rotating a fourth driven spocket pinion 35 which is mounted onto the axle 24 of the third wheel 25 with a pneumatic tire for unitary rotation therewith it should be pointed out that the four pinions 30 32 33 and 35 all have the same diameter two lugs 36 see fig8 are fastened underneath one of the side members of the framework 18 namely the one which is located towards the train of gearing and at the level of the mid perpendicular of the spacing distance between the axles of the pneumatic tires to rotatably support small tightener or tensioning pinions 37 which are meshing internally with the chains 31 and 34 respectively the shafts of these tightener or tensioning pinions 37 are mounted within elongated adjusting openings 36a through the aforesaid supporting lugs 36 all the pinions and sprocket wheels may for example have a double toothing and the chains may be provided with double rollers the aforesaid train of gearing 27 to 37 could of course be replaced with any other suitable drive means the pneumatic control circuit for operating the motor 7 of the device of the invention will now be described the aforesaid pneumatic motor 26 may be mounted onto a supporting plate 38 secured crosswise or transversely onto the side members of the framework 18 the pneumatic motor 26 is connected by cables or fluid flow hose ducts 39 see fig6 to a pneumatic distribution control box 40 comprising a forward backward motion operating lever 40a and supported upon the upper subframe 6 by a transverse part 41 the control box 40 further comprises a leakage flow.