Abstract: Heavy load-carrying vehicles such as concrete trucks occasionally require the temporary addition of extra load-carrying wheels in order to reduce load concentrations transmitted to highways to within allowable limits as set by statute and/or regulatory agencies in various states. This invention is directed to a load-carrying unit adapted for attachment to the frame of a heavy load-carrying vehicle such as a concrete truck or the like for mounting outside of both sides of the frame thereof for movement between retracted and load-sharing positions. The load-carrying unit includes first and second wheel assemblies which are movable between the retracted and load-sharing positions independently of each other, each wheel assembly including wheels mounted for rolling engagement with the highway and for rotation about an approximately vertical axis located forwardly of the wheels.