14 but at the same time the end roll means 11 is free to assume a position determined by the front portion of the track means 20 which is guided around the end roll means 11 so that the front end roll means 11 itself need not necessarily turn through the same angle as that through which the connecting means 13 is turned about the axis determined by the pivot 14 as is schematically shown in fig2 and fragmentarily illustrated in fig1 in the central vertical plane occupied by the horizontal axis a a the endless track means 20 fixedly carries at its inner surface an elongated endless substantially non stretchable band or equivalent track reinforcement 24 this central non stretchable band 24 may be made of a suitable metal for example and is fixed with the inner surface of the endless track 20 a guide means 23 cooperates with the band 24 this guide means 23 includes a pair of lower rollers situated next to the lower inner surfce of the endless track 20 and engaging opposite sides of the band 24 as well as a pair of upper rollers 23 situated directly over the rollers 23 shown in fig2 and engaging the opposite sides of the band 24 next to the upper inner surface of the endless track means 20 the shafts which carry the rollers 23 are in turn fixedly carried by suitable rigid arms which are fixed to the frame means 10 in the manner schematically illustrated in fig1 and 2 thus rollers or wheels 23 serve.