Brake Disc Warping

[KenC]

I recently overhauled my brakes, which included lightly used Zimmerman Cross-drilled rotors, rebuilt front and rear calipers, and BMA SS brakelines.

The system has been working wonderfully, but I've started to notice that there are one or two rotors that are slightly warped... I think the warping is very slight because I only feel it at low speeds when coming to a stop. This all started 2+ months after my install, so I know the rotors weren't warped before I installed them.

Is there a way to diagnose, while driving, which rotors are warped? My intuition is that it is probably one, or both, of the rears as there is NO shimmy in the steering when I feel the braking pulsation (which occurs only at low speeds).

What would possibly cause them to warp quickly like this? I personally rebuilt the rear calipers, but can't imagine that I messed up as it's such a simple mechanism. Is there a way to tell if a caliper is sticking (assuming that would cause a premature warp)?

Gracias,
Ken

[nando]

warped rotors are unlikely, even under heavy track use E30 rotors don't warp very easily. Since they were used, I'd bet on uneven pad deposits. it will feel the same. how did you break them in?

[StereoInstaller1]

Or shitty CABs combined with pad deposits

[KenC]

Or shitty CABs combined with pad deposits

M3 CABs with about 20k on them.

[KenC]

warped rotors are unlikely, even under heavy track use E30 rotors don't warp very easily. Since they were used, I'd bet on uneven pad deposits. it will feel the same. how did you break them in?

The pads were the only thing I carried over from my last set-up. I guess I assumed incorrectly that I needed to pay attention carefully breaking them in. I just daily drove them after putting them in... nothing too hard. However, I did have to do some "60+mph then fucking mash the pedal to floor to get the air out of the abs unit" stops immediately after putting them in.

So assuming it's uneven pad deposits, how do I solve that?

[jlevie]

Remove the rotors and sand them with 120grit Silicon Carbide paper. Before re-installing the rotors clean all rust from the hubs.

[KenC]

Remove the rotors and sand them with 120grit Silicon Carbide paper. Before re-installing the rotors clean all rust from the hubs.

Any special process for this? Is silicon carbide paper widely available?

[jlevie]

I use small angle grinder for this fitted with an SC sanding disk. But you can do it (slowly) by hand. You can find SC paper at most any auto parts house or home improvement center.

[KenC]

So is it easy to see where the deposits are? Or just sand the whole thing? I don't have access to an angle grinder so I'll be doing it by hand. 120 grit seems pretty course.

[StereoInstaller1]

M3 CABs with about 20k on them.

So much for that idea, right?

You might wanna peek at them anyway, Ken.