Patent Publication Number: US-8113531-B2

Title: Personal mobility vehicle having a pivoting suspension with a torque activated release mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-In-Part Application of U.S. patent application Ser. No. 11/504,968, filed Aug. 16, 2006, now U.S. Pat. No. 7,896,394, issued Mar. 1, 2011 and entitled MIDWHEEL DRIVE WHEELCHAIR WITH INDEPENDENT FRONT AND REAR SUSPENSION, which claimed priority from U.S. Provisional Patent Application Ser. No. 60/709,307, filed Aug. 18, 2005, entitled MIDWHEEL DRIVE WHEELCHAIR WITH INDEPENDENT FRONT AND REAR SUSPENSION, and also from U.S. Provisional Patent Application Ser. No. 60/799,529, filed May 11, 2006, entitled MIDWHEEL DRIVE WHEELCHAIR WITH INDEPENDENT FRONT AND REAR SUSPENSION; and also claims the benefit of U.S. Provisional Application No. 61/007,137, filed Dec. 11, 2007, the disclosures of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to suspension systems for use with personal mobility vehicles. In particular, this invention relates to a pivoting suspension system having a torque actuated suspension release mechanism for use with a powered wheelchair. 
     Power-driven personal mobility vehicles are known in the art and may include vehicles such as, for example, scooters and wheelchairs. Some power-driven personal mobility vehicles, particularly certain configurations of power-driven wheelchairs, are known to include suspension systems to improve ride and stability characteristics. One type of power-driven, personal mobility vehicle is a center drive wheelchair that typically includes a base unit having a frame, two spaced-apart drive wheels, and a plurality of caster wheels. The drive wheels are located generally near the longitudinal center of the base. The caster wheels are usually supported on longitudinally extending suspension arms that may be mounted for pivotal movement relative to the frame. The base may include a suspension system to control the relative movement of the drive wheels and the caster wheels in reaction to obstacles or uneven terrain. In some center drive wheelchair configurations, the drive motor is connected to the caster suspension arm in order to urge the arm and caster wheel over an obstacle. Such drive motor and suspension arm arrangements rely on the torque reaction of the motor to lift the caster wheel over the obstacle. The lifting movement of the suspension arm is typically in an upward direction toward the wheelchair seat. The motor engages the suspension arm and transfers the torque reaction load to the suspension arm, to urge it in an upward direction by the reaction force of the motor. 
     SUMMARY OF THE INVENTION 
     This invention relates to a wheelchair having a frame and a front pivot arm pivotally mounted to the frame at a front pivot point, the front pivot arm having a caster for supporting the frame. A rear pivot arm is pivotally mounted to the frame at a rear pivot point, the rear pivot arm having a caster for supporting the frame. A ground engaging mid-wheel drive wheel is connected to the frame. A linkage connects the front and rear pivot arms to each other in a manner such that an upward or downward rotation of one of the pivot arms about its pivot point causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. 
     According to this invention there is also provided a wheelchair having a frame, a ground engaging mid-wheel drive wheel connected to the frame, and a front pivot arm pivotally mounted to the frame at a front pivot point, the front pivot arm having a caster for supporting the frame, the front pivot arm being independent of the drive wheel. A rear pivot arm is pivotally mounted to the frame at a rear pivot point, the rear pivot arm having a caster for supporting the frame, the rear pivot arm being independent of the drive wheel. A linkage connects the front and rear pivot arms to each other in a manner such that an upward or downward rotation of one of the pivot arms about its pivot point causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. 
     According to this invention there is also provided a wheelchair that has a frame, a front pivot arm pivotally mounted to the frame at a front pivot point, the front pivot arm having a caster for supporting the frame, and a rear pivot arm pivotally mounted to the frame at a rear pivot point, the rear pivot arm having a caster for supporting the frame. A ground engaging mid-wheel drive wheel is connected to the frame. The front and rear pivot arms are configured in a manner such that an upward or downward rotation of one of the pivot arms about its pivot point causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. 
     This invention further relates to a suspension system for a wheelchair that includes a frame, and a suspension unit including a front suspension arm pivotally supported on the frame. A front caster wheel is mounted on the front suspension arm for relative pivotal movement therewith. A torque arm pivotally supports a drive unit relative to the frame. The torque arm including a suspension lock portion that selectively engages the suspension unit such that when the drive unit pivots relative to the frame the suspension lock portion becomes disengaged from the suspension unit, thereby enabling the front suspension arm to pivot relative to the frame. 
     According to this invention there is described herein a suspension system for a wheelchair including a base having a frame. A drive unit, having a motor and a gear box, is connected to a drive wheel for rotation of the drive wheel relative to the base. The drive unit supported by a torque arm for pivotal movement relative to the frame. The torque arm includes a suspension lock portion. A suspension unit includes a front suspension arm that is pivotally supported on the frame and a front caster wheel mounted on the front suspension arm for relative pivotal movement. The suspension lock portion of the torque arm is movable, upon rotation of the torque arm, into and out of selective engagement with the suspension unit such that torque applied to the drive wheel selectively disengages the suspension lock portion from the suspension unit. 
     The invention still further relates to a suspension system for a wheelchair that includes a base unit and a front caster wheel mounted on a front suspension arm that is pivotally mounted to the base unit. A torque arm supports a drive wheel and a motor. The torque arm is pivotally mounted to the base unit in a manner that enables the torque arm to pivot when the motor generates torque. The torque arm is configured for selective engagement with the front suspension arm to selectively block pivoting of the front suspension arm. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded, side elevational view of a personal mobility vehicle including a base unit having a suspension system. 
         FIG. 2  is a perspective view of the base unit of the personal mobility vehicle of  FIG. 1 . 
         FIG. 3A  is a side elevational view of the base unit of  FIG. 2 . 
         FIG. 3B  is a side elevational view of the base of  FIG. 3A  showing the suspension system in a deflected condition. 
         FIG. 4  is a perspective view of a suspension system portion of the base unit of  FIG. 3  showing the relative movement of components of the suspension system. 
         FIG. 5  is a side elevational view, similar to  FIG. 3 , of another embodiment of a suspension system of a personal mobility vehicle. 
         FIG. 6A  is a side elevational view of another embodiment of a suspension unit that is part of a suspension system, similar to  FIG. 4 . 
         FIG. 6B  is a side elevational view of another embodiment of a suspension unit that is part of a suspension system, similar to  FIG. 6A . 
         FIG. 7  is a side view in elevation of another embodiment of a personal mobility vehicle configured as a center wheel drive power wheelchair and having a base, similar to the personal mobility vehicle of  FIG. 1 . 
         FIG. 8  is a side view in elevation of an alternative embodiment of a base of a wheelchair similar to the base of  FIG. 2 , with the one of the drive wheels removed for clarity. 
         FIG. 9  is a plan view in elevation of the base of  FIG. 8 . 
         FIG. 10  is a side view in elevation of the suspension of the wheelchair. 
         FIG. 11  is an exploded view in elevation of the suspension of the wheelchair. 
         FIG. 12  is a side view in elevation of the suspension as the wheelchair is overcoming an obstacle. 
         FIG. 13  is a side view in elevation of a cross-over beam configuration of the wheelchair suspension. 
         FIG. 14  is an exploded view in elevation of the suspension of  FIG. 13 . 
         FIG. 15  is a side view in elevation of the suspension of  FIG. 13  as the wheelchair is overcoming an obstacle. 
         FIG. 16  is a side view in elevation of an electronic configuration of the wheelchair suspension. 
         FIG. 17  is an exploded view in elevation of the suspension of  FIG. 16 . 
         FIG. 18  is a side view in elevation of the suspension of  FIG. 16  as the wheelchair is overcoming an obstacle. 
         FIG. 19  is a side view in elevation of a gear linkage configuration of the wheelchair suspension. 
         FIG. 20  is an exploded view in elevation of the suspension of  FIG. 19 . 
         FIG. 21  is a side view in elevation of the suspension of  FIG. 19  as the wheelchair is overcoming an obstacle. 
         FIG. 22  is a side view in elevation of a rotating members configuration of the wheelchair suspension. 
         FIG. 23  is an exploded view in elevation of the suspension of  FIG. 22 . 
         FIG. 24  is a side view in elevation of the suspension of  FIG. 22  as the wheelchair is overcoming an obstacle. 
         FIG. 25  is a side view in elevation of an elongated link configuration of the wheelchair suspension. 
         FIG. 26  is an exploded view in elevation of the suspension of  FIG. 25 . 
         FIG. 27  is a side view in elevation of the suspension of  FIG. 25  as the wheelchair is overcoming an obstacle. 
         FIG. 28  is a side view in elevation of a third link configuration of the wheelchair suspension. 
         FIG. 29  is an exploded view in elevation of the suspension of  FIG. 28 . 
         FIG. 30  is a side view in elevation of the suspension of  FIG. 28  as the wheelchair is overcoming an obstacle. 
         FIG. 31  is a side view in elevation of an angled link configuration of the wheelchair suspension. 
         FIG. 32  is an exploded view in elevation of the suspension of  FIG. 31 . 
         FIG. 33  is a side view in elevation of the suspension of  FIG. 31  as the wheelchair is overcoming an obstacle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is illustrated in  FIG. 1  a power-driven wheelchair  10  that includes a control device  15 , a seating system  20 , and a power-driven base unit  30 . Though described in the context of a power-driven wheelchair  10 , the various embodiments may be used in any environment for the purposes described below. The control device  15  may be a joystick, examples of which are known in the art, to provide an interface between the user and the power-driven base  30  for operation of the wheelchair  10 . The seating system  20  includes a seat back  22 , a seat base  24 , and a seat frame  26 . The seating system  20  may be mounted to the power-driven base unit  30  by cooperating mounting points  28   a  and  28   b , though any type of connection may be provided if desired. 
     The base unit  30  includes a frame  32  that supports a pair of spaced-apart drive wheels  33 , though only one is shown in  FIGS. 1 and 2 . The base unit  30  also includes front caster wheels  34  and rear caster wheels  36 . The front caster wheel  34  is supported by a front fork  35  for rotational and pivot movement relative to the base unit  30 . In a similar manner, the rear caster wheel is supported by a rear fork  37  for rotational and pivot movement relative to the base unit  30 . A pivot head assembly  38  may provide pivotal movement of the front and rear forks  35  and  37 , respectively, by way of bearings or bushing elements.  FIG. 2  shows the base unit  30  with one of the drive wheels  33  removed to reveal a suspension unit  40 . The suspension unit  40  is shown and will be described as a right side suspension unit  40 . It is to be understood that a mirror image, left side suspension unit is provided on the opposite side of the base unit  30 . The right and left side suspension units  40  operate the same manner and may also move independently of each other. 
     As shown in  FIGS. 2 and 3 , the suspension unit  40  includes a front suspension arm  42  that supports the front fork  35 , by way of the pivot head  38 , to allow pivotal movement of the front caster wheels  34  about a vertical axis. The suspension unit  40  further includes a rear suspension arm  44  that, likewise, supports the rear fork  37 , by way of the pivot head  38 , to allow pivotal movement of the rear caster wheels  36  about a vertical axis. The embodiment shown in  FIG. 2  includes a first link arm  46  that is connected between the front and rear suspension arms  42  and  44  by first and second pivot points  48  and  50 . A second link arm  52  is fixed between the front and rear suspension arms  42  and  44  by mounting points  54  and  56 . The first and second link arms  46  and  52  provide coordinated movement of the rear suspension arm  44  when the front suspension arm  42  moves in reaction to an obstruction, as shown in  FIG. 4  and as will be explained in detail below. The front and rear suspension arms  42  and  44  are coordinated for concurrent or simultaneous movement when the obstruction is encountered. In other words, when the front suspension arm  42  is urged up to overcome an obstacle the rear suspension arm  44  also moves in a similar direction at the same time. Additionally, the front and rear suspension arms  42  and  44  may move concurrently during any articulation, though such is not required. A similar suspension unit is disclosed in U.S. Published Patent Application No. 2007/0039766, published Feb. 22, 2007, which is hereby incorporated by reference in its entirety. 
     The distance between the pivot point  48  and the mounting point  54  of the front suspension arm may be varied to produce a different amount of movement, or a suspension deflection ratio, between the front and rear suspension arms  42  and  44 . This suspension deflection ratio may compensate for differences in length, or other differences, between the front and the rear suspension arms  42  and  44  to raise both caster wheels  34  and  36  off of the ground by the same amount. Likewise, the distance between the pivot point  50  and mounting point  56  of the rear suspension arm may be varied in a similar manner to produce the same effect. Alternatively, the pivot points and mounting points  48 ,  54  and/or  50 ,  56  may be varied to allow the rear suspension arm  44  to move by a different amount in reaction to movement of the front suspension arm  42 . 
     The base unit  30  further includes a drive unit, shown generally at  58 . The drive unit  58  includes a motor  60  and a gear box  62 , examples of which are known in the art. The motor  60  engages the gear box  62  to provide rotational movement of the drive wheel  33  in response to commands from the control device  15 . The drive unit  58  is illustrated as a right side drive unit and it should be understood that a corresponding, mirror-image left drive unit is also provided. The control device  15  coordinates the right and left drive units  58  to provide direction and propulsion to the wheelchair  10  in response to the control device  15 . A wheel flange  64  is coupled to and extends from the gear box  62  to support the drive wheel  33  for rotation. 
     The gear box  62  is shown connected to the frame  32  by a drive unit mount, shown generally at  66 . A motor stop  67  is positioned between the frame  32  and the drive unit  58 . The motor stop  67  is illustrated as a cylindrical protrusion connected to the frame  32  by a bolt, though any suitable structure may be used to limit movement of the drive unit  58 . The drive unit mount  66  includes a bracket  68  that is fixed to the frame  32 . The fixed bracket  68  includes a pivot point  70  that supports a torque arm  72  for relative pivotal movement therewith. The torque arm  72  is illustrated as an angled bracket structure having a drive mount portion  74  and a suspension lock portion  76 . The drive unit  50  is mounted on the drive mount portion  74 . The torque arm  72 , however, may be any structure suitable to pivotally support the drive unit  58  and selectively prevent movement of the suspension unit  40 , if desired. 
     The suspension lock portion  76  selectively contacts a suspension stop  78 . When the suspension lock portion  76  of the torque arm  72  contacts the suspension stop  78 , movement of the front and rear suspension arms  42  and  44 , in an upward vertical direction toward the seat  24  and relative to the base frame  32 , is prevented. In other words, when the suspension lock portion  76  of the torque arm  72  contacts the suspension stop  78 , the front casters  34  are substantially prevented from being raised off the ground. The suspension stop  78  is illustrated as a cylindrical protruding knob that is bolted to the front suspension arm  42 . The suspension stop  78 , however, may be any structure or component feature, connected to or integrally formed with a portion of the suspension unit, to restrict or permit suspension movement in response to the torque reaction of the drive unit  58 . For example, the suspension stop  78  may be a point directly on the front suspension arm  42 , the rear suspension arm  44 , or any of the link arms  46  and  52 , if desired. The suspension stop  78  may further be configured as a bearing element such that when the suspension lock portion  76  is moved slightly out of the locking position, the suspension stop  78  may be in general rolling contact against a lower portion of the torque arm  70 . 
     When the suspension lock portion  76  is pivoted away from the suspension stop  78 , the front and rear suspension arms  42  and  44  are permitted to articulate in reaction to encountered terrain irregularities. A spring/damper mechanism, shown as a shock absorber  80 , is connected between the base frame  32  and the front suspension arm  42  to provide a reactive suspension force when the wheelchair  10  is driven over obstacles. The shock absorber  80  is pivotally connected to the front suspension arm  42  at the suspension stop  78 . The opposite end of the shock absorber  80  is connected to the frame  32  at an upper suspension mount  82 , as shown in  FIG. 2 . The shock absorber  80  may be embodied as any type of suspension mechanism that supports a suspension component for relative movement with respect to the frame. Once the front and rear suspension arms  42  and  44  are free to articulate, the shock absorber  80  compresses during a forward moving encounter with an obstacle. The shock absorber  80  then provides a reactive force to bias the suspension unit  40  to return to a neutral or near-neutral position. 
     During typical operation of the wheelchair  10  over generally flat or level terrain or in a deceleration condition, the drive unit  58  may contact the motor stop  67 , though such is not required. When the wheelchair  10  is moving at a relatively constant speed (i.e. near zero acceleration) or in a decelerating condition, the suspension lock portion  76  of the torque arm  72  engages the suspension stop  78 , and the front suspension arm  42  is in a locked position. The engagement of the torque arm  72  against the suspension stop  78  is further made by the weight of the user being transmitted through the suspension unit  40  to the ground. When in the locked position, the reactive movement of the front and rear suspension arms  42  and  44  is restricted. In this position, suspension isolation of minor road irregularities may be provided largely by the seat  24  and the deflection characteristics of the caster wheels  34  and  36  and the drive wheels  33 . The caster wheels  34  and  36  and the drive wheels  33  may be provided as pneumatic tires having a soft ride and low force deflection characteristic, though such is not required. The suspension locked position provides the wheelchair  10  with a substantially rigid suspension having a stable ride characteristic over a generally flat or non-obstructed terrain. The tires of the caster wheels  34  and  36  and the drive wheels  33  provide sufficient isolation from minor bumps for rider comfort. 
     In an alternative embodiment, a gap  75  may be provided between the suspension lock portion  76  and the suspension stop  78  during normal operation. The gap may be in the range of 2-3 millimeters, though any relative spacing may provided if desired. The gap  75  between the suspension lock portion  76  and the suspension stop  78  allows a small amount of movement of the front and rear suspension arms  42  and  44  when the wheelchair  10  is operating at a relatively constant speed (i.e. near zero acceleration) or in a decelerating condition. In this arrangement, the motor stop  67  may be adjusted to contact the drive unit  58  and thus establishing the gap  75  to provide an additional degree of terrain isolation from the shock absorber  80 . The gap  75 , however, may be provided by other adjustment mechanisms if so desired. Thus, the movement of the front and rear caster wheels  34  and  36  may be controlled by limiting the gap  75  between the suspension locking portion  76  and the suspension stop  78 . 
     Referring now to  FIG. 4 , the general movements of points of the suspension unit  40  and the drive unit  58  are indicated by various arrows, as will be explained below. These suspension movements are typically encountered when the front caster wheel  34  traverses an obstacle having a height H such as, for example, a door threshold, a curb, or other abrupt surface irregularity. If the height H of the obstacle is high enough, relative to the diameter of the front caster wheel  34 , the forces developed to overcome the obstacle will cause the drive unit  58  to pivot, or otherwise move, relative to the frame  32 . The movement of the drive unit  58  is a reaction to the torque applied to the drive wheels  33  in order to overcome the inertia of the wheelchair  10  when traversing the obstacle. In an example of operating such a wheelchair  10 , the user may drive up to the obstacle and bring the front caster wheel  34  in contact with the obstacle. As the user actuates the joystick  15  to drive the wheelchair  10  over the obstacle, the drive unit  58  increases the torque applied to the drive wheel  33 . Since the wheelchair  10  has an inertia due to its mass and the resistance provided to overcome the obstacle, the torque applied to the drive wheels  33  reacts at the drive unit mount  66 . In this reaction, as the drive wheel  33  transfers torque to the ground or other surface without slipping, the drive unit  58  applies a reactive load, indicated by arrow  100  in  FIG. 4 , causing the torque arm  72  to rotate about the pivot point  70  as indicated by arrow  102 . 
     As the torque arm  72  begins to rotate, the suspension lock portion  76  moves away from the suspension stop  78  in a direction indicated by arrow  104 . As the suspension lock portion  76  disengages from the suspension stop  78 , the blockage of movement of the front suspension arm  42  relative to the frame  32  is removed. With suspension stop  78  released, the front suspension arm  42  is free to move in response to the force from the obstacle and the reaction of the shock absorber  80 , similar to conventional reactive suspension systems, examples of which are known in the art. Before the inertia of the wheelchair  10  against the obstacle is overcome, the applied torque causes the drive unit  58  to rotate about the pivot point  70 , thus moving the suspension lock portion  76  away from the suspension stop  78 . As the drive torque begins to overcome the inertia of the wheelchair  10  against the obstacle, the front suspension arm  42  is free to rotate in a counterclockwise direction about the suspension stop  78 , as shown in  FIG. 4 . The freed movement of the front suspension arm  42  allows the front caster wheel  34  to move generally in the direction of arrow  106  (i.e. up and over the obstacle of height, H). The front caster wheel  34  begins to traverse the obstacle by rising up the distance H. As the front suspension arm  42  rotates counterclockwise (as viewed in the drawings), the link arm  46  moves in the direction of arrow  108 , and about the pivot point  48 . The link arm  52  functions as a stiffening element and may be fixed to the front and rear suspension arms  42  and  44 . The suspension guide  84  may cooperate with a frame component  86 , as shown in  FIG. 2 , to control various movements of the rear suspension arm  44  and may further act to limit suspension travel, though such is not required. 
     When the front caster wheel  34  is raised up, the link arm actuates the rear suspension arm  44  through pivot points  48  and  50  to move generally in a direction indicated by arrow  112 . The upward movement of the rear caster wheel  36  allows the drive wheel  33  to remain loaded by the vehicle/user weight and in sufficient contact with the ground to maintain tractive effort. This prevents slipping of the drive wheels  33  under torque by precluding a bridging effect between the front and rear caster wheels  34  and  36 , respectively. As shown in  FIGS. 2-4 , the drive unit  58  may include a motor limiter  88  that limits the amount of deflection of the drive unit  58  relative to the rear suspension arm  44 . The amount of deflection limited by the motor limiter  88  defines a maximum gap between the suspension lock portion  76  and the suspension stop  78  during operation. While illustrated as a boss formed on a portion of the rear suspension arm  44 , the motor limiter  88  may be any other structure capable of defining or controlling an upper limit of torque reaction deflection of the drive unit  58 . Alternatively, the motor limiter  88  may be adjustable to vary the distance from the motor  60 , thus altering the maximum allowable excursion of the drive unit  58 . This, in turn, also limits the amount of upward movement of the front and rear suspension arms  42  and  44 . 
     Referring now to  FIG. 5 , there is illustrated another embodiment of a suspension unit, shown generally at  240 . The suspension unit  240  is shown in a similar arrangement to the suspension unit  40 , described above. Only those elements necessary to provide an understanding of the operation of the suspension unit  240  will be explained in detail. Where possible, similar reference numbers will be used to identify similar features or elements. The suspension unit  240  is supported for relative movement on a base frame  232 . The suspension unit  240  includes a front suspension arm  242  that supports a front caster wheel  234  and a front fork  235 , as in the embodiment described above. A rear suspension arm  244  supports a rear caster wheel  236  and a rear caster fork  237  in a similar manner. The front suspension arm  242  is connected to the rear suspension arm  244  by a single link arm  246  at a front pivot point  248  and a rear pivot point  250 . The front and rear suspension arms  242  and  244  include adjustment points  290  and  292 , respectively, though such are not required. The adjustment points  290  and  292  may provide an additional degree of suspension geometry adjustment or to change the rates of relative movement of the front and rear suspension arms  242  and  244 . Additionally, the link arm  248  may adjustable, by way of a threaded turnbuckle (not shown) to vary the geometry of the suspension unit  240 . A suspension guide  284 , similar to suspension guide  84 , may be provided as described above, to maintain the path of travel and the position of the rear suspension arm  244 . 
     The embodiment of the suspension unit  240  operates in a manner similar to that of the suspension unit  40  described above. A drive unit  258  is supported by a torque arm  272  for rotation about a pivot point  270 . As the drive unit  258  deflects under the torque reaction loads, the torque arm  272  rotates about the pivot point  270 . This movement creates or increases a gap between a suspension locking portion  276  and a suspension stop  278  to provide suspension movement, as described above. The suspension movement is controlled by a shock absorber  280  in a manner known in the art. A motor stop  267  may be adjusted to change the contact point of the drive unit  258  relative to the frame  232 . The change in this contact point sets a gap between the suspension locking portion  276  and the suspension stop  278  in order to add another degree of isolation. 
     In another embodiment illustrated in  FIG. 6A , an adjustable actuating link  388  may be directly connected between a rear suspension arm  344  and a drive unit  358  such that deflection of the drive unit  358  applies an articulating force to the rear suspension arm  344  as the front suspension arm (not shown) is unlocked or freed to react to the obstacle. The adjustable actuating link  388  is illustrated as being located at a pivot point  350 . However, the adjustable actuating link  388  may be located generally between a mounting point  356  and the pivot point  350 . Additionally, other locations generally at the pivot point end of the rear suspension arm  344  may be used if desired. The articulation force applied to the rear suspension arm  344  by the adjustable actuating link  388  may be added in a progressive manner based on the deflection of the drive unit  358  and the power required to overcome the obstacle. Such an arrangement may define a first range of motion of the drive unit  358  where the suspension lock portion (not shown) moves away from the suspension stop (not shown). This first range of motion enables the front suspension arm to move, or otherwise react, in response to the obstacle. The second range of motion provides contact between the drive unit  358  and the rear suspension arm  344  to add a force component to the suspension unit  340  causing the front suspension arm to be assisted in overcoming the height, H of the obstacle. 
     In another embodiment illustrated in  FIG. 6B , a resilient actuating link  488  is shown having a resilient member such as a spring or rubber bumper. The resilient actuating link  488  may provide a proportional transfer of actuation force to a rear suspension arm  444  based on the spring rate of the resilient member portion of the resilient actuating link  488 . The drive unit  458  may contact the resilient actuating link  488  and compress the resilient portion thus applying a force that is proportional to the amount of deflection of the resilient actuating link  488 . The resilient actuating link  488  is illustrated as being located at a pivot point  450 . However, the resilient actuating link  488  may be located generally between a mounting point  456  and the pivot point  450 . Additionally, other locations generally at the pivot point end of the rear suspension arm  444  may be used if desired. 
     Referring now to  FIGS. 7-9 , there is illustrated another embodiment of a center wheel drive power wheelchair, shown generally at  506 , and configured with a suspension  508 . The wheelchair  506  includes a base  509  and a frame  510  supporting two center drive wheels  514  mounted for rotation and aligned along a horizontal axis, normal to the direction of fore/aft motion, and two drives  512  for powering the center drive wheels  514 . The frame  510  supports a seat  516  for the wheelchair occupant. On each side of the wheelchair a front pivot arm  520  is pivotally mounted to the frame  510  at a front pivot point  522 . The front pivot arm  520  includes a front caster  518  to support the frame  510 . On each side of the wheelchair a rear pivot arm  524  is pivotally mounted to the frame  510  at a rear pivot point  530  as shown in  FIG. 8 . The rear pivot arm  524  includes a rear caster  526  to support the frame. The embodiment of the center wheel drive power wheelchair, shown in  FIGS. 7-12 , includes front casters  518  and rear casters  526 . However, it should be understood that the term “casters” includes casters, idler wheels and anti-tip wheels. The drive wheels  514  can be mounted from the frame  510  by means of pivot arms, not shown, but such pivot arms are optional. 
     As shown in  FIGS. 8-12 , each front pivot arm  520  includes a front link point  534  located to the front of the front pivot point  522 . The rear pivot arm  524  includes a rear link point  536  located to the front of the rear pivot point  530 . It can be seen that when the front pivot arm  520  pivots upward relative to the frame  510  on the front pivot point  522 , the front link point  534  moves up and the front caster  518  is raised. Likewise, when the rear pivot arm  524  pivots relative to the frame  510  on the rear pivot point  530 , the rear link point  536  moves down and the rear caster is raised. 
     The center wheel drive power wheelchair suspension  508  includes a connecting linkage  528  which connects the front pivot arm  520  at the front link point  534  to the rear pivot arm  524  at the rear link point  536 . Although the connecting linkage  528  shown in  FIGS. 8-12  is a straight member, it should be understood that the connecting linkage  528  may be any means of connecting the front pivot arm  520  at the front link point  534  to the rear pivot arm  524  at the rear link point  536 . The connecting linkage  528  is configured in such a way that an upward or downward rotation of one of the pivot arms  520  or  524  about its respective pivot point  522  or  530  causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. Therefore, if the front caster  518  is raised up, the front pivot arm  520  will pivot clockwise, when viewing the left hand side of the wheelchair as shown in the drawings, about its pivot point  522 . This will cause the corresponding movement of the rear pivot arm  524  in a counterclockwise rotational movement about its pivot point  530 . Counterclockwise rotation of the rear pivot arm  524  causes the rear caster to be raised from the ground. In summary, the connecting linkage  528  connects the front and rear pivot arms  520 ,  524  to each other in a manner such that an upward or downward rotation of one of the pivot arms about its pivot point causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. Another result of the suspension  8  is that when the front caster wheels  518  are lifted up, the rear caster wheels  526  are also lifted up. 
     The front and rear pivot arms can be configured so that the ratio of the upward angular rotation of the front pivot arm to corresponding upward angular rotation of the rear pivot arm is approximately 1:1. In other embodiments, the ratio of angular rotation of the front pivot arm to corresponding angular rotation of the rear pivot arm is different from 1:1. For example, the ratio can be greater than 1:1 so that a 30 degree angular rotation of the front arm  520  results in a 20 degree angular rotation of the rear arm  524 . 
     The connecting linkage  528  can be provided with a notch  529  to conform to the structure of the pivot point apparatus at pivot point  522 , as shown in  FIG. 11 . 
     An optional feature of the suspension  508  is the use of a resilient member  532 , as shown in  FIGS. 8 and 10 , which is connected to hold or urge the suspension  508  in or to a desired position. In a specific embodiment of the invention, the resilient member is a spring  532  that connects the connecting linkage  528  and the front pivot arm  520 , at the front link point  534 , to the frame  510 . The spring  532  urges the connecting linkage  528  and the front pivot arm  520  toward the frame  510 , and hence provides a home position or neutral position for the suspension  508 . As various members of the suspension  508  pivot, the spring  532  is stretched (or compressed), thereby biasing the suspension into a neutral position. One end of the spring  532  is connected to the connecting linkage  528  and the front pivot arm  520  at the front link point  534 , which is forward of the front pivot point  522 , and the other end to the frame  10  at the frame spring point  538 . The resilient member  532  provides resistance to movement of the linkage  528  and the front pivot arm  520  relative to the frame  510 . It should be understood that the resilient member  532  may be any means of providing resistance or a biasing force to movement of the connecting linkage  528  and the front pivot arm  520  relative to the frame  510 . The resilient member  532  need not be connected to the frame  510  at frame spring point  538 , but can connect the connecting linkage  528  and the front pivot arm  520  to other members. Also, the spring can be connected solely to the connecting linkage or solely to the front pivot arm  520 . 
     An exploded view of the center wheel drive power wheelchair suspension  508  is shown in  FIG. 11 . The front pivot arm  520  includes a front pivot arm forward segment  540  located forward of the front pivot point  522 . The rear pivot arm  524  includes a rear pivot arm forward segment  542  located forward of the rear pivot point  530  and a rear pivot arm rearward segment  544  located rearward of the rear pivot point  530 . 
     As shown in  FIG. 10 , the front caster  518 , the rear caster  526 , and the center drive wheels  514  are normally all in constant contact with the ground. However, it should be appreciated that under normal conditions continuous contact with the ground by the front caster  518  and rear caster  526  is not required for the operation of this suspension system. 
     In an optional embodiment, the front pivot point  522  and the rear pivot point  530  are located within the outline or envelope  539  of the center drive wheel  514 , as shown in  FIG. 10 , to allow the pivot points to be as close to the ground as possible. The envelope is the region corresponding to the outline of the drive wheel. It is advantageous to locate the pivot points of the linkage arms within the envelope of the center drive wheels  514  because this will minimize ground clearance problems while ensuring the resultant force generated by contacting an obstacle acts toward lifting the caster front. 
     Referring now to  FIG. 12 , the ability of the center wheel drive power wheelchair  506  to overcome an obstacle will now be described. As the center wheel drive power wheelchair  506  encounters an obstacle  546 , the front caster  518  contacts the obstacle  546 , and a force F fc  is created on the leading edge  548  of the front caster due to the momentum of the wheelchair  506  in the forward direction. Force F fc  causes an upward movement of the front caster  518 . The upward movement of the front caster causes an upward rotation of the front pivot arm  520  about the front pivot point  522 . As the front pivot arm  520  pivots about the front pivot point  522  (clockwise, as shown in  FIG. 12 ), the front pivot arm  520  causes the front link point  534  to rotate in a clockwise direction. As the front link point  534  rotates in a clockwise direction, the connecting linkage  528  connected to the front pivot arm  520  at the front link point  534  also moves in a clockwise rotational direction. Rotational movement of the connecting linkage  528  is resisted by the resilient member  532 . As the connecting linkage  528  moves in a clockwise direction, the rear link point  536  moves downward. As the connecting linkage  528  moves in a clockwise direction and the rear link point  36  moves downward, the connected rear pivot arm  524  is forced to rotate (counter-clockwise as shown in  FIG. 12 ) about the rear pivot point  30 . Counter-clockwise rotation of the rear pivot arm  524  about the rear pivot point  530  results in an upward rotation of the rear pivot arm rearward segment  544 . The upward rotation of the rear pivot arm rearward segment  544  results in a lifting of the rear caster  526 . 
     Summarizing the action of the center drive power wheelchair suspension  508 , a force on either the front caster  518  or the rear caster  526 , results in the lifting of that caster and a rotation of the respective pivot arm. The rotation of the pivot arm about its pivot point results in a movement of the connecting linkage  528 , which connects the front pivot arm  520  and the rear pivot arm  524  to each other in a manner such that an upward or downward rotation of one of the pivot arms about its pivot point causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. This action causes the front caster  518  and the rear caster  526  to lift, thereby causing the center drive wheels  514  to maintain contact with the ground. While  FIG. 12  describes the ability of the center wheel drive power wheelchair  506  to overcome an obstacle  546  in the forward direction, the center wheel drive power wheelchair  506  has the ability to overcome an obstacle  546  in either the forward or rearward direction. 
     In the embodiment disclosed in  FIGS. 7-12 , the connecting linkage  528  is shown as a straight member. However, the connecting linkage  528  can be configured in numerous other shapes. As will be explained below, examples of different configurations of the connecting linkage  528  include a cross-over beam, an elongated member, a gear linkage, rotatable members connected by a belt or chain, a cross-over beam with a third link, an electronic system, a hydraulic system, a pneumatic system, a curved member or any equivalent means. 
     It can be seen that when the wheelchair encounters rough terrain, where the drive wheel  514  travels over a depression or low spot, the raising of the front and rear wheels  518 ,  526  will maintain the drive wheels  514  in contact with the ground. It also can be seen that the front caster  518  and the rear caster  526 , as well as their respective pivot arms  520  and  524 , are independent of the drive wheels  514  and any suspension for the drive wheels. 
     In another embodiment of the center wheel drive power wheelchair suspension, as shown in  FIGS. 13-15 , a suspension  508 A is configured in the form of a cross-over beam linkage. In this embodiment, the suspension  508 A includes a front cross-over beam  620 , which contains a first pin slot  650 , and which pivots about a front pivot point  622 . The suspension  508 A also includes a rear cross-over beam  624 , which contains a second pin slot  652 , and which pivots about a rear pivot point  630 . The front cross-over beam  620  and the rear cross-over beam  624  are connected to each other by a connecting pin  654  that extends into the first pin slot  650  and the second pin slot  652 . The connection of the front cross-over beam  620  and the rear cross-over beam  624  by the connecting pin  654  is configured in such a way that an upward or downward rotation of one of the cross-over beams  620  or  624  about its respective pivot point  122  or  130  causes rotation of the other cross-over beam about its pivot point in an opposite rotational direction. Therefore, if the front caster  518  is raised up, such as by an impact with the obstacle  546 , the front cross-over beam  620  will pivot in a clockwise direction, when viewing the left hand side of the wheelchair as shown in the drawings, about its pivot point  622 . This will cause a corresponding movement of the rear cross-over beam  624  in a counterclockwise rotational movement about its pivot point  630 . Counterclockwise rotation of the rear cross-over beam  624  causes the rear caster  526  to be raised from the ground. The pin  654  can be any mechanism suitable to connect the slots  650 ,  652  together to allow the beams  620  and  624 , respectively, to be connected in a pivotable manner. For ease of description, similar part numbers will be used in describing similar parts in the various embodiments. 
     In another embodiment of the center wheel drive power wheelchair suspension, as shown in  FIGS. 16-18 , a suspension  508 B has an electronic linkage configuration. In this embodiment, the suspension  508 B includes a front pivot arm  720  that is mounted for pivoting relative to the frame  510  about front pivot point  722 . The front pivot arm  720  includes a connection point  774 . The suspension  508 B also includes a rear pivot arm  724  mounted for pivoting relative to the frame  510  about a rear pivot point  730 . The rear pivot arm  724  contains a rear connection point  776 . The front pivot arm  720  and the rear pivot arm  724  are connected to each other by an electronic linkage  728  at the front connection point  774  and the rear connection point  776 , respectively. The electronic linkage  728  is configured to sense the upward or downward rotation of one of the pivot arms  720  or  724  about its respective pivot point  722  or  730  and subsequently to cause rotation of the other pivot arm about its pivot point in an opposite rotational direction. Therefore, if the front caster  518  is raised up, such as by encountering an obstacle  546 , the front pivot arm  720  will pivot in a clockwise direction. Such rotation is sensed by the electronic linkage  728 , about its pivot point  722  and the electronic linkage  728  will cause the corresponding movement of the rear pivot arm  724  in a counterclockwise rotational movement about its pivot point  730 . Counterclockwise rotation of the rear pivot arm  724  causes the rear caster  26  to be raised from the ground. The electronic linkage can be a mechanism that senses the rearward or downward movement of connection point  774 , or forward or downward motion of the connection point  776 . The electronic linkage  728  can be freely suspended between the arm  720  and the arm  724 . Alternatively, it can be connected to the frame  510  in any suitable manner. The connection between the arms  720 ,  724  and the electronic linkage can be purely electronic, in which case an inclinometer or other similar device can be incorporated into the system to communicate the presence of a pivoting motion for one of the arms  720 ,  724 . 
     Other mechanisms can be used for sensing the motion or rotation of one of the arms  720  and  724 , and causing the other of the arms to pivot. Although the linkage  728  shown in  FIGS. 16-18  has been described as an electronic linkage, it should be understood that the linkage  728  may be any means of sensing rotational movement of rotational movement of one of the pivot arms  720  or  724  and to subsequently cause rotation of the other pivot arm  720  or  724  including a hydraulic system or a pneumatic system. For example, the system could include solenoids activated by pivoting of one of the arms  720 ,  724 , with the other arm provided with a counter-rotating pivoting motion by the action of a motor. Optionally, the electronic linkage  728  includes a resilient member, not shown, to hold or urge the suspension  8 B in or to a desired position. Also, the electronic linkage  728  itself can act as a resilient member to hold or urge the suspension  508 B in or to a desired position. It should be understood that a separate resilient member, comprising any means of holding or urging the suspension  508 B in or to a desired position, may be used. 
     In another embodiment of the center wheel drive power wheelchair suspension, as shown in  FIGS. 19-21 , a suspension  508 C includes a gear linkage. In this embodiment, the suspension  508 C includes a front pivot arm  820  which contains a front gear rack  864 , and which pivots about a front pivot point  822 . The suspension  508 C also includes a rear pivot arm  824  containing a rear gear rack  866 , which pivots about the rear pivot point  830 . The front pivot arm  820  and the rear pivot arm  824  are connected to each other as the front gear rack  864  engages the rear gear rack  866  at the gear rack intersection  868 . The connection of the front gear rack  864  and the rear gear rack  866  at the gear rack intersection is configured in such a way that an upward or downward rotation of one of the pivot arms  820  or  824  about its respective pivot point  822  or  830  causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. Therefore, if the front caster  18  is raised up, such as by encountering an obstacle  546 , the front pivot arm  820  will pivot in a clockwise direction, when viewing the left hand side of the wheelchair as shown in the drawings, about its pivot point  822 . This will cause the corresponding movement of the rear pivot arm  824  in a counterclockwise rotational movement about its pivot point  830 . Counterclockwise rotation of the rear pivot arm  824  causes the rear caster  526  to be raised from the ground. An optional feature of the suspension  508 C is the use of a resilient member  832 , which is connected to the front pivot arm  820  at the spring point  837  and to the frame  510  at the frame spring point  838 . The resilient member  832  is configured to hold or urge the suspension  508 C in or to a desired or neutral position. Although the resilient member  832  shown in  FIGS. 19-21  is a spring, it should be understood that the resilient member  832  may be any means of holding or urging the suspension  508 C in or to a desired position. It is to be understood that the gear mechanism with gear racks  864 ,  866  can be any mechanism suitable for causing rotation or pivoting of one of the arms  820 ,  824  in response to the pivoting of the other arm. 
     As shown in  FIGS. 22-24 , a suspension  508 D for the center wheel drive power wheelchair can be configured with belts, chains or other power transmission members to tlc together the rotation or pivoting of the suspension members. In this embodiment, the suspension  508 D includes a front pivot arm  920 , which contains or is connected to a front pulley  970 . The front pivot arm is pivotally mounted at front pivot point  922  for pivoting with respect to the frame. The suspension  508 D also includes a rear pivot arm  924  containing a rear pulley  972 . The rear pivot arm  924  is mounted for pivoting with respect to the frame  510  about the rear pivot point  930 . The front pivot arm  920  and the rear pivot arm  924  are connected to each other by a belt  928  that engages the front pulley  970  and the rear pulley  972 . The connection of the front pulley  970  and the rear pulley  972  by the belt  928  is configured in such a way that an upward or downward rotation of one of the pivot arms  920  or  924  about its respective pivot point  922  or  930  causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. Therefore, if the front caster  518  is raised up, such as would occur if an obstacle  546  is encountered, the front pivot arm  920  will pivot in a clockwise direction, when viewing the left hand side of the wheelchair as shown in the drawings, about its pivot point  922  and cause the corresponding movement of the rear pivot arm  924  in a counterclockwise rotational movement about its pivot point  930 . Counterclockwise rotation of the rear pivot arm  924  causes the rear caster  526  to be raised from the ground. While the suspension  508 D is shown configured with the belt  928  to connect the front pulley  970  with the rear pulley  972 , it should be understood that any transmission means, such as a chain or cord, can be used to transmit rotation from the pulleys  970  and  972  to each other. 
     An optional feature of the suspension  508 D is the use of a resilient member  932  which is connected between the suspension  508 D and the frame  510 . A resilient member, such as a spring  932 , connects the front pivot arm  920  at the spring point  937  and to the frame  510  at the frame spring point  938 . The spring  932  is configured to hold or urge the suspension  508 D in or to a desired position. It should be understood that the spring  932  may be any means, such as an elastic member or elastic band, capable of holding or urging the suspension  508 D in or to a desired position. 
     Although the suspension  508 D shown in  FIGS. 22-24  illustrates the pivot arms  920  and  924  as pivoting on pivot points  922  and  930  respectively, the arms could alternatively be set up as pivoting at pivot points  922 A and  930 A, which are positioned at the center of the pulleys  970 ,  972 . 
     In another suspension of the center wheel drive power wheelchair, as shown in  FIGS. 25-27 , a suspension  508 E includes a linkage in the form of an elongated member. In this embodiment, the suspension  508 B includes a front pivot arm  1020  which contains a first pin slot  1050 . The front pivot arm  1020  pivots about a front pivot point  1022 . The suspension  508 E also includes a rear pivot arm  1024  which contains a second pin slot  1052 , and which pivots about a rear pivot pin  1030 . The front pivot arm  1020  and the rear pivot arm  1024  are connected to each other by an elongated member  1056 . The elongated member  1056  is rotatably mounted at the front pivot point  1022  and the rear pivot point  1030 . The elongated member  1056  is also connected to the front pivot arm  1020  by a first link pin  1054  which extends through the first pin slot  1050  in the front pivot arm  1020 , and through the front slot  1058  in the elongated member  1056 . Similarly, the elongated member  1056  is connected to the rear pivot arm  1024  by a second link pin  1055  which extends through the second pin slot  1052  in the rear pivot arm  1024 , and through the rear slot  1060  in the elongated member  1056 . 
     The elongated member  1056  is a flexible member. The connection of the elongated member  1056  to the front pivot arm  1020  and to the rear pivot arm  1024  by the link pins  1054  and  1055  is configured in such a way that an upward or downward rotation of one of the pivot arms  1020  or  1024  about its respective pivot point  1022  or  1030  causes a movement or displacement of the elongated member  1056  that in turn causes a rotation of the other pivot arm about its pivot point in an opposite rotational direction. The movement or displacement of the elongated member  1056  can be a bending due to the torque or bending forces applied by the upward movement of the front arm  1020  or rear arm  1024 . Therefore, if the front caster  518  is raised up, such as shown in  FIG. 27  where the wheelchair  506  has encountered an obstacle  546 , the front pivot arm  1020  will pivot in a clockwise direction, when viewing the left hand side of the wheelchair as shown in the drawings, about its pivot point  1022 . This causes a downward flexing or rotation of the elongated member and causes the corresponding movement of the rear pivot arm  1024  in a counterclockwise rotational movement about its pivot point  1030 . Counterclockwise rotation of the rear pivot arm  1024  causes the rear caster  526  to be raised from the ground. In this embodiment of the invention, the elongated member  1056  connects the front pivot arm  1020  and the rear pivot arm  1024  as well as acts as a resilient member in the suspension  508 E by resisting motion and returning the system to a neutral position as it flexes. 
     As shown in  FIGS. 28-30  a center wheel drive power wheelchair suspension  508 F includes a cross-over beam linkage with a resilient connection and an optional third link. In this embodiment, the suspension  508 F includes a front cross-over beam  1120  which pivots about a front pivot point  1122 . The suspension  508 F also includes a rear cross-over beam  1124  which pivots about a rear pivot point  1130 . The front cross-over beam  1120  and the rear cross-over beam  1124  are optionally connected to each other by a third link  1128 . The front crossover beam  1120  includes an elongated slot  1140 , and the rear crossover beam includes a corresponding elongated slot  1142 . The third link  1128  also includes an elongated slot  1144 . When the front cross-over beam  1120  and the rear cross-over beam  1124  are assembled with the third link  1128 , the elongated slots  1140 ,  1142  and  1144  are all aligned and held in a connected configuration by a linking pin  1148 . 
     The connection of the front cross-over beam  1120  and the rear cross-over beam  1124  by the third link  1128  is configured in such a way that an upward or downward rotation of one of the cross-over beams  1120  or  1124  about its respective pivot point  1122  or  1130  causes rotation of the other cross-over beam about its pivot point in an opposite rotational direction. Therefore, if the front caster  518  is raised up, as would be the case upon impact with an obstacle  546 , the front cross-over beam  1120  will pivot in a clockwise direction, when viewing the left hand side of the wheelchair as shown in the drawings, about its pivot point  1122  and cause the corresponding movement of the rear cross-over beam  1124  in a counterclockwise rotational movement about its pivot point  1130 . Counterclockwise rotation of the rear cross-over beam  1124  causes the rear caster  526  to be raised from the ground. 
     In an alternate configuration of the suspension  508 F, a resilient member, such as an elastic band  1132 , can be positioned around the front and rear cross over beams  1120 ,  1124 , to hold them together and urge them into a neutral position. When the elastic band or other resilient member is employed, the optional third link is not necessary. 
     In yet another suspension configuration, as shown in  FIGS. 31-33 , the suspension  508 G includes a curved member linkage. In this configuration the suspension  508 G includes a front pivot arm  1220  which contains a front link point  1234 , with the front pivot arm  1220  being configured to pivot about a front pivot point  1222 . The suspension  508 G also includes a rear pivot arm  1224  containing a rear link leg  1246 . The rear pivot arm is mounted to pivot about the rear pivot point  1230 . The front pivot arm  1220  and the rear pivot arm  1224  are connected to each other by a connecting linkage  1228 . The connecting linkage  1228  connects to the front pivot arm  1220  at the front link point  1234  and to the rear pivot arm  1224  at the rear link leg  1246 . The connection of the front pivot arm  1220 , the rear pivot arm  1224  and the connecting linkage  1228  is configured in such a way that an upward or downward rotation of one of the pivot arms  1220  or  1224  about its respective pivot point  1222  or  1230  causes rotation of the other pivot arm about its pivot point in an opposite rotational direction. Therefore, if the front caster  518  is raised up, the front pivot arm  1220  will pivot in a clockwise direction, when viewing the left hand side of the wheelchair as shown in the drawings, about its pivot point  1222 . This will cause the corresponding movement of the rear pivot arm  1224  in a counterclockwise rotational movement about its pivot point  1230 . Counterclockwise rotation of the rear pivot arm  1224  causes the rear caster  526  to be raised from the ground. 
     As shown, the connecting linkage  1224  is a curved member. However, the connecting member  1224  can be of any shape or form that connects the front pivot arm  1220  to the rear pivot arm  1224  and can transmit rotational movement of one pivot arm to an opposite rotational movement in the other pivot arm. An optional feature of the suspension  508 G is the use of a resilient member  1232 , which is connected at one end to the front pivot arm  1220  at the spring point  1238 , and at the other end to the frame  510 . In this embodiment, the resilient member  1232  is a spring which is configured to hold or urge the suspension  508 G in or to a desired position, but it should be understood that the resilient member  1232  can be any means to hold or urge the suspension  508 G in or to a desired position. 
     While the various suspension configurations above illustrate only the left side of the suspension, it is to be understood that the suspension actually includes both a left and a right suspension. Also, an optional feature of any of the suspensions described above is the use of a resilient member configured to hold or urge the suspension in or to a desired position. The resilient member can be a spring, an elastic band, or any means of holding or urging the suspension  508  in or to a desired position. 
     It is to be understood that the term “caster” includes idler wheels as well as casters. Also, the mid-wheel drive wheel, which is usually positioned underneath the approximate center of gravity of the wheelchair and occupant, can be positioned anywhere between the front caster  518  and the rear caster  526 . Further, although the suspension systems disclosed are configured so that when the front pivot arm  520  is raised the rear pivot arm  524  is also raised, the suspension  528  can be configured in an opposite manner, wherein when the front arm  520  is raised, the rear pivot arm is lowered relative to the frame. Also, the suspension  8  can be configured so that the rear pivot arms can be disconnected and therefore not mounted for pivoting in response to the pivoting of the front pivot arm. In yet another configuration, the connecting linkage  528  is configured in an adjustable manner so that adjustments in the suspension  508  can be readily made. The adjustment feature can include a telescoping configuration, an angle change configuration, or any other configuration that allows adjustability. Also, although the suspension  508  has been described in terms of a front pivot arm  520  with front caster  518 , a rear pivot arm  524  with rear caster  526 , and a drive wheel, typical use on a wheelchair will include such a suspension on each side of the wheelchair (left and right), so that there is a pair of front pivot arms  520  with front casters  518 , a pair of rear pivot arms  524  with rear caster  526 , and a pair of drive wheels. 
     The principle and mode of operation of this invention have been described in its preferred embodiments. However, it should be noted that this invention may be practiced otherwise than as specifically illustrated and described without departing from its scope.