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C3 Rear Brake Rotors



The rear brake pads on the '71 needed to be replaced, and I had a leaking caliper. When I took the rotors to have them cut, NAPA said that one was under spec already and the other couldn't be cut or it would become under spec. So we purchased new Raybestos rotors & calipers & installed them in April. We thought we were doing a good thing...

I just took her in to get the tires balanced & rotated and have the stripped stud fixed, and when my 'Vette mechanic heard what we had done, he said that I'm going to wind up losing my rear brakes because of "run out", because 'Vette rear axles aren't straight. He said you should never replace the rear rotors on a Vette, even if they're under spec!

Now I'm scared to drive her to Bloomington tomorrow. :(
Does anyone have experience with this "issue"? What should I do??:confused

Barb :w
Sounds like your getting a runaround from your mechanic.
" vette axles aren't straight ". News to me.
Keep the rotors.....replace the mechanic.
I'm sure I'm not quoting him correctly, what he was trying to say is that the rear axles are, or get, "bent" or out of true, so the rotors get "run out". Anyway, I think we're probably going to go for the drive anyway. Haven't had any issue since April, driving it about once or twice per week, so I think we're probably OK. But he sure had me flustered:(
Anyway, thanks billagroom and Stingray 6974 for some straight talk.

Barb :w
i may be wrong but my 71 bb does not have rear axles. it has half shafts bolted to a carrier bearing. when i replaced the trailing arms i went with new rotors and calipers and dialed in the rotors to the carrier to keep the rotors from wobbling . i would think if not dialed in they would cause the brake pads to surge and the wheel to have excess run out. i would also think a bent half shaft would cause a very bad vibration that would increase with speed as harmonics set up. i am sure you would feel this in the stearing wheel and shifter, and in a short time it would destroy the carrier bearings. mike would probably be able to better explain this. robert
Barb, I have replaced numerous rear rotors, I have never heard of such a thing are have had a problem. As far as the rear axles not beeing straight, I really don't doubt that, too much. But, they weren't straight when it left St Louis, so what differance does it make:confused If you have trouble, I'll go home and get the trailer and fix it myself.............See ya in Bloomington..........Steve
Barb I agree with everyone else, lose the mechanic. The only thing that I was told recently that may cause an otherwise "good" rotor to run out is the wheel lug nuts being improperly torqued. Generally this is caused be an inexperienced mechanic/tire changer who leans on the air impact wrench til the bolt is ready to strip...but it happens. In order to correctly evaluate run out on the rear rotors, the wheel must be removed and the wheel nuts re-installed to the correct value. Then set up a dial indicator and rotate the rotor and watch the indicator. From the sound, this guy may not know what a dial indicator is. Go to Bloominton you should be fine.
Maybe the bent shafts the mechanic was talking about is the camber rods. For some reason, I've seen a lot of these bent and I really hope it wasn't by someone trying to do an alignment :)
I hate to show my ignorance here...

...but what does the term "run out" mean in application to brakes and rotors?

Made it safely!

Thanks all for your input! Hubby & I decided to do the drive to Bloomington (about 155 miles each way), and she did a GREAT job! (I just wish she had 5th gear...) The brake pedal felt just as firm at the end as it did in the beginning. So I guess I should stop worrying?

Unfortunately, we didn't get there til a little after 1pm, so we missed the meeting at the gate. Didn't see any CAC shirts walking around, either, so I didn't get to meet anyone. Bummer! But it was still a great day, and Bloomington was packed full of great looking 'Vettes!

Silver, I found this explanation while searching for answers myself:
I think this is what my mechanic was trying to tell me.

Thanks again everyone!
Barb :w
ssvett said:
If you have trouble, I'll go home and get the trailer and fix it myself.............See ya in Bloomington..........Steve

What a great guy! Wish we could have met but we were running too late (as usual)...
Barb :w
brake rotor/disk runout

Heidi & Ken:
Glad you got into your new house and back online. The rotor is essentially a flat steel plate and the caliper is a clamp that grips both side of the plate when brakes are applied. Kinda like gripping the head & tail of a large coin between your thumb and index finger ... while the coin rotates. When new or after proper resurfacing, the rotor is nearly perfectly flat. But heat causes it to warp ... no longer flat ... kinda like the raised relief on a coin. A warped rotor's raised areas (Lateral runout) are not as pronounced as a coin's bumps and letters ... but the caliper pads grip this warped area more tightly than the smoother areas ... and you get a shudder and intermittant grabbing ... even undue tire skidding. When the waped rotors are turned in a brake shop's lathe (or cut or ground or resurfaced or machined) the raised areas are cut down and the surface is flat once again. Rotors can only be cut down so far ... after that they become too thin are unstable ... in use they warp even more quickly and might even fracture. The rotor's warpage is usually far too small to detect with naked eye ... a dial indicator (kinda like a micrometer) that measures in thousandth's of an inch is placed against the mounted rotor ... the rotor is hand-spun and the dial indicator registers the amount of lateral runout. The micrometer is used to measure the thickness of the rotor ... to see if it is too thin.
run out

...so it is basically a measurement indicator of 'run out' of room on the rotor? ;)
I did understand the explanations, thanks.

runout ... other that warpage

Regardless, Corvette or not ... front or rear ... The flat rotor is mounted to a flat surface. That flat surface MUST run at true 90 degrees perpendicular to the axis of rotor's rotation. If the front spindle/hub or rear spindle/hub has bad (loosely worn) bearings in it ... the flat rotor will still have undue lateral runout because the hub/spindle it's mounted to is flopping about the axis... varying plus -minus 90 degrees. So, Barb's brake mechanic is not entirely wrong ... it's just that he should verify the rear disk's (and rear spindle bearing) runout before unduly alarming the customer. Most lateral runout is due to warped rotors ... but loose bearings are seen as a cause also. Agreed, machines to cut rotors on car are not so common ... but MANY imports require such a lathe to properly do the job ... lotta imports over the last 25 years .... there are a few out there. The vette's rear stub axles/spindles and rotors can be chucked up in a large machine shop lathe (screw machine) and cut/trued as a matched set. But if the stub axles are true ( they can be checked separately for runout) ... and the bearings are good and adjusted correctly ... and the rotors are flat ... then everybody should work happily together.

Don't feel bad about not seeing anyone at Bloomington. I was there and somehow missed everybody. Just talked to Steve and found out that we must have just missed each other. Maybe next year we will be big enough to support a booth there. Then we will have a place to meet.

Re: runout ... other that warpage

Jack said:
The vette's rear stub axles/spindles and rotors can be chucked up in a large machine shop lathe (screw machine) and cut/trued as a matched set. But if the stub axles are true ( they can be checked separately for runout) ... JACK:gap

Sorry, another stupid question - what type of shop would I have to go to in order to get this checked? Apparently it isn't something my "ex"mechanic can do, would it be a brake specialty shop or restoration shop or... ???
Since we just put brand new rear rotors on, I'd like to make sure everything else is A-OK so I don't have any surprises...

Barb :w

Chances are, everything's ok anyway w/ stub axles and runout ... I'm with Steve... axles're usually straight when they leave St. Louis ... I'll add they're pretty darn hard to bend too! Another possible cause of runout are debris, corrosion, burrs on the mating flanges of stub axles & rotors ... but easily filed or ground off. But again, everything's probably AOK 'cuz if not, you'd probably be feeling that pedal pulse/shudder. If you're gonna have it checked out, I'd call both vette & brake shops ... ask if they have a dial indicator and can use it to measure runout while rotors ON car ... also ask if they are experienced and can measure the "play" in the rear wheel bearings (those bearings your rear wheel spindles/stub shafts run in) ... both can be checked almost simultaneously if the tech knows what to look for ... it AIN'T rocket science. A good, experienced brake shop should be capable ... ask the right questions ... and this is something you might want to witness (shop insurance issues or not) because you'll both learn from it and get that warm fuzzy "verification" feeling. BTW, the camber rods are another matter entirely ... they can be bent during an accident or agricultural excursion ... and I've seen a few that were intentionally bent by alignment "pros." BTW, I'm simply a hobbyist mechanic these days, but I've got my own dial indicator and magnetic stand for just this kinda stuff ... a cheapo under $30. I've got other DI's & mikes costing nearly $200 ... but not for use under a street car. There are folks that are comfortable with checking runout while holding a pencil against the rotor ... if I didn't have a DI, I'd use a pencil too ... got the T-shirts. Again, probably all's well since you don't mention pedal pulse/grab.
Thank you

Thank you for the positive feedback! The pedal feels nice & solid, no pulsing (even after the 300+ mile highway roundtrip), so I'm feeling pretty good about it right now.

I'm really beginning to think that my "ex"mechanic might have just been upset that I didn't pay him to do it for me...:(

Again, thanks for the info!
Barb :w
Hi 78 Ragtop

Nice car, who modified it to a convertible ?
Is this an easy task to do ?

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