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Rear derailleur arm moving

garethjonesgarethjones Posts: 57
edited June 2014 in MTB workshop & tech
When I am in the middle of my cassette, the arm on the rear derailleur, at the point of the bottom jockey wheel, moves back and forth. Any ideas what is going on?


  • Chunkers1980Chunkers1980 Posts: 8,035
    Only in the middle of the cassette?

    A bit more explanation needed really.
  • yes, so regardless of the front chain ring, the derailleur arm jolts back and forth quickly when I am about half way up the cassette. Moving up or down, keeps the arm stationary.
  • Chunkers1980Chunkers1980 Posts: 8,035
    Still don't get it!

    Have a read of Parktools to get your terminology right.
  • njee20njee20 Posts: 9,613
    Either the chain is catching on something and pulling it forward, normally it's on the chainrings though, bit odd. Or something isn't round which should be.
  • I will make a video at lunch
  • Chunkers1980Chunkers1980 Posts: 8,035
    Good idea. A few angles.
  • The RookieThe Rookie Posts: 27,747
    By arm, do you mean the actuation arm that the cable bolts to, the cage that holds the jockey wheels, or the parallelogram that goes between the static portion and the cage?

    If you mean the cage that is usually due to stiff links in the chain or another cause of chain suck (where the chain 'sticks' to the chainwheel rather than releasing cleanly as it should.
  • the cage the holds the jockey wheels
  • bobbyglossbobbygloss Posts: 314
    Is the chain jumping on the cassette, as if trying to change up a gear then dropping back down? Either indexing fault, bent tooth on the cassette, or something like a bit of stick wedged in the cassette?
  • njee20njee20 Posts: 9,613
    Or stiff chain.
  • bikaholicbikaholic Posts: 350
    Normally, you would use the b-tension screw on the RD to minimise cage judder. The screw position should be at the optimum point for when the RD is on the biggest cog and on the smallest cog - any cog inbetween, therefore, should not be affected by cage judder.

    So, the problem is either chain related (check for bent and stiff links), cog related (check for damaged teeth) or chain/cog teeth mesh issues.

    The latter is something akin to chainsuck in that after every few links or so, the chain falls out of synch with the cog teeth. This can even happen on a new bike if the relative position between hanger and wheel axle is out of working tolerance (ie the hanger is too long, too short or is bent forwards or backwards parallel with the line of symmetry of the frame).
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