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I have changed the derailleur on my daughters back, bought like for like.

Now fitted.

When I try to pedal the derailleur, it flicks backwards and the bike won’t pedal forward, it remains stuck.

Any advice would be greatly appreciated.

  • 1
    Did you change the chain or is that the original chain?
    – Criggie
    Apr 16, 2020 at 9:08
  • 2
    Something does look very out of position, almost like something is twisted around on itself, but I can't tell what from here. frustrating.
    – Swifty
    Apr 16, 2020 at 9:24
  • 3
    Chain is threaded incorrectly though the pulleys
    – mattnz
    Apr 16, 2020 at 9:36
  • 4
    I am not sure that the chain is correctly threaded through the pulleys. I can’t see it going through the upper of the two pulleys in the photo. Also, it somehow looks like it’s mounted upside down, because I can see the cable bolt on top. All the road RDs I’ve used put the bolt on the bottom of the derailer body. Can the OP say what model derailer it is?
    – Weiwen Ng
    Apr 16, 2020 at 13:29
  • 1
    When it was installed the tension spring got placed incorrectly. Apr 16, 2020 at 15:10

3 Answers 3


I think we can rule out the chain being too long. The OP replaced the derailleur but does not state the chain was replaced. Also, that does not look like a clean and shiny new chain.

The chain is threaded through the cage correctly, it's just that the derailleur had rotated 90 degrees rearward so that the cage is sitting behind the sprockets rather then underneath them. If the derailleur was rotated back it's possible to see that the chain would run in front of the top jockey wheel and behind the lower one.

Here is my theory: the cage is not tensioning the chain, it should be trying to rotate clockwise under spring tension but is not. I think the cage was somehow rotated clockwise before the chain was threaded through it, unloading its spring.

To fix, remove the chain, rotate the cage anti clockwise so that its spring is wound up, rethread the chain through the cage and rejoin. The chain should be under quite a bit of tension when you join it.


Old thread, but yikes.

No, chain is not threaded correctly. Jockey wheel at rear has no chain around it!

  • 1
    I thought that too when this was first posted. But the lowest jockey wheel visible is definitely the lower one, because of the guard-part of the cage. Somehow the whole deraillleur has inverted, like a knee bending the wrong way. It is definitely not right.
    – Criggie
    Mar 6, 2022 at 7:02

Looking at it again, I think all that's happened is the mounting bolt was done up with the derailleur in the wrong place. It's also possible the bolt is loose, hence "flicking back" rather than stuck back.

I reckon the cure is to loosen the mounting bolt, correctly position the derailleur, with the B screw pressing against the hanger, then tighten the mounting bolt again. It should be properly tight - there's a chance it was only done up lightly and worked looser in the first rides, meaning it would be likely to fall off.

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