lobes290
Well-known member
I was told by somebody that when you get a vette rotor turned, you have to get it shimed correctly, or it could cause a low pedal, is this true, and do you have to shim a new rotor
Brakes; Rotor Shims
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Vettehead Mikey
Well-known member
Shimming rotors is semi-bubba. The new rotor needs to be surfaced while attached to the hub. They were designed to be machined as a pair to minimise run out.
Why are you replacing/turning yours? Most time the originals outlast the vehicle and never need turning.
Why are you replacing/turning yours? Most time the originals outlast the vehicle and never need turning.
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lobes290
Well-known member
Im not but on of the rear rotors was turned-not by me, and i was told it needed to be shimmed and that this was the cause of my spongy brakes, so i was wondering if i could just buy a new rotor and not have to shim it, instead of shimming the turned rotor(because i dont know how). thanks
iron cross
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I have never heard of shimming rotors. When they are machined normally equal amounts are taken from each side to true them up to prevent any chance of a pulsing brake pedal.
Hib Halverson
Technical Writer for Internet & Print Media
I doubt anyone still makes brake disc shims for the C2/3 disc brakes and even if you can find some, they are a bandaid fix used to mask the real problem which is run-out. It's certainly not unusual that your problem is in the rear as the rear brakes are the most problematic from a run-out and pedal-feel standpoint.
Spongy brake pedal feel on a 65-82 is probably caused by air in the system. Servicing the 65-82 disc brake system in a manner solves all problems with air getting into the system can sometimes be a complex task.
There are some articles on the Idaho Corvette page that discuss servicing 65-82 brakes in great detail. See:
http://www.idavette.net/hib/BBfHInet12.htm
http://www.idavette.net/hib/BBfHInet13.htm
Spongy brake pedal feel on a 65-82 is probably caused by air in the system. Servicing the 65-82 disc brake system in a manner solves all problems with air getting into the system can sometimes be a complex task.
There are some articles on the Idaho Corvette page that discuss servicing 65-82 brakes in great detail. See:
http://www.idavette.net/hib/BBfHInet12.htm
http://www.idavette.net/hib/BBfHInet13.htm
It's tough if not impossible to find a shop that can turn the rear or for that matter, even the front rotor/hub assemblies on the car.
The front's are easier if you can find a shop that can mount the rotor/hub on a lathe and turn them as a unit. Just turning the rotor itself won't guarantee that the hub/rotor assemblies will run true. That's why the factory riveted them on and turned them as a unit.
Years ago, shimming the rotors was "standard" practice. Once you drill out the rivets and replace the rotor, that rotor is not "trued" to the assembly.
Also, sloppy rear wheel bearings can contribute to runout. Mount a dial indicator and check it out. If your runout is greater than about .003, you're going to have air pumping/soft pedal.
O Ring and Zero Tolerance calipers are designed to compensate for rotor runout. Hope this helps. Chuck
The front's are easier if you can find a shop that can mount the rotor/hub on a lathe and turn them as a unit. Just turning the rotor itself won't guarantee that the hub/rotor assemblies will run true. That's why the factory riveted them on and turned them as a unit.
Years ago, shimming the rotors was "standard" practice. Once you drill out the rivets and replace the rotor, that rotor is not "trued" to the assembly.
Also, sloppy rear wheel bearings can contribute to runout. Mount a dial indicator and check it out. If your runout is greater than about .003, you're going to have air pumping/soft pedal.
O Ring and Zero Tolerance calipers are designed to compensate for rotor runout. Hope this helps. Chuck
GTR1999
Well-known member
As posted, it's not easy to get a rotor to have little runout. I have installed new rear rotors that had .004-.005 runout out of the box. The only way to get them pefectly true is to turn them bolted or riveted to the spindle-which requires complete disassembly. The shops rebuilding the arms have a lathe large enough to do this, just about every garage or dealer will not-nor will they have the understanding of what to do.
So what do you do if you're doing brakes at home and the rear bearings are good?
Well, start by getting a good dial indicator and learning how to use it properly. Then check the rotor runout with the bearings adjusted for the fronts and the 1/2 shafts disconnected for the rears. Move the calipers out of the way so you have the rotors moving freely. Check the runout throught 360* for each one, if you hav emore then .003" then you'll have to shim them if they are otherwise good(no grooves,warping,cracks,etc). I use a stainless steel shim kit and punch set which cost about $80. The shim sizes run between .001" and .020" you'll have to play with them a bit to get them to the area you want but it can be done. The last ones I did, I was able to get them down to .0005"-.001"
Most,if not all the local garages I've dealt with over the past 25 years working on these cars bolt on a new rotor or turn the old ones and then install new pads,bleed the system and let it go. If it comes back,they'll charge you again and do the same thing until they try and tell you "thats the way corvettes are,every one we work on is the same" Thank you Mr Bubba!!
So what do you do if you're doing brakes at home and the rear bearings are good?
Well, start by getting a good dial indicator and learning how to use it properly. Then check the rotor runout with the bearings adjusted for the fronts and the 1/2 shafts disconnected for the rears. Move the calipers out of the way so you have the rotors moving freely. Check the runout throught 360* for each one, if you hav emore then .003" then you'll have to shim them if they are otherwise good(no grooves,warping,cracks,etc). I use a stainless steel shim kit and punch set which cost about $80. The shim sizes run between .001" and .020" you'll have to play with them a bit to get them to the area you want but it can be done. The last ones I did, I was able to get them down to .0005"-.001"
Most,if not all the local garages I've dealt with over the past 25 years working on these cars bolt on a new rotor or turn the old ones and then install new pads,bleed the system and let it go. If it comes back,they'll charge you again and do the same thing until they try and tell you "thats the way corvettes are,every one we work on is the same" Thank you Mr Bubba!!
Bolisk
Well-known member
At the risk of sounding stupid. I have some questions.
I'm not completely mechnically un-inclinded. . .I have rebuilt many of the compoents of my 70 vette (including the complete front suspension rebuild) with great success. Now, I'm about to up the anti, and start in on my trailing arms. So, with that being said. . .
Can someone explain what "Rotor Runout" actually is? I'm assumiong its the ammount the rotor wobbles in relation to the spindle when spinning. Is that correct? Related to that, can someone post detailed descriptions on exacty how you would check the rotor runout (with photos if availble). Do you check the runout at different radius's. . .or just near the end of the rotor. . .etc. . .etc. . .etc...
Can someone explain exactly how you check the "end play" of the rear wheel berrings. I understand how the bearing setup tool works. Since I haven't started, I'm not sure how to actually check the end play. In post's it looks like people are putting the barring assembly in a vice (wheel side down) then placing the bearing setup tool. . .with the bearings. . .shims. . .etc. . .into the assembly. Quesitons are, are you pushing the bearing setup tool up thought the bottom. . .or from the top. . .does it matter, and how do you ensure that it stays in place. . .or is it just obvious when I get to that point? (i'm guessing you place the outer bearing on the tool, shove it up from the bottom, then place the inner bearing and shims on top, and tourq to 100ft/bls.) Are the races already installed in the assembly at that point? lastly, how do you "exactly" check the end play. I know you put a dial indicator against what looks like a washer. . .zero the indicator. . .but then what do you do? Wrap on the bottom of the setip tool with a hamer or your hand to see how much the setup tool moves in and out of the assembly?
I realize this may be trivial to poeple who are familure with these types of tools. . .but I've never done this before.
Lastly where do I get the shims the Gary used to adjust the rotors. . .pluss the counter sink bolts that Gary used to attach the rotor to the spindle.
Regards,
JonR
I'm
I'm not completely mechnically un-inclinded. . .I have rebuilt many of the compoents of my 70 vette (including the complete front suspension rebuild) with great success. Now, I'm about to up the anti, and start in on my trailing arms. So, with that being said. . .
Can someone explain what "Rotor Runout" actually is? I'm assumiong its the ammount the rotor wobbles in relation to the spindle when spinning. Is that correct? Related to that, can someone post detailed descriptions on exacty how you would check the rotor runout (with photos if availble). Do you check the runout at different radius's. . .or just near the end of the rotor. . .etc. . .etc. . .etc...
Can someone explain exactly how you check the "end play" of the rear wheel berrings. I understand how the bearing setup tool works. Since I haven't started, I'm not sure how to actually check the end play. In post's it looks like people are putting the barring assembly in a vice (wheel side down) then placing the bearing setup tool. . .with the bearings. . .shims. . .etc. . .into the assembly. Quesitons are, are you pushing the bearing setup tool up thought the bottom. . .or from the top. . .does it matter, and how do you ensure that it stays in place. . .or is it just obvious when I get to that point? (i'm guessing you place the outer bearing on the tool, shove it up from the bottom, then place the inner bearing and shims on top, and tourq to 100ft/bls.) Are the races already installed in the assembly at that point? lastly, how do you "exactly" check the end play. I know you put a dial indicator against what looks like a washer. . .zero the indicator. . .but then what do you do? Wrap on the bottom of the setip tool with a hamer or your hand to see how much the setup tool moves in and out of the assembly?
I realize this may be trivial to poeple who are familure with these types of tools. . .but I've never done this before.
Lastly where do I get the shims the Gary used to adjust the rotors. . .pluss the counter sink bolts that Gary used to attach the rotor to the spindle.
Regards,
JonR
I'm
Vettehead Mikey
Well-known member
Think of runout as being similar to a car with a wobbly wheel. We've all seen that at one time or another. In the case of a rotor surface, any lateral wobbling past approx. 006" is more than the brake pad, pistons and piston seals can deal with and the result is the pads being pushed back from the rotor surface. The result is a low pedal that needs to be pumped to take up the gap and push the pads back against the rotor surface. An extreme case will push the pads back with such force that air will be sucked into the caliper around the edges of the lip seals. Frequent need to bleed the brakes is the result.
The cause of the rotor runout can be from any one of three or more sources
1) warped rotor (very rare on C2/C3 Corvettes)
2) replacement rotor installed and not machined as a set with the spindle
3) worn or improperly set up trailing arm bearings
It's possible to have more than one of these conditions at the same time. Measure and correct bearing end play first. With no grease and no seals in place, assemble the bearings and spacers using the set up tool instead of the real spindle and torque to required setting. The assembly should spin freely. Holding the trailing arm securely so that the shaft is vertical, place the indicator of the measuring instrument on any horizontal surface of the spindle (tool) assembly and set the dial to zero. Using hand pressure only move the tool along it's axis looking for free play. If this clearance is not within spec, increase or decrease the spacer size as required. Take lots of measurement in lots of places so that you're sure you've got repeatable consistent results.
Once final bearing assembly is done, but without the rotor in place measure the mating surface of the spindle (just for fun and practice). Most likely you will observe some indication variations, or wobble as you turn the spindle. Even if a rotor is dead flat, the is wobble will telegraph to the brake pad surface and lead to the problems illustrated above. Although it possible to move the rotor to one other position and still have the parking brake adjustment holes line up, most likely the runout will not go away. The GM factory dealt with this issue by riveting the rotor to the spindle flange and then machining them as a set. More common and what specialists like Gary R does is add shims between the rotor and spindle surface to cancel the runout. Gary goes one step further by bolting the new rotor in place.
To answer your runout measuring question, the tip of the tool should rest somewhere on the normal brake pad swept surface. The closer to the outer edge, the higher the readings will be and therefore be more accurate. Again, the more readings you take the better idea you will have as to what needs to be corrected.
The cause of the rotor runout can be from any one of three or more sources
1) warped rotor (very rare on C2/C3 Corvettes)
2) replacement rotor installed and not machined as a set with the spindle
3) worn or improperly set up trailing arm bearings
It's possible to have more than one of these conditions at the same time. Measure and correct bearing end play first. With no grease and no seals in place, assemble the bearings and spacers using the set up tool instead of the real spindle and torque to required setting. The assembly should spin freely. Holding the trailing arm securely so that the shaft is vertical, place the indicator of the measuring instrument on any horizontal surface of the spindle (tool) assembly and set the dial to zero. Using hand pressure only move the tool along it's axis looking for free play. If this clearance is not within spec, increase or decrease the spacer size as required. Take lots of measurement in lots of places so that you're sure you've got repeatable consistent results.
Once final bearing assembly is done, but without the rotor in place measure the mating surface of the spindle (just for fun and practice). Most likely you will observe some indication variations, or wobble as you turn the spindle. Even if a rotor is dead flat, the is wobble will telegraph to the brake pad surface and lead to the problems illustrated above. Although it possible to move the rotor to one other position and still have the parking brake adjustment holes line up, most likely the runout will not go away. The GM factory dealt with this issue by riveting the rotor to the spindle flange and then machining them as a set. More common and what specialists like Gary R does is add shims between the rotor and spindle surface to cancel the runout. Gary goes one step further by bolting the new rotor in place.
To answer your runout measuring question, the tip of the tool should rest somewhere on the normal brake pad swept surface. The closer to the outer edge, the higher the readings will be and therefore be more accurate. Again, the more readings you take the better idea you will have as to what needs to be corrected.
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