Patent Publication Number: US-6220377-B1

Title: Load support shifting vehicle

Description:
The present application claims priority from and is a continuation application of Lansberry, U.S. patent application Ser. No. 09/292,673, filed Apr. 14, 1999, now U.S. Pat. No. 6,044,921, and also claims priority to Lansberry U.S. Provisional Patent Appln. Ser. No. 60/082,184, filed Apr. 17, 1998, the entirety of each of which are hereby incorporated into the present application by reference. 
     The present application claims priority to U.S. Provisional Application of Lansberry, Ser. No. 60/082,184, filed Apr. 17, 1998, the entirety of which is hereby incorporated into the present application by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to vehicles and, more particularly, to a vehicle for use on a wide range of terrain, including uneven and/or steep terrain having a variety of soil conditions. The vehicle of the present invention offers many advantages over conventional vehicles and can replace conventional vehicles in performing a variety of tasks. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     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 &#39;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 &#39;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 &#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary schematic side elevational view of a vehicle embodying the principles of the present invention, showing a wheel assembly comprising a central pair of wheels and a side portion of a track assembly in fragmentary view to illustrate a vehicle power source mounted within the track assembly; 
     FIG. 2 is schematic top plan view of the vehicle of FIG. 1 showing the pair of wheels of the wheel assembly in flanking relation to the track assembly; 
     FIG. 3 is a schematic view of a portion of a second embodiment of the wheel assembly showing a half axle thereof pivotally mounted to a main frame structure of the track assembly shown in fragmentary view; 
     FIG. 4 is a view similar to FIG. 3 showing a half axle of a third embodiment of the wheel assembly pivotally mounted to a main frame structure of the track assembly shown in fragmentary view; 
     FIG. 5 is a schematic front view of a mechanical linkage suspension system for the wheel assembly mounted to the main frame structure shown in fragmentary view; 
     FIG. 6 is a view similar to FIG. 5 showing a torsion bar suspension system for the wheel assembly mounted to the main frame structure shown in fragmentary view; 
     FIG. 7 is an isolated perspective view of a portion of a wheel assembly showing a transaxle thereof and a plurality of coil spring members of a coil spring suspension system mounted thereto; 
     FIG. 8 is a view similar to FIG. 7 showing a plurality of leaf spring members of a leaf spring suspension system mounted to the wheel assembly; 
     FIG. 9 is a view similar to FIG. 7 showing a plurality of fluid filled cylinders of a suspension system mounted to the wheel assembly; 
     FIG. 10 is a perspective view of a rigid axle assembly of the vehicle; 
     FIG. 11 is a schematic view of a side of the track assembly showing a block therein which indicates a general area in which a repositionable subframe member may be mounted to provide a movable support structure for an axle assembly of the wheel assembly; 
     FIG. 12 is a schematic top plan view of an embodiment of the vehicle which incorporates a plurality of telescoping axle members in the wheel assembly thereof showing portions of the wheel assembly in phantom lines to reveal internal portions thereof; 
     FIG. 13 is an enlarged sectional view of a portion of a wheel assembly which incorporates telescoping axle members showing a portion of the main frame structure in fragmentary view; 
     FIG. 14 is a schematic illustration of a first portion of an electronically controlled hydraulic circuit of the vehicle of the invention for driving the wheels and central track thereof; 
     FIG. 15 is a schematic illustration of a second portion of the electronically controlled hydraulic circuit of FIG. 14 of the vehicle of the invention for controlling steering angle and wheel ground pressure; 
     FIG. 16 is a schematic illustration of a third portion of the electronically controlled hydraulic circuit of FIG. 14 of the vehicle of the invention showing stack valve sets for front and rear attachments; 
     FIG. 17 is a schematic illustration of a fourth portion of the electronically controlled hydraulic circuit of FIG. 14 of the vehicle of the invention for oil conditioning and for powering attachments; 
     FIG. 18 is a schematic representation of a first portion of an alternative embodiment of an electronically controlled hydraulic circuit for the vehicle showing a portion of a locomotion circuit for powering the flanking wheels of the vehicle; 
     FIG. 19 is a schematic representation of a second portion of the alternative embodiment of the electronically controlled hydraulic circuit of FIG. 18 showing a steering and ground pressure control circuit; 
     FIG. 20 is a schematic representation of a third portion of the alternative embodiment of the electronically controlled hydraulic circuit of FIG. 18 showing an attachment circuit; 
     FIG. 21 is a schematic representation of a fourth portion of the alternative embodiment of the electronically controlled hydraulic circuit of FIG. 18 showing a cooling and filtration circuit; 
     FIG. 22 is a fragmentary schematic front elevational view of a vehicle showing portions of the axle assembly mounted to a center wheel drive motor assembly shown in exploded view mounted to a wheel; 
     FIG. 23 is a view similar to FIG. 22 showing the axle assembly mounted to the wheel with an off-set gear drive assembly; 
     FIG. 24 is a fragmentary schematic front elevational view of a portion of the carrier vehicle showing various alternative positions in which an axle member thereof could be pivotally mounted to the main frame structure of the vehicle, including a first position shown in solid lines and second and third positions shown in phantom lines; 
     FIG. 25 is a fragmentary schematic front elevational view of a portion of the vehicle showing an example of a height adjustable axle assembly; 
     FIG. 26 is a schematic top plan view of the track assembly and the wheel assembly showing two alternative drive assemblies for the track and wheel assemblies including a chain or belt driven drive assembly and a gear driven assembly; 
     FIG. 27 is a schematic view of the track assembly and the wheel assembly showing the wheel assembly mounted to a central drive shaft of the track assembly, 
     FIG. 28 is a schematic front elevational view of the track assembly and wheel assembly of FIG. 27; 
     FIG. 29 is a schematic view of the track assembly and a flanking track assembly mounted to a central portion of the track assembly; 
     FIG. 30 is a schematic side elevational view of a portion of an embodiment of the vehicle showing a centrally positioned operator compartment and illustrating a plurality of alternative points of attachment for a wheel assembly; 
     FIG. 31 is a schematic illustration of the vehicle of the invention having a vertically movable axle assembly; 
     FIG. 32 is a schematic side elevational view of a tandem vehicle embodying the principles the present invention; 
     FIG. 33 is a front schematic view of the vehicle on a sloped surface showing the wheel assembly configured and positioned to provide lateral support therefor 
     FIG. 34 is a schematic side elevational view of an alternative embodiment of the vehicle showing a track drive drum, a wheel drive motor, a drive shaft and an axle housing in phantom lines; 
     FIG. 35 is a schematic front elevational view of the vehicle of FIG. 34 showing a track drive motor, a plurality of wheel drive motors and a plurality of drive shafts in phantom lines. 
     FIG. 36 is a partial top view showing an alternative axle assembly with an arm extending generally rearwardly along side the track; and 
     FIG. 37 is a partial side view of the alternative axle assembly of FIG.  36 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring to the drawings, a vehicle, generally designated by the reference numeral  10 , is shown therein embodying the principles of the present invention. The vehicle  10  includes a main frame structure, generally indicated at  12 , in the form of a vehicle chassis, having fore and aft end portions,  14  and  16 , respectively. In the illustrated embodiment, an operator compartment  18  is mounted to the fore end portion  14  of the frame structure  12  and a winch assembly, generally indicated at  20 , is mounted to the aft end  16 . 
     The vehicle  10  includes a driving track assembly, generally indicated at  22 , mounted to the frame structure  12 . The driving track assembly  22  may comprise one or more endless track belts. In the illustrated embodiment, the driving track assembly  22  is in the form of a single, centrally mounted track unit having a conventional ground engaging endless track or belt  26  extending in the longitudinal direction. The track assembly  22  may be of conventional configuration, such as the Trackman, manufactured by Goodyear Rubber and Tires may be employed. 
     Both a two wheel rear drive track assembly and an all wheel drive track assembly are within the scope of the invention. It is preferred that the track assembly be powered and rotated by a conventional lug and sprocket-type arrangement in which a plurality of track wheel sprockets engage lugs on the track or belt. Alternatively, the track assembly may be of the friction-type in which the track wheels of the track assembly frictionally engage the track or belt. 
     The track  26  may be a rubber track, a steel track or may be constructed using various resilient, elastomeric composite or synthetic materials, but rubber is preferred. The bounds or inner periphery of the track  26  defines an interior space or envelope  28  which will be described below in further detail. The track  26  may have various profiles, as shown by the track assemblies  22 ,  822 ,  922 ,  1022  and  1322  in the exemplary embodiments of the vehicle  10  depicted in FIGS. 1,  27 ,  30 ,  31  and  34 , respectively. 
     In the embodiment illustrated in FIG. 1, a plurality of hydraulic motors are included in the vehicle  10 , including at least one central hydraulic motor, generally designated MA (shown schematically in FIG.  14 ), that drives the track  26  of the driving track assembly  22 . A single motor MA, for example, can be mechanically coupled to a forward drive wheel  23  of the track assembly  22  or to a rear drive wheel  25  of the track assembly. Likewise, both wheels  23 ,  25  can be provided with a motor MA for bi-directionally rotating the track  26  with respect to the main frame structure  12 . The motor MA is conventional and can represent, for example, a Heavy Duty XL-1120cc radial piston motor, with hub mount, manufactured by Rotary Power. Preferably, however, the motor MA is a closed loop hydraulic motor such as a model MT50 hydraulic motor manufactured by Poclain. This Poclain motor has a 120 cubic inch/revolution displacement and generates a maximum hydraulic pressure of 6500 p.s.i. Each hydraulic motor in the vehicle, including motor MA, can be of the fixed displacement-type and capable of “free wheeling”, unpowered operation. 
     It is within the contemplation of the invention to employ a conventional mechanical transmission to drive the track  26  in lieu of hydraulic power. 
     FIG. 1 also shows that a pair of secondary driving and steering assemblies, generally designated  35 , is mounted centrally on opposing lateral sides of the track assembly  22 . The secondary driving and steering assembly  35  is shown in fragmentary view therein to show an internal combustion engine  52  that is described hereinbelow. 
     FIG. 2 shows a top plan view of the vehicle  10  of FIG.  1 . It can be appreciated that each secondary driving and steering assembly  35  includes a ground engaging structure in the form of a rotatable wheel  36   a  and  36   b,  respectively. The wheels flank the track assembly  22 . Wheels  36   a,    36   b  are the preferred ground engaging structures, although a set of smaller tracks may be used as shown in FIG.  29 . The wheels  36  are rotatably mounted on an axle assembly, generally designated  38 . The axle assembly  38  is comprised of two axially aligned, independent first and second axle structures, generally designated  32  and  34 , respectively. As set forth in detail below, it is contemplated to mount the axle assembly  38  in various ways to the track assembly  22  and in various positions with respect to the track assembly  22 , but in the embodiment of the vehicle  10  shown in FIG. 2, axle assembly  38  of the wheel assembly  35  is mounted in fixed relation with respect to and in the general center of the frame structure  12 . 
     The axle assembly  38  has opposing ends  37  and  41 , respectively, that are disposed generally transverse to the longitudinal extent of the track  26  extending outside of the envelope  28  and adjacent the track  26 . Each axle structure  32 ,  34  includes an axle housing designated  27  and  29 , respectively. It can be appreciated from the phantom representation of a central portion of the axle assembly  38  in FIG. 2 that each axle structure  32 ,  34  is secured to a central portion of the main frame structure  12  which is also represented in phantom. Each wheel  36  is rotatably coupled to a respective end  37 ,  41  of the axle assembly  38  and each is provided with a hydraulic motor MB and MC, respectively, each of which is enclosed within a respective housing  27 ,  29  and engages a hub structure at the free ends  37 ,  41  of the axle assembly through a half axle drive shaft  43  to affect the independent, bi-directional rotation of each wheel  36 . 
     As shown in the side elevation view of FIG. 1, the wheels  36  are slightly below the ground engagement portion  49  of the track  26  so that each wheel  36  engages the ground surface (not shown) at a position slightly lower than does the track  26 . Because of this lower relative positioning, the wheels are deeper in the ground when the vehicle is traveling on a soft surface as, for example, on topsoil or snow, than the track  26 . The functional advantage of the deeper positioning of the wheels than the track  26  with respect to the ground surface will become apparent below. It will also be explained below that it is within the scope of the invention to provide each wheel  36  with the ability to be horizontally and/or vertically repositioned with respect to the main frame structure  12 . This repositioning of the wheels of the wheel assembly  35  can be effected by a suspension mechanism, a powered mechanism or a combination of both. 
     The wheels  36  preferably include rubber tires. However, it can be appreciated that steel wheels can also be employed. Thus, it is within the contemplation of the invention to employ a rubber track and rubber tires, or a steel track and steel wheels, or rubber tires and a steel track, or steel wheels and a rubber track. The material used for the track and wheels/tires may be selected based upon the particular vehicle application. 
     In the illustrated embodiment, a separate and independent hydraulic motor, generally designated MB and MC, respectively, in FIG. 2, is included respectively within each axle structure  32 ,  34  to power each wheel and to affect its independent, bidirectional rotation. The motors MB, MC are preferably identical and conventional, and each can be, for example, a Series 90 75cc 2-speed motor manufactured by Sauer Sundstrand Company of Ames, Iowa. The motors MB, MC can be variable displacement-type motors and can be operated at either maximum or minimum displacement as determined by the operator. Preferably, each motor MB, MC is a closed loop hydraulic motor, the preferred closed loop motor being a Series 90 hydraulic motor manufactured by the Ross Company. These Ross Series 90 motors have a 17 cubic inch/revolution displacement and generate a maximum fluid pressure of 3000 p.s.i. These motors are currently available from the Surplus Center of Lincoln, Nebr. as item number 9-1894-18. 
     Although two motors MB, MC are provided in the wheel assembly  35  of the embodiment of the vehicle  10  illustrated in FIGS. 1 and 2, it is within the scope of the invention to provide the appropriate number of hydraulic motor(s) to operate the particular axle assembly selected to affect the independent rotation of each wheel  36 . 
     Although only a single pair of wheels  36  is provided in the illustrated embodiment, it is within the scope of the invention to provide the track assembly  22  with two or more pairs of wheels  36  on separate and operationally independent axle assemblies  38 ; for example, a front pair of wheels and a rear pair of wheels could be included in such arrangement as to provide both driving and steering functions. 
     It will also become apparent that it is within the scope of the invention to mount the wheel assembly on the track assembly  22  at the fore and/or aft ends as shown in phantom in FIG.  1 . 
     As will be described hereinbelow, motors MA, MB, MC are powered by a central engine assembly and are generally controlled by a control assembly  42  located in the operator compartment  18 . Control of the operation of the vehicle  10 , including speed, steering and operation of the winch and other attachments (not shown), is affected through the control assembly  42 . The control assembly  42  is in electrical communication with a conventional electronic control module  44  (shown schematically in FIGS. 14-15, but not shown in FIG.  1 ). 
     When the operator manipulates the control assembly  42  electronic signals are sent to the control module  44  as inputs. The control module  44 , in response, sends command signals to appropriate hardware described herein to control the operation of the hydraulic motors, including motors MA, MB, MC. 
     As will be discussed below when the operation of the vehicle is considered, the vehicle is turned by rotating the wheels  36  at different rates of rotation by driving motors MB and MC at different rates relative to one another. In order to turn to the right, the left wheel  36   b  is rotated at a relatively faster rate than the right wheel  36   a  Likewise, turning to the left is accomplished by rotating the right wheel  36   a  at a relatively faster rate than the left wheel  36   b.  This steering is achieved by the manipulation of a steering assembly  40  located in the operator compartment that includes a steering wheel  45  mounted on a steering wheel shaft (not shown). 
     A rotary potentiometer is connected to the steering wheel shaft to communicate to the control module  44  (shown schematically in FIG. 14) the desired direction and turning radius of a turn. It can be appreciated, therefore, that there is no direct mechanical connection between the wheels  36  of the wheel assembly  35  and the steering wheel shaft. Any conventional rotary potentiometer can be used on the steering wheel to communicate the steering direction and turn radius to the control module. The potentiometer can be, for example, a 50,000 kΩ 0.5% Helipot potentiometer. As will become apparent, when the steering wheel is turned, the potentiometer indicates to the control module  44  the desired direction and magnitude of the turn. The control module  44  sends electrical signals to individual pumps associated with the wheel motors MB, MC that drive the wheels  36   a,    36   b  to cause different wheel rotational speeds in the wheels  36   a,    36   b.  Using an electronic control module like control module  44  to independently control hydraulic motor speed MA, MB, MC is conventional and well known to one skilled in the art. 
     A power drive structure or engine assembly, generally indicated at  50 , is provided for the vehicle  10  and includes the power operated equipment necessary to drive the wheels  36  and the track  26 . The engine assembly  50  is best seen in FIG.  1 . In the illustrated embodiment, the engine assembly  50  includes a power unit, generally indicated at  51 , comprised of an internal combustion engine  52 , a pump drive gear box  54  coupled to the engine  52  and a plurality of hydraulic transmission pumps including pumps  55 ,  56 ,  57  and  234  (shown schematically in FIGS. 15-16) coupled to the gear box  54 . In the embodiment shown in FIGS. 1 and 2, the power unit  51  is mounted within the envelope  28 . It is to be understood, however, that this is exemplary only and that it is within the scope of the invention to mount the power unit  51  outside of the envelope  28 . The power unit  51  could be mounted, for example above the track assembly  22 , behind the track assembly, or in front of the track assembly. It can be appreciated, however, that mounting power unit  51  within the envelope  28  results in the vehicle generally having a lower and more centrally positioned center of gravity. As will be explained, when the power unit  51  is located within the envelope  28  it can be movably mounted for repositioning relative to the main frame structure  12 . This movable mounting makes it possible to reposition the center of gravity of the vehicle, which may improve vehicle maneuverability in rugged terrain or may facilitate turning of the vehicle. The wheel assembly can also be movably mounted on the same platform within the envelope  28  as will be considered hereinbelow. When the power unit  51  is mounted outside of the envelope  28 , the area within the envelope can be used for other purposes. For example, when the vehicle is used for harvesting crops, the power unit  51  can be mounted outside the envelope and a removable container or hopper can be mounted therein. In this configuration, the power unit  51  may be mounted above the envelope  28  so that the hopper can be mounted therein and a wheeled implement or attachment, such as a mechanical harvesting device, could be attached behind and pulled by the vehicle. 
     The internal combustion engine  52  drives the hydraulic pumps  55 ,  56 , and  57  that are fluidly connected, respectively, to the track hydraulic motor MA to drive the track  26 , and to the two hydraulic motors MB and MC of the axle assembly  38  to drive the wheels  36 . 
     The internal combustion engine  52 , the hydraulic pumps  55 ,  56 ,  57  and the gear box  54  may be conventional, commercially available parts. The engine  52  may be a conventional Model 4045T PowerTech 4.5L engine manufactured by Deere Power Systems of Waterloo, Iowa. The pump gear box  54  which is mounted to the engine  52  may be a Series 28000 Double 6 inch type, manufactured by Funk Manufacturing Company. The hydraulic pumps may be a Series 90, 4.5 cu in/rev displacement hydrostatic pumps, manufactured by Sauer Sundstrand Company. Each pump  55 ,  56 ,  57  is capable of 6500 p.s.i. maximum fluid pressure and each includes an integral charge pump and cross-over relief valves. The gear box  54  is powered by the engine  52  and operates a plurality of pumps, including pumps  55 ,  56  and  57 . As will be explained and as one skilled in the art will understand, the vehicle  10  is driven by the hydrostatic pumps  55 ,  56 , and  57  delivering hydraulic fluid, such as oil, to the motors MA, MB and MC. 
     As noted above, it can be appreciated that in lieu of or in combination with one or more of the hydrostatic pumps  55 ,  56  and  57  and the hydraulic motors MA, MB, MC, the power drive structure  50  may include a mechanical mechanism such as a mechanical transmission to facilitate driving the track  26  and/or the wheels  36 . Also, it is contemplated that the hydraulic motors MA, MB, and MC may be driven from a single hydrostatic pump. 
     The axle assembly  38  of the wheel assembly  35  of the embodiment shown in FIGS. 1 and 2 is rigidly secured to the main frame structure  12  of the track assembly  22  and the wheels  36 , thereof, are rotatably but non-pivotally attached thereto. It is within the scope of the invention, however, to provide an axle assembly having pivotally attached steerable wheels, to provide a wide range of axle assembly structures and to provide a wide variety of ways of mounting these various axle structures to the main frame structure  12  of the track assembly  22 . 
     An example of the use of an axle assembly with pivotally attached, steerable wheels is disclosed in U.S. Pat. No. 5,615,748, issued Apr. 1, 1997 to the present inventor, which is hereby incorporated into the present application by reference in its entirety. An example of non-pivotally attached, rotatable wheels is shown in FIGS. 1 and 2 herein as noted above. 
     The axle assemblies contemplated include an axle assembly which has a single drive shaft that extends through the envelope  28  of the track assembly  22  (referred to as a single axle), a pair of half-shaft suspensioned or powered axles (referred to as a half-shaft axles) or a transaxle having two independent drive shafts which are engaged in a well known manner with a central gear box. The gear box allows differential rates of wheel rotation in a known manner. 
     Each of these axle types can be mounted to the main frame structure  12  of the vehicle  10  with various suspension systems that use, for example, coil springs, leaf springs, fluid filled cylinders or similar spring-like structures to provide improved control when maneuvering the vehicle  10  and improved traction between the wheels and the ground surface. Additionally or alternatively, each of these axle types can be mounted to the main frame structure  12  using various powered supports so that the axle assembly  38  can be repositioned within the envelope  28  to reposition the wheels for various applications of the vehicle  10 , examples of which are considered hereinbelow. 
     Each axle type can, for example, be provided with a vertical axle moving device in the form of a hydraulic mechanism mounted between the axle structure and the main frame structure  12  of the track assembly  22  which hydraulic mechanism, when actuated vertically, repositions the axle structure with respect to the track assembly  22 . 
     The advantages of repositionable wheels can be appreciated from consideration of the functions performed by the wheels  36  of the wheel assembly  35 . The wheels generally provide lateral support, steering ability and propulsive force for the vehicle  10 . In the illustrated embodiment of FIGS.  1  and  2 , the wheels  36  are disposed at a position below the ground surface engagement portion  49  of the track  26  so that the wheels  36  will engage the ground at positions slightly deeper than the engagement portion  49  of the track  26 . This lower position improves wheel ground contact and therefore wheel traction and steering. 
     Wheel contact with the ground, especially on an uneven or soft ground surface, can be improved by providing a suspension system that provides an axle biasing device to bias the axle assembly downwardly toward the ground or by a providing a powered axle moving device to reposition the wheels downwardly toward the ground to maintain ground contact even when the track assembly  22  is at an angle relative to the ground surface or the ground surface is uneven. Because the vehicle  10  frequently operates on uneven terrain or soft ground, it is desirable to provide further embodiments of the vehicle  10  that incorporate secondary driving or wheel assemblies that pivot, wheel assemblies that include suspension systems, secondary driving or wheel assemblies that move with respect to the track assembly  22  or various combinations thereof. 
     FIGS. 3-11 show exemplary alternative embodiments of some of the axle assemblies and various suspension systems or powered supports that can be used. These embodiments are intended to show some of the axle types, suspension mountings and wheel hub mountings that can be used in a secondary driving assembly, but it is understood by one skilled in the art that all possible combinations taught by these representative embodiments are within the scope of the present invention and are not shown because they are too numerous and would be readily apparent to one skilled in the art from examination of the illustrated embodiments. 
     FIG. 3 shows an example of an independent wheel drive system, generally designated  133 , that can be used to provide independent powered vertical movement of the wheels of the secondary driving assemblies  135  with respect to the main frame structure  12 . The main frame structure is shown in fragmentary view and the tires and rims which are normally mounted on the hubs  131  are not shown in FIG. 3 to more clearly show portions of the system. Although only one axle assembly  132  is shown in FIG. 3, it can be appreciated that each secondary driving assembly  135  includes an axle assembly pivotally mounted to the tractor frame  12 . The axle assembly on the opposite side of the track assembly  22  is a mirror image of the axle assembly  132  shown therein. 
     The axle assembly  132  is pivotally mounted on a rod member  142  that is rigidly mounted between two pivot ear members  136  rigidly attached to the main frame structure  12 . A vertical axle moving device in the form of a conventional hydraulic cylinder  146  has a portion pivotally attached to the axle assembly  132  and a portion attached to an upper portion of the main frame structure  12  (not shown in FIG.  3 ). The hydraulic cylinder  146  is fluidly communicated with an hydraulic pump  234  (shown schematically in FIGS. 15-16) which is operatively connected to the gearbox  54  of the power unit  51 . The hydraulic pump supplies pressurized fluid to the cylinder  146  in order to extend the cylinder and pivot the axle assembly  132  about the mounting rod member  142  to lower the vertical position of the wheel  36  associated therewith with respect to the ground engagement portion  49  of the track  26 . The cylinder  146  also retracts to raise the wheel  36  associated therewith. 
     Each axle assembly  132  of the vehicle  10  pivots independently of the other axle assembly  132  (and of the other axle assemblies  132  if more than two are mounted to the track assembly  222 ). A hydraulic motor such as motor MB or MC is provided within the housing of the axle assembly  132  to independently bi-directionally rotate its associated wheel  36 . 
     It is within the scope of the present invention for the independent hydraulic powered pivoting of each axle structure to be under the control of a human operator of the vehicle  10  or, alternatively, to be controlled by appropriate electrical and logic circuitry which may be included in the vehicle  10  for this purpose to control the pivoting of the wheels automatically in response to, for example, the rotation of the steering wheel  45  to steer the vehicle or a sensing of uneven ground conditions. 
     Alternatively, the cylinder  146  could be an axle biasing device in the form of a conventional suspension cylinder that provides damping and a spring force directed downwardly on the axle assembly  132  to thereby pivot the associated wheel arcuately downwardly in a biased manner to hold it in contact with the ground on uneven terrain. A plurality of fluids are known to provide damping; typical fluids used in these cylinders include air, inert gasses of various types and viscous liquids such as oil. 
     FIG. 4 shows another example of a suspension system for use with a wheel assembly. A first end of an axle assembly  148  is pivotally mounted on pivot ears  147  that are fixedly attached to the main frame structure  12 . A vertical axle moving device in the form of a fluid filled cylinder  146 , which may be a hydraulically powered cylinder, is connected between the axle assembly  148  and the frame structure  12  in a conventional manner to provide a mechanism for maintaining the wheel (not shown) on a second end of the axle assembly  148  in contact with the ground surface. Alternatively, the cylinder may be a biasing device that biases the wheel into ground engagement. 
     A suspension arm  150  is mounted between a central portion of the axle structure  148  and an edge portion of the main frame structure  12 . The arm  150  is a metal spring structure and provides resilient spring support for the axle structure  148 . A drive mechanism (not shown) such as a hydraulic motor MB or MC is mounted within the axle structure  148 . This suspension arm  150  and fluid cylinder  146  could also be used with a transaxle on a single axle. 
     FIG. 5 shows a mechanical linkage suspension structure, generally designated  151 , for a secondary driving assembly. Two axle assemblies  152  are pivotally mounted by pivot pins  154  to a portion of the frame structure  12  (shown schematically). A central portion of a pair of elongated arms  156  are pivotally secured to the frame structure  12  by a common pin  158  to form a scissors-like support structure  159 . Spring or fluid filled damper members  160  are connected across each end of the scissors-like mounting structure  159 . A rigid linkage bar member  162  is pivotally mounted between the mounting structure  159  and an adjacent axle structure  152 . The bar members  162  cooperate with the mounting structure  159  to transmit the spring force of the springs or dampers  160  to the pivotally mounted axle structures  152 . These springs or dampers may also be considered to be axle biasing devices that bias the axle downwardly in to ground contact. 
     FIG. 6 illustrates another spring-biased suspension arrangement for a wheel assembly. Axle assemblies  164  are pivotally mounted to the frame structure  12  by pins  166 . A resiliently flexible torsion bar assembly  168  mounted to the main frame structure  12  is resiliently attached to the axle assemblies  164  by suspension spring members  170 . More specifically, the spring members  170  are attached between, respectively, forwardly and rearwardly extending flange portions  172 ,  174  of the torsion bar assembly  168  and a portion of the axle structure  164  associated therewith. Again, these suspension spring members may be considered axle biasing devices that bias the axle assemblies downwardly to maintain the wheels in ground contact. 
     FIGS. 7-9 show isolated perspective views of three suspension systems that can be included in a transaxle-type axle assembly, generally designated at  176 . The tires and rims are not shown in these figures to more clearly illustrate the axle assembly  176 . Many conventional, commercially available transaxles can be used in the vehicle and examples of specific embodiments are cited hereinbelow. The transaxles and suspension systems associated therewith shown in FIGS. 7-9 can be constructed entirely from conventional, commercially available parts. Each axle assembly  176  includes a conventional central differential structure  178 , two transversely extending conventional axle structures  180 ,  181  mounted thereto and two conventional hub structures  182  mounted to the free ends of the axle structures  180 ,  181 . The axle assembly shown in FIGS. 7-9 can be, for example, a drive axle with non-pivoting steering wheels such as any of axle models 12D0736, 12D0840, 14D1550, 14D1139, 14D1441, 15D1441, 15D1841, 1632149, 19D2746, 21D3747, 21D4354, 21D5073, 25D6847 or 25D7060 manufactured by Clark Components International, a Business Unit of Clark Equipment Company, of Statesville, N.C. or, alternatively, a pivotally steering drive axle such as any of axle models 12S0738, 12S0840, 14S1035, 14S1139, 14S1441, 16S2149, 16S1841, 16F1841, 16F1937 also manufactured by Clark Components International. Each Clark axle listed above can be provided with two hydraulic motors, such as MB and MC, in a conventional manner to independently power each wheel  36   a,    36   b.  Other commercially available transaxles that are contemplated include Series 70 Transaxles manufactured by the Sauer Sundstrand Company, specifically Sauer Sunstrand Model 310-0750 or Model 210-2510L. Axles manufactured by Fluidrive, Inc. can also be used. 
     FIG. 7 shows a coil spring suspension system, generally designated  183 , in isolation that can be used with the axle assembly  176 . The coil spring suspension system  183  includes a plurality of coil spring members  184 , which can be conventional coil springs, mounted on a portion of the axle assembly  176 . These coil springs  184  bias the axle assembly  176  downwardly so as to maintain the wheels in ground contact and thus may be considered axle biasing devices. It can be appreciated that the free end of each spring member  184  is mounted to the frame structure  12  in a conventional manner. Because FIG. 7 is exemplary, one skilled in the art will understand that it is within the scope of the invention to use a coil spring suspension system with any of the axle types disclosed herein, including single axles, half axles and transaxles. 
     FIGS. 8 and 9 show, respectively, a leaf spring suspension system, generally designated  186 , and a damped or, alternatively, hydraulically controlled, fluid filled cylinder suspension system, generally designated  188 . The exemplary suspension systems in FIGS. 8 and 9 are mounted on a transaxle, but it is understood that each suspension system can be used with any axle type. Again, these leaf springs  186  or cylinders  188  bias the axle downwardly and thus may be considered axle biasing devices. 
     The leaf spring suspension system  186  of FIG. 8 includes conventional leaf spring members  190  secured to the axle assembly  176  by conventional means. The free ends of each leaf spring member  190  are secured to the main frame structure  12  (not shown) of the vehicle  10  using any means known to one skilled in the art. 
     The cylinder suspension system  188  shown in FIG. 9 is a passive damped suspension system or, alternatively, a hydraulically powered system that actively repositions the axle assembly with respect to the track assembly in response to commands from the control module  44 . When a damping system is constructed according to FIG. 9, the fluid filled cylinders  192  can have any known fluid means including air, inert gas, oil or other viscous liquid. Connection of the cylinder between the axle assembly and the frame structure  12  can be accomplished by any conventional method. 
     An example of a single rigid axle assembly, generally designated  200 , is shown in FIG.  10 . The exemplary embodiment shown is a Model 1150 steerable axle (i.e., the axle  200  is provided with pivotally steerable wheels) manufactured by the John Deere Corporation of Waterloo, Iowa. This rigid axle, when installed in the vehicle  10 , can be rigidly attached to the frame structure  12  or movably mounted for selective powered repositioning within the envelope  28  of the track assembly. The axle assembly  200  can be movably mounted within the envelope  28  in a plurality of ways. The assembly  200  can be mounted for vertical movement with respect to the frame structure  12  by using a pneumatic bag device (not shown). 
     This mounting for selective repositioning by using a pneumatic bag is fully disclosed in the above-incorporated &#39;748 patent. It is contemplated to use basically the same mounting in the vehicle described herein. It is also contemplated to movably mount any of the axle assemblies disclosed herein using any of the methods disclosed in the &#39;748 patent for movably mounting an axle assembly within the envelope. The concept of movably mounting an axle assembly is not intended to be limited to the rigid axle structure shown in FIG.  10 . 
     These embodiments which use the pneumatic bag or the movable member are also intended to illustrate a general concept of providing a movable structure within the envelope  28  of the track assembly  22  that supports the wheel assembly. This general concept is illustrated schematically in FIG. 11 which shows a movable subframe  204  movably mounted within the track assembly  222 . It is contemplated to provide the track assembly  222  with a movable subframe  204  which defines a movable sub-envelopee  206  within the larger envelope  128 . The power unit  51 , the wheel assembly  38  or both could be mounted within the subframe  204 . A plurality of movable subframes and a plurality of methods and structures for mounting the subframe within the envelope  28  are disclosed in the incorporated &#39;748 patent and will not be repeated herein. 
     This subframe  204  can be movably mounted within the frame structure  12  using bearings, a pneumatic device such as a pneumatic bag, can be slidably mounted on a ramp structure or mounted for movement with respect to the track assembly  222  using any means known to one skilled in the art. The subframe  204  may be used with any axle assembly disclosed herein and may contain the axle assembly alone or may contain the axle assembly and portions of the power drive structure (not shown in FIG. 12) such as the engine  52 , the gear box  54 , and/or the pumps  55 ,  56 ,  57 , as well. It can be appreciated that moving the subframe  204  moves the center of gravity and repositions the wheels  36  with respect to the track assembly  222 . The subframe concept therefore represents a means for placing the repositioning of the center of gravity and the wheels under the control of the operator and/or a programmable logic control unit. This can aid in propelling and turning and stabilizing the vehicle. 
     It can be appreciated that a vertically moving wheel assembly can lift a portion of the track off of the ground so that the vehicle is supported by the two wheels and a portion of the track. This facilitates turning and minimizes destruction of the ground surface because a significant portion of the track assembly  22  is not engaged with the ground as it is turned. By rotating the wheels in opposite directions, for example, while the track is rotating or freewheeling, the vehicle can turn in place. 
     A telescoping axle assembly, generally designated  208 , is shown in FIGS. 12-13. This axle assembly  208  provides the ability to independently reposition the wheels  210  laterally outwardly with respect to the track assembly  222  and lock them in a new adjusted operating position. FIG. 12 is a top plan view of the vehicle showing the telescoping wheel axle assembly  208  partially in phantom lines to reveal the internal structure thereof. Each axle assembly  208  includes a frame mounting portion in the form of an outer sleeve member  212  pivotally attached to the frame structure  213  and a wheel mounting portion in the form of an inner sleeve member  214  slidably mounted therein for telescopic movement with respect thereto. 
     The inner and outer sleeve members  212 ,  214  are generally cylindrical hollow metal structures. The sleeve members  212 ,  214  are releaseably locked together by a locking pin  216 . A first end of a conventional hydraulic cylinder  218  is secured to the outer sleeve  212  and a second end of the cylinder  218  is secured to the inner sleeve  214  to effect the telescoping movement of the inner sleeve  214  with respect to the outer sleeve  212 . 
     A motor  220 , drive shaft  223 , hub  224  and a wheel  210  are mounted to a distal end of each drive unit  208  to independently rotate the wheels  210  to drive and steer the vehicle. The wheel  210  and hub  224  can be non-pivotally attached to the inner sleeve  214  as shown for non-angular steering in the manner described below or, alternatively, can be pivotally attached to provide the vehicle with angular steering as in the &#39;748 patent. 
     An enlarged cross sectional view of a single drive unit  208  on one side of the track assembly  222  is shown in FIG.  13 . The inner and outer sleeves  212 ,  214  are provided with a plurality of aligned apertures  226  for releaseably locking the inner sleeve  214  to the outer sleeve  212  with the locking pin  216  to lock the wheel  210  in a given horizontal position with respect to the track assembly  222 . 
     The outer sleeve member  212  can be rigidly attached to the main frame structure  213  or pivotally attached thereto as shown in FIGS. 12-13. A pivot rod  228  pivotally secures an apertured mounting structure  230  integral with the outer sleeve between apertured ear members  232  integral with the frame structure  213 . When pivotally attached to the main frame structure  12 , any one of the suspension systems shown or referenced herein can be used with each drive unit, including leaf springs, coil springs and fluid filled cylinders. Alternatively, any one of the powered axle assembly mounting structures disclosed herein or the equivalents thereof can be used to provide powered pivoting movement for the telescoping axle assembly with respect to the main frame structure  213 . When a suspension system is used, for example, the suspension system can be mounted between main frame structure  213  and the outer sleeve  212  or, alternatively, between the main frame structure  213  and the inner sleeve  214 . No suspension system or powered system is shown in FIGS. 12-13, however, in order to more clearly show the telescoping feature, although any of the suspension systems shown in FIGS. 3,  4 ,  5  or  6  can also be used with this telescoping drive unit. 
     The inner and outer sleeves  214 ,  212  are preferably constructed of steel and may both be hollow, rectangular-shaped or cylindrical structures. One or more steel locking pins  216  may be used with each telescoping unit  208 . 
     The telescoping wheels  210  of the present invention can be independently repositioned horizontally easily by a single person. For example, a single wheel  210  can be horizontally repositioned with respect to the track assembly  222  by driving the track assembly  222  on, for example, a slightly raised surface so that one of both of the wheels  210  are suspended above the ground surface, removing the appropriate lock pin  216 , actuating the fluid filled cylinder  218  to lengthen or shorten the same as appropriate to telescopically displace the inner sleeve  214  with respect to the outer sleeve  212 , and replacing the locking pin  216  when the desired operating position of the wheel  210  with respect to the track assembly  222  is realized. 
     Alternatively, if the outer sleeve  212  is pivotably mounted to the frame structure  213 , the outer sleeve  212  can be pivoted upwardly while the vehicle is disposed on a level surface to raise the wheel  210  associated therewith out of contact with the ground or, alternatively, to at least reduce the ground pressure on the wheel  210  while the wheel is still in contact with the ground. Then, after the locking pin  216  is removed, the wheel  210  can be horizontally repositioned by using the hydraulic cylinder  218  to telescopically displace the inner sleeve  214  with respect to the outer sleeve  212 . The locking pin  216  is then reinserted to lock the sleeves  212 ,  214  together. A worker may have to assist the telescopic repositioning of the inner sleeve  214  by lifting, pushing or moving the associated wheel  210  side-to-side depending on several factors such as the weather and wheel weight, but the horizontal repositioning of the wheels can be accomplished quickly by one or two people. 
     It is within the scope of the present invention to provide any conventional means for powering the telescoping movement of the inner sleeve with respect to the outer sleeve. The fluid cylinder is preferably fluidly communicated to the hydraulic pump  234  (shown schematically in FIGS. 15-16) operatively connected to the gear box  54  of the power unit  51 . 
     The telescoping driving units  208  provide many advantages and make the carrier vehicle  10  adaptable for a wide range of uses. When the vehicle  10  is used to cultivate or harvest row crops, for example, the telescoping ability of the drive units  208  can accommodate a wide range of crop row widths. Increased wheel assembly width provided by the telescoping structure also increases lateral stability when the vehicle  10  is used on uneven or sloped terrain. When the telescoping function is used in combination with any of the described suspension systems, the vehicle  10  provides positive ground contact between the wheels  210  and the ground surface even on severely uneven terrain. 
     It can be appreciated that the choice of whether to include various features of the vehicle  10  depends on the use to which the vehicle will be put. For example, an embodiment of the vehicle  10  in which the outer sleeves  212  are rigidly attached to the frame structure  213  and without a suspension system may be appropriate for row cropping on level fields and other uses on level ground. On the other hand, the pivotal and suspension mounting of the drive units  208  may be more appropriate when the vehicle will encounter rough, uneven or unknown terrain. 
     FIGS. 14-17 are schematic illustrations of a plurality of hydraulic circuits for the vehicle  10 . Each component represented in the hydraulic circuits represents a conventional, readily available part. One skilled in the art will appreciate that conventional hydraulic lines are represented by solid lines that connect the conventional hydraulic components and that conventional electrical connections are represented by broken lines drawn from the control module  44  to various components within the circuit. 
     It will be recalled that it is preferred that the vehicle locomotion is effected by three separate hydraulic motors MA, for driving the track  26  and second and third motors, generally designated MB and MC, respectively, for the driving the flanking wheels  36 . Hydraulic circuit diagrams which include the motors MA, MB, MC are shown in FIG.  14 . Each motor MA, MB, MC has a separate closed loop hydrostatic transmission circuit, generally designated  230 ,  231  and  232 , respectively, and each motor is provided with a separate hydraulic pump respectively designated  55 ,  56  and  57 . 
     Each hydrostatic pump  55 ,  56 ,  57  has an integral charge pump system and cross-over relief valves. Each charge pump system provides the make-up fluid required for leakage compensation and cooling; each charge pump system also provides the hydraulic pressure needed to affect the hydraulic displacement in response to a command signal as explained hereinbelow. 
     Alternatively, a functionally equivalent locomotion system can be constructed which uses a plurality of hydraulic motors, as, for example, motors MA, MB, MC, operated by a single hydrostatic pump. In a single pump locomotion system, a plurality of conventional hydraulic valves are used to in a manner well known to one skilled in the art to correctly proportion the flow of hydraulic fluid among the plurality of motors of the locomotion system. A single pump locomotion system, however, is less efficient than the preferred multiple pump system. 
     One skilled in the art will recognize that the multiple pump hydraulic system shown in FIGS. 14-17 is more efficient than a single pump system because the hydraulic pumps  55 ,  56  and  57  need only generate as much pressure as is required by the motor associated therewith. A single pump system, on the other hand, has only one pump and one valve control. This reduces efficiency because the pressure demand for the locomotion hydraulic circuit would equal the maximum required by the associated hydraulic motors plus the loss caused by the valve control. 
     When a multiple pump system is used, the pump displacements of the hydrostatic pumps are controlled by a conventional speed, steering and ground control module, generally designated  44 . The control module  44  can be, for example, a Campbell Mpde; CR10X-2M programmable control module. This control module  44  is a programmable logic device which can accept both digital and analog inputs and generates both digital and analog outputs. The control module  44  is easily reprogrammable to change the relationships between the inputs and outputs. As will be explained, the control module  44  accepts input signals from various instruments in the control assembly  42 , as for example the steering wheel, and generates output controls signals to control such vehicle functions as ground pressure and steering cylinder position. 
     The flow rate of hydraulic fluid to the driven motors MA, MB, MC of the locomotion system, and, hence, the individual motor speeds, is proportional to pump displacement of pumps  55 ,  56 ,  57 , respectively. The logic circuitry within the control module  44  determines individual pump  55 ,  56 ,  57  displacements, and hence, individual motor MA, MB, MC speed, as functions of target speed and direction selected by the vehicle driver. 
     For example, when the vehicle driver wishes to negotiate a right turn, he turns the steering wheel  45  to the right. A steering wheel position signal is generated in response by the rotary potentiometer and sent to the control module  44 . In response, the logic circuitry in the control module  44  generates appropriate control signals to increase or decrease the displacement of the pumps  55 ,  56  and  57 . If the right turn is gradual, for example, the left pump MB displacement would be increased, the track pump  55  displacement may be adjusted and the right pump MC displacement may be decreased. If, on the other hand, the right turn is sharp, the right pump MC may be set to zero displacement or, alternatively, reversed which would, in turn, reverse the rotational direction of the right wheel  36   a.    
     A separate, single pump hydraulic circuit provides hydraulic power for steering (for embodiments of the vehicle which are provided with pivotally steerable wheels), for ground pressure control for the flanking wheels  36 , oil conditioning, and for motors to drive attachments mounted on the vehicle. A schematic diagram for such a single open loop pump hydraulic circuit is shown in FIGS. 15-17. A pressure compensated pump  234 , a relief valve  236 , a check valve  238 , a solenoid valve  240  and an accumulator  242  are shown as part of both the attachment and steering circuits, generally designated  244  and  246 , respectively, in FIGS. 15-16. The pump  234 , is preferably realized by connecting two small pumps in parallel because this arrangement is less expensive than one large pump. Pump  234 , however, will be discussed as a single pump. It can be appreciated that this open loop pump  234  provides fluid for all tractor functions except locomotion. These functions include steering (when pivotally steerable wheels are provided), ground pressure control, attachment drives and oil conditioning (i.e., filtering and cooling). Pump  234  can be two 45L07PC25 pumps commercially available from the Steffen Company whose main office is located at 623 West 7 th  Street, Sioux City, Iowa 51103. Pump  234  can also be a Series 45 pump manufactured by the Sauer Sunstrand Company. The Series 45 pump has a 3.5 cubic inch per revolution displacement and a 3000 p.s.i. maximum pressure. The Sunstrand Series 45 pump is pressure compensated and has a horsepower limiting option. 
     The relief valve  236  limits the hydraulic pressure in the circuit to within a safe upper limit. The check valve  238  prevents pressurized oil stored for steering and ground pressure control (priority functions) from being used to operate any attachments. The solenoid operated on/off valve  240  discharges the accumulator in response to a signal from a conventional timing circuit. The timing circuit may, for example, signal the solenoid valve  240  to discharge the accumulator a predetermined amount of time after the engine has been turned off. This decrease in hydraulic pressure facilitates safe maintenance or repair of any of the components of the ground pressure and steering circuit. The solenoid valve  240  is conventional and can be a Waterman 12CR5S-F12-T6; the Waterman valve has a capacity of less than five gallons per minute and a maximum p.s.i. of 3000. 
     The accumulator  242  can be a conventional Tobul 9A30-40 accumulator manufactured by the Tobul Company commercially available from Winco Fluid Power, Inc., 2955 Terwood Road, Willow Grove, Pa. 19090. The accumulator  242  preferably has a 10 gallon total capacity and is a piston type accumulator. The accumulator(s)  242  in the circuit store energy in the form of pressurized fluid to allow the pump flow rate capacity to be lowered by a longer term “maximum average”. Peaks in the flow demand for steering and for following ground contour would be provided by the accumulators. The accumulators also provide a limited amount of emergency fluid pressure for steering. 
     Control signals generated by the control module  44  control the ground pressure of the flanking wheels  36 . More specifically, the ground pressure control signals are sent to conventional valve drivers  248  and  250 , sometimes also referred to as valve amplifier cards, which, in turn, control conventional proportional pressure control valves  252  and  254 . The proportional pressure control valves  252 ,  254  control the pressure applied to the cap end of the wheel cylinders  256  and  258 . The schematically illustrated wheel cylinders  256  and  258  may be the fluid cylinder  146  shown in FIGS. 3 and 4 which controls the ground pressure and vertical positioning of an associated wheel. 
     The pressure reducing valves  252 ,  254  can be HYDRAFORCE Proportional Electric Pressure Reducing Valves model TS10-36A-M-8T-N-12DG and the valve drivers  248 ,  250  can be HYDRAFORCE Proportional Control Amplifiers model 7114920. The proportional pressure control valves  252 ,  254  control pressure to the ground pressure control cylinders  256 ,  258  and hence control the pressure between the tires and the ground (and hence between the track and the ground also). The amplifier cards  248 ,  250  receive control signals from the control module  44  and send proper pulse width modulation signals to the valves  252 ,  254 . 
     A sample of the control logic for the ground pressure function is as follows. Generally, to turn the vehicle, the driver turns the steering wheel in the desired turn direction. The farther the steering wheel is turned in a given direction, the shorter the turning radius. To negotiate a significant turn, ground pressure under the wheels would be increased in order to reduce the ground pressure of the central track. The conventional rotary potentiometer (not shown in the schematic drawings) interconnected to the steering wheel shaft (not shown in the drawings) generates an appropriate electrical input signal that is sent to the control module  44  which input signal indicates the desired turn direction and turn radius. 
     The control module  44 , in response, sends a control signal to the valve amplifier cards  248  and  250 . The valve amplifier cards  248 ,  250  control the proportional pressure control valves  252 ,  254  which control the pressure applied to the cap end of the wheel cylinders  256 ,  258  in a conventional manner. The pressure applied to the wheel cylinders  256 ,  258  forces the wheels downwardly with respect to the central track to thereby relieve the weight carried by the track as the vehicle turns. 
     A ground pressure control override system is provided which allows the operator to control the maximum pressure exerted by the ground pressure control cylinders  256 ,  258 . Left and right wheel ground pressure override input signals are sent to the control module  44 . The override system provides the operator with the capability to “dial in” or program in override system characteristics appropriate for the current driving conditions. For example, the operator may lessen ground pressure of both flanking wheels in response to soil conditions or may lessen ground pressure of one wheel, the uphill wheel, when driving across, that is, generally perpendicular to, the gradient of a slope. 
     When steerable wheels are provided, the flow of hydraulic fluid to the steering cylinders  260  and  262  is controlled by proportional valves  268  and  270  which are, in turn, powered by amplifier cards  264  and  266 . The amplifier cards  264 ,  266  are actuated by control signals from the control module  44 . Feedback position transducers  272  and  274  provide feedback to the control module  44  based on the position of the steering cylinders  260 ,  262 . The position transducers can be, for example, Linear Motion Potentiometers model number M1326-8-103 manufactured by the Maurey Company commercially available from Servo Systems Company, 115 Main Road, P.O. Box 97, Montville, N.J. 07045-0097 as stock number PR259. The position transducers  272 ,  274  provide feedback to the control module  44  which indicates the actual position of the steering cylinders and, therefore, tire angle. Comparisons of the actual position of the wheels with the desired position based on the rotational position of the steering wheel determine the control signals sent to the valve controllers  264 ,  266 . The proportional valves  268 ,  270  are conventional and can be Model KDG4V35-2C-15x manufactured by the Waterman Company. These valves  268 ,  270  have ten gallon per minute capacity and a maximum pressure of 3000 p.s.i. The valve drivers  264 ,  266  are conventional and can be model APCI/10-12 drivers manufactured by the Waterman Company. The drivers  264 ,  266  accept an analog input and PWM output devices. 
     As shown in FIGS. 15-17, the vehicle includes hydraulic circuits for attachments which require low hydraulic flow, for attachments which require large hydraulic flow and for oil conditioning. Lift attachments and other with low flow requirements are powered by the open loop pump  234  that also powers the steering and ground pressure circuits. All of the valves in the low flow hydraulic circuit are of relatively small capacity, typically 10 g.p.m.. Front attachments are controlled by a stackable valve set which includes valves  278 ,  280 ,  282 , and  284 . Connections to the attachments are made through quick coupler sets  286 . A plurality of coupler sets are indicated at several locations in the schematic diagrams; coupler sets with identical reference numerals are identical structures. Two valves  282  and  284  provide forward, reverse and hold functions depending upon which solenoids  290 ,  292 ,  294  or  296  are powered. 
     The solenoids are powered by simple on/off rocker type switches or by any other appropriate switching device. The valves are conventional; valve  278  is a solenoid operated, three position directional valve such as Model D64V35-xx-MFW manufactured by the Waterman Company. It has a 10 g.p.m. capacity, a 3000 p.s.i. maximum pressure and is a float center type valve. The valves  282 ,  284  are solenoid operated, 3 position directional valves such as model D64V35-xC-MFW manufactured by the Waterman Company. These valves  282 ,  284  have a 10 g.p.m. capacity, a 3000 p.s.i. maximum pressure and are of the closed center type. The quick couplers  286  are conventional female adapters which have ASAE standard dimensions and a 3000 p.s.i. maximum pressure capability. 
     One pair of valves  278  and  280  provide lift, lower, float and hold positions for a blade, a feller head or other attachment. If no solenoid is energized the associated attachment is held in a stationary position. Energizing solenoid  298  provides float. Energizing  300  and  298  provides lift and energizing  300  and  302  provides attachment lowering. Valve  280  is conventional and is a solenoid operated, 2 position directional valve; valve  280  can be a Waterman model D64V35-2A-MFW which has a 10 g.p.m. capacity and a 3000 p.s.i. maximum pressure. 
     The rear attachment stackable valve set is similar to the front attachment circuit. Connections to the attachments are made through a plurality of quick coupler sets  286  located at various positions in the hydraulic circuit. Three valves  304 ,  306  and  308  provide forward, reverse and hold functions depending on which solenoids  310 ,  312 ,  314 ,  316 ,  318 , and  320  are energized. One valve  322  provides float, forward and reverse functions for control of the winch depending upon which solenoid is energized, none,  324  or  326 , respectively. Valve  322  is a solenoid operated, three position directional valve such as Model D64V35-xx-MFW manufactured by the Waterman Company. It has a 10 g.p.m. capacity, a 3000 p.s.i. maximum pressure and is a float center type valve. Valves  304 ,  306 ,  308  are solenoid operated, 3 position directional valves such as model D64V35-xC-MFW manufactured by the Waterman Company. These valves  304 ,  306 ,  308  have a 10 g.p.m. capacity, a 3000 p.s.i. maximum pressure and are of the closed center type. 
     Attachments which require a large fluid flow such as a feller head or winch are powered by an open loop pump  328  shown in FIG.  17 . The relief valve  330  limits the pressure in the circuit to a safe upper limit. If no solenoids are energized, the oil from pump  328  flows through the air cooler  332  and the filter  334 . The air cooler  332  removes heat from the oil. The filter  334  removes particulate matter from the oil and helps maintain a clean supply of oil in the reservoir. The air cooler and the filter can both be conventional parts. The air cooler can be a model AOHM-35 oil cooler manufactured by Thermal Transfer Products, Ltd. The open loop hydraulic pump  328  is conventional and can be, for example, a Series 45 pump made by the Sunstrand Company. This pump has a 3.5 cubic inch/revolution displacement and a 3000 p.s.i. maximum pressure. This pump is pressure compensated and has a horsepower limiting option. The adjustable relief valve  330  is conventional and has a 3000 p.s.i. maximum pressure rating. The return line filter  334  is conventional and preferably has a 35 g.p.m., 500 p.s.i. capacity. 
     When the solenoids associated with valves  336  and  338  are energized, the rear attachment (e.g., a winch) is powered. When the solenoids associated with valves  336  and  340  are energized, the front attachment (e.g., a feller head) is powered. The oil that flows from pump  328  always flows through the conditioning loop which is comprised of the cooler and the filter, even if the oil is returning from a motor circuit. A flow control valve  342  controls the flow rate to the motor circuits and hence controls the motor speed. The connection between the power unit and the motor circuits is effected through the quick coupler pairs  344  which are shown in two places in the schematic drawings. 
     The adjustable relief valve  330  has a 3000 p.s.i. maximum pressure capacity. The valves  336 ,  338 ,  340  are conventional and can represent solenoid operated, 2 position directional valves manufactured by the Waterman Company, model 21C-1S-F12-T16. These valves have a 35 g.p.m. capacity and a 3000 p.s.i. maximum pressure capacity. The quick coupler  344  is conventional and is preferably a female adapter with a 35 g.p.m. capacity and a 3000 p.s.i. maximum pressure capacity. 
     An alternative embodiment of the vehicle hydraulic system is shown in FIGS. 18-21. This embodiment is similar to the preferred embodiment shown in FIGS. 14-17 and identical part are given identical reference numbers but are not discussed in detail. All components represented schematically in FIGS. 14-21 are conventional, commercially available parts. 
     FIG. 18 represents a portion of the locomotion circuit for the vehicle which shows the hydraulic locomotion circuits for the left and right wheel hydrostatic transmission circuits. The locomotion circuit, generally designated  346 , include separate hydraulic circuits for the left wheel drive assembly, generally designated  348 , the right wheel drive assembly, generally designated  350 , and the central track assembly (not shown). The left and right wheel circuits  348 ,  350  include separate left and right wheel drive motors  331  and  333 , respectively; a separate center track drive motor (not shown) is provided in the central track circuit. Separate pumps  56  and  57  are provided for each motor  331  and  333 , respectively; a separate pump (not shown) is provided for the central track drive motor (not shown). One skilled in the art will appreciate that any conventional hydraulic circuit configuration which includes a motor and pump on a closed loop circuit for the central track, such as the one shown in FIG. 14 herein, can be provided to drive the central track  22 . The control and operation of the locomotion system shown in FIG. 14 are similar to that of the preferred embodiment. As explained above, the locomotion system could be replaced by a system that includes a single pump which drives the three motors, but this decreases efficiency. 
     The alternative steering and ground pressure control circuit is shown in FIG.  19 . The pressure compensated pump  352 , relief valve  354 , check valve  356 , manual valve  358 , and accumulator  360  are also shown on the attachment and oil conditioning circuits shown in FIGS. 19-20, but are the same physical devices. The manual valve  358  allows the accumulator to be discharged for maintenance or repair of components in the ground pressure and steering circuit. 
     One skilled in the art will recognize that the control module  44 , valve drivers  248  and  250 , pressure control valves  252  and  254 , and the wheel cylinders  256  and  258  function in essentially the same way described above to change ground pressure and/or steer the vehicle. 
     Similarly, the steering cylinders (optional)  260  and  262 , valves  268  and  270 , amplifier cards  264  and  266  and feedback position transducers  272  and  274  function in essentially the same way as described above when discussing the preferred embodiment of the hydraulic circuit. 
     An attachment circuit, generally designated  362 , is shown in FIG.  20 . The attachment circuit  362  includes a front attachment circuit, generally designated  364 , and a rear attachment circuit, generally designated  366 . The attachments are powered by an open loop pump  352 . This pump  352  also powers the steering and the ground pressure circuits  246 . All valves in the attachment circuit  362  are of relatively small capacity, preferably ten (10) gallons per minute, with two exceptions, valves  368  and  370 . 
     Front attachments (such as a bulldozer blade or various mine detecting equipment) are controlled by a stackable valve set comprised of the valves  372 ,  374 ,  376 ,  378  and  368 . Connections to the attachments are made through a plurality of quick coupler sets indicated by the reference numerals  380  and  382 . 
     Two valves  376 ,  378  provide forward, reverse and hold functions depending on which solenoids  384 ,  386 ,  388  or  390  are powered. Powering of the solenoids  384 ,  386 ,  388 , and  390  is controlled by conventional off/on rocker-type switches located in the operating compartment. One pair of valves  372  and  374  provide lift, lower, float and hold positions for a blade, feller head or other attachment. When no solenoid is energized, the attachment is held stationary. Energizing solenoid  394  results in float. Energizing solenoids  392  and  394  lift and energizing solenoids  396  and  392  results in attachment lowering. 
     A larger capacity valve  368  is used to power a motor circuit. This valve is of approximately thirty five (35) gallon per minute capacity. The motor circuit is used to power various attachments such as a feller head saw. 
     The rear attachment circuit  366  includes a rear attachment stackable valve set  400 . This valve set  400  is similar to the front attachment circuit. Connections to rear attachments are made through a plurality of quick coupler sets, generally designated  380  and  382 . Three valves  402 ,  404 , and  406 , provide forward, reverse and hold functions, depending on which solenoids  408 ,  410 ,  412 ,  414 ,  416 , and  418  in the rear attachment circuit  366  are energized. A valve  420  in the rear attachment circuit provides float, forward and reverse functions for a winch control, depending upon which solenoid associated therewith, none,  422  or  426 , is energized. A larger capacity valve  370  is used to selectively power a motor circuit for an attachment such as a winch. 
     An oil conditioning circuit is shown in FIG.  21  and is generally designated  428 . A fraction of the hydraulic oil flowing from the open loop pump  352  is directed to the conditioning loop for oil cooling and oil filtration. The pressure compensated flow control valve  430 , the oil to air cooler  432  and the filter  434  function in this circuit as described above in reference to the preferred circuit. 
     FIG. 22 is a front elevational view of the vehicle  10  showing the flanking wheels  536  in exploded relationship to the track assembly  522 . The operator compartment is not shown to more clearly show the invention. A hollow cylindrical portion  532  of the axle assembly  538  secured to the main frame structure (not visible in FIG. 17) extends transversely therefrom essentially parallel to the ground surface  537 . It will be understood that this hollow cylindrical portion  532  is a schematic representation of a portion of the axle assembly  538  and represents any of the axle structural types disclosed herein, both those that provide independent wheel rotation to effect steering through differential wheel speed or, alternatively, pivoting steerable wheels, including a half axle, a telescoping half axle, a transaxle with independent wheel rotation or a rigid axle structure. 
     A reference line designated  539  indicates the vertical position of the center of the wheel cylinder which is the rotational center of each wheel  536  and of the center of a center drive hub assembly  540  which may include a hydraulic motor unit to drive the associated wheel. It can be appreciated that the hollow cylindrical portion  532  of the axle assembly  538  and the hub assembly  540  are axially aligned with the reference line  539  which passes through the rotational center of each wheel  536 . The hollow cylindrical portion  532  of the axle assembly  538  attaches to the center of the center drive hub assembly  540 . 
     FIG. 23 is a view similar FIG. 22 but showing that an offset gear or chain wheel drive assembly  542  associated with each wheel  536  can be used in lieu of center drive hub assembly  540 . Each wheel  536  is independently powered by a motor (not shown), as for example a hydraulic motor, to power the offset gear drive assembly  542 . The hollow cylindrical portion  544  of the axle assembly  546  is fixed to the main frame structure (not visible) of the track assembly  522  at a higher vertical position than the position shown in FIG.  17 . The reference line  539  is in the same vertical position relative to the ground surface, the track assembly  522  and the wheels  536  as in FIG.  22 . The reference line passes through the center of the wheel cylindrical of each wheel  536  which is the rotational center of each wheel. 
     It can be appreciated that the hollow cylindrical portion  532  attaches to the wheel drive assembly  542  at a position off set from the center thereof and that a plurality of cooperating gear structures couple the motor (not shown) to the hub (not shown). It will be understood that the offset drive assembly  542  can be used with any of the axle assemblies shown or described herein including transaxles, half axles and rigid axles and can be used in conjunction with any of the suspension systems and/or any of the methods of mounting the axle assembly to the main frame structure of the track assembly. 
     It will be readily understood by one skilled in the art that both the center drive hub assembly  540  and the offset drive assembly  542  are schematic representations of broad categories of hub assemblies and are meant to be exemplary and not limiting. 
     FIG. 24 illustrates that it is within the scope of the present invention to provide a plurality of embodiments of a high clearance axle assembly, generally designated  638 , for the vehicle  10  which provide a wide range vertical clearance heights for a transverse portion  641  of the axle assembly  438  which extends between the track assembly  622  and an adjacent wheel  636 . The various representative embodiments of the high clearance axle assembly  638  shown in FIG. 24 are designated  638   a,    638   b  and  638   c  for reference and are schematic representations which show the range of structures contemplated and within the scope of the invention. A first representative embodiment designated  638   a  and drawn in phantom lines shows that is it within the scope of the invention to mount the axle assembly  638  above the track assembly  622  and for each end of the axle assembly  638  distal from the track assembly  622  to be mounted to an offset gear-type hub assembly (not shown). 
     FIG. 24 illustrates the configurations of two additional exemplary embodiments  638   b  and  638   c  of the axle assembly. These embodiments illustrate that the axle assemblies  638   b,    638   c  can be secured to the track assembly  622  at various vertical locations within the envelope  28  and that the distal ends of each assembly  638   b,    638   c  can be mounted at the center of the hub assembly (not shown) as in  638   a  or below the hub assembly (not shown). It can be appreciated that each embodiment of the axle assembly  638  represented in FIG. 24 is of the half axle type and each wheel  636  is independently, bi-directionally rotatable by, for example, a separate hydraulic motor associated with each wheel  636 . 
     An advantage of the axle assembly  638  configurations shown in FIG. 24 is that various amounts of vertical clearance is provide between the transverse portion  641  of each axle assembly  638  and the ground. High amounts of clearance are advantageous in some applications of the vehicle  10  such as row cropping. It is contemplated that at least one embodiment of the vehicle  10  will be used for row cropping where the track assembly is driven between a row of crops and the wheels  636  are driven between adjacent spaces between rows. It will be understood that a row of crops will pass under each transverse portion  641  as the vehicle is driven between the crop rows. 
     FIG. 24 should not be taken to imply that all of the configurations of the axle assembly  638  would be on a single vehicle  10 ; rather, FIG. 24 illustrates in schematic form the various structural configurations the wheel assembly could assume and a possible position of each configuration of the wheel assembly on the vehicle  10 . 
     The configuration of the axle assembly  638  shown in FIG. 24 is a basic configuration and it within the scope of the invention to provide a plurality of variations upon this structure by providing, for example, means for moving the axle assembly  638  vertically and/or longitudinally with respect to the track assembly  622  or means for repositioning the wheels  636  horizontally inwardly and outwardly with respect to the track assembly  222  by providing, for example, telescoping portions of the axle assembly  38  associated with each wheel  636 . It is also contemplated to incorporate any of the suspension systems disclosed herein in the high clearance axle assembly  638 . Some of these contemplated variations are shown schematically in FIG.  20 . 
     More specifically, FIG. 25 shows that either an independent suspension system can be combined with a high clearance axle assembly  638  configuration or a powered pneumatic cylinder can be combined with the axle assembly  638 , or both. A cylinder structure  646  on each side of the track assembly  622  is mounted between the main frame structure and a portion of the axle assembly  638  and each cylinder structure  646  can represent either a damping cylinder or a hydraulic cylinder. When a suspension system is included in the axle assembly  638 , one way in which the suspension may be provided is by providing a damping cylinder  646 . Additional alternative embodiments of the suspension system can be provided by incorporating any of the suspension systems described or shown herein, including the suspension systems shown in FIGS. 7-9 in the high clearance axle assembly. 
     In the embodiments of the axle assembly  638  in which the cylinder  646  of FIG. 25 is a hydraulic cylinder, each inner portion  641  of the axle assembly  638  is pivotally connected to the main frame and can be vertically pivotally repositioned with respect to the track assembly  622  by extending or retracting the respective cylinder  646 , each of which is in fluid communication with a hydraulic pump (such as pump  234 ) operatively connected to the gear box  54  of the power unit  51 . Retracting the cylinder  646  pivots the inner portion  641  upwardly so that it may pass over particularly high rows of crops. 
     FIG. 26 is a schematic top plan view of the vehicle  10  that illustrates that a plurality of alternative mechanical drive mechanisms such as sprockets, belts or drive shafts can be incorporated in the vehicle  710  to transmit power from a power source mounted within or outside of the envelope (not visible in FIG. 26) of the track assembly  722  to drive the track  726  alone or to drive the track  726  and wheels  736 , depending on the application to which the vehicle  710  is put and the operating conditions of the vehicle. The power source can be an internal combustion engine, a flywheel assembly or any other power source disclosed herein or know to one skilled in the art. It can be appreciated that, depending on the power source of the vehicle  710 , the wheels  736  may drive the track  726  or the track  726  may drive the wheels  736 . Various combinations using mechanical clutches or reversing clutches are within the scope of this invention. 
     A drive system is represented schematically in a block diagram indicated at  751 . This block diagram represents a drive chain or drive belt  753  in the form of a closed loop interengaged with sprocket structures (not shown) or similar structures on a plurality of shafts  754 ,  756 ,  758  mounted at the center axes of front, middle and rear drive wheels  760 ,  762 , and  764 , respectively. The drive wheels  760 ,  762   764  are rotatably mounted within the envelope of the track assembly  722  to drive the track  726 . All of these components would usually be located in the area marked off by block  751 . However, in order to easily illustrate the various components, they are being shown outside of the block  751 . In this drive system  752  one of the shafts, as for example the central shaft  756 , is powered and the drive chain  753  transmits power from the powered shaft to the other shafts,  754  and  758  in this example. Clutch assemblies, one associated with each wheel  736 , facilitate independent rotation of each wheel  736  for steering of the vehicle  710 . 
     An alternative drive system, generally designated  766 , is shown on the right hand side of FIG.  26 . This drive system  766  can be installed in the location indicated by the blocks  751  and  752  and comprise a plurality of gear boxes  768 , drive shafts  770 ,  772  and  774 , (mounted in a manner similar to the drive shafts  754 ,  756  and  758 , respectively) and clutch assemblies  776 . It is contemplated to drive one of the shafts, as, for example, the central shaft  772 , to selectively drive the shafts  770  and  774  by engaging the gears within the respective gear boxes  768 . 
     FIGS. 27-28 shows a side elevational schematic view of a track assembly  822  showing a large central drive wheel  824  at the center of the envelope  828 . The central drive wheel  824  is mounted on a central shaft  830  which is connected through a clutch assembly (not shown) and driven by the power unit (not shown). The central shaft  830  is connected through a series of gear structures and a clutch mechanism to each central flanking wheel  836  for selective, independent and variable speed rotation of the same. 
     It is contemplated to embody the central shaft structure as a straight through power shaft structure which drives the track  826  and the flanking wheel  836 . When a straight through drive shaft is used, the central track drive wheel  824  and the two flanking wheels  836  are driven by a single central rigid axle. The rigid axle is driven by a power source such as a hydraulic motor or, alternatively, by an internal combustion engine which is coupled thereto by a conventional mechanical transmission. When the straight through power shaft structure is used the axes of rotation of the central wheel  826  and the two wheels  836  are linearly aligned. Vehicle steering can be accomplished by pivotally steering the wheels  836  or alternatively by rotating the wheels  836  at different rotational speeds relative to one another. When steering is provided by driving the wheels  836  at different rotational speeds, each wheel  836  can be individually clutched to provide independent free wheel operation for each wheel  836 . Steering would then be accomplished by free wheeling one wheel and turning the vehicle by powering the other wheel. 
     The forward vehicle speed would be controlled by the hydraulic motor speed or, alternatively, by the gearing of the transmission in a conventional manner if a mechanical transmission were used. 
     One skilled in the art will also understand that this central shaft structure can be supported by any of the suspension systems disclosed herein and that because the central shaft structure is coupled to each wheel  836  through a clutch structure, the wheels are rotationally independent. 
     FIG. 29 is a schematic view similar to FIG. 27 except that FIG. 29 includes two central flanking track assemblies  810  that can be operated at different rotational speeds to steer the vehicle  10  left or right in either the forward or the reverse directions. The flanking tracks  840  are each capable of independent freewheel operation or opposite rotation to provide the turning modalities. Each flanking track  840  is preferably oval as shown in FIG. 29 in order to minimize the ground contact area and the ensuing damage to the ground when turning. 
     Each flanking track  840  can be independently powered by, for example, a separate hydraulic motor provided in each flanking track assembly. The hydraulic motors may be like motors MB and MC discussed above. The flanking tracks  840  can be either on opposite ends of a single transaxle or can each be mounted to the free end of a half axle which is pivotally or rigidly mounted to the main frame structure of the track assembly. Any appropriate suspension system or, alternatively, powered repositioning system, including those previously discussed, can be used with the flanking tracks. A telescoping axle can also be used with the flanking tracks. Likewise, any of the fixed or adjustable high clearance axle assemblies previously discussed may also be used in conjunction with the flanking tracks as well. 
     The embodiment of the vehicle  910  shown in FIG. 30 illustrates various exemplary vertical positions that an axle assembly, generally designated  938 , can assume with respect to the main frame structure  912  of the track assembly  922 . These vertical positions are indicated by a series of circles, designated  938   a - 938   e.  Each circle is a schematic representation of an end view of the axle structure which extends from the main frame structure to the wheel hub. These various vertical positions can be realized in one of two ways: 1) by mounting the axle assembly  938  fixedly in one of these positions, in which case the circles  938   a - 938   e  would represent alternative positions at which the axle assembly is mounted to the main frame structure of the  922 , or 2) by movably mounting the axle assembly  938  within the envelope  928 , for selective vertical repositioning within the envelope  928  in which case the circles  938   a - 938   d  would represent various adjusted operating positions of a single axle assembly. In the latter case, the axle assembly  938  could occupy any vertical position within the range of positions represented by  938   a - 938   d.  It can be seen in FIG. 30 that the axle assembly  938  may also be mounted above the envelope  928  as mentioned above. 
     The axle assembly that is represented schematically in FIG. 30 can include any embodiments disclosed herein, including all telescoping embodiments. The embodiment in FIG. 30 also includes a centrally positioned operator compartment  955  which is covered by a rollover protection structure  957  which is preferably made of steel and is strong enough to resist significant deformation in the event of a rollover-type accident. It is also contemplated to provide a carrier vehicle constructed without the operator compartment, with the operator compartment located inside the envelope of the track assembly  222 , or with the operator compartment located outside of the envelope in a forward, central or rearward position. It is contemplated to provide a vehicle without an operator compartment when the vehicle  10  will be used for computer controlled or remote controlled operations when a human operator is not occupying a portion of the vehicle. 
     FIG. 31 shows another embodiment of the carrier vehicle  1010  of the invention. The vehicle  1010  is employed as a bulldozer having a bulldozer attachment  1110 . The axle assembly  1130  shown is a rigid axle like the one designated  200  shown in FIG.  10  and is movable vertically within the envelope of the track assembly by, for example, using a pneumatic bag mounting as disclosed in the &#39;748 patent which has been incorporated in its entirety by reference. With reference to FIGS. 10 and 31, the pneumatic air bag device (not shown) may be mounted between an upper frame member of the frame structure  1112  and the hinged saddle  203  that extends to the back of the axle assembly  204 . The saddle  203  is coupled to the frame structure  1112  via hinge pins  205 . 
     Upon inflation, the air bag of the device pushes the axle assembly  200  and the wheels  1036  downwardly within the envelope, thereby raising the track  1126  off the ground at the fore end of the vehicle  1010  and increasing the ground bearing pressure at the wheels. Thus, the wheels  1036  and only the aft end of the track  1026  are in ground contact. In this position, it can be appreciated that turning of the wheels  1036  would result in turning of the vehicle  1010  about a small turning radius because the tractive forces are only at the wheels  1036  and at the aft end of the track  126 . Because a portion of the driven track  1026  is out of ground contact, improved mobility of the vehicle  1010  is achieved. In addition, raising the fore and of the track  1026  off the ground reduces damage to the soil as the vehicle turns. Limit stops  201  (FIG. 10) may be provided between the air bag device and the axle assembly  200  for restricting oscillatory movement of the axle assembly  1130 . When the air bag of the device is caused to deflate, the axle assembly  1130  will move to a vertical position such that the track  1026  and the wheels  1136  are each in full contact with the ground. 
     It can be appreciated that when any of the axle assemblies described herein is movably mounted within envelope  1028  shown in FIG. 31, the distribution of weight and tractive effort between the wheels  1036  and the track  1026  can be adjusted for improved driving and steering of the vehicle  1010 . 
     In addition, other devices, such as hydraulic or air cylinders (not shown) coupled between, for example, the portions of the axle assembly  1130  and the main frame structure  1112  of the track assembly  1022 , may also be used to raise or lower the axle assembly  1130  within the envelope  1028  of the track assembly  1122 . 
     The power source shown schematically in FIG. 31 can represent an internal combustion engine, an electromechanical flywheel assembly, an electric generator, a battery, a fuel cell, a human powered mechanical power source or any type of energy storage device known to one skilled in the art. Also, the tractive drive or drives and the wheels drive or drives may be either electrically powered, hydraulically powered, mechanically powered, or powered by a combination thereof. 
     With reference to FIG. 32, it is within the scope of the present invention for the axle assembly  1235  and the power unit  1251  to be fixedly mounted on a movable member, shown schematically and generally indicated at  1253  in FIG. 32, so as to be selectively movable within the envelope  1228  in both the horizontal and vertical directions. The details of the construction, mounting and operation of the movable member  1253  are fully disclosed in the previously incorporated &#39;748 patent and will not be repeated in detail herein. As shown in FIG. 32, the axle assembly  1238  and power unit  1251  are mounted on the movable member, generally indicated at  1253  so as to advantageously change the fore and aft center of gravity per the load or the attachment or to compensate for various surface conditions which may prevail, and to change the driving and steering geometry in any working conditions. 
     With continued reference to FIG. 32, a tandem carrier vehicle  1200  is shown therein, embodying the principles of the present invention. The tandem vehicle  1200  includes a front powered vehicle  1210 , which is substantially similar to the vehicle  10  of FIG.  1 . However, vehicle  1210  is provided with an articulated connection  214  at the aft end thereof. A rear vehicle  1212  is coupled to vehicle  1210  at the connection  1214 . Each vehicle  1200 ,  1210 , is provided with individually powered flanking wheels and each wheel in both vehicles is controlled by an appropriate control assembly in the operator compartment of the vehicle. 
     It is within the scope of the invention to provide vehicles  1210  and  1212  with track profiles which are shorter than the profile of the single vehicle  10  of FIG.  1 . Further, the vehicle  1210  may be provided with a fixed axle assembly  1235 , a vertically movable axle assembly or a movable axle assembly and movable power unit, with driven and steerable or non-steerable wheels, as discussed above. Any of these axle assemblies may optionally include any suspension system disclosed herein. Vehicle  1212  preferably has a fixed, axle assembly  1232 . 
     The track and wheels of the rear vehicle  1212  are driven by the power drive structure  1251  of vehicle  1210 . Thus, the rear vehicle  1212  includes the appropriate hoses originating from vehicle  1210  to provide the hydraulic power necessary to drive the track and wheels of the first and second vehicles simultaneously. The rear vehicle  1212  may also be provided with wheels capable of both different rates and directions of rotation to independently steer the rear vehicle  1212 . 
     For the best results, the wheels  36  should be disposed at the center of gravity of the vehicle to increase their tractive force. Thus, even though the driven track  26  will try to maintain movement of the vehicle in the particular direction of vehicle travel, when the wheels  36  are rotated at different rotational rates to steer the vehicle  10 , the track  26  will turn. This occurs because the wheels  36  exert sufficient ground bearing pressure to turn the track  26  and change the vehicle&#39;s direction of travel by virtue of the different rotational rates of the wheels  36 . FIG. 1, however, shows that the wheels can be positioned at the fore or aft end of the vehicle, depending on the way the vehicle will be used. 
     Since the vehicle of the invention may traverse uneven terrain, there may be a need to compensate for any vehicle yaw. If one wheel rises with respect to the other, for example, if one wheel runs over a stump, oscillation of the vehicle and track mount may occur. This oscillation or yaw can be compensated for by providing, for example, shock absorbers, springs, or rubber blocks between the frame structure and wheel axles. 
     Because the axles in many of the above described embodiments are pivotally attached to the main frame structure of the track assembly and because the wheel assembly can be constructed to provide lateral support and stability for the track assembly, a plurality of embodiments of the vehicle  10  can easily be constructed for stable operation on sloped surfaces. FIG. 33 schematically shows an exemplary embodiment of the vehicle with the wheels  30  thereof vertically repositioned for use on a sloped surface. The vehicle of FIG. 33 is positioned across the gradient of a sloped surface and each transversely extending portion of the axle assembly  38  is independently pivotable with respect to the track assembly. Any of the disclosed pivoting mechanisms described herein can be used to pivotally mount each axle assembly  38  to the main frame structure of the track assembly  222 . 
     The pivotal movement of each axle assembly  38  is under the control of the vehicle operator so that the operator can easily position each wheel to maintain the track assembly  222  in an essentially upright position. Any of the disclosed suspension systems can be used with the pivoting wheels to increase stability and maximize the ground engagement on sloped and other surfaces. 
     FIGS. 34 and 35 are schematic representations of the locomotion system for the track assembly  1322  and the flanking wheels. The front elevational schematic of FIG. 35 shows the general locations of the track drive motor  1330  and the wheel drive motors  1331 ,  1332 . The positions of the track drive motor and the wheel drive motors within the envelope is indicated in the side elevational schematic of FIG. 34. A conventional drive track drum  1340  indicated in phantom lines. 
     The hydrostatic transmission motor  1330  is directly coupled to the track drive drum  1340 . The linear velocity of the track, and hence the vehicle velocity, is proportional to the flow rate of the fluid in the driving motor. Because the flow rate of the fluid is proportional the fluid displacement of the associated pump, a change in the pump displacement changes the track velocity. Motors  1330 ,  1331  and  1332  are conventional and can be the same commercial embodiments listed above for motors MA, MB and MC, respectively. 
     FIGS. 36 and 37 show top and side views, respectively, of an alternative secondary driving assembly  1500 . Only one secondary driving assembly  1500  is shown, but it is to be understood that there is a secondary driving assembly  1500  mounted on each side of the vehicle. The secondary driving assembly  1500  includes an axle assembly  1502 , a wheel  1504 , and a vertical axle moving device in the form of a hydraulically operated extensible and retractable piston  1506 . The axle assembly  1502  has a frame mounting portion  1508  pivotably mounted to the main frame structure (which may be the frame of any embodiment of the vehicle described hereinabove) and a wheel mounting portion  1510  on which the wheel  1504  is rotatably mounted. The wheel mounting portion  1510  houses a hydraulic motor, such as motors MB and MC discussed above, for rotating the wheel  1504  using pressurized fluid from the engine assembly of the vehicle (which may be any of the engine assemblies or power drive structures described hereinabove). 
     The axle assembly  1502  also includes an arm  1512  that extends generally rearwardly from the frame mounting portion  1508  to the wheel mounting portion  1510  alongside the track assembly (not shown), preferably parallel to the track assembly. An arm connecting bracket  1514  extends upwardly from the arm  1512  and a frame connecting bracket  1516  is provided on the frame structure. One end of the hydraulic cylinder (preferably the end of its cylinder  1518 ) is pivotably connected to the frame connecting bracket  1516  and the other end of the hydraulic cylinder (preferably the end of its extendible rod  1520 ) is pivotably connected to the arm connecting bracket  1514 . 
     To move the wheel  1504  generally downwardly, pressurized fluid is supplied to the cylinder  1506  such that the rod  1520  thereof extends to pivot the arm  1512  about the axle assembly&#39;s frame connecting portion  1508 . As shown in FIG. 37, the arm  1512  will pivot in a clockwise manner. This action moves the wheel  1504  downwardly in a generally vertical manner. Conversely, to move the wheel  1504  generally upwardly, pressurized fluid is withdrawn from the cylinder  1506  such that the rod  1520  thereof retracts to pivot the arm  1512  about the axle assembly&#39;s frame connecting portion  1508 . During this motion, the arm  1512  pivots counterclockwise as shown in FIG. 37 to move the wheel  1504  upwardly in a generally vertical manner. The advantage of this arrangement is that the wheel  1504  remains vertically aligned throughout its entire movement, thereby maintaining the bottom surface of the wheel  1504  generally parallel to the ground. In contrast, with axle assemblies that pivot about axes that extend parallel to the track, the vertical alignment of the wheels constantly changes, causing unnecessary wear on certain portions of the wheels and effecting its traction capabilities. 
     When the vehicle is used as an anti-personnel mine vehicle, the vehicle platform is a relatively small unit and is operated remotely. The low center of gravity and the two wheel/one track arrangement enable the vehicle to traverse the most difficult terrain. The vehicle has a very small footprint pressure of approximately one p.s.i. for a 21,000 twenty one thousand pound prototype. This ensures minimal disruption of the ground. The vehicle&#39;s low center of gravity allows it to travel side ways on slopes which are up to sixty degrees. The vehicle can make a three hundred and sixty degree turn in place. Sealed motor drives which have been developed by Solomon Technologies for marine applications make the vehicle usable in a broad range of environments from desert regions to swamps. 
     The preferred drive motors for demining operations have the trade name Electric Wheel. These motors contain highly efficient DC permanent magnet motors that have a transmission incorporated in the motor. The Electric Wheel is a relatively simple drive that has fewer than ten moving parts. The transmission portion ensures that sufficient torque is available for rugged terrain. 
     The preferred power plant is an electromechanical battery that has been developed and patented by U.S. Flywheel Systems of Newberry Park, Calif. These power plants are generally disclosed in U.S. Pat. Nos. 4,370,899, 5,124,605, 5,268,608, and 5,614,777, each of which are herein incorporated by reference in their entirety. These batteries have an energy density storage capacity twice that of a conventional lead acid batteries and a power density five to ten times greater than lead acid batteries. 
     The flywheel design results in an energy storage system that is safe, can be easily charged from a variety of electrical sources and can provide a reliable source of clean, stable electrical energy. The system uses a small commercially available internal combustion engine that is in the range of five to ten horsepower. The overall unit is therefore small, light and efficient. This results in a smaller vehicle platform that can access a wider range of places such as forests, rocky terrain and so on than currently available and used landmine vehicles. 
     The flywheel provides electric power to drive an electric motor. The electric motor powers a conventional gear box which drives the hydraulic pumps. Alternatively, the flywheel can drive an electric motor to power the wheels or it can drive transmission pump motors. The flywheel design also has ample energy not only to move the vehicle but also to supply electricity for detection and communications equipment on the vehicle. Enviromeasure has successfully developed gamma ray detection instruments and neutron activation analysis devices for rapid chemical composition analysis. These instruments can be used in various applications including identification of agricultural grains and the location of buried deplete uranium shells on military firing ranges. Because these instruments look for specific signature signals for elements found in explosives such as nitrogen and chlorine false positive signals are kept to a minimum. 
     It is contemplated to use multiple gamma ray detectors on a vehicle to obtain the profile of the ordinance buried in a particular area and to pinpoint their locations and depths. Because of the small cross-section capture of neutrons and gamma rays in soil, landmines buried up to a foot can be detected. The detection system built by Enviromeasure for detecting buried depleted urnamium shells was tested at the capital Yuma firing range and it accurately detected buried shells at speeds from one to two feet per second. 
     It is contemplated to use this same detection system for the land mine detection vehicle. Similar travel speeds should be attained. Because of recent advances in gamma ray detectors and neutron generators and because of advances made in computer processing for analytical instruments, the overall size of the detection system incorporated in a land mine detection vehicle would be less than fifty pounds and would have a power requirement of from one hundred to two hundred watts. 
     This detection system could easily be mounted on the above specified small platform vehicle. It is also contemplated to use the vehicle for mapping of detected mines using a GPS (Global Positioning System) satellite system, remote operation using a combination of video and artificial intelligence to traverse a particular area, automated destruction of located detected land mines, a radar sensor system to compliment the neutron activation analyzer and the development of multiple robot units that are controlled by a central mobile command station. 
     It is also contemplated to modify the hybrid electric flywheel powered system for use in the lumber, agriculture, construction and mining industries. This hybrid electric vehicle would have the tractive and propulsive of a bulldozer, the mobility of a four wheel loader and a lower footprint pressure than any tractor currently on the market. The vehicle would also have a smaller internal combustion engine and fewer components than any conventional vehicle. This means that the vehicle would be lighter, more efficient, less expensive and easier to maintain than conventional vehicles. 
     It is also contemplated to use the neutron activation detection system mounted on the vehicle for various environmental and industrial site inspections to locate various environmental contaminants. 
     The vehicle may also be used in the location of various metal ores during deep mining or strip mining operations. The vehicle of the present invention may also be designed so as to be suitable for unmanned underwater operations. Such an unmanned underwater vehicle would be equipped with appropriate technologies for use in very shallow water (VSW)/surface zone (SZ) mine countermeasure missions (MCM). 
     The vehicle would be equipped with appropriate sensory, communications, navigation, signaling and neutralization equipment to enable the detection, classification, identification, mapping/reporting and neutralization of moored, bottomed and buried mines in the near shore very shallow water (VSW) region in support of Explosive Ordnance Disposal and VSW mine countermeasures missions. VSW missions are nominally considered to be 10 feet to 40 feet in depth. 
     It can be appreciated that the vehicle  10  of the invention can be modified without departing from the principles of the invention. For example, although hydraulic power is disclosed for operating the vehicle, other known modes of power can be used. It is also within the contemplation of the invention to adjust the size of the vehicle so as to be employed as a child&#39;s toy. Thus, manual power, such as pedaling, or battery power may be used to propel such a vehicle. Further, the vehicle may be adapted for use as a recreational vehicle. 
     In addition, although the engine and/or power unit is disclosed as being mounted within the envelope of the track, it is within the contemplation of the invention to mount the engine and/or power unit anywhere on the vehicle. For example, the engine and/or power unit may be mounted near the operator&#39;s compartment. Further, the engine and/or power unit need not be movable, but may be fixed with respect to the main frame structure. 
     It thus will be seen that the objects of this invention have been fully and effectively accomplished. It will be realized, however, that the foregoing preferred embodiments of the present invention have been shown and described for the purposes of illustrating the structural and functional principles of the present invention and are subject to change without departure from the spirit and scope of the appended claims.