Should I worry? Brake rotors....

[Alan_Myers]

Hi all,

I unpacked a new set of rotors for my TR4, installed them and noticed one in particular appeared to have a lot of runout. Checked it with a dial gauge and found .012-.013 approx. The other wasn't visually obvious but tested to also have about .008 runout.

Since the service manual said .002 maximum allowable, off I went to a local shop that turns rotors to have them trued up. Paid my $30 and waited overnight. Fitting them up today, I thought I could still see some wobble in the worst one. Once the pads went in, I could hear it dragging, too. So I again attached the dial gauge and found the two rotors measure .010 and .007 runout, respectively. The local shop didn't seem to do very much with them!

Do you think I should take them back and ask that they try again?

Can anyone suggest anything else that might be causing the runout? The rotors were turned while fully installed on the hubs, of course, with brand new bearing races installed. Matching, new bearings went back in today. The axles are old, but appear to be in fine shape. I checked, checked and triple checked bearing adjustment. Both wheels are currently spinning smoothly with no hint of play. The rest of the suspension is all newly rebuilt, but unlikely to have any effect on the rotors, anyway.

In fact, I'm not sure any of these other things could cause the rotors to appear out of true. Or, if .010 and .007 are really enough to worry about. Perhaps when the new brake pads bed in they will cure some of it, anyway. (I've got semi-metallic on the car right now, but will be changing to Kevlar pads when they arrive.)

What do you think?

Thanks in advance!

Alan

 

[piman]

Hello Allan,

2 thousandths of an inch is a tight tolerance particularly if you measure the run out at the maximum diameter. However, are you sure the shop did not remove the discs to skim them. If so I would look carefully at the hub mounting faces. Do you know if the shop used a centre lathe or a special brake disc lathe? Certainly I would go back to the shop and tell them about that runout.
Another thing you could try is to re-mount the discs at a different orientation. I would try the shop first though.

Alec

 

[Alan_Myers]

Hi Alec,

You're right! It could make quite a difference where you measure runout. I always test about an inch in from the edge.

Yes, I'm sure the shop didn't unmount the rotors. I'd painted the retaining bolts along with center of the rotor and the hubs, and there was no sign anyone had loosened any of the bolts. They wanted them mounted to do the turning, but I didn't get info about what type of tools they used to do the job. It's primarily a good quality parts store that provides a few simple (supposedly) machining services.

I always make a habit of painting rotor/hub assemblies after they have been bolted together. For one thing, it makes future inspection quick and easy. If any bolts loosen, it's pretty obvious due to the cracked paint. Of course, I use locking Locktite *and* locking washers or safety wire whenever possible, so they shouldn't loosen!

Painting after assembly also seals the seams, helps prevent water getting in, rust starting and makes future disassembly a little easier.

I'm considering just using these disks as they are, for now, to see if they bed in okay. Thinking about it after I posted my question, the only thing I can point to that might cause runout would be bent axles. These particluar axles are used, probably original to the car, although they look to be in very super shape. I suppose it's possible they got tweaked a bit sometime in the past 40 years.

I've never changed the stub axles in a TR before, but I understand it's possible. I've got a spare set of vertical links with badly scored axles, maybe I'll get those fixed up and try them sometime, to see if it makes a difference.

Thanks for your ideas!

Alan

 

[Banjo]

A bent stub axle can cause an alignment issue or a clearance issue between the caliper and rotor but it won't cause runout. the stub axle would have to be turning with the rotor to do that. I would suspect a bent hub or debris between the hub and rotor if its not in the rotor itsself still turning the rotor on the hub should have taken care of that

 

[Alan_Myers]

Hi Banjo,

Thanks for your reply and suggestions.

At this point, I'm guess the stub axles must be off a little. The rotors are new and the hubs were stripped to bare metal before new bearings were pressed in. The rotors and hubs were assembled prior to painting, torqued as specified and then painted to seal all joints (maksed off the working surfaces of the rotors, of course. In fact, just about everything is new except for the vertical link, axles and hubs themselves.

So, I'm a little baffled. But, I'm going to give it a try with the slight amount of runout and see if I can live with it. The old brakes were probably worse, but I don't recall... the last time I drove the car was in 1986, according to the license plates. It's been in covered storage since and is only now getting ready to see the light of day again.

Thanks again!

Alan

 

[piman]

Hello Alan,
from what you say my guess is that they used a centre lathe, and it is not easy to get a cast hub with a hollow centre to run absolutely true. I can only assume that they did not set it up to the degree of accuracy required. I have a centre lathe in my workshop and I would have taken the discs off to machine them.
I agree with Banjo, the stub axle is not the cause.

Alec

 

[vettedog72]

Disk or hub? You might try marking the apex of the run out on the disk and then remove, rotate 180 degrees and replace the disk. If the disk still has the run out apex in the same place, I think you have a bad disk. On the other hand, if the disk run out is 180 degrees from where it was, you have a hub problem. I am thinking you cleaned the hub surface making sure there are no bumps or lumps on the surface and arournd the studs. Hope to hear how it turns out.