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  1. #1
    mtbr member
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    XTR M950 rear derailleur b-tension problem

    I have an 8 speed XTR M950 rear derailleur with a XTR M950 rear derailleur with a problem: I can't get the b-tension high enough to prevent rubbing between the upper pulley and high cogs in the following combinations: 32x30 and 22X30, using a 44/32/22 front and 11x30 rear setup. I just rebuilt the derailleur hoping it would help but it didn't do anything other than make it a little worse, as I set the cage pivot spring at a higher tension which I later realized/discovered would make it worse as the b axle pivot springs and cage pivot springs are in opposition. So I took it apart again to make sure I had the b-axle spring in the correct way and lowered the cage pivot spring tension. The derailleur is several years old but in excellent shape otherwise. Unfortunately after much searching I can't find either a replacement b spring or complete b axle assembly for that model available anywhere. I may shorten the chain by a link but that's the most I could do and i'm not sure if that will help much. Other than buying a new der. has anyone run into this problem and found a convenient solution? I'm also thinking of also getting a longer M4x12 b tension screw to crank up the tension some more.

  2. #2
    Do It Yourself
    Reputation: Homebrew's Avatar
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    Get a longer screw and thread it in backwards as the head is much larger and less likely to slip. I've used allen head screws from the hardware store but I forget the exact size.
    Long Live Long Rides

  3. #3
    Ride Responsibly
    Reputation: LWright's Avatar
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    double check the chain length, too long will do what you describe

  4. #4
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    I didn't end up shortening the chain because I figured out even taking out a link could result in a derailleur blowup in a 44x30 combo, which I would never normally shift into but sometimes you do by mistake. I solved it by putting the cage spring tension at the lower setting and getting a longer b adjustment screw (M4x16), 4mm longer than the stock one. and it's a 3mm hex head so it's easier to adjust than the stock phillips head. basically i had to go two or three turns past the extreme high adjustment I could get with the shorter stock bolt. it's too bad there aren't available more of a selection of b-axle assemblies or springs, clearly mine (XTR M950) is just old and tired and if i could find the exact replacement i would prefer getting a new spring. maybe some of the other b axle springs from similar models that are available would work but one never knows with shimano stuff whether it is going to be compatible without direct comparison in hand.

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