Not talking about ridge (or I think you really mean groove on the rotor).
If you can picture a vented rotor, it has 2 "plates" with the vented part
between the 2 plates. In my C220, one plate is much thinner. I do not
think it has anything to do with better cooling. I believe these rotors
are first made with the same thickness plates and then one plate ground
down in order to make it fit the application. The reason I say this is I
happen to purchase discs from NAPA. The NAPA disc plates had the same
thickness. So I tried putting them on. The NAPA disc fit but the inner
side was hitting the disc shroud. After further investigation, I realized
it was because the inner plate was thick. My point is that this might have
been an after thought by MB. Furthermore, it seems to me that having an
uneven plate thickness like this encourages warping more easily on the
I returned the NAPA discs and got ATE discs. I advised NAPA that their
discs are not compatible.
I'm just not convinced that the reason the plate thickness are not the
same is due to better performance. If that were true then all the rotors
out there would be like so. Any thoughts???