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I was just getting ready to try dressing the front rotors to remove the high spot friction material and level it when I noticed something very odd. I'm going to work my way up in abrasives until I'm convinced the surface is perfectly the same all the way around.
Oddly, what I just noticed is that these opposite pad sized areas of slightly thinner thickness (.0007") begin precisely at the balance milling notch on the edge of both rotors and extend in the direction of forward driving rotation. These are supposedly decent quality Bosch Quietcast rotors with relatively little mileage on them. The milling on one is fairly significant, about a 3-1/4" long area that's a good 1/16" deep in the middle. It's about half that length on the other rotor, which is more typical of the milling I've seen over the years.
Why would these areas start at the same point on both rotors??? One thought was it has something to do with their milling process that slighlty affects the surface. My only other theory is that when the brakes are hot, the metal expansion in the milled areas is a hair less and may cool a hair different than the rest of the vented rotor. Over time, the build-up of friction material will be greater where the rotor is thickest and hottest. If this was in any way a production issue with the milling, I would expect Bosch would know about this by now.
Any other theories?
I think this is worthy of bringing to Bosch's attention in case it's a problem caused by the way the milling was done. The only contact listed is a toll-free number so we'll see how far I get or whether I even get a response.
Based on this, for the moment I am no longer recommending Bosch rotors or Bosch Quietcast pads unless they can convince me it was somehow my problem and that it was a total coincidence it was the same on both rotors. I now have a pair of Zimmermann's on the shelf waiting to be used if the problem comes back again after dressing the Bosch rotors. At this point, I'm treating the Bosch rotors as an experiment. I've got also got relatively new Quietcast rotors on my other 940 that have a hair of pulsatoion, so I'm going to be checking those out as soon as the garage is clear.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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