Standard rear derailleurs have a pulley cage pivot that is free to move in both directions, but companies started adding one-way clutches about ten years ago to help keep the chain from falling off when mountain biking on rough terrain. They still have a spring inside to keep the chain tensioned as usual, but the clutch keeps the cage from rotating forward when you hit bumps. On the whole, they do a fantastic job, and it’s almost impossible these days to find a modern performance-oriented mountain bike rear derailleur that doesn’t have one.

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