Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

[Bob S.]

Hi All -

In searching a number of threads on the disc brake C2 control arm assy, I note the concern of not being able, or the unlikelihood of reacheiving proper perpendicularity between the spindle and brake rotor after rivet removal and rotor reassembly.

Isn't rotor/spindle perpendicularity established by the mating of the outboard spindle flange surface and the inner surface of the rotor hat section? It appears that if everything is reassembled as it came apart, and the mating surfaces are prepared by cleaning, deburring, rust removal, etc the correct tolerances would be reestablished.

I know I must be missing something.........you guys have vastly more experience on this than I do......I've never even dissassembled one (yet).

Or without the rivets could something like a design / manufactuing flaw allow the inside corner of the rotor hat section to bind on the spindle fillet radius thus preventing the wheel nut tightening from pulling the rotor / spindle surfaces together properly??

Please help me understand. I'm trying to keep out of trouble.

Thanks,
Bob

[Wayne P.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

The problem occurs when using a different rotor. Replacing yours in the original orientation should not be a problem.

[Michael W.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

Or without the rivets could something like a design / manufactuing flaw allow the inside corner of the rotor hat section to bind on the spindle fillet radius thus preventing the wheel nut tightening from pulling the rotor / spindle surfaces together properly??

Much simpler than that. Many spindles have mating faces that are flat and of the correct diameter, but not perpendicular to the shaft.

To compensate, the friction surface of the rotor was finish machined after the spindle and rotor were attached as an assembly.

If the original rotor is kept with the original spindle and in the correct orientation then all is well. The difficulty starts when either component is replaced or the respective positioning is altered.

[Bob S.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

Thanks guys. What you say makes perfect sense.

But Joe's point is that even if the parts are reassembled in their original position / orientation a rotor runout problem can be created.....

That's the part that escapes my aging, feeble mind.

Bob

[Stephen L.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

Why not assemble the rotor and stub shaft using the lugnuts and washers (No wheel). Then check the rotor runout. If it's NOT in tolorance, then decide how to fix it...... rotating to the next bolt set..... resurfacing.... etc.

[Joe L.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

Thanks guys. What you say makes perfect sense.

But Joe's point is that even if the parts are reassembled in their original position / orientation a rotor runout problem can be created.....

That's the part that escapes my aging, feeble mind.

Bob

Bob-----


In general, no problem will be created if the spindle or rotor are not replaced and the rotor is returned to its original orientation on the spindle.

However, returning the rotor to its EXACT original position is not always so simple as it might seem. There is a certain amount, albeit small, of "play" between the stud holes in the rotor and the wheel studs. So, it's possible to SLIGHTLY move the rotor relative to the spindle after the rotor is installed on the spindle. As I say, this is a very small amount of movement and it USUALLY has no real effect on TIR. However, in some circumstances it may have an effect and that effect may be enough to put the TIR out of spec, especially if it was marginal to begin with.

In addition, it's also CRITICAL that there is nothing interfering with the seating of the rotor on the spindle. This may also seem quite easy to do, but it may not be so easy. Even the slightest of burrs, corrosion particles, metal particles, etc. can have a significant effect. For example, if as little as a 0.001" "high spot" is created at the spindle/rotor interface, that can translate to .002-.003" TIR on the rotor. In addition, if there was originally some sort of burr, metal shaving etc. that was present before the rotor and spindle were originally riveted together and machined AND THAT BURR, METAL SHAVING, ETC. IS REMOVED, that can also create a problem.

I believe that the problems associated with re-achieving original or in-spec TIR when the rotor is removed from the spindle is the reason that about 1974 GM changed its service procedures and recommended that the rotor not be removed from the spindle except when replacement of the rotor or spindle was required. Earlier service procedures had cautioned that the rotor be returned to the same position on the spindle as when removed but, apparently, GM found that this was not sufficient to ensure in-spec TIR.

As I say, MOST of the time, returning an original rotor to an original spindle and indexed as-original will result in no problems. But, NOT ALWAYS.

One interesting little fact is that in later service manual editions although GM cautioned against drilling out the rotor rivets and separating the rotor from the spindle except when rotor or spindle replacement was necessary, they never provided any instruction, at all, with respect to how to correct excessive TIR if a rotor or spindle had to be replaced. They were SILENT on that issue.

[Joe R.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

If I recall correctly, there is only one position of the rear rotor that properly aligns the parking brake adjustment holes through the rotor and spindle. So, the option of rotating the rotor to reduce runout only works for the front rotors.

[Joe L.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

If I recall correctly, there is only one position of the rear rotor that properly aligns the parking brake adjustment holes through the rotor and spindle. So, the option of rotating the rotor to reduce runout only works for the front rotors.

Joe-----


A new parking brake adjustment hole through the spindle flange can be drilled if the rotor is re-indexed to the spindle. However, I've never really had much luck using re-indexing to correct rotor runout. If one is installing new rotors on an existing spindle or, even, installing a new rotor on a new spindle, I think that trying to re-index the rotor to eliminate runout is a complete waste of time. New rotors usually have minimal runout whereas GM spindles, original or SERVICE, usually have lots of runout. So, no mater where one indexes a new rotor, the runout is going to be about the same.

[Jim T.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

Bob I don't know what your problem is that requires removing the disc from the spindle. However you stated you wanted to stay out of trouble.
The one way I know of to stay our of trouble and the end result is having a Corvette with a firm brake pedal all the time, replaced emergency brake pieces, new bearings, new control arm bushing, and new paint. All you have to do is remove the brake caliper, and then remove the control arm assembly and send it to www.bairs.com. Bairs will even send you wooden boxes to ship it in. You will be able to enjoy many thousands miles of driving pleasure.

[Rob A.]

Re: Rear Rotor Perpendicularity to Spindle after Rivet removal - Joe Lucia's concern

If you send your trailing arm assemblies to Bairs, make sure you tell them what finish you want on the various components(if that is a concern to you) or some will be finished with stainless steel colored paint. They will finish them as you want, NCRS specs, bare steel left that way, etc., but only if you specify. It's a lot easier to tell them before they are finished, than to have to remove paint after they're assembled.