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When shifting on the front chain rings the chain can sometimes get jammed on the teeth causing it to get stuck between crank and chain stay. This is also called “chain suck”. This occurs mainly when shifting down onto the two smallest chain rings.
The cause may be bent teeth on your chain rings or an incorrectly adjusted front derailleur.
However, this is mainly the result of worn chain rings. On old and worn chain rings the cause is mainly an edge that has formed and when shifting the chain gets stuck causing the chain to get jammed between chain stay and crank.
If the chain is completely wedged in between crank and chain stay, then you’ll need to take off the crank to free it.