Abstract:
A power transmission apparatus for an engine of a vehicle which can suppress the changing frequency of the ratio of a continuously variable transmission by compensating for driving force upon low speed rotation upon which the torque generated by an engine is low, and can reduce the overall width of the engine to raise the degree of freedom in mounting of the engine to achieve reduction of the cost. A power transmission apparatus for an engine for a vehicle which includes a starting clutch for smoothly connecting rotation of a crankshaft to a transmission upon starting of the vehicle, a hydrostatic continuously variable transmission for performing speed change depending upon a capacity difference between a swash plate hydraulic pump and a swash plate hydraulic motor to transmit rotation of the crankshaft at a reduced speed to a driving wheel, and a slider for moving a ball screw back and forth to change the angle of the swash plate of the swash plate hydraulic motor. The starting clutch can be a torque converter.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a power transmission apparatus for an engine of a vehicle, and more particularly to a power transmission apparatus for a vehicle such as an off-road vehicle.  
         BACKGROUND  
         [0002]    A power transmission apparatus for an engine of a vehicle which includes a hydrostatic continuously variable transmission is known. In the hydrostatic continuously variable transmission, rotation of a crankshaft is transmitted to a transmission through a starting clutch of the centrifugal clutch type, and speed change is performed depending upon a capacity difference between a swash plate hydraulic pump and a swash plate hydraulic motor. Consequently, the rotation of the crankshaft is transmitted at a reduced speed to a driving wheel. In the power transmission apparatus, a speed change driving shaft is rotated by a stepping motor to move a speed change driving member back and forth along the speed change driving shaft to change the angle of the swash plate of the swash plate hydraulic motor to perform speed change. See, for example, Japanese Patent Laid-Open No. 2001-343060.  
           [0003]    In the prior art described above, control of the vehicle speed is performed by setting of an engine speed and a ratio (change gear ratio) of a hydrostatic continuously variable transmission. Therefore, upon low speed rotation upon which the torque generated by the engine is low, it is necessary to change the ratio of the continuously variable transmission by a great amount to the low ratio side. Further, in order to prevent a stall of the engine and prevent the vehicle speed from being rendered unstable, it is necessary to change the ratio frequently. As a result, in order to increase the stroke amount of the speed change driving member which moves along the speed change driving shaft, it is necessary to set the length of the speed change driving shaft long. Thus, there is a problem that this increases the scale of the stepping motor and gives rise to an increase in cost. Further, in order to cope with frequent changes of the ratio, it is necessary to raise the durability of the stepping motor and associated cost.  
           [0004]    Further, the hydrostatic continuously variable transmission can be used also as a multi-step transmission by fixing the angle of the swash plate of the swash plate hydraulic motor. In this instance, however, similar problems apply also when a driver changes the ratio in accordance with the vehicle speed or the slope of the road surface. Further, significant problems can be associated with an off-road vehicle which runs on an irregular ground which exhibits frequent variations of the road surface situation.  
         SUMMARY  
         [0005]    Therefore, the present invention provides a power transmission apparatus for an engine of a vehicle which can suppress the changing frequency of the ratio of a continuously variable transmission by compensating for driving force upon low speed rotation upon which the torque generated by an engine is low, and can reduce the overall width of the engine to raise the degree of freedom in mounting of the engine to achieve reduction of the cost.  
           [0006]    In accordance with one aspect, a power transmission apparatus for an engine of a vehicle can include a starting clutch for smoothly connecting rotation of a crankshaft to a transmission upon starting of the vehicle, a hydrostatic continuously variable transmission for performing speed change depending upon a capacity difference between a swash plate hydraulic pump and a swash plate hydraulic motor to transmit rotation of the crankshaft at a reduced speed to a driving wheel, and a speed change driving member for moving a speed changing driving shaft back and forth to change the angle of the swash plate of the swash plate hydraulic motor. In addition, the starting clutch can be a torque converter.  
           [0007]    By the configuration described above, the driving force upon low speed rotation of the engine whereupon the torque generated by the engine is low can be compensated by a torque amplification action of the torque converter.  
           [0008]    According to another aspect, the power transmission apparatus for an engine of a vehicle can also be configured such that the crankshaft is disposed in a forward and backward direction of the vehicle while an axial line of a cylinder block is disposed in a substantially upward and downward direction, and an axis of the hydrostatic continuously variable transmission is set to a position higher than that of an axis of the crankshaft while an axis of the speed change driving shaft is disposed in the inside of an angle defined by a line segment passing the axis of the hydrostatic continuously variable transmission and the axis of the crankshaft and the axial line of the cylinder block.  
           [0009]    With the configuration, even if a torque converter, which typically can have an increased outer diameter when compared with a centrifugal clutch, is incorporated, the hydrostatic continuously variable transmission can escape to a position higher than the position of the crankshaft. Further, the overall width of the engine can be reduced by disposing the speed change driving shaft in the inside of the angle defined by the line segment passing the axis of the hydrostatic continuously variable transmission and the axis of the crankshaft and the axial line of the cylinder block.  
           [0010]    The above summary is not intended to describe each disclosed embodiment or every implementation of the present invention. Figures and the detailed description that follow more particularly exemplify embodiments of the invention. While certain embodiments will be illustrated and described, the invention is not limited to use in such embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Aspects of the invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings.  
         [0012]    [0012]FIG. 1 is a side elevational view of a four-wheeled vehicle according to an embodiment of the present invention.  
         [0013]    [0013]FIG. 2 is a schematic sectional view of a power unit according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 3 is a longitudinal sectional view of the power unit of FIG. 2 taken along a plane parallel to several axes including a crankshaft and a driving shaft of a hydrostatic continuously variable transmission.  
         [0015]    [0015]FIG. 4 is a sectional view of a ratio changing mechanism in accordance with an embodiment of the present invention.  
         [0016]    [0016]FIG. 5 is a schematic diagrammatic view of the power unit in accordance with an embodiment of the present invention as viewed from the front.  
         [0017]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0018]    In the following, the present invention is described with reference to the drawings taking an embodiment thereof that is applied to an off-road vehicle as an example.  
         [0019]    Referring to FIG. 1, a four-wheeled vehicle for running on an irregular ground is configured such that a pair of left and right front wheels (driving wheels)  2  and a pair of left and right rear wheels (driving wheels)  3  are provided on the front side and the rear sides of a body frame  1 , respectively. A power unit  4  including a four-cycle engine and a transmission integrated with the engine is supported at a central portion of the body frame  1 . The power unit  4  is of a longitudinal type in which a crankshaft  5  extends in a forward and backward direction of the vehicular body. The four-wheeled vehicle is of a four-wheeled drive type and includes an output shaft  6  provided at a lower portion of the power unit  4  and extending in parallel to the crankshaft  5 . The output shaft  6  drives the front wheels  2  via a front wheel propeller shaft  7  and drives the rear wheels  3  via a rear wheel propeller shaft  8 .  
         [0020]    The front side of a crankcase  10  which composes the power unit  4  is covered with a front case cover  11 , and the rear side of the crankcase  10  is covered with a rear case cover  12 . The front case cover  11  and the rear case cover  12  cooperatively form the crankcase  10 . The crankcase  10  is divided forwardly and backwardly into a front case  10   a  and a rear case  10   b.    
         [0021]    A cylinder block  13 , a cylinder head  14 , and a cylinder head cover  15  are mounted at an upper portion of the crankcase  10 . A carburetor  16  is connected to an intake port of the cylinder head  14 , and an air cleaner  17  is connected to the carburetor  16  from the rear side. An exhaust pipe  18  is connected to an exhaust port of the cylinder head  14 .  
         [0022]    An oil cooler  20  is disposed forward of the power unit  4 . The oil cooler  20  communicates with an oil pump provided on the crankcase  10  through a feed side hose  21 . Further, the oil cooler  20  communicates with the oil pump provided in the crankcase  10  through a return side hose  22 . In FIG. 1, reference numeral  23  denotes a cooling fan,  24  a handle bar,  25  a fuel tank, and  26  a saddle type seat. Reference numeral  27  denotes an oil tank, which is attached directly to the front face of the front case cover  11 . The oil tank  27  is connected to the oil cooler  20  through the feed side hose  21  and the return side hose  22  and is communicated also with the oil pump built in the power unit  4 .  
         [0023]    The power unit  4  is described below with reference to FIG. 2. In particular, reference numeral  30  denotes a valve,  31  a piston, and  32  a connecting rod. A torque converter (starting clutch)  33  is provided at one end side of the crankshaft  5  to which the connecting rod  32  is attached. The torque converter  33  has a known structure including a pump shell  33   a,  a turbine runner  33   b,  and a stator  33   c.  The pump shell  33   a  is secured to the crankshaft  5 , and the turbine runner  33   b  is connected to a primary driving gear  34 . An ACG  35  is provided on the other end side of the crankshaft  5 .  
         [0024]    The crankshaft  5  is supported for rotation by a journal wall  36   a  integral with the front case  10   a  and a journal wall  36   b  integral with the rear case  10   b  through main bearings  37   a  and  37   b,  respectively. A hydrostatic continuously variable transmission  40  is built in the crankcase  10  that forms an engine section of the power unit  4 . Approximately one-half of the hydrostatic continuously variable transmission  40  in its lengthwise direction is located between the main bearings  37   a  and  37   b.    
         [0025]    The hydrostatic continuously variable transmission  40  includes a hydraulic pump  42  of the swash plate type driven by a primary driven gear  41  held in meshing engagement with the primary driving gear  34 . The hydrostatic continuously variable transmission  40  further includes a hydraulic motor  44  of the swash plate type driven by oil discharged from the hydraulic pump  42  for outputting rotational force at a changed speed to a driving shaft  43  that is a transmission shaft. The hydraulic motor  44  and the hydraulic pump  42  are disposed in parallel on the driving shaft  43 . It is to be noted that, in FIG. 2, the internal structures of the hydraulic pump  42  and the hydraulic motor  44  are omitted for the convenience of illustration. The driving shaft  43  is disposed with an axis thereof directed in parallel to and in register with the crankshaft  5  in the forward and backward direction of the vehicle body. An oil passage  45  is formed along the axis of the driving shaft  43  in such a manner as to pass through the drive shaft  43  in the lengthwise direction. An end of the driving shaft  43  is directly connected by spline-coupling to a main shaft  47  of a multi-step transmission  46 .  
         [0026]    A first-speed driving gear  48  and a second-speed driving gear  49  are integrally provided on the main shaft  47 . The gears  48  and  49  are disposed for meshing engagement with a first-speed driven gear  51  and a second-speed driven gear  52 , respectively. The first-speed driven gear  51  and the second-speed driven gear  52  rotate on a countershaft  50  disposed in parallel to the main shaft  47 . A reverse driven gear  53  is provided for rotation on the counter shaft  50 . The reverse driven gear  53  is rotated in the direction reverse to the rotational direction of the first-speed driven gear  51  and the second-speed driven gear  52  through a reverse idle gear meshed with the first-speed drive gear  48  on a separate shaft not shown.  
         [0027]    Shifters  54  and  55  are spline-coupled to the counter shaft  50  for movement in the axial direction. When the shifter  54  is at a position moved leftward in FIG. 2, rotation of the first-speed driven gear  51  is transmitted from the counter shaft  50  to a final driving gear  56  integrally provided at an axial end of the countershaft  50 . The rotation is further transmitted to the output shaft  6  via a final driven gear  57  provided on the output shaft  6  and held in meshing engagement with the final drive gear  56 .  
         [0028]    When the shifter  55  is at the position moved leftwardly, rotation of the second-speed driven gear  52  is similarly transmitted to the output shaft  6 , thereby achieving the second-speed drive mode. On the other hand, when the shifter  54  is at another position moved rightward, rotation of the reverse driven gear  53  is transmitted to the countershaft  50  to reversibly rotate the counter shaft  50  to reversibly rotate the output shaft  6 , thereby achieving a backward drive mode. An oil passage  58  is formed along the axis of the main shaft  47  and extends through the main shaft  47  in a communicating relationship with the oil passage  45  of the drive shaft  43 . Another oil passage  59  similar to the oil passage  58  is formed along the axis of the countershaft  50 . The oil passage  59 , however, is configured such that an inner end thereof is closed up and an open end thereof on the outer side is exposed to an oil passage  60  formed in a thick wall portion of the rear case cover  12 . Oil having passed through the main shaft  47  is supplied to the oil passage  60 . Oil supplied from the oil passage  59  is supplied, via an oil passage provided in the rear case cover  12  separately from the oil passage  60 , to the ACG  35  and a valve mechanism of the cylinder head  14  to lubricate the ACG  35  and the valve mechanism. An oil passage  62  is formed also along the axis of the crankshaft  5 . Oil is supplied to the oil passage  62  via an oil passage provided in the front case cover  11  and lubricates the torque converter  33  serving as a starting clutch and the bearing portions of the crankshaft  5 .  
         [0029]    Now, the hydrostatic continuously variable transmission  40  is described with reference to FIG. 3. The hydrostatic continuously variable transmission  40  changes the speed of rotation of the crankshaft  5  in accordance with a difference in capacity between the hydraulic pump  42  and the hydraulic motor  44  to transmit the rotation at a reduced speed to the front wheels  2  and the rear wheels  3 .  
         [0030]    The hydraulic pump  42  and the hydraulic motor  44  which compose the hydrostatic continuously variable transmission  40  include housings  70  and  71 , respectively. The housing  70  is formed integrally as part of the front case cover  11  while the housing  71  is formed integrally as part of the front case  10   a.  The driving shaft  43  is supported at the opposite ends thereof for rotation on the housings  70  and  71  through bearings  72  and  73 , respectively.  
         [0031]    The hydraulic pump  42  includes an input side rotary section  74  supported for rotation on the driving shaft  43  through a bearing  75 . The input side rotary section  74  rotates integrally with the primary driven gear  41 . A fixed swash plate  76  is supported for rolling contact with an inner periphery of the input side rotary section  74  through bearings  77  and  77 ′ such that it is inclined with respect to an axial direction of the driving shaft  43 . A pump cylinder  79  is provided in an opposing relationship to the fixed swash plate  76  on the driving shaft  43 . A plurality of pump plunger holes  80  is disposed annularly around the axis of the pump cylinder  79  in the pump cylinder  79 . A pump side plunger  78  is provided for back and forth movement in each of the pump plunger holes  80  and slideably contacts at an end thereof with the fixed swash plate  76  to perform an oil sucking stroke and an oil discharging stroke. The input side rotary section  74  is supported for relative rotation on an outer circumference of the pump cylinder  79  through a bearing  81 .  
         [0032]    Meanwhile, the hydraulic motor  44  includes a swash plate holder  83  of a substantially cup shape supported for rolling movement in a concave curved face portion  82  formed in the housing  71 . A movable swash plate  86  is supported for rolling movement in the concave curved face through bearings  84  and  85 . A swash plate is formed from the swash plate holder  83  and the movable swash plate  86 .  
         [0033]    A number of motor side plungers  87  equal to the number of the pump side plungers  78  are moved back and forth in motor plunger holes  89  from and toward the surface of the movable swash plate  86  to perform a projecting stroke and a retreating stroke. The motor plunger holes  89  are disposed annularly around an axis of a motor cylinder  88  provided on the axis of the driving shaft  43  similarly.  
         [0034]    The motor side plungers  87  are projected by pressure oil discharged by the pump side plungers  78  and press the surface of the movable swash plate  86  to apply rotational force to the motor cylinder  88 . Since the inner circumferential face of the motor cylinder  88  is held in a spline-coupled state with the outer periphery of the driving shaft  43 , the motor cylinder  88  outputs the input power from the primary driven gear  41  at a changed speed to the driving shaft  43 . The change gear ratio in this instance can be adjusted by varying the inclination of the movable swash plate  86 , and the inclination of the movable swash plate  86  can be varied freely by rotating the swash plate holder  83 . The motor cylinder  88  is supported at the outer periphery thereof for rotation on the housing  71  through a bearing  90 .  
         [0035]    More specifically, the change gear ratio of the driving shaft  43  with respect to the primary driven gear  41  (input side rotary section  74 ) is given by the following expression:  
         [0036]    change gear ratio=(capacity of hydraulic motor  44 )/(capacity of hydraulic pump  42 )  
         [0037]    Accordingly, if the capacity of the hydraulic motor  44  is varied from its maximum to zero, then the change gear ratio can be changed from a maximum value (the low gear state) to 1 (the top gear state).  
         [0038]    The pump cylinder  79  and the motor cylinder  88  are integrated with each other at a great diameter portion  91  at a middle portion therebetween. A number of pump side valves  92  and motor side valves  93  equal to the number of pump side plungers  78  and motor side plungers  87  are disposed annularly in a juxtaposed relationship in two rows on the great diameter portion  91 . The pump side valves  92  and the motor side valves  93  are disposed for movement in radial directions. The pump side valves  92  and the motor side valves  93  open and close communicating portions of an inner side passage  94  and an outer side passage  95  with the pump plunger holes  80  and the motor plunger holes  89 , respectively. The inner side passage  94  and the outer side passage  95  are formed concentrically on the inner side of the great diameter portion  91 .  
         [0039]    In particular, in the suction stroke of the pump side plungers  78 , the pump side valves  92  establish communication between the pump plunger holes  80  and the inner side passage  94  but interrupt the communication between the pump plunger holes  80  and the outer side passage  95 . In the discharge stroke, the pump side valves  92  operate reversely. Similarly, in the projecting stroke of the motor side plungers  87 , the motor side valves  93  establish communication between the motor plunger holes  89  and the outer side passage  95  but interrupt communication between the motor plunger holes  89  and the inner side passage  94 . In the retreating stroke, however, the motor side valves  93  operate reversely.  
         [0040]    Subsequently, a ratio changing mechanism  120  for changing the inclination angle of the movable swash plate  86  to change the change gear ratio is described with reference to FIG. 4. A link arm  63  projects to the outside of the housing  71  from the swash plate holder  83  in which the movable swash plate  86  is accommodated for rolling movement. The link arm  63  is connected at an end thereof for pivotal motion to a slider (speed change driving member)  65  on a ball screw (speed change driving shaft)  64  by means of a pin  63   a.  The inclination of the movable swash plate  86  can be varied by rotating the ball screw  64  forwardly or reversely to move the slider  65  axially in the leftward or rightward direction. The ball screw  64  is supported at the opposite ends thereof for rotation by bearings  67  and  68  on stays  66   a  and  66   b  integrated with the housing  71 . A driven gear  69  is attached to an end of the ball screw  64 .  
         [0041]    The driven gear  69  is driven by an output gear  102  of an electric motor  101  through a torque limiter  100 . The torque limiter  100  includes a rotary shaft  104 . A second speed reduction gear  105  having a diameter smaller than that of the driven gear  69  is provided at an end of the rotary shaft  104  and held in engagement with the driven gear  69 . A first speed reducing gear  106  having a diameter greater than that of the output gear  102  of the electric motor  101  is provided at the other end of the rotary shaft  104  and held in meshing engagement with the output gear  102 .  
         [0042]    The first speed reducing gear  106  includes a cylindrical member  108  for being coupled to or uncoupled from the rotary shaft  104  through a plurality of friction plates (not shown). The friction plates are pressed from the second speed reduction gear  105  side by a set spring  109  in the form of a coil spring to form a friction plate clutch mechanism.  
         [0043]    In an ordinary state, torque within preset load of the set spring  109  is transmitted between the output gear  102  of the electric motor  101  and the driven gear  69 . Accordingly, in the ordinary state, rotation of the output gear  102  is transmitted from the first speed reducing gear  106  to the rotary shaft  104  through the cylindrical member  108  and the friction plates on the inner side of the cylindrical member  108 . The rotation is further transmitted from the second speed reduction gear  105  to the ball screw  64  through the driven gear  69 .  
         [0044]    As a result, when the ball screw  64  rotates, the slider  65  moves in response to the rotation of the ball screw  64  and pivots the swash plate holder  83  through the link arm  63  to change the inclination of the movable swash plate  86  supported on the inner side of the swash plate holder  83  thereby to adjust the change gear ratio. On the other hand, if the transmission torque between the output gear  102  and the driven gear  69  exceeds the preset load of the set spring  109 , then a slip appears between the plurality of friction plates. Consequently, the rotation of the first speed reducing gear  106  is not transmitted to the rotary shaft  104  but is interrupted.  
         [0045]    Subsequently, arrangement of the ratio changing mechanism  120  is described with reference to FIG. 5.  
         [0046]    [0046]FIG. 5 is a schematic arrangement view of interior parts as viewed from the front side of the vehicle body.  
         [0047]    The cylinder block  13  is attached to an upper portion of the crankcase  10 , and the cylinder head  14  and the cylinder head cover  15  are attached to an upper portion of the cylinder block  13 . The oil tank  27  is provided on the front face of the crankcase  10 . It is to be noted that the exhaust pipe  18  is attached to the cylinder head  14 .  
         [0048]    The crankshaft  5  is disposed in the inside of the crankcase  10 , and the torque converter  33  is attached to the crankshaft  5 . The driving shaft  43  of the hydrostatic continuously variable transmission  40  is disposed on the right side of the crankshaft  5  above a horizontal line H which passes an axis C of the crankshaft  5 . The output shaft  6  is disposed below the horizontal line H of the crankshaft  5 .  
         [0049]    An axis B of the ball screw  64  of the ratio changing mechanism  120  is disposed in the inside of an included angle α. The included angle α is defined by a line segment L 1  passing an axis K of the driving shaft  43  of the hydrostatic continuously variable transmission  40  and the axis C of the crankshaft  5  and an axial line L 2  of the cylinder block  13  disposed in a substantially upward and downward direction. The included angle α is set within a range from 60 to 90 degrees and preferably set to approximately 75 degrees. Meanwhile, an included angle β defined by a line segment L 3  interconnecting the axis K of the driving shaft  43  of the hydrostatic continuously variable transmission  40  and the axis B of the ball screw  64  and the axial line L 2  of the cylinder block  13  is set within a range from 0 to 15 degrees and preferably set to approximately 10 degrees.  
         [0050]    According to the embodiment described above, the torque converter  33  is provided as a starting clutch. Consequently, the driving power upon low speed rotation of the engine whereupon the torque generated by the engine is low can be compensated by a torque amplification action of the torque converter  33 . Accordingly, the ratio of the hydrostatic continuously variable transmission  40  need not be changed frequently, and an engine stall upon low speed rotation is substantially reduced and further stabilization of the vehicle speed can be realized. Accordingly, the embodiment described above can be applied suitably to an off-road vehicle which runs on an irregular ground at a low engine speed.  
         [0051]    Consequently, the hydrostatic continuously variable transmission  40  is released from frequent ratio changing operations also where it is used as a multi-step transmission by fixing the angle of the movable swash plate  86  of the hydraulic motor  44 .  
         [0052]    Further, since the driving power upon low speed rotation of the engine whereupon the torque generated by the engine is low can be compensated by a torque amplification action of the torque converter  33 , the stroke amount of the slider  65  which moves along the ball screw  64  can be designed smaller as much. Accordingly, miniaturization of the stepping motor can be realized, and the arrangement space can be reduced. Consequently, frequent ratio change is eliminated and the durability of the stepping motor can be improved. Accordingly, the requirement for a countermeasure for raising the durability is eliminated and reduction of the cost can be realized.  
         [0053]    Further, the crankshaft  5  is disposed such that it extends in the forward and backward direction of the vehicle and the axial line L 2  of the cylinder block  13  is disposed in the substantially upward and downward direction. Furthermore, the axis K of the driving shaft  43  of the hydrostatic continuously variable transmission  40  is set to a position higher than the horizontal line H passing the axis C of the crankshaft  5 . In addition, the axis B of the ball screw  64  of the ratio changing mechanism  120  is disposed in the inside of the included angle α defined by the line segment L 1  passing the axis K of the hydrostatic continuously variable transmission  40  and the axis C of the crankshaft and the axial line L 2  of the cylinder block  13 . Consequently, it is possible to dispose the hydrostatic continuously variable transmission  40  and the ratio changing mechanism  120 , which significantly affects the widthwise dimension of the vehicle, rather near to the cylinder block  13  to reduce the overall width of the engine. Consequently, the mounting facility of the engine and the balance of the vehicle in the leftward and rightward direction can be improved.  
         [0054]    It is to be noted that the present invention is not limited to the embodiment described above, and for example, equally applies to a three-wheeled vehicle. Further, while a four-wheeled drive vehicle is described as an example, the present invention can be applied also to a two-wheeled drive vehicle.  
         [0055]    The above specification, examples and data provide a complete description of the manufacture and use of various aspects of the invention. Many alternative embodiments of the invention can be made without departing from the spirit and scope of the invention.