Front rotor runout

[Brandon K.]

Most of the discussion on rotor runout that we have on this page concerns rear bearing assemblies, but I'm having trouble on the front. I installed new bearings and a new rotor on the left front, and the runout is way up there, around 15 thousandths(.015) at the rotor. I checked the hub runout, and its about 5 thousandths (.005). Is this too high, and how can I get it lower? Does it mean that the spindle is bent,or the bearing cups are cocked? I tried two rotors, and they both gave the same high runout. Indexing didnt seem to help any.

I'm open to suggestions. I dont think this runout is acceptable, and I'm not sure that machining the rotors (which I dont want to do on brand new rotors) will be the answer.

thanks in advance. Brandon

[Joe L.]

Re: Front rotor runout

Brandon----

The problems that you are experiencing are EXACTLY the reason that I always recommend replacing the front rotors as a complete unit with the hub. Yes, I know that this is expensive, but it eliminates this problem. GM NEVER sold Corvette front rotors as a seperate unit without the hub, although their aftermarket Delco parts system may have to be competitive with other aftermarket suppliers.

To properly correct this problem with what you have you will be forced to have the rotors machined as an assembly with the hubs. Fortunately, for front rotors this can be done on a brake lathe. The main problem for you is that you have so much runout that after machining your rotors may be below "discard" thickness. You see, when the factory machined these assemblies, they started with an UNFINISHED rotor with plenty of "meat" and they were machined and trued in one operation to "new" rotor thickness of 1.250".

Otherwise, you will need to have the hub rotor flange surface trued by a machine shop(most brake shops won't be able to help you on this type machining operation), then mount the rotor to the trued hub and check for runout. It should be minimal, but, if not, you will need to have the rotor/hub ASSEMBLY now trued on the brake lathe to final specification. Inasmuch as the hub will have been previously trued, this operation will not now require a "cut" that gets you to below "discard" thickness. All this work will be expensive, though; especially the machine shop work on the hubs. So, all of the "savings" which are generated by replacing just the rotors and not the complete assemblies will be lost. And then some. That's why I have TWO SETS of front rotor/hub assemblies in my collection as a hedge against future need after GM stops making them.

An alternative to machining of the hub would be to use shims to correct the gross runout problem, then have the assembly trued on a brake lathe. I don't recommend this approach, though. Not only is it time consuming, but it can create other problems.

One other thing that I should have mentioned is that you need to make sure first that your bearing races are FULLY SEATED in the hub before you do ANY of this. If the bearing races are NOT FULLY SEATED, you will measure runout that does not really exist on the rotor/hub assembly. Also, when you measure the runout, make sure that the spindle nut is at zero end play(light preload), NOT the running spec of .001-.006".

[Michael W.]

Re: Front rotor runout

Hi Brandon,

I saw your note on CRPL also. The discussions here regarding rear rotor run out apply equally to front rotors. Your .015" is almost guaranteed to give you problems with pad knock back, and possibly air in the system also. The run out of .005" on the hub is pretty well normal and does not indicate that anything is amiss. The hubs were not machined with any great care, as the disks were later mated and riveted, then machined as a pair to give the final runout of .005" as measured on the rotor. The only reasons the rivets were there was to allow machining as a pair, and to make transportation and assembly easier.

You probably have a pair of perfectly flat rotors but by measuring the run out further from the axle, are getting a larger actual reading.

The only remedy to this is to do what GM did originally, secure the disk to the hub (reversed wheel nuts work perfectly) and have a shop machine them as a pair take off the minimum amount to get the runout in spec.

As a side note, there was a string recently about one vendor of disks selling units that were more true than another vendor. The whole point is irrelevant as you almost always have to machine them anyway.

Mike

[Brandon K.]

Re: Front rotor runout

Thanks for the advice, guys. I'm going to do some experimenting with different rotors, and if that doesnt yield any results, will send the hubs to Bairs, to have them rivited and turned as an assembly.

Brandon

[howard]

Re: Front rotor runout

Brandon

try moving the rotor position on the hub,you might get close to desired runout!

[Brandon K.]

Re: Front rotor runout

thanks. already tried indexing, but didnt help.

BK

[Rob Brainard]

Re: Front rotor runout

Brandon, You stated that the hubs showed a .005" runout. Based on the fact that the rotor is larger in diameter, the runout would be worse even if the rotors were dead on. Why not have the hubs trued so that they have zero runout, then install the rotors and check. Make sure when you are checking the runout that the rotors are fastened as if a wheel was installed [not just with lug nuts]. I've had this happen with rear axle flanges where the flanges have had runout as bad as .060".