US20110018222A1

US20110018222A1 - Caster Strut, Wheelchair Frame and Wheelchair

Info

Publication number: US20110018222A1
Application number: US12/809,807
Authority: US
Inventors: Michael Knopf
Original assignee: Sunrise Medical GmbH and Co KG
Current assignee: Sunrise Medical GmbH and Co KG
Priority date: 2007-12-21
Filing date: 2008-12-22
Publication date: 2011-01-27
Also published as: EP2219582A1; WO2009080347A4; WO2009080346A4; WO2009080348A4; EP2222265A2; WO2009080348A1; WO2009080347A2; DE202008017981U1; EP2222265B1; ATE553737T1; WO2009080346A1; US20110025014A1; WO2009080347A3; EP2219582B1; US8490994B2

Abstract

A wheelchair has a frame including left and right frame members, each frame member having a first leg configured to support a seat, and a second leg configured to support front caster wheels, the first leg extending substantially horizontally and the second leg extending substantially downward from the first leg. The wheelchair also has a caster strut attached to the second leg of each frame member. The caster strut has first and second strut sections. The first strut section is fixed to the second leg and arranged substantially horizontally. The second strut section is arranged substantially vertically and provides, at a lower end, a mounting for the caster wheel. The caster strut further contains a curved section intermediate the first and second strut sections.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Patent Application Ser. No. 61/008,806, filed Dec. 21, 2007, and entitled LIGHTWEIGHT WHEELCHAIR, and PCT application, entitled CASTER STRUT, WHEELCHAIR FRAME AND WHEELCHAIR. This application is the National Phase of International Application PCT/EP2008/011015 filed Dec. 22, 2008 which designated the U.S. and that International Application was published in English under PCT Article 21(2) on Jul. 2, 2009 as International Publication Number WO 2009/080346. PCT/EP2008/011015 claims priority to U.S. Provisional Application No. 61/008,806, filed Dec. 21, 2007. Thus, the subject nonprovisional application claims priority to U.S. Provisional Application No. 61/008,806, filed Dec. 21, 2007. The disclosures of both applications are incorporated herein by reference.

TECHNICAL FIELD

This invention relates to lightweight wheelchairs, caster struts for wheelchairs, and frames for wheelchairs.

BACKGROUND OF THE INVENTION

Manual wheelchairs are comprised of a wheelchair frame that is supported on the ground by two front caster wheels and two rear drive wheels. The drive wheels are large relative to the front caster wheels. The frame supports a seat for the wheelchair user. In lightweight wheelchairs, the drive wheels are configured to allow the user to propel and steer the wheelchair by rotating the large rear drive wheels by hand. Lightweight wheelchairs are usually configured to be folded and disassembled for ease of transport.
Wheelchair frames for manual wheelchairs are commonly made of bent tubing of aluminum or other metals or alloys. At the front of the wheelchair frame, the tubing is often configured to accommodate the wheelchair user's feet and a footrest The tubing also provides a suitable structure to mount the front caster wheels. It would be advantageous if the arrangement of the front portion of frames for wheelchairs could be improved.

SUMMARY OF THE INVENTION

According to this invention, there is provided a wheelchair having a frame including left and right frame members, each frame member having a first leg configured to support a seat, and a second leg configured to support front caster wheels, the first leg extending substantially horizontally and the second leg extending substantially downward from the first leg. The wheelchair also has a caster strut attached to the second leg, the caster strut having first and second strut sections, the first strut section being fixed to the second leg and arranged substantially horizontally, and the second strut section being arranged substantially vertically and providing at its lower end a mounting for the caster wheel. The caster strut further contains a curved section intermediate the first and second strut sections.
According to this invention, there is also provided a wheelchair having a frame including left and right frame members, each frame member having a first leg configured to support a seat, and a second leg configured to support front caster wheels. The first leg extends substantially horizontally and the second leg extends substantially downward from the first leg. The wheelchair also has a caster strut attached to the second leg, the caster strut having first and second strut sections and a curved section. The curved section is intermediate the first and second strut sections. The first strut section is attached to the second leg. The cross-section of the first strut section is larger than the cross-section of the second strut section.
According to this invention, there is also provided a wheelchair having a frame including left and right frame members. Each frame member has a first leg configured to support a seat and a second leg configured to support front caster wheels. The first leg extends substantially horizontally and the second leg extends substantially downward from the first leg. The wheelchair also has a caster strut attached to the second leg. The caster strut has first and second strut sections and a curved section intermediate the first and second strut sections, the first strut section being attached to the second leg. The first strut section has a varying cross-sectional area with the largest cross-sectional area at the point where the first section is attached to the second leg. The cross-sectional area of the first section tapers to a smaller cross-sectional area as the distance from the second leg increases.
Various advantages 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 a perspective view of a wheelchair, with the seat, backrest, and one of the drive wheels partially removed for clarity.

FIG. 2 is a side view of the wheelchair frame.

FIG. 3 is a perspective view of the wheelchair frame and backrest frame.

FIG. 4 is a view of an end of a caster strut for attachment to members of the wheelchair frame.

FIG. 5 is a perspective view of the caster strut of FIG. 4.

FIG. 6 is a perspective view from another angle of the caster strut of FIGS. 4 and 5.

FIG. 7 is a side view of a tubular structure for forming the caster strut at an intermediate stage of production.

DETAILED DESCRIPTION OF THE INVENTION

The lightweight wheelchair described in this specification and illustrated in the accompanying drawings includes a caster strut that is configured to make the wheelchair easier to handle. This is particularly true when the wheelchair is being transported. The caster strut is sufficiently strong to be able to support the weight of the wheelchair and the user. In one embodiment, the caster struts allow the caster wheels to be placed closer to the rear wheels, thereby improving the maneuverability of the wheelchair.
Referring to FIGS. 1-3, a wheelchair 1, shown here by way of example, is supported by left and right rear main wheels 2, 3 and left and right caster wheel assemblies 4, 5, comprising caster wheels 6, 7. The wheelchair 1 includes a base frame, shown generally at 8 a, and a backrest frame, shown generally at 8 b.
As shown in FIG. 3, the base frame 8 a includes left and right side frame members 9, 10 arranged on opposite sides of a longitudinal wheelchair central axis 48, and aligned with a direction “d” of forward displacement, or forward direction of the wheelchair 1. The left and right side frame members 9, 10 are generally L-shaped. In other words, the wheelchair 1 has an open frame. The sideframe members 9, 10 each include a longitudinally extending first leg 11, 12 and a downwardly extending second leg 14. Each first leg 11, 12 transitions into a second leg via a respective curved section 15, 16. The first legs 11, 12 extend generally horizontally, and the second legs 13, 14 depend downwardly and forwardly. It is noted that the angle of the first legs 11, 12 to the horizontal can be adjusted by adjusting the height of the first legs 11, 12 relative to the ground. They are thus only predominantly oriented in a horizontal direction, and but not exclusively oriented that way. Additionally, the directional labels such as downwardly, forwardly, horizontally and the like are for description purposes relative to the drawings and not limitations to the scope of the invention.
The first legs 11, 12 are arranged for supporting a seat (not shown) of the wheelchair 1. In particular, a seat sling (not shown) can be slung between the first legs 11, 12 of the sideframe members 9, 10, on top of which a seat cushion (not shown) of any suitable shape or configuration may be placed.
The curved sections 15, 16 of the sideframe members 9, 10 are curved in multiple planes, such that the second legs 13, 14 are separated from each other by a shorter distance than the first legs 11, 12 of the sideframe members 9, 10. Thus, the seat can be relatively wide, whereas the second legs 13, 14 of the sideframe members 9, 10 contribute to providing support for the wheelchair occupant's legs. The second legs 13, 14 have longitudinal axes 50, 52, as shown in FIG. 3.
The second legs 13, 14 are supported by the caster wheel assemblies 4, 5 via caster struts 17, 18 in which the caster wheel assemblies 4, 5 are partially accommodated. As shown in FIG. 3, the caster struts 17, 18 are attached at respective longitudinal ends 17 a, 18 a (18 a is not shown in FIG. 1, for clarity) to the sides of the respective second legs 13, 14 at a position spaced above the lower ends of the second legs 13, 14. The caster struts 17, 18 place the caster wheels 6, 7 at a wider distance from the longitudinal wheelchair central axis 48 of the wheelchair 1 when compared to the second legs 13, 14, to provide stability. The caster struts 17, 18 are also attached to second legs 13, 14 of the sideframe members 9, 10 at a slightly rearward angle with respect to the second legs 13, 14 so as to be oriented at least partly in a direction opposite to the direction d of displacement of the wheelchair 1, i.e. closer to the rear wheels 2, 3. This improves the maneuverability of the wheelchair 1 by shortening the wheelbase. In one embodiment, the amount of the rearward directional component relative to a line perpendicular to the central longitudinal axis 48 is within the range of from about 5 degrees to about 40 degrees. In another embodiment, the rearward directional component is within the range of from about 15 degrees to about 25 degrees. Other angles of orientation may be used.
Because the wheelchair 1 has an open frame, it is supported by the caster wheels 6, 7 only via the caster struts 17, 18 and second legs 13, 14. There is no other connection between the caster wheel assemblies 4, 5 and the first legs 11, 12 of the sideframe members 9, 10. Similarly, the caster struts 17, 18 form the only connections between the caster wheel assemblies 4, 5 and the sideframe members 9, 10.
In one embodiment, the rear wheels 2, 3 are positioned as close together as possible, and in particular, as close to the sideframe members 9, 10 as possible. The seat should be wide enough to accommodate an occupant comfortably, but, overall, the wheelchair 1 should be narrow for better maneuverability and access to buildings, transport means and the like. This is only possible if the set camber of the rear wheels 2, 3 is maintained when the wheelchair 1 is occupied. There must be no “sagging” of the frame, and in particular, no rotation or torsion of the sideframe members 9, 10 under the weight of the occupant. Otherwise, the wheels 2, 3 would angle inwards at the top towards the first legs 11, 12 and run against them or against side guards (not shown) mounted to the first legs 11, 12 of the sideframe members 9, 10. To prevent such sagging, and to provide rigidity for the wheelchair base frame 8 a, the first legs 11, 12 are directly connected by a cross-brace 19 and indirectly by an axle tube 20. Additionally, a foot rest assembly 24 a with left and right foot rest frame members 21, 22, in general alignment with and connected to the second legs 13, 14, includes at least one cross-member 23 interconnecting the left and right foot rest frame members 21, 22. In other embodiments, the cross-member 23 is eliminated and instead a different cross-member (not shown) may be situated below the foot plate 24.
For adjustment of the height of the frame 8 a relative to the foot plate 24 it is contemplated that the left and right foot rest frame members 21, 22 of the foot rest assembly 24 a be accommodated within the second legs 13, 14 in telescoping manner. Other methods of connecting the foot rest frame members 21, 22 to the second legs 13, 14 may be used. The telescopic movement of the left and right foot rest frame members 21, 22 can be fixed in a desired position by any suitable means. In one embodiment, the foot rest frame members 21, 22 can be fixed within the second legs 13, 14 in one of a number of positions, and secured in place, for example, by biased pins in the foot rest frame members 21, 22, arranged to cooperate with any of a series of holes in the second legs 13, 14 of the sideframe members 9, 10.
The first legs 11, 12 of the sideframe members 9, 10 are each linked to the axle tube by a respective axle plate 25, 26 to which a respective axle tube clamp 27, 28 is connected in one of a number of pre-determined positions on the axle plate. The interconnection between the axle tube clamp 27, 28 and the axle plate 25, 26 allows for movement of the axle tube 20 between the different pre-determined positions at varying distances relative to the first legs 11, 12 of the sideframe members 9, 10. The positions are at varying distances to the seat. In this manner, the rear seat height can be adjusted because the axle tube 20 accommodates camber tubes (not shown in detail) for holding axles of the rear wheels 2, 3.
At least one of the camber tube and the axle is removable from the axle tube 20 so that the rear wheels 2, 3 can be taken off the wheelchair frame 8 a when the wheelchair 1 needs to be transported, as is shown in FIG. 2.
The back rest frame 8 b is pivotably connected to the base frame 8 a by a connection mechanism comprising, in this example, left and right angle plates 31, 32, that enables left and right backrest frame members 28, 29 to be fixed at any of several angles relative to the first legs 11, 12 of the sideframe members 9, 10. In particular, as shown in FIG. 2, the backrest frame members 28, 29 can be folded and fixed in a position in which their longitudinal central axes are predominantly aligned with longitudinal central axes of the first legs 11, 12 of the sideframe members 9, 10. In this configuration, the wheelchair 1 can be transported easily, e.g. in the trunk of a car. Optionally, the wheelchair 1 can be carried with one hand by the cross brace 19 between the sideframe members 9, 10 or a similar cross-brace provided between the backrest frame members 28, 29. In the configuration of FIG. 2, the caster struts 17, 18 are quite exposed. Due to their shape, however, there is reduced risk of their causing damage or injury.
FIGS. 4-6 show the right caster strut 18 by way of example, the other caster strut 17 being a mirror image. FIG. 7 shows the caster strut 18 at an intermediate stage of its production.
The caster strut 18 is a tubular structure with, as viewed along its longitudinal central axis, a first section 34, a second section 36, and a curved section 35, by way of which the first section 34 transitions into the second section 36. The caster strut 18 can be made of composite materials, metal or a metal alloy. Suitable materials include aluminium-scandium alloys, aluminium alloys from the 7000 series, particularly aluminium 7003, or aluminium from the 6000 series. Aluminium 7000 has a relatively high tensile strength, which allows one to bend a tubular member into a shape with a curved section 35 having a small radius of curvature. If made of a composite, the caster strut 18 can be chemically bonded to the second leg of the side frame member. If made of metal or a metal alloy, it may alternatively be welded. In a quite different embodiment to the one illustrated herein, a stub (not shown) may be provided on the second leg, and the caster strut 18 may be attached onto the stub, e.g. mechanically. In yet another different embodiment, cooperating and interlocking attachment means are provided. The illustrated embodiment is relatively lightweight and strong, particularly when welded to the second leg 13, 14 of the side frame member 9, 10. The length of the various sections 34, 35, and 36 may vary. In one embodiment, the length of the first section 34 is within the range of about 1.5 times to about 3 times the length of the second section 36. In another embodiment, the length of the first section 34 is within the range of about 1.8 times to about 2.2 times the length of the second section 36.
Although the first section 34 is substantially horizontal, it may be positioned at a slight angle to the horizontal, sloping downward as it extends from the second leg 14. In one embodiment, the first section 34 has a slope within the range of from about 10 degrees to about 20 degrees to the horizontal.
With reference to FIGS. 4-6, the end 18 a of the first section 34, at which the caster strut 18 terminates, is provided with upper and lower cut- outs 37, 38, that are adapted to the profile of the second leg, which is partially received in the cut- outs 37, 38. When attached, the second leg 14 is thus partially enveloped by the end 18 a of the caster strut 18. As a result, there is a relatively large contact area between the caster strut 18 and the second leg 14. This relatively large contact area facilitates bonding, particularly by welding, of the caster strut 18 to the second leg 14 along the edges of the cut- outs 37, 38. The area is also relatively large because the first section 34 of the caster strut 18 has a substantially oval cross-sectional shape, as shown in FIG. 4. In alternative embodiments, the cross-section can be teardrop shaped or T-shaped. The oval shape can be produced from cylindrical tubing having a circular cross section.
Referring to FIG. 4 in particular, the end 18 a of the caster strut 18, where the first section 34 terminates, is shaped (by way of the cut- outs 37, 38 in this case) to attach to the second leg 14 with a certain orientation relative to the longitudinal axis 52 of the second leg 14. The oval cross-section of the first section 34 has a largest diameter or dimension “A” in a direction along a major axis 39, which is predominantly parallel to the longitudinal axis 52 of the second leg 14 when attached. Thus, no separate buttresses, fins, or similar strengthening support structures are required between an underside 40 of the first section 34 and the second leg 14 of the side frame member 10.
The illustrated caster strut 18 may be manufactured by starting with a round tube, reducing the tube diameter, at least at an end 41 that will become the lower end of the second section 36, bending the tube to provide a bent section corresponding to the curved section 35, and adjusting the cross-sectional shape (including the diameter) of the first section 34 to give it a generally oval-shaped end. The adjustment can be carried out using the technique of hydroforming to widen the first section 34 and give it an oval shape. Alternatively, if less widening is required, a circular precursor of the first section 34 can simply be pressed from one side. The result of these steps is shown in FIG. 7. Subsequently, the cut- outs 37, 38 are provided, as well as apertures 42, 43 and a slot 44, the purpose of which will be explained below. Other manufacturing techniques may be used to manufacture the caster strut 18. In another embodiment, the thickness of the tube varies along a portion of the first section 34.
Referring in particular to FIG. 7, it can be seen that, in a longitudinal cross-sectional plane parallel to the plane in which the curve of the curved section 35 lies, the first section 34 tapers towards the curved section 35. The major diameter A, shown in FIG. 4, of the oval cross-section at a first position P1, shown in FIG. 7, near the end 18 a of the first section 34 is larger than the major diameter of the oval cross-section at a second point P2 nearer the curved section 35. In the illustrated embodiment, a minor diameter “B”, shown in FIG. 4, remains generally the same along the length of at least the first section 34 of the caster strut 18. In other embodiments, the minor diameter B may vary. Therefore, in one embodiment of the caster strut 18, the first strut section 34 has a varying cross-sectional area with the largest cross-sectional area at the point P1 where the first section 34 is attached to the second leg 14, and with the cross-sectional of the first section 34 tapering to a smaller cross-sectional area as the distance from the second leg 14 increases.
A radius of curvature, “R”, of the center line of the curved section 35 is smaller than or equal to four times the diameter of the caster strut 18 at a transition from the first section 34 to the curved section 35. More particularly, radius R is equal to or smaller than three times the diameter. The taper between P1 and P2 is provided to make this radius R smaller in absolute terms and still provide sufficient stiffness. The radius of curvature R cannot generally be made very small in relative terms, i.e. relative to the diameter of the caster strut 18 in the plane of the curve, because such a curve cannot be provided by bending without risk of tearing the tubular structure. To reduce this risk, the bend can be made with the tubular structure at a temperature corresponding to a generally ductile, or almost ductile, state of the material of the tube forming caster strut 18.
In contrast to the first section 34 of the caster strut 18, the second section 36 has a generally circular cross-section, at least at the lower end of the caster strut 18 where it provides an aperture for inserting part of the caster wheel assembly 5.
It will be appreciated from the description provided above that the first, second and curved sections, 34, 35, and 36 respectively, are continuations in the longitudinal direction of each other, so that they are sections of a single tubular member. Thus, there are no joints between the sections that need to be strengthened.
The second section 36 of the caster strut 18 has a longitudinal axis 45. In one embodiment, the caster wheel assembly 5 will be at least partly housed in lower end 41 of the second section 36 of the caster strut 18. Caster wheel assemblies 4, 5 are known in the art. Such a caster wheel assembly 4, 5 includes a swivel post (not shown) suspended on a pivot axle that is inserted through the apertures 42, 43 about which, when in use, the swivel post can rotate to at least a limited extent. Thus, when the rear seat height above ground is adjusted by displacing the axle tube clamp 27 along the axle plate 25, 26, the angle of the swivel post relative to a longitudinal central axis 45 can be adjusted, so that the swivel post remains generally aligned with the vertical.
The slot 44 is provided to serve as a guide for guiding movement of a member attached to the swivel post of the caster wheel assembly 5, and is oriented to guide the member along a trajectory around the longitudinal central axis 45 of the second section 36. Ends 46, 47 of the slot 44 limit this trajectory to a length corresponding to a rotation of less than 180 degrees, in particular less than 90 degrees.
The caster strut 18 thus facilitates adjustment of the wheelchair 1 to the needs of the individual user, as well as contributing to the rigidity and low weight of the wheelchair frame 8 a. This is particularly due to the small radius of curvature R of the curved section 35 which makes the wheelchair 1 compact when folded and partly disassembled for transport. In that state, the risk of damage or injury is kept low by the smooth surface of the caster struts 17, 18.
The cutouts 37, 38 provide the end 18 a of the caster strut 18 with a shape for direct contact with the second leg 14. As can be seen in the embodiment shown in FIG. 6, cutouts 37, 38 do not completely cut back all of the original end of the tube. Instead, there are remainder portions 54, 56, that are not cut out. The cutouts 37, 38, extend around the perimeter of the oval within the range of from about 60 percent to about 90 percent of the entire perimeter of the oval. In other embodiments the cutouts 37, 38, extend around the perimeter of the oval within the range of from about 70 percent to about 80 percent of the entire perimeter of the oval. The remainder portions 54, 56 provide a stronger connection to the second leg 14 when the caster strut 18 is welded to the second leg 14. In other embodiments, the reminder portions are not present. The non-circular cross-section provides additional strength in at least one longitudinal cross-sectional plane of the caster strut 18, e.g. the plane in which bending forces occur. All this is made possible without resorting to an increase in the diameter of the second legs 13, 14, of the left and right frame members 11, 12, thus contributing to a low overall wheelchair frame weight.
If the end 18 a of the caster strut 18 at which the first longitudinal section 34 terminates is configured, and in particular shaped, to attach to the second leg 14 with a certain orientation of a longitudinal axis 52 of the second leg 14, and the non-circular cross-section has a larger diameter in a direction predominantly parallel to the longitudinal axis 52 of the second leg 14, when attached, then the caster strut 18 is particularly strengthened in the longitudinal cross-sectional plane of the caster strut 18 in which bending forces occur. This plane is predominantly oriented in the vertical direction when the wheelchair 1 is in use. The diameter of the caster strut 18 can be made smaller in transverse longitudinal cross-sectional planes, which keeps the weight down and allows attachment to the left or right frame member 11, 12, having a second leg 13, 14, of limited diameter.
An advantage of the one-piece caster strut 18 is that it can avoid the sharp edges sometimes present in two-part caster struts. The outside surface of the caster strut 18 can be essentially smooth, so that there is minimized risk of its catching on anything as the wheelchair frame is handled. The absence of angled joint sections (e.g. T-pieces) between the first and second longitudinal sections 34, 36, makes the caster strut 18 relatively lightweight. By providing a caster strut 18 that is attached with an end 41 to a second leg 14 of one of left and right frame members 9, 10, arranged on opposite sides of a central axis 48, with each frame member including a first leg 12 for supporting a seat that transitions into a second leg 14 depending downwardly, the wheelchair frames are more compact when the rear wheels 2, 3 are removed, and are lighter than those with closed side frames. Because the first longitudinal section 34 of the caster strut 18 terminates at an end 41 of the caster strut where the first longitudinal section 34 is attached to the second leg at an angle thereto, the caster struts 17, 18 allow the caster wheels 6, 7 to be placed further apart, improving stability. The second legs 12, 13 of the left and right frame members 9, 10 can be placed closer together to support an occupant's legs. Moreover, the caster struts 17, 18 allow the caster wheels 6, 7 to be placed closer to the rear wheels 2, 3, improving the maneuverability of the wheelchair 1.
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.

Claims

1-15. (canceled)

16. A wheelchair having a frame including left and right frame members, each frame member having a first leg configured to support a seat, and a second leg configured to support front caster wheels, the first leg extending substantially horizontally and the second leg extending substantially downward from the first leg, and a caster strut attached to the second leg, the caster strut having first and second strut sections, the first strut section being fixed to the second leg and arranged substantially horizontally, and the second strut section being arranged substantially vertically and providing at a lower end a mounting for the caster wheel, the strut further containing a curved section intermediate the first and second strut sections.

17. The wheelchair of claim 16 in which the caster strut has a tubular structure, with one portion of the caster strut having a cross-section different from another portion of the caster strut.

18. The wheelchair of claim 17 in which the one portion of the caster strut is the first strut section, and the cross-section is non-circular.

19. The wheelchair of claim 18 in which the cross-section is oval.

20. The wheelchair of claim 3 in the second leg has a longitudinal cross-sectional, and in which the largest diameter of the cross-section is predominantly parallel to the longitudinal axis of the second leg.

21. The wheelchair of claim 18 in which the second longitudinal section has a substantially circular cross-section.

22. The wheelchair of claim 17 in which the first longitudinal section tapers toward the curved section in at least one longitudinal cross-sectional plane.

23. The wheelchair of claim 17 in which the caster strut has a caster strut longitudinal end, with the caster strut being attached to the second leg at the caster strut longitudinal end of the caster strut.

24. The wheelchair of claim 17 in which the second longitudinal section includes at least one guide for guiding movement of a member attached to a swivel post of the caster wheel assembly, the guide oriented to guide the member along a trajectory around a longitudinal central axis of the second longitudinal section.

25. The wheelchair of claim 17 in which each caster strut is attached to a second leg at an angle so as to be oriented at least partly in a direction opposite to the forward direction.

26. The wheelchair of claim 17 in which the thickness of the tube first section varies along a portion of the first section.

27. The wheelchair of claim 17 in which the cross-sectional shape of the first section varies along a portion of the first section.

28. A wheelchair having a frame including left and right frame members, each frame member having a first leg configured to support a seat, and a second leg configured to support front caster wheels, the first leg extending substantially horizontally and the second leg extending substantially downward from the first leg, and a caster strut attached to the second leg, the caster strut having first and second strut sections and a curved section intermediate the first and second strut sections, the first strut section being attached to the second leg, and the cross-section of the first strut section being larger than the cross-section of the second strut section.

29. The wheelchair of claim 28 in which the cross-section of the first and intermediate strut sections is circular.

30. The wheelchair of claim 28 in which the cross-sectional shape of the first section varies along a portion of the first section.

31. The wheelchair of claim 28 in which the cross-section of the firsts section at the point of attachment to the second leg is oval.

32. The wheelchair of claim 28 in which the curved section has a centreline with a radius that is smaller than or equal to four times the diameter of the caster strut at a transition from the first section the curved section.

33. The wheelchair of claim 28 in which the radius is equal to or smaller than three times the diameter.

34. The wheelchair of claim 28 in which the length of the first strut section is within the range of about 1.5 to about 3 times the length of the second strut section.

35. A wheelchair having a frame including left and right frame members, each frame member having a first leg configured to support a seat, and a second leg configured to support front caster wheels, the first leg extending substantially horizontally and the second leg extending substantially downward from the first leg, and a caster strut attached to the second leg, the caster strut having first and second strut sections and a curved section intermediate the first and second strut sections, the first strut section being attached to the second leg, and the first strut section having a varying cross-sectional area with the largest cross-sectional area at the point where the first section is attached to the second leg, and with the cross-sectional area of the first section tapering to a smaller cross-sectional area as the distance from the second leg increases.