I'am not found of using an Idler. However they do work and take observing, meaning do study it!!!! 4000 miles on my first build, wore out three idlers so far fixing on my fourth. I didn't get mine to completly line up. It acts serves a teinsioner, and a derailer. Hope this helps. One more thing get a ball bearing one from Thatsdax, the stocks ones didn't impress me the bearing is nicer, and quiter last longer too! You do want to line the chain as best you can