I'm trying to sort out pad dragging / noises in Avid BB-7 mechanical disc brakes and I'm wondering how much runout in the rotor is acceptable? Right now I have about 0.05 mm (measured with a Park DT-3i).
0.05mm is nothing. Stop worrying about it. As long as rotor does not make noise or drags extensively on the pads, you'll be fine.
Rotors change shape every time you ride: you apply breaks a little longer than couple seconds, rotor heats up and expands, then cools down and shrinks. On extended periods of braking rotors can heat up enough to burn your fingers when you touch it (done it myself unfortunately). And when this level of temperature change is applied all sorts of transformation happens to the level of warping/runout. So you can spend all the time in the world to make rotor as true as possible, then you go for a ride and it all changed and rotor is nothing like you've left is last time when you fiddled with it.
I am truing a rotor right now and it started with enough brake rub to stop the wheel from free spinning in a stand. I didn't get a number but estimate it had over 2mm of runout.
After 15 minutes of rough tweaking, it has 0.16mm of runout. This results in slight pad rub for me. The rotor visibly wobbles when the wheel is free-spun.
Finally after another 30 minutes effort, it is down to 0.065mm of runout, and while not perfect, its good enough.
Your 0.05mm (0.004 inches) should be fine and any further issues will be in your caliper, alignment or brake pad offset.
The BB5 and BB7 are single-sided calipers, and the pad setup is notoriously twitchy.
- Set the outer pad wide open by tightening in the barrel adjusters.
- Use a T25 torx through the wheel and tweak the dial so the inside/non-moving pad is just clearing the rotor. This should be done after bedding in brake pads. When the wheel will free-spin, go back to the ouside/moving pad.
- Then twiddle the barrel adjusters to just-not rub. If you run out of adjust, you might need to move the inner cable inside its clamp.
At no point should you need to relax the caliper's mounting bolts - that will upset the parallelism of the pistons to the disk.
Truing a rotor with a cheap chinese dial indicator, on a knockoff noga stand which is magged to a cheap mill-vise, which is clamped around a tine of an old steel fork, mounted vertically in a bench vise. With some clamps added for confidence, I suspect the magnet was not doing much.
I usually replace my disk brake pads when I have 0.5 mm thickness. The reason I do that is because they can easily be smashed by the jaws to remove the braking material ( which will eventually fall apart ). After that it will result in a friction between the disk and the metal of the pads, which is very very bad.
I change the disk when it stops braking when I use a new disk brake pads. But for the thickness I should say that It depends how much are you loading them. As you say above disk with thickness of 0.05 mm is very hardcore ;). You should really think of replacing that thin disk, because it can really surprise you when it breaks.