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The chain likes to jump from the front sprocket, it is my impression that it makes no difference if the chain lies on the inner or outer cogs. Since I am unsure about the root cause I ask here for some pointers to troubleshoot the issue.

Bike:

The bike in question is an old 20" kid bike from a garage sale, history is unknown except that it is at least 7 years old. Former maintenance was neglected. It has an 1x6 transmission. The entire transmission is rather old and not the fanciest one. I do not have much experience adjusting the derailleur but in the end I was able to change smoothly to every gear (adjustment was done with the bike upside-down). The chain is well lubricated and has no stiff links. The entire transmission (front/rear sprockets, pulleys) moves smoothly.

Observations:

  • Observing my son riding the bike on the lawn I see the chain jumping a bit too much for my liking (amplitude about 10cm), I am not sure if that is fine.
  • When I tried to reproduce the issue I was able to "block" the chain between one of the rear sprockets and the jockey (upper pulley), I presume the issue was missing clearance between them. AFAIK there is no option to adjust that distance nor the spring tension.

Update

Closer inspection revealed that the large rear cog is definitely worn: The rear cassette will be replaced.

Update #2

I replaced both cogs, chain and derailleur (including cables). The chain behaves a bit better but still jumps from time to time from the front cog. The chain had a stiff link where I joined it, but that got resolved. The new front cog has no wobble. The rear derailleur is properly adjusted (smooth shifting to every position).

Frankly, I have no idea on how to proceed (Will post pictures of the new setup). Perhaps I should mount a chain guard?

Additional thought: My son is just learning to shift. Perhaps I am overthinking that issue. Could it be that misuse (perhaps he is playing with the shift lever while standing) provoke most of the chain jumps? If so, would a chain guard limit the impact?

Rear derailleur Rear derailleur Entire transmission, chain jumped

  • Hi Martin, welcome! A few clarifying questions. Has this issue been happening for a long time, or did it just crop up recently? Also, can you tell if the chain is more likely to slip off the front chainring if it's on the innermost rear sprocket, or the outermost? And finally, do you know the approximate age of the chain? – hairboat Aug 27 at 16:35
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    Also, you may be able to adjust the distance between the largest cog and the top of the pulley by looking for the B-screw. Not sure if that will solve the whole issue, but it would be an easy first thing to try. – hairboat Aug 27 at 16:40
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    The chain appears to be "stretched" (worn out), and that large rear cog looks to be fairly worn as well. – Daniel R Hicks Aug 27 at 17:07
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    @hairboat thank you. I don't know if the issue happened for a long time as the bike was bought a month ago. My suspicion is that the issue happens on all gears. I also think that the chain got never replaced and was neglected the entire time. I will change rear cassette, derrailleur and chain and will report back then. – Martin Aug 27 at 20:44
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    The most tension on the derailleur cable is when the bike is in 1st gear. If the bike was stored for a long time in a low gear (like 1st), the derailleur cable may have stretched a bit. This also happens within a week or so of having a new cable installed. Just tightening up the cable tension may help. There may be a barrel style cable tension adjuster on the shifter (sometimes also on the derailleur). If you back it out, that'll add a bit more tension to the cable. – mbmast Aug 27 at 20:58
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There are a few reasons that might cause the jumping of chains.

  1. Chain or sprocket is worn out (Buy new ones).
  2. The derailleur is misaligned (Needs adjustment).
  3. The wheels are not fitted properly making it a bit slanted (clean the crevices and fit the wheel properly).

Check all of these and comment back if all are good.

  • Agreed. My bet would be #1, the chain is worn and maybe the cassette too, especially if OP doesn't know the history of the bike. Or, all of the above! – hairboat Aug 28 at 14:48
  • Update: Replaced chain and both sprockets, chain got better, still jumps from time to time, will look today evening and comment back – Martin Sep 10 at 8:12

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