side of yoke 46 so that reciprocation of the piston rod within the cylinder will effect oscillating movement of the yoke 46 once again as indicated by arrow 48 since the leading idler wheels 23 are journaled about axle 53 which extends through yoke 46 pivoting of yoke 46 by piston and cylinder arrangement 50 causes both of the idler wheels 23 to be turned accordingly such rotation of the idler wheel changes the stress exerted against the interior of belt 21 and thereby causes a minor change in the direction of travel of the tractor an alternative construction for enabling the adjustment of the idler and bogie wheels as an entire unit is shown in fig5 and 6 of the drawings a bearing support point is provided on the inner side of yoke 30 and suitably dependent from it two pairs of plates 41 wherein each pair straddles the bearing support point and bearing support 35 respectively are also provided the plates 41 connected to the bearing support point and the bearing support 35 that face bearing beam 40 are connected to bearing beam 40 by a pair of inwardly extending arms 42 in the same manner as shown in fig3 in this construction the steering means for altering the orientation of the idler wheels is comprised of a linear motor 60 that is secured onto the face of one of the plates 41 that are connected to bearing support 35 an eccentric shaft 61 with horizontal axis 65 extends through plates 41 that are connected to bearing support 35 the central portion of shaft 61 is contained within the ball socket 45 which has.