Abstract: A semi-trailer includes a self-steering rear axle assembly and a stationary axle assembly located at least 10 feet in front of the self-steering axle assembly and at least the minimum distance from the tractor front axle. The wheels on the self-steering axle assembly are carried on king pin assemblies which are movable between forward and reverse positions so that the self-steering axle wheels track the stationary axle wheels during turning in both the forward and reverse directions. The self-steering axle assembly preferably is retractable so the wheels can be raised when not needed. In one embodiment, the tractor has only a front axle and a single drive axle and all axles have a load rating of at least one-fourth of the load rating of the semi-trailer truck unit so that a maximum gross load can be hauled even though the unit has only four axles.