It seems me, that the chain is too long.

Probably this is the reason, why the chain falls sometimes.

By how many links I should shorten the length?

P.S. Probably the reason for it, that my 52T sprocket had broken and I replaced it by 50T sprocket.

PHOTO UPDATED after adjusting tension on a B-Screw on rear derailleur.

enter image description here

  • Did the "new" 50T sprocket come with this chain? or is this the old chain?
    – EarlGrey
    May 13, 2020 at 8:11
  • @EarlGrey, no chain was old. I've also updated the photo.
    – Michael D
    May 13, 2020 at 8:58
  • 2
    Unrelated - your bike isn't locked in that new photo. The front wheel is, but it looks possible to unbolt the axle, fold the bars down, and work the chain over the bars/stem. The fix I use is to push the bike further forward into the stand, and run the chain through the frame loop over the BB as well as through the front wheel and rack parking stand. I also park it with the main latch unlocked and the bike slightly folded, so it looks busted.
    – Criggie
    May 13, 2020 at 12:57
  • 1
    @MichaelD looks fine to me. There are a bunch of questions about sizing a chain, like bicycles.stackexchange.com/questions/2376/setting-chain-length but I prefer to simply put the bike in big chainring, big cog, and make sure it doesn't bind up. In that gear the derailleur cage is cocked forward quite a lot, but it should not run out of travel.
    – Criggie
    May 14, 2020 at 13:35

1 Answer 1


You can determine the proper chain length by following the technique used to size chains when they are installed - in a modify way, as you will have a joined chain.

Before doing that make sure the B-screw is adjusted correctly so that the upper jockey wheel is not hitting the largest sprocket.

Put the rear derailleur on the largest sprocket, take a couple of chain links and pull the chain taught (it may help if you have someone hold the derailleur cage to remove its tension. You should be able to pull two plates together so that they would be joinable with 2 links (one inner, one outer) of axtra chain between them.

I'd also check the spring tension in your derailleur, both the A pivot where the cage rotates and the B pivot where the derailleur rotates on the hanger.

  • B-screw is adjusted correctly. Upper jockey wheel is not hitting the largest sprocket. From the photo it looks that the length is right or a bit longer, than needed?
    – Michael D
    May 14, 2020 at 9:36

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