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Recently I replaced crank, chains and cassette on my about 4 years old trekking bike. All parts are low priced Shimano parts, just as the ones I replaced. New crank and cassette have lesser numbers of teeth, but still sport 3 and 8 rings.

Now, when I cycle about a kilometer, the chain starts to click, like when its shifting, and even jumps, like the teeth of the cassette are worn. The first kilometer is fine, but then this behaviour sets in, no matter, how I finetune the gearshift. This behaviour can't be reproduced on a bike mounting stand, only under normal use. Bumpy roads seem to enhance the issue.

Does someone have an idea, where this issue might come from, and what can be done to solve it? I thought about trying to replace the small cogs on the derailleur, could they be the problem? All the cables (inner & outer) are the original ones, but they worked fine before I replaced the drivetrain.

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  • What exact rear derailleur are you using, and what speeds is it intended for? You don't want to put a 6-7-8 speed chain into a derailleur that is intended for 8-9-10 speed cassettes.
    – Kaz
    Jul 14, 2023 at 18:01
  • @Kaz Is that even possible, isn't 6-7-8 speed always "narrow-wide" chains, so CN-HG40 (or HG71 as e-bike spec mm). 9/10/11 speed are different spec/width?
    – DoNuT
    Jul 15, 2023 at 13:46
  • @Kaz everything's basic Shimano stuff for (6-7-)8 speed.
    – Erik
    Jul 17, 2023 at 7:48

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Correct length of the new chain, are you sure that it is not sized too long?

It is just a guess but bumpy roads worsening the issue indicates that there might be slack and the chain bouncing makes things worse and skips cogs.

Probably best checked if it is under enough tension on the smallest chainring and smallest cog combination because the rear derailleur must extend the most in this chain position. As least, there should be no visible slack and the rear derailleur should fully compensate for that.

For chain sizing, I'll just quote our favorite tool company: https://www.parktool.com/en-int/blog/repair-help/chain-length-sizing

The largest cog/chainring method worked fine for me on a 3x7 Shimano Tourney groupset, recently.

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    I haven't considered that, will check it this evening at home. Could be quite possible the issue.
    – Erik
    Jul 14, 2023 at 10:01
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    It was the length of the chain. Thanks for the hint.
    – Erik
    Jul 16, 2023 at 14:40
  • @Erik I'm glad I could actualy help because that's what this forum is about, no writing out the most elaborate answer that touches on ten other things but not the actual problem. How many links did you have to take out?
    – DoNuT
    Jul 16, 2023 at 19:35
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    I should have taken out 4 links, but I took out 6 links, and not some gear combinations aren't smooth anymore. Still, it works for now, and the next chain is up in 6 months anyway.
    – Erik
    Jul 17, 2023 at 7:47
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    @Erik OK, one link too much probably isn't an issue but four definitely are. I know the tutorials are often a bit confusing but generally, you need an overlap of compatible links. Keep in mind that a now too short chain might have increased wear on the drivetrain, but to be fair: 8-speed Shimano components don't cost a fortune.
    – DoNuT
    Jul 17, 2023 at 7:55

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