A reclinable chair

Abstract: A chair (10) includes a supporting frame (20,22) and a seat portion (14) which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion, forward of the transverse fold. The seat portion (14) is supported above the supporting frame by its rearward portion. The chair (10) also includes a reclinable back portion (16) and a recline mechanism with which the back portion (16) is connected for reclining action of the back portion(16), The recline mechanism is operably linked to the rearward portion of the seat portion (14) such that on reclining action of the back portion (16), the rearward portion is moved to increase in rearward tilt angle and to obtain a net increase in height above the supporting frame (20,22), with a consequent folding of the seat portion about the transverse fold line under the weight (W) of the occupant.

Description: The present invention relates to a reclinable chair. In particular, although not exclusively, the invention relates to a synchro-tilt type chair in which the seat portion tilts rearwardly in synchronism with reclining action of the back portion. The invention is described primarily in the context of commercial office chairs. However, the invention is not limited in its application to commercial office chairs and may have application to any other type of seating such as public seating for theatres, aircraft or domestic seating. Background to the Invention Reclining office chairs are well known. There are certain disadvantages associated with the conventional form of reclining office chair. One of the disadvantages is that as the occupant of the chair reclines rearwardly, his head drops in height. Therefore, the eye level of the chair's occupant will not be maintained constant. This may pose a difficultly if the occupant is working at a computer terminal where it is desirable to maintain a constant eye level relative to the screen. Additionally, in meetings it is also desirable to maintain a constant eye level relative to the other attendees of the meeting. Any person who undergoes a dip in eye level may effectively drop out of the conversation. Another difficulty with conventional reclining chairs is that relative movement between the back portion and the seat portion may lead to frictional grabbing of occupant's shirt, thereby pulling out the occupant's shirt from his trousers. US Patent No. 4,596,421 discloses an office chair in which a seat portion and a back rest portion are pivotally connected to a supporting plate. The chair includes a coupling lever that couples the seat and back rest portions such that movement of the seat portion is followed by movement of the back rest portion. US Patent No. 5871,258 is in respect of a reclining office chair. The seat portion of the chair has a front portion connected to a rear portion by a resilient section in order that the rear portion carries most of the occupant's weight. The seat portion is operably connected to the reclining mechanism such that as the back portion reclines, the rear portion of the seat also tilts but additionally moves in a downward and forward motion. It will be appreciated that this further only serves to exacerbate the problem of tipping eye level. In this case, not only is the occupant's head dropping on account of their reclining action but also, the rear portion of the seat supporting the occupant's weight is also moving downwardly, with the practically certain result that the eye level of the occupant will dip during reclining action. US Patent No. 5,314,237 raises the vertical height of the seat support during recline and thereby claims to achieve consistent vertical eye level. However, the chair disclosed in this US patent suffers from another shortcoming. As the seat portion lifts, the forward edge of the seat portion will accordingly be raised and thereby act as a hard edge bearing against the back of the occupant's knees. This can lead to circulatory problems for the occupant and/or lifting of the users feet from the floor with consequent poor posture. Flexing of seat backs in the lumbar region of the user is also a desirable feature of modem office chairs. Chair occupants come in a wide range of different sizes and weights and it is therefore necessary for chair manufacturers to produce a chair which caters for a wide range of occupant sizes and weights. A larger, weightier person will be able to flex a chair back easily. On the otherhand, a person of light build may only be able to flex the back portion with a high degree of force. Accordingly, a person of light build may not receive much satisfaction from the feature of a flexible back portion. Another common feature of reclinable chairs is the use of recline springs to resist rearward recline. Adjustment mechanisms are often provided to adjust the spring tension of the recline springs to suit the build of the occupant of the chair. Where such adjustment mechanism operate directly against the action of the spring e.g. by way of a rotatable knob, generally a large number of turns of the knob are required in order to gradually stiffen the spring. Otherwise, the knob would be too stiff to turn in order to bring about the required adjustment. It is therefore an object of the present invention to provide a chair which overcomes or at least addresses some of the foregoing disadvantages. Summary of the Invention In accordance with a first aspect of the present invention there is provided a chair according to claim 1. In order to achieve a foldable seat portion, the seat portion may be flexible. The seat portion may be constructed of a flexible material such as plastic. In a preferred form of the invention, the seat portion may comprise a panel which has apertures, e.g. slots to enhance its flexibility. The slotted pattern may extend across the entirety of the panel with a specific arrangement of slots provided to increase comfort for the seat occupant. For example, the slotted panel may have the slots arranged to accommodate the ischial protuberosities of the occupant. Alternatively, the slotted pattern may simply exist in a specific zone to provide flexing about the transverse fold. The transverse fold may be shaped as a straight line, depending upon the arrangement of the slots or apertures in the seat panel or according to the manner in which the seat portion is supported. The transverse fold may alternatively take the shape of a curve lying in the plane of the seat portion. Where the seat portion takes the form of a panel, stiffening webs may be provided which offer little resistance to flexing towards the forward edge of the seat portion and greater resistance to flexing towards the rear of the seat portion. The resistance offered may progressively increase from the front edge of the seat portion towards the rear. Accordingly, the stiffening webs may be tapered to offer the varying resistance. In an alternative less preferred form of the invention, the seat portion may comprise the forward portion and the rearward portion being articulated. In a preferred form of the invention, the rearward portion of the seat portion is supported, at least in part, by the recline mechanism while the forward portion is unsupported. The depth position of the seat portion may be adjustable relative to the back portion and/or the supporting base. Accordingly, the positioning of the transverse fold may be variable as a function of the seat depth position. For example, the seat portion may be moveable forward/backward relative to guides forming part of the recline mechanism with the forward edge of the guides or a transition in curvature defining the transverse fold. The ease of folding may be dependent upon the depth position of the seat portion. As described above, this may be achieved by the seat portion having an increased resistance to folding in the directly rearwardly from the forward edge of the seat portion. The recline mechanism preferably interconnects the seat portion, the supporting frame and the back portion. In a most preferred form, the recline mechanism is in the form of a four bar linkage. The four bar linkage may be replicated on each side of the chair. Therefore, the following description of the four elements of the four bar linkage may apply to single elements or alternatively to duplicated elements on opposite sides of the chair. The first linkage is in the form of a main support. The main support may be selectively height adjustable by the user. However, the main support is in normally fixed disposition relative to the supporting frame. In the most preferred form of the invention, the main support is supported at the top of a height adjustable gas spring extending upwardly as part of the supporting frame. The second linkage of the four bar linkage may be the seat portion itself. Where the seat portion is depth adjustable, then the second linkage may comprise a guide for the depth adjustment. The third linkage of the four bar linkage preferably comprises a front support linkage extending between the main support and the second linkage. The fourth linkage is preferably in the form of a drive linkage which is pivotable about a drive axis through the main support, being connected to the second linkage and being operably linked to be driven about the drive axis by rearward recline action of the back portion. Preferably, the back portion is also supported from the main support. The back portion is preferably attached to a back attach portion which is pivotally connected to the main support at a recline axis. The recline axis of the back portion is preferably below the seat portion. In a most preferred form of the invention, the recline axis is below the ischial protuberosities of the occupant. Preferably, the back portion is biased against reclining action by a recline biasing device. This may be in the form of a one or more springs. In a most preferred form of the invention, the biasing force is adjustable. In a preferred embodiment of the invention there may be two back extension arms extending from the back portion. These extension arms could be an integral part of the back attach portion or alternatively could be rigidly connected thereto. With the two extension arms pivotally connecting the back portion to the main support, the one or more springs are preferably held by one or both of the back extension arms, with the spring(s) acting against the main support. Preferably there are two springs in the form of leaf springs. Preferably, the first spring has a predetermined spring rate (or spring constant). The second spring may be clamped against the first spring with the combination having a resultant spring rate with the degree of clamping being variable to adjust the resultant spring rate. Preferably, the second spring has a high spring constant in its unclamped state in order that only a small clamping adjustment is required to bring about an appreciable change in the resultant spring rate of the combination. One or more recline abutment surfaces may define the recline limit of the back portion. Preferably, the recline abutment surfaces are provided on one or both of the back extension arms and the main support. Furthermore, there may be provided one or more forward abutment surfaces which define the forward position of the back portion. Preferably, the forward abutment surfaces are disposed on one or both of the back extension arms and the main support. In a most preferred form of the invention, one or both of the back extension arms include a pin which travels within a slot of the main support. The slot has a base which engages against the pin when the pin reaches a position of travel within the slots corresponding to the forward position of the back portion. Additionally, cushioning may be provided to cushion the abutment between the forward abutment surfaces. This may comprise an O-ring encircling the pin. Desirably, the invention also includes a recline lock, to lock the back portion against reclining action. The recline lock may be selectively lockable by the user. In a preferred form of the invention, the recline lock acts against a lock abutment surface on one or both of the back extension arms. Preferably, the recline lock is in the form of a push rod/bar which, when selectively operated by the user acts against the lock abutment surfaces of both extension arms at the same time. Another preferred feature of the invention is that the back portion is flexible or at least flexible at a part corresponding to the lumbar region of the occupant. Preferably the flexibility, i.e. the stiffness, is adjustable. The flexibility may be adjustable selectively although it is preferred that the adjustment takes place automatically in response to the weight imparted by the occupant on the seat portion. Preferably, the larger the weight, the greater the stiffness imparted to the back portion. Preferably, the adjustment can be achieved through the use of a tensionable biasing device provided to act against the flexible back portion, with a varying degree of tension to impart a varying degree of stiffness to the back portion. For example, the biasing device may be in the form of a spring. Preferably, there are two flat springs lying against the back portion at a lower region thereof adjacent the connection of the back portion to the back attach portion. Preferably, the tensioning of the biasing device is achieved by means of an interconnecting linkage which in response to the occupant's weight on the seat portion tensions the biasing device by a corresponding amount. Preferably, the interconnecting linkage interconnects the biasing device with the drive linkage. In a most preferred form of the invention, where the biasing device is in the form of a leaf spring lying against the back portion, the leaf spring is connected to a spring carrier forming part of the interconnecting linkage, the spring carrier being pivotally mounted to the back attached portion in a manner whereby the weight of the occupant on the seat portion is transferred through to the spring carrier so as to bend the leaf spring against the back portion. As there may be two four bar linkages provided on opposite sides of the chair, there may accordingly be provided two interconnecting linkages with two spring carriers receiving two leaf springs. The back portion may include a back frame which, in its lower regions defines a rearwardly facing channel. Preferably, each leaf spring engages within the channel on a respective side of the back frame. Preferably, each interconnecting linkage also includes two push links, each interconnecting linkage also includes two push links, each interconnecting the associated spring carrier with the associated drive linkage. The back attach portion may be in the form of a housing ie the back attach housing. The spring carrier(s) and the push link(s) may be at least partly received within the back attach housing. Each leaf spring and associated spring carrier may be of integral construction. The supporting frame may be of any type. Preferably, the supporting frame is of the conventional type with a central support and a plurality of radiating legs with castors. The supporting frame may incorporate a height adjustable gas spring. A tension limit may be provided to prevent over-tensioning of the tensionable biasing device. For example, rotation of the spring carrier may be stopped against the back attach housing. This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. The invention consists in the foregoing and also envisages constructions of which the following gives examples. Brief Description of the Figures In order that the invention may be more fully understood, some embodiments will now be described by way of example with reference to the Figures in which: Figure 1 is a perspective, partially exploded view of a chair in accordance with a first preferred embodiment of the chair; Figure 2a is an exploded perspective view of a back portion of the chair shown in Figure 1; Figure 2b is a perspective view of a back attach casting forming part of the back portion of the chair illustrated in Figure 2a; Figure 3 is an assembled view of a lower portion of the back portion of the chair illustrated in Figure 2; Figure 4 is a perspective view of a main transom of the chair of Figure 1; Figure 5 is a perspective view of an assembly from the underside of the main transom illustrated in Figure 4; Figure 6 is a perspective view of the assembled chair looking down upon the main transom illustrated in Figure 4; Figure 7 illustrates an adjustable clamp; Figure 8 is a plan view of the cam for the adjustable clamp; Figure 9 is an enlarged perspective view of a portion of the main transom illustrated in Figure 4; Figure 10 is a perspective view of the chair of Figure 1 from the underside with the main transom removed, illustrating certain components of a recline lock; Figure 11 is a graph illustrating the change in resistance to backward recline achievable by the adjustable clamp illustrated in Figures 6-8; Figure 12 is a perspective view of a control lever for the recline lock; Figure 13 is a perspective view of a modified form of the back extension arm in accordance with the second preferred embodiment of the chair; Figure 14 is a perspective view of a modified form of the main transom from above in accordance with the second preferred embodiment of the chair; Figure 15 is a perspective view of a modified form of the transom of Figure 14 from below; Figure 16 is a perspective view illustrating the modified form of the back extension arm of Figure 13 in assembly with the modified form of the main transom of Figures 14 and 15; Figure 17 is a perspective view of a modified form of a first recline spring in accordance with the second preferred embodiment of the chair; Figure 18 is a perspective view illustrating the first recline spring of Figure 17 in assembly with the back extension arms and the main transom together with a second recline spring; Figure 19 is a diagrammatic illustration of a first adoptable position of the first recline spring; Figure 20 is a diagrammatic illustration of a second adoptable position of the first recline spring; Figure 21 is a diagrammatic illustration of a third adoptable spring position of the first recline spring; Figure 22 is a perspective view similar to Figure 18 with the first recline spring in the third adoptable spring position; Figure 23 is a diagrammatic view illustrating engagement between a part of the first recline spring and a part of the main transom; Figure 24 is a graphical illustration of the change in spring constant as the first recline spring of the second embodiment is rotated through the three adoptable spring positions illustrated in Figures 19 to 21; Figure 25 is a more detailed view of the assembly as in Figures 18 and 16, with additional parts removed for clarity; Figure 26 is a further perspective view of the modified form of the back extension arm 70' of Figure 13, shown from another angle; Figure 27 is a further exploded view of parts making up the back portion of the first embodiment; Figure 28 is a perspective view from the rear of the assembled parts illustrated in Figure 27; Figure 29 is a perspective view illustrating in exploded fashion, a spring carrier and a leaf spring as used in the first embodiment; Figure 30 is a perspective view of the chair of the first embodiment from the side rear, with certain parts removed for clarity; Figure 31 is a schematic view of the main elements of the recline mechanism of the chair of the first embodiment; Figure 32 is a side view of a seat guide, being one of the elements shown in Figure 31; Figure 33 is a side view of the chair of the first embodiment illustrated in Figure 1, illustrating the arrangement of the main links with occupant weight applied to the seat portion; Figure 34 is a side view as per Figure 33, except with the occupant weight removed from the seat portion. Figure 35 is a side view of the chair of Figure 1, illustrating the recline action of the chair; Figure 36 is an exploded view of the parts making up the back portion according to the second preferred embodiment of the chair; Figure 37 is a front perspective view of a detail of the back attach casting forming part of the back portion of the chair according to the second preferred embodiment; Figure 38 is a perspective view of the leaf spring as used in the second embodiment; Figure 39a is a rear perspective view of the assembled parts of Figure 36; Figure 39b is a perspective view of a supplementary spring forming part of the back portion of the chair; Figure 39c is a perspective view of a push link forming part of the recline mechanism of the second embodiment; Figure 39d is cross-sectional view of a detail of the back portion assembled with the push link of Figure 39c; Figure 40 is a front perspective view of the back frame together with the back extension arms and recline springs of Figure 25 assembled with the back frame; Figure 41a is a perspective view of the chair according to the second embodiment from the rear, with certain parts removed for clarity; Figure 41b is a perspective view of a detail of Figure 41a; Figure 42 is a schematic view of the main elements of the recline mechanism of the chair according to the second embodiment; Figure 43 is a perspective underside view of the seat guide, one of the main elements of the recline mechanism of the chair according to the second embodiment; Figure 44 is a side view of the main parts of the recline mechanism of the chair according to the second embodiment; Figure 45 is a side view as per Figure 44, except with the seat added; Figure 46 is a perspective view of a seat panel which may be used with either the first or second embodiment of chair; Figure 47 is a perspective view of the underside of the seat panel shown in Figure 46; Figure 48 is a plan view of the underside of the seat panel illustrated in Figure 46; Figure 49 is a perspective view of a detail of the underside of the seat panel illustrated in Figure 47; Figure 50 is a schematic longitudinal sectional view through the middle of the seat panel illustrated in Figure 46; Figure 51 is a schematic view of the side edge; Figure 52 is a schematic transverse sectional view through the seat panel at approximately 150 mm forward of the rear edge; Figure 53 is a schematic transverse sectional view at approximately 120 mm from the front edge; Figure 54 is a schematic view of the front edge of the seat panel illustrated in Figure 46; Figure 55 is a perspective view of the chair according to the first embodiment with the seat panel removed to show a seat depth adjustment mechanism; Figure 56 is a perspective view showing similar detail to Figure 55; Figure 57 is a perspective view with the seat panel removed, showing the workings of the seat depth adjustment mechanism; Figure 58 is a side view of a portion of the chair with the seat panel in an extended position; Figure 59 is a side view of a portion of a chair illustrated in Figure 58 with the seat panel in a retracted position; Figure 60 is an underside perspective view of the portion of the chair illustrated in Figures 58 and 59 illustrating the seat depth adjustment mechanism; Figure 61 is a perspective view of the chair according to a second embodiment with the seat panel removed to show a seat depth adjustment mechanism; Figure 62a is a different perspective view showing a similar detail to Figure 61; Figure 62b is a perspective view of the opposite side the seat guide to that shown in Figure 43; Figure 62c is a perspective view of the seat guide as shown in Figure 62b except with a portion removed. Figure 63 is a side view of a portion of the chair with the seat panel in a retracted position; Figure 64 is a side view of the portion of the chair of Figure 63 with the seat panel in an extended position; Figure 65 is an underside view of the portion of the chair illustrated in Figures 63 and 64 illustrating the seat depth adjustment mechanism. Figures 66 to 80 show views relating to a lumbar support mechanism, not forming part of the present invention Figure 81 is a perspective view of an upright member of the back frame, cut-through to show the cross-section; Figure 82 is a perspective view of a piece of insert strip; Figure 83 is an assembled view in cross-section of the upright member of the back frame and the insert strip; Figure 84 is a perspective view of a preferred form of a wheeled base; Figure 85 is an underside perspective view of the leg assembly forming part of the wheeled base illustrated in Figure 84; Figure 86 is a perspective view of a castor forming part of the mobile base illustrated in Figure 84; Figure 87 is a perspective view of an axle assembly forming part of the castor illustrated in Figure 86; Figure 88 is a perspective view of a topper pad, not forming part of the present invention. Figure 89 is a schematic bottom view of a slightly modified form of the seat panel; and Figure 90 is a perspective, partly exploded view of a chair in accordance with the second preferred embodiment of the chair. Description of Preferred Embodiment First Embodiment Since the Figures illustrate the chair from various different angles as convenient to explain certain parts, an arrow marked "F" has been inserted into the drawings where appropriate. Accordingly the terms forward, rearward, left side and right side should be construed accordingly. Figure 1 illustrates an office chair 10 including a main assembly having a seat portion 14 and a back portion 16. The seat portion 14 and the back portion 16 are supported above the ground by a supporting frame including a wheeled base 18 and a central support column 20. The central support column 20 houses a pneumatic spring (not shown) for height adjustment of the seat portion 14 in conventional fashion. The pneumatic spring is connected to the main transom 22 of the chair which is illustrated in Figure 4. The main transom 22 extends transversely across the chair and is connected to the pneumatic spring by way of central spring connection ring 23. Figure 1 also illustrates two detachable arm assemblies 24. The arm assemblies 24 each include an upper armrest 26 which is padded for user comfort. Each arm assembly 24 includes an upright support structure 28. The armrest 26 is mounted to the upper end of the upright support structure 28. The lower end of the upright support structure has an elongate attachment portion 30 extending inwardly therefrom at a downwardly inclined angle relative to the upright support structure 28. The elongate attachment portion 30 is releasably engaged within one end of the main transom 22. The manner of attachment is not significant to the present invention and the reader may refer to our co-pending Patent Application US Serial No..........., the details of which are incorporated herein by reference. Back Portion The back portion 16 is defined by a peripheral frame 34 which is approximately rectangular in shape, as shown in Figure 2. In the finished chair the peripheral frame 34 has a mesh fabric stretched over it in a manner described more fully in connection with Figures 81 to 83. Within the opening defined by the rectangular peripheral frame 34, a lumbar support mechanism 36 is provided Figure 2 illustrates more clearly the form of the peripheral frame 34. The peripheral frame 34 is constructed of a flexible plastics material such as injection moulded reinforced polyester. The peripheral frame 34 is of integral construction and comprises two upright members 38, a top beam 40 and a bottom beam 42. The upright members 38 are bowed with a gentle serpentine curve sweeping forwardly in the upward direction and then rearwardly beyond the lumbar region. This is a shape which is comfortable to the chair occupant. The upright members 38 include channels 44 which are open in the direction facing rearwardly as shown in Figure 28. The upright members 38 are also joined by an intermediate back beam 46. The back beam 46 supports the lumbar support mechanism 36. Rigidly connected to the lower end of the peripheral frame 34 is a back attach casting 48. The back attach casting 48 is an integrally cast component as shown in Figure 2b. The back attach casting 48 includes two pairs of sprigs 50 which engage with aligned apertures 52 provided at the bottom of the upright members 38. This enables the lower region of the peripheral frame 34 to be securely fixed to the back attach casting 48. An additional snap fitting (not shown) may be provided. The back attach casting 48 also includes 2 pairs of opposed walls 54 on opposite sides (more clearly seen in Figure 27). Each pair of spaced walls 54 defines a forwardly extending channel 64 in which a spring carrier 60 is received. Each pair of opposed walls 54 includes aligned slots 56. The spring carrier 60 (to be described more fully in connection with Figure 27) has pins 62 on opposite sides to engage with the aligned slots 56. Furthermore, the back attach casting 48 includes two forwardly extending hollow projections 66. The hollow projections 66 each define a socket 68. Two back extension arms 70 are welded within respective sockets 68 of the hollow projections 66. Referring to Figure 3 for greater clarity, each back extension arm 70 includes a forward nose portion 72 and a chin portion 74. An extension arm aperture 75 extends through the back extension arm 70 in a position rearwardly of the nose portion 72 and the chin portion 74. Reference is now made to Figure 4 which illustrates the main transom 22 which extends transversely across the chair as already explained. The main transom 22 is supported on a pneumatic spring at central spring support ring 23. The main transom is a beam-like construction of diecast aluminium with pivot features 76 formed at opposite ends. At each end, the pivot features comprise opposed supporting webs 78. The opposed supporting web 78 have rear aligned apertures 80. In the assembled chair, the extension arm aperture 75 of one of the back extension arms is aligned with the rear aligned apertures 80 on one side of the main transom to receive a main pivot pin (not shown) therethrough. Likewise the other back extension arm 70 is pivotally attached to the main transom 22 on the other side. Each back extension arm is pivotable about the associated main pivot pin and the recline axis R of the back portion 16 is thereby defined. Recline Limits As mentioned above, a nose portion 72 is defined forwardly of each back extension arm 70. The nose portion 72 has two bosses 84 extending sideways from the flanks of the nose portion 72. The bosses 84 are receivable within facing slots 86 in the opposed supporting webs 78. Each of the facing slots 86 has a base formed therein. During rotation of the back extension arm 70 about pivot R, the bosses 84 move within respective ones of the facing slots 86. In the forward most position of the back portion 16 in its pivoting action about the recline axis R, the bosses 84 will bottom out at the bases of the slots 86 thereby defining forward limits. This is referred to as the "forward active position"of the back portion 16. The chin portion 74 of each back extension arm 70 includes a first abutment surface 88 for engagement with a second abutment surface 90 (see Figure 9) provided as part of the rear wall of the main transom 22. On each side, when the first abutment surface 88 engages with the second abutment surface 90, the rearward recline limit of the back portion 16 of the chair will be thereby defined. It would not be possible for the chair portion 16 to recline back any further once the two abutment surfaces come into engagement although flexing of the peripheral frame is still possible in this position. One end of the main transom 22 illustrating the pivot features 76 in greater detail can be seen in Figure 7. Recline Biasing Device Referring to Figure 3 the inner flanks of the chin portions 74 of both back extension arms 70 include facing aligned slots 92, the left one of which can be seen in the Figure. A first recline spring 94 in the form of an elongate bar or leaf spring has each end received in a respective one of the facing slots 92. As shown in Figure 4, the main transom 22 has a reaction surface 98 against which the first spring 94 engages. The reaction surface 98 is centrally disposed and has a depth corresponding to the depth of the first spring 94. The reaction surface 98 forms part of an integrally formed projection extending rearwardly from the main transom 22. As the back portion 16 reclines rearwardly about the recline axis R, the first recline spring 94 engages against the reaction surface 98, thereby biasing the back portion 16 against reclining action. A second recline spring 96 also has one end received in one of the facing slots 92. However, the second recline spring 96 is somewhat shorter than the first recline spring 94 so the second end of the second recline spring 96 is not received within the other facing slot 92 (see Figure 10). As shown, the second spring is also in the form of a elongate spring bar or leaf spring. The second spring 96 lays behind the first spring 94, against the first spring 94, for at least half the length of the first spring 94. An adjustable clamp 100 (see Figure 7) is provided to clamp the free end of the second spring 96 against the first spring 94 and thus alter the curvature of the second spring 96 and thereby alter its spring resistance. The second spring 96 is disposed such that increased clamping against the first spring will act to increase its resistance to bending. The net force biasing the back portion against recline will thereby be the sum of the spring force provided by the first spring 94 and the spring force provided by the second spring 96. With the second spring more tightly clamped to the first spring 94, the resultant spring resistance will be higher than for a more relaxed clamping between the two springs. The first spring 94 has a factory set spring rate. The second spring 96 is selected to have a high spring rate, greater than the spring rate of the first spring 94. Thereby, a small adjustment of the clamping between the first spring 94 and the second spring 96 will bring about an appreciable change in the spring resistance of the second spring 96. The adjustable clamp 100 is illustrated in Figure 7. The adjustable clamp 100 includes a U-shaped bracket 101 which extends around the two recline springs 94, 96. A cam 102 is mounted on axle 103 extending between the two legs of the U-shaped bracket 101. The axle 103 is journaled for rotation about an axis 104. The cam 102 includes four cam surface portions 105a, 105b, 105c and 105d as shown in Figure 8. The cam surface portions are substantially flat as indicated and each is spaced a different amount from the cam axis 104. The spacing decreases in the clockwise direction around the cam 102 from 105a through to 105d. The cam 102 bears against the free end of the second spring 96. The chair occupant can adjust the position of the cam to determine which of the cam surface portions 105a-105d will bear against the free end of the second spring 96. A progressively higher clamping force and hence higher resultant spring rate of the second spring can be obtained as the occupant rotates the cam 102 through to the maximum setting at 105a. At 105e, an extension to the cam 102 is provided to prevent over rotation of the cam 102. A knob 103b is provided for user adjustment of the cam 102. The change in the net spring force over distance is illustrated graphically in Figure 11 for each of the positions of the cam 102. In position 1, the clamping is such that no force is contributed from the second spring 96. The first spring thereby offers an initial resistance of typically 10 kg. As the cam position is adjusted, the second spring contributes to the overall force so that the initial resistance to recline is increased above 10 kg, say approximately 11 kg. It will be appreciated that in changing the force offered by the second spring from 0 kg to approximately 1 kg, it is only necessary to act against a maximum of approximately 1 kg of force offered by the second spring 96. This is considerably lesser force than if the first spring 94 was adjusted to increase its initial resistance from 10 kg to 11 kg since the whole of the spring force would need to be acted against to bring about the required adjustment. In the particular embodiment described in which the first and second springs 94, 96 lay flat against each other, adjustment of the second spring 96 may bring about some change in the spring constant of the first spring. However, this is not graphically illustrated in Figure 9. Recline Lock Figure 5 illustrates a recline lock which may be operated selectively by the user to prevent the back portion from reclining. As can be seen in Figure 4, the main transom 22 includes four rearwardly extending projections 106. The recline lock comprises an elongate lock bar 107 which has four slots 108 arranged therein, with the lengthwise direction of the slots 108 arranged in the lengthwise direction of the bar 107. The slots 108 each receive one of the rearwardly extending projections 106 as shown in Figure 5. The elongate lock bar 107 is slidable from side to side between a recline lock position and a recline operative position. The projections 106 received in the slots 108 thereby define the limit of travel of the elongate lock bar 107. The elongate lock bar 107 is biased toward the recline operative position by spring 109. The elongate lock bar 107 can be seen in Figure 10 in which the main transom 22 has been removed for greater clarity. The lock bar 107 has at each end a rearwardly extending lock bit 110. The lock bits 110 thereby move from side to side with the movement of the elongate lock bar 107. Each lock bit is moveable into a recline lock position whereby the lock bit 110 is engaged against a recline locking face 112 provided on the chin portion 74 of the back extension arms. The left-hand side lock bit 110 (shown on the right in the figure) moves from a recline operative position in which is it clear of the associated back extension arm 70, to a position in which it is engaged against the recline lock face 112 on the associated arm 70. The arrangement in connection with the right hand lock bit 110 (shown in the left in the figure) is slightly different. It can be seen that the associated extension arm 70 has the recline lock face 112. Additionally, the associated arm 70 is provided with the rebate 114 adjacent to the recline lock face 112. In the recline lock position, the lock bit 110 is engaged with the recline lock face 112 whereas in the recline operative position, the left lock bit 110 is received within the rebate 114. When the lock bit is received within the rebate 114, the associated back extension arm 70 can still pivot freely about the recline axis. Figure 12 illustrates the lock bar control lever 116 which is mounted underneath the seat portion 14 in a forward position on the left hand side. The lever 116 is connected to cable actuator 118. The cable actuator 118 is connected to a control cable 120 which operates in the conventional fashion. The control cable 120 controls the position of the elongate lock bar 107 (see Figure 5). The cable actuator 118 is rotatable by operation of the control lever 116. The cable actuator 118 has a dimple provided on the forward edge which is engageable with the two position detent 122. The dimple 121 is locatable in either of two positions, the first of which corresponds to the recline lock position of the elongate lock bar 107, and the second of which corresponds to the recline operative position of the elongate lock bar 107. The user thus selects whether the recline lock is on or off according to the position of the lock bar control lever 116. Modified Form of Back Extension Arms, Main Transom, Recline Springs and Recline Lock - Second Embodiment Many of the parts described in connection with the second embodiment will be similar in many respects to corresponding parts in the first embodiment. Where the parts are essentially equivalent, like reference numerals are used. Where the parts differ in construction but perform an equivalent or analogous function, a prime (') will be used following the relevant reference numeral. Figure 13 illustrates a modified form of one of the back extension arms 70'. The back extension arm 70' has a forked forward end forming a right fork 93c and a left fork 93d with an extension arm aperture 75' extending transversely through both forks. Two such back extension arms 70' are rotatably mounted about the recline axis R to the main transom 22' as shown in its modified form in Figure 14. From Figure 15, it can be seen that the main transom 22' has pivot features 76' formed at opposite ends. At each end, the pivot features include a pair of spaced supporting webs in the form of inner and outer lobes 78' through which extends aligned apertures 80'. The alignment of the apertures 80' defines the recline axis R about which the back extension arms 70' pivot. A pin inserted through each pair of apertures 80' mounts each back extension arm 70' to the main transom 22'. The inner lobe 78' is inserted between the forks 93c, 93d of the associated back extension arm 70'. From Figure 13, it can be seen that the rearward end of the upper abutment surface 93 has a skid 93e which engages with complementary ramp 76a on the main transom 22'. The ramp 76a is curved with a centre of curvature centred on the recline axis R. This defines a potential pinching point where the occupant of the chair might jam his fingers or shirt tails etc. Therefore outer lobe 78' extends rearwardly beyond the ramp 76a to act as a guard. Figure 16 illustrates one of the back extension arms 70' rotatably mounted to the main transom 22'. Figure 13 illustrates an alternative form of recline lock mechanism. It can been seen that the forward end of the back extension arm 70' is provided with a substantially flat upper abutment surface 93 comprised of a forward surface portion 93a, forward of the recline axis R and a rearward surface portion 93b, rearward of the recline axis R. In assembly of the back extension arm 70' with the main transom 22', the abutment surface 93 lies underneath an upper portion of the main transom (see Figure 16). The rearward surface portion 93b thus defines the forward recline limit which will be reached when the back extension arm 70' pivots so that the rearward surface portion 93b abuts the underside of the main transom 22'. Conversely, the rearward recline limit will be defined when arm 70' rotates such that the forward surface portion 93a abuts the underside of the main transom 22'. The engagement between the forward surface portion 93a and the underside of the main transom 22' thus defines the rearward recline limit. A recline lock may be operated selectively by the user to prevent the back portion from reclining or to set an intermediate recline limit. As seen in Figure 13, the forward end of the back extension arm 70' is formed with a transversely extending slide 70a in which is slidably mounted a key 107a. The slide 70a has a substantially closed inner end 70c which has an V-shaped slot 70b. A spring (not shown) is received in the slide 70a between the key 107a and the closed end 70c to bias the key 107a outwardly away from the closed end 70c. The key 107a is slidable within the slide against the action of the spring by means of a cable connected to the inner end of the key 107a which is adjustable in the same manner described in Figure 12 (see also Figure 62). The key has first and second abutment surfaces 107b and 107c. When the key 107a is in the innermost position (relative to the chair as a whole) illustrated in Figure 13, then the first abutment surface 107b does not interfere with the reclining action of the back extension arm 70' as already described. This is referred to as the hyper-recline position, allowing recline of 15°. As already explained, the forward end of the back extension arm 70' is forked as shown to define right and left forks 93c, 93d. As the key 107a is moved into a position whereby the first abutment surface 107b is aligned with the right fork 93c then the first abutment surface 107b will interfere with the recline action of the back extension arm because the first abutment surface 107b will hit the underside of the main transom 22' before the forward surface portion 93a normally would. This allows recline of 12°. When the key 107a is moved so that the second abutment surface 107c is aligned with the right fork 93c then the second abutment surface 107c is disposed such that any recline of the back extension arm 70' is prevented or at least largely prevented. A recline lock is thereby defined. Figure 14 illustrates the manner by which the keys 107a may be moved in unison. A cable 120' is connected between a cable actuator 118' (see Figure 62) and cable amplification mechanism 410 mounted on the rearward extension 22a of the main transom 22. The cable amplification mechanism 410 includes a pair of pivotally mounted amplifiers 412 which have intermeshed teeth for synchronous operation. One of the amplifiers 412 has a rearward amplifier extension 414 to which the end of the cable 120' is connected. The cable 120' passes through cable guide 416. As the cable 120' operates on the rearward amplifer extension 414 to move it downwardly from the perspective shown in Figure 14, the intermeshing amplifiers 412 will be driven to rotate so that their remote ends move towards each other. The remote ends of the amplifiers 412 are connected by respective cables to respective ones of the keys 107a. This cable connection is depicted by phantom line 418. In Figure 13, it can be seen that the side of the back extension arm 70' includes two bores 92a and 92b which face like bores on the facing side of the other back extension arm (not shown). Bore 92a is cylindrical and bore 92b is rectangular as shown. As shown in Figure 18, first and second recline springs 95, 97 extend between the facing bores. The second recline spring 97 is in the form of an elongate bar, the ends of which are received in facing bores 92b of the two back extension arms 70'. The main transom 22' includes a rearward extension 22a having a bearing block 98' seated in a complementary recess on the upper surface of the rearward extension 22a. The bearing block 98' defines a complementary recess to receive a central portion of the second recline spring 97. As the back extension arms 70' recline relative to the main transom 22', the second recline spring 97 is caused to bend downwardly at its ends while the intermediate portion is held fixed by being seated in the bearing block 98' on the main transom 22'. The second recline spring 97 thus resists rearward recline and biases the back extension arms 70' toward the forward recline limit. The second recline spring 97 is preloaded at the forward recline limit by being slightly bent. This is achieved by having the centres of the bores 92b slightly below the centre of the spring in the recess of the bearing block 98'. The first recline spring 95 operates on a similar principle but is somewhat more complex. The first recline spring 95 is illustrated in greater detail in Figure 17 and comprises a spring portion 95a, in the form of a flat bar. The outer ends of the first recline spring 95 are fitted with cylindrical bosses 99a to be received in the facing cylindrical bores 92a provided in the back extension arms 70'. Additionally, a central cylindrical boss 99b is fitted onto the bar 95a. The central boss 99b is slotted to allow the bar 99a to pass through. As shown in Figure 18, the central cylindrical boss 99b is seated in a semi-cylindrical recess provided in the bearing block 98' on the main transom 22'. The bearing block 98' may be provided with upstands at its sides to locate the boss 99b relative to its seat in the bearing. The flat bar spring portion 95a provides resistance to recline through its inherent resistance to bending about a bending axis arranged transversely to the length of the spring 95. It will be appreciated that with the configuration of the ends of the first spring 95 and the central cylindrical boss 99b bearing against the main transom 22', the bending axis will be defined which extends generally transverse to the longitudinal axis of the spring 95. The arrangement is such that no pre-load is applied to flat spring portion 95a in the forward active position. The central recess in the bearing block 98' and the cylindrical bores 92a are thus aligned for this reason. The first recline spring 95 is adjustable to change the spring rate. This is achieved by rotating the first spring 95 about the longitudinal axis of the spring through the use of paddle 99c which is fixed onto the spring bar portion 95a. It can be seen from the cross-sectional views shown in Figures 19 to 21 that the spring portion 95a has a thickness and a width dimension, the width dimension being greater than the thickness dimension. In Figure 19, the spring 95 is oriented so that the width dimension is arranged substantially parallel to the bending axis. This represents the 'easy' spring position. In Figure 20, the thickness dimension is arranged diagonally to the transverse bending axis. Such an arrangement will present a greater resistance to bending about the transverse axis. This accordingly represents the "medium" spring position. Furthermore, in Figure 21, the width dimension is arranged transversely to the bending axis. Such an arrangement presents the greatest resistance to bending and is thus deemed the "hard" position for the first recline spring 95. The first recline spring 95 is thus adjustable through 90° to provide three adoptable spring positions at each of which the spring exhibits a different spring rate. This is visually depicted in Figure 24 which illustrates graphically the change in net spring force over distance as the spring is adjusted between easy (A), medium (B) and hard (C). Furthermore, Figure 18 illustrates the first spring 95 in the easy position whereas Figure 22 illustrates the first spring 95 in the hard position. Referring to Figure 23, in order to locate the first recline spring 95 in the adoptable spring positions, locators are provided in the form of grooves 99d provided in the cylindrical boss 99b. A complementary rib 99e is disposed in the semi-cylindrical recess of the bearing block 98a. The rib 99e can engage with any one of the complementary grooves 99d to accordingly locate the first spring 95 in that position. It may be necessary to remove most of the loading on the first spring 95 in order to change the spring position. Accordingly, it may be necessary to bring the back portion to the forward active position to achieve this. Figure 25 illustrates in greater detail the form of the cylindrical bosses 99a on the first spring 95. The end of each boss is cut away to define a semi-circular rebate 99d thereby defining a diametrical abutment face 99e. As can be seen in Figure 26, the end of bore 92a is provided with a projecting quadrant 92c. With the boss 99a assembled in the bore 92a, the quadrant 92c projects into the semi-circular rebate 99d. The spring 95 is rotatable through 90° between a first rotatable limit where one face of the quadrant 92c abuts against one half of the diametrical abutment face 99e and a second rotatable limit where the other face of the quadrant 92c abuts against the other half of the diametrical abutment face 99e. The interaction between the quadrant 92c and the diametrical abutment face 99e limits the rotation of the spring 95 to 90°. In Figure 26, the two bores 92a and 92b are shown as formed directly in the sides of the back extension arms 70. It is also envisaged that a plastic insert could be fitted into the side of the arm 70 with the bores 92a and 92b formed in the insert. Stiffness adjustment of Peripheral Frame - First Embodiment Figure 27 illustrates a further exploded view of parts assembled with the peripheral frame 34. As described previously, a back attach casting 48 is fixed to the back of the peripheral frame 34. The back attach casting 48 has two upright channels 64 arranged at either end, each defined by opposed walls 54. The opposed walls 54 have aligned slots 56 arranged therein for receipt of pins 62 provided on a spring carrier 60. The specific form of the spring carrier 60 is illustrated more clearly in Figure 29. The spring carrier 60 is in the form of an elongate member which is approximately square or rectangular in cross section with the pins 62 being arranged on opposite sides. One end of the member is provided with a rebate 124. The other end of the spring carrier is forked for pivotal connection with another linkage as will subsequently be explained. The forked end has aligned apertures 126. The rebate 124 has spaced threaded bores 130 provided therein. A leaf spring 128 has a lower end 131 shaped to be received within the rebate 124. The lower end 131 has two spaced apertures 133 provided therein. These apertures 133 align with the threaded bores 130 provided on the spring carrier so that the leaf spring 128 may be securely fastened to the spring carrier 60. From the lower end 131 in the upwards direction, the leaf spring 128 gradually increases in width with a slight tapering in thickness, although overall the leaf spring 128 is of generally elongate configuration as shown. The leaf spring 128 is constructed from high tensile spring steel. As can be seen in Figure 27, there are two spring carriers provided on opposite sides of the back portion, each received within a respective one of the channels 64 and mounted for pivotal movement about an axis defined through the bases of the aligned slots 56. Figure 28 illustrates the assembled combination whereby each of the leaf springs lie against the back of the peripheral frame 34 in a respective channel 44. As already described the peripheral frame 34 has a degree of flexibility. By rotating the spring carrier about pins 62 so that the forked end 125 moves rearwardly, the leaf spring 128 will be caused to act against the lower portion of the peripheral frame thereby increasing its stiffness against rearward flexing. The two spring carriers act in unison in a manner which will be described in connection with Figures 30 to 34. The stiffness of the lower portion of the peripheral frame 34 can thereby be adjusted by adjustment of the position of the spring carrier 60. Further, the channels 64 in which each of the spring carriers 60 are received are closed rearwardly by a rear wall 135 of the back attach casting 48. The rear wall 135 defines a stop against which the forked ends 125 of the spring carriers engage, thereby defining the maximum rotation of the spring carrier 60 and thus the maximum stiffness which can be imparted by the leaf spring 128 to the peripheral frame 34. Figure 30 illustrates the main elements of the recline mechanism. The back attach casting 48 has been removed for clarity, together with the right back extension arm 70. The left back extension arm 70 is shown in position pivotally connected to the main transom 22. The forked end 125 of each spring carrier 60 is connected to a push link 139. Reverting to Figure 3, it can be seen that the lower portion of the peripheral frame 34 has an access opening 143 to enable the push link 139 to engage with the forked end 125 of the spring carrier 60 disposed within the assembled back attach casting 48. The forward end of the push link 139 is connected to a drive link 141 (see Figure 30) which is one element of a four bar linkage which will be understood more fully from a consideration of the schematic illustration of Figure 31. Figure 31 illustrates only one four bar linkage and it will be apparent to the reader that two such four bar linkages are provided, one on each side of the chair 10. The drive link 141 extends at an inclined upwards angle from its connection with push link 139. The drive link 141 is curved along its length with the centre of the curve being disposed rearwardly and upwardly. The drive link 141 is mainly of rectangular cross section. The drive link 141 is pivotally connected at an intermediate location along its length to the main transom 22 for pivoting motion about the recline axis R. Specifically, the drive link 141 is pivotally connected to lie adjacent to the outer one of the opposed supporting webs 78 of the main transom 22. A common pivot pin (not shown) interconnects both of the opposed supporting webs 78, the back attach arm 70 through aperture 75, and the drive link 141. The main transom 22 forms another element of the four bar linkage. As has already been explained, the main transom 22 is centrally mounted to the supporting frame at the top of the central support column 20 which incorporates a height adjustable pneumatic spring 145. The height adjustment 145 is selectively operable by the chair occupant. However, the main transom 22 is normally stationary relative to the supporting frame. The seat portion 14 is slidably mounted to a seat guide 149 in a manner which will be described more fully in connection with Figures 55 to 60. The seat guide 149 thereby forms another element of the four bar linkage. The upper end of the drive link 141 is pivotally connected to the seat guide 149. Another link in the form of a front support link 151 interconnects the seat guide 149 and the main transom 22. The front support link 151 is of generally rectangular cross section and, like the drive link 141 is curved along its length with the centre of curvature disposed upwardly and rearwardly. From Figure 30 it can be seen that both ends of the drive link 141 are forked. The lower end is forked to accommodate the lower end of the push link 139. The upper end of the drive link 141 is also forked. The seat guide also has a dependent lobe 155 as shown in Figure 32. The forked upper ends of drive link 141 are disposed on each side of the lobe 155 and the inner fork is pivotally connected between the lobe 155 and the side wall of the seat guide 149. The outer fork is fanned in shape for aesthetic reasons and the pivotal connection does not extend therethrough. Likewise, the upper end of the front support link 141 is also forked with the inner fork being pivotally connected between a seat guide 149 and another lobe 157 (see Figure 32), with the outer fork being of fanned shape. The lower end of the front support link 151 is pivotally connected on the outside of the outer one of the opposed supporting webs 78 (see Figure 4) by means of a pin (not shown) extending through aligned forward apertures 153 on the forward end of the opposed supporting webs 78. It will be appreciated that the connection of the lower end of the drive link 141 and the front support link 151 are blind connections as shown for aesthetic reasons. Operation of Recline Mechanism The operation of the recline mechanism will now be explained in connection with Figure 31. Reference is only made to the four bar linkage elements on one side of the chair. The reader will appreciate that the elements are duplicated on the other side of the chair. As already stated above, the back portion 16 is reclinable about recline axis R. First and second recline springs bias the seat portion 16 into the forward active position. In the unoccupied state, the arrangement of the elements of the four bar linkage is determined by the spring tension of leaf spring 128. The natural resiliency of the leaf spring 128 will tend to straighten the leaf spring 128 thereby urging the spring carrier 60 in a clockwise direction about the pins 62. This determines the position of the push link in the unoccupied state of the chair. With no force exerted on the seat guide 149, the elements of the four bar linkage will be held in an unoccupied position on account of the natural resiliency of the spring 128 acting through push link 139. When a user bears weight W against the seat portion 14, this will be taken up by the seat guide 149 whereby the drive link 141 will be driven to rotate in an anticlockwise direction around recline axis R. This will cause the push link 139 to move generally upwardly and rearwardly thereby rotating spring carrier 60 anticlockwise about pivot pins 62. The lower portion of the peripheral frame 34 is rigidly held within back attach casting 48 which is stopped in its forward active position as already explained. With anticlockwise rotation of the spring carrier 60, the leaf spring 128 will be caused to bend with the upper part pushing against the back of the peripheral frame 34. Depending upon the flexibility of the peripheral frame 34, the occupant's weight will be taken up by a spring tension in leaf spring 128 as it flexes against the back of the peripheral frame 34. This has the effect of stiffening the back portion against rearward flexing. It will be appreciated that the tension imparted to leaf spring 128 will depend upon the weight of the user W applied to the seat portion 14. The greater the weight W, the greater the tension taken up by the leaf spring 128 and thus the greater the degree of stiffness imparted to the leaf spring 128 to resist rearward flexing of the peripheral frame 34. Accordingly, the stiffness of the peripheral frame 34 will be adjusted according to the weight W of the chair occupant. If the occupant's weight W exceeds a predetermined level then the leaf spring 128 will be tensioned to a point where the forked end 125 of the spring carrier 60 engages against the rear wall 135 of the back attach casting 48. This provides a limit to the amount of tension imparted to the leaf spring 128. The limit is reached at about 80 kg. Figure 33 illustrates the downward motion of the seat guide 149 as the user applies weight W. When the occupant alights from the chair, the seat portion 14 will move upwardly as indicated by arrow U in Figure 34. As already mentioned, the gentle serpentine shape of the peripheral frame 34 is designed to correspond with the shape of the occupant's spine for the comfort of the occupant. With the flexing action of the back portion, the ergonomics of the chair are further enhanced because this enables the occupant to exercise his spine. The general health of a person's spine is enhanced by movement. The stiffness of the back portion in rearward flexing is adjusted according to the occupant's weight. Therefore, within a certain range, the ease of rearward flexing will correlate to the weight of the occupant. Therefore, a light person will be able to obtain full benefit from the rearward flexing action by applying a light force against the peripheral frame. Also, a heavier person will encounter a greater resistance to flexing, ensuring that the peripheral frame is not too floppy for a large person. The chair is designed so that the occupant will be able to obtain deflection through flexing in the range of 80 mm to 120 mm. Figure 35 illustrates the reclining action of the chair 10. When the user applies their weight to the seat portion 14, the seat portion will move downwardly as already described and adopt a position just above the seat guide 149 as illustrated by the solid lines. Once a user has applied their weight to the seat portion 14, the leaf spring 128 takes up a corresponding amount of spring tension whereupon the spring carrier 60 and the push link 139 will adopt a more or less fixed position relative to the back attach casting 48. Therefore, as the user leans against the back portion 16, the back attach casting 48, spring carrier 60, push link 139 act in unison driving the drive arm 141 to rotate in a clockwise direction through push link 139. The arrangement of the four bar linkage is such that the seat guide 149 will adopt a position with a net increase in height and with an increase in rearward tilt angle compared to the occupied position of the seat guide 149 before recline. In practice, there may be some slight shifting between the leaf spring 128, the spring carrier 60 and the push link 139. Since the seat portion 14 undergoes a net increase in height with the rearward recline action, the occupant's weight W will be counteracting the recline action, together with the bias applied by the first and second recline springs 94, 96. The weight of the occupant W will therefore be a variable factor in the ease with which the back portion 16 reclines. If the adjustable second recline spring 96 is set at a constant level then a heavier person will encounter a greater resistance to reclining action than a lighter person. This establishes an automatic correlation between the weight of the person and the resistance to the reclining action. For a large proportion of people who fit within physical norms this automatic adjustment may be sufficient. However, people come in all different shapes and sizes and therefore additional adjustment is required through the use of the clamping adjustment as explained previously. For example, a very tall, light person may obtain leverage through their height which makes the back portion 16 fall back too easily against their low weight W. The net increase in height also has the advantage of raising the occupant during recline so that the eye level of the chair occupant can be maintained even though he is undergoing a reclining action. Once the chair is fully reclined (as determined by the first abutment surface 88 engaging against second abutment surface 90), the peripheral frame will still be able to flex under additional force applied by the chair occupant. As already mentioned, it is considered that the peripheral frame will be capable of undergoing deflection in the range of 80 mm to 120 mm. During the recline action, it is considered that the weight of the user against the back portion will bring about a deflection of up to 20 mm. Therefore, once the recline limit is reached, the occupant still has further deflection available through flexing of the peripheral frame in the range of 60 to 100 mm. As explained subsequently in connection with Figures 55 to 60, the seat portion 14 is only supported by the seat guide 149 at a rear portion thereof with a forward portion being unsupported. As shown in Figure 32, a transition point 161 is disposed behind the forward edge 160 of the seat guide 149. The transition point 161 marks the boundary between the planar upper surface 178 of the seat guide 149 and a forwardly inclined lead surface 285. The seat portion 149 is foldable transversely at this location. The transition point 161 hence defines the division between the rearward portion and the forward portion of the seat portion 14. Since the seat portion 14 is slidable forwardly and rearwardly for seat depth adjustment as will be explained in connection with Figures 55 to 60, the division between rearward portion and forward portion of the seat will vary as a function of seat depth. Figure 35 illustrates the changing curvature of the back portion 16 and seat portion 14 in recline. The solid lines indicate the forward active position in the occupied configuration. The dotted lines illustrate the reclined position. As the back portion 16 reclines, the seat guide 149 attains a net increase in height and an increased rearward tilt. This effectively cups the occupant's derrière, negating any inclination to slide forwardly during the recline action. The seat portion 14 is also flexible and since the occupant's derrière is undergoing a net increase in height together with increased rearward tilt, a greater amount of weight from the occupant's legs will be brought to bear against the forward portion of the seat portion 14. Accordingly, the seat portion 14, will be allowed to fold transversely at the transition point 161 on the seat guide 149. To achieve maximum benefit from the cupping action, the occupant ought to adjust the seat depth so that with his derriere abutting the back portion, transition point 161 approximately corresponds to the gluteal fold of the occupant's derriere. Therefore, during recline, the occupant's derriere will be cupped between the rear portion of the seat portion 14 and a lower region of the back portion 16 while the forward portion of the seat drops forwardly under the weight of the occupant's legs. Locating the transverse fold at the gluteal fold of the occupant ensures that undesirable pressure will not be brought to bear against the back of the occupant's legs. Modified Form of Back Portion - Second Embodiment Figure 36 illustrates in exploded fashion a modified form of the back portion 16'. As with the previous embodiment, the back portion 16' includes a flexible peripheral frame 34' which is connected to a back attached casting 48'. In this embodiment, the spring carriers have been obviated and instead there are two unitary leaf springs 128' which bear against the back of the peripheral frame 34'. Additionally, two supplementary springs 450 are also provided, the function of which will be explained. Figure 39c illustrates the modified form of the push link 139'. The push link is arcuate in configuration. At one end, the push link has an aperture 452 to which it can be pivotally connected to drive link 141' (see Figure 41a and 41b). At the other end of push link 139' is a stepped region 454 having a first abutment face 456 and a second abutment face 458. Forwardly of the stepped region 454 is a first pair of gliders 460. Each glider of the pair 460 is disposed on opposite side faces of the push link 139'. Disposed directly below the first pair of gliders 460 is a second pair of gliders 462 disposed on opposite side faces of the push link 139'. Referring to Figure 37, one side of the back attach casting 48' is shown in greater detail. The back attach casting 48' incorporates two pairs of sprigs 50' which engage with aligned apertures (not shown) in the peripheral frame 34' for assembly purposes. As with the previous embodiment, spaced walls 54' define a forwardly extending channel 64' in which the leaf spring 128' is housed in a manner which will be explained. The forwardly extending channel 64' includes two forwardly extending tracks 464 on opposite sides of the channel 64'. The tracks 464 each comprise a substantially horizontal ledge 466 which terminates in a downwardly extending flange 468 in the assembled configuration of the push link 139' and the back attach casting 48', the first pair of gliders 460 are disposed to glide along the top surface of the associated ledges 466 whereas the second pair of gliders 462 passes underneath the bottom surface of the associated ledges 466. As can be seen from Figure 39c, each of the second pair of gliders 462 has a flat abutment surface 470 which abuts against the inside of the downwardly extending flange 468. This defines the forward limit in the sliding movement of the push link 139' relative to the tracks 464. Figure 39d illustrates the assembled configuration of the push link 139', the back attach casting 48', the leaf spring 128', the supplementary spring 450 and the peripheral frame 34'. The operation of the recline mechanism has already been described in connection with Figure 31 and the operation is not substantially different in the second embodiment and thus can be understood by reference to Figure 31 already described. When a user's weight bears against the seat portion 14, this will be taken up by the seat guide 149 whereby the drive link 141 will be driven to rotate in an anti-clockwise direction about the recline axis R. In the present embodiment, rotation of the drive link 141 will cause the aperture in the push link 139' to move generally upwardly and rearwardly. This causes a consequent sliding of the first and second pair of gliders 460, 462 along the tracks 464. The supplementary spring 450 and the leaf spring 128' are arranged such that the first abutment face 456 will come into contact with the supplementary spring 450 prior to the second abutment face 458 coming into contact with the leaf spring 128'. This means that up to a predetermined threshold of the user's weight W, the push link 139' will bear against the supplementary spring 450. The supplementary spring 450 does not have a bearing on the stiffness of the peripheral frame 34'. Therefore, up to a predetermined threshold of the users weight W, there will be no stiffening effect on the peripheral frame 34'. After the predetermined threshold is reached, which is about 50 kg, the second abutment face 458 of the push link 139' will come into contact with the leaf spring 128'. The leaf spring 128' has an initial slightly bent configuration as illustrated in Figure 39d. The leaf spring 128' bears against spring seat 474 disposed at the top of the forwardly extending channel 64' as can be seen in Figure 37. The spring seat 474 is concave from side to side to position the leaf spring 128' while being convex from top to bottom as illustrated in cross section in Figure 39d. By being forwardly convex as illustrated, the spring seat 474 defines a point about which the leaf spring 128 bends as the push link 139' moves rearwardly in its tracks 464. Similar to the first embodiment, as the spring 128' is pushed from its lower end to flex about spring seat 474, above the spring seat 474 it will bear against the back of the peripheral frame 34' thereby increasing the stiffness of the peripheral frame 34'. Furthermore, as with the first embodiment, at a certain point the push link 139' and/or the leaf spring 128' will bear against the back attach casting 48' where upon no further movement will be possible. This will define the tension limit for the leaf spring 128'. Figure 39b illustrates in greater detail the form of the supplementary spring 450. The supplementary spring is in the form of a leaf spring having an enlarged head formation 478 which includes two bights 480 on opposite edges. The bites 480 cooperate with facing complementary locating blocks 482 disposed on opposite sides of the forwardly extending channel 64. Figure 41a illustrates certain components of the recline mechanism although the peripheral frame 34' and the back attach casting 48' have been removed for clarity. As in the previous embodiment, the drive link 141' is pivotally mounted to the main transom 22' at an intermediate location. The opposite end of the drive link 141' to that which the push link 139' is attached is pivotally connected with the seat guide 149'. Similarly, the front support link 151' is connected between the seat guide 149' and the main transom 22'. In this embodiment, the drive link 141' and the front support link 151' are also curved about one or more upright axes as well as being curved about a horizontal transverse axis as described with the first embodiment. This renders a more complex shape for the seat guide 149' as depicted in Figure 43. Seat Panel - First and Second Embodiments Figure 46 is a perspective view of a preferred form of the seat portion 14 which is appropriate for use with either embodiment of the chair. The seat portion 14 is in the form of a flexible plastic panel, whose flexibility is enhanced by the arrangement of slots as indicated. The plastic panel may be injection moulded plastic such as TPR. It will be noted that while the seat panel 14 is depicted in the computer generated drawings of Figures 47-49 to be a flat panel, the seat panel is in fact dish shaped as can be seen from the schematic views illustrating the various cross-sections in Figures 50 to 54. Figure 50 is a longitudinal section through the middle of the seat panel 14 illustrating the general curved configuration with a rolled over edge. The edge drops by an amount of dimension A. Figure 51 illustrates the side edge of the seat panel 14. The side edge is flatter than the middle section. Additionally, the forward edge dips down a dimension B, where B is larger than A. Figure 52 illustrates a transverse sectional view at about 150 mm from the rear of the seat whereas the view Figure 53 depicts the transverse cross sectional view 120 mm from the front edge. This is essentially a flat shape. Therefore, the rear part of the seat behind 120 mm from the front edge is essentially dished for user comfort whereas in front of this, the seat portion inclines downwardly in the forward direction. Additionally, as can be seen in Figure 54, the front edge is also curved so as to incline downwardly toward the sides. The illustrations in Figures 50-54 are merely indicative of the moulded shape of the seat panel 14. The seat panel is also flexible to accommodate the occupant and to respond to movement of the occupant. The arrangement of slots in the seat panel 14 as shown in Figure 46 is designed to enhance the flexibility of the seat panel 14. The arrangement of slots in the forward half of the panel is designed to facilitate folding along the transverse fold. In particular, it can be seen that the slots are arranged in a series of spaced sinuous lines 163 extending transversely across the seat portion 14 with the central part being shaped convex forwardly with the outer parts being shaped concave forwardly. The lines of slots 163 are discontinuous. As already explained, the seat portion 14 is dished at least in a rearward part. This dishing may be accentuated by the occupant in the seat. The series of spaced sinuous lines 163 enables the seat panel 14 to fold transversely, even though the rear part is dished. Furthermore, at the front corners, the slotted pattern 164 is such as to extend diagonally across the corners following the curvature of the transverse sinuous lines 163. In this way, if the user moves a leg to one of the forward corners then the diagonal arrangement of the slots 164 will enable the forward corner to fold under the weight of the occupant's leg. In the rear half of the panel, the slots are arranged in a pattern to accommodate the ischial protuberosities of the occupant. In particular, the slotted pattern provides two spaced, approximately rectangular zones 162 whose locations correspond to the ischial protuberosities of the occupant (assuming the occupant is properly seated with an appropriate seat depth adjustment). The two zones 162 interrupt the transverse slot pattern. Each zone is comprised of slots arranged in a series of longitudinally extending, transversely spaced sinuous lines. The lines of slots are discontinuous. The longitudinal arrangement of slots in each zone 162 enables the remaining material between the longitudinal lines of slots to spread apart thereby creating pockets, one for each ischial protuberosity of the seat occupant. Figure 47 illustrates longitudinal stiffening webs 165 provided on the underside of seat panel 14. There are five stiffening webs, two disposed along the opposite side edges. A further two are disposed on each side at 60 mm from the corresponding side edge. Another is centrally disposed. The longitudinal stiffening webs are constant in height from the back edge of the seat portion until the taper start point 164 from where they progressively reduce in height until a taper finish point 166. (The central web however terminates early) The seat portion 14 accommodates a depth adjustment as will be explained in connection with Figures 55 to 60. The seat portion folds transversely about the transition point 161 on the seat guide 149. It will be appreciated that if the seat panel 14 is located in a rearward position in order to suit a small person then the depth of the stiffening ribs in the region at the transition point 161 is shallow thereby offering little resistance to flexing. Generally, this suits a small, light weight person. However, for a larger person, the seat panel will be disposed further forwardly in relation to the seat guide 149. The depth of the stiffening ribs in the location of the transition point 161 will be deeper, thereby offering increased resistance to bending. This suits a larger, heavier person. The start taper point 164 is at a position which corresponds to the transition point 161 when the seat is at its full forward position to suit a large person. The taper finish point 166 is at a position corresponding to the transition point on the seat guide 149 with the seat in the rear most position to suit a small person. The taper start point 164 and the taper finish point 161 define a transition zone therebetween. The transverse fold may be disposed at a range of positions within the transition zone, dependent on seat depth adjustment. The pattern of transversely extending sinuous lines of slots extends for at least the transition zone. Figure 47 also illustrates transverse stiffening webs 168. The stiffening webs 168 follow the pattern of the transversely arranged sinuous slots 163. As already explained, the seat panel is moulded in a dished shape. However, it is desirable to limit curvature, especially about a longitudinal axis at the front part of the seat portion. Accordingly, the transverse stiffening webs 168 help to retain the shape of the front part without inhibiting the transverse folding action under the weight of the user. Additionally, a back web is provided along the back of the seat panel 14 on the underside as shown in Figure 47. Figure 49 illustrates in greater detail the arrangement of features along one side edge. Between the two longitudinal webs 165 is a series of spacer blocks 270 extending in a line between the taper start point 164 and the taper finish point 166. Between each of the spacer blocks 270 is a wedge-shaped gap 272 widening towards the top. As will be explained in connection with Figures 55 to 60, the seat panel 14 sits atop a seat carriage 167. Depending upon the position of the seat carriage 167 relative to the seat guide 149, there will normally be a forward portion of the seat guide 149 (including the lead surface 285) in front of the seat carriage 167. A rear part of the seat panel 14 is secured atop the seat carriage 167 so that forwardly of the seat carriage 167 there will be a gap between the seat guide 149 and the seat panel 14. The spacer blocks 270 extend into this gap. As the seat panel 14 folds, the spacer blocks 270 bear against the top of the seat guide 149. It can be seen that the spacer blocks 270 also taper off in height as shown. Furthermore, the spacer blocks 270 will define the maximum curvature of the seat panel along the transverse fold since once the side walls of the wedge- shaped gaps 272 engaged with each other, further curvature will be prevented. A guard also extends alongside the spacer blocks 270 to provide a barrier against the user's fingers being trapped. Seat Depth Adjustment Mechanism Figure 55 illustrates the main elements of the seat depth adjustment mechanism. The seat guide 149 is one of the elements of the four bar linkage discussed previously. There are two seat guides 149 disposed on opposite sides of the chair. The two seat guides 149 provide a guide for a slidable seat carriage 167. A rear part of the seat panel 14 illustrated in Figures 47-54 is attached to the carriage 167. The rear half only of the seat panel 14 is attached to the seat carriage 167. The seat panel 14 may be moved forwardly and rearwardly by the sliding action of the seat carriage 167 on the seat guide 149. As shown in Figure 49, rearwardly of the spacer blocks 270 on the underside of the seat panel 14 is a longitudinally extending rib 274 and then a short tab 276 spaced rearwardly of the longitudinally extending rib 274. The rib 274 engages within a channel 278 (see Figure 55) of the seat carriage 167 and the tab 276 is a snap fit connection within the recess 280 located rearwardly on the seat carriage 167. Furthermore, four spaced retention tabs 282 engage against soffit 284 of the carriage 167. The retention tabs 282 retain the seat panel 14 engaged with the seat carriage 167 while the longitudinal rib is the main load bearing part. Figure 55 also illustrates the controls for the height adjustable pneumatic spring 145. A height adjustment control lever 169 is mounted for pivotal motion on the outside of the right hand seat guide 149. The pivotal motion of the height adjustment control lever 169 is replicated by the height adjustment control actuator 170 which is connected to one end of a control cable 172. The other end of the control cable 172 is connected to the top end of pneumatic gas spring 145. As the user lifts the height adjustment control lever 169, the control cable 172 releases the gas spring in the conventional known manner and the chair occupant adjusts the height of the seat portion 14 to suit his requirements. Figure 56 is a further detailed view of the left side of the seat carriage 167. The seat guide 149 includes a plastic seat guide liner 176. The seat guide liner is of elongate configuration with an upper glide surface 178 and an inner glide surface 180. The inner glide surface 180 is spaced from the inner side of the metal part seat guide 149 with a peripheral wall 182 maintaining the inner glide surface 180 in spaced configuration therefrom. The seat guide liner 176 is thereby hollow behind the inner glide surface 180. The upper glide surface 178 is received within a rebate in the upper surface of the metal part of the seat guide 149 in order that the upper glide surface 178 is contiguous with the upper surface of the metal part of the seat guide 149. The seat guide liner 176 provides a bearing surface for easy sliding of the seat carriage 167. As such, the seat guide liner 176 may be comprised of nylon or acetal. The reader will appreciate that a symmetrical arrangement is provided on the right hand side of the chair. The seat carriage 167 is of unitary cast aluminum construction and comprises two spaced slides, each of which engages with a respective seat guide 149. Each slide is of a generally L-shaped configuration having an upright glide surface 186 on an inner wall for sliding engagement with the inner glide surface 180 and a horizontal glide surface 187 for engaging with the upper glide surface 178. The carriage is of a symmetrical configuration about a central upright longitudinally extending plane of the chair. The two slides provided on the right and left are thereby of opposite configuration. The two slides are joined by transversely extending bearers 190. The inner glide surface 180 is moulded with a series of archlets which extend from the inner glide surface 180. The archlets 184 protrude inwardly (relative to the chair as a whole) to bear against the upright glide surface 186 of the seat carriage 167. The archlets may be arranged in any pattern but preferably they are staggered along the length of the inner glide surface 180. Both of the seat guide liners 176 have inwardly extending archlets bearing against the associated upright glide surfaces of 186 of the carriage 167. The archlets 184 thereby act against the carriage to centre the carriage 167 centrally between the two seat guides 149. Furthermore, in the event that the parts are not accurately tooled, the resilient archlets 184 will take up any slack between the upright glide surface 186 and the inner glide surface 180. This assists to prevent jamming of the carriage 167 within the seat guides 149. Figure 57 illustrates the control for seat depth adjustment. The inner wall of both slides 185 have a lower edge with a series of spaced notches 192. A seat depth adjustment bar 194 has two teeth 196, each arranged at opposite ends of the bar 194. The seat depth adjustment bar 194 is moveable between a latched position in which the teeth 196 engage in a respective one of the notches 192 and an unlatched position in which the carriage 167 is free to slide along the seat guide 149. The seat depth adjustment bar 194 is controlled by a seat depth adjustment button 200. The seat depth adjustment button 200 is moveable from the latched position against the bias of a spring (not shown) to move the seat depth adjustment bar 194 into the unlatched position whereby the teeth 196 no longer engage in the notches 192. The seat carriage 167 can then be slid to an appropriate seat depth whereupon the occupant releases the seat depth adjustment button 200 to enable the teeth 196 to engage with the closest of the notches 192. A seat depth stop 174 (Figure 55) formed as a dependent projection from the seat carriage 167 determines the forward position of the seat carriage 167 as it engages with the adjustment bar 194 or sleeves 158 receiving the ends of the adjustment bar 194. The rear limit is defined by a pin (not shown) extending inwardly from the seat guide 149 to engage within a slot of the seat carriage 167. The slot is machined to define a stop to engage with the join in the rear most position of the seat portion. Figures 58 and 59 illustrate the extended and retracted positions respectively of the seat portion 14. Seat DP pth Adjustment - Second Embodiment Figure 61 and 62 illustrate a modified form of the seat carriage 167' and the seat guide 149'. The seat carriage 167' is a unitary cast aluminum construction with two spaced slides as explained with the first embodiment, each of which engage with a respective seat guide 149'. The two slides are joined by a unitary deck construction having a series of transversely extending ribs as shown. As with the previous embodiment, the seat guides 149' include seat guide liners 176' having an upper glide surface 178' and an inner glide surface 180' to slidably engage with the respective slide of the seat carriage 167'. The seat guide liners 176' will be described in greater detail in connection with Figure 62b and 62c. As shown in Figure 61, the second embodiment of the chair includes a control lever 169' on the right hand side (left hand side of the figure). This lever 169' is a dual actuator for both the seat height adjustment and seat depth adjustment. The control lever 169 is mounted for pivotal motion on the outside of the right hand seat guide 149'. The control lever 169' effects the operation of a dual actuator 170' mounted on the inside of the right hand seat guide 149'. The actuator 170' includes a first actuator portion 170a and a second actuator portion 170b. The first actuator portion 170a is connected to cable 172' which connects to the top end of a pneumatic gas spring 145'. As the user raises the control lever 169', the control cable 172' releases the gas spring in the conventional known manner and the chair occupant adjusts the height of the seat portion 14 to suit his requirements. The second actuator portion 170b is connected via cable 488 to a pivotable pawl 490. The pawl is engageable between any one of a plurality of teeth provided on a rack 492 formed on the underside of the seat carriage 167'. The pawl and rack arrangement 490, 492 is also duplicated on the other side of the seat carriage 167' as shown in Figure 62. The cable 488 passes from the right hand pawl 490 around to the other side of the seat carriage 167' for simultaneous operation of the two pawls 490. The user depresses the control lever 169' to operate the second actuator portion 170b to pivot the two pawls against a bias out of engagement with the teeth of the associated rack 492. The seat carriage 167' can then be slid to an appropriate seat depth where upon the occupant releases the control lever 169' to enable each of the pawls 490 to engage with the associated rack 492. Figure 61 also illustrates a forward cover 495 which is shaped in a serpentine manner for aesthetic purposes to extend in front of the main transom 22'. The cover 495 is joined to the seat guides 149' on each side through the use of integrally formed bosses 497 which can be seen in Figure 62b and Figure 62c. As already explained, the seat guide 149' illustrated in Figure 62b includes a seat guide liner 176'. The seat guide liner 176' includes an upper glide surface 178' and an inner glide surface 180'. Thus, the seat guide liner 176' is essentially L-shaped in configuration. The inner glide surface 180 is formed with a series of spaced integral resilient projections 500. The integral resilient projections 500 are directed inwardly. The seat guide liner 176' is supported on a metal supporting part of the seat guide liner as shown in Figure 62c. The inner glide surface 180 is disposed in spaced configuration from the inside of the supporting part of the seat guide 149'. Additionally, the supporting part of the seat guide 149' includes three spaced rests 502. The integral resilient projections 500 are shaped like ramps, the ends of which engage against the associated rest 502. The majority of the inner glide surface 180' is thereby resiliently held in spaced configuration from the supporting part of the seat guide 149'. It can been seen in Figure 59 of the first embodiment that a gap exists between the top surface of the seat guide 149 and the spacer blocks 270 which extend from the seat panel 14. This gap might be one in which the occupant can get their fingers caught. Accordingly, a movable comb like formation 504 is incorporated into the seat guide liner 176' as shown in Figure 62b. The comb like formation 504 has an upper surface continuous with the upper glide surface 178' and dependent prongs 506 which extend downwardly. The prongs are receivable into a series of corresponding pits 508 formed in the metal supporting part of the seat guide 149'. The movable comb like formation 504 is resiliently flexible and would normally extend to fill the gap between the leading edge 285 of the seat guide 149' and the dependent spacer blocks 270'. For instance, see Figure 63 although in Figure 63, the occupant's weight is not yet bearing on seat panel 14 and thus the seat panel 14 has not yet come to rest on top of the comb like formation 504. Additionally, the dependent spacer blocks are not visible in this view because the seat panel 14 has a peripheral guard to prevent jamming of fingers in the V-shaped gaps of the spacer blocks 270'. When the user's weight bears forwardly of the seat panel 14, the spacer blocks 270' will come to bear against the comb like formation 504 which will deflect as the seat portion 14 folds about the transverse fold. In this way, the comb like formation 504 presents an additional guard to mitigate the likelihood of user's fingers being caught between the seat panel 14 and the seat guide 149'. However, the comb like formation 504 does not interfere with the transverse folding of the seat panel 14. Figure 63 illustrates the seat panel 14 in its inward retracted position whereas Figure 64 illustrates the seat panel 14 located in its outer most extended position. Upholstery Figure 81 illustrates the preferred cross section for the upright members 38 of the peripheral frame 34. As has been described previously, the uprights of the peripheral frame each include a rearwardly open channel 44 in which the leaf spring 128 resides as has been explained previously. The upright member 38 also includes a second rearwardly open channel 252 of much narrower configuration than the first mentioned rearwardly open channel 44. The second rearwardly open channel 252 receives an attachment strip 254. The attachment strip 254 is of extruded resilient plastics material in the form shown. The attachment strip 254 has a longitudinal extending lip 550 which engages with retainer portions 552 provided along one of the walls of the channel 252 to assist in holding the attachment strip 254 within the channel 252. The attachment strip 254 also includes a part 258 which extends over the edge of the channel 252 when the lip 550 is engaged with retainer portions 552. The mesh fabric 260 is sized so that with the attachment strip 254 secured within the second rearwardly open channel 252 on both sides of the back portion 16, the mesh fabric 260 will be relatively taut across the peripheral frame. The top of the mesh fabric 260 is also held within a top rearwardly open channel 253, in the same manner. The bottom of the mesh fabric 260 is held within a bottom rearwardly open channel 255 in the same manner. The attachment strip 254 is a unitary strip extending around the entire periphery of the peripheral frame 34. As already explained, the peripheral frame 34 is of flexible construction, particularly around the region corresponding to the lumbar region of the occupant. Additionally, the mesh fabric is drawn taut across the peripheral frame 34. It is important that the frame does not flex so as to draw in the upright members 38 of the peripheral frame 34 due to the tautness of the mesh fabric 260. Accordingly, the back beam 46 is positioned so as to correspond approximately with the lumbar region of the seat occupant. This maintains the spacing of the upright members 38, particularly in the lumbar region where the frame 34 bends. The bending of the peripheral frame 34 close to the lumbar region of the occupant is encouraged by the serpentine shape of the peripheral frame 34 as well as being encouraged by the cantilevered connection of the peripheral frame 34. The mesh fabric 260 may have a degree of resiliency but this is somewhat limited. It is preferable that the mesh fabric should be able to maintain tension over a reasonably long period of time. It is desirable that the mesh fabric 260 is not overly stretched. For this reason, it is desirable that the neutral axis of bending be close to the front surface of the upright members 38 of the peripheral frame 34. Accordingly, the cross section of the peripheral frame 34 is designed to have the bulk of material on the forward face so that bending occurs as close as possible toward the forward face of the upright member 38. In bending, there will be some compression of the walls defining the channel 252 in the lumbar region. Additionally, there may be some flexing of the two walls of the channel 252 towards each other. Wheeled Base Figure 84 illustrates a preferred form of the wheeled base 18. The wheeled base includes five radially extending legs 300. Each of the legs is supported by a respective castor 302. As more clearly illustrated in Figure 85, the five legs 300 make up an unitary cast leg assembly. Each leg is elongate and substantially plate-like in thickness, strengthened by a strengthening web 304 extending longitudinally along each leg 300. The strengthening webs 304 terminate at their inner ends at a centrally disposed annular boss 306. At their outer ends, each of the legs 300 is provided with an integrally formed dependent connector 308. Each dependent connector 308 is in the form of a socket or sleeve. As the legs are substantially plate-like in configuration, the end of each leg 300 terminates in a clip-on bumper 301 comprised of resilient plastic or rubber material. Figure 86 illustrates the form of the castor 302. Each castor 302 comprises two spaced wheel portions 312. The wheel portions 312 are rotatably mounted on an axle 314 forming part of an axle assembly 316 illustrated in Figure 87. The axle assembly 316 incorporates the axle 314, a connector pin 318 and an intermediate body portion 320 interconnecting the axle 314 and the connector pin 318. The wheel portions 312 are received on opposite ends of the axle 314 and rotatably held there by means of a snap-fitting. In the assembled configuration illustrated in Figure 86, the connector pin 318 is disposed between the two wheel portions 312. Furthermore, there is a further gap provided between the connector pin 318 and the wheel portions 312 to receive at least part of the dependent connector 308. The connector pin 318 releasably engages with the dependent connector 308 enabling the pin to rotate within the dependent connector 308 about the longitudinal axis of the pin 318. A snap-fit connection may be provided therebetween. In assembled configuration of the leg 300 and the castor 302, only a small clearance need be provided between the underside of the leg 300 and the top of the castor 302. This provides for a compact arrangement of low height (typically less than 65mm), causing minimal disruption to the movement of the chair occupant's feet under the seat portion. Figure 89 illustrates in schematic form, the underside of the slotted seat panel 14. Mounted to the underside of the seat panel 14 is a scabbard which is curved in form. The scabbard 350 houses an instruction slide 352 which is also curved and slides in and out of the scabbard at one end. From above, the instruction slide 352 has printed indicia thereon providing user instructions to the seat occupant. The foregoing describes embodiments of the present invention and features not forming part of the present invention and modifications may be made thereto without departing from scope of the appended claims.

Claims: A chair comprising: a supporting flame (18, 20, 22, 22'); a seat portion (14) having a rearward portion and a forward portion, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion (16); and a recline mechanism (48, 48', 70, 70', 141, 141', 139, 139') with which the back portion is connected for reclining action of the back portion, characterised in that the seat portion is foldable about a transverse fold to define the rearward portion behind the transverse fold and the forward portion forward of the transverse fold, and in that the recline mechanism is operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, with a consequent folding of the seat portion about the transverse fold line under the weight of the occupant. The chair as claimed in Claim 1, wherein the recline mechanism is operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, in addition to obtaining a net increase in height above the supporting frame, the rearward portion is moved to increase the rearward tilt angle. The chair as claimed in Claim 1 or Claim 2, wherein the seat portion is constructed of a resiliently flexible material. The chair as claimed in Claim 3, wherein the seat portion comprises a panel which has apertures (162, 163, 164) to enhance its flexibility. The chair as claimed in any one of Claims 1 to 4, wherein the seat portion takes the form of a panel and stiffening webs (165) are provided which offer resistance to folding towards the rear of the seat portion and lesser amount of resistance to flexing towards the forward edge of the seat portion. The chair as claimed in Claim 5, wherein the stiffening webs are tapered to offer the progressively increasing resistance to folding from the front edge of the seat portion towards the rear. The chair as claimed in any one of the preceding claims, wherein the rearward portion of the seat portion is supported, at least in part, by the recline mechanism and the forward portion is substantially unsupported. The chair as claimed in any one of Claims 1 to 7, wherein the recline mechanism comprises a four bar linkage comprising four elements as follows: a main support (22, 22') forming part of the supporting frame; a second linkage comprising the seat portion or a guide (149, 149') relative to which the seat portion is selectively movable; a front support linkage (151, 151') extending between the main support and the second linkage; and a drive linkage (141, 141') pivotable about a drive axis (R) through the main support, the drive linkage being connected to the seat portion or guide and being operably linked to be driven about the drive axis by rearward recline action of the back portion to bring about the net increase in height of the second linkage on rearward recline action of the back. The chair as claimed in Claim 8, wherein two such four bar linkages are defined on opposite sides of the chair. The chair as claimed in Claim 8 or Claim 9, wherein the main support is selectively height adjustable. The chair as claimed in any one of claims 1 to 7, wherein the seat portion is adjustable in position between a forward extended position and a rearward retracted position such that the positioning of the transverse fold is variable as a function of the seat depth position. The chair as claimed in Claim 11, wherein the recline mechanism includes at least one guide and the seat portion is slidable relative to the guide between the extended and retracted positions, the guide including a fixed portion (161) about which the seat portion folds. The chair as claimed in Claim 12, wherein the guide has an upper surface (178, 178') having a forward portion (285, 504) with an upwardly facing convex shape, whereby a transition in curvature defines the transverse fold position of the seat portion. The chair as claimed in Claim 12 or Claim 13, wherein the recline mechanism comprises a four bar linkage comprising four elements as follows: a main support forming part of the supporting frame; the guide; a front support linkage extending between the main support and the guide; and a drive linkage pivotable about a drive axis through the main support, the drive linkage being connected to the guide and being operably linked to be driven about the drive axis by rearward recline action of the back portion to bring about the net increase in height of the guide on rearward recline action of the back. The chair as claimed in Claim 14, wherein two such four bar linkages are defined on opposite sides of the chair. The chair as claimed in Claim 14 or Claim 15, wherein the back portion is pivotally connected to the main support at a recline axis. The chair as claimed in Claim 16, wherein the recline axis is located below the seat portion. The chair as claimed in Claim 17, wherein the recline axis is located below the ischial protuberosities of the chair occupant. The chair as claimed in any one of claims 14 to 18, wherein the back portion is biased against reclining by a recline biasing device (94, 95, 96, 97). The chair as claimed in Claim 19, wherein the recline biasing device comprises one or more springs. The chair as claimed in Claim 20, wherein two back extension arms (70, 70') substantially rigidly extend from the back portion and pivotally connect the back portion to the main support, the one or more springs being held by one or both of the back extension arms, with the spring(s) acting against the main support. The chair as claimed in any one of Claims 19 to 21, wherein the biasing force of the recline biasing device is adjustable. The chair as claimed in Claim 22, wherein two springs are provided, being a first spring (94) and a second spring (96), both of which operate in the manner of leaf springs with the second spring being clampable against the first spring with the combination having a resultant spring rate, with the degree of clamping being variable to adjust the resultant spring rate. The chair as claimed in any one of Claims 1 to 13, wherein the back portion comprises a flexible frame which is flexible or at least flexible at a part corresponding to the lumbar region of the occupant. The chair as claimed in Claim 24, wherein the flexibility of the back portion is adjustable. The chair as claimed in Claim 25, wherein the flexibility adjustment takes place automatically in response to the weight imparted by the occupant on the seat portion with, at least beyond a predetermined threshold in weight, the larger the weight, the greater the stiffness imparted to the back portion. The chair as claimed in Claim 26, wherein the flexibility adjustment takes place through the use of a tensionable biasing device comprising at least one leaf-type spring (128, 128') lying against the back portion at a lower region thereof. The chair as claimed in Claim 27, wherein the recline mechanism comprises a four bar linkage comprising four elements as follows: a main support forming part of the supporting frame; a second linkage comprising the seat portion or a guide relative to which the seat portion is selectively movable; a front support linkage extending between the main support and the second linkage; and a drive linkage pivotable about a drive axis through the main support, the drive linkage being connected to the second linkage and being operably linked to be driven about the axis by rearward recline action of the back portion to bring about a net increase in height of the second linkage on rearward recline action of the back; and further wherein an interconnecting linkage interconnects the leaf-type spring with the drive linkage such that, at least beyond the predetermined threshold, the weight on the seat causes the leaf-type spring to flex against the back portion to impart greater stiffness thereto. The chair as claimed in Claim 28, wherein a supplementary spring (450) is provided, whereby weight on the seat portion up to the predetermined threshold causes flexing of the supplementary spring. The chair as claimed in Claim 28 or Claim 29, wherein two four bar linkages are defined on opposite sides of the chair, there being two such leaf-type springs on opposite sides of the chair with two such interconnecting linkages, wherein each interconnecting linkage is disposed directly between the associated leaf-type spring and the associated drive link. The chair as claimed in any one of Claims 28 to 30, wherein a tension limit(48', 125, 128', 135, 139') is provided to prevent over-tensioning of the tensionable biasing device.

Inventors: FIFIELD JON LEONARD, PENNINGTON MARK RUNDLE, STEWART ROBERT BRUCE, FIFIELD, JON LEONARD, PENNINGTON, MARK RUNDLE, STEWART, ROBERT BRUCE
Applicants: FORMWAY FURNITURE LTD, FORMWAY FURNITURE LIMITED
Application year: 2001
Main classification: A47C1
IPC codes: A47C1, A47C7, A47C3, A47C5