Abstract:
A modular wheelchair assembly is adapted to be reconfigured between a fixed angle of tilt, a dynamically adjustable tilt-in-space configuration with the tilt axis being near the user&#39;s knees and a dynamically adjustable tilt-in-space configuration with the tilt axis being near the user&#39;s center of gravity. The reconfiguration is achieved by modifying a limited number of selectively removable components of the support assembly or the pivot assembly of the wheelchair.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/838,492 filed Aug. 14, 2007 now U.S. Pat. No. 7,871,094, all of which is incorporated herein in its entirety by this reference thereto. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to wheelchairs. More particularly, the invention relates to a modular wheelchair assembly that is configurable to different tilt configurations and to features thereof. 
     BACKGROUND OF THE INVENTION 
     The designs of most wheelchairs are optimized to accommodate a particular level of disability. Persons with low disability tend to use relatively inexpensive wheelchairs that have no seat tilt or a fixed seat tilt and a footrest assembly that easily accommodates self-propulsion using the occupant&#39;s feet. 
     Persons with moderate disability may prefer a wheelchair that allows for optional self-propulsion but that can be tilted to offer a range of seating angles. Tilting the seat provides pressure relief to the occupant, reduces discomfort associated with sitting for long periods of time, and provides passive correction for deformities. The ability to self propel using the feet may be preserved despite various tilt angles by providing the axis of rotation near the front of the seat such that the distance from the knees to the ground remains relatively constant. A disadvantage of such a configuration is the force required in order to move the weight of the occupant about the axis of rotation. This is sometimes compensated for by a pneumatic assist mechanism extending between the base of the chair and the seat frame as described in commonly owned U.S. Pat. No. 6,447,064. 
     High disability individuals typically require a wheelchair with deeper tilt angles to improve trunk stability and head control. Some such wheelchairs also use mechanical actuators to accommodate the significant force sometimes required to move the weight of the occupant through deep tilt angles. It is also known in the prior art to minimize the effort required to tilt the occupant by providing a pivot point as close as possible to his center of gravity. U.S. Pat. No. 7,007,965 provides an example of such a system. 
     While various tilt configurations may be suited to particular types or levels of disability, many individuals suffer from disabilities that progress over time. Over the course of such a disability, the occupant may graduate through 3-4 different types of wheelchairs, each having different attributes. For example, a no-tilt or fixed tilt wheelchair may be used at the early onset of disability, a self-propellable tilting wheelchair can be used when the disability becomes moderate, and a deep tilt wheelchair can be used in the later stages of disability. 
     The present invention addresses the need for a reconfigurable modular wheelchair that is capable of being selectively configured in a fixed tilt configuration, a dynamic tilt-in-space configuration with the axis of rotation near the occupant&#39;s knees, or a dynamic tilt-in-space configuration with the axis of rotation near the occupant&#39;s center of gravity, as required to accommodate the evolving needs of the occupant. 
     SUMMARY OF THE INVENTION 
     The wheelchair according to the invention comprises a base frame, a seat frame, and interchangeable interface components adapted to assemble the base frame and seat frame to one another according to either a fixed tilt configuration, a dynamic tilt-in-space configuration with the axis of rotation near the front of the seat, or a dynamic tilt-in-space configuration with the axis of rotation near the center of gravity of the occupant. 
     In a first configuration, the wheelchair comprises a base frame assembly pivotally connected to a seat frame assembly about cooperating pivot elements at a pivot point located near the knees of the occupant. A support assembly extends between a base crossbar assembly and a seat crossbar assembly. The support assembly comprises a bracket removably attached to one of such crossbar assemblies, and an interface element attached between the brace and the other one of such crossbar assemblies. The interface element includes a plurality of attachment points arrayed to share a constant radius in relation to the pivot point. The selection of the attachment point allows the base frame and the seat frame assemblies to be assembled at a variety of fixed tilt angles to thereby provide adjustable static positioning for the user requiring minimal support and correction. 
     In a second configuration, the wheelchair again provides a pivot point near the knees of the occupant through cooperating pivot elements on the base frame and seat frame assemblies. A support assembly extending between the base crossbar assembly and the seat crossbar assembly comprises a bracket and a bias mechanism such as a gas strut to enable the occupant to be lifted from a low tilt angle more easily than would be the case without the mechanism. In one aspect, the pneumatic mechanism includes a bell crank arrangement to converts the longitudinal force from the gas strut to an upward force to lift the seat frame and to modulate the degree of resistance provided at different tilt angles as the centre of gravity of the occupant moves forward or backward and to translate. 
     In a third configuration, the chair may be tilted about an axis that approximately coincides with the centre of gravity of the occupant. The tilting is provided by suspending the seat frame from an axis of rotation supported on the base frame. This configuration has the advantage of making it very easy to tilt the wheelchair and obviates the need for pneumatic mechanisms or actuators. 
     The invention is also directed to a drive wheel system wherein the wheel lock assembly and the anti-tip assembly are connected to the axle mounting plate such that the change of drive wheel position on the base frame does not require consequent adjustment of the lock and anti-tip assemblies. 
     In yet a further aspect, the invention is directed to a telescoping crossbar assembly comprising an outer sleeve having a base with a non-straight cross-section, a hollow inner shaft having a base with a cross-section conforming to said non-straight cross-section, and a pair of aligned fastener holes in said outer sleeve, one of said fastener holes having a larger diameter than the other. 
     In yet a further aspect, the invention is directed to a mounting assembly for securing fasteners to an elongated hollow member such as a side tube of a wheelchair. The hollow member has a plurality of fastener apertures extending longitudinally of the hollow member. An elongated insertion member is adapted to be longitudinally inserted and retained in the hollow member. A plurality of nuts are retained in several seats provided along the length of the insertion member such that when it is inserted and retained in the hollow member with the nuts aligned to the fastener apertures, fasteners inserted into the apertures will engage the nuts and be retained without the need to traverse the opposing wall of the hollow member. 
     In a further aspect, the wheelchair has a seat frame assembly comprising opposed seat rails and at least one seat crossbar assembly extending between them. A base frame assembly comprises opposed base rails and at least one base crossbar assembly extending between the base rails. A forward portion of the seat frame assembly and a forward portion of the base frame assembly is adapted to receive opposed removable pivot assemblies to pivot the seat frame assembly in relation to the base frame assembly. The seat and base crossbar assemblies are adapted to removably receive a support assembly extending between them. A forward portion of each of the seat rails is adapted to selectively attach a pivot member thereto and a rearward portion of each of the base rails is adapted to receive a removable pivot arm thereon enabling reconfiguration of the wheelchair between a pivot point near the user&#39;s knees and a center of gravity pivot point. 
     In another aspect, a fixed tilt wheelchair comprises a seat frame assembly and a base frame assembly. A first pivot element is removably attached to a forward portion of the seat frame assembly. A second pivot element is removably attached to a forward portion of the base frame assembly and the first and second removable pivot elements, when installed, cooperate to define a pivot point between them. A removable support assembly is connected between the seat frame assembly and the base frame assembly, the support assembly being configurable to define any one of a plurality of predetermined relative pivot angles between the seat frame and base frame assemblies. 
     Another aspect of the invention relates to a mounting assembly for an elongated hollow member. An elongated hollow member has a plurality of fastener apertures extending transversely of the hollow member. An elongated insertion member is adapted to be longitudinally inserted and retained in the elongated hollow member, the insertion member having a plurality of seats for retaining nuts therein. A plurality of nuts are seated in the seats and the insertion member is inserted into the hollow member to align said nuts with the fastener apertures. 
     In another aspect a dynamically tiltable wheelchair comprises a seat frame assembly, a base frame assembly, a first pivot element removably attached to a forward portion of the seat frame assembly and a second pivot element removably attached to a forward portion of the base frame assembly. The first and second removable pivot elements, when installed, cooperate to define a pivot point between them, said first and second pivot elements being operatively secured to one another. A removable support assembly is connected between the seat frame assembly and the base frame assembly, the support assembly comprising bias means between the seat frame assembly and the base frame assembly. 
     In another aspect the bias means comprises an extendible element one end of which is pivotally secured to a bell crank, and said bell crank is retained in operative relationship to said base frame assembly. 
     In another aspect, a dynamically tiltable wheelchair comprises a seat frame assembly having opposed seat rails and a base frame assembly having opposed base rails. A pivot arm is removably secured to a rearward portion of each of the base rails, said pivot arm extending upwards above said seat frame assembly. A hanger member is removably secured to a rearward portion of each of the seat rails and extends upwardly. The pivot arm and the hanger member cooperate to define a pivot point near the expected center of gravity of a wheelchair occupant for pivoting the seat frame assembly in relation to the base frame assembly. 
     In a method aspect of the invention, the wheelchair may be reconfigured from a fixed tilt configuration to a dynamically tiltable configuration. By removing from the fixed tilt configuration an element that renders a removable support assembly configurable to any one of a plurality of predetermined relative pivot angles between the seat frame and the base frame. A biasing mechanism is also installed that provides a mechanical advantage in tilting the seat frame in relation to the base frame. 
     In another method aspect, the wheelchair is reconfigurable from a first dynamically tillable configuration where the pivot axis is near the front of the wheelchair to a second dynamically tiltable configuration where the tilt axis is near the expected center of gravity of an occupant. The first dynamically tiltable configuration comprises a seat frame, a base frame, a removable forward pivot assembly pivotally attaching the forward portion of the seat frame to the forward portion of the base frame and a removable support assembly connected between the seat frame and the base frame, the support assembly comprising a biasing mechanism that provides a mechanical advantage in tilting the seat frame in relation to the base frame. The reconfiguration is accomplished by disengaging the forward pivot assembly and installing a center of gravity pivot assembly comprising a pivot point near the expected center of gravity of a seated occupant. 
     In another aspect, the invention comprises a kit for a reconfigurable wheelchair system comprising a seat frame assembly, a base frame assembly and a plurality of alternative support assemblies for supporting the seat frame assembly on the base frame assembly. 
     In an aspect of the invention relating to the crossbar assembly, there is provided a telescoping crossbar assembly for rigidly extending between structural elements at selectable degrees of extension comprising a hollow outer tube, an inner tube slidably receivable in the outer tube, the inner tube and the outer tube having generally corresponding cross-sectional shapes. A plurality of fasteners extend through the outer tube and the inner tube, each of the fasteners having a body portion and a head portion larger than the body portion, and wherein the head portion bears on the inner tube through an aperture in the outer tube. 
     In yet another aspect, the invention is a drive wheel assembly for a wheelchair comprising a mounting element adapted to be adjustably secured to a component of a base frame in one of a plurality of alternative positions. A wheel mountable on the mounting element and a rod having a wheel lock assembly mounted thereon is attached to the mounting element. Adjustment of the mounting element in relation to the base frame maintains the position of the wheel lock assembly in relation to the wheel without requiring separate adjustment thereof. 
     In a further aspect of the invention, there is provided a crossbar mounting system for the crossbar between opposed rails. The crossbar has a substantially hollow tube having opposed apertures therein and the rail has at least one aperture extending therethrough. An insert is adapted to be inserted through said opposed apertures, said insert having at least one aperture adapted to receive a fastener extending through said aperture and said rail for securing said crossbar to the rail. 
     The foregoing was intended as a broad summary only and of only some of the aspects of the invention. It was not intended to define the limits or requirements of the invention. Other aspects of the invention will be appreciated by reference to the detailed description of the preferred embodiment and to the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred embodiment of the invention will be described by reference to the drawings thereof in which: 
         FIG. 1  is a front perspective view of the TF configuration of the wheelchair of the preferred embodiment; 
         FIG. 2   a  is a side elevation of the TF configuration at a neutral (horizontal) tilt angle; 
         FIG. 2   b  is a side elevation of the TF configuration at a different tilt angle than in  FIG. 2   a;    
         FIG. 3  is a bottom rear perspective view of the TF configuration; 
         FIG. 4  is a front perspective view of the T20 configuration; 
         FIG. 5   a  is a side elevation of the T20 configuration at a neutral (horizontal) tilt angle; 
         FIG. 5   b  is a side elevation of the T20 configuration at a different tilt angle than in  FIG. 5   a;    
         FIG. 6  is a front perspective view of the T50 configuration; 
         FIG. 7   a  is a side elevation of the T50 configuration at a neutral (horizontal) tilt angle; 
         FIG. 7   b  is a side elevation of the T50 configuration at a different tilt angle than in  FIG. 7   a;    
         FIG. 8  is a perspective view of the base frame, seat frame, support and pivot assemblies of the TF configuration; 
         FIG. 8   a  is a perspective view of the base frame assembly of the TF configuration, with the interface mount secured to the rear base crossbar assembly; 
         FIG. 9  is a perspective view of the base frame, seat frame, support and pivot assemblies of the T20 configuration; 
         FIG. 9   a  is a partially sectioned side view of the support assembly of the T20 configuration; 
         FIG. 10  is a perspective view of the base frame, seat frame, support and pivot assemblies of the T50 configuration; 
         FIG. 10   a  is a top perspective view of the base frame assembly of the T50 configuration, including the pivot arms mounted thereon; 
         FIG. 10   b  is a partially sectioned, top perspective view of the support (lock) assembly for the T50 configuration; 
         FIG. 11  is a perspective view of a pivot support; 
         FIG. 12  is a side elevation of a pivot support; 
         FIG. 13  is a perspective view of a bracket used in the T20 and T50 configurations; 
         FIG. 14  is an exploded view showing the crossbar assembly and the mounting of the crossbar on a rail; 
         FIG. 15  is a bottom perspective view of the seat frame, pivot and support assemblies for the T50 configuration, using transit tie-down brackets; 
         FIG. 16  is an exploded view of the pivot assembly in relation to the base and seat frame assemblies in the T50 configuration; 
         FIG. 17  is a cross-sectional view of the crossbar assembly according to the preferred embodiment; 
         FIG. 18  is a partially sectioned view of the inner tube of the crossbar assembly; 
         FIG. 19  is a perspective view of a transit tie-down bracket; 
         FIG. 20  is side and end elevations of the transit tie-down bracket; 
         FIG. 21  is a side elevation of a rear crossbar mount; 
         FIG. 22  is a partially exploded view of rear base rail mounting system; 
         FIG. 23  is an exploded view of the rear (drive) wheel mounting assembly; 
         FIG. 24  is a partially sectioned view of the rear wheel mounting assembly; 
         FIG. 25  is a partially sectioned view of the rear wheel mounting assembly, including the wheel lock assembly; 
         FIG. 26  is an exploded view of back cane mounting assembly for the TF and T20 configuration; and, 
         FIG. 27  is a side elevation of a pivot hanger bracket. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 ,  2   a ,  2   b  and  3  illustrate the preferred embodiment of the fixed tilt configuration of the wheelchair according to the invention, which in this disclosure will be referred to as the “TF” configuration. In TF configuration, the wheelchair is set at one of several possible angles of tilt about a pivot axis  10  near the knees of the occupant.  FIGS. 2   a  and  2   b  illustrate two alternative fixed tilt angles for the TF configuration. 
       FIGS. 4 ,  5   a  and  5   b  illustrate the preferred embodiment of the dynamic tilt-in-space configuration of the wheelchair, in which the axis of rotation  12  is provided near the front of the seat frame assembly  14 . The preferred embodiment of this configuration is designed to ensure that the front of the occupant&#39;s knees move upward only a very small amount as the chair undergoes a full range of tilt of up to 20 degrees. In this disclosure, this configuration will be referred to as the “T20” configuration.  FIGS. 5   a  and  5   b  illustrate two different degrees of tilt for the T20 configuration. 
       FIGS. 6 ,  7   a  and  7   b  illustrate the preferred embodiment of the dynamic tilt-in-space configuration of the wheelchair, in which the axis of rotation  16  is provided near the center of gravity of the occupant. The preferred embodiment of this configuration is designed for tilt angles of up to 50 degrees. In this disclosure, that configuration will be referred to as the “T50” configuration.  FIGS. 7   a  and  7   b  illustrate two different degrees of tilt for the T50 configuration 
     Each of the TF, T20 and T50 configurations is built around a set of sub-assemblies that is common to each of the configurations, and that are adapted to receive interchangeable components to modify the wheelchair to the desired configuration. 
     The principal sub-assemblies that are modified to effect a change in the configuration of the wheelchair are the support assemblies for providing load-bearing support between the base frame assembly and the seat frame assembly (or to lock the seat frame against pivoting), and the pivot assemblies that provide a pivot connection between the base frame assembly to the seat frame assembly.  FIGS. 8 ,  9  and  10  illustrate the base frame and seat frame assemblies for the TF, T20 and T50 configurations respectively, including their associated support and pivot assemblies. 
     TF Configuration 
     Referring to  FIG. 8 , a seat frame consists of an assembly  18  comprising a left and right seat rails  20 ,  22  joined by front and rear seat crossbar assemblies  24 ,  26 . A base frame consists of an assembly  28  comprising left and right base rails  30 ,  32  joined by front and rear base crossbar assemblies  34 ,  36 . In the TF configuration, seat frame assembly  18  is set in pivoted relation to the base frame assembly  28  about opposed pivot points (only pivot point  38  is visible in  FIG. 8 ) located near the front of the left and right seat rails  20 ,  22 . The pivot points are located between 1⅝ and 6⅝ inches from the forward edge of a seat pan that is secured, as intended, to the seat crossbar assemblies  24 ,  26 . to The pivot assembly in the TF configuration generally comprises two pivot elements that cooperate to define pivot point  38  between them: pivot supports  40  and pivot hanger brackets  42 . 
     The support assembly  44  for the TF configuration is attached between the front and rear seat crossbar assemblies  24 ,  26  and the rear base crossbar assembly  36 . Support assembly  44  comprises brace bracket  46  and an interface mount element  48 . Different degrees of relative tilt between the seat frame and base frame are achieved by connecting the lower end of brace bracket  46  to one of several attachment points  50  on interface mount  48  that is in turn removably attached to the inner tube  52  of the rear base crossbar assembly  36 . The attachment points comprise apertures  50  arrayed at different angular positions along an arc of constant radius in relation to the pivot points  38 . Inner tube  52  of the rear base crossbar assembly  36  includes a centrally located aperture  54  to receive a removable fastener  56  for attachment of the interface mount  48  thereto. A better view of the interface mount  48  is provided in  FIG. 8   a.    
     Referring to the pivot assembly, the pivot support  40  is illustrated in detail in  FIGS. 11 and 12 . Pivot support  40  consists of a seating block one surface  58  of which is shaped to conform to the inside of a seat rail, and further including a downwardly extending tab  60  having an pivot pin hole  62  therethrough to receive a pivot pin. Seating block  40  includes two spaced apertures  64 ,  66  for receiving fasteners  68 ,  70  that are used to secure the front seat crossbar assembly  24  to the rail as will be discussed in more detail below. 
     The pivot hanger bracket  42  (shown in detail in  FIG. 27 ) similarly includes a pivot pin hole to receive a pivot pin at pivot point  38 . Pivot hanger bracket  42  also has a base  72  through which extend two apertures for receiving fasteners  74 ,  76  used to attach the lower end of the bracket to the front base crossbar assembly  26 . The pivot hanger bracket  42  is preferably provided with an oblong aperture  78  in the body thereof so as to be used as a transit tie-down bracket for optional use in securing the wheelchair to tie-down stations in vehicles. The pivot hanger bracket  42  has a portion thereof that is shaped to mate with a seat provided in a partial sleeve  80  that is welded to the front portion of each rail. 
     T20 Configuration 
       FIG. 9  illustrates the base frame, seat frame, support and pivot assemblies for the T20 configuration. As in the case of the TF configuration, the seat frame assembly  82  and the base frame assembly  84  are connected at pivot points  86  by means of pivot supports  88  mounted to the left and right seat rails  94 ,  96  and pivot hanger brackets  98 ,  100  mounted to the left and right base rails  102 ,  104 . The pivot supports and pivot hanger brackets of the TF and T20 configurations are identical. 
     The T20 configuration uses a different support assembly than does the TF configuration. The T20 support assembly  106  comprises a bracket  108  (slightly different from the TF bracket  46 ) attached to the front and rear seat crossbar assemblies and to the rear base crossbar assembly by means of a bell crank  110  pivotally mounted to the inner tube  112  of the rear base crossbar assembly. The bell crank serves to modulate the degree of resistance provided at different tilt angles and to accommodate the change in spatial relationship between the bracket and the base frame as the seat frame is tilted. 
     Referring to  FIGS. 9 ,  9   a  and  13 , bracket  108  has spaced shoulders  114 ,  116 . The front  118  of the bracket includes a bridge  120  extending between the shoulders  114 ,  116 . Bridge  118  has a fastener aperture for attachment of one end of a gas strut. 
     A gas strut  120  is mounted between the shoulders of the bracket. One end of gas strut  120  is secured to bridge  118  by means of a shoulder bolt while the other end is attached to another shoulder bolt  122  extending through the medial portion of the bell crank  110 . A trigger  124  is provided to control the gas strut. Because the gas strut is connected to the center of the bell crank, a pivoting of the base frame  84  in relation to the seat frame  82  will also cause a translation of the lower end of the bell crank in relation to the vertical plane. Such translation is accommodated by connecting the lower end of the bell crank to a slide  126  mounted on a guide tang  128  that is attached to the inner tube  112  of the rear base crossbar assembly by means of a fastener threaded through a suitable aperture in the inner tube  112 . 
     T50 Configuration 
       FIG. 10  illustrates the base frame, seat frame, support (lock) and pivot assemblies for the T50 configuration. In the T50 configuration, the forward pivot point that was a feature of the TF and T20 configurations is not present and the pivot hanger brackets are not used in the T50 configuration. The pivot supports on the seat rails may be replaced by transit tie-down brackets  132  (see  FIG. 15 ) according to whether the wheelchair is intended to be attachable to tie-downs on public and private transit vehicles. The transit tie-down brackets also double as crossbar mounting elements. The pivot hanger brackets that would normally be seated in partial sleeve  80  on the base rails are replaced by filler blocks  134 . 
     The pivot assembly for the T50 configuration comprises a pivot arm  136  extending up from each of left and right base rails  138 ,  140  to a height  142  above the seat pan. In the preferred embodiment, apart from being secured to the rails, each pivot arm is also braced by attachment to the rear base crossbar assembly  144 . The seat frame assembly is supported about pivot pins  146  at the upper end of the pivot arms by means of opposed pivot hanger plates  148  that are attached to the left and right seat rails  150 ,  152  and that are pivotally suspended from the pivot pins  146 . 
     The pivot arm  148  comprises a base  154  having a surface conforming to the rear of the base rail (see  FIG. 15 ). Apertures  156  are provided in the base  154  to enable the base to be secured by fasteners to selected apertures  158  in a longitudinal recess  160  formed in the rear portion of the base rails. Vertically spaced apertures  162  are adapted to secure the pivot arm  136  to the rear base crossbar assembly  144 . In the preferred embodiment, the pivot arm  148  extends generally upward to a forwardly extending elbow  164  to avoid interfering with the hardware used to secure the seat frame, then upwards to the pivot point  142 . 
     Pivot pin  146  extends through the pivot arm  136  and through the pivot aperture of the pivot hanger plate  148 . 
     The height of the pivot point  142  is selected by reference to the expected center of gravity of the occupant, as calculated using publicly available anatomical data. In the preferred embodiment, the height of this point is about 6.75 inches (171.4 mm) above the seat pan. Such height has been selected by accounting for a typical seat cushion of about 2″ in thickness and an anatomically typical occupant. 
     The precise location in the horizontal plane of the center of gravity of a occupant tends to vary more than does its location in the vertical plane. The invention accommodates such variation by providing means to adjust the horizontal position of the back rest and of the seat pan in the fore and aft directions. This allows the occupant or installer to optimize the coincidence of the pivot point  142  at the top of the pivot arm with the center of gravity of the occupant. A matrix of apertures  166  (see  FIG. 6 ) is provided along the edge of the seat pan  168  allowing the seat pan to be located at different fore and aft positions in relation to the seat frame assembly. The pivot arm  136  is also adapted to be set at various horizontal positions on the base rails, for example to change the wheel base load distribution and to clear interference of the front rigging and front casters. 
     Referring to  FIG. 16 , the pivot hanger plate  148  has a broad base  170  that tapers to a pivot aperture  172  in the top of the plate forming a generally triangular shape that can also serve as a guard to prevent the occupant&#39;s clothing from coming into contact with the rear wheel. The base  170  of the pivot hanger plate includes a bottom portion  174  that conform to the top surface of the rail  176 , and a downwardly extending flange  178  shaped to abut the outside of the rail. The flange  178  includes a plurality of apertures  180  the rearmost five of which are used to receive fasteners for releasably securing the back cane mounting to the pivot hanger plate  148  and the rail  178 . Two of the apertures are to receive fasteners extending through the hanger plate  148 , the rail  176 , the transit tie-down bracket  184  (for tansit-ready chairs only) and a threaded insert  186  (see  FIG. 14 ) extending laterally through the sleeve tube  188  of the rear crossbar assembly. A tab  190  extends downward from the center of the flange and is securable to the transit tie-down bracket by means of a fastener. 
     A plurality of cane mounting apertures are provided at the rear of the pivot hanger plate including three sets of apertures  192  arranged in diverging arcs. The apertures are used to mount a back cane at various angles and positions in relation to both the rail  176  and the pivot hanger plate  148 . 
     Referring again to  FIG. 10 , the preferred embodiment, the support assembly for the T50 configuration comprises the same bracket as in the T20 configuration, as well as an extendible lock rod  193  attached between the rear ends  194  of the shoulders of the bracket. The rear end of the lock rod is pivotally attached to a rod mount  196  attached to the inner tube  198  of the base crossbar assembly. A trigger  200  is provided to selectively lock the rod against retraction or extension to prevent rocking of the seat frame about the pivot points. 
     Crossbar Assemblies 
     In order to provide adjustability in the width of the wheelchair, each of the seat and base crossbar assemblies are telescope assemblies in which an inner tube  202  is received within opposed sleeve tubes  188  as may appreciated by reference to  FIGS. 14 and 17 . The inner and sleeve tubes have generally corresponding cross-sectional shapes and dimensions to facilitate the telescoping function. 
     Inner tube  202  is hollow save for a series of ribs  189  extending along the central longitudinal axis of the tube. A series of apertures  204  adapted to receive fasteners  206  are located between the ribs. The ribs provide rigidity against deformation when the inner and sleeve tubes are brought into engagement with one another by means of head screws  206  extending through selected ones of the apertures  208  and corresponding apertures in the sleeve tube. 
     A feature of the invention is the means by which the crossbar assemblies may be secured in a given telescoped position with a high degree of rigidity. Rather than the head of a fastener bearing on one side of the sleeve tube and a nut bearing on its opposite side, the invention provides apertures  208  in the top wall  210  of the sleeve tube  188  that are larger than the aligned apertures in the bottom wall (not visible) of the sleeve tube and that are sufficiently large that the head of the fastener bears directly on the top wall  212  of the inner tube  202 . This allows the inner tube  202  to bear against the inner bottom surface  214  of the sleeve tube thereby providing a great deal of friction against relative displacement. 
     In addition, the inner tube shape and dimensions are selected to accommodate a small degree of elastic deformation of the inner tube to further lock the inner tube against the sleeve tube when the positioning fasteners are tightened. In the preferred embodiment, this is accomplished by providing non-flat mating bottom walls  216 ,  218  of the inner and sleeve tubes respectively such that any deformation of the inner tube will result in several points and angles of contact between them. In the preferred embodiment such non-flat portions comprises opposed, spaced protuberances  220 ,  222 . 
     In order to accommodate the elastic deformation of the inner tube, a small dimensional gap  224  or tolerance is provided between the inner and sleeve tube contact surfaces. It will be appreciated that the extent of the gap is selected according to the elastic range of the inner tube but it should not be so large as to allow plastic deformation to occur. The telescoping joint mechanism ensures that the joint stays tight even with continuous variations in loading (fatigue). Plastic deformation of the inner tube would compromise the joint integrity and allow the joint to become loose over time. 
     Crossbar Mounting 
     Referring to  FIG. 14 , each seat crossbar assembly is secured to each rail by a mounting element  130  that interfaces between the rail  226  and the sleeve tube  188  of the crossbar assembly and by fasteners  228  that extend through the rail and the mounting element  130  to engage an insert  186  seated laterally through the sleeve tube. 
     The front seat crossbar mounting elements for all configurations are the pivot supports  40 . One side of each mounting element conforms to the inside of a rail  226 , and the opposite side is shaped to engage the outer end of the sleeve tube  188 . Two apertures  230  are provided in the mounting element and are spaced to correspond to the spacing of two mounting holes  232  in the rail so that fasteners  228  may be received through the rail and through the mounting element. The fasteners engage insert  186  that extends laterally through the hollow inside of the sleeve tube. 
     The mounting elements for the rear seat crossbar assemblies for all configurations consist of either a simple mounting element  130  as in  FIG. 14  or a transit tie-down bracket  132 , best illustrated in  FIGS. 19 and 20  that conform on one side to the inside of the rail and are configured on the other side to engage the end of the crossbar assembly. 
     In the case of the base crossbar assemblies, securement to the rails is by means of components that conform to a part of the rail and that include a seat to receive and secure the end of the crossbar assembly by means of fasteners. In the case of the TF and T20 configurations, the front base crossbar assembly is seated in and against partial sleeve  80  and the rear base crossbar assembly is seated in and against rear crossbar mount  81 . Rear crossbar mount  81  is shaped to conform to the outside and top of the rear portion of the rail, including recess  160 . Rear crossbar mount  81  also has a flat surface  83  for receiving and securing the end of the crossbar o assembly, as seen in  FIG. 21 . The rear base crossbar assembly is oriented such that its transverse breadth lies in the vertical plane. This allows attachment of the interface mount  48 , the slider assembly  126 ,  128  or the mechlok rod mount  196  (as the case may be) to be attached to the inner tube by a fastener through an aperture traversing the width of the inner tube. 
     In the case of the T50 configuration, the front base crossbar assembly is seated against a seat in a forward crossbar mount  80  that conforms to part of the front of the base rail and that has a seat adapted to receive and secure the end of the crossbar assembly. 
     The rear base crossbar assembly of the T50 configuration is seated in a seat provided on the inside of the base of the pivot arm  136  and is secured by two screws  162 . 
     Base Rail Mounting System 
     The invention provides adjustability of the wheel base as well as the location of the pivot point in the horizontal plane for the T50 configuration by a longitudinal recess  160  journaled in the rear portion of each base rail  233 . Referring to  FIG. 22 , a plurality of aligned apertures  234  along the interior of the recess receive fasteners  236  that are used to secure the rear (drive) wheel axle mounting plate  238 , crossbar mounts or the base of the pivot arms as the case may be. The relative front to back position of those components can be adjusted by selecting the appropriate apertures. The edges of the channel include grooves  240  adapted to receive clip-on masking caps  242  (see for example  FIG. 5   a ) to provide an aesthetic cover for those portions of the channel that are not otherwise covered by one of the foregoing components. 
     A mounting assembly is provided for securing the fasteners within the hollow interior of the rail. An elongated rod  244  is adapted to be longitudinally inserted and retained in the hollow rail  233 . A plurality of nuts  246  are retained in several spaced seats  248  provided along the length of the rod such that when it is inserted and retained in the rail with the nuts aligned to the fastener apertures  234 , fasteners  236  inserted into the apertures will engage the nuts and be retained without the need to traverse the opposing wall of the rail. This arrangement also avoids potential problems that might arise from securing the fasteners directly to the rail itself. As the rail and the fasteners may be of different materials, the potential for reaction between them is reduced by the invention. 
     The forward ends of the rails terminate in a caster clamp for retaining a standard caster assembly. The forward ends of the seat rails terminate in a front rigging hanger with an insertion tube adapted to telescope a selected depth into the front end of the rail and a vertically oriented sleeve adapted to receive a standard footrest assembly. 
     Drive/Wheel Assembly 
     The rear (drive) wheel assembly is illustrated in  FIGS. 23 ,  24  and  25 . The wheel assembly includes an axle mounting plate  250  secured to the base rail  252  and means to mount each of the wheel, the wheel lock assembly and the anti-tip assembly directly onto the axle mount. This allows the position of the wheel on the frame to be adjusted by changing the location of the axle mounting plate, rather than needing to separately adjust an anti-tip assembly  254 , an axle mounting plate and a wheel lock assembly  256 . 
     The axle mounting plate  250  has a base  258  with an inner dimension corresponding to the outer shape of the rail including the recess, and an extension  260  having a plurality of aligned vertical positioning apertures  262  for receiving a rear wheel axle receiver  264  in any one of several vertical positions: The axle mounting plate  250  is secured to the rail  252  by fasteners  266  extending through apertures in the base of the axle mounting plate and through apertures provided in the recess  160 . 
     The axle receiver  264  is inserted through a selected one of the vertically aligned apertures  262  according to the preferred ground clearance for the base frame of the wheelchair. A wheel lock tube  268  is secured between the rear wheel  270  and the axle mounting plate  250  by means of a mounting piece  272  that is adapted to provide a secure mating seat  274  for the side of the axle mount. A clearance aperture  276  through the mounting piece  272  provides a passageway for the axle receiver. The end of the axle receiver is threaded so as to receive a nut  278  used to tighten the wheel lock tube (through the mounting piece) to the axle mount. The axle  280  is inserted through the nut and the mounting piece  272  and into the hollow interior of axle receiver  264 . The end of axle  280  includes retainers  282  that project out of the end of axle receiver to hold the axle therein. 
     Retainers  282  are biased and may be manually depressed to allow the axle to be disengaged from the axle receiver. Upon doing so, removal of the nut is all that is required in order to remove the axle receiver  264  and mounting piece  272  so as to be able to reposition the axle receiver into a different vertical positioning aperture  262 . 
     The invention provides a simple means of repositioning the height of the rear wheel  270  in relation to the base frame with a minimum of tools and effort. In addition, since the anti-tip assembly  254  and the wheel lock assembly  256  are both mounted on the wheel lock tube  268  which in turn is mounted to the axle mounting plate, it is possible to adjust the horizontal position of the rear wheel on the base rail by repositioning the axle mount without the need to separately readjust the anti-tip assembly or the wheel lock assembly. 
     Back Cane Mounting 
     A back cane assembly illustrated in  FIG. 26 . The assembly  284  is adapted to be mounted in various angular and fore and aft positions by providing a back plate  286  having plurality of suitable apertures to accommodate different orientations and positions of the cane  290 . Back plate  286  is secured to the inside of the seat rail  294  by means of two fasteners  296  on the inside of the back cane. A number of fore and aft positions can be selected using a plurality of apertures  298  provided on the rail. The cane is then secured to the back plate by a pivot fastener  300  and by a second fastener  302  inserted through one of several apertures  288  provided in an arc about the pivot fastener  300 , thereby enabling the cane to be mounted at different angles in relation to the rail. 
     In the TF and T20 configurations, a second back plate  302  is provided on the outside of the rail and all fasteners extend through both the inside and outside back plates. In the case of the T50 configuration, the outside back plate  302  is omitted but the pivot hanger plate  148  is provided with corresponding apertures and fulfills the same function as the outside back plate does in the TF and T20 configurations. 
     Converting from TF to T20 or T50 
     Reconfiguring a wheelchair from the TF fixed tilt configuration to a dynamically tiltable configuration (i.e. to either the T20 or the T50) is generally accomplished as follows. 
     The TF configuration comprises a support assembly (brace bracket  46  and interface mount element  48 ) connected between the seat frame assembly  18  and the base frame assembly  28 . The interface mount element  48  is configurable by the selection of different attachment points  50  to define any one of a plurality of predetermined relative pivot angles between the seat frame and the base frame. The interface mount element  48  is first removed by removing fasteners  56  and  57 . A bias mechanism is then installed to provide a mechanical advantage in tilting the seat frame in relation to the base frame, thereby providing a dynamically tiltable wheelchair. In the case of the T20, the bias mechanism is the assembly consisting of gas strut  120 , bell crank  110  and slide  126 . One end of gas strut  120  is secured to bell crank  110 . The other end of the gas strut is secured to the seat frame, or more particularly to a bracket  108  that is associated with the seat frame. Bell crank  110  is secured (through slide  126  and guide tang  128 ) to the same attachment point that received fastener  56  in the TF configuration. In the case of the T50m the bias mechanism is a mech lok. 
     Converting from T20 to T50 
     The conversion to the T20 to the T50 configuration involves both a change of the pivot assembly and of the support assembly. 
     In the T20, the pivot assembly consists of cooperating pivot elements, namely pivot supports  88  and pivot hanger brackets  98 ,  100 , each of which is removably attached to the chair by fasteners  68 ,  70  or fasteners  74 , 76 . The pivot assembly of the T20 is removed by disengaging fasteners  68 ,  70 ,  74  and  76 . A new centre of gravity pivot assembly is installed by mounting pivot arms  136  on the base rails and mounting pivot hanger plates  148  to the seat rails, and pivotally connecting the pivot arms to the hanger plates by pivot pins  146 . The pivot arm is secured to the base rails by inserting fasteners in to apertures that extend to the pivot arm and in to the base rails. The hanger plates are mounted by securing fasteners to the plate and into apertures in the seat rails. If desired, a transit tie down bracket can be installed where the pivot hanger brackets would normally be attached in the T20 configuration. 
     The preferred embodiment of the invention has been described in some detail. However, those skilled in the art will appreciate that various modifications to the constructional details of the embodiment may be practiced without departing from the spirit and scope of the invention, which scope is properly defined by the claims that follow. The following claims are nonetheless to be considered part of the disclosure herein.