Turning Rotors: a case study...

yes indeed! it's all about the first owner; all the others can go hang.

buddy of mine used to work at one of their r&d facilities. all their time & money was going into cost control [which is ok] and life limitation [which is not, since the price differential between say ford & honda is minimal]. if a ford was 1/3 the price of a honda, i'd have no problem with it, but i'm wierd like that.

Realistically, there isn't much that can be done. The customer can't be forced to pay for repairs he doesn't want, and restraining either the customer or his property isn't allowed - those are crimes. If a peace officer shows up in time he can take the car off the road but a citizen legally can't (at least not in the US - in any state I know if.)

When I was in aviation we would occasionally see planes in obviously unairworthy condition come through and all we could do was inform our FAA safety inspector.

Mike

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Some states yes, not this one (we wish there was!)

It has to do with the quality of the OEM part. Sometimes they are better, other times (as in the Taurus) there worse. BMW, VW Volvo and now even Ford Focus have a extremely soft metal for the rotors. Replace them with the pads they wear that quick.

Well said.

That's what I suspected, i.e., a lack of parallelism. Thanks for confirming it.

Eric

I replaced the front rotors on our '85 Volvo with aftermarket rotors from NAPA and pads from Volvo when money was tight (the Volvo pads don't rattle like aftermarket pads do). Now the pads are about half worn and the rotors are severely worn.

Mike

A rotor that is "warped", not simply scored, is a very poor candidate for resurfacing. The metal's properties have been changed by the level of heat it has been exposed to, causing a permanent change in its structure. After machining, once heat is applied, it will revert to its prior condition.

I went through this issue with my wife's Malibu, arguing with the Chevrolet service manager every step of the way. They resurfaced the rotors 3 times to cure pedal shudder, but it never worked because the rotors were warped. In this case. it was a design flaw by GM - they made them too thin in the first place, and paired them up with lousy pads. When I finally gave up dealing with them and did the job myself, the rotors were ~0.003" thicker than the bare minimum, suggesting that they were able to shave off a minimal amount of metal each time they turned them. But so what? The rotors were junk the first time they warped.

OTOH, a rotor that is merely scored can be effectively resurfaced and returned to service, providing that not too much metal is removed. I've done this on cars when the pad wore too far and "kissed" ther rotor. A competent technician can do this.

Doug

sorry - unless this disk has been operated well into red heat, you're not going to get much change in microstructure. you can however have problems with a bad disk if it was not heat treated properly after casting. when operated hot, /that/ disk will distort. more likely is that there are simply different cooling rates on different parts of the disk leading to local temporary distortion. that shouldn't happen on a disk of sufficient thickness and whose internal vanes have not lost too much material through rust.

my experience is that the disk cutting process is far from perfect - those cutting machines see heavy usage, and you can mount the same disk up a dozen times and get 13 different centers.

if you do a basic surface scratch test for these disks vs. others of "harder" metal, you'll not find much difference. what i know for sure is that some oem pads have a high silica content [with some aftermarket pad producers copying their lead]. silica is an aggressive abrasive. this is specified by the manufacturers you cite allegedly to eliminate disk glazing and cope with surface rust on salted roads, and it does have some benefit for those purposes. reality however is that it's all about life limitation.

i used to work in a shop and had access - that's how i know these things can be difficult to center, giving inconsistent results.

Your opinion and a dollar will get you a cup of coffee. That's all it's worth . Repeated heating and cooling cycles will definitely change the distribution of carbon atoms in an iron matrix - and it doesn't have to be "red" hot. The temperatures created by the crappy pads used by GM did the job in less than 15,000 miles.

If that's your experience, that's too bad. You need to find a better technician, or learn to do it yourself.

Ahhh, New equipment.... Some are sweet!

Thanks for "confirming" suspicion of lack of parallelism are superfluous - Stephen H had told us of it:

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Why bring up coaxiality, or lack thereof? It is irrelevant as long as the contact surface with the pad gets machined.

Relevant is that the machined surface and the mounting surface are perpendicular to the axis, and it is therefor important that the mounting surfaces are clean. This is the area you have to concentrate on, not centering. No wonder why you got "inconsistent results."

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Actually, he is talking about himself. He does not understand the physical relationship between runout (I believe this is the name - I am no mechanic) and perpendicularity of axis and contact surfaces - he concentrated on irrelevant coaxiality!:

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In a number of messages (maybe in all of his "cutting rotor" messages) jim beam emphasized the importance of centering the disks when their surfaces are being machined. This is what I was replying to - centering, within limits, is irrelevant, but I used the wrong term "coaxiality." Once again, important is the "perpendicularity of axes and contact surfaces," and anything that throws this off will cause problems.

In fact, there is the possibility that jim beam, too, is talking about this when he writes about the importance of centering: in a message at another list he wrote of the importance of cleaning the mounting surfaces of the lathe and of the disk.

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In a number of messages (maybe in all of his "cutting rotor" messages) jim beam emphasized the importance of centering the disks when their surfaces are being machined. This is what I was replying to - centering, within limits, is irrelevant, but I used the wrong term "coaxiality." Once again, important is the "perpendicularity of axes and contact surfaces," and anything that throws this off will cause problems.

In fact, there is the possibility that jim beam, too, is talking about this when he writes about the importance of centering: in a message at another list he wrote of the importance of cleaning the mounting surfaces of the lathe and of the disk.

Another factor is also how the disk is actually cut. Not only is each axis perpendicular to each other, but the surfaces for the pads to be parallel. A poor cutting bit, taking off too much at a time, or a crossfeed that is too high can cause these kinds of problems.

I just went through all of this with Chrysler, as the Jeep Cherokee's from

2000-2003 are notorious for premature warping of the front rotors (TSB issued). They (Chrysler) went through the trouble of changing the calipers and pads to another design in accordance with the TSB recommendations, but only cut the rotors (which I argued against), and they were warped again within 4K miles. I gave up arguing with them and purchased and installed a set of high performance rotors and pads.

you need to read around a bit more. if the disk plane is not exactly perpendicular to the rotation axis, on a floating [single piston] caliper, you have pulsing in the hydraulics because of momentum differences due to the mass of the caliper vs. the piston. with a fixed caliper and 2 [or 4 or 6] pistons, the mass on each side is the same and there's little net effect.