Originally Posted by
strock
You can use a shorter-cage RD by running a chain that is too short to allow you to use your large-large combinations, but if you accidentally try to shift into, say, 44/34, when the chain isn't long enough, it can really mess you up. Don't ask me how I know.
That's a dangerous and potentially damaging approach, as you apparently found out. A better way is to size the chain to allow big-big and, if necessary make it too long for the small chainring and one or two of the smallest cogs. If you inadvertently use those gears, all that happens is the chain goes slack but no damage occurs.