Box components DH cassette spacer. Why? Ghost shifting can happen on rough tracks, even with a clutch rear derialleur. And having that chain jump up into your spokes is a great way to bring a great race run or even a sublime ride to a screaching halt. As well as ruin your wheel. Yeah, there are ghetto fixes to that problem but this elegent solution by Box will only set you back $44 and should last the life of your next couple bikes. Available in black and orange is the new black.