Patent Publication Number: US-7712834-B2

Title: Back support for seating unit

Description:
RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 11/532,784, filed Sep. 18, 2006 now U.S. Pat. No. 7,427,105, entitled BACK CONSTRUCTION FOR SEATING UNIT, which is a continuation of application Ser. No. 11/048,091, filed Feb. 1, 2005 now U.S. Pat. No. 7,131,700, entitled BACK CONSTRUCTION FOR SEATING UNIT, which is a continuation of application Ser. No. 10/945,838, filed Sep. 21, 2004 now U.S. Pat. No. 7,114,777, entitled CHAIR HAVING RECLINEABLE BACK AND MOVABLE SEAT, which is a continuation of application Ser. No. 10/439,409, filed May 16, 2003, entitled SEATING UNIT WITH VARIABLE BACK STOP AND SEAT BIAS (now U.S. Pat. No. 6,817,668), which is a continuation of application Ser. No. 10/376,535, filed Feb. 28, 2003 now U.S. Pat. No. 6,905,171, entitled SEATING UNIT INCLUDING NOVEL BACK CONSTRUCTION, which is a continuation of application Ser. No. 10/214,543, filed Aug. 8, 2002, entitled SEATING UNIT INCLUDING NOVEL BACK CONSTRUCTION (now U.S. Pat. No. 6,749,261), which is a continuation of application Ser. No. 09/921,059, filed Aug. 2, 2001, entitled SEATING UNIT INCLUDING NOVEL BACK CONSTRUCTION (now U.S. Pat. No. 6,460,928), which is a divisional of application Ser. No. 09/694,041, filed Oct. 20, 2000, entitled SEATING UNIT INCLUDING NOVEL BACK (now U.S. Pat. No. 6,349,992), which is a continuation of application Ser. No. 09/491,975, filed Jan. 27, 2000, entitled BACK FOR SEATING UNIT (now U.S. Pat. No. 6,367,877), which is a continuation of application Ser. No. 09/386,668, filed Aug. 31, 1999, entitled CHAIR CONTROL HAVING ADJUSTABLE ENERGY MECHANISM (now U.S. Pat. No. 6,116,695), which is a divisional of application Ser. No. 08/957,506, filed Oct. 24, 1997, entitled CHAIR WITH RECLINEABLE BACK AND ADJUSTABLE ENERGY MECHANISM (now U.S. Pat. No. 6,086,153). 

   This application is also related to the following co-assigned patents and applications. The disclosure of each of these patents and applications is incorporated herein by reference in its entirety: 
   
     
       
         
             
             
             
           
             
                 
             
             
               TITLE 
               PATENT NO. 
               ISSUE DATE 
             
             
                 
             
           
          
             
               Chair Including 
               5,975,634 
               11/02/99 
             
             
               Novel Back Construction 
             
             
               Chair With Novel Seat Construction 
               5,871,258 
               02/16/99 
             
             
               Chair with Novel Pivot Mounts and 
               5,909,923 
               06/08/99 
             
             
               Method of Assembly 
             
             
               Synchrotilt Chair with 
               5,979,984 
               11/09/99 
             
             
               Forwardly Movable Seat 
             
             
               Seating Unit with Reclineable Back 
               6,394,549 
               05/28/02 
             
             
               And Forwardly Movable Seat 
             
             
               Seating Unit with Novel 
               6,394,548 
               05/28/02 
             
             
               Seat Construction 
             
             
               Seating Unit with Novel Pivot Mounts 
               6,318,800 
               11/20/01 
             
             
               And Method of Assembly 
             
             
               Back for Seating Unit 
               6,394,545 
               05/28/02 
             
             
               Seating Unit with Novel Pivot 
               6,318,800 
               11/20/01 
             
             
               Mounts and Method of Assembly 
             
             
               Seating Unit with Novel Seat 
               6,394,548 
               05/28/02 
             
             
               Seating Unit with Reclinable Back 
               6,394,549 
               05/28/02 
             
             
               And Forwardly Movable Seat 
             
             
                 
             
          
         
       
     
   
   BACKGROUND 
   The present invention concerns seating units having a reclineable back, and more particularly concerns seating units having a reclineable back with flexible lumbar region. 
   A synchrotilt chair is described in U.S. Pat. No. 5,050,931 (to Knoblock) having a base assembly with a control, a reclineable back pivoted to the control, and a seat operably mounted to the back and control for synchronous motion as the back is reclined. This prior art chair incorporates a semi-rigid flexible shell that, in combination with the chair support structure, provides a highly-controlled postural support during the body movements associated with tasks/work (e.g., when the back is in an upright position) and during the body movements associated with recline/relaxation (e.g., when the chair is in a reclined position). This prior art chair moves a seated user&#39;s upper body away from the user&#39;s work surface as the user reclines, thus providing the user with more area to stretch. In fact, moving around in a chair and not staying in a single static position is important to good back health in workers whose jobs require a lot of sitting. However, users often want to remain close to their work surface and want to continue to work at the work surface, even while reclining and relaxing their body and while having continued good postural support. Further, workers often want to selectively choose the amount of maximum recline. In other words, workers often want to lean backward (i.e. recline) a small amount in an intermediate recline position, and yet simultaneously stay an appropriate distance from their work surface. also, workers prefer not to “fight” with the chair to stay in the intermediate partial-recline positions. 
   Modern customers and chair purchasers also demand a wide variety of chair options and features, and a number of options and features are often designed into chair seats. It is important that such options and features be incorporated into the chair construction in a way that minimizes the number of parts and maximizes the use of common parts among different options, maximizes efficiencies of manufacturing and assembling, maximizes ease of adjustment and the logicalness of adjustment control positioning, and yet that results in a visually pleasing design. 
   Accordingly, a chair construction solving the aforementioned problems is desired. 
   SUMMARY OF INVENTION 
   In one aspect of the present invention, a seating unit comprises a base, a seat operably supported on the base, and a back frame pivoted to the base. The seating unit further includes a flexible back with an upper section pivoted to the back frame and a lower section that defines with the upper section a forwardly protruding lumbar portion for lumbar support to a seated user. The lower section is operably connected to one of the seat, the back frame, or the base with the connection being made at a location forward of a center region of a lower edge of the flexible back. A spring mechanism is connected to a center of the back frame and to a location near the lower edge of the flexible back. 
   In another aspect of the present invention, a method of seating comprises providing to a user of a base and a seat operably supported on the base, and a back frame which is pivoted to the base. The method also includes providing a flexible back that has an upper section pivoted to the back frame and a lower section that together define a forwardly protruding lumbar portion for lumbar support to the user, with the lower section being pivoted to the seat, the back frame, or the base at a location forward of a center region of the lower edge of the flexible back. The method includes connecting a spring mechanism between a center of the back frame and to the flexible back at a location near the lower edge and biasing the forwardly protruding lumbar portion using the spring mechanism to a forwardly protruding shape providing optimal lumbar support to the seated user. 
   These and other features and advantages of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 

   
     DESCRIPTION OF FIGURES 
       FIGS. 1-3  are front, rear, and side perspective views of a reclineable chair embodying the present invention; 
       FIGS. 4A and 4B  are exploded perspective views of upper and lower portions of the chair shown in  FIG. 1 ; 
       FIGS. 5 and 6  are side views of the chair shown in  FIG. 1 ,  FIG. 5  showing the flexibility and adjustability of the chair when in the upright position and  FIG. 6  showing the movements of the back and seat during recline; 
       FIG. 7  is a front view of the chair shown in  FIG. 1  with an underseat aesthetic cover removed; 
       FIG. 8  is a top view of the control including the primary energy mechanism, the moment arm shift adjustment mechanism, and the back-stop mechanism, the primary energy mechanism being adjusted to a relatively low torque position and being oriented as it would be when the back is in the upright position so that the seat is in its rearward at-rest position, the back-stop mechanism being in an intermediate position for limiting the back to allow a maximum recline; 
       FIG. 8A  is a perspective view of the base frame and the chair control shown in  FIG. 8 , some of the seat and back support structure being shown in phantom lines and some of the controls on the control being shown in solid lines to show relative locations thereof; 
       FIG. 9  is a perspective view of the control and primary energy mechanism shown in  FIG. 8 , the primary energy mechanism being adjusted to a low torque position and shown as if the back is in an upright position such that the seat is moved rearwardly; 
       FIG. 9A  is a perspective view of the control and primary energy mechanism shown in  FIG. 9 , the primary energy mechanism being adjusted to the low torque position but shown as if the back is in a reclined position such that the seat is moved forwardly and the spring is compressed; 
       FIG. 9B  is a perspective view of the control and primary energy mechanism shown in  FIG. 9 , the primary energy mechanism being adjusted to a high torque position and shown as if the back is in an upright position such that the seat is moved rearwardly; 
       FIG. 9C  is a perspective view of the control and primary energy mechanism shown in  FIG. 9 , the primary energy mechanism being adjusted to the high torque position but shown as if the back is in a reclined position such that the seat is moved forwardly and the spring is compressed; 
       FIG. 9D  is a graph showing torsional force versus angular deflection curves for the primary energy mechanism of  FIGS. 9-9C , the curves including a top curve showing the forces resulting from the high torque (long moment arm engagement of the main spring) and a bottom curve showing the forces resulting from the low torque (short moment arm engagement of the main spring); 
       FIG. 10  is an enlarged top view of the control and primary energy mechanism shown in  FIG. 8 , including controls for operating the back-stop mechanism, the back-stop mechanism being shown in an off position; 
       FIG. 11  is an exploded view of the mechanism for adjusting the primary energy mechanism, including the overtorque release mechanism for same; 
       FIG. 11A  is a plan view of a modified back-stop control and related linkages;  FIG. 11B  is an enlarged fragmentary view, partially in cross-section, of the circled area in  FIG. 11A ; and  FIG. 11C  is a cross-sectional view taken along the line XIC-XIC in  FIG. 11A ; 
       FIG. 12  is a side view of the back assembly shown in  FIG. 1  including the back frame and the flexible back shell and including the skeleton and flesh of a seated user, the back shell being shown with a forwardly-convex shape in solid lines and being shown in different flexed shapes in dashed and dotted lines; 
       FIG. 12A  is an enlarged perspective view of the back frame shown in  FIG. 4A , the back frame being shown as if the molded polymeric outer shell is transparent so that the reinforcement can be easily seen; 
       FIGS. 12B and 12C  are cross-sections taken along lines XXIIB-XXIIB and XXIIC-XXIIC in  FIG. 12A ; 
       FIGS. 12D-12I  are views showing additional embodiments of flexible back shell constructions adapted to move sympathetically with a seated user&#39;s back; 
       FIG. 12J  is an exploded perspective view of the torsionally-adjustable lumbar support spring mechanism shown in  FIG. 4A , and FIG.  12 JJ is an exploded view of the hub and spring connection of  FIG. 12J  taken from an opposite side of the hub; 
       FIG. 12K  is an exploded perspective view of a modified torsionally-adjustable lumbar support spring mechanism; 
     FIGS.  12 L and  12 LL are side views of the mechanism shown in  FIG. 12K  adjusted to a low torque position, and FIGS.  12 M and  12 MM are side views of the mechanism adjusted to a high torque position,  FIGS. 12L and 12M  highlighting the spring driver, and FIGS.  12 LL and  12 MM highlighting the lever; 
       FIG. 12N  is a fragmentary cross-sectional side view of the back construction shown in  FIG. 12 ; 
       FIG. 13  is a cross-sectional side view taken along lines XIII-XIII showing the pivots that interconnect the base frame to the back frame and that interconnect the back frame to the seat frame; 
       FIG. 13A  is a cross-sectional side view of modified pivots similar to  FIG. 13 , but showing an alternative construction; 
       FIGS. 14A and 14B  are perspective and front views of the top connector connecting the back shell to the back frame; 
       FIG. 15  is a rear view of the back shell shown in  FIG. 4A ; 
       FIG. 16  is a perspective view of the back including the vertically-adjustable lumbar support mechanism shown in  FIG. 4A ; 
       FIGS. 17 and 18  are front and top views of the vertically-adjustable lumbar support mechanism shown in  FIG. 16 ; 
       FIG. 19  is a front view of the slide frame of the vertically-adjustable lumbar support mechanism shown in  FIG. 18 ; 
       FIG. 20  is a top view, partially in cross-section, of the laterally-extending handle of the vertically-adjustable lumbar support mechanism shown in  FIG. 17  and its attachment to the slide member of the lumbar support mechanism; 
       FIG. 21  is a perspective view of the depth-adjustable seat shown in  FIG. 4B  including the seat carrier and the seat undercarriage/support frame slidably mounted on the seat carrier, the seat undercarriage/support frame being partially broken away to show the bearings on the seat carrier, the seat cushion being removed to reveal the parts therebelow; 
       FIG. 22  is a top view of the seat carrier shown in  FIG. 21 , the seat undercarriage/rear frame being removed but the seat frame slide bearings being shown and the seat carrier depth-adjuster stop device being shown; 
       FIG. 23  is a top perspective view of the seat undercarriage/rear frame and the seat carrier shown in  FIG. 21  including a depth-adjuster control handle, a linkage, and a latch for holding a selected depth position of the seat; 
       FIGS. 24 and 25  are side views of the depth-adjustable seat shown in  FIG. 21 ,  FIG. 24  showing the seat adjusted to maximize seat depth, and  FIG. 25  showing the seat adjusted to minimize seat depth;  FIGS. 24 and 25  also showing a manually-adjustable “active” thigh support system including a gas spring for adjusting a front portion of the seat shell to provide optimal thigh support; 
       FIG. 26  is a top view of the seat support structure shown in  FIGS. 24 and 25  including the seat carrier (shown mostly in dashed lines), the seat undercarriage/rear frame, the active thigh support system with gas spring and reinforcement plate for adjustably supporting the front portion of the seat, and portions of the depth-adjustment mechanism including a stop for limiting the maximum forward and rearward depth adjustment of the seat and the depth-setting latch; 
       FIG. 26A  is a cross-section taken along line XXVIA-XXVIA in  FIG. 26  showing the stop for the depth-adjuster mechanism; 
       FIGS. 27 and 28  are top and bottom perspective views of the seat support structure shown in  FIG. 26 ; 
       FIGS. 29 and 30  are top and bottom perspective views of a seat similar to that shown in  FIG. 26 , but where the manually-adjustable thigh support system is replaced with a passive thigh support system including a leaf spring for supporting a front portion of the seat; and 
       FIG. 31  is a bottom perspective view of the brackets and guide for supporting ends of the leaf spring as shown in  FIG. 30 , but with the thigh-supporting front portion of the seat flexed downwardly causing the leaf spring to flex toward a flat compressed condition. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1  with a person seated in the chair. However, it is to be understood that the invention may assume various alternative orientations, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as unnecessarily limiting, unless the claims expressly state otherwise. 
   A chair construction  20  ( FIGS. 1 and 2 ) embodying the present invention (sometimes referred to herein as a “seating unit”) includes a castored base assembly  21  and a reclineable back assembly  22  pivoted to the base  21  for movement about a stationary back-tilt axis  23  between upright and reclined positions. A seat assembly  24  ( FIG. 6 ) is pivoted at its rear to the back  22  for movement about a seat-tilt axis  25 . Seat-tilt axis  25  is offset rearwardly and downwardly from the back-tilt axis  23 , and the seat  24  is slidably supported at its front on the base  21  by linear bearings, such that the seat  24  slides forwardly and its rear rotates downwardly and forwardly with a synchrotilt movement as the back  22  is reclined (see  FIG. 6 ). The synchronous motion initially moves the back to seat at an angular synchronous ratio of about 2.5:1, and when near the fully reclined position moves the back to seat at an angular synchronous ratio of about 5:1. The seat  24  and back  22  movement during recline provides an exceptionally comfortable ride that makes the seated user feel stable and secure. This is due in part to the fact that the movement keeps the seated user&#39;s center of gravity relatively constant and keeps the seated user in a relatively balanced position over the chair base. Also, the forward slide/synchronous motion keeps the seated user near his/her work during recline more than in previous synchrotilt chair constructions, such that the problem of constantly scooting forward after reclining and then scooting rearward when moving toward an upright position is greatly reduced, if not eliminated. Another advantage is that the chair construction  20  can be used close to a wall behind the chair or in a small office, with less problems resulting from interference from office furnishings during recline. Still further, we have found that the spring  28  for biasing the back  22  toward an upright position can be potentially reduced in size because of the reduced rearward shifting of a seated user&#39;s weight in the present chair. 
   The base  21  includes a control housing  26 . A primary energy mechanism  27  ( FIG. 8 ) is operably positioned in control housing  26  for biasing the seat  24  rearwardly. Due to the interconnection of the back  22  and the seat  24 , the rearward bias of the seat  24  in turn biases the back  22  toward an upright position. Primary energy mechanism  27  ( FIG. 8 ) includes a main spring  28  positioned transversely in the control housing  26  that operably engages a torque member or lever  54 . The tension and torque provided by the main spring  28  is adjustable via an adjustable moment arm shift (MAS) system  29  also positioned substantially in the control housing  26 . A visual cover  26 ′ ( FIG. 1 ) covers the area between the control housing  26  and the underside of the seat  24 . The back assembly  22  includes a back support or back frame  30  ( FIG. 4A ) with structure that defines pivots/axes  23  and  25 . A flexible/compliant back shell construction  31  is pivoted to back frame  30  at top connections  32  and bottom connections  33  in a manner providing an exceptionally comfortable and sympathetic back support. A torsionally-adjustable lumbar support spring mechanism  34  is provided to bias the back shell  31  forwardly into a forwardly-convex curvilinear shape optimally suited for providing good lumbar pressure. A vertically-adjustable lumbar support  35  ( FIG. 16 ) is operatively mounted on back shell  31  for vertical movement to provide an optimal shape and pressure location to the front support surface on back  22 . The seat  24  is provided with various options to provide enhanced chair functions, such as a back-stop mechanism  36  ( FIG. 8 ) which adjustably engages the seat  24  to limit recline of the back  22 . Also, the seat  24  can include active and passive thigh support options (see  FIGS. 24 and 30 , respectively), seat depth adjustment (see  FIGS. 28 and 25 ), and other seat options, as described below. 
   Base Assembly 
   The base assembly  21  ( FIG. 1 ) includes a floor-engaging support  39  having a center hub  40  and radially-extending castored legs  41  attached to the center hub  40  in a spider-like configuration. A telescopingly-extendable center post  42  is positioned in center hub  40  and includes a gas spring that is operable to telescopingly extend the post  42  to raise the height of the chair. The control housing  26  is pan shaped ( FIG. 11 ) and includes bottom panels and flanged sidewalls forming an upwardly-open structural member. A notch  43  is formed in one sidewall of the housing  26  for receiving a portion of the adjustable control for the MAS system  29 . A front of the housing  26  is formed into an upwardly-facing U-shaped transverse flange  44  for receiving a transverse structural tube  45  ( FIG. 8A ), and a hole  46  ( FIG. 11 ) is formed generally adjacent flange  44 . The transverse tube  45  is welded to the flange  44  and extends substantially horizontally. A reinforcement channel  47  is welded in housing  26  of base assembly  21  immediately in front of transverse structural tube  45 . A frustoconical tube section  48  is welded vertically to reinforcement  47  above hole  46 , which tube section  48  is shaped to mateably and securely engage the upper end of extendable center post  42 . A pair of stiff upwardly-extending side arms  49  (sometimes also called “struts” or “pods”) are welded to the opposing ends of transverse tube  45 . The side arms  49  each include a stiff plate  50  on their inside surface. The plates  50  include weld nuts  51  that align to define the back-tilt axis  23 . The housing  26 , transverse tube  45 , and side arms  49  form a base frame that is rigid and sturdy. The sidewalls of the housing  26  include a lip or flange that extends along their upper edge to reinforce the sidewalls. A cap  52  is attached to the lips to form a stationary part of a linear bearing for slidably supporting a front of the seat. 
   Primary Energy Mechanism and Operation 
   It is noted that the housing  26  shown in  FIGS. 9-9C  and  10  is slightly longer and with different proportions than the housing of  FIGS. 8 ,  8 A, and  11 , but the principles of operation are the same. The primary energy mechanism  27  ( FIG. 8 ) is positioned in housing  26 . The primary energy mechanism  27  includes the spring  28 , which is operably connected to the seat  24  by an L-shaped torque member or bell crank  54 , a link  55 , and a seat-attached bracket  56 . The spring  28  is a coil spring transversely positioned in housing  26 , with one end supported against a side of housing  26  by a disc-shaped anchor  57 . The anchor  57  includes a washer to support the end of the spring  28  to prevent noise, and further includes a protrusion that extends into a center of the end of the spring  28  to securely grip the spring  28 , but that allows the spring  28  to be compressed and to tilt/flex toward a side while the torque member or bell crank  54  is being pivoted. The L-shaped torque member or bell crank  54  includes a short leg or lever  58  and a long leg  59 . The short leg  58  has a free end that engages an end of the spring  28  generally proximate a left side of housing  26  with a washer and protrusion similar to anchor  57 . Short leg  58  is arcuately shaped and includes an outer surface facing the adjacent sidewall of housing  26  that defines a series of teeth  60 . Steel strips  61  are attached to the top and bottom sides of the short leg  58  and have an outer arcuate surface that provides a smooth rolling bearing surface on the leg  58 , as described below. The arcuate surface of the strips  61  is generally located at about the apex or the pitch diameter of the gear teeth  60 . The short leg  58  extends generally perpendicular to a longitudinal direction of spring  28  and the long leg  59  extends generally parallel the length of spring  28 , but is spaced from the spring  28 . Link  55  ( FIG. 8 ) is pivoted to an end of long leg  59  and is also pivoted to the seat-attached bracket  56 . 
   A crescent-shaped pivot member  63  ( FIG. 11 ) includes an arcuate roller bearing surface that rollingly engages the curved surface of steel strips  61  on short leg  58  to define a moving fulcrum point. Pivot member  63  also includes a rack of teeth  64  configured to mateably engage the teeth  60  on short leg  58  to prevent any slippage between the interfacing roller bearing surfaces of leg  58  and pivot member  63 . Pivot member  63  is attached to a side of the housing  26  at the notch  43 . When the seat  24  is in a rearward position (i.e., the back is in an upright position) ( FIG. 9 ), the long leg  59  is located generally parallel and close to the spring  28  and the short leg  58  is pivoted so that the spring  28  has a relatively low amount of compression. In this position, the compression of spring  28  is sufficient to adequately bias the seat  24  rearwardly and in turn bias the back frame  30  to an upright position for optimal yet comfortable support to a seated user. As a seated user reclines, the seat  24  is moved forwardly ( FIG. 9A ). This causes the L-shaped torque member or bell crank  54  to roll on pivot member  63  at the fulcrum point in a manner compressing spring  28 . As a result, spring  28  provides increasing force resisting the recline, which increasing force is needed to adequately support a person as they recline. Notably, the short leg  58  “walks” along the crescent-shaped pivot member  63  a short distance during recline, such that the actual pivot location changes slightly during recline. The generous curvilinear shapes of the short leg  58  and the pivot member  63  prevent any abrupt change in the support to the back during recline, but it is noted that the curvilinear shapes of these two components affect the spring compression in two ways. The “walking” of the short leg  58  on the pivot member  63  affects the length of the moment arm to the actual pivot point (i.e., the location where the teeth  60  and  64  actually engage at any specific point in time). Also, the “walking” can cause the spring  28  to be longitudinally compressed as the “walking” occurs. However, in a preferred form, we have designed the system so that the spring  28  is not substantially compressed during adjustment of the pivot member  63 , for the reason that we want the adjustment to be easily accomplished. If adjustment caused the spring  28  to be compressed, the adjustment would require extra effort to perform the adjustment, which we do not prefer in this chair design. 
   As discussed below, the pivot member  63  is adjustable to change the torque arm over which the spring  28  operates.  FIG. 9B  shows the primary energy mechanism  27  adjusted to a high torque position with the seat  24  being in a rearward position (and the back frame  30  being in an upright position).  FIG. 9C  shows the primary energy mechanism  27  still adjusted to the high torque condition, but in the compressed condition with the seat  24  in a forward position (and the back frame  30  being in an upright position). Notably, in  FIGS. 9B and 9C , the pivot member  63  has been adjusted to provide a longer torque arm on lever  58  over which the spring  28  acts. 
     FIG. 9D  is a graph illustrating the back torque generated by spring  28  as a function of the angle of recline. As apparent from the graph, the initial force of support can be varied by adjustment (as described below). Further, the rate of change of torsional force (i.e., the slope) varies automatically as the initial torsional force is adjusted to a higher force, such that a lower initial spring force results in a flatter slope, while a higher initial spring force results in a steeper slope. This is advantageous since lighter/smaller people not only require less support in the upright position of the chair, but also require less support during recline. Contrastingly, heavier/larger people require greater support when in upright and reclined positions. Notably, the desired slope of the high and low torque force/displacement curves can be designed into the chair by varying the shape of the short leg  58  and the pivot member  63 . 
   The crescent-shaped pivot member  63  ( FIG. 11 ) is pivotally supported on housing  26  by a bracket  65 . The bracket  65  includes a tube section  66  and a configured end  67  with a juncture therebetween configured to mateably engage the notch  43  in the side of housing  26 . The configured end  67  includes a pair of flanges  68  with apertures defining an axis of rotation  69  for the pivot member  63 . The pivot member  63  is pivoted to the flanges  68  by a pivot pin and is rotatable around the axis  69 . By rotating the pivot member  63 , the engagement of teeth  60  and  64  and the related interfacing surfaces change in a manner causing the actual pivot point along short leg  58  of L-shaped torque member or bell crank  54  to change. (Compare  FIGS. 9 and 9B .) As a result, the distance from the end of spring  28  to the actual pivot point changes. This results in a shortening (or lengthening) in the torque arm over which the spring  28  operates, which in turn results in a substantial change in the force/displacement curve (compare the top and bottom curves in  FIG. 9D ). The change in moment arm is relatively easily accomplished because the spring  28  is not compressed substantially during adjustment, since the interfacing surface on pivot member  63  defines a constant radius around its axis of rotation. Thus, adjustment is not adversely affected by the strength of spring  28 . Nonetheless, the adjustment greatly affects the spring curve because of the resulting change in the length of the moment arm over which the spring  28  operates. 
   Pivoting of the pivot member  63  is accomplished through use of a pair of apertured flanges  70  ( FIG. 11 ) on the pivot member  63  that are spaced from axis  69 . An adjustment rod  71  extends through tube section  66  into configured end  67  and is pivoted to the apertured flanges  70 . Rod  71  includes a threaded opposite end  72 . An elongated nut  73  is threaded onto rod end  72 . Nut  73  includes a washer  73 ′ that rotatably engages an end of the tube section  66 , and further includes a configured end  74  having longitudinally-extending ribs or slots shaped to mateably telescopingly engage mating ribs  75  on a driving ring  76 . A handle  77  is rotatably mounted on tube section  66  and is operably connected to the driving ring  76  by an overtorque clutch ring  78 . Clutch ring  78  includes resilient fingers  79  that operably engage a ring of friction teeth  80  on the driving ring  76 . Fingers  79  are shaped to frictionally slip over teeth  80  at a predetermined torsional load to prevent damage to components of the chair  20 . A retainer  81  includes resilient legs  81 ′ that snappingly engage the end  74  of the nut  73  to retain the driving ring  76  and the clutch ring  78  together with a predetermined amount of force. A spacer/washer  82  rides on the end of the nut  73  to provide a bearing surface to better support the clutch ring  78  for rotation. An end cap  83  visually covers an end of the assembly. The end cap  83  includes a center protrusion  84  that snaps into the retainer  81  to forcibly keep the resilient legs of the retainer  81  engaged in the end of the nut  73 . 
   In use, adjustment is accomplished by rotating the handle  77  on tube section  66 , which causes nut  73  to rotate by means of clutch ring  78  and driving ring  76  (unless the force required for rotation of the nut  73  is so great that the clutch ring  78  slips on driving ring  76  to prevent damage to the components). As the nut  73  rotates, the rod  71  is drawn outwardly (or pressed inwardly) from the housing  26 , causing the pivot member  63  to rotate. Pivoting the pivot member  63  changes the point of engagement (i.e. fulcrum point) of the pivot member  63  and the short leg  58  of the L-shaped torque member or bell crank  54 , thus changing the moment arm over which the spring  28  acts. 
   Back-Stop Mechanism 
   The back-stop mechanism  36  ( FIG. 8 ) includes a cam  86  pivoted to the housing  26  at location  87 . The cam  86  includes stop surfaces or steps  88 , detent depressions  89  that correspond to surfaces  88 , and teeth  90 . The steps  88  are shaped to mateably engage the seat-attached bracket  56  to limit the rearward rotation of the back frame  30  by limiting the rearward movement of the seat  24 . This allows a seated user to limit the amount of recline to a desired maximum point. A leaf spring  91  ( FIG. 10 ) is attached to the housing  26  by use of a U-shaped finger  92  that slips through a first hole and hooks into a second hole in the housing  26 . The opposite end of the leaf spring includes a U-shaped bend  93  shaped to mateably slidably engage the detent depressions  89 . The depressions  89  correspond to the steps  88  so that, when a particular step  88  is selected, a corresponding depression  89  is engaged by spring  91  to hold the cam  86  in the selected angular position. Notably, the steps  88  (and the depressions  89 ) are located angularly close together in the area corresponding to chair positions close to the upright position of the back frame  30 , and are located angularly farther apart in the area corresponding to more fully reclined chair positions. This is done so that seated users can select from a greater number of back-stopping positions when near an upright position. It is noted that seated users are likely to want multiple back-stopping positions that are close together when near an upright position, and are less likely to select a back-stopping position that is near the fully reclined chair position. 
   The cam  86  is rotated through use of a control that includes a pivoting lever  94 , a link  95 , and a rotatable handle  96 . The pivoting lever  94  is pivoted generally at its middle to the housing  26  at location  97 . One end of the pivoting lever  94  includes teeth  98  that engage teeth  90  of cam  86 . The other end of lever  94  is pivoted to rigid link  95  at location  97 ′. Handle  96  includes a body  101  that is rotatably mounted on tube section  66  of MAS pivot bracket  65 , and further includes a flipper  99  that provides easy grasping to a seated user. A protrusion  100  extends from the body and is pivotally attached to link  95 . 
   To adjust the back-stop mechanism  36 , the handle  96  is rotated, which rotates cam  86  through operation of link  95  and lever  94 . The cam  86  is rotated to a desired angular position so that the selected step  87  engages the seat-attached bracket  56  to prevent any further recline beyond the defined back-stop point. Since the seat  24  is attached to the back frame  30 , this limits recline of the back  22 . 
   A modified control for operating the back-stop cam  86  is shown in  FIG. 11A . The modified control includes a pivoting lever  94 A and rotatable handle  96 A connected to the handle  96 A by a rotary pivot/slide joint  380 . The lever  94 A includes teeth  381  that engage cam  86  and is pivoted to housing  26  at pivot  97 , both of which are like lever  94 . However, in the modified control, link  95  is eliminated and replaced with the single joint  380 . Joint  380  includes a ball  381  ( FIG. 11B ) that extends from the lever  94 A. A snap-on “car” or bearing  382  includes a socket  383  for pivotally engaging ball  381  to define a ball-and-socket joint. The bearing  382  includes outer surfaces  384  that slidably engage a slot  385  in a radially-extending arm  386  on handle  96 A ( FIG. 11C ). The joint  380  operably connects the handle  96 A to the lever  94 A, despite the complex movement resulting from rotation of the handle  96 A about a first axis, and from rotation of the lever  94 A about a second axis that is skewed relative to the first axis. Advantageously, the modified control provides an operable interconnection with few parts, and with parts that are partially inside of the control housing  26 , such that the parts are substantially hidden from view to a person standing beside the chair. 
   Back Construction 
   The back frame  30  and back shell  31  ( FIG. 12 ) form a compliant back support for a seated user that is particularly comfortable and sympathetic to back movements of the seated user, particularly in the lumbar area of the back  22 . Adjustment features on the assembly provide further comfort and allow a seated user to customize the chair to meet his/her particular needs and preferences in the upright through reclined positions. 
   The back frame  30  ( FIG. 12A ) is curvilinearly shaped and forms an arch across the back area of the chair  20 . A variety of constructions are contemplated for back frame  30 , and accordingly, the present invention should not be improperly limited to only a particular one. For example, the back frame  30  could be entirely metal, plastic, or a combination thereof. Also, the rigid internal reinforcement  102  described below could be tubular, angle iron, or a stamping. The illustrated back frame  30  includes a looping or arch-shaped internal metal reinforcement  102  and an outer molded-on polymeric skin or covering  103 . (For illustrative purposes, the covering  103  is shown as if it is transparent ( FIG. 12A ), so that the reinforcement  102  is easily seen.) The metal reinforcement  102  includes a looping intermediate rod section  104  (only half of which is shown in  FIG. 12A ) having a circular cross-section. Reinforcement  102  further includes configured ends/brackets  105  welded onto the ends of the intermediate section  104 . One or two of T-shaped top pivot connectors  107  are attached to intermediate section  104  near a top portion thereof. Notably, a single top connector  107 , when used, allows greater side-to-side flexibility than with two top connectors, which may be desired in a chair where the user is expected to often twist his/her torso and lean to a side in the chair. A pair of spaced-apart top connectors  107  provide a stiffer arrangement. Each connector  107  ( FIG. 12B ) includes a stem  108  welded to intermediate section  104  and includes a transverse rod section  109  extended through stem  108 . The rod section  109  is located outboard of the skin or shell  103  and is adapted to snap-in frictionally and pivotally engage a mating recess in the back shell  31  for rotation about a horizontal axis, as described below. The present invention is contemplated to include different back frame shapes. For example, the inverted U-shaped intermediate section  104  of back frame  30  can be replaced with an inverted T-shaped intermediate section having a lower transverse member that is generally proximate and parallel the belt bracket  132 , and a vertical member that extends upwardly therefrom. In a preferred form, each back frame of the present chair defines spaced-apart lower connections or apertures  113  that define pivot points and a top connection(s)  107  forming a triangular tripod-like arrangement. This arrangement combines with the semi-rigid resiliently-flexible back shell  31  to posturally flexibly support and permit torsional flexing of a seated user&#39;s torso when in the chair. In an alternative form, the lower connections  113  could occur on the seat instead of the back of the chair. 
   The configured ends  105  include an inner surface  10 ′ ( FIG. 13 ) that may or may not be covered by the outer shell  103 . In the illustrated back frame  30  of  FIGS. 12A and 4A , the reinforcement  102  is substantially covered by the shell  103 , but a pocket is formed on an inside surface at configured ends  105  at apertures  111 - 113 . The configured ends  105  include extruded flanges forming apertures  111 - 113  which in turn define the back-tilt axis  23 , the seat-tilt axis  25 , and a bottom pivotal connection for the back shell  31 , respectively. The apertures  111  and  112  ( FIG. 13 ) include frustoconically-shaped flanges  116  defining pockets for receiving multi-piece bearings  114  and  115 , respectively. Bearing  114  includes an outer rubber bushing  117  engaging the flanges  116  and an inner lubricous bearing element  118 . A pivot stud  119  includes a second lubricous bearing element  120  that matingly slidingly engages the first bearing element  118 . The stud  119  is extended through bearing  114  in an outward direction and threaded into welded nut  51  on side arms  49  of the base frames  26 ,  45 , and  49 . The bearing element  118  bottoms out on the nut  51  to prevent over-tightening of the stud  119 . The head of the stud  119  is shaped to slide through the aperture  111  to facilitate assembly by allowing the stud to be threaded into nut  51  from the inboard side of the side arm  49 . It is noted that the head of stud  119  can be enlarged to positively capture the configured end  105  to the side arm  49  if desired. The present arrangement including the rubber bushings  117  allows the pivot  23  to flex and compensate for rotation that is not perfectly aligned with the axis  23 , thus reducing the stress on the bearings and reducing the stress on components of the chair such as on the back frame  30  and the side arms  49  where the stud  119  is misaligned with its axis. 
   The lower seat-to-back frame bearing  115  is similar to bearing  114  in that bearing  115  includes a rubber bushing  121  and a lubricous bearing element  122 , although it is noted that the frustoconical surface faces inwardly. A welded stud  123  extends from seat carrier  124  and includes a lubricous bearing element  125  for rotatably and slidably engaging the bearing element  122 . It is noted that in the illustrated arrangement, the configured end  105  is trapped between the side arms  49  of base frames  26 ,  45 , and  49  and the seat carrier  124 , such that the bearings  114  and  115  do not need to be positively retained to the configured ends  105 . Nonetheless, a positive bearing arrangement could be readily constructed on the pivot  112  by enlarging the head of the stud  119  and by using a similar headed stud in place of the welded stud  123 . 
   A second configuration of the configured end of back frame  30  is shown in  FIG. 13A . Similar components are identified by identical numbers, and modified components are identified with the same numbers and with the addition of the letter “A.” In the modified configured end  105 A, the frustoconical surfaces of pivots  111 A and  112 A face in opposite directions from pivots  111  and  112 . Pivot  112 A (including a welded-in stud  123 A that pivotally supports the seat carrier  124  on the back frame  30 ) includes a threaded axial hole in its outer end. A retainer screw  300  is extended into the threaded hole to positively retain the pivot assembly together. Specifically, a washer  301  on screw  300  engages and positively retains the bearing sleeve  125  that mounts the inner bearing element  122  on the pivot stud  123 A. The taper in the pocket and on the bearing outer sleeve  121  positively holds the bearing  115 A together. The upper pivot  111 A that pivotally supports the back frame  30  on the side arms  50  of the base frame is generally identical to the lower pivot  112 , except that the pivot  111 A faces in an opposite inboard direction. Specifically, in upper pivot  111 A, a stud  119 A is welded onto side arm  50 . The bearing is operably mounted on the stud  119 A in the bearing pocket defined in the base frame  30  and held in place with another washered screw  300 . For assembly, the back frame  30  is flexed apart to engage bearing  115 , and the configured ends  105 A are twisted and resiliently flexed, and thereafter are released such that they spring back to an at-rest position. This arrangement provides a quick assembly procedure that is fastenerless, secure, and readily accomplished. 
   The present back shell system shown in  FIGS. 12 ,  15 , and  16  (and the back systems of  FIGS. 12D-12I ) is compliant and designed to work sympathetically with the human back. The word “compliant” as used herein is intended to refer to the flexibility of the present back especially in the lumbar area (see FIGS.  12  and  12 F- 12 I) or a back structure that provides the equivalent of that flexibility (see  FIGS. 12D and 12E ), and the word “sympathetically” is intended to mean that the back moves in close harmony with a seated user&#39;s back as the chair back  22  is reclined and when a seated user flexes his/her lower back and posturally supports the seated user&#39;s back. The back shell  31  has three specific regions, as does the human back, those being the thoracic region, the lumbar region, and the pelvic region. 
   The thoracic “rib cage” region of a human&#39;s back is relatively stiff. For this reason, a relatively stiff upper shell portion ( FIG. 12 ) is provided that supports the relatively stiff thoracic (rib cage) region  252  of a seated user. It carries the weight of a user&#39;s torso. The upper pivot axis is strategically located directly behind the average user&#39;s upper body center of gravity, balancing his/her back weight for good pressure distribution. 
   The lumbar region  251  of a human&#39;s back is more flexible. For this reason, the shell lumbar region of back shell  31  includes two curved, vertical-living hinges  126  at its side edges ( FIG. 15 ) connected by a number of horizontal “cross straps”  125 ″. These straps  125 ″ are separated by widthwise slots  125 ′ allowing the straps to move independently. The slots  125 ′ may have radiused ends or teardrop-shaped ends to reduce concentration of stress. This shell area is configured to comfortably and posturally support the human lumbar region. Both side straps  125 ″ are flexible and able to substantially change radius of curvature from side to side. This shell region automatically changes curvature as a user changes posture, yet maintains a relatively consistent level of support. This allows a user to consciously (or subconsciously) flex his/her back during work, temporarily moving stress off of tiring muscles or spinal disc portions onto different ones. This frequent motion also “pumps” nutrients through the spine, keeping it nourished and more healthy. When a specific user leans against the shell  31 , he/she exerts unique relative pressures on the various lumbar “cross straps.” This causes the living hinges to flex in a unique way, urging the shell to conform with a user&#39;s unique back shape. This provides more uniform support over a larger area of the back improving comfort and diminishing “high pressure points.” The cross straps can also flex to better match a user&#39;s side-to-side shape. The neutral axis of the human spine is located well inside the back. Correspondingly, the “side straps” are located forward of the central portion of the lumbar region (closer to the spine neutral axis), helping the shell flexure mimic human back flexure. 
   The pelvic region  250  is rather inflexible on human beings. Accordingly, the lowest portion of the shell  31  is also rather inflexible so that it posturally/mateably supports the inflexible human pelvis. When a user flexes his/her spine rearward, the user&#39;s pelvis automatically pivots about his/her hip joint and the skin on his/her back stretches. The lower shell/back frame pivot point is strategically located near but a bit rearward of the human hip joint. Its nearness allows the shell pelvic region to rotate sympathetically with a user&#39;s pelvis. By being a bit rearward, however, the lumbar region of the shell stretches (the slots widen) somewhat less than the user&#39;s back skin, enough for good sympathetic flexure, but not so much as to stretch or bunch up clothing. 
   Specifically, the present back shell construction  31  ( FIG. 4A ) comprises a resiliently-flexible molded sheet made from polymeric material such as polypropylene, with top and bottom cushions positioned thereon (see  FIG. 4A ). The back shell  31  ( FIG. 16 ) includes a plurality of horizontal slots  125 ′ in its lower half that are located generally in the lumbar area of the chair  20 . The slots  125 ′ extend substantially across the back shell  31 , but terminate at locations spaced from the sides so that resilient vertical bands of material  126  are formed along each edge. The bands of material or side straps  126  are designed to form a naturally forwardly-convex shape, but are flexible so that they provide an optimal lumbar support and shape to a seated user. The bands  126  allow the back shell to change shape to conform to a user&#39;s back shape in a sympathetic manner, side to side and vertically. A ridge  127  extends along the perimeter of the shell  31 . A pair of spaced-apart recesses  128  are formed generally in an upper thoracic area of the back shell  31  on its rearward surface. The recesses  128  ( FIGS. 14A and 14B ) each include a T-shaped entrance with the narrow portion  129  of the recesses  128  having a width for receiving the stem  108  of the top connector  32  on the back frame  30  and with the wider portion  130  of the recesses  128  having a width shaped to receive the transverse rod section  109  of the top connector  32 . The recesses  128  each extend upwardly into the back shell  31  such that opposing flanges  131  formed adjacent the narrow portion  129  pivotally capture the rod section  109  of the T-top connector  107  as the stem  108  slides into the narrow portion  129 . Ridges  132  in the recesses  128  frictionally positively retain the top connectors  107  and secure the back shell  31  to the back frame  30 , yet allow the back shell  31  to pivot about a horizontal axis. This allows for the back shell  31  to flex for optimal lumbar support without undesired restriction. 
   A belt bracket  132  ( FIG. 16 ) includes an elongated center strip or strap  133  that matches the shape of the bottom edge of the back shell  31  and that is molded into a bottom edge of the back shell  31 . The strip  133  can also be an integral part of the back shell or can be attached to back shell  31  with screws, fasteners, adhesive, frictional tabs, insert-molding techniques, or in other ways of attaching known in the art. The strip  133  includes side arms/flanges  134  that extend forwardly from the ends of strip  133  and include apertures  135 . The torsional adjustment lumbar mechanism  34  engages the flanges  134  and pivotally attaches the back shell  31  to the back frame at location  113  ( FIG. 4A ). The torsional adjustment lumbar spring mechanism  34  is adjustable and biases the back shell  31  to a forwardly-convex shape to provide optimal lumbar support for a seated user. The torsional adjustment lumbar spring mechanism  34  cooperates with the resilient flexibility of the back shell  31  and with the shape-changing ability of the vertically-adjustable lumbar support  35  to provide a highly-adjustable and comfortable back support for a seated user. 
   The pivot location  113  is optimally chosen to be at a rear of the hip bone and somewhat above the seat  24 . (See  FIG. 12 .) Optimally, the fore/aft distance from pivot location  113  to strip  133  is approximately equal to the distance from a seated user&#39;s hip joint/axis to his/her lower spine/tail bone region so that the lower back  250  moves similarly and sympathetically to the way a seated user&#39;s lower back moves during flexure about the seated user&#39;s hip joint. The location  113  in combination with a length of the forwardly-extending side flanges  133  causes back shell  31  to flex in the following sympathetic manner. The pelvic supporting area  250  of the back shell construction  31  moves sympathetically rearwardly and downwardly along a path selected to match a person&#39;s spine and body movement as a seated user flexes his/her back and presses his/her lower back against the back shell construction  31 . The lumbar support area  251  simultaneously flexes from a forwardly-concave shape toward a more planar shape. The thoracic support area  252  rotates about top connector  107  but does not flex a substantial amount. The total angular rotation of the pelvic and thoracic supporting areas  250  and  252  are much greater than in prior art synchrotilt chairs, which provides substantially increased comfort. Notably, the back shell construction  31  also flexes in a horizontal plane to provide good postural support for a seated user who twists his/her torso to reach an object. Notably, the back frame  30  is oriented at about a 5° rearward angle from vertical when in the upright position, and rotates to about a 30° rearward angle from vertical when in the fully reclined position. Concurrently, the seat-tilt axis  25  is rearward and at an angle of about 60° below horizontal from the back-tilt axis  23  when the back frame  30  is in the upright position, and pivots to almost vertically below the back-tilt axis  23  when the back frame  30  is in the fully reclined position. 
   Back constructions  31 A- 31 F ( FIGS. 12D-12I , respectively) are additional constructions adapted to provide a sympathetic back support similar in many aspects to the back shell construction  31 . Like back construction  31 , the present invention is contemplated to include attaching the back constructions  31 A- 31 F to the seat or the base frame at bottom connections. Specifically, the illustrated constructions  31 A- 31 F are used in combination with back frame  30  to provide a specific support tailored to thoracic, lumbar, and pelvic regions of a seated user. Each of the back constructions  31 A- 31 F are pivoted at top and bottom pivot connections  107  and  113 , and each include side arms  134  for flexing about a particularly located lever pivot axis  113 . However, the back constructions  31 A- 31 F achieve their sympathetic back support in slightly different ways. 
   Back construction  31 A ( FIG. 12D ) includes a cushioned top back support  255  pivoted at top pivot connection  107 , and further includes a cushioned bottom back support  256  pivoted at bottom location  113  by the belt bracket  132  including side flanges  134 . Top and bottom back supports  255  and  256  are joined by a pivot/slide connection  257 . Pivot/slide connection  257  comprises a bottom pocket formed by a pair of flanges  258 , and top flange  259  that both slides and pivots in the pocket. A torsional lumbar support spring mechanism  34  is attached at bottom pivot location  113  and, if desired, also at connection  107  to bias top and bottom back supports  255  and  256  forwardly. The combination provides a sympathetic back support that moves with a selected user&#39;s back to match virtually any user&#39;s back shape, similar to the back shell construction  31  described above. 
   Back construction  31 B ( FIG. 12E ) includes a top back support  261  pivoted at top connection  107 , a bottom back support  262  pivoted at lower connection  113  on belt bracket side flange  134 , and an intermediate back support  262  operably positioned therebetween. Intermediate back support  262  is pivoted to bottom back support  262  at pivot  263 , and is slidably pivoted to top back support  261  at pivot/slide joint  264 . Pivot/slide joint  264  is formed by top flanges  265  defining a pocket, and another flange  266  with an end that pivots and slides in the pocket. Springs are positioned at one or more joints  107 ,  113 , and  264  to bias the back construction  260  to a forwardly-concave shape. 
   Back construction  31 C ( FIG. 12F ) is similar to back shell construction  31  in that it includes a sheet-like flexible shell with transverse lumbar slits. The shell is pivoted at top and bottom connections  107  and  113  to back frame  30 . The shell of back construction  31 C is biased toward a forwardly-convex shape by a torsional lumbar support spring mechanism  34  at bottom pivot  113  and at top pivot  107 , by a curvilinear leaf spring  271  in the lumbar area of the shell, by a spring  272  that presses the shell forwardly off of an intermediate section of back frame  30 , and/or by a vertical spring  273  that extends from top connection  107  to a rear pivot on belt bracket side flange  134 . 
   Back construction  31 D ( FIG. 12G ) includes a transverse leaf spring  276  that spans between the opposing sides of back frame  30 , and that biases the lumbar area of its back shell  277  forwardly, much like spring  272  in the back construction  270 . Back construction  31 E ( FIG. 12H ) includes vertical leaf springs  279  embedded in its back shell  280  that bias the lumbar area of back shell  280  forwardly, much like springs  271  in back construction  270 . Notably, back construction  278  includes only a single top pivot connection  107 . Back construction  31 F ( FIG. 12I ) includes a vertical spring  282  connected to a top of the back frame  30 , and to belt bracket  132  at a bottom of its back shell  283 . Since the back shell  283  is forwardly convex, the spring  282  biases the shell  283  toward an even more convex shape, thus providing additional lumbar support. (Compare to spring  273  on back construction  31 C,  FIG. 12F .) 
   It is contemplated that the torsional lumbar support spring mechanism  34  ( FIG. 12I ) can be designed in many different constructions, but includes at least a spring operably connected between the back frame  30  and the back shell  31 . Optionally, the arrangement includes a tension adjustment device having a handle and a friction latch to provide for tension adjustment. The spring biases the belt bracket  132  rotationally forward so that the back shell  31  defines a forwardly-convex shape optimally suited for lumbar support to a seated user. By rotating the handle to different latched positions, the tension of the spring is adjusted to provide an optimal forward lumbar force. As a seated user presses against the lumbar area of back shell  31 , the back shell  31  flexes “sympathetically” with a movement that mirrors a user&#39;s spine and body flesh. The force of the bands of material  126  in the shell  31  provide a relatively constant force toward their natural curvilinear shape, but when combined with the torsional lumbar support spring mechanism  34 , they provide a highly-adjustable bias force for lumbar support as the user leans against the lumbar area. It is noted that a fixed non-adjustable spring biasing the back belt of the back shell flex zone directly could be used, or that an adjustable spring only adjustable during installation could be used. However, the present adjustable device allows the greatest adjustment to meet varying needs of seated users. Thus, a user can assume a variety of well-supported back postures. 
   In the present torsional lumbar support spring mechanism  34  ( FIG. 12I ), belt bracket  132  is pivoted to back frame  30  by a stud  290  that extends inboard from back frame  30  through a hole  291  in belt bracket side flange  134 . A bushing  292  engages the stud  290  to provide for smooth rotation, and a retainer  293  holds the stud  290  in hole  291 . A base  294  is screwed by screws  294 ′ or welded to back frame  30 , and includes a protrusion  295  having a sun gear  296  and a protruding tip  297  on one end. A hub  298  includes a plate  299  with a sleeve-like boss  300  for receiving the protrusion  295 . The boss  300  has a slot  301  for receiving an inner end  302  of a spiral spring  303 . The body of spring  303  wraps around protrusion  295 , and terminates in a hooked outer end  304 . Hub  298  has a pair of axle studs  305  that extend from plate  299  in a direction opposite boss  300 . A pair of pie-shaped planet gears  306  are pivoted to axle studs  305  at pivot holes  307 . A plurality of teeth  308  are located in an arch about pivot holes  307  on the planet gears  306 , and a driver pin  309  is located at one end of the arc. A cup-shaped handle  310  is shaped to cover gears  306 , hub  298 , spring  303 , and base  294 . The handle  310  includes a flat end panel  311  having a centered hole  312  for rotatably engaging the protruding tip  297  of base  294 . A pair of opposing spirally-shaped recesses or channels  313  are formed in the end panel  311 . The recesses  313  include an inner end  314 , an outer end  315 , and an elongated portion having a plurality of detents or scallops  316  formed between the ends  314  and  315 . The recesses  313  mateably receive the driver pins  309 . The hooked outer end  304  engages fingers  317  on belt bracket  132 , which fingers  317  extend through an arcuate slot  318  in the configured end  105  of back frame  30 . 
   Handle  310  is rotated to operate torsional lumbar support spring mechanism  34 . This causes recesses  313  to engage driver pins  309  on planet gears  306 . The planet gears  306  are geared to sun gear  296 , such that planet gears  306  rotate about sun gear  296  as the driver pins  309  are forced inwardly (or outwardly) and the planet gears  306  are forced to rotate on their respective pivots/axles  305 . In turn, as planet gears  306  rotate, they force hub  298  to rotate. Due to the connection of spiral spring  303  to hub  298 , spiral spring  303  is wound tighter (or unwound). Thus, the tension of spring  303  on belt bracket  132  is adjustably changed. The detents  316  engage the driver pins  309  with enough frictional resistance to hold the spring  303  in a desired tensioned condition. Due to the arrangement, the angular winding of spiral spring  303  is greater than the angular rotation of handle  310 . 
   In a modified torsional lumbar support spring mechanism  34 A ( FIG. 12K ), a base bracket  244 A is attached to configured end  105 A of back frame  30 . A lever  306 A and driver  298 A are operably mounted on base bracket  244 A to wind a spiral spring  303 A as a handle  310 A is rotated. Specifically, the base bracket  244 A includes a pivot pin  290  that pivotally engages hole  291  in belt bracket  132 . A second pin  317  extends through arcuate slot  318  in configured end  105 A, which slot  318  extends around pivot pin  290  at a constant radius. Two pins  360  and  361  extend from base bracket  244 A opposite pivot pin  290 . The driver  298 A includes an apertured end  362  with a hole  363  for rotatably engaging center pin  360 . The end  362  includes an outer surface  364  with a slot therein for engaging an inner end  365  of spiral spring  303 A. The outer end  365  is hook-shaped to securely engage pin  317  on the belt bracket  132 . A finger-like stud  366  extends laterally from the outer end  367  of driver  298 A. 
   Lever  306 A includes a body with a hole  368  for pivotally engaging pin  361 , and a slot  369  extending arcuately around hole  368 . A pin  370  extends from lever  306 A for engaging a spiral cam slot  313 A on an inside surface of cup-shaped handle  310 A. A tooth  371  on lever  306 A is positioned to engage stud  366  on driver  298 A. Hole  372  on handle  310 A rotatably engages the pivot pin  360  on base bracket  244 A. 
   Handle  310 A is rotatable between a low tension position (FIGS.  12 L and  12 LL) and a high tension position (FIGS.  12 M and  12 MM). Specifically, as handle  310 A is rotated, pin  370  rides along slot  313 A causing lever  306 A to rotate about hole  368  and pivot pin  361 . As lever  306 A rotates, tooth  371  engages pin  366  to rotate driver  298 A about pin  360 . Rotation of driver  298 A causes the inside end  365  of spring  303 A to rotate, thus winding (or unwinding) spring  303 A. The arrangement of driver  298 A, lever  360 A, and handle  310 A provide a mechanical advantage of about 4:1, so that the spiral spring  303 A is adjustably wound with a desired amount of adjustment force on the handle  310 A. In the illustration, a rotation of about 330° of the handle  310 A produces a spring tension adjustment winding of about 80°. 
   Optionally, for maximum adjustability, a vertical adjustable lumbar system  35  ( FIG. 16 ) is provided that includes a slide frame  150  ( FIG. 19 ) that is generally flat and that includes several hooked tabs  151  on its front surface. A concave lumbar support sheet  152  ( FIG. 16 ) of flexible material such as spring steel includes a plurality of vertical slots that form resilient leaf-spring-like fingers  153  along the top and bottom edges of the sheet  152 . The (optional) height adjustable back support sheet  152  is basically a radiused sheet spring that can, with normal back support pressures, deflect until it matches the shape of the back shell beneath it. In doing so, it provides a band of higher force across the back. This provides a user with height-adjustable localized back support, regardless of the flexural shape of the user&#39;s back. Thus, it provides the benefits of a traditional lumbar height adjustment without forcing a user into a particular rigid back posture. Further, the fabric or upholstery on the back is always held taunt, such that wrinkles are eliminated. Stretch fabric can also be used to eliminate wrinkles. 
   A user may also use this device for a second reason, that reason being to more completely adapt the back shell shape to his/her own unique back shape. Especially in the lower lumbar/pelvic region, humans vary dramatically in back shape. Users with more extreme shapes will benefit by sliding the device into regions where their back does not solidly contact the shell. The device will effectively change its shape to exactly “fill in the gap” and provide good support in this area. No other known lumbar height adjuster does this in the manner described below. 
   Four tips  154  on fingers  153  form retention tabs that are particularly adapted to securely engage the hooked tabs  151  to retain the sheet  152  to the slide frame  150 . The remaining tips  155  of the fingers  153  slidably engage the slide frame  150  and hold the central portion  156  of the concave sheet forwardly and away from the slide frame  150 . The slide frame  150  is vertically adjustable on the back shell  31  ( FIG. 16 ) and is positioned on the back shell  31  between the back shell  31  and the back cushion. Alternatively, it is contemplated that the slide frame  150  could be located between the back cushion and under the upholstery covering the back  22 , or even on a front face of the back  22  outside the upholstery sheet covering the back  22 . By adjusting the slide vertically, this arrangement allows a seated user to adjust the shape of the lumbar area on the back shell  31 , thus providing a high degree of comfort. A laterally-extending guide  157  ( FIG. 19 ) is formed at each of the ends of the slide frame  150 . The guides  157  include opposing flanges  158  forming inwardly-facing grooves. Molded handles  159  ( FIG. 20 ) each include a leg  160  shaped to mateably telescopingly engage the guides  157  ( FIGS. 17 and 18 ). The handles  159  further include a C-shaped lip  160  shaped to snappingly engage and slide along the edge ridge  127  along the edge of back shell  31 . It is contemplated that other means can be provided for guiding the vertical movement of the slide frame  150  on back shell  31 , such as a cord, a track molded along but inward of the edge of the back shell, and the like. An enlarged flat end portion  161  of handle  159  extends laterally outwardly from molded handle  159 . Notably, the end portion  161  is relatively thin at a location  161 ′ immediately outboard of the lip  160 , so that the handle  159  can be extended through a relatively thin slot along the side edge of the back  22  when a cushion and upholstery sheet are attached to the back shell  31 . 
   The illustrated back  22  of  FIG. 12  includes a novel construction incorporating stretch fabric  400  sewn at location  401  to a lower edge of the upholstery sheet  402  for covering a front of the back  22 . The stretch fabric  400  is further sewn into a notch  406  in an extrusion  403  of structural plastic, such as polypropylene or polyethylene. The extrusion  403  is attached to a lower portion  404  of the back shell  31  by secure means, such as snap-in attachment, hook-in attachment, rivets, screws, other mechanical fasteners, or other means for secure attachment. The foam cushion  405  of the back  22  and the vertically-adjustable lumbar support device  35  are positioned between the sheet  402  and back shell  31 . It is contemplated that the stretch fabric will have a stretch rate of at least about 100%, with a recovery of at least 90% upon release. The stretch fabric  400  and sheet  402  are sewn onto the back  22  in a tensioned condition, so that the sheet  402  does not wrinkle or pucker despite the large flexure of the lumbar region  251  toward a planar condition. The stretch fabric  400  is in a low visibility position, but can be colored to the color of the chair if desired. It is noted that covering  402  can be extended to cover the rear of back  22  as well as its front. 
   Primary Seat Movement, Seat Undercarriage/Support Frame and Bearing Arrangement 
   The seat  24  ( FIG. 4B ) is supported by an undercarriage that includes a seat front slide  162  and the seat carrier  124 . Where seat depth adjustment is desired, a manually depth-adjustable seat frame  163  is slidably positioned on the seat carrier  124  (as is shown in FIGS.  4 B and  21 - 30 ). Where seat depth adjustment is not desired, the features of the seat frame  163  and seat rear carrier  124  can be incorporated into a single component, such as is illustrated in  FIG. 29  by frame member  163 ′. A seat shell  164  ( FIG. 4B ) includes a buttock-supporting rear section  165  that is positioned on the seat carrier  124 . The buttock-supporting rear section  165  carries most of the weight of the seated user, and acts somewhat like a perch in this regard. The seat shell  164  further includes a thigh-supporting front section  166  that extends forwardly of the seat frame  163 . Front section  166  is connected to rear section  165  by a resilient section  167  strategically located generally under and slightly forward of a seated user&#39;s hip joint. The resilient section  167  has a plurality of transverse slots  168  therein. The slots  168  are relatively short and are staggered across the seat shell  164 , but are spaced from the edges of the seat shell  164 , such that the band of material  169  at the edges of the seat shell  164  remains intact and uninterrupted. The bands  169  securely connect the front and rear sections  166  and  165  together and bias them generally toward a planar condition. A seat cushion  170  is positioned on seat frame  163  and is held in place by upholstery sheet and/or adhesive or the like. 
   Slide  162  ( FIG. 4B ) includes a top panel  171  with C-shaped side flanges  172  that extend downwardly and inwardly. A linear lubricous cap  173  is attached atop each sidewall of housing  26  and a mating bearing  174  is attached inside of C-shaped side flanges  172  for slidably engaging the lubricous cap  173 . In this way, the slide  162  is captured on the housing  26  for fore-to-aft sliding movement. The seat-attached bracket  56  is attached under the top panel  171  and is located to operate with the back-stop mechanism  36 . An axle  174 ′ is attached atop the top panel  171  and includes ends  175  that extend laterally from the slide  162 . 
   Seat carrier  124  ( FIG. 4B ) is T-shaped in plan view. Seat carrier  124  is stamped from sheet metal into a “T” shape, and includes a relatively wide rear section  176  and a narrower front section  177 . Embossments such as elongated embossments  178 ,  179 , and  180  are formed in sections  176  and  177  along with side-down flanges  181  and side-up flanges  182  to stiffen the component. Two spaced-apart stop tabs  183  and a series of latch apertures  184  are formed in the front section  177  for reasons discussed below. The welded studs  123  are attached to side-up flanges  182  and extend laterally. As discussed above, the studs  123  define the seat-tilt axis  25  at this location. 
   Seat frame  163  ( FIG. 4B ) is T-shaped, much like the seat carrier  124 , but seat frame  163  is shaped more like a pan and is generally larger than the seat carrier  124  so that it is better adapted to support the seat shell  164  and seat cushion  170 . Seat frame  163  includes a front portion  185  and a rear portion  186 . The front portion  185  includes a top panel  187  with down flanges  188  at its sides. Holes  189  at the front of down flanges  188  form a pivot axis for the active thigh flex device  190  described below. Other holes  191  spaced rearwardly of the holes  189  support an axle that extends laterally and supports a multi-functional control  192  for controlling the seat depth adjustment and for controlling the active thigh flex device  190 . The center of front portion  185  is raised and defines a sidewall  193  ( FIG. 23 ) having three apertures  194 - 196  that cooperate to pivotally and operably support a depth latch  197 . A depression  198  is formed in the center of front portion  185  and a slot  200  is cutout in the center of the depression  198 . A T-shaped stop limiter  199  ( FIG. 26 ) is positioned in the depression  198  and screw-attached therein, with the stem  201  of the limiter  199  extending downwardly through the slot  200  ( FIGS. 26 and 26A ). An inverted U-shaped bracket  203  is attached to the wide rear section  176 . The U-bracket  203  ( FIG. 28 ) includes apertures for pivotally supporting one end of a gas spring  204  used in the active thigh flex support device  190  described below. The rear section  176  ( FIG. 23 ) includes a U-shaped channel section  205  that extends around its perimeter and an outermost perimeter flange  206 , both of which serve to stiffen the rear section  176 . Flat areas  205 ′ are formed on opposing sides of the rear section  176  for slidably engaging the top of rear bearings  209 . 
   Seat Depth Adjustment 
   A pair of parallel elongated brackets  207  ( FIG. 4B ) are attached under the forwardly-extending outer sides of the U-shaped channel section  205  for slidingly supporting the seat frame  163  on the seat carrier  124 . The elongated Z-brackets  207  form inwardly-facing C-shaped guides or tracks ( FIG. 21 ) that extend fore-to-aft under the seat frame  163 . A bearing member is attached inside the guides of bracket  207  to provide for smooth operation if desired. Two spaced-apart front bearings  208  ( FIG. 4B ) and two spaced-apart rear bearings  209  are attached atop the seat carrier  124 , front bearings  208  being attached to front section  177 , and rear bearings  209  being attached to rear section  176 . The rear bearings  209  are configured to slidably engage the guides in brackets  207 , and further include a tongue  210  that extends inwardly into the C-shaped portion of the C-shaped guides. The tongue  210  captures the seat frame  163  so that the seat frame  163  cannot be pulled upwardly away from the seat carrier  124 . The front bearings  208  slidably engage the underside of the front section  187  at spaced-apart locations. The front bearings  208  can also be made to capture the front portion of the seat frame  163 ; however, this is not deemed necessary due to the thigh flex device, which provides this function. 
   The depth adjustment of seat  24  is provided by manually sliding seat frame  163  on bearings  208  and  209  on seat carrier  124  between a rearward position for minimum seat depth (see  FIG. 24 ) and a forward position for maximum seat depth (see  FIG. 25 ). The stem  201  ( FIG. 26A ) of limiter  199  engages the stop tabs  183  in seat carrier  124  to prevent the seat  24  from being adjusted too far forwardly or too far rearwardly. The depth latch  197  ( FIG. 23 ) is T-shaped and includes pivot tabs  212  and  212 ′ on one of its arms that pivotally engages apertures  194  and  195  in seat frame  163 . The depth latch  197  further includes a downwardly-extending latching tooth  213  on its other arm that extends through aperture  195  in seat frame  163  into a selected one of the series of slots  214  ( FIG. 26 ) in the seat carrier  124 . A “stem” of the depth latch  197  ( FIG. 23 ) extends laterally outboard and includes an actuation tab  215 . Multi-function control  192  includes an inner axle  217  that supports the main components of the multi-function control. One of these components is an inner sleeve  218  rotatably mounted on axle  217 . The handle  219  is connected to an outer end of the inner sleeve  218  and a protrusion  220  is connected to an inner end of the inner sleeve  218 . The protrusion  220  is connected to the actuation tab  215 , such that rotation of the handle  219  moves the protrusion  220  and pivots the latch  197  about latch pivots  194  and  195  in an up and down disconnection. The result is that the latching tooth  213  is released from the series of slots  214 , so that the seat  24  can be adjusted to a new desired depth. A spring on inner sleeve  218  biases the latch  197  to a normally engaged position. It is contemplated that a variety of different spring arrangements can be used, such as by including an internal spring operably connected to inner sleeve  218  or to latch  197 . 
   Seat Active Thigh Angle Adjustment (with Infinitely Adjustable Gas Spring) 
   A front reinforcement plate  222  ( FIG. 28 ) is attached to the underside of the thigh-supporting front section  166  of seat shell  164 . A Z-shaped bracket  221  is attached to plate  222  and a bushing  223  is secured between the bracket  221  and the plate  222 . A bent rod axle  224  is rotatably supported in bushing  223  and includes end sections  225  and  226  that extend through and are pivotally supported in apertures  190  of down flanges  189  of seat frame  163 . The end section  226  includes a flat side, and a U-shaped bracket  227  is non-rotatably attached to the end section  226  for supporting an end of gas spring  204 . The U-shaped bracket  227  is oriented at an angle to a portion of the bent rod axle  224  that extends toward bushing  223 , such that the U-shaped bracket  227  acts as a crank to raise and lower the thigh-supporting front portion  166  of seat shell  164  when the gas spring  204  is extended or retracted. Specifically, the gas spring  204  is operably mounted between brackets  227  and  203 , so that when extended, the front thigh-supporting section  166  of seat shell  164  is moved upwardly to provide additional thigh support. Notably, the thigh-supporting section  166  provides some flex even when the gas spring  204  is locked in a fixed extension, so that a person&#39;s thighs are comfortably supported at all times. Nonetheless, the infinite adjustability of this active thigh support system provides an improved adjustability that is useful, particularly to people with shorter legs. 
   The gas spring  204  ( FIG. 28 ) is self-locking and includes a release button  233  at its rear end that is attached to the bracket  203  for releasing the gas spring  204  so that its extendable rod is extendable or retractable. Such gas springs  204  are well-known in the art. The multi-functional control  192  ( FIG. 3 ) includes an actuator for operating the release button  233 . Specifically, the multi-functional control  192  includes a rotatably outer sleeve  229  ( FIG. 23 ) operably positioned on the inner sleeve  218  and a handle  230  for rotating the outer sleeve  229 . A connector  231  extends radially from an inboard end of outer sleeve  229 . A cable  232  extends from the connector  231  on outer sleeve  229  to the release button  233  ( FIG. 28 ). The cable  232  has a length chosen so that when outer sleeve  229  is rotated, the cable  232  pulls on the release button  233  causing the internal lock of the gas spring  204  to release. The release button  233  is spring biased to a normally locked position. A seated user adjusts the active thigh flex support system by operating the handle  230  to release the gas spring  204 . The seated user then presses on (or raises his/her legs away from) the thigh-supporting front portion  166  of the seat shell  164  causing the gas spring  230  to operate the bent rod axle  217  to re-adjust the thigh-supporting front portion  166 . Notably, the active thigh support system  190  provides for infinite adjustment within a given range of adjustment. 
   Also shown on the control  192  ( FIG. 10 ) is a second rotatable handle  234  operably connected to a pneumatic vertical height adjustment mechanism for adjusting chair height by a Bowden cable  235 , sleeve  235 ′, and side bracket  235 ″. The details of chair height adjustment mechanisms are well known, such that they do not need to be discussed herein. 
   The seat shell  164  and its supporting structure ( FIG. 4B ) is configured to flexibly support a seated user&#39;s thighs. For this reason, the seat cushion  170  includes an indentation  170 A located slightly forwardly of the seated user&#39;s hip joint ( FIG. 12 ). The upholstery covering the seat cushion  170 B includes a tuck or fold at the indentation  170 A to allow the material to expand or stretch during downward flexing of the thigh support region since this results in a stretching or expanding at the indentation due to the fact that the top surface of the upholstery is spaced above the hinge axis of flexure of the seat shell  164 . Alternatively, a stretch fabric or separated front and rear upholstered cushions can be used. 
   Seat Passive/Flexible Thigh Support (without Gas Spring) 
   A passive thigh flex device  237  ( FIG. 30 ) includes a reinforcing plate  238  attached to the underside of the thigh-supporting front portion  166  of seat shell  164  ( FIG. 4B ). A pair of L-shaped stop tabs  239  ( FIG. 29 ) are bent downwardly from the body of the plate  238 . The L-shaped tabs  239  include horizontal fingers  240  that extend rearwardly to a position where the fingers  240  overlap a front edge  241  of the seat frame  163 . Bushings  242  are positioned inside the L-shaped tabs  239  and include a notch  243  engaging the front edge  241 . A curvilinearly-shaped leaf spring  244  is positioned transversely under the reinforcing plate  238  with the ends  245  of the leaf spring  244  engaging recesses in the top of the bushings  242 . The leaf spring  244  has a curvilinear shape so that it is in compression when in the present passive thigh flex device  237 . When a seated user presses downwardly on the thigh-supporting front portion  166  with his/her thighs, the leaf spring  244  bends in the middle causing the reinforcing plate  238  to move toward the front edge  241  of the seat frame  163 . When this occurs, the fingers  240  each move away from their respective bushings  242  ( FIG. 31 ). When the seated user releases the downward pressure on the thigh-supporting front portion  166 , the spring  244  flexes toward its natural bent shape causing the bushings  242  to move back into engagement with the fingers  240  ( FIG. 30 ). Notably, this passive thigh flex device  237  allows the user to flex the lateral sides of the thigh-supporting front portion  166  of the seat shell  164  independently or simultaneously. The degree of flexure of the passive thigh flex device  237  is limited by the distance that bushings  242  can be moved in L-shaped tabs  239 . 
   In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise.