Abstract:
A control system for a lawnmower includes a handlebar connected to the lawmnower for controlling direction and speed of travel of the lawmower based on a position of the handlebar relative to the lawnmower. The control system is connected to a drive system of the lawnmower such that changing the elevation of the handlebar adjusts operation of a number of wheel drive units and turning of the handlebar relative to the lawnmower adjusts operation of one wheel drive unit relative to another wheel drive unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Provisional Application Ser. No. 60/743,610 filed on Mar. 21, 2006, titled “Steering System For A Lawnmower” and the disclosure of which is incorporated herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a steering system and, more particularly, relates to a steering system for a self-propelled lawnmower. 
     2. Discussion of the Related Art 
     Lawnmowers equipped with dual hydraulically-powered drive systems that propel the drive wheels on each side of the mower are known in the grooming a lawns. Turning is achieved by driving each drive wheel at different speeds. They can even be turned in different directions for a very sharp or even a “zero-turn” radius, where the mower pivots about the midpoint drive wheel axis. This kind of drive wheel steering system is more maneuverable than traditional vehicle steering systems that employ one set of wheels, either front or rear, which turned in unison while other drive wheels are powered to drive the mower in a forward or rearward direction. 
     Hydraulically-powered drive systems typically include a separate variable displacement hydraulic pump for each of the drive wheels. Each variable displacement pump is typically an axial-piston type which includes a tilting swash plate which can vary the pump discharge rate from a zero flow, also referred to as neutral, up to a maximum flow or reverse direction flow to a particular maximum. 
     The pumps are controlled by an operator controlled steering mechanism. Several such mechanisms are known, all of which have disadvantages. 
     For example, U.S. Pat. No. 5,822,961, entitled “Quick Adjustment for Straight Ahead Travel for a Wheel-Steered Lawn Mower,” discloses a pistol-grip type of steering mechanism for adjusting a dual-hydraulically-powered walk-behind mower. Right and left steering control levers can be squeezed to control steering of the walk-behind mower. 
     A drawback of this type of steering control mechanism is that it requires squeezing with the operator&#39;s fingers to maneuver the mower. Repeated squeezing motions causes fatigue and increases the likelihood of carpal tunnel syndrome. Vibrations are also transmitted to the operator&#39;s hands. 
     Another drawback is that this type of steering control is not adjustable for variable sizes of hands. The physics of operating larger machinery requires larger steering linkages, which are more difficult to operate with smaller hands. This further increases fatigue and the likelihood of carpal tunnel syndrome. 
     U.S. Pat. No. 5,913,802, entitled “Single Lever Drive Wheel Steering Power Lawn Mower,” discloses a self-propelled mower equipped with a pair of variable flow hydraulic pumps controlled by a T-shaped steering handle. The T-shaped steering handle is configured to rotate about a vertical axis so as to transmit control forces to the variable displacement pumps so as to control the steering of the lawn mower. The handlebar can also be rotated or twisted about a horizontal axis to control the speed of the mower from a stationary neutral position or a reverse direction position. This steering control mechanism has similar drawbacks to that described above for the pistol-type grip lever. Repeated twisting motions with the wrists to control steering of the mower causes fatigue and increases the likelihood of carpal tunnel syndrome. Another drawback is the difficulty encountered in simultaneously twisting the grip and rotating a grip handle to maneuver a turn of the mower. 
     In addition, speed limiters for traditional lawnmowers must be operated independently of the steering system, complicating operation and making the lawnmower more difficult to control during a turn. 
     SUMMARY OF THE INVENTION 
     In light of the foregoing, a steering assembly is desired that improves the state of the art by overcoming the aforesaid problems of the prior art. 
     It is also desired to provide a steering system that reduces or dampens the transfer of vibration from the engine to the steering handle of the mower. 
     It is also desired to provide a steering system that reduces the forces required to operate the mower, thereby further reducing the transfer of vibration to the operator&#39;s grip. 
     It is also desired to provide a mower having a steering system with a steering handle operatively connected to the first and second drives, where rotational movement of the steering handle from a central longitudinal position steers the mower to turn in the direction from a forward direction of travel, and where movement of the steering handle in a vertical direction controls forward propulsion of the mower. 
     It is also desired to provide a steering system for a mower having a speed limiter assembly that includes speed limiter reference bar variably positioned relative to the steering handle, where engagement of the steering handle along the speed limiter reference bar automatically slows a speed of the mower maneuvering a turn relative to a speed of the mower in the forward direction. 
     It is also desired to provide a speed limiter assembly having a speed limiter reference device that is variably positioned to adjust a maximum speed of the mower. 
     It is also desired to provide a method of operating a mower that includes at least some of the steps in accordance with the foregoing summary. 
     In accordance with an aspect of the invention, at least some of these desires are fulfilled by providing a steering assembly for a zero turn lawnmower or other vehicle that is handlebar or yoke based rather than squeeze grip based. A handle is coupled to the swash plate actuators of left and right pumps by linkages that result in forward, rearward, and/or turning motion of the lawnmower depending on the direction and magnitude of handle movement. For instance, pivoting of the handle about a horizontal axis upwardly or downwardly from a neutral position may result in reverse or forward propulsion of the lawnmower at speed proportional to the magnitude of pivoting. Pivoting of the handle about a vertical axis may result in turning of the lawnmower to the left or the right. The handle or another type of steering device may cooperate with an adjustable speed limiter that can be actuated to set a maximum speed of the lawnmower. The speed limiter may be configured to cause the lawnmower to automatically decelerate through a turn. The handle and speed limiter may be provided either singly or in combination and may apply to either riding or walk behind lawnmowers and even to other hydraulically powered self propelled machines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate a preferred and exemplary embodiment of the invention. 
       In the drawings: 
         FIG. 1  is a left-side elevational view of a lawnmower equipped with a control system according the present invention. 
         FIG. 2  is a rear end elevational view of the lawnmower shown in  FIG. 1 . 
         FIG. 3  is a right side elevational view of a portion of the lawnmower shown in  FIG. 1 . 
         FIG. 4  is a side elevational view of a drive and control linkage system of the lawnmower shown in  FIG. 1 . 
         FIG. 5  is an elevational view of a portion of the drive and control linkage system shown in  FIG. 5 . 
         FIG. 6  is detailed view of the portion of one of the control linkages shown in  FIG. 5 . 
         FIG. 7  is a detailed view of a second of the control linkages shown in  FIG. 1  with the respective links oriented in a first position. 
         FIG. 8  is a view similar to  FIG. 7  with the control linkages moved to a second position. 
         FIG. 9  is an elevational view of an intermediary support of the drive and control system shown in  FIG. 4 . 
         FIG. 10  is a side elevational view similar to  FIG. 3  showing the orientation of a handlebar relative to a speed reference bar when a speed control lever is oriented in a first position. 
         FIG. 11  is a view similar to  FIG. 10  and shows the orientation of the handlebar relative to the speed reference bar when the speed control leveler is positioned in a second position. 
         FIG. 12  is a detailed view of an adjustor of the drive and control system shown in  FIG. 3 . 
         FIG. 13  is a view similar to  FIG. 2  and shows the position of the handlebar relative to the speed reference bar during a turning operation of the lawnmower. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawings illustrate a preferred exemplary embodiment of a preferred embodiment of the invention as incorporated into a walk behind zero turn lawnmower  20 . Understandably, one of ordinary skill in the art will appreciate that the control system as disclosed herein is usable with other equipment configurations such stand-on mowers, riding mowers, or other equipment categories such as power buggies, utility carts, or the like. Generally, various configurations of these types of machines are generically defined by the term utility vehicle or a vehicle have more uses that for merely recreational transportation. With respect to the embodiment of a self-propelled zero turn walk behind mower illustrated in the accompanying drawings, it will be appreciated that like reference numerals represent like parts throughout the drawings. 
     Referring to  FIGS. 1-3 , walk-behind, self-propelled lawnmower  20  includes a frame  22  having an engine  24 , a control system  26 , and a mower deck  28  supported thereon. A pair of independently-driven drive wheels  40  and  42  and a pair of casters  44 ,  46  are attached to frame  22  to elevate mower deck  28  above a cutting surface. Drive wheels  40 ,  42  are operatively connected to engine  24  and control system  26  to drive lawnmower  20 . Casters  44 ,  46  are undriven and are pivotally attached at a front end  47  of frame  22  relative to a forward direction of travel of mower  20 . Deck  28  of this embodiment is positioned between drive wheels  40  and  42  and casters  44  and  46  and is a multi-blade cutting deck  28  and includes multiple rotating cutting blades which are positioned and driven in a conventional manner. Understandably, deck  28  could alternatively be a single blade cutting deck. It is also appreciated that deck  28  is movably attached to frame  22 , thereby allowing a user to specify a distance of deck  28  from a cutting surface to provide a number of desired cutting heights. A deck adjustment lever  30  allows an operator to raise and lower deck  28  when the operator is positioned proximate control system  26 . 
     Referring specifically to  FIG. 2 , a pair of variable flow hydraulic displacement pumps  48 ,  50  are positioned rearward of the drive wheels  40  and  42  and provide the hydraulic pressure associated with the operation of each of the drive wheels  40  and  42 , respectively. Each hydraulic displacement pump  48 ,  50  is mechanically interconnected to impel a hydraulic, low speed, high-torque motor  52 ,  54  to drive the associated drive wheel  40 ,  42 . Collectively, each side pair of a pump and a motor forms a drive of lawnmower  20 . That is, pump  48  and motor  52  form a first drive and pump  50  and motor  54  form a second drive. Preferably, each variable displacement pump  48 ,  50  is an axial-piston type pump which includes an internal tilting swash plate (not shown) which can be rotated to vary the pump discharge rate from a zero flow, also referred to as neutral, up to a maximum flow in either the forward or reverse directions. A pair of calibration bodies, or tuning control dials  49 ,  51  extend from mower  20  and are configured to allow an operator to calibrate the output of each respective pump  48 ,  50  such that mower  20  moves in a straight line when no turning function is being performed. Such operation is discussed further below with respect to  FIG. 5 . 
     Control system  26  is positioned proximate an operator control area  56 . Control system  26  includes a handlebar  58  and a speed control linkage  60  which controls operation of pumps  48 ,  50 . Handlebar  58  is attached to frame  22  and the remainder of control system  26  such that an operator can control the direction and speed of lawnmower  20  by simply moving handlebar  58  with respect to lawnmower  20 . Handlebar  58  includes a left or first grip portion  62  and a right or second grip portion  64  that are located at generally opposite ends of handlebar  58 . A yoke assembly  66  connects handlebar  58  to the frame  22  of the lawnmower  20  such that the handlebar  58  can move in a side-to-side or lateral direction and an up-and-down or vertical direction. As described further below with respect to  FIGS. 10-13 , the position of handlebar  58  relative to lawnmower  20  controls the operation of motors  52 ,  54  and therefore, controls the direction and speed of travel of lawnmower  20 . Understandably, the lateral and vertical movement of handlebar  58  could be facilitated with any of a linear, pivotal, or swivel connection between handlebar  58  and control system  26 . That is, it is appreciated that various connection modalities are available to provide the lateral and vertical movement of handlebar  58  beyond the assembly connection configuration shown in the appended drawings. 
     A grip system  65  is included in each grip portion  62 ,  64  of handlebar  58 . Each grip system  65  is configured to be contacted by an operator controlling mower  20 . Each grip system  65  preferably requires a minimum or a threshold contact to maintain continued operation of mower  20 . That is, the grip must be engaged for the lawnmower  20  to run under certain operating conditions, such as when the mower is not in neutral. Preferably, grip system  65  is configured to monitor handlebar  58  to detect the presence of an operator. More preferably, this monitoring occurs at intervals of, e.g., approximately every 0.5 seconds, although other durations are envisioned. Grip system  65  relies on a minimum threshold contact by the operator to allow movement of mower  20 . A preferred grip is the Senso-GRIP® as manufactured by MAGURA®. Accordingly, handlebar  58  is grip pressure independent and configured to allow operation of mower  20  even though an operator&#39;s hand(s) may not be generally wrapped about either one of grip portions  62 ,  64 . That is, grip system  65  functions as an operator presence detector rather than an operator&#39; grip pressure detector. This pressure independence provides for reduced operator effort and fatigue in interacting with mower  20 . Additionally, grip system  65  provides for a control interface with a reduced number of movable parts. Furthermore, as grip system  65  is grip pressure independent, handlebar  58  reduces the potential for operators&#39; developing carpal tunnel syndrome as a result of repeatably compressing a lever actuated dead-man like switch which generally include a pair of oppositely oriented levers that must be squeezed together. Grip system  65  could be configured to operate according to various principles including piezoelectric, pneumatic, humidity, or capacitance. Furthermore, grip system  65  could be integrated in a number ways with mower  20 . That is, grip system  65  can be configured to allow starting and operation of engine  24  and only interfere with the operation of mower  20  when deck  28  is operating or mower  20  is moving. Preferably, if either the mower is moving or the deck is operating, if grip system  65  does not detect the contact presence of an operator at one of grip portions  62 ,  64 , grip system  65  interrupts operation of mower  20 , engine  24 , and/or deck  28 . Grip system  65  is integrated with control system  26  to terminate operation of engine  24  thereby stopping operation of mower  20 . 
     An interlock, a speed limiter, or reference bar  68  extends from lawnmower  20  generally beneath handlebar  58 . Reference bar  68  is movably attached to mower  20  to define a range of motion of handlebar  58  during various operating condition of mower  20 . Limiting the range of motion of handlebar  58  with reference bar  68  limits the operating speed that can be attained as determined by the position of reference bar  68  relative to handlebar  58 . During high speed operation of mower  20 , reference bar  68  allows for a greater range of motion of handlebar  58  and during lower speed operation of mower  20 , reference bar  68  limits the range of motion of handlebar  58  thereby limiting the operational speed of mower  20 . 
     As best shown in  FIG. 10 , reference bar  68  is operationally connected to speed control linkage  60  to provide a variable distance  69  between handlebar  58  and reference bar  68 . Reference bar  68  has a generally curvilinear shape that extends between a pair of ends  71 ,  73  that are pivotably connected to mower  20 . End  73  is operationally connected to speed control linkage  60  so that the relative position of the speed control linkage  60  defines the position of reference bar  68  relative to handlebar  58 . Variable distance  69  determines a range of motion of handlebar  58  and, thereby, an operating speed range of lawnmower  20 . Understandably, although reference bar  68  is shown as extending rearward of lawnmower  20  generally beneath handlebar  58 , other constructions are envisioned and within the scope of the claims. That is, reference bar  58  could be configured to interact with any of a number of links of control system  26  or interact with handlebar  58  without otherwise extending from lawnmower  20 . The term “bar” therefore should not be narrowly construed to be limited to any particular shape. 
     As shown in  FIG. 3 , speed control linkage  60  includes a control lever  70 , a control plate  72 , and a reference bracket  74 . As described further below with respect to  FIGS. 10 and 11 , operator manipulation of control lever  70  controls the maximum operating speed of pumps  48 ,  50 . Understandably, control lever  70  could be configured to also control the operating condition of engine  24 . Manipulation of control lever  70  relative to control plate  72  determines, in part, the position of reference bar  68  relative to handlebar  58 . A number of notches or catches  76  are formed in control plate  72  and are configured to maintain control lever  70  in the desired position relative to lawnmower  20 . Such a construction allows an operator to maintain two hand contact with handlebar  58  after the operator has configured lawnmower  20  for desired operation. 
     A brake system  78  [ FIGS. 2 and 3 ] is also positioned proximate speed control linkage  60 . Brake system  78  includes a brake lever  80  that is pivotably connected to frame  22  of lawnmower  20 . A brake linkage  82  extends between brake lever  80  and a drive interlock  83  that is constructed to interfere with the operation of drive wheels  40 ,  42  so that lawnmower  20  cannot move when brake system  78  is engaged. Orienting speed control linkage  60  and brake lever  80  on a common side of lawnmower  20  allows an operator to adjust the operating condition of lawnmower  20  while maintaining at least one hand in engagement with handlebar  58 . 
       FIG. 4  is an elevational view of a drive and control linkage system of control system  26  of lawnmower  20 .  FIGS. 5-9  show respective portions of control system  26  in various orientations achieved during operation of lawnmower  20 . Referring to  FIGS. 4-9 , and as previously mentioned, the swash plate of each pump  48 ,  50  is rotated by a shaft  84 ,  86  that extends out of a side of the pumps  48 ,  50 , respectively. Each shaft  84 ,  86  is driven to rotate by a swash plate drive linkage or lever assembly  88 ,  90  to which an associated drive rod  92 ,  94  is connected. 
     Each lever assembly  88 ,  90  is biased to a neutral position by a return spring  93 ,  95 . As best seen in  FIGS. 4-6 , the lever assemblies  88 ,  90  are inverted relative to one another such that the return spring  93  of left pump  48  is positioned above the shaft  84  of left pump  48  and the return spring  95  of the right pump  50  is positioned below the shaft  86  of the right pump  50 . This permits motion of the drive rods  92 ,  94  in the same direction (i.e. up or down) to result in turning of the shafts  84 ,  86  in the same direction despite the fact that the pumps  48 ,  50  are facing each other and, therefore, of opposite orientations. 
     Handlebar  58  is rotatable about generally vertical axis to steer the lawnmower  20 . As shown in  FIG. 2 , a left linkage  96  and a right linkage  98  extend from handlebar  58  generally between grip portions  62 ,  64 . The left linkage  96  is located on one side of a longitudinal center line  105  of lawnmower  20  generally opposite right linkage  98 . Referring to  FIGS. 2 and 4 , handlebar  58  is mechanically connected to a generally vertical steering shaft  110  via the linkages  96 ,  98  to rotate with a steering shaft  110  about the axis of the steering shaft  110 . This relation controls steering of the mower  20  in the left and right turn directions. The forward end of left generally linkage  96  is pivotally connected by a flex joint  115  at an upper end of a first or left vertical drive rod  120  that is mechanically connected to the lever assembly  88  of hydraulic pump  48 . In a similar manner, the forward end of the right linkage  98  is pivotally connected by a flex joint  125  at an upper end of a second or right generally vertical drive rod  130  that is mechanically connected to the lever assembly  90  of the hydraulic pump  50 . Each flex joint  115  and  125  allows movement of the inter-coupled linkages in multiple directions. 
     Each flex joint may, for instance, comprise a simple pivotable hinge joint or a more involved multi-directional pivot such as a ball and socket connection. As shown, each flex joint  115  includes a shoulder  116  that is pivotably connected to frame  22  by adjoining plates  117 . A number of fasteners  118  secure respective plates  117  to frame  22  while allowing drive rods  120 ,  130  to move relative thereto. As shown in  FIG. 13 , a spring  119  is engaged with each drive rod  120 ,  130  and returns the respective drive rods to a neutral drive position when there is no operator interaction with handlebar  58 . An adjuster  121 , such as a pair of nuts, is operationally engaged with drive rods  120 ,  130  and configured to allow tuning of the return force associated with each of spring  119  of each vertical drive rod  120 ,  130  respectively. Such a construction secures drive rods  120 ,  130  to mower  20  while allowing the rods  120 ,  130  to be translated rotationally and linearly with respect to frame  22 . That is, such a construction allows the adjustment of each of drive rods  120 ,  130  throughout the operational range of movement of handlebar  58 . 
     Referring to  FIGS. 1 and 2 , handlebar  58  is vertically movable to adjust the operating speed of the lawnmower  20 . Preferably, handlebar  58  is pivotable about a horizontal axis  123  as shown in  FIG. 4 . In the preferred embodiment, the pivot axis is located between the left and right grip portions  62 ,  64  and the flex joints  115  and  125  and intersects the generally vertical axis of the steering shaft  110 . Handlebar  58  pivots about the horizontal axis such that downward motion of the left-hand and right-hand grip portions  62 ,  64  of the handlebar  58  causes both the left and right drive rods  120  and  130  to rise or move in an upward direction to drive the lawnmower forward at a rate that is proportional to the pivot stroke of the handlebar  58 . Similarly, upward motion of grip portions  62 ,  64  causes downward movement of both the left and right drive rods  120  and  130 , again at a rate that is at least generally proportional to the structure of the handlebar  58 . As discussed in more detail below, maximum forward speed is limited by handlebar contact with the reference bar  68 . As discussed above, vertical movement of handlebar  58  translates drive rods  120 ,  130  relative to flex joints  115 ,  125 , respectively. 
     Referring to  FIGS. 4-9 , the lower end of each left and right generally vertical drive rods  120  and  130  is pivotally coupled to a V-shaped linkage  160  and  162  which are operatively connected to pumps  48 ,  50 , respectively. Each V-shaped linkage  160  and  162  includes a forward leg  164  and a rearward leg  166  that extend in generally opposite directions from a vertex or elbow  185  of the linkage  160 ,  162 . The lower end of each drive rod  120  and  130  is pivotally coupled to the forward leg  164  of a respective V-shaped linkage  160  and  162 . Each of the pair of drive rods  92 ,  94  has a first end that is pivotally coupled to each V-shaped linkage  160 ,  162  proximate elbow  185  and a second end that is pivotably connected to a swash plate drive linkage  194 ,  196  associated with each pump  48 ,  50 . The rearward leg  166  of each V-shaped linkage  160  and  162  is pivotally coupled to an L-shaped control pivot plate  200  and  202 . 
     As shown best in  FIG. 4 , each L-shaped control pivot plate  200  and  202  generally includes a horizontal leg  204  and a vertical leg  210  which are generally perpendicular to one another. An elbow  215  is formed at the intersection of the horizontal leg  204  and the vertical leg  210 . The free end of each horizontal leg  204  of each L-shaped control pivot plate  200 ,  202  is pivotally coupled to the rearward leg  166  of each V-shaped linkage  160 ,  162 , respectively. The elbow  215  of each L-shaped control pivot plate  200  and  202  is pivotably coupled to frame  22  of mower  20  such that each L-shaped control pivot plate  200 ,  202  can rotate about a generally horizontal axis  127 . 
     Referring to  FIGS. 4 and 12 , a pivotable swivel  209  is attached to a free end of each vertical leg  210  of each L-shaped control pivot plates  200  and  202 . Each pivotable swivel  209  includes a body  211  having an opening  220  that is constructed to engage a generally horizontally aligned steering rod  225 ,  227 . A stem  212  extends from body  211  and pivotably engages a respective pivot plate  200 ,  202 . A forward end of each steering rod  225  and  227  includes a head  230  that is configured to operatively engage the pivotable swivel  209  attached to the L-shaped control pivot plate  200  and  202 , respectively such that each of steering rods  225 ,  227  is slidably engaged a respective swivel  209 . An opposite rearward end of each steering rod  225  and  227  is pivotally coupled to a yoke  235  located at a lower end of the generally vertical steering shaft  110 . A spring  236  is engaged with each steering rod  225 ,  227  between head  230  and yoke  235  and biases steering rod  110  to a neutral position when no lateral force is exerted on handlebar  58 . 
     Yoke  235  is generally triangular and defines a rearward vertex  240  and left and right forward vertices  245 ,  247 . The steering shaft  110  is fixedly attached at the rear vertex  240  such that the yoke  235  rotates with rotation of the steering shaft  110 . The left vertex  245  is pivotally coupled at a rear end of the left steering rod  225 , while the right vertex  247  is pivotally coupled at a rear end of the right steering rod  227 . Yoke  235  is constructed to communicate the multi-directional translation of handlebar  58  to the linkages associated with the operation of pumps  48 ,  50  and the swash plates thereof. During a turning operation rotation of steering shaft  110  compresses one of springs  236  associated with one of steering rods  225 ,  227  depending on the direction of the turning operation. If the operator releases the turning pressure of handlebar  58 , springs  230  return steering rods  225 ,  227 , and therefore steering shaft  110  and handlebar  58 , to a neutral, or non-turning orientation. 
     Having generally described the construction of the control system  26 , the following is a general description of the operation of lawnmower  20 . For purposes of example only, assume that handlebar  58  is positioned in neutral such that there is no forward reverse movement of the mower  20 . 
     Analogous to steering a bicycle, clockwise rotation of handlebar  58  causes mower  20  to turn to a right direction and counterclockwise rotation of handlebar  58  causes mower  20  to turn to in a left direction. Referring to  FIGS. 10-13  with respect to the position of handlebar  58 , and the remaining Figs. for the associated linkage, and first to right-hand turning, the steering movement of the handlebar  58  is communicated to yoke  235  through rotation of the vertical steering shaft  110  about its axis. Yoke  235  converts the rotating moment of the vertical steering shaft  110  to linear movement of both the left and right steering rods  225  and  227  generally in a fore and aft direction relative to the frame  22 . Specifically, clockwise rotation of handlebar  58  and yoke  235  causes forward movement of the left steering rod  225 , and rearward movement of the right steering rod  227 . The forward moving left steering rod  225  slides through the opening  220  in the swivel  209  attached to the left L-shaped control pivot plate  200 , resulting in no movement of L-shaped control pivot plate  200 . The left drive wheel  40  therefore continues to rotate at the commanded speed. Rearward movement of the right steering rod  227  causes head  230  of right steering rod  227  to operatively engage the swivel  209  connected to the right L-shaped control pivot plate  202 . The engagement of the right steering rod  227  with swivel  209  of the L-shaped control pivot plate  202  causes pivot plate  202  to rotate about axis  127  as noted in  FIG. 4 . The downward rotating movement of the horizontal leg  204  of the L-shaped control pivot plate  202  causes the drive rod  94  to pivot the swash plate drive linkage downwardly about its axis of rotation, thereby reducing the operating speed of right drive wheel  42  and causing mower  20  to turn in right direction due to the speed differential between drive wheels  40 ,  42 . 
     In a similar manner, counterclockwise rotation of the handlebar  58  rotates steering shaft  110  thereby causing yoke  235  to rotate in a counterclockwise manner. The counterclockwise movement of yoke  235  causes generally rearward linear motion of the left steering rod  225 , and forward linear motion of the right steering rod  227 . The rearward moving head  230  of left steering rod  225  engages against the swivel  209  of the left L-shaped control pivot plate  200  and causes downward rotation of L-shaped control pivot plate  200 . Concurrently, right steering rod  227  passes through opening  220  of swivel  209  of the right L-shaped control pivot plate  202  thereby preventing right steering rod  227  from interfering with the position of the right side L-shaped control pivot plate  202 . The downward movement of L-shaped control pivot plate  200  moves the left drive rod  190  and left swash plate drive linkage  90  downwardly, thereby reducing the operational speed of drive wheel  40  and causing the mower  20  to turn left due to the speed differential between drive wheels  40 ,  42 . 
     Comparatively, when speed control lever  70  is located in the neutral position, both pumps  48 ,  50  are disabled provided handlebar  58  is maintained in a central neutral position. When handlebar  58  is moved to one side or the other, a respective pump  48 ,  50  is enabled while the other pump remains in the neutral position. This configuration allows lawnmower  20  to pivot about a non-driven wheel through operation of only one of drive wheel  40 ,  42  while the other drive wheel is free to rotate. Additionally, further lateral, or steering translation of handlebar  58 , causes one of springs  236  associated with steering rods  225 ,  227  to be fully compressed against the associated pivot  209  and to thereafter translate the corresponding pivot plate  200 ,  202  in a reverse direction with respect to the other of the pivot plates  200 ,  202 . As a result, continued translation of handlebar  58  results in counter rotation of pivot plates  200 ,  202  which yields reverse directional operation of pumps  48 ,  50 . Such a configuration allows mower  20  to perform a counter tire rotation and yields a zero-turn radius turn function without detrimentally affecting the supporting surface with tire skidding. 
     When the drive system is shifted out of a neutral configuration, vertical movement of handlebar  58  relative to a neutral position controls the forward and reverse linear travel of mower  20 . Downward rotational movement of handlebar  58  about the horizontal axis from a neutral position (illustrated by arrow  248  in  FIG. 11 ) causes upward movement of the left and right vertical drive rods  120 ,  130 . The upward movement of the vertical drive rods  120 ,  130  from a neutral position drives the forward leg  164  of both V-shaped linkages  160  and  162  upwardly about the rearward leg  166  so as to cause upward movement of the left and right drive rods  92 ,  94  and swash plate drive linkages  88  and  90 . Upward movement of each of the left and right swash plate drive linkages  88  and  90  about its axis  238  of rotation ( FIG. 5 ) causes forward propulsion of corresponding the left and right hydraulic pumps  48 ,  50 , which drives forward motion of the motors and drive wheels  40  and  42  so as to cause forward movement of the mower  20  at a speed proportional to the magnitude of the movement of steering handlebar  58 . 
     In an opposite manner, upward movement of the handlebar  58  causes downward movement of both the left and right drive rods  120 ,  130 . The downward movement of the left and right drive rods  120 ,  130  causes downward rotational movement of the leg  164  of both V-shaped linkages  160  and  162  about the rearward leg  166  of the V-shaped linkages  160  and  162 , which causes downward movement of the left and right drive rods  92 ,  94  and swash plate drive linkages  88  and  90  so as to cause reverse propulsion of the left and right hydraulic pumps  48 ,  50  and corresponding reverse motion of the drive wheels  40  and  42 . The mower  20  moves in a reverse direction at a speed proportional to the magnitude of the movement of handlebar  58 . 
     Referring now specifically to FIGS.  3  and  10 - 13 , a vertical stroke of handlebar  58  is determined by the position of the reference bar  68  and, thus, the stroke of control lever  70  relative to control plate  72 . Speed control linkage  60  is constructed such that control lever  70  can engage any one of the number of catches  76  formed in control plate  72 . Speed control linkage  60  is constructed to be single hand operable and configured to retain an operating orientation of control system  26 . That is, an operators setting of control lever  70  orientates reference bar  68  relative to handlebar  58  and thereby defines an operational speed range of mower  20 . As described further below, control linkage  60  is also configured to require two distinct operator actions to facilitate increasing the operating speed of mower  20 , but only one action to reduce the operating speed of mower  20 . Such a construction allows an operator to quickly decelerate mower  20  in the event deck  28  encounters an obstruction. Such a configuration allows control lever  70  to mechanically interlock at selected positions relative to control plate  72  so as to define the maximum operating speed of pumps  48 ,  50  and thereby the maximum operating speed of wheels  40 ,  42 . 
     Control plate  72  includes an elongated slot  264  which receives a lateral sliding dog  270  attached to control lever  70 . When the control lever  70  engages against a rear end  266  of the slot  264 , the mower  20  is in neutral such that there is no uniformly forward propulsion of the hydraulic pumps  48 ,  50  and no correlated uniformly forward movement of the drive wheels  40  and  42 . Although this neutral position of control lever  70  prevents forward travel of mower  20 , handlebar  58  can be moved laterally relative to mower  20 , thereby allowing mower  20  to be turned even through the lateral drive control lever is in a neutral configuration. Forward movement of the control lever  70  towards a front end  268  of the slot  264  causes increasing propulsion speed of the hydraulic pumps  48 ,  50  to be communicated to drive wheels  40  and  42 . That is, the further forward control lever  70  is positioned relative to slot  264 , the faster mower  20  can travel. 
     Each of catches  76  is constructed to operatively engage slideable dog  270  attached to control lever  70 . Dog  270  is slideable in a downward direction but is retained in engagement with a respective catch  76  by a biasing spring  272 . Each of the series of catches  76  includes a rear portion that is at less of an angle relative to a front portion of the catches  76  such that the control lever  70  can be rotated in a rearward direction along the slot  264  without requiring downward movement of the dog in the vertical direction. Such a construction allows an operator to simply grab and pull the control lever  70  in a rearward direction to slow or stop the mower  20  without manipulation of dog  270 . To permit higher speed operation of the mower  20 , the operator must manipulate dog  270  of control lever  70  to pass the forward wall of a respective catch  76 . Thus, moving the control lever  70  in the forward direction requires a two stage operation that hinders unintended mower acceleration and prevents the control lever from undesirably slipping in the forward direction and jumping to a higher speed, when traveling over rough terrain. 
     The lower end of the control lever  70  is pivotably attached to a forward end of an elongated speed limiter link  280  about an axis  281 . A rear end of the elongated speed limiter link  280  includes an elongated slot  282  configured to receive a fastener  284  that pivotally couples the elongated speed limiter link  280  to a forward vertex  288  of a triangular-shaped speed limiter reference bracket  290 . An upper rearward most vertex  292  of the speed reference limiter reference bracket  290  is pivotally coupled to the frame  22  of the mower  20 . End  73  of reference bar  68  is connected to the triangular-shaped speed limiter reference bracket  290  generally below vertex  292  such that positioning of control lever  70  also controls the orientation of reference bar  68  relative to handlebar  58 . 
     As best shown in  FIGS. 5-10  and  11 , the exemplary reference bar  68  has a curvilinear-shape, is located underneath handlebar  58 , and extends generally across the width of operator control area  56 . Movement of control lever  70  relative to the control plate  72  causes a correlated up-and-down movement of the reference bar  68  relative to handlebar  58  via the elongated speed limiter link  280  and the triangular-shaped speed limiter reference bracket  290 . The reference bar  68  forms a variable position stop below which handlebar  58  cannot move. When the control lever  70  is positioned against the rear end  266  of the elongated slot  264  of the control plate  72 , the steering handlebar  58  cannot move past a position in which both of the hydraulic motors move from their neutral position in a forward direction. When the control lever  70  is positioned at the forward end  268  of the elongate slot  264 , the handlebar  58  can move downward to its full possible stroke, permitting propulsion at the maximum possible speed. 
     Comparing  FIGS. 11 and 13 , it can be seen that when control lever  70  is located in its rearmost position ( FIG. 13 ), reference bar  68  is maintained in relatively close proximity to handlebar  58 . Comparatively, when control lever  70  is moved to its forward most position ( FIG. 11 ), a substantial gap is formed between handlebar  58  and reference bar  68  thereby providing handlebar  58  with a greater range of motion. As control lever  70  is oriented in the locking positions associated with catches  76 , reference bar  68  varies the range of movement of handlebar  58 . Accordingly, the position of control lever  70  and reference bar  68  define a range of motion of handlebar  58 . The range of motion of handlebar  58  defines an operating speed range of mower  20  that is within the maximum operating range of the lawnmower. 
     In operation, the position of the reference bar  68  is dictated by the position of speed limit control lever  70 . When control lever  70  is in any position between neutral (all the way back) or full speed (all the way forward), handlebar  58 , when not interfered with by an operator, maintains a handlebar neutral position wherein handlebar  58  is generally axially centered relative to mower  20  and is a variable distance above reference bar  68 . As described above, downward movement of the handlebar  58  from a neutral position causes forward propulsion of both pumps  48 ,  50  thereby providing a forward driving force to drive wheels  40  and  42 . The position of the reference bar  68  limits the downward movement of the handlebar  58  and, thereby, limits the maximum forward speed of the mower  20 . When control lever  70  is oriented in the neutral position, reference bar  68  is positioned generally immediately under handlebar  58 , thereby preventing handlebar  58  from moving downward and thereby preventing forward movement of mower  20 . The reference bar  58  also reduces speed during a turn. Specifically, when pivoting the steering handle to turn the mower  20 , engagement of the handlebar  58  against the upwardly inclined curvilinear surface of the reference bar  68  drives handlebar  58  upwardly from its central position, thereby causing mower  20  to automatically decelerate through a turn. More specifically, as illustrated in  FIG. 13 , one of the left-hand and the right-hand grip portions  62 ,  64  of the handlebar  58  maintains contact along the curvilinear shape of the reference bar  68 , while a gap  310  is created between the other of the left-hand and right-hand grip portions  62 ,  64  of the handlebar  58  and the reference bar  68 . This automatic speed reduction avoids undesired skidding of drive wheels  42 ,  44  during a turning operation, reduces wear of drive wheels  42 ,  44 , and reduces the potential of wheels  42 ,  44  tearing up a lawn. 
     When mower  20  is configured for full speed operation (as shown in  FIG. 11 ), control lever  70  is moved to the full forward position of control plate  72  thereby rotating reference bar  68  down and away from handlebar  58 . Accordingly, handlebar  58  is free to be moved throughout its full range of motion. Comparatively, when control lever  70  is located in its neutral position (as shown in  FIG. 13 ), reference bar  68  is rotated to the position generally directly below handlebar  58 . Regardless, of the orientation of control lever  70 , handle  58  is allowed to laterally translate relative to reference bar  68  thereby allowing turning operation of pumps  48 ,  50 , or normal operation of one pump and destroked operation of the other pump. The curvilinear construction of reference bar  68  allows for destroked operation of the pump  48 ,  50  positioned generally opposite the direction of translation of handlebar  58  and provides for the increased destroking of one of the pumps as handlebar  58  is moved away from the central axis position. During a turning operation, a predetermined gap  310  is formed between the side of handlebar  58  and reference bar  68  positioned toward a turn direction. The rise of handlebar  58  associated with the engagement of handlebar  58  with reference bar  68  on a side away from the direction of the turn causes the pumps  48 ,  50  to automatically destroke and thereby slow down the forward propulsion of the mower  20 . 
     As the control lever  70  is moved forward and increases the range of operational speed of the mower  20 , reference bar  68  is lowered relative to its neutral position. The lowered position of the reference bar  68  relative to handlebar  58  increases the angle of the inclination of the ends of the reference bar  68 , causing a correlated increase in destroking of the hydraulic pumps  48 ,  50  and reduction in speed of the mower  20  when maneuvering the turn. In more simplified terms, the higher that the speed limiter control lever  70  is positioned, the greater the incline of the ends of the reference bar  68  and the greater the destroking of the driving pumps  48 ,  50  when maneuvering a turn. Hence, the magnitude and rate of speed reduction during a turn are, generally speaking, directly proportional to the speed of the mower  20 . According, lawnmower  20  is not only simple and efficient to operate, but the training required of new operators to relatively simple. Accordingly, lawnmower  20  can be efficiently produced and is simple to operate. 
     Therefore, one embodiment of the invention includes a steering assembly for a mower that has first and second drives configured to move first and second drive wheels, respectively. The steering assembly includes a steering handle configured to steer the mower in a turn direction relative to a forward direction of travel. Movement of the steering handle in a vertical direction varies an output of the first and second drives thereby varying operation of the first and second drive wheels. An interlock is variably positioned relative to the steering handle such that a position of the interlock is associated with a position of a speed limiter control lever that is configured to control a maximum speed of the first and second drives. 
     Another embodiment of the invention includes a mower having a cutting deck that is supported by a frame and a first and a second drive. The first drive is configured to operate a first drive wheel and the second drive is configured to operate a second drive wheel that is located opposite the first drive wheel. A steering control assembly includes a steering handle that is operatively connected to the first and the second drives. Lateral movement of the steering handle steers the mower in a turn direction relative to a forward travel direction and movement in the steering handle in a vertical direction controls forward movement of the mower. 
     A method of steering a vehicle according to another embodiment of the invention includes moving a steering handle in a vertical direction to drive a vehicle one of a forward and rearward direction of travel and moving the steering handle in a lateral direction from a central longitudinal position to cause the vehicle to turn. 
     As indicated above, many changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of some of these changes is discussed above. The scope of others is apparent from the appended claims.