You can run disc brakes on the stock axle or the narrow version. If you want the axle narrowed it's personal preferance. Some disc kits widen the front end 1/2" per side. I offer a narrowed axle only if the guy wants to keep his front end the stock width with disc brakes. Also some guys are wanting to run wheels with more offset to the out side and like the wheels pulled in a little. I sell mostly the stock width axles. They seem to be most popular