Rims generally need to be centered over the hub*. Our dishing gauge allows you to verify that it is, and if it's not, tells you how far out it is. The donuts on either end allow you to use the tool with a tire mounted on the rim. Fit wheels from 16" to 29" *Some rim & hub combinations, on some bikes, are purposely dished to one side. It's rare, and if it's something you need to be concerned about you're likely already aware of it.