Rotor runout . . . HELP!
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
Rotor runout . . . HELP!
I don't get it? What am I missing? This is my first time shimming the rotor runout and I'm not sure what to do next. I indexed the rotor by mounting the dial indicator on the frame and placing the needle on the backside of the rotor.
I made sure the needle was positioned at 90* to the rotor.
To have consistent measurements, I moved the dial indicater to the backside of the rotor so I would not have to move the dial each time I rotated to the next index. I torqued the lug nuts to 40lbs each time. When I measured the total runout as I indexed the rotor, it was within .001 each time.
I was expecting there to be a difference from one side of the rotor to the other and I would shim the low side. But what does it meant when each index/hole is that close?
I made sure the needle was positioned at 90* to the rotor.
To have consistent measurements, I moved the dial indicater to the backside of the rotor so I would not have to move the dial each time I rotated to the next index. I torqued the lug nuts to 40lbs each time. When I measured the total runout as I indexed the rotor, it was within .001 each time.
I was expecting there to be a difference from one side of the rotor to the other and I would shim the low side. But what does it meant when each index/hole is that close?
Melting Slicks
Joined: Nov 2003
Posts: 2,362
Likes: 1
From: San Antonio TX
I am sorry as I do not see a problem here? If you are within .001 leave it alone 
As for both sides of the roter they should be the same. Now go on to the next 3 rotor's
From what I can remember you would like to shoot for .003 run out or less.
As for both sides of the roter they should be the same. Now go on to the next 3 rotor's From what I can remember you would like to shoot for .003 run out or less.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
No, I don't think I'm doing something right here. You can see the .018/.019 "wobble". My understanding is that I'm trying to get that down to .002. I'm not sure if my measring process is wrong or what. It's just not making sense to me.
Does this mean it doesn't matter which hole/lug combination I use from my index, just pick one and shim to .002? Did I just answer my own question?
Does this mean it doesn't matter which hole/lug combination I use from my index, just pick one and shim to .002? Did I just answer my own question?
Burning Brakes
Joined: Jan 2006
Posts: 1,152
Likes: 23
From: Kawartha Lakes and PEC, Ontario
No, I don't think I'm doing something right here. You can see the .018/.019 "wobble". My understanding is that I'm trying to get that down to .002. I'm not sure if my measring process is wrong or what. It's just not making sense to me.
Does this mean it doesn't matter which hole/lug combination I use from my index, just pick one and shim to .002? Did I just answer my own question?
Does this mean it doesn't matter which hole/lug combination I use from my index, just pick one and shim to .002? Did I just answer my own question?
Maybe it's the way you're describing it but it doesn't seem like there's a problem.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
No, I don't think I'm doing something right here. You can see the .018/.019 "wobble". My understanding is that I'm trying to get that down to .002. I'm not sure if my measring process is wrong or what. It's just not making sense to me.
Does this mean it doesn't matter which hole/lug combination I use from my index, just pick one and shim to .002? Did I just answer my own question?
Does this mean it doesn't matter which hole/lug combination I use from my index, just pick one and shim to .002? Did I just answer my own question?
The dial indicator should be in constant contact with the rotor surface and positioned so that the indicating needle is set in the middle of it's travel. The dial should then be rotated so that the needle is at 0 (zero). Rotate the rotor slowly through 360 degrees and record the variations from zero as either positive + or negative - numbers.
The delta will indicate how much runout you have and where the high and low spots are. Index the rotor to a new orientation to the hub and try again to see if it's better or worse. Pick the best indexing and shim accordingly. Better yest would be to have the rotor and bub machined as a set like the factory did.
I don't think the measurements you have to date are of much use.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
My numbers are the total runout or the delta between the pos & neg. I set the lowest spot to zero and rotated the rotor which gave me the .018. Then I pulled the rotor and rotated one hole and measured again. This time the delta was .019. I repeated the process for all five holes and the results are what you see. Do I need to note for each hole the values to the left and to the right of 0? Then which one do I select to shim?
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Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
Sounds like this is the part of the process that I'm missing. I'm not sure which is the best index to use. Unless I have a revelation and figure it out tonight, I'll record my numbers and post so you guys can look at with me. I can be pretty hard-headed, but I usually get there.
Race Director
Joined: Feb 1999
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From: Lenoir City Tennessee
Ci 6, 8 & 10 Veteran
St. Jude Donor '05-'06-'07-'08-'09-'10-'11-'12-'13-'14-'15
No need to remove the rotor and turn it to the next hole each time. When you get the runout, find the low spot, add a shim under the rotor at the low spot. Put your lug nuts back on and torque, reread the runout. This should get you closer to the goal. Keep working like this and you should get there.
I hope this makes sense.
I hope this makes sense.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Sounds like this is the part of the process that I'm missing. I'm not sure which is the best index to use. Unless I have a revelation and figure it out tonight, I'll record my numbers and post so you guys can look at with me. I can be pretty hard-headed, but I usually get there.
It seems that there is only .001" difference from the best indexing to the worst. This indicates that your rotor (like most) is almost dead flat. Pick the best of the lot and start shimming from there. You want to add shims at the spot where the rotor is closest to the dial indicator.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
It seems that there is only .001" difference from the best indexing to the worst. This indicates that your rotor (like most) is almost dead flat. Pick the best of the lot and start shimming from there. You want to add shims at the spot where the rotor is closest to the dial indicator.
Melting Slicks
Joined: Apr 2002
Posts: 2,337
Likes: 14
From: Norfolk VA
Hmm, Maybe I got lucky. When I replaced all the rotors on my Vette last year I did not check for run out and my brakes don't suck air. Replaced RF and RR calipers, replaced all bleeders with check ball (speed) bleeders, bled system with Motive pressure bleeder, and viola!!! Nice firm pedal. Car stops on a dime and gives you 9 cents change. (hope I didn't jinx myself) Good luck.
Thanks for the help!
Limping across the line
Joined: Oct 2002
Posts: 5,299
Likes: 109
From: Peoria Az
Redman I was just looking at the other pics you put up on Photobucket.
I cant tell from the pics, but you did put the rubber seal around the spindle between the knuckle and the backing plate didn't you?
Could be just the way its pictured but it looks like the seal is on top of the backing plate.
I cant tell from the pics, but you did put the rubber seal around the spindle between the knuckle and the backing plate didn't you?
Could be just the way its pictured but it looks like the seal is on top of the backing plate.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Take the rotors to a good auto parts and have them machine cut a few thousands to make the rotors true and be done with it. I cant understand why so many people with Vettes dont get it....
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
Redman I was just looking at the other pics you put up on Photobucket.
I cant tell from the pics, but you did put the rubber seal around the spindle between the knuckle and the backing plate didn't you?
Could be just the way its pictured but it looks like the seal is on top of the backing plate.
I cant tell from the pics, but you did put the rubber seal around the spindle between the knuckle and the backing plate didn't you?
Could be just the way its pictured but it looks like the seal is on top of the backing plate.
Yeah, I just looked at the pictures myself and it does kind of look like it's on top, but it under the shield.
Last edited by redman76; Apr 2, 2009 at 02:58 PM.
Race Director
Joined: Jul 2001
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Without being rude, they don't get it cause it's not true. The OP has already determined that his new rotor was flat within .001". I've never seen a new rotor that was out of spec and even if it were dead flat it won't help a hub that's got a surface that's not perpendicular to the shaft.