Patent Publication Number: US-9894998-B2

Title: Tiltable stool

Description:
TECHNICAL FIELD 
     The present invention generally relates to an article of furniture, and more particularly, to a tiltable stool or chair which self-adjusts to the weight of a user. 
     BACKGROUND 
     Articles of furniture such as stools or chairs which allow a user to rock forward, backward and sideways are generally known. A tiltable stool is typically configured to be used on a generally horizontal surface such as a floor. The stool comprises a top section providing seat and a base section comprising a rounded bottom surface configured to support the stool on the floor. 
     The base section may be a weighted base which has a downwardly convex lateral surface area to support the stool upon contacting the floor when the stool is tilted out of its normal upright position. In that normal position the stool rests on the floor with a flat or concave area of its base. The weighting of the base is so chosen that the center of gravity of the stool comes to lie inwardly of the perimeter of the central contact area in its tilted state. The support areas may be contiguous, thus forming part of a continuous annular surface, or may be peripherally spaced apart, as by being individually disposed on three or more legs projecting generally radially from the base. Such stools are generally described in U.S. Pat. No. 3,312,437 and in US patent application publication US 2013/0320727. 
     The conventional stools typically assume an upright normal position when unoccupied. The upright position is obtained by a resetting force which acts on the stool when tilted out of its normal upright position. The resetting force is caused by coordinating the center of gravity of the stool with the fixed shape of its base such that the center of gravity assumes its lowest position when the stool is upright. Typically, the resetting force is selected based on a desired characteristic of an unoccupied stool. 
     The rounded base of conventional stools have several disadvantages: They may cause noise when the stool is tilted, they require a relatively large and heavy base. The base may slide or roll away due to a small contact surface with the floor, and the stool generally provides insufficient support for a user when tilting out of the upright position, making it undesirable or even dangerous in particular for elderly users. 
     Attempts have been made to address the inherent disadvantages of a fixedly formed rounded base by using an inflatable base. An exemplary seating arrangement having an inflatable rubber ring is disclosed in U.S. Pat. No. 6,644,742. The inflatable base requires occasional reinflation, which is not practical. It may also be prone to outgassing and cause an undesirable odor. 
     Also, bases made of foam have been proposed, but those do not address the lack of support for a user to maintain a generally upright seating position and do not adjust to a user&#39;s weight. 
     SUMMARY 
     An improved tiltable stool provides soft and comfortable dynamic seating without jeopardizing safety and stability. The stool is intended to be used while keeping both of a user&#39;s feet on the ground. When tilted out of a normal position the stool provides a stabilizing force which aides in maintaining a stable seating position. The stabilizing force of the stool increases approximately exponentially with the tilt angle of the stool out of the normal, typically upright, position. The stabilizing force also increases with the weight of a user. At a given tilt angle the stabilizing force increases approximately linearly with the weight of the user, thus making the stool self-adjust to the user&#39;s weight. The improved stool provides a similar seating experience for both light and heavy users: All users can easily tilt out of the normal position, while experiencing a stabilizing force with increasing tilt angle that corresponds to the user&#39;s weight. The improved tiltable stool provides dynamic seating flexibility similar to that of an exercise ball, but eliminates the inherent instability of sitting on a ball. Exercise balls have been associated with severe injuries when users have lost their balanced and fallen over backward. When the stool is unoccupied the stabilizing force is small and affected only by the weight of the stool, but sufficient to return a tilted stool into a normal position. 
     The improved stool comprises a seat, a body structure, and a base. The base comprises an annular elastic base member having a downwardly convex outer surface and a downwardly concave inner surface. The annular elastic base member is held in a base structure. The body structure extends between the seat and the base. Deformation of the annular elastic base causes a stabilizing force which pushes the tiltable stool towards a normal position when the tiltable stool or chair is tilted out of the normal position. An upper section of the annular elastic base is substantially cylindrical and firmly connected to the base structure. The downwardly convex outer surface of a lower section of the annular elastic base member rests on the floor. When the stool is upright a contact area between the annular elastic base member and the floor is substantially ring-shaped, and grows outwardly with an increasing weight placed on the seat. 
     Preferably, the annular elastic base member has a tapered cross-sectional shape with downwardly decreasing thickness. Typically, the normal position of the stool is upright. When the stool is upright the outer surface of an upper section of the annular elastic base member is substantially vertical. However, the normal position may also be selected such that the stool is biased out of the upright position. 
     A tongue-and-groove connection may be used to connect the annular elastic base member to the base structure. A circular tongue at the upper end of the annular elastic base member engages a corresponding circular groove of the base structure. The annular elastic base may be press-fitted, glued, welded, or mechanically fixed to the base structure. In particular, an electrically conductive disc may be disposed within the circular groove of the base structure. The annular elastic base member may be welded to the base structure by applying an electric current through the electrically conductive disc. Alternatively, the tongue of the annular elastic base may be inserted into the circular groove of the base structure by cooling the annular elastic base to reduce the width of the circular tongue. 
     The annular elastic base may be made of various elastic materials, and is preferably made of plastic, which may be reinforced by glass fibers. In particular, the annular elastic base may be made of thermoplastic polyurethane (TPU). 
     In alternative embodiments the stool may comprise an annular elastic base having an outwardly convex, substantially “j”-shaped cross section. Deformation of the annular elastic base affects a stabilizing force which increases with a tilt angle between a tilted seating position and the upright position. The stabilizing force may increase approximately exponentially with the tilt angle. The stabilizing force increases, preferably linearly, with a weight that is applied to the seat. 
     In yet another alternative embodiment the annular elastic base has a substantially “o”-shaped cross section. The annular elastic base may then comprise a pressurized cavity between an outer wall and an inner wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective top view of an exemplary tiltable stool. 
         FIG. 2  is a perspective bottom view of the stool as in  FIG. 1 . 
         FIG. 3  is a cross sectional view of an exemplary tiltable stool, showing the stool in a tilted position. 
         FIG. 4  is a detailed cross sectional view of an exemplary annular elastic base under various loads. 
         FIG. 5  shows diagrams illustrating the relationship between load and deformation of the annular elastic base and between tilt angle and stabilizing force. 
         FIG. 6  is a cross sectional view of an alternative profile of an annular elastic base under two selected loads. 
         FIG. 7  shows a “j” profile of an annular elastic base under various loads. 
         FIG. 8  shows an “o” profile of an annular elastic base under various loads. 
         FIG. 9  shows exemplary contact areas of the annular elastic base with the floor under various loads. 
         FIG. 10  is a cross section view through an exemplary annular elastic base in a normal (upright) position and in a tilted position. 
         FIG. 11  is a perspective bottom view of an exemplary annular elastic base with additional ridges. 
         FIG. 12  is a perspective bottom view of an exemplary annular elastic base in a tilted position. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1, 2 and 3 , a tiltable stool  1  comprises a seat  2 , a base  3 , and an elongated body structure  4  between the base  3  and the seat  2 . The stool  1  may comprise a height adjustment mechanism including an adjustment lever  7  to adjust the length of the body structure  4 . The body structure  4  may comprise a pillar assembly and defines a vertical axis  13  of the stool  1 . 
     The base  3  may comprising a base structure  6  connected to an annular elastic base member  5 . The annular elastic base member  5  is configured to rest on the floor  14 . The stool  1  is tiltable in any direction by deforming the annular elastic base member  5 . When a tilting force is applied to the seat, the seat is moved from a normal position into a dynamic seating position. Typically, the normal position is upright. In the upright position the vertical axis  13  of the stool  1  is perpendicular to the floor  14 . In response to a tilting force the annular elastic base member  5  is deformed, and the vertical axis  13  of the stool  1  is tilted by a tilt angle α out of the normal position. The annular elastic base member  5  may be rotationally symmetrical and extend around a central opening. 
       FIG. 3  shows a cross section of the stool  1  in a tilted seating position. Here, the stool  1  is tilted to the right by a tilt angle α out of the upright position. The annular elastic base member  5  is deformed and the height of the annular elastic base varies. As shown, the annular elastic base member  5  is compressed in the direction of the tilt (right side of  FIG. 3 ) and expanded opposite the direction of the tilt (left side of  FIG. 3 ). Compression of the annular elastic base member  5  causes a stabilizing force Fs at the seat  2  which counteracts the tilting force. In a stable tilted position the weight of the user Fw is countered by an axial force Fa in the direction of the vertical axis  13  and the stabilizing force Fs which is perpendicular to the axial force Fa. 
     The annular elastic base member  5  extends from a substantially circular upper end  38  to a lower end  39  around an opening. The diameter of the annular elastic base member  5  at the upper end  38  is larger than the diameter of the annular elastic base member  5  at the lower end  39 . Beneficially, the diameter at the upper end  38  is between approximately 1.2 and 1.6 times that of the diameter at the lower end  39 . The annular elastic base member  5  has a downwardly convex outer surface  10  and a downwardly concave inner surface  11 . The outer surface  10  of the annular elastic base contacts the floor  14  at a contact area  15 . The contact area  15  extends at a distance around the vertical axis  13 . A tongue  36  may be formed at the upper end  38  of the annular elastic base member  5  which engages a corresponding groove  37  in the base structure  6  to firmly connect the annular elastic base member  5  to the base structure  6 . 
       FIG. 4  shows the deformation of an exemplary annular elastic base member  5  under various loads in more detail. When a first force F 1  of approximately 400 N/m is applied, the annular elastic base member  5  has a first height h 1  and a first cross sectional shape  311 . The first force F 1  correlates to a person weighing about 40 kg sitting on a stool to which an annular elastic base member with a diameter of 320 mm is attached. As the stool is more heavily loaded, the annular elastic base member  5  is deformed. When loaded with a second force F 2  of approximately 800 N/m, corresponding to a person with a weight of 80 kg sitting on the stool, the annular elastic base member assumes a second cross sectional shape  312 . The height of the annular elastic base is reduced by a first deformation d 2  to a second height h 2 . When further loaded with a third force F 3  of approximately 1200 N/m, corresponding to a person with a weight of about 120 kg sitting on the stool, the annular elastic base member  5  assumes a third cross sectional shape  313 . Under the third load F 3  the height of the annular elastic base is reduced by a second deformation d 3  to a third deformed height h 3 . 
     The relationship between deformation d and load F of an annular elastic base is generally shown in  FIG. 5 . The relationship  500  between deformation d and load F is non-linear. More specifically, with increasing deformation d the load F grows approximately exponentially. The non-linear relationship between load F and deformation d allows users of vastly different weight to use a stool equipped with the annular elastic base, without significantly affecting the overall height of the stool. 
     Tilting a stool by an angle α out of the normal position causes a stabilizing force opposite the direction of tilt. The relationship between tilt angle α and stabilizing force Fr is generally shown in curves  501 ,  502  and  503  shown in  FIG. 5 . As illustrated, the stabilizing force is zero when the stool is in an upright position, allowing a user to easily tilt. The stabilizing force Fr increases with increasing tilt angle α as shown in curves  501 ,  502  and  503 . The different curves show the stabilizing force for users of different weight. As shown, the first curve  501  illustrates the stabilizing force for a heavy user weighing 120 kg. The second curve  502  illustrates the stabilizing force for a medium user weighing 80 kg. The third curve  503  illustrates the stabilizing force for a light user weighing 40 kg. At a given tilt angle α′ the stabilizing force F r1  experience by the heavy user is larger than the stabilizing force F r2  experienced by the medium user, which in turn is larger than the stabilizing force F r3  experienced by the light user. This desired effect provides a similar seating experience for users of various weights. It provides additional stabilizing support for heavier users without impeding the ability of lighter users to reach the same tilt angle. 
       FIG. 6  shows an alternative cross sectional shape  601 ,  602  of the annular elastic base member  5  in an unloaded state and in a loaded state. The annular elastic base member  5  preferably has an inwardly curved cross sectional shape  601 ,  602 . The annular elastic base member  5  may be tapered, having a downwardly decreasing thickness. The thickness of the annular elastic base member  5  may decrease from a first thickness t 1  at an upper, substantially cylindrical section  325  of the annular elastic base member  5  to a lower thickness t 2  at a lower end section  326  of the annular elastic base member  5 . The tapered profile of the annular elastic base member  5  supports the desired load/deformation characteristic as shown in  FIG. 5 . Thinner portions of the annular elastic base member  5  close to its lower end  39  bend more easily than upper portions close to its upper end  38 . 
     The uneven thickness of the annular elastic base member  5  allows a stool equipped therewith to be used equally by a very light user, such as a child, and a heavy adult. In case of a child, only the lower, thinner, portions of the annular elastic base member  5  will bend. When used by an adult, the thicker, upper, portions of the annular elastic base member  5  will also bend. In both cases the user will experience a similar “feel” of the stool&#39;s stabilizing force. 
     Under extreme load the lower end  39  of the annular elastic base member  5  may bend through completely and come to rest against the base structure  6 . This limits the maximum deformation of and prevents damage to the annular elastic base member  5  when exposed to extreme loads. 
     The annular elastic base member  5  may be shaped outwardly convex and inwardly concave. The downwardly convex outer surface  10  and a downwardly concave inner surface  11  meet at the lower end  39  of the annular elastic base member. The lower end  39  of the annular elastic base member may extend into a stiffening ring  40  as shown in  FIG. 4 . The stiffening ring  40  at the lower end  39  helps stabilizes the annular elastic base member  5  at the contact area  15 . The stiffening ring  40  protects the lower end  39  from kinking when the stool  1  is loaded and/or tilted out of its normal position. The stiffening ring  40  is preferably an integral component of the annular elastic base member  5 . 
     To prevent kinking, the lower end  39  of the annular elastic base member  5  may also be bent upwardly, such that the contact area  15  between the annular elastic base member  5  and the floor is below and radially outward of the lower end  39  of the annular elastic base member  5 . 
     When a stiffening ring  40  is used to strengthen the lower end  39  of the annular elastic base member  5 , the unloaded annular elastic base  5  may contact the floor at the stiffening ring  40  as is shown in  FIG. 4 . This arrangement has been found beneficial when working with relatively thick material. When using relatively thinner cross sectional shapes it has been found beneficial to provide the unloaded contact area  15  of the annular elastic base  5  radially outwardly and below the stiffening ring  40  as is shown in  FIG. 10 . 
     Opposite the lower end  39  an upper end  38  of the annular elastic base member  5  may be formed as a tongue  36  which engages a corresponding groove  37  in the base structure  6 . The tongue  36  may be slightly wider than the groove  37  and during assembly the tongue  36  may be press-fitted into the groove  37  with the help of a tool that secures the annular elastic base member  5  during insertion into the base structure  6 . The tongue may be further secured with self-tapping screws. As shown in  FIG. 11 , mounting holes  12  may be provided and are preferably circumferentially spaced at the base structure  6  to secure radially outwardly directed screws into an upper portion of the inner surface  11  of the annular elastic base member  5 . 
     Alternatively, the tongue  36  of the annular elastic base member  5  may be assembled to the groove  37  of the base structure  6  by gluing or welding. In particular, the annular elastic base member  5  may be ultrasonically welded to the base structure  6 . Welding may also be achieved by inserting an electrically conductive disc into the groove  37  or by molding an electrically conductive element into the tongue  36 . An electric current may then be applied to the electrically conductive disc in order to cause resistive heating. The electric heating causes the surface of the tongue  36  to meld and weld to groove  37 . 
     Yet another assembly option is to cool the annular elastic base member  5  to a temperature significantly below room temperature, causing the tongue  36  to shrink. Cooling may for example be affected by directing a cold gas onto the tongue  36  just before insertion into the groove  37 . The groove  37  may be dimensioned such that the cooled tongue  36  can be easily inserted thereto, but is firmly held within the groove  37  once the tongue  36  warms back up to room temperature, expanding within the groove  37 . Experiments have shown that load on the stool  1  reinforces the tongue-and-groove connection between the annular elastic base member  5  and the base structure  6 , so that reinforcement of the connection by welding or gluing is not critical and may not be necessary at all. 
     The cross-sectional profile and the material of the annular elastic base member  5  are coordinated to provide a desired seating experience. The annular elastic base member  5  may be made of thermoplastic polyurethane (TPU), rubber, thermoplastic polyolefin (TPO), fiberglass enforced polyamide (PA) or fiberglass enforced polyurethane (PU). The selection of material requires a trade-off decision between cost and functionality. Experiments including durability tests have shown, that a thermoplastic polyurethane with 90 Shore hardness provides the required robustness at an affordable price. An annular elastic base member  5  made of softer TPU with 75 Shore hardness would require about twice the amount of material as one made of TPU with 90 Shore hardness. 
     The following configuration of the annular elastic base member  5  has been found to be particularly beneficial for users having a weight between 40 kg and 150 kg, which is a typical market requirement:
         Material: TPU   Mass: 600 grams   Hardness: 90 Shore   Diameter at the upper end  38 : 333 mm   Diameter at the lower end  39 : 245 mm   Height h 1  without load: 56 mm (including the tongue  36 )   Height without load: 48 mm (not including tongue  36 )   Tongue  36  dimensions: 8×8 mm   Thickness t 1  at an upper section  325 : 13 mm   Thickness t 2  at a lower section  326 : 2 mm   Thickness (diameter) of the stiffening ring  40 : 8 mm       

     An alternative substantially “j”-shaped cross-sectional profile  701 ,  702 ,  703  of an annular elastic base member  5  under three different loads is shown in  FIG. 7 . This profile may be used in combination with harder materials such as fiber-enforced polyamide or polypropylene. The shown “j”-shaped profile  701 ,  702 ,  703  has a more even thickness and consequently bends more readily compared to the tapered profile  601  shown in  FIG. 6 . While this is generally not preferred as it increases the risk of kinking, the “j”-shaped profile  701 ,  702 ,  703  may be manufactured more cost effectively, and may hence be a viable alternative for cost sensitive products. 
     Yet another alternative cross-sectional profile  801 ,  802 ,  803  is substantially “o”-shaped as shown in  FIG. 8 . An annular elastic base member  5  having a substantially “o”-shaped cross-sectional profile  801  may be manufactured by extrusion, and thus be an alternative to molded profiles. Alternatively, the annular elastic base member  5  having a substantially “o”-shaped cross-sectional profile  801  may be formed by rotational molding or by blow molding. The substantially “o”-shaped profile  801  may comprise a cavity  804  which may be filled with air or another gas and may be pressurized. 
       FIG. 9  illustrates the shape of the contact area  15  of the annular elastic base  5  with the floor  14  under various loads. An unloaded stool may have a contact area  15  that assumes a first circular shape  331 . For reference, the position  335  of the upper end  38  of the annular elastic base member  5  is shown in a dashed line in  FIG. 9 . When the stool is symmetrically loaded and upright the contact area grows outwardly but remains circular. Consequently, the contact area  15  may assume a second circular shape  332  that has larger diameter than the first circular shape  331  when additional weight is placed on the stool. When tilted, the contact area may assume a substantially oval shape  333 . As shown, the substantially oval shaped contact area  333  has a longer inner lever l i  than outer level l o . The inner level l i  is measured from the intersection of the vertical axis  13  with the floor  14  to the contact area in the direction of the tilt. The outer lever l o  is measured from the intersection of the vertical axis  13  with the floor  14  to the contact area in the direction opposite the tilt. The contact area  15  remains within the confines of the position  335  of the upper end  38  of the annular elastic base member  5  under all loads, and whether loaded symmetrically or asymmetrically. 
     When the an annular elastic base member  5  is used with additional circumferentially spaced radially extending ridges  42 , the contact area  15  consists of a plurality of circumferentially spaced radially extending surfaces  336  that are arranged in an approximately circular shape  331 ,  332  or approximately oval shape  333  as explained above. 
     The stool may be tilted beyond its dynamic seating envelope of about 10 degrees. In that case the substantially oval shaped contact area  333  opens up at the inner end and eventually assumes an approximately crescent-shape  334 . When tilting the stool beyond its dynamic seating envelope a user will remain a stable position by applying force to his legs. Advantageously, the so tilted stool does not have a tendency to roll sideways. Further, the elasticity of the annular elastic base provides good friction on the floor and thus prevents the stool from sliding backward. 
     When in its upright position the annular elastic base member  5  has a substantially ring-shaped, circular, contact area  15  with the floor. Increasing deformation of the annular elastic base member  5  causes the diameter of this ring-shaped contact area  15  to grow outwardly. This increases the effective lever arm  1  of the annular elastic base member  5 . When the stool  1  is tilted out of its normal position the contact area  15  changes from a circular shape toward an approximately oval shape. Beneficially, the effective inner lever l i  in the direction of the tilt is growing larger, while the effective outer lever l o  opposite the direction of tilt is getting smaller. This effect amplifies the stabilizing force of the stool and contributes to its stability. 
     The shape of the contact area as shown in  FIG. 9  is further illustrated in the cross sectional view of  FIG. 10 . Here, the annular elastic base  5  is shown in a symmetrically loaded or upright position  8  and in a tilted position  9 . In the tilted position  9  the annular elastic base member  5  is tilted by an angle α out of the upright position  8 . The comparison of the symmetrically loaded upright position  8  with the tilted position  9  shows the relative change of the inner lever l i  and the outer level l o . As shown, the inner lever l i  grows in the direction of the tilt and the outer lever l o  becomes shorter in the direction opposite the tilt. 
     The maximum elastic deformation of the annular elastic base member  5  may allow spring-loaded tilt of the stool of up to about 10 degrees and be associated with the equivalent of a symmetrical load of 200 kg. A user may tilt the stool beyond its dynamic seating envelope up to about 45 degree. This is achieved by lifting the backward portion of the annular elastic base member  5  into the air. Tilting the stool up to about 45 degrees allows a user to conveniently pick up articles from the floor. The shape of the annular elastic base member as described before provides a relatively smooth and seamless transition from dynamic deformation (up to approximately 10 degrees) to lifting the backward portion of the base into the air (between approximately 10 degrees and 45 degrees). 
       FIG. 11  shows an annular elastic base member  5  with additional circumferentially spaced radially extending ridges  42  at the contact area of the annular elastic base with the floor. The radially extending ridges  42  have been found to prevent noise that might otherwise be caused by deformation of the annular elastic base member  5 . Noise may stem from vibrations that may be caused when the annular elastic base member  5  deforms while a user tilts the associated stool. The radially extending ridges  42  may also prevent a vacuum from forming under a stool equipped with the annular elastic base member  5 . This is particularly relevant if the annular elastic base member  5  and the base structure  6  form an upwardly sealed surface. In that case, the annular elastic base member  5  and the base structure  6  could act a large suction cup, which must be prevented. Prevention is achieved by the ridges  42  which segment the contact area of the annular elastic base  5  with the floor  14  and allow air to pass through air channels between the ridges  42 . Instead of ridges  42 , grooves (not shown) may be applied to the annular elastic base member  5  in an equivalent circumferentially spaced and radially extending arrangement to provide the desired air channels.  FIG. 11  shows a configuration with 44 ridges. Alternative configurations may choose to use more or fewer ridges, for example between 20 and 80 ridges. 
       FIG. 12  shows a deformed annular elastic base member  5  under extreme load. The lower end  39  of the annular elastic base member  5  here bends through completely and comes to rest against a stop surface  320  of the base structure  6 . This limits the maximum deformation and thereby prevents damage of the annular elastic base member  5  when exposed to extreme loads. The extremely loaded stool remains slightly elastic based on compression of the material of the annular elastic base  5 . Damage to the floor is prevented as the base structure  6  does not contact the floor even under extreme load. 
     While the present invention has been described with reference to exemplary embodiments, it will be readily apparent to those skilled in the art that the invention is not limited to the disclosed or illustrated embodiments but, on the contrary, is intended to cover numerous other modifications, substitutions, variations and broad equivalent arrangements that are included within the spirit and scope of the following claims. For example, while this specification and the claims refer to a stool, it should be understood that the invention can equally be applied to a chair or other tiltable article of furniture. 
     Alternative Embodiments 
     A tiltable stool, comprising:
         a seat;   an annular elastic base member; and   an elongated body structure extending between the seat and the base.       

     The tiltable stool as above, wherein the annular elastic base is rotationally symmetrical. 
     The tiltable stool as above, wherein the annular elastic base member comprises a downwardly convex outer surface and a downwardly concave inner surface extending around a central opening. 
     The tiltable stool as above, further comprising a height adjustment mechanism. 
     The tiltable stool as above, wherein the elongated body structure comprises a pillar assembly and defines a vertical axis of the stool. 
     The tiltable stool as above, wherein the annular elastic base member is an integral molded component. 
     The tiltable stool as above, wherein the annular elastic base member rests on the floor. 
     The stool as above, wherein the stool can be tilted in any direction by deforming the annular elastic base. 
     The stool as above, wherein the stool can be tilted in any direction by deforming the annular elastic base within a dynamic seating enveloped of approximately 10 degree tilt. 
     The stool as above, wherein the stool can be tilted beyond the dynamic seating envelope by lifting a backward portion of the annular elastic base into the air. 
     The stool as above, wherein the stool wherein the stool has an upright normal position. 
     The stool as above, wherein deformation and/or compression of the annular elastic base member causes a stabilizing force. 
     The stool as above, wherein the stabilizing force increases with the weight of a user. 
     The stool as above, wherein the stabilizing force at a given tilt angle increases approximately linearly with the weight of a user. 
     The stool as above, wherein the stabilizing force increases with the tilt angle of the stool. 
     The stool as above, wherein the stabilizing force increases approximately exponentially with the tilt angle of the stool. 
     The stool as above, wherein the height of the annular elastic base member decreases in the direction of tilt when the stool is tilted. 
     The stool as above, wherein the height of the annular elastic base member increases in the direction opposite tilt when the stool is tilted. 
     The stool as above, wherein a tongue is formed at an upper end of the annular elastic base member. 
     The stool as above, wherein the tongue engages a corresponding groove in the base. 
     The stool as above, wherein the tongue has a height of approximately 8 mm and a thickness of approximately 8 mm. 
     The stool as above, wherein the tongue is press-fitted into the groove. 
     The stool as above, wherein the stool is configured to support a user having a weight between 40 kg and 150 kg. 
     The stool as above, wherein the annular elastic base is configured to support a load between approximately 400 N/m and 1500 N/m. 
     The stool as above, wherein the annular elastic base member has an inwardly curved cross section. 
     The stool as above, wherein the annular elastic base member is tapered having a downwardly decreasing thickness. 
     The stool as above, wherein the annular elastic base member is tapered having a downwardly decreasing thickness. 
     The stool as above, wherein the annular elastic base member is made of thermoplastic polyurethane. 
     The stool as above, wherein the annular elastic base member has a mass of approximately 600 grams. 
     The stool as above, wherein the annular elastic base member has a hardness of approximately 90 Shore. 
     The stool as above, wherein the annular elastic base member has a diameter at the upper end of approximately 333 mm. 
     The stool as above, wherein the annular elastic base member has a diameter at the lower end of approximately 245 mm. 
     The stool as above, wherein the annular elastic base member has a diameter at the upper end that is approximately 1.4 times its diameter at the lower end. 
     The stool as above, wherein the annular elastic base member has a diameter at the upper end that is between 1.2 and 1.6 times its diameter at the lower end. 
     The stool as above, wherein the annular elastic base member has height h 1  without load of approximately 56 mm including a tongue. 
     The stool as above, wherein the annular elastic base member has height without load of approximately 48 mm, not including a tongue. 
     The stool as above, wherein the annular elastic base member has a thickness t 1  at its upper end of approximately 13 mm. 
     The stool as above, wherein the annular elastic base member has a thickness t 2  at its lower end of about 2 mm. 
     The stool as above, wherein a lower end of the annular elastic base member touches a stop surface when fully deformed under heavy load. 
     The stool as above, wherein deformation of the annular elastic base is limited by a stop surface in the base which supports the lower end of the annular elastic base under heavy loads. 
     The stool as above, wherein the annular elastic base is secured to the base by screws. 
     The stool as above, wherein the annular elastic base is secured to the base by self-tapping screws. 
     The stools as above, wherein mounting holes are provided and circumferentially spaced at the base structure to secure radially outwardly directed screws into an upper portion of the inner surface of the annular elastic base member. 
     The stools as above, wherein the lower end of the annular elastic base member is bent upwardly, such that the contact area between the annular elastic base member and the floor is below and radially outward of the lower end of the annular elastic base member. 
     The stool as above, wherein a stiffening ring is provided at the lower end of the annular elastic base member. 
     The stool as above, wherein the stiffening ring has a diameter of approximately 8 mm. 
     The stool as above, wherein the annular elastic base member contacts the floor at the stiffening ring when the stool is unoccupied. 
     The stool as above, wherein the annular elastic base member contacts the floor radially outwardly and axially below the stiffening ring when the stool is unoccupied. 
     The stool as above, wherein the annular elastic base member has an approximately circular contact area with the floor when the stool is upright. 
     The stool as above, wherein the radius of the approximately circular contact area with the floor increases as weight is placed onto the stool. 
     The stool as above, wherein the contact area of the annular elastic base member with the floor is approximately oval when the stool is tilted. 
     The stool as above in a tilted position, wherein an outer lever l o  measured from the intersection of the vertical axis with the floor to the contact area in the direction opposite the tilt is shorter than an inner lever l i  measured from the intersection of the vertical axis with the floor to the contact area in the direction of the tilt. 
     The stool as above, further comprising circumferentially spaced radially extending ridges at the contact area of the annular elastic base with the floor. 
     The stool as above, comprising between 20 and 80 ridges. 
     The stool as above, further comprising circumferentially spaced radially extending grooves at the contact area of the annular elastic base with the floor. 
     The stool as above, comprising between 20 and 80 grooves.