Originally Posted by
bikinfool
So you have matched adapters to your rotor size?
Yes. That's not the problem.
Originally Posted by
5b00
I believe that I have the same combination of rotor (SM-RT78 from Nashbar) and brake (Avid BB7 road). I also had problems with weak, pulsating brakes. If you look closely, the brake track on the XT rotor is narrower than is typical. This means that your pads are probably hitting the arms, which causes the pulsing sound.
Using the XT rotor, the stopping power is about the same as the cantilevers that came on my Crosscheck-- not terrible but not as good as I'd like. The pulsing goes away and I have better stopping power when using an Avid Roundagon or a Clean Sweep G3. This solution works for both 6-bolt hubs and Centerlock hubs using a Shimano adapter (SM-RTAD10). A cheaper solution that doesn't work as well is to shim the brake mount out by one millimeter, so that less of the pad is hitting the arms.
The braking isn't weak, and it isn't a "pulsing"...nor is it rapid enough to be paired with the phenomenon you are describing. Rather, there is a definite "grab" at a particular, and regular, point of the wheel rotation. Having done my best to clean the rotor, I feel I have to accept the theories offered above, that the rotor is quite possibly thicker in that location, or that there is some form of localized distortion that has eluded eyeball detection so far.