Turn rotors completely flat?

chevrolet

#1

I’m replacing the front brake pads on my 1985 Chevy 30 Motor Home with 60,000 miles on it. I took the rotors in to have them resurfaced because they had a slight warp to them. When I got them back, I noticed that they still had a little warp, probably about 1.5mm. I took them back to the shop and asked them to take a little more off (there’s still plenty of thickness, so I’m not worried about exceeding the allowance.) When I got them back again, there’s still a detectable warp of perhaps 1mm. The shop guy said that’s as close as he can get it, and that I’d never even notice it on such a large vehicle anyway. Does that sound reasonable, or should I have insisted that he get the warp completely out?


#2

How are you (and the shop) determining that the rotors are warped? I have never been able to eveball a warped rotor no matter how bad the warpage.Now determining the rotor has a “lip” that is easy to see.


#3

Check for rust on the rotor mounting surfaces. Also, it’s good to snug up lug nuts a little at a time and finish with a torque wrench. I always use a torque wrench now; took the pulsation out of my brakes by doing that.

It would be less than usual if both sides are warped the same amount.

Your brake guy is feeding you rubbish. If a TIR (Total Indicator Reading) of 1mm is the best he can do, then he should take setup and machining lessons.


#4

One and half millimeters is huge and I’m also curious as to how you’re determining this figure.

One MM is roughly .040 of an inch and it’s claimed that it’s “as close as he can get it” and you would “never even notice it”?
Rotors that are .005 out of whack will shake you to pieces so .040 is a bit unbelievable.

Something is being misinterpreted here.


#5

Thanks for these three quick replies. Here’s how I determined the rotor is warped: The rotor and wheel hub are all cast as a single unit - it’s not a floating rotor. (this is a 1 ton vehicle with 8 lugs per wheel) I mounted the unit on the wheel bearing spindle during reassembly. Once mounted, I braced a small tool against a part of the suspension and spun the rotor. the surface touched the tool at the same spot each revolution and was about a mm away when the other end came around. I checked both sides of the rotor and the point where it touched on that surface coincided with where it didn’t touch on the first side. As a control, I set this “indicator” tool at the edge of the flat-face surface where the lug bolts are, and it maintained a constant touch against that surface, which indicates to me that the bearings, etc. are perpendicular to the unit as cast.

Bottom line is that all three of your replies make me believe that it should be turned all the way flat - the shop was apparently either feeding me a line or is just not competent to do it right - or both.

Thanks!


#6

Does the rotor look something like this?

http://cgi.ebay.com/ebaymotors/Front-Brake-Rotor-GMC-Chevrolet-/220629990479?pt=Motors_Car_Truck_Parts_Accessories&fits=Make%3AChevrolet|Model%3AC30&hash=item335e927c4f

I’ve machined countless rotors and have never seen a rotor out of whack as badly as the ones you describe. Generally, what one does when surfacing a rotor is make 1 or 2 fast cuts to give one an idea of how badly a rotor may be warped. The preliminary cuts will usually reveal quickly whether it’s time to chunk the rotors rather than waste time cutting them.

Since I’ll have to theorize a bit on this my feeling is that the rotors were not chucked up in the brake lathe straight and the ensuing cuts were made at an angle due to this oversight.
Many brake lathes use cones to center the rotors on the brake lathe shaft. If loose rust is present or care is not used when mounting the rotor/hub on the cones this will cause the bits to cut the rotor at an angle.
Other possibilities (less likely) would be a problem with the brake lathe itself or a distorted cone.

Really, that is about the only way I could see the rotors being off that much but my gut feeling is that it’s an operator error. Based on comments about being as close as he can get it and not noticing it on a large vehicle he does sound a bit suspect. Hope that helps.


#7

With this front axle/hub assembly you’re going to see a slight runout of the rotor once it’s mounted. When the rotor is mounted to the axle, the axle nut is tightened while turning the rotor to preload the tapered bearing. Once the axle nut is tight, it’s loosened about a 1/4 turn to set the tapered bearing adjustment. So with this bearing design you’re going to see a slight runout at the rotor.

Tester


#8

You could assemble everything and put your testing technique too the test,what I mean is,see if the brake pedal pulsates.This would not be the first time brakes were assembled and had to come back apart.


#9

Thanks again for the good input, oldschool & tester. Saturday is my next opportunity to get back to it - I’ll take 'em back off the rig and try another shop to 1.) see if they really are still warped, and 2.) get em done right.


#10

When a hub type rotor is mounted properly in the races the machining is very true. If the rotor is mounted with a cone at either end, or worse both ends, some runout is inevitable. The cones are often used when the proper adapters are not available.


#11

Also, once the rotor is turned improperly it is somewhat difficult to true it unless it is mounted correctly in the races. Insist that the machinist has the proper equipment and uses it.