a tractor 1 in accordance with the present invention comprises an axle 2 shown in fig3 and 4 and thereto coupled wheels 3 equipped with hydraulic motors , a 5th wheel 4 coupled to axle 2 in a known manner through a cylinder 6 and possibly a bearing 5 , an auxiliary wheel 9 coupled to a supporting structure 10 through cylinder 11 , an engine space 8 , and a driver &# 39 ; s cab 7 . the cylinder 6 is fixed to the axle 2 in such a manner that it cannot rotate around the axle 2 . the engine space 8 accommodates the motor , a hydraulic pump , tanks and other devices and the driver &# 39 ; s cab 7 accommodates the controls of the tractor 1 , a bench etc . while the tractor is not coupled into a trailer to be transported or similar , the auxiliary wheel 9 is by means of the cylinder 11 pressed to a supporting position shown in fig4 for the transfer of the tractor 1 in an unloaded state . the auxiliary wheel 9 is of a so - called free rotating type seeking its direction according to the direction where the tractor 1 moves . the 5th wheel 4 may be equipped with the bearing 5 at the point where it is connected to the cylinder 6 . in fig1 a known transfer device 12 has been attached to the tractor 1 through the 5th wheel 4 . the transport device 12 is in a lowered position . by adjusting the cylinder 6 conveniently the transport device 12 can be lifted as shown in fig2 . the transfer device 12 may be e . g . a lifting wagon for the handling of platforms with legs or it may be a semitrailer or some other device equipped with a known standardized kingpin 13 . by regulating the hydraulic motors of the wheels 3 conveniently it is possible to make the tractor 1 turn in relation to the kingpin 13 and thus steer the combination of the tractor and the transfer device 1 - 12 . fig5 illustrates a cross - sectional view of the coupling of the tractor 1 and the transfer device 12 shown in fig1 and 2 . the 5th wheel 4 comprises a known aperture 17 or similar , through which the kingpin 13 is guidely directed into the space inside the 5th wheel where there is a fork 14 operated by cylinders 15 and 16 . cylinders 15 and 16 are arranged to move the fork in a horizontal direction and vertical direction , respectively , so that the loop of the fork 14 is capable of gripping the kingpin 13 . the fork 14 is used to grip the kingpin 13 in a known manner . the kingpin 13 is by means of the cylinder 16 pressed into contact with the 5th wheel 4 . by choosing the force of the cylinder 16 appropriately it is possible to eliminate a substantial part of the forces directed towards the kingpin coupling in a driving situation . if the pulling force of the cylinder 16 is chosen to be equal to the force with which the transfer device 12 bears against the 5th wheel 4 , all inner forces produced by acceleration or braking at the coupling point will theoretically be eliminated . the 5th wheel 4 may be equipped with such known sensors by means of which a force concerned at any given time can be determined , and on the basis of this information the force of the cylinder 16 is adjusted to a desired level by means of known devices and methods . the pressing of the 5th wheel 4 against the surroundings of the kingpin 13 of the transfer device 12 upon pulling by means of the cylinder 16 by the kingpin 13 will substantially increase friction at the connecting point . to ensure a smooth turning the joint between the 5th wheel 4 and cylinder 6 may be equipped with a bearing 5 having considerably greater dimensions than a conventional 5th wheel and kingpin combination and thus a better transmission capacity . it is possible to grip the kingpin 13 in many other ways , too , and not only by means of a fork but also by such known ways of gripping in which the total gripping area of the kingpin 13 is utilized . in addition to the method of pulling directly by means of the cylinder 16 the force acting on the kingpin 13 may also be produced by using different lever and other mechanisms . the 5th wheel 4 may , in addition to the bearing 5 , also be equipped with a known spring that always turns an unstressed 5th wheel 4 to the position shown in fig3 . on the other hand , the 5th wheel 4 may also be of such construction that the kingpin 13 is coupled substantially in vertical direction and not so much guidedly on lateral level as conventionally . in this case the upper surface of the 5th wheel 4 is rather a level disc having an aperture fit for the kingpin 13 and possibly slightly conic guiding surfaces in the center . in any case it is to be understood that the invention has above been described with reference to a few preferred embodiments only , however , not wanting to restrict the applications of the invention to these embodiments only , several modifications being possible within the scope of the inventive idea as defined by the following claims .