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  • 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? Commented Aug 27, 2019 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. Commented Aug 27, 2019 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. Commented Aug 27, 2019 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. Commented Aug 27, 2019 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. Commented Aug 27, 2019 at 20:58