Chain slap is something we all have to live with, but Shimano's ZEE RD-M640 Shadow Plus makes it a whole lot less cringe-inducing by including its now-famous clutch technology. By increasing spring tension when it's engaged, the clutch reduces slap and the risk of dropping a chain, making it a killer choice for riders who spend decent chunks of their time pounding through rock gardens and over rugged trails. When the clutch is off, the reduced tension makes it easy to pop the derailleur out of the way for wheel removal. It's made of aluminum and steel, so it's durable enough to be knocked around with no ill effects, and comes in both a DH and Freeride model, so you can dial it in for the type of riding you do most often.