Abstract: A semi-trailer is connected to an agricultural-type tractor by means of a coupling dolly. The coupling dolly has a chassis which is supported by wheels at its rear end and is connected to a weight transfer hitch at its front end. The semi-trailer is connected to the dolly by means of a universal coupling supported between the ends of the dolly chassis and having mutually intersecting rearwardly disposed pitching, rolling and yawing axes. The weight transfer hitch is pivotally connected to the tractor for articulation about a horizontal forwardly disposed pitching axis extending transversely of the tractor, between the axes of the front and rear wheels of the tractor and below the level of the axis of the rear wheels of the tractor.