Abstract: A trailing unit is pivotally supported by a tractor or leading unit at the front and a sub-chassis at the rear. Pivoting of the sub-chassis is controlled in accordance with turning of the leading unit so that the angle between sub-chassis and trailing unit is equal and opposite to the angle between leading unit and trailing unit. Power is communicated between leading unit and sub-chassis by a front shaft on the leading unit, a rear shaft on the sub-chassis, and an intermediate shaft connected to them by universal joints at positions equidistant from the pivot axes of the leading unit and sub-chassis.