The tractor unit of an articulated, freight-carrying road vehicle has two rear axles, one (24) of which is driven by an engine (15) mounted rearwardly of the driven axle. The axis of the fifth wheel coupling (13) to the trailer is situated forwardly of the driven axle so that the centre of gravity of the tractor unit is in the vicinity of or is to the rear of the fifth wheel axis. The engine is connected at its rear end with a chassis (10) of the tractor unit and is supported by the rear axles so that the engine moves relative to the chassis when the rear axles move relative to the chassis.