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  1. #1
    mtbr member
    Join Date
    Nov 2004

    centering a magura rotor

    I have Martas and am having a problem getting the rotor centered on the rear. The slave pad rubs on the rotor. I have no shims, and had a shop use the Gnann-o-matt, but I question if it was faced properly since the rotor is still not centered. I have gently pryed the pads, and when I reset them pumping the lever, the slave pad rubs the rotor.The bike is ridable, but does have drag. The rear brake also squeals which progressively worsens over the course of a ride. The squeal sometimes becomes a vibrating hum through the whole bike. I am sure this is related to the centering issue. I have the front brake dialed in and it performs flawlessly. Should I try another Gnann-o-matter or is there a problem with the slave pad not retracting enough or something else? Any suggestions are appreciated.

  2. #2
    Do It Yourself
    Reputation: Homebrew's Avatar
    Join Date
    Jan 2004
    Is it not centered in the caliper? Or not centered between the pads? If the rotor isn't centered in the caliper and needs to be moved out more then you need to machine the tabs down some more but this is very unlikely. The rear typically needs a 1mm or two of shims. So unless your frame is out of whack, then this shouldn't be a problem on the rear (front is another story).

    If the rotor is centered in the caliper but the pads are not centered, then you need to microadjust. This involves holding the side that too close to the rotor in while you pump the brake. You can either bend the rotor a bit or insert something like a clean feeler gauge, business card or whatever while you pump the brakes. You might have to go back and forth a couple times to get it perfect.

    The pistons not moving at the same time is a separate issue. You can try to reanimate the seals by taking the pads out and VERY CAREFULLY pumping the pistons out and then pushing them back in many times. You should probably clean the piston area with Clean Streak and put some Blood around the pistons as your doing this. This should help the pistons to move together.

    Good luck.
    Long Live Long Rides

  3. #3
    Reputation: Monte's Avatar
    Join Date
    Dec 2003
    What Homebrew said, and using a Gnann-o-matt tool won't center the caliper on the rotor, it just makes the pads parallel to the rotor. You need to shim the caliper over to center the rotor between the pads.

    mcm 64

  4. #4
    mtbr member
    Join Date
    Nov 2004

    magura's advice

    Thanks for the input. The problem seems to be a sticking piston. The outer pad remains up against the rotor. I have tried microadjusting with a business card. As soon as I take the card out and press the brake lever, the outer pad pushes up against the rotor and stays there. Perhaps the rotor is not centered between the pads. Magura suggests doing the Gnann-o-matt again (due to the squealing), and if this doesn't work to treat it for a sticky piston.In that case I will try Homebrew's suggestion with the Blood.


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