run out revisited

[76L82]

Okay...understanding that I am quite lazy, I am looking to address this run out issue once and for all. Now, I suspect that I could go to the parts store and get a micrometer (or whatever its called!) and try to measure the rotors...cursing the whole way.

Now, if in the worse case scenario I will need to have the rotors turned...why not just bite the bullet...have them turned and end the issue of whether or not the rotors are warped in the first place?? heck, I have changed almost everything thing else, why not be perfect?

On the serious side...if measuring for runout essentially is going to tell me if my rotors are warped...wouldn't taking them to be turned to start with get me there that much quicker? Any thoughts here would be much appreciated...as I can not go on with having spongy or fading brakes for much longer!

 

[JohnZ]

It's highly unlikely that your rotors are warped (if they're the stock OEM rotors); they have plenty of meat, and very seldom warp. What's more likely (if you're experiencing the familiar "air-pumping" problem on the rears) is that your rear wheel bearings have worn beyond limits and play in the spindles is causing lateral movement of the rotors.

If your rotor surfaces look OK and thickness is within limits, there's no reason to replace them if a dial indicator runout check shows them to be within .003" or so; if they're beyond that, you need to determne if the cause is the rotors or excessive play in the wheel bearings (spindle end play spec in the Shop Manual is .001"-.008", but that's pretty generous - .001"-.003" is preferred). Changing the rotors on a C2/C3 isn't just a "bolt 'em on and go" kind of job like it is on other cars with floating calipers.

 

[Barry70]

I have had some experience with runout over the years.
Johnz is correct.

However, most of the problems I have experienced is the fact that the spindle itself is not true.
(Dealing with rear only)

A little history....
GM (or supplier) assembled the rotor and spindle as a unit. The rotors/spindle were "trued" at manufacture as a unit.

If, at any time the, the rivets are drilled and the rotor is separated from the spindle and then reinstalled in a different location, then you most likely will experence runout, as the relationship between the rotor and spindle has been lost.

If you put on new rotors, this will not help, as the runout is likely coming from the face of the spindle.

Turning the old rotors will not work as well, same reason.

To really determine if runout is present you need to invest in a dial guage and check for runout.
Check the spindle first, then install the rotor and check again.
In some cases, trying the rotor on the spindle in different locations will fix the problem.
If the spindle has a slight warp in one direction and the rotor is also out a bit, then if they are installed together in the opposite direction, then, yes, two wrongs do make a right.

If you suspect you do have runout, then you could be in for a long period of brake problems until you address the runout issue.

Hope this information helps.
Barry

 

[76L82]

JOhnZ and Barrry - Once again..you guys on this board have come to my rescue!! This makes all the sense in the world to me now. I would have to guess that my wheel bearings are probably gone.

When I bought the car I was amazed to see that all 4 rotors had the original factory rivets in them...nothing had been done on this car since it was new. I made the usual first attempts at trying to fix the problem....new pads, new brake lines and new calipers all the way around. This seemed to give me more pedal at first when the brakes were bled but after a test run of not more than 15 -20 miles...after each stop, the pedal would work its way down to the floor.

I understand eactly what you are saying. If the basis from which "true" is determined...the spindles...is beyond tolerance...then anything else you bolt to them, regardless if its new or not. will not bring the unit back into correct alignment.

What has been so trying for me is that before I took on the Vette I had a couple of successes putting more conventional vehicles, Mustangs, Camaro's, back together. Never having owned a Vette before..I have really been taken by surprise how much more sophisticated the suspension components are. That being said...I still love wrenching on it.