Originally Posted by
dstrong
but anything bigger would likely require going to a long cage derailleur (I'm assuming the current der is a short cage) and probably a new chain.
No. Cage length has nothing to do with capacity - the largest cog a derailleur can handle. Cage length only affects how much chain slack there can be in the drive train.
frisky, Shimano RDs are all rated at 27T capacity, but typically handle 30T without problems. Larger than that and you need a MTB derailleur. You may or may not need to lengthen your chain. When you put a bigger cassette on, measure it. I often swap a 23T and 27T, and need to make no chain adjustments.
Oops, I see joejack got here first.