Jockey
Wheels

A jockey wheel is known under a couple of different names, including a guide pulley. A jockey wheel is in charge of gear shifting on a bike, and guides the chain when the gears are shifted via the derailleur. There are many reasons to buy a new jockey wheel, it may be that the jockey wheel has been snapped or broken in a crash, and is being replaced alongside other transmission components - or it may be that the jockey wheel needs replacing as it is worn out, the most obvious way to tell that a jockey wheel is ready for replacement, is sharp or pointy teeth.