BACKGROUND OF THE INVENTION 
     The present invention relates to a folding unicycle that can be easily attached to a wheelchair solely by the wheelchair occupant while the occupant is seated in the wheelchair, thereby converting the wheelchair into a hand cranked tricycle. 
     Several prior art unicycle devices have been suggested for converting a wheelchair into a hand cranked tricycle. However, most of these devices are not foldable for easy transport and storage, and all have complicated means for attaching the unicycle to the wheelchair, which attachment means cannot be operated solely by a wheelchair occupant while seated in the wheelchair. 
     U.S. Pat. No. 3,485,510 describes a hand driven unicycle 20 for attachment to the front of a wheelchair 22 to form a tricycle. The attachment means includes a unicycle frame member 26 having a rear engagement end 28 attached to a bearing means 40 which is affixed to a transverse member 36 the ends of which are connected to a quick connect/disconnect means 32. The unicycle described in the &#39;510 patent cannot be folded, and attachment requires modification of the existing wheelchair, including removal of the front wheels, which cannot be easily done by a wheelchair user. 
     U.S. Pat. No. 4,316,616 describes a hand driven unicycle for attachment to the front of a wheelchair to form a tricycle. A generally rectangular rear framework formed by telescoping horizontal tube members 31, 32, 34, 35, 36 and 37 and vertical tube members 33 and 38 is removably attachable to the front frame portion of the wheelchair. An upper bar 49 and lower, telescoping bar 52, 53 attaches the rear framework to the sleeve 11 of the unicycle. A tension spring 57 is attached to the outer section of bar 53 and to the outer end of an L-shaped arm 59 secured to the lower end of steering post 9. Tension spring 57 automatically aligns the main wheel 12 in the central longitudinal plane of the wheelchair and attachment assembly when the crank arms 18 are released. The unicycle described in the &#39;616 patent cannot be folded and does not have an attachment means which can be easily operated by the wheelchair user. 
     U.S. Pat. No. 4,471,972 describes a foldable, hand driven unicycle 100 for attachment to the front of a wheelchair W to form a tricycle. The unicycle attachment means includes a frame 150 with attachment means 110 (FIG. 2) or 120 (FIGS. 7 and 8) for attaching the unicycle to the various frame members 61 of the frame W of a wheelchair. The attachment means 120 shown in FIGS. 7 and 8 (involving inserting frame member 55′ into a slot on the wheelchair engaging frame 120) which raises the castor wheels above the ground when locked into place. Frame members 52 and 55 of the unicycle can be folded upwardly from the position shown in FIGS. 1 and 2 to that shown in FIG. 3 for storage. The attachment means cannot be easily operated solely by a wheelchair user. 
     U.S. Pat. No. 4,483,548 describes a hand driven unicycle 10 for attachment to the front of a wheel chair to form a tricycle. A front tubular housing 50 has a mounting post 51 rotatable within housing 50. Mounting post 51 has a wheel 13 rotatably mounted thereon. Two rearwardly extending main posts 64 and 65 are fixedly attached at one of their ends to tubular housing 50. The other end of post 64 is fixedly attached to tube 29 of a frame attachable to the wheelchair, and the other end of post 65 is slidably received into tube 66 secured to tube 26 of the attachment frame. Post 65 is adjusted within tube 66 a distance to cause the small castor wheels 70 to be lifted off the ground. The unicycle of the &#39;548 patent is not foldable, and is not connectable to the wheelchair with attachment means that can be easily operated solely by the wheelchair user. 
     U.S. Pat. No. 4,720,117 describes a hand driven unicycle 10 for attachment to the front of a wheelchair 12 to form a tricycle by an attachment frame 14. The unicycle is not foldable and is connectable to the wheelchair by use of four separate attachment means which are difficult to manipulate solely by the wheelchair user. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a foldable unicycle that is attachable to a wheelchair to form a hand driven tricycle. The unicycle has upper and lower drive train frames and means for attaching the unicycle to a wheelchair. 
     The means for attaching the unicycle to a wheelchair includes a positioning stand that holds the unicycle and its attachment means in a position adapted to receive a hitch means attached to the wheelchair frame. Latching means releasably latch together the attachment means and hitch means. The positioning stand of the unicycle and the front castor wheels of the wheelchair are lifted off the ground during latching of the unicycle attachment means to the hitch means of the wheelchair. 
     For easy transport and storage of the unicycle, folding means are provided for folding the upper drive train frame downwardly into a position adjacent the lower drive train frame. Folding means are also provided for folding the unicycle attachment means downwardly into a position adjacent the lower drive train frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a right side elevation view of the folding unicycle of the present invention shown attached to a wheelchair (outlined in phantom); 
     FIG. 2 is a front elevation view of the folding unicycle of the present invention; 
     FIG. 3 is a right side elevation view of the frame of the folding unicycle of the present invention; 
     FIG. 4 is an enlarged left side elevation view of the upper drive train subassembly frame of the folding unicycle of the present invention; 
     FIG. 5 is an enlarged top plan view of the drive train pivot axle of the folding unicycle of the present invention; 
     FIG. 6 is an enlarged right side elevation view of the upper portion of the lower drive train subassembly; 
     FIG. 7 is a left side elevation view of the unicycle wheel; 
     FIG. 8A is an enlarged right side elevation view of the wheelchair attachment means positioned adjacent the attachment receptor means of the wheelchair; 
     FIG. 8B is an enlarged right side elevation view of the wheelchair attachment means of the folding unicycle of the present invention shown in its initial contact position with the attachment receptor means of a wheelchair; 
     FIG. 8C is an enlarged right side elevation view of the wheelchair attachment means of the folding unicycle of the present invention shown with its stirrup swiveled into position behind the attachment receptor means of a wheelchair; 
     FIG. 8D is an enlarged right side elevation view of the wheelchair attachment means of the folding unicycle of the present invention shown in full locking engagement with the attachment receptor means of a wheelchair; and 
     FIG. 9 is a right side elevation view of the folding unicycle of the present invention shown in its fully folded position. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The foldable unicycle  10  of the present invention can be easily attached to a wheelchair, such as wheelchair  12  shown in phantom in FIG. 1, by a wheelchair occupant while the occupant (not shown) is seated in the wheelchair. 
     The frame of foldable unicycle  10  is shown in FIG.  3  and includes an upper drive train subassembly  20 , a lower drive train subassembly  40  and a wheelchair attachment subassembly  100 . 
     Upper drive train subassembly  20  includes a head tube  21 , an upper steering tube  22  adjustably received and locked within head tube  21 , a crank axle housing  23  located at the outer end of upper steering tube  22 , and a folding mechanism sleeve  24  (formed of two half-sleeves  24   a  and  24   b ) attached to the lower end of head tube  21  (as best seen in FIG.  4 ). Half-sleeves  24   a  and  24   b  are, substantially, one-half of a tubular cylinder. Upper steering tube  22  can be adjusted inwardly and outwardly relative to head tube  21  to accommodate the trunk height of the wheelchair occupant, and locked into position by suitable locking means, such as ring clamp  19  located over a slot (not shown) in the wall of head tube  21 . Such adjustment may require the addition or substraction of links from chain  84 . 
     As best seen in FIG. 2, a crank axle  25  extends through crank axle housing  23  and is suitably journaled in bearings located at each end (not shown) of crank axle housing  23 . Right and left crank arms  26  and  26 ′ are each attached at one end to the right and left outer ends of crank axle  25 , respectively, and right and left handles  27  and  27 ′ are attached to the other ends of right and left crank arms  26  and  26 ′, respectively. An upper drive sprocket subassembly is attached to the right side of crank axle  25  at a location between right crank arm  26  and the right end of crank axle housing  23 , said upper drive sprocket subassembly including hand crank drive sprocket  28  and associated guard  29 . 
     Crank axle housing  23 , crank axle  25 , right and left crank arms  26 ,  26 ′, and hand crank drive sprocket  28  are available as a single unit called “Mountain Drive” manufactured by Schlumph, and described in U.S. Pat. No. 5,609,071. 
     As best seen in FIGS. 3 and 6, lower drive train subassembly  40  includes fork tube  41 , lower steering tube  42  extending through fork tube  41 , an upper ball bearing retainer assembly  43  and a lower ball bearing retainer assembly  43 ′. Upper and lower ball bearing retainer assemblies  43  and  43 ′ can be any of those typically used in American-type bicycle headsets. 
     A notched head tube rest stop  49  (best seen in FIG. 2) extends outwardly and upwardly from the front side of fork tube  41 . 
     The upper ends of right and left fork blades  44  and  44 ′ are attached to the lower end of lower steering tube  42 , such as by welding. 
     It is known in the bicycle art that a problem arises when the steering bar is not directly above the front or steering wheel. When the steering is moved behind the wheel and the angle between the steering device and the wheel becomes more obtuse, the wheel develops a tendency to flop to one side unless held in the upright position. The more obtuse the angle between the steering unit and the effected wheel, the more pronounced this tendency becomes. In order to reduce this problem, fork blades  44  and  44 ′ have a substantial rake angle between their upper and lower portions, the rake angle being between about 75 degrees and about 90 degrees. 
     Right and left strut members  45  and  45 ′ extend between the lower and upper ends of right and left fork blades  44  and  44 ′, respectively, and provide additional rigidity to the lower drive train frame. 
     Right and left dropout members  46  and  46 ′ are attached to the lower ends of right and left fork blades  44  and  44 ′, respectively, and have slots  47 ,  47 ′ for receiving axle  48  of wheel  50 . Although shown as separate parts, right and left dropout members  46  and  46 ′ may be made integral with the lower ends of right and left fork blades  44 ,  44 ′, such construction being common in many bicycles. 
     Drive wheel  50  is a conventional bicycle wheel, and includes a pneumatic tire  51  mounted on a rim  52  supported by a plurality of spokes  53  attached at one end to the flanges of a hub  54  and at the other end to the rim  52  in a manner conventional in the bicycle art. 
     Drive wheel  50  includes a conventional sprocket (freewheel) cluster  55  (shown as being located on the right side) attached to axle  48 , and a conventional derailleur  56  (such as the Shimano “DeFore LX”) attached to right dropout member  46 . The sprocket cluster  55  may contain any number of gears between about three and nine, preferably about six. 
     Derailleur  56  communicates a lower drive chain  57  with one of the gears of the sprocket cluster  55 . Derailleur  56  is operated by means of derailleur cable  58  attached at one end thereto and at its other end to a conventional derailleur adjustment lever  59  mounted on head tube  21 . 
     Drive wheel hub  54  (best seen in FIG. 7) contains a coaster brake subassembly having a lower brake arm  60  (shown as being located on the left side). A brake cable  62  is attached at one end to lower brake arm  60  and at its other end to an upper brake arm  86  (FIG.  5 ). 
     Drive wheel  50 , hub  54  (including the coaster brake subassembly with lower brake arm  60 ), and sprocket cluster  55  are available as a single unit manufactured by Sachs as the “Orbit HT”. 
     As best seen in FIGS. 5 and 6, a cam clamp arm  70  is attached to the upper outer end of lower steering tube  42 , such as by welding. A conventional quick release cam clamp  71  for the upper drive train latch  72  is attached to the upper surface of cam clamp arm  70 . Cam clamp  71  includes a cam clamp axle housing  73 , a cam clamp axle  74 , a cam clamp lever arm  75  attached to the outer end of axle  74 , and clamping members  76 ,  76 ′. The space between clamping members  76 ,  76 ′ and the adjacent ends of axle housing  73  allow the forked ears of upper drive train latch  72  to be received around axle  74  (as best seen in FIG. 3) and clamped into place by rotating cam clamp lever arm  75  into its locking position. 
     A tubular bottom bracket axle mechanism housing  80  is attached to an outer wall portion of lower steering tube  42  extending above fork tube  41 , such as by welding. An axle mechanism  81  extends through tubular bottom bracket axle housing  80 . Axle mechanism  81  includes an axle (not shown) that is journaled therein by use of suitable bearings to allow the axle to rotate within axle mechanism  81 . 
     As best seen in FIG. 5, the right side outer end of the axle of axle mechanism  81  has outer and inner intermediate drive sprockets  82  and  83  attached thereto in a manner such that rotation of outer intermediate drive sprocket  82  causes inner intermediate drive sprocket  83  to rotate. Outer intermediate drive sprocket  82  communicates with hand crank drive sprocket  28  by means of upper drive chain  84  (shown in phantom in FIGS. 1 and 5) and inner intermediate drive sprocket  83  communicates with derailleur  56  and sprocket cluster  55  by means of lower drive chain  57  (shown in phantom in FIGS.  1  and  5 ). 
     Axle mechanism  81  is the type used in the bottom bracket of coaster braking type bicycles, and allows outer and inner sprockets  82  and  83  to remain immobile during coasting and to actuate coaster braking through upper coaster brake mechanism  85  when cranks  26 ,  26 ′ are reversed from their normal clockwise driving direction. 
     Upper coaster brake mechanism  85  includes an upper brake arm  86  that communicates with lower brake arm  60  by means of brake cable  62 . The entire coaster brake mechanism, including that portion contained in wheel hub  54  and upper coaster brake mechanism  85 , can be any conventional bicycle coaster brake mechanism. 
     Upper drive train subassembly  20  is rotatably attached to axle mechanism housing  80  by placing the two half sleeves  24   a  and  24   b  around the housing and inserting and tightening the four lock bolts  30  to the point where subassembly  20  can still be rotated about housing  80 . Half sleeves  24   a  and  24   b  have suitable cutouts (not shown) to allow lower steering tube  42  to be attached to axle mechanism housing  80 . 
     As best seen in FIGS. 3 and 9, wheelchair attachment subassembly  100  of unicycle  10  includes an anchor member  102  and a swivel arm  110  pivotally attached thereto. 
     Anchor member  102  is attached at its inner end to fork tube  41 , such as by welding. At its outer end, anchor member  102  is split into a pair of identical and parallel lower arms  103  (only the right lower arm  103  being shown) and identical and parallel forked upper arms  104  (as best seen in FIG. 9, only the right upper arm  104  being shown). 
     Lower arms  103  each have an opening passing therethrough for receiving a pivot bolt  105  therethrough. Bosses attached to the lower inner end of swivel arm  110  (not shown) have openings passing therethrough which are in alignment with the openings in lower arms  103 , and pivot bolt  105  extends through all of these openings. A nut (not shown) is attached to the outer end of pivot bolt  105 . As a result of this construction, swivel arm  110  can be pivoted away from anchor member  102  about pivot bolt  105 , as best seen in FIG.  9 . 
     Upper arms  104  each have a forked terminus (see FIG. 9) adapted to be received by cam clamp  112 , which is attached to the top inner end of swivel arm  110  such as by welding. Cam clamp  112  is not shown in detail, but is identical in construction and operation to cam clamp  71  shown in FIG.  5 . Swivel arm  110  can be pivoted upwardly from the position shown in FIG. 9 so that the forked termini of upper arms  104  engage cam clamp  112  and can be locked into place in the position shown in FIG. 3 by turning the cam lever of cam clamp  112  to its locking position. 
     A rectangular hitch block  114  is attached to the rear end of swivel arm  110 , such as by welding. 
     As best seen in FIGS. 2 and 3, a positioning stand  116  is located inwardly of hitch block  114 . Positioning stand  116  is, essentially, an inverted U-shaped tubular member with its base welded to swivel arm  110 . Right and left legs  117 ,  117 ′ extend downwardly from the base with their rubber covered tips  118 ,  118 ′ in contact with the ground  200  (see FIG. 2) prior to attachment of unicycle  10  to wheelchair  12 . In this position, drive wheel  50  and legs  117 ,  117 ′ form a tripod to maintain unicycle  10  in the standing position shown in FIG.  2 . 
     An attachment latching mechanism includes a bifurcated lever arm  120  pivotally attached at its lower end to pivot pin  122  and having a covered handle  124  at its upper end. An essentially rectangular-shaped attachment stirrup  126  is pivotally attached at its upper end to bifurcated lever arm  120 . An essentially rectangular stirrup stop member  128  is pivotally attached at its upper end to bifurcated lever arm  120  above attachment stirrup  126  and surrounds swivel arm  110 . 
     Right and left stabilizing spring members  129  extend between, and are attached to, anchor member  102  and right and left fork blades  44 ,  44 ′ (only the right spring member  129  being shown in FIGS.  1  and  3 ). Similarly, a third stabilizing spring member  129  (not shown) extends between anchor member  102  and the base of steering tube  42 . Stabilizing spring members  129  reduce the tendency of the wheel  50  to turn from side to side during hand pedaling, and increases the tendency of the wheel  50  to point straight ahead regardless of the user&#39;s stroke technique without limiting the turning radius of the wheel. 
     Having both crank arms  26 ,  26 ′ facing in the same direction, as shown in FIG. 2, has also been found to help reduce wobbling since pedaling power is being applied to right and left crank arms  26 ,  26 ′ in the same direction at the same time. However, crank arms  26 ,  26 ′ may be positioned in opposite directions, such as shown in U.S. Pat. No. 4,720,117 and others. 
     Wheelchair  12  has a hitch means  130  which includes a tubular tongue  131  having a hitch socket  132  attached to the outer end thereof, such as by welding. Hitch socket  132  includes a generally rectangular hitch plate  133  having a slotted lip  134  extending outwardly from its upper edge, a ramp  135  extending downwardly and outwardly from its lower edge, and a locking ledge  136  extending along its inner lower edge. Tongue  131  is attached to the wheelchair frame by right and left side bracket members  137 , only the right side bracket members  137  being shown in FIG.  1 . Where the wheelchair  12  is of the folding type, right and left side bracket members  137  can be made to be foldable relative to the wheelchair frame and/or tongue  131 . 
     FIGS.  1  and  8 A- 8 D show how the attachment subassembly  100  attaches unicycle  10  to a wheelchair  12 . In FIGS. 8A-8D, positioning stand  116  has been omitted for the sake of clarity. In attaching unicycle  10  to wheelchair  12 , unicycle  10  is placed in the tripodal position shown in FIG.  2 . Hitch means  130  of wheelchair  12  is then brought into close proximity to the wheelchair attachment subassembly  100  of unicycle  10 , in the direction shown by the arrow in FIG.  8 A. The bottom of hitch block  114  engages ramp  135  at an angle, as shown in FIG.  8 B. Handle  124  is pulled backwardly towards the wheelchair occupant, in the direction shown by the arrow in FIG. 8B, and hitching stirrup  126  raised into position behind hitch plate  133 , in the direction shown by the arrow in FIG.  8 C. Handle  124  is then pushed forwardly, in the direction shown by the small arrow in FIG. 8D, and hitching stirrup  126  hand guided into place on top of locking ledge  136  behind hitch plate  133 . As handle  124  is pushed forwardly, the upper edge of hitch block  114  is pulled into engagement with the slot in slotted lip  134  with the bottom edge of hitch block  114  resting against ramp  135  with hitching stirrup  126  resting on ledge  136  and in tight engagement with the back of hitch plate  133 . During the hitching operation, i.e., during forward movement of handle  124 , the longitudinal axes of swivel arm  110  and tongue  131  move upwardly in the direction shown by the large arrow in FIG.  8 D and are brought into alignment, thereby lifting the small front castor wheels  14  of wheelchair  12  upwardly and out of engagement with the ground  200  and into the position shown in FIG.  1 . Simultaneously, positioning stand  116  is pulled upwardly and out of engagement with the ground  200 , as also shown in FIG.  1 . 
     To disengage the unicycle  10  from wheelchair  12  the procedure just described is reversed. 
     Foldable right and left foot rests  140  and  140 ′ are rotatably attached to right and left fork blades  117  and  117 ′, respectively. Both foot rests  140  and  140 ′ may be folded up against their respective fork blades in the direction shown by the arrow in FIG.  2 . Footrests  140  and  140 ′ may be those manufactured by Quickie and sold as their “foam covered tubular style” footrests for wheelchairs. Suitable straps (not shown) may be used to hold the user&#39;s feet in place on the foot rests  140  and  140 ′. 
     When it is desired to store or transport unicycle  10 , the unicycle can be folded into the position shown in FIG.  9 . The procedure in folding unicycle  10  is to first disengage cam clamp  72  by turning cam clamp lever  75  to its released position, and lowering upper drive train portion  20  to the position shown in FIG.  9 . In this folded position, head tube  21  rests against the notch in head tube stop rest  49 . Next, cam clamp  112  is disengaged by turning its cam clamp lever to its release position, and lowering the wheelchair attachment subassembly  110  to the position shown in FIG.  9 . 
     To reassemble unicycle  10  to the operating position shown in FIGS. 1 and 2, the procedure just described is reversed. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.
A foldable unicycle attachable to a wheelchair to form a hand driven tricycle. The unicycle has upper and lower drive train frames. The means for attaching the unicycle to a wheelchair includes a positioning stand that holds the unicycle and its attachment means in a position adapted to receive and engage hitch means attached to the wheelchair frame. The positioning stand of the unicycle and the front castor wheels of the wheelchair are lifted off the ground during engagement of the unicycle attachment means to the hitch means of the wheelchair. The upper drive train frame includes means to allow it to be folded downwardly into a position adjacent the outer face of the lower drive train frame, and the unicycle attachment means includes means to allow it to be folded downwardly into a position adjacent the inner face of the lower drive train frame.