Abstract: A semi-trailer construction including forward coupling structure for universal coupling and support from a towing vehicle, a rear portion from which wheeled running gear is supported and an intermediate portion dependingly supporting supplemental wheeled running gear is provided. The supplemental wheeled running gear includes a mounting plate supported beneath the semi-trailer frame for oscillation relative thereto about a vertical axis and horizontal shifting relative to the frame along a predetermined path extending through the axis of oscillation of the mounting plate relative to the frame. The mounting plate supports a transverse axle assembly therefrom having support wheels journaled thereon and the axle assembly is disposed generally normal to the aforementioned path.