Abstract:
A shock absorbing suspension for the large rear wheels of a wheelchair is disclosed. A fork element is hinged at one end to a clamping member which attaches to a horizontal frame member of the wheelchair. A cylindrical solid polymer shock absorber cushions the movement of the fork element toward the clamping member. The free end of the fork element includes an axleway for the axle of a large wheelchair wheel. The movement of the fork element relative to the clamping member is limited by abutting surfaces on the fork element and the clamping member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from copending provisional patent application entitled “Rear Suspension for Wheelchair”, Ser. No. 60/412,073 filed Sep. 19, 2002. The disclosure of provisional patent application Ser. No. 60/412,073 is hereby incorporated in its entirety. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention pertains to wheelchairs and particularly to suspension of the large rear wheels of wheelchairs which are manually operable as well as those that are powered by self-contained motors. 
   In the typical manual wheelchair each rear wheel is a large wheel mounted to the frame of the wheelchair without any provision for absorption of shocks or damping of vibration of the wheelchair. In such a wheelchair, shocks and vibrations arising from irregularities and defects in the traveled surface are transmitted through the rear wheels to the wheelchair occupant. One attempt to reduce this phenomenon is shown in U.S. Pat. No. 4,455,031 wherein a shock absorber interconnects the seat of the wheelchair with a frame on which the large rear wheels are mounted. The present invention provides simplified shock absorbing apparatus for each large rear wheel to reduce the shock and vibration felt by the user of a wheelchair traversing a surface. 
   SUMMARY 
   The present invention provides a rear suspension for a wheelchair which can be retrofitted to a wheelchair or installed at initial manufacture. The invention is directed toward a manually operated wheelchair, especially a sport wheelchair, which is driven by the rider pushing the large rear wheels by hand. However the invention is equally applicable to a powered wheelchair. 
   Each rear suspension element is mounted to a horizontal side bar of the wheelchair frame such that the hub of each back wheel is located slightly below the horizontal side bar to which the suspension element is mounted. Each rear suspension element includes the clamp for mounting the suspension element to the horizontal side bar of the wheelchair frame, the clamp having a base and an upper member which cooperate to surround the side bar. A fork element is hinged to the base of the clamp at one end of the base. The fork includes an axle housing to receive the axle of the rear wheel it suspends. The fork extends rearward along the horizontal side bar below the base of the clamp. An upright polymer cylinder is trapped between the bottom of the base of the clamp and the fork to provide a shock absorber. Rotation of the fork relative to the hinge is limited by surfaces on the fork and on the base near the hinge such that the fork cannot rotate away from the clamp base to the extent that the polymer cylinder will fall out of place. Similarly other abuttable surfaces prevent the fork from rotating toward the clamp base to an extent which might cause excessive compression of the polymer cylinder. 
   The fork includes a large axle housing to receive the axle of the large wheel. The fork has two separated tines which are joined at their lower edges by a transverse wall which carries a recess which receives the lower end of the polymer cylinder. Each tine terminates with a ring which, with the ring of the other tine, forms the large axle housing. Each ring contains a small gap in its circumference and the distal parts of the lower edges of the rings are joined by a web. The web may be drawn toward the transverse wall of the fork by a screw which can be adjusted, making the large axle housing adjustable in circumference so that the axle of the large rear wheel can be held securely within the axle housing of the fork. 
   The bottom of the clamp base includes a stub having a cylindrical hollow which can receive the upper end of the polymer cylinder to retain it. The hinge and fork are arranged slightly off center below the clamp so that movement of the suspension element is not interfered with by the frame of wheelchair. 
   It is an object of the invention to provide a shock absorbing apparatus to reduce the transmission of bumps and shocks from the large rear wheels of a wheelchair to the rider in the wheelchair. 
   It is a further object of the invention to provide an improved manual wheelchair which is less uncomfortable for the user to ride over irregular surfaces. 
   It is also an object of the invention to provide a shock absorbing rear suspension which can be easily added to an existing wheelchair. 
   These and other objects will become apparent from examination of the description which follows. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       FIG. 1  is a front elevation of a wheelchair equipped with the invention. 
       FIG. 2  is a perspective of the invention mounted to the frame of a wheelchair and supporting the axle of the rear wheel of a wheelchair, the frame being cut away. 
       FIG. 3  is an exploded perspective of the invention of  FIG. 1 . 
       FIG. 4  is an enlarged front elevation of the base member of the invention. 
       FIG. 5  is an enlarged front elevation of the fork member of the invention. 
       FIG. 6  is a front perspective of an alternative embodiment of the polymer shock absorber of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  discloses a wheelchair  55  equipped at each rear wheel  57  thereof with a rear suspension element  2  according to the present invention. A horizontal frame member  51  on each side of the wheelchair  55  has one of rear suspension elements  2  clamped thereto. Each rear wheel  57  is mounted at its axle  50  to a rear suspension element  2 . 
     FIG. 2  shows a perspective of the invention rear suspension element  2  suspending rear wheel axle  50  (with the rear wheel removed) of a manual wheelchair. The invention is shown mounted to a horizontal frame member  51  (cut away) of the manual wheelchair. 
   Referring now to  FIGS. 2–5 , the invention comprises a clamp base  4  which may be secured to horizontal frame member  51  of the wheelchair by use of enclosing member  3  which may be securely fastened to clamp base  4  by use of bolts  10 . Enclosing member  3  includes longitudinal notch  28  into which elongate ridge  38  of clamp base  4  is received before bolts  10  are installed. Clamp base  4  and enclosing member  3  cooperate to surround tubular frame member  51  and retain the invention to horizontal frame member  51 . As is readily understood, clamp base  4  and enclosing member  3  may be fixed to a frame member of many different wheelchairs and such wheelchairs can be easily retrofitted with the rear suspension elements  2 . 
   Clamp base  4  includes an axle housing  12  mounted therebelow at one longitudinal end  13  thereof. Near opposing end  14  of clamp base  4  is formed a vertically oriented receiver  15  which includes a hollow which receives upper end  16  of polymer shock absorber  6 . Polymer shock absorber  6  is retained between clamp base  4  and fork  5  and comprises an elongate solid cylinder of compressible polymeric material which is uniform in construction. Shock absorber  6  is constructed to be robust in resisting longitudinal compression and may be one to three inches long and about one inch in diameter. Polymer shock absorber  6  may be formed of VIBRATHANE™ polyurethane supplied by Uniroyal Co. or of many other resilient polymer compounds which can absorb longitudinal compressive forces of approximately three hundred fifty pounds. 
   Fork  5  comprises a pair of parallel spaced apart tines  19 ,  20  which are joined at the lower edges  36  by transverse wall  23 . Each tine  19 ,  20  includes pivot pin openings  24 ,  25  respectively at pivot end  29  of fork  5 . Transverse wall  23  includes seat  26  which receives the lower end  27  of shock absorber  6 . Seat  26  comprises a cylindrical recess in transverse wall  23 . Free end  30  of fork  5  is provided with wheel axle openings  31 ,  32  on tines  19 ,  20  respectively into which the axle  50  of the rear wheel of the wheelchair is to be received. With shock absorber  6  uncompressed, fork  5  rests at about twenty-two degrees of angular separation from the longitudinal axis of clamp base  4 . 
   Fork  5  is retained to clamp base  4  by pivot pin  8  which passes through pivot pin openings  24 ,  25  and is received in pivot bushings  7  which are received in opposing ends of axle housing  12 . Pivot pin  8  is retained in bushings  7  and within axle housing  12  by cap screws  9  mounted at opposing ends of pivot pin  8 . Fork  5  is pivotable about pivot pin  8  over a limited range permitted by adjoining stop faces  17 ,  18  on each of tines  19 ,  20  respectively of fork  5 . First stop faces  17  are joined to second stop faces  18  at an obtuse angle of at least about one hundred thirty degrees. Axle housing  12  is provided with third and fourth stop faces  21 ,  22  (see  FIG. 4 ) on opposing ends thereof. Third stop faces  21  join fourth stop faces  22  at an angle exceeding the angle separating first and second stop faces  17 ,  18  by at least approximately ten degrees. Third stop faces  21  abut first stop faces  17  of tines  19 ,  20  when shock absorber  6  is decompressed and limit the downward rotation of fork  5  about pivot pin  8 . Conversely, when fork  5  is forced upward by a road surface feature, compression of shock absorber  6  is limited when second stop faces  18  limit rotation of fork  5  about pivot pin  8  by their abutment on fourth stop faces  22 . 
   Fork  5  may include voids  33  in each of tines  19 ,  20  to reduce weight. Voids  33  also allow access to the polymer shock absorber for its positioning within seat  26 . 
   Fork  5  is formed of rigid metal or polymeric materials and is cast or formed such that tines  19 ,  20  and transverse wall  23  are integral and of uniform composition, it being anticipated that fork  5  will be formed as a one-piece unit. Fork  5  is provided with a generally linear lower edge  36  and each of tines  19 ,  20  has an upper edge  37  which is generally linear before sloping upward to join second stop face  18 . Lower edge  36  slopes upward at approximately forty-five degrees near pivot end  29  of fork  5 . 
   As may be best seen in  FIG. 4 , receiver  15  is slanted at its lower end  35  to facilitate placement of upper end  16  of polymer shock absorber  6  therein such that the longitudinal axis of polymer shock absorber  6  rests at essentially a perpendicular to transverse wall  23 . Therefore, the longitudinal axis of polymer shock absorber  6  is not aligned with the longitudinal axis of receiver  15  when shock absorber  6  is decompressed. Receiver  15  is a hollow sleeve which receives the upper end  16  of polymer shock absorber  6 . 
   Each of wheel axle openings  31 ,  32  is defined by a ring  41 ,  42  of tines  19 ,  20  respectively and each of rings  41 ,  42  comprises a gap  40  in its lower region. Provision of gaps  40  allows rings  41 ,  42  to be adjustable in circumference so that they can be tightened around wheel axles  50  of the large rear wheels of the wheelchair. A web  43  joins the distal lower portions of rings  41 ,  42 . Web  43  may be drawn toward transverse wall  23  to reduce or increase gaps  40  by use of a screw  44  extending through web  43  and threaded into transverse wall  23 . Rib  34  may be integrally formed on the underside of transverse wall  23  to provide structure to receive screw  44 . 
   In order to avoid interference of the rear wheels with frame components of the wheelchair  55  upon which rear suspension element  2  is mounted, it is preferable that fork  5  be offset from the axis of the frame member  51  such that fork  5  may pivot about pivot pin  8  without striking or rubbing any part of wheelchair  55 . Hence the longitudinal axis of fork  5  is spaced apart laterally approximately ¼ inch from the axis of cylindrical recess  39  of clamp base  4  when suspension element  2  is oriented vertically, as would be the normal orientation when mounted to a wheelchair. When the rear suspension element  2  is mounted to a wheelchair, the preferred orientation of the rear suspension element  2  is such that the pivot end  29  of fork  5  is forward of the free end  30  of the fork  5 . 
   An alternative embodiment polymer shock absorber  60  is illustrated in  FIG. 6 . In this embodiment, body  47  of shock absorber  60  is of enlarged cross section while stubs  48  and  49  are of smaller cross section and upper stub  48  will fit within the hollow of receiver  15  and lower stub  49  will fit within seat  26  of transverse wall  23 . Shock absorber  60  may be utilized in place of polymer shock absorber  6  when a heavier person will be the user of the wheelchair on which the rear suspension elements  2  are mounted. 
   It can be seen that shock absorber  6  or alternate shock absorber  60  are disposed generally vertically within rear suspension element  2  and serve to dampen vertical movement of the fork  5  to reduce the jarring of the wheelchair occupant as the large wheels  57  pass over irregularities of the street or floor on which the wheelchair  55  is operated. 
   Many variations will be apparent to those skilled in the art. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced other than as specifically described.