A Chain Device is a part used to help eliminate chain suck and chain slap when riding over rough terrain or landing from big drops making them very popular with Trials bike and MTB riders. There are a few different fitments, some frames have ISCG mounts which are lugs on the drive-side of the bottom bracket shell and allow direct fitment to the frame, other chain devices fit in between the bottom bracket and frame and require a specific bottom bracket with a larger drive-side cup, and there is also the type that clamp around the seat tube in place of the front mech. We also have Chain guards here also known as 'bash guards', these you stick to your bottom chainstay just below your chain to stop the chain bouncing on the chainstay and chipping the paint.