Vehicles which operate over adverse terrain have traditionally employed track drive systems. Track driven bulldozers, loaders, excavators, cranes and other relatively heavy industrial vehicles have been in use for decades, as have armored tanks and other track driven military vehicles. The tracks comprise a number of rigid tread elements pivotally interconnected in series to form bands which are supported for movement around predetermined courses on the sides of the vehicle. The tracks are in fixed lateral positions and propulsion is effected by movement of the tread elements. Steering is accomplished by differential drive of the tracks.
Track drive systems are advantageous in some respects and disadvantageous in several others. The relatively large and long contact area of the tracks enables a heavy vehicle to negotiate uneven terrain without difficulty. On the other hand, a power plant of considerable size is required to overcome the resistance generated by the large contact area of the tracks. Further, since the tracks are constructed of rigid tread elements, at least some surface damage occurs during operation of a track driven vehicle, and particularly during maneuvering of the vehicle in close quarters.
Track driven vehicles are thus more suitable for operation over rough terrain rather than over a paved or prepared surface. The inability to operate over a paved surface without causing undue damage thereto can restrict the usefulness and versatility of vehicles incorporating track drive systems. Such vehicles are typically specialized, complex and thus expensive, but limited somewhat in their utility due to the surface damage caused by track drive systems. Even when operated over smooth surfaces, the speeds attainable by such vehicles tend to be relatively slow due to the track drive systems.
More recently, wheeled vehicles capable of operation over both smooth and rough terrain have been developed. U.S. Pat. No. 3,799,362, entitled Six Wheel Material Handling Vehicle and assigned to the assignee hereof, illustrates a successful example of one such vehicle. Other vehicles incorporating multiple wheels and the principle of skid steering have been developed especially for use with particular mechanisms. In addition, multiple wheeled undercarriage assemblies employing lower middle wheels to facilitate skid steering have been developed. Heretofore, however, there has not been available a complete multi-wheeled undercarriage unit adapted to be interchanged with a track drive undercarriage to make a particular mechanism operable over smooth terrain with little or no damage thereto, as well as over adverse terrain.