for the brake lathe was resting on that rust.
If no other cause for the runout can be found, clean all the rust buildup off, then take them back to the machine shop and have them set them up again on their lathe. They can attach a dial indicator to measure exactly how much runout there is, but if it's enough that you can feel it, it's enough that you'll be able to see it easily too.
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Saturday, March 20th, 2010 AT 4:19 PM
