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1979 Corvette. I am replacing the front calipers (New Delco) and rotors and pads (Powerstop z23). Disc runout has measured +.015. I reset the the disc to nearly opposite the 0 degree mark and the 0 degree mark is on the same place as before on the hub. So I don’t think it is the rotor. If I explained this correctly where is the problem? Is it the hub bushings, the hub, both or something else? I defer to the forum geniuses. Thanks!
In the beginning, At the factory. The rotors were riveted to the hubs and then machined. There is the problem.
2 fixes. One. shims behind rotor. two. mobile disc grinding service, where they machine the rotors on the car.
Wow! Australia! Thanks 4-vette! I looked for shims on the web but they aren’t available. It seems a shame to have new good rotors machined to fix this. New hubs and / bushings is not an option? Thanks from Jacksonville FL. Doug
You actually can cut front rotors on a standard Brake Lathe with the hubs secured to the rotor via the studs....just make sure you mark where to index. I haven't seen this done in years though as Brake Lathes are going away and most just shim now anyway......
There is also a special tool set up to turn the rotors while mounted on the car.
But, over all the thing to do is to try indexing the rotors for the least amount of run out, then shim.
Some people use beer can shims, but I prefer the tapered shims available at Napa, etc.
Thanks Sayfoo! For some reason the tapered shims are no longer available. Everyone is out of stock. My impeccable timing strikes again. But beer can shims sound right up my alley. I will resist the temptation for now.
Depending on the rotor/hub match up you most likely will need various size shims from 002-008" Sometimes one shim will work other times you may need two. SS shim stock and punch set is what I use. A good shim and punch set will set you back $60-$80 depending on where you buy them. If there is a old school hardware store around they may have shim stock. I never use those tapered shims, I always make up my own and they don't have to be fancy, just a thin washer type shim on a stud. For years guys have over thought this or said it won't work, I done it 100's of times getting new and used rotors in under 0025" every time.
You can cut new rotors but they now come 005 thinner then original so that doesn't leave a lot to remove. An experienced operator can do it but a moron, like the guy who cut my rotors 35 years ago, can wreck them. But if you find a good shop with a brake lathe they can be trued to the hubs. It would be best if the rotor was riveted or bolted to the hub so they stay the same after machining, I would look for some thin shims though, I haven't had tocut a rotor in years.
Yeah, the rotors almost have to be machined on the car. Lot of shops can do that. Get them as close to 'true' ahead of time, as you can. I would AVOID machining the hubs or rotors on a standard bench-top brake lathe. I've had terrible results on multiple occasions where I've done that. The brake lathe spins the hub and rotor on it's spindle, but does NOT spin the hub on the hub's bearings. You're at the whim of the technician who like to walk away and come back and give them to you. I've done it. Waste of time/money ...despite what the shop will claim (been there). The hub/rotor really should spin on it's bearings to get it 'true' and that's kind of the tricky part.
Or you could fab a simple jig like I did and machine the hub faces 100% 'true' (pic below). And any 'good' rotor will run true thereafter.
Thanks Mark, Unfortunately the pic didn't show up on my phone. I will check if it did on my tablet later tonight. I look forward to seeing it as I have thought about that. I am guessing it would be a balance issue if I used a hand grinder and just did the best I could? The shim will be fairly large to make up the .015 from .000. Sheesh!
Ha ha ...try the picture again. I had to remove and put up the 'right' picture.
With the jig, the drill in the background rotates the hub (drill peg goes into that 3/4" (20mm) wide bar below the hub), and a piece of heater hose on that peg provides grip. Obviously there are a number of ways to skin a cat here. I would have used an electric grinder, but my air grinder bearing is super tight and I thought a little easier to mount. But yeah, otherwise and electric grinder was my first choice. I thought it would actually work better -- a guy could lock the motor 'on' and adjust the speed with a router speed controller. I only had 2 hubs to machine so I did what I did.
I previously took a hub to a machine shop (bench brake lathe). They A) removed too much material, B) there was a 'ridge' from their cutting bit skipping over the holes, and C) ultimately it had as much run-out as when I took it in! I had to buy another (used) hub off ebay for $75. My rig I can grind off as little as necessary and get it 100% right. The finished surface is very nice. Lot nicer than the bench brake lathe was.
This wouldn't necessarily work for the rear hubs ...a guy would need to come up with a different system for them, or make a jig that would work both ways. Rear spindles would be a lot easier for a shop to chuck in a lathe and run a couple passes, and get true.
Mark, I am sure it is my phone but still no photos. I can check my tablet when home around midnight. I look forward to seeing it. Thanks for your input!
I have seen the picture! Now if you could fix my phone to display this forums images that would also be helpful. That is some great work Mark! It is a little out of my league or maybe a lot out of my league. I am apparently a junior member for good reason. We will see what I can do with your ideas. I may just tackle it by hand with a die grinder or resort to some kind of shimming.
10-19-2021, 03:40 PM
Disc runout
