Most conventional vehicles such as loaders or bulldozers are either driven by four wheels or by two tracks.  Typical track-propelled vehicles employ a pair of spaced steel or rubber endless tracks that are driven to move the vehicle over the ground.  Vehicles that are solely wheel-propelled typically include a pair of front wheels and a pair of rear wheels that are rotatably driven to propel the vehicle.  The wheels of the wheel-propelled vehicles are generally large and have tread designs that aid in moving the vehicle over sand, clay and mud.  These conventional vehicles, although capable of moving over terrain having a variety of soil conditions, frequently become stuck because all of the tractional forces and propelling surfaces are not always put to the ground. 
Conventional four wheel vehicles and conventional two track vehicles often cause environmental damage when used in natural areas.  Recently, environmental concerns have been raised about the disruption of the topsoil which occurs when conventional loader/bulldozer-type vehicles are operated on the topsoil, sand or other soft terrain of sensitive natural areas.  For example, in the tree harvesting industry, construction industry and/or the agricultural industry, the operation of conventional vehicles of the type described may cause significant damage to the topsoil, which in turn may result in the formation of ruts which may lead to soil erosion. 
U.S. Pat.  No.  5,615,748 discloses a vehicle that has a central track and a pair of flanking wheels disposed on opposing sides of the track.  The flanking wheels are mounted for pivotal movement about a pair of generally vertically extending axes. 
As discussed above, the advantage of a two-track tractor vehicle over a four-wheeled tractor vehicle is its traction and stability.  On the other hand, the advantage of a four-wheel tractor vehicle over a two-track tractor vehicle is in its ease of handling and maneuverability. 
To a considerable extent, the tractor vehicle of my '748 patent achieves the advantages of both two track and four wheeled tractor vehicles.  This is because it provides a central track for traction and stability and two outrigger wheels for ease of handling and maneuverability. 
The outrigger wheels of my '748 tractor vehicle were steerable wheels of the type having an upright steering axis and a steering mechanism capable of turning both wheels generally in unison about their steering axes to effect turning movement. 
It has been found that enhanced ease of handling and maneuverability can be achieved by utilizing outrigger wheels which are steered by changing the relative driving speed between the two outrigger wheels rather than by moving them in unison about upright steering axes.  Further enhancement can be obtained by mounting the outrigger wheels for vertical movement and utilizing hydraulic cylinders and a control system therefor to maintain them in ground contact. 
It has also been understood that the '748 tractor vehicle enhanced as aforesaid because it includes a central track and two outrigger wheels is uniquely set up to enable a substantial portion of the load support to be shifted between the central track and the outrigger wheels.  For example, if the hydraulic cylinders which keep the outrigger wheels in ground contact are adjusted so that a substantially low pressure condition exists, the central track will support most of the vehicle load on the ground.  As the pressure conditions in the hydraulic cylinders are increased, more and more load will be assumed by the outrigger wheels.  This substantial shift in load support occurs without any shifting of the load itself or any tilting of the frame.  In contrast, the only way load support can be shifted between the two tracks of a two-track tractor or the four wheels of a four-wheel tractor is to shift the load itself or tilt the frame.  It has been found this unique load support shifting capability made possible by the use of hydraulic cylinders to keep the independently driving outrigger wheels on opposite sides of the central track in ground contact, enables traction and stability to be enhanced while at the same time further enhancing the ease of handling and maneuverability of the tractor vehicle. 
In accordance with the principles of the present invention, the advantages discussed above are achieved by providing a vehicle comprising a main frame; an engine assembly constructed and arranged to generate power; and a driving track assembly supporting the main frame.  The driving track assembly includes an endless ground engaging track.  The driving track assembly is connected to the engine assembly in power supplying relation and is constructed and arranged to move the track in driving engagement with the ground using power from the engine assembly so as to move the vehicle along the ground.  A pair of secondary driving and steering assemblies are mounted on opposing lateral sides of the track assembly in flanking relation.  The secondary driving and steering assemblies each include a ground engaging driving and steering structure.  The ground engaging driving and steering structures and the track assembly are mounted within the vehicle so as to enable the track assembly and the driving and steering structures to be moved generally vertically relative to one another to shift a distribution of vehicle weight between the driving and steering structures and the track.  In the illustrated embodiments, the ground engaging driving and steering structures are moved downwardly relative to the driving track assembly, but the scope of this aspect of the invention is not limited in such a manner.  The secondary driving and steering assemblies are operatively connected to the engine assembly in power supplying relation and are constructed and arranged to move the ground engaging driving and steering structures in driving engagement with the ground using power from the engine assembly so as to assist the track assembly in moving the vehicle along the ground. 
The vehicle of this aspect of the invention further comprises power-operated structure operatively connected to the engine assembly in power supplying relation.  The power-operated structure is constructed and arranged to affect the generally vertical relative movement between the driving and steering structures and the track assembly using power from the engine assembly to shift the distribution of the vehicle weight.  An electronic controller is communicated to the power-operated structure and each of the secondary driving and steering assemblies.  A steering input device is communicated to the electronic controller.  The steering input device is operable to transmit a steering signal to the controller indicative of a selected angle at which the vehicle is to be steered.  The controller is operable to process the steering input signal and responsively control the secondary driving and steering assemblies and the power-operated structure to affect a vehicle steering operation wherein (1) the power-operated structure affects the generally vertical relative movement between the track assembly and the driving and steering structures so as to shift the distribution of vehicle weight from the track to the driving and steering structures, thereby providing the driving and steering structures with increased traction during movement thereof in driving engagement with the ground, and (2) the secondary driving assemblies move the driving and steering structures in driving engagement with the ground to thereby steer the vehicle at the selected angle.  In the preferred embodiment, the steering is affected by as a result of the movement imparted to the vehicle by one of the driving and steering structures being different than the movement imparted to the vehicle by the other of the driving and steering structures to thereby turn the vehicle at the selected angle; however, in the broadest aspects of the invention, steering may be affected by pivoting the driving and steering structures about generally vertical axes. 
A more preferred aspect of my invention provides a vehicle comprising a main frame; a driving track assembly mounted in supporting relation to the main frame, and a pair of secondary driving and steering assemblies mounted on opposing lateral sides of the track assembly in flanking relation.  The driving track assembly includes an endless ground engaging track that extends in a vehicle driving direction and a track operating motor constructed and arranged to move the track in driving relation to the ground.  Each of the secondary driving and steering assemblies comprises a vertically movable mounting structure constructed and arranged to be moved in a generally vertical direction relative to the main frame independently of the mounting structure of the other secondary driving and steering assembly; a ground engaging structure movable in driving and steering relation to the ground mounted on the mounting structure such that movement of the mounting structure in the generally vertical direction moves the ground engaging structure generally vertically relative to the main frame; a secondary operating motor mounted to the mounting structure for generally vertical movement therewith, the secondary operating motor being constructed and arranged to move the ground engaging structure in driving and steering relation to the ground; and a power-operated mounting structure mover constructed and arranged to move the mounting structure in the generally vertical direction so as to move the ground engaging structure thereon generally vertically to vary an amount of ground bearing pressure the ground engaging structure applies to the ground. 
An engine assembly is constructed and arranged to supply power to each of (1) the track operating motor of the track assembly, (2) the secondary operating motors of the secondary driving and steering assemblies, and (3) the mounting structure movers of the secondary driving and steering assemblies.  An electronic controller is operable to control (1) an amount of power supplied from the engine assembly to the track operating motor, (2) an amount of power supplied from the engine assembly to one of the secondary operating motors, (3) an amount of power supplied from the engine assembly to the other of the secondary operating motors, (4) an amount of power supplied from the engine assembly to one of the mounting structure movers, and (5) an amount of power supplied from the engine assembly to the other of the mounting structure movers.  The controller is operable to receive speed signals indicative of a selected speed at which the vehicle is to be driven and responsively control the amount of power supplied from the engine assembly to the track operating motor of the driving track assembly and the secondary operating motors so as to move the vehicle at the selected speed during the vehicle driving operation. 
A steering input device is communicated to the electronic controller.  The steering input device is operable to transmit a steering signal to the controller indicative of a selected angle at which the vehicle is to be turned.  The controller is operable to process the steering input signal and responsively control (1) the respective amounts of power supplied from the engine assembly to the secondary operating motors so as to affect a vehicle steering operation wherein the movement imparted to the vehicle by one of the ground engaging structures is different than the movement imparted to the vehicle by the other of the ground engaging structures to thereby turn the vehicle at the selected angle, and (2) the respective amounts of power supplied from the engine assembly to the mounting structure movers so as to affect a ground pressure varying operation wherein the ground bearing pressure applied by and the load support carried by the ground engaging structures is increased during the vehicle steering operation to thereby decrease the ground bearing pressure applied by and the load support carried by the track assembly. 
Other objects, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.