Rotor runout Options

[Olympic 914]

As one of my winter projects.

I had a slight pulsing in the brake, really only feel it just when coming to a stop. no noticeable pulling to either side.


So I hooked up the dial indicator to check the rotor runout. rears were .004 and .005
right front was .004 left front was .016

For what its worth I am running 911SC struts with A calipers. the rotors are separate from the hub. i did carefully torque the new rotors down to 18ft lbs in a star pattern when installing them.


Thinking I have found my Smoking Gun in a warped rotor, I ordered 2 new front rotors and new pads, although the old pads only had 16K on them and still looked very good. Figured new rotors get new pads.

So I get the left side back together today and was working on the right side, when I got the idea to check the runout.

Well wouldn't you know it the left side with a NEW rotor had .016 runout. WTF??

Right side back at .004, same as before.

any one have this kind of problem before?

the only thing I can think of is, when I installed the bearing races back when I first put them on, one of them wasn't seated all the way. or somehow got in there slightly crooked.

guess I will take it back apart and try to drive the races in further.

we shall see what happens.

[Olympic 914]

Pulled the bearings out and tried to reseat the races, made no difference.

And it seems that the bearing preload remains constant when spinning the rotor.

Marked the high and low spots on the rotor and hub.

Pulled the rotor off and dressed the mating surface of Hub/Rotor with a file, in case there was a high spot. also re-clocked the rotor to see where the high spot moved to.

The high spot stayed with the mark on the hub. So I guess my old rotor was probably OK,... too late now I already sent it off to the scrap pile.

So now I have to figure out what is going on with the hub. The races look like they are seated ok. but I may have to investigate further.

I recently moved and I don't have my lathe set up yet,

If I can devise a way to hold it I may be able to indicate off the race edge to see if it really is in there crooked.

I can't imagine that the rotor mounting surface on the hub was machined wrong. IDK

a small difference at the bearing would translate to much larger at the rotors edge.

[914sgofast2]

Your hub may be warped which is causing the runout. There is a YouTube Channel about this called "BrakeAlign" which discusses the problem. They used to sell special tapered shims to correct the problem, although they may now be out of business because their website is dormant/for sale. Maybe you can take the hub off and have a machine shop reface it so it is not warped.

I found that YouTube channel on Sunday afternoon because I was just looking for a "how to use a dial indicator" instructions.

[PatMc]

Pulled the bearings out and tried to reseat the races, made no difference.

And it seems that the bearing preload remains constant when spinning the rotor.

Marked the high and low spots on the rotor and hub.

Pulled the rotor off and dressed the mating surface of Hub/Rotor with a file, in case there was a high spot. also re-clocked the rotor to see where the high spot moved to.

The high spot stayed with the mark on the hub. So I guess my old rotor was probably OK,... too late now I already sent it off to the scrap pile.

So now I have to figure out what is going on with the hub. The races look like they are seated ok. but I may have to investigate further.

I recently moved and I don't have my lathe set up yet,

If I can devise a way to hold it I may be able to indicate off the race edge to see if it really is in there crooked.

I can't imagine that the rotor mounting surface on the hub was machined wrong. IDK

a small difference at the bearing would translate to much larger at the rotors edge.

first, confirm your races are installed correctly... but if they're not, that won't result in runout so much as bearing wear.

The problem is very likely corrosion/damage to the rotor mating surface of the hub. I'd find a good machine shop with a brake lathe, throw the hub in the lathe (with no rotor) and take a light cut to face off the backside of the hub. You might need to make a mandrel and use a real lathe, depending on whether or not the brake lathe geometry will get the cutter that close to the spindle....some will, some won't.

Moving the rotor outboard 0.005" or however much you need to remove the hub isn't going to affect anything.

One thing you can do to PROVE where the runout is coming from....take your runout measurements again and mark them right on the face of the rotor with a sharpie...mark your zero and mark your high spot (write the number on the rotor face). Then mark the rotor hat and the hub....take the rotor off the hub and re-clock it 2 lug holes over, put it back together and make the same measurements again. If your marks are the second measurement are about 2 lug holes off your marks from the first measurement....your hub needs to be machined. If your marks from both first and second measurement match up, then you've got a bad rotor.

[Olympic 914]

Pulled the bearings out and tried to reseat the races, made no difference.

And it seems that the bearing preload remains constant when spinning the rotor.

Marked the high and low spots on the rotor and hub.

Pulled the rotor off and dressed the mating surface of Hub/Rotor with a file, in case there was a high spot. also re-clocked the rotor to see where the high spot moved to.

The high spot stayed with the mark on the hub. So I guess my old rotor was probably OK,... too late now I already sent it off to the scrap pile.

So now I have to figure out what is going on with the hub. The races look like they are seated ok. but I may have to investigate further.

I recently moved and I don't have my lathe set up yet,

If I can devise a way to hold it I may be able to indicate off the race edge to see if it really is in there crooked.

I can't imagine that the rotor mounting surface on the hub was machined wrong. IDK

a small difference at the bearing would translate to much larger at the rotors edge.

One thing you can do to PROVE where the runout is coming from....take your runout measurements again and mark them right on the face of the rotor with a sharpie...mark your zero and mark your high spot (write the number on the rotor face). Then mark the rotor hat and the hub....take the rotor off the hub and re-clock it 2 lug holes over, put it back together and make the same measurements again. If your marks are the second measurement are about 2 lug holes off your marks from the first measurement....your hub needs to be machined. If your marks from both first and second measurement match up, then you've got a bad rotor.

I did re-clock the rotor and the high spot remained with the mark on the hub.

I then dressed the mating surface of both the rotor and the hub with a flat file, that got the runout down to 0.0125 from 0.016

So it looks like I will have to machine the hub.

I will work on setting up the lathe today and see what I can do.

if I don't like the setup, I'll have to find a machine shop

[ClayPerrine]

It might be simpler to find a replacement hub.

[Olympic 914]

Got the hub setup in the lathe, indicating off of the machined surface inside the hub.
Then decided to check the races.
Outer race was good but the inner race was not seated correctly.
I tried again to drive it in more without success.
So drove it out, I will source a new inner bearing.
Then put the hub in the oven and race in the freezer before installing .

We shall see how this plays out.