Tractor with rear castor wheels

An agricultural tractor comprising:

• a tractor frame for movement in a direction of forward working movement of the tractor across a field;
• the tractor frame having support members at a forward end of the frame, in respect of the direction of working movement, arranged for attachment to an implement to be moved across the ground with the implement supported across and in front of the forward end;
• a pair of first ground wheels mounted on the frame at the forward end of the frame at positions spaced transversely of the frame;
• the first ground wheels being mounted on the frame at fixed angles parallel to each other and parallel to a center line of the frame;
• a pair of second ground wheels mounted on the frame at a rearward end of the frame at positions spaced transversely of the frame;
• each of the second ground wheels being mounted on a respective castor assembly providing, for the respective wheel:
• a transverse axle of the wheel,
• a pivot member defining a vertical pivot axis,
• and a mounting link interconnecting the pivot member and the axle such that the axle is located below the pivot member and, in respect of a the forward direction of movement, in a plane radial to the vertical pivot axis and rearwardly of the vertical pivot axis;
• the pivot member being arranged to allow rotation of the mounting link around the pivot axis so that the tractor can move in the direction of forward working movement with the first wheels forward and the second wheels trailing and in a second direction of movement opposite to the direction of forward working movement with the second wheels forward and the first wheels trailing;
• each of the first ground wheels being driven by a respective drive motor which allows variable speed in bath the first and second directions such that steering of the tractor is effected by a differential in speed between the first wheels with the second wheels following the steering in a castoring action;
• and a pair of damper cylinders each connected between the frame and a respective one of a pair of levers where each lever is arranged at a respective one of the castor assemblies so as to provide a damping force tending to restrict rotation of the respective second wheel about the respective vertical pivot axis;
• the damper cylinders and the levers being arranged such that the damping force for each castor assembly varies at different angles around the vertical pivot axis due to changes in mechanical advantage as the lever pivots around the vertical pivot axis and is at a maximum value when the second wheels are parallel to the center line and the tractor is moving in the direction of forward working movement and is at a maximum value when the second wheels are parallel to the center line and the tractor is moving in the opposite direction;
• the frame including a rear axle on which the second ground wheels are mounted;
• the rear axle including a center portion and two adjustment portions on which the second ground wheels are mounted with the adjustment portions being adjustable relative to the center portion to change spacing between the second ground wheels;
• each of the damper cylinders being attached to a bracket which is carried on the center portion of the rear axle and is movable relative thereto to accommodate adjustment of the respective adjustment portion.