Tried my best searching for this--extensive Google searches and an email to StopTech which was not answered:
What is the minimum rotor thickness for StopTech rotors? My ST60/ST40 BBK comes with 380x35mm front rotors and 355x32mm rear rotors. I'm on my third set of Street Performance pads in just under a year with the BBK setup, and I'm wondering when I need to consider replacing the rotors, or "friction rings" as StopTech calls them (thanks for pointing this out, Richard
Richard at M-World did a recent DIY for replacing the friction ring component of the rotor assembly (rotor assembly consists of aluminum rotor hat secured to iron friction rings). I thought I may have seen somewhere that 2mm is the allowed wear for the rotor thickness before they should be replaced, but I can't find this reference. I believe Richard indicated he was replacing them after three pad changes.
It's not an insignificant concern because replacing the friction rings alone (and leaving the rotor hat as is) is upwards of $600 per axle. I realize it's a wear item, but I don't want to replace them prematurely and throw out good pieces.
Once I can get a digital caliper to measure my current rotor thickness, I'll have a better idea of how much wear the rotors have seen at this point. In my three pad set changes I've replaced all four pads at a time each time, but it was only the fronts that were seeing the majority of the wear. Each time the rear pads still had quite a bit of material left on them (there I go again, throwing out good pieces prematurely...). So I would assume that my rear rotors have not seen the wear that my fronts have, and so my strategy will probably be to just replace the front friction rings at this point.
I started thinking about this because track season is approaching and I'll likely try the Pagid RS29s now as an upgrade from the StopTech Street Performance pads.
Any thoughts or comments welcome.