i have no idea on chains either. i can see when they are loosening [they sag at the bottom, and usually move quite a bit when lifted back up] and i know to get em seen to. last thing you want is to have it jump off on you. if you do notice its loose, get it tightened asap. id suggest checking it when on long rides, where you are spending most of the day in the saddle on the open roads.
when mine came off, it was just like getting my pants caught in a bicycle chain: all motion stopped, except for the coasting. chain came off and got caught in the guard. twisted the guard all to hell. part of it was jammed, and what wasnt, was hanging loose around the sproket.
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