Originally Posted by
SJX426
Bianchigirll -
A clutch-type rear derailleur has a mechanism at the top of the cage that adds some resistance to the cage moving. This acts to keep better tension on the chain over rough terrain or when suspension movement results in chain growth or shrinkage. The clutch is engaged using a lever at the bottom knuckle.
They came into being from the 1X chain ring thing; the clutch keeps the chain from bouncing off the front. Most have a way to unlock it for wheel removal. In conjunction with that are 1X chainrings that have fat & skinny teeth alternated to help keep the chain on.