This axle allows you to increase your wheel diameter without cutting your rails. Plus it puts two wheels next to the rails and keeps the weight inboard. You can go from a 6.375" wheel to an 8" wheel or 7.25" wheel to 9". For this axle to mount properly, you need an adjuster that pushes from the front and an axle slot that is 1" high. This will not fit the XP/G4 rails.