Blaine correct me if i'm wrong ! but every time I've had rotors turned they warp within 10k miles, so now I just change them out .... am I doing wrong ?
They aren't warping again, and thickness, or the ever-so-slight reduction in thickness from resurfacing has absolutely no affect on heat capacity of the rotor...we're dealing with a miniscule amount of material.
The key word here is "runout".
First, the way most brake pads (applicable to TJs) work:
We're dealing with Adherent friction. This is why pads and new rotor surfaces require bed in. During this process, the pads will deposit a layer of their own material down onto the rotor surface....ideally in an even manner (which is why you don't want to come to complete stop while the brakes are hot during the first few initial heat cycles). Once we've established the transfer layer, as the pad is pressed against a surface of the same material on the rotor, there is breaking and recreation of crystaline bonds to create friction. Effectively, we have material at the molecular level moving from the rotor to the pad and back and forth. No pad will be 100% adherent...under certain circumstances we might get a bit of abrasive action out of it, but when it gets back into its happy temp range, it will act adherent again and maintain the friction transfer layer on the rotor. Think of this transfer layer as sort of a snowball that wears....constantly being broken down and rebuilt somewhat as we drive.
Now, if we take a rotor that spins dead true with zero lateral runout (in and out wobble as it turns), the above theory typically works out quite well. If we have an excessive amount of runout (more than .003" or so in this case), the transfer layer in the high spots will eventually become established to a different level than it is on the rest of the rotor, due to the high spots contacting the pads at slightly higher pressures and more often than the rest of the rotor surface. Since the quality of the transfer layer more or less determines our friction levels, this results in a "judder", and will snowball from there.
Back to machining rotors...
Bench brake lathes are great tools at the hands of someone that knows what they're doing....the problem is, very few understand the objective.
Before you machine a rotor....it should be removed from the hub and the mating surfaces (inside of the rotor hat and the hub surface) cleaned of all rust/schmoo/etc...) and then the rotor reinstalled to measure runout using a dial indicator. You can 5 different positions to see which one gets you as close to zero as possible....when you find that position, mark a hole/stud so you know that position, then mark the zero spot and the high spot on the friction surface. When this rotor is chucked up on the brake lathe, use the dial indicator to make sure that you are duplicating the on-hub conditions. Your zero spot and your high spot should match what you saw on the hub....if it doesn't, either the hub was bent or the brake lathe adapters are fucked.
Brake lathe adapters need to be machined while they're on the lathe arbor, after having index marks put on the arbor and the adapters, to "match" them so that the adapter surface has zero runout when indexed to the arbor....this is rarely the case. More often than not, a perfectly good and new rotor with near zero runout can be installed on a brake lathe and it will look "warped" and usually take a couple passes to get a complete 360° cut....this is not because the "cheap chinese rotor" was warped out of the box, as many will try to convince you, it's because the brake lathe adapters are fucked up, and have just destroyed your new (or good used) rotor.
"But they're nice and smooth when I put them on the car"....of course they are, both surfaces are nice and new and parallel to each other. Any runout will be un-noticed because the transfer layer will take time to become inconsistent and since you had the caliper off and had to knock the slide sleeves unstuck to get it back on with new pads, the caliper (for the time being) will float back and forth and follow the rotor without any hydraulic feedback...for the time being.
Over the course of 5-10K miles though, this runout condition will develop into an uneven surface condition and a thickness variation (the transfer layer has dimension) and will rattle your teeth out.
New rotors are the safer way to go because an ignorant lathe operator hasn't had the chance to fuck them up yet, but installed on a rusty/bent/etc hub, they will not last long. Get a dial indicator at Harbor freight and a magnetic base (or some sort of fixture) and make sure runout is under .002 or .003" whenever a rotor goes on a hub....and if using a brake pad suitable for the operating temp range (like Blaine's pads), your pulsation/judder/"warped" rotor problems should go away.
