Abstract:
This application relates to rear suspension system for bicycles, such as freeride, downhill and trail-style mountain bikes. The suspension system includes a rear suspension for coupling the rear wheel of the bicycle to the bicycle frame, a rear shock absorber disposed entirely in front of the seat tube of the frame and a linkage for operatively coupling the rear suspension to the rear shock absorber. The linkage intersects the longitudinal axis of the seat tube for transmitting rear suspension forces around the seat tube to the rear shock absorber. The seat tube is preferably continuous and is disposed so that the bicycle seat is adjustable over a full range of seat positions. The seat tube orientation ensures that the seat is movable between optimum biomechanical positions for both uphill pedaling and aggressive downhill freeriding. Additionally, the seat tube and rear suspension system are configured so that the rear wheel and rear suspension will not contact the seat tube, seat, or bicycle rider even when the rear suspension system is in a fully compressed, forward position. The shock absorber is preferably oriented in an orientation to maximize its stroke length. For example, the shock absorber may be oriented generally parallel to a top tube of the bicycle frame.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority on U.S. patent application No. 60/528,725 filed 12 Dec. 2003 which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     This application relates to an improved rear suspension system for bicycles.  
       BACKGROUND  
       [0003]     High-end mountain bikes typically have both rear and front suspension systems to assist in traversing uneven terrain. This is particularly the case for freeride, downhill and trail bikes which are designed to -descend steep and uneven mountain terrain, often at high speeds. Many rear suspension systems designed for freeriding and other biking applications are known in the prior art. Such systems generally include a rear suspension permitting a limited degree of travel of the rear wheel relative to the bicycle frame and a rear shock absorber for absorbing suspension forces. The range of rear wheel travel that is permitted by existing suspension systems varies, but is typically within the range of about 5 to 12 inches measured vertically (although some designs permit travel outside this range).  
         [0004]     While many mid and long rear wheel travel bicycle designs are known, most designs have rear suspension systems which compromise overall bike performance and versatility. Two primary problems caused by existing designs are (a) undesirable contact between the rear tire or suspension linkages and the bicycle seat or seat tube when the rear suspension system is under full compression; and (b) a restricted range of adjustability of the seat position.  
         [0005]     U.S. Pat. Nos. 5,509,679, 5,678,837 and 5,899,480 owned by Specialized Bicycle Components, Inc. of Morgan Hill, Calif. describe two primary types of four bar linkage rear suspension systems, commonly referred to as the walking beam and low linkage designs. The above-noted patents are incorporated herein by reference in their entireties. The walking beam design is illustrated in  FIG. 1  and includes a rocker arm link extending in a near horizontal orientation and a rear shock absorber having a travel axis extending in a near vertical orientation. The rocker arm link pivots on the seat tube portion of the bicycle frame and extends between an upper end of the rear shock absorber and an upper end of the seat stays. The main manufacturing advantages of the walking beam design are that the frame of the bike can be built out of a typical “triangle” shape, common in the bicycle industry, the pivot point for the rocker arm link can be conveniently arranged to mount to the seat tube, and the rear shock can be conveniently pivotally connected to the top of the bottom bracket area. The main performance advantage of the walking beam design is that the seat tube is straight and continuous and allows for a full adjustment range of the seat (i.e. saddle) height. This is important because the seat needs to be raised to the correct biomechanical position to allow for effective pedaling performance when climbing up hills and crossing non-technical terrain and the seat needs to be lowered substantially (typically by 4 to 8 inches or more depending upon the rider&#39;s height and body proportions) so that the rider can safely and effectively traverse difficult or challenging terrain and obstacles.  
         [0006]     The walking beam design is suitable for bicycle suspension frames with up to approximately 6 inches of vertical rear wheel travel. However, as the rear travel gets longer than about 6 inches, several problems arise with the walking beam design. As the rear wheel travel path starts to come forwards, towards the front of the bike, this movement combined with the generally rearward sloping seat tube causes the rear wheel and the seat tube to collide before the rear wheel has finished its travel. Further, the relatively high linkage arrangement (when compared to the low linkage four bar design described below) causes the rocker arm link (when the rear suspension is under substantial or full compression) to interfere with the low rearward position of the rider&#39;s “bottom” (a position needed to ride challenging terrain) and also the rear seat when the seat is lowered.  
         [0007]     Longer travel suspension frames work better with rear shock absorber stroke length ratios that match their travel. If the ratio of rear wheel travel versus shock stroke length increases beyond a favourable ratio, then the relatively short stroke shock absorber will be less effective in its ability to control the movement of the rear suspension and will be potentially more prone to failure. Additionally, higher rear shock absorber spring rates are required, which reduces the “suppleness” of the rear suspension&#39;s feel. As the rear wheel&#39;s travel increases, the rocker arm link pivot must also be moved up “higher” along the seat tube, to accommodate the longer stroke rear shock absorber (which is fixed near the bottom bracket of the front triangular frame as shown in  FIG. 1 ). Additionally, the rocker arm link needs to be longer to accommodate the greater length of the shock absorber stroke. This exacerbates the problems discussed above concerning the interference of the rear wheel and rocker arm link with the frame&#39;s seat tube, seat, and the rider&#39;s bottom. These effects are increased even more in the case of smaller frame sizes designed for smaller riders, because there is less room for the suspension elements to move due to a seat that is in an overall lower position (because the rider is smaller).  
         [0008]     Low linkage four bar rear suspensions as exemplified by the Specialized FSR design shown in  FIG. 2  also exhibit several limitations. On the one hand, such low linkage designs do not suffer from rear wheel and rocker arm link interference issues as described above in connection with the walking beam configuration. This is because the rocker arm link is typically mounted in a lower, diagonal to near vertical position, and the rear shock absorber is mounted in a diagonal to near horizontal position ( FIG. 2 ). Additionally, the seat tube position can be arbitrary, as it is typically mounted to the end of a cantilevered “beam”, instead of being welded in line with the bottom bracket axis. However, on the other hand the low linkage design requires that the rear shock “interrupt” the seat tube and hence causes the seat tube to be cut off at the shock absorber location. This severely limits the range of seat and seat post adjustability. This is a major problem on bikes designed for technical riding where the seat needs to be substantially lowered so that it is out of the way of the rider&#39;s body movement, such as when traversing uneven downhill terrain. To get the seat low enough, technical riders typically cut their seat post to a shorter length. However, this leaves the seat post too short and the seat cannot be raised enough to provide a biomechanically correct pedaling position for the rider, particularly when climbing hills. A full range of adjustable seat post positions is desirable for the reasons specified above.  
         [0009]     Other existing rear suspension systems designed for technical riding, such as single pivot swing arm and virtual pivot point designs, also suffer from various drawbacks. For example, rear suspensions having long swing arms typically have lower overall lateral rigidity than other designs. Some long travel virtual pivot point designs include interrupted seat tubes, or have full length seat tubes which are bent to provide sufficient rear wheel clearance, but such designs restrict the range of seat height adjustability. Other four bar virtual pivot point designs have elongated seat stays which extend past the seat tube to allow for a full length seat tube. However, increasing the length of the seat stays also sacrifices the overall lateral stability of the rear suspension system which is not desirable.  
         [0010]     The need has therefore arisen for an improved rear suspension system for bicycles which maintains the advantages of prior art designs while eliminating or substantially reducing their disadvantages.  
       SUMMARY OF INVENTION  
       [0011]     In accordance with the invention, a rear suspension system for a bicycle having a rear wheel and a frame is provided. The frame includes a seat tube having a longitudinal axis. The system includes a rear suspension pivotally coupling the wheel to the frame; a rear shock absorber disposed entirely forwardly of the seat tube; and a linkage coupling the rear suspension to the shock absorber for transferring forces therebetween, wherein the linkage extends in an orientation intersecting a plane passing through the seat tube parallel to the longitudinal axis.  
         [0012]     The linkage is configured to transmit the suspension forces around the seat tube. In one embodiment the linkage may straddle the seat tube. For example, the linkage may comprise first and second linkage elements disposed on opposite sides of the seat tube. The linkage preferably has a length exceeding the diameter of the seat tube and is pivotably coupled to both the rear suspension and the shock absorber. In one embodiment the rear suspension may include one or more pivot points, where the most forward one of the pivot points is located rearwardly of the longitudinal axis.  
         [0013]     Preferably the seat tube of the bicycle frame is continuous and the shock absorber has first and second ends coupled to the frame forwardly of the seat tube. In one embodiment the first end of the shock absorber is pivotably coupled to the frame and the second end of the shock absorber is pivotably coupled to the linkage. In one particular configuration, the frame comprises a top tube, a head tube and a down tube in addition to the seat tube, and the first and second ends of the shock absorber are coupled to the down tube.  
         [0014]     In one embodiment, the bicycle frame is orientable in a vertical plane and the shock absorber may be supported so that its longitudinal axis extends in a plane at an angle of between about 45-90° relative to the vertical plane. In one embodiment the shock absorber may extend in a horizontal or near-horizontal orientation to maximize its stroke length. In a particular embodiment the shock absorber may extend beneath and parallel to a top tube of the bicycle frame.  
         [0015]     In one particular embodiment, the rear suspension may comprise a four bar suspension design including a chain stay link and a seat stay link coupled to the rear wheel and a rocker arm link coupled to the seat stay link and extending forwardly therefrom. In one embodiment the linkage is coupled to the rocker arm. In alternative embodiments the linkage is alternatively coupled to the seat stay link or the chain stay link.  
         [0016]     The suspension system is particularly useful to mid to long rear wheel travel bicycle designs where the range of vertical travel of the suspension system exceeds approximately 5 inches.  
         [0017]     The application also relates to a bicycle having a rear wheel and a frame orientable in a vertical plane, the frame comprising a seat tube having a longitudinal axis. The bicycle further includes a rear suspension for pivotally coupling the rear wheel to the frame, wherein the rear wheel is movable within a range of travel between a most rearward position and a most forward position. The bicycle further includes a rear shock absorber disposed entirely forwardly of the seat tube and a linkage coupling the rear suspension to the rear shock absorber, wherein the linkage extends in an orientation intersecting a plane passing through the seat tube parallel to the longitudinal axis.  
         [0018]     Preferably the rear wheel and the rear suspension system are spaced apart from the seat tube in the most forward position and the longitudinal axis of the seat tube extends at an angle substantially tangential to the most forward position of the rear wheel. In one embodiment the longitudinal axis of the seat tube extends at an angle between about 50-75° degrees relative to a horizontal plane passing through the seat tube and extending perpendicular to the vertical plane. More preferably, the longitudinal axis of the seat tube extends at an angle of between about 55-65° degrees relative to the aforesaid horizontal plane.  
         [0019]     The seat tube of the frame is preferably continuous. The frame may also include a down tube and a bottom bracket mounted at a lower end of said down tube for receiving a crank assembly of the bicycle. The bottom end of the seat tube is offset forwardly as compared to conventional frame designs such that the longitudinal axis of said seat tube intersects the down tube at a location forward of the bottom bracket.  
         [0020]     The invention may also relate to a kit for coupling a rear suspension to a shock absorber using the linkage of the applicant&#39;s invention. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]     In drawings which illustrate embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way,  
         [0022]     FIGS.  1  is a side elevational view of a rear suspension system of the prior art configured in a walking beam four bar subassembly.  
         [0023]     FIGS.  2  is a side elevational view of a rear suspension system of the prior art configured in a low linkage four bar subassembly.  
         [0024]      FIG. 3  is a side elevational view of a bicycle having the rear suspension system of the invention.  
         [0025]      FIG. 4  is an enlarged side elevational view of the applicant&#39;s rear suspension system of  FIG. 3 .  
         [0026]      FIG. 5 ( a ) is an enlarged rear isometric view of the applicant&#39;s rear suspension system of  FIG. 3  in an uncompressed configuration.  
         [0027]      FIG. 5 ( b ) is an enlarged isometric view of the applicant&#39;s rear suspension system of  FIG. 3  under full compression.  
         [0028]      FIG. 6  is a schematic view showing the relative orientation of the bicycle subframe and rear suspension system of  FIGS. 3-5  when the suspension system in a fully compressed configuration.  
         [0029]      FIG. 7  is a schematic view showing the bicycle subframe and rear suspension of  FIGS. 3-5  when the suspension system is in an uncompressed configuration.  
         [0030]      FIG. 8  is a schematic view showing the relative orientation of the bicycle subframe and rear suspension system in an alternative embodiment of the invention.  
         [0031]      FIG. 9  is a schematic view showing the relative orientation of the bicycle subframe and rear suspension system in a further alternative embodiment of the invention.  
         [0032]      FIG. 10  is a schematic view showing the relative orientation of the bicycle subframe and rear suspension system in a further alternative embodiment of the invention. 
     
    
     DESCRIPTION  
       [0033]     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0034]     This application relates to a bicycle  10  having an improved rear suspension system  12  ( FIG. 3 ). Bicycle  10  includes a rear wheel  14 , a front wheel  16  and a frame  18  coupled to wheels  14 ,  16  for supporting a rider. As described in detail below, rear suspension system  12  pivotably couples rear wheel  14  to frame  18 . Front wheel  16  is coupled to frame  18  by means of front forks  20 . Forks  20  may optionally include front shock absorbers  21 .  
         [0035]     As used in this patent application, the terms “front”, “forward”, and forwardly” mean toward the front end of bicycle  10  and the terms “rear, “rearward”, and rearwardly mean toward the rear end of bicycle  10 . Similarly, the words “top”, “upper”, “upward” and “upwardly” mean toward the upper portion of bicycle  10  when it is in an ordinary riding orientation with wheels  14 ,  16  resting on a support surface, as shown in  FIG. 3 . The words “bottom”, “lower”, “loward” and “lowardly” refer to the lower portion of bicycle  10  when it is in the ordinary riding orientation  
         [0036]     In the illustrated embodiment, bicycle frame  18  has a generally triangular configuration and comprises a top tube  22 , a down tube  24  and a seat tube  26 . Frame  18  also includes a head tube  28  for receiving forks  20  and handlebars  30  at the front end of bicycle  10 . A seat post  32  supporting a seat  34  is slidably coupled to seat tube  26 . The position of seat post  32  is adjustable to alter the height of seat  34 . Although the present invention is described with reference to the triangular-shaped frame  18  of  FIG. 3 , the invention is equally applicable to other frame designs, including monocoque frames (such as is shown in  FIG. 2 ).  
         [0037]     Bicycle frame  18  further includes a bottom bracket  35  located at the bottom end of down tube  24  for receiving a pedal assembly (not shown) including a crank arm and bottom bracket axle. Most conventional bicycle frames having a triangular configuration have a seat tube which is coincident with the bottom bracket as shown in  FIG. 1  (i.e. the longitudinal axis of the seat tube intersects the bottom bracket). However, in the Applicant&#39;s invention, the position of seat tube  26  is shifted forwardly so that the axis of seat tube  26  intersects down tube  24  at a location spaced forwardly and upwardly from bottom bracket  35 . This feature is described in further detail below. The Applicant&#39;s frame  18  may further include a bottom bracket block  37  for supporting bottom bracket  35  and elements of rear suspension system  12  at a location rearwardly of seat tube  26 .  
         [0038]     Rear suspension system  12  is illustrated in detail in  FIGS. 4 and 5 . System  12  may include a rear suspension  40 , a rear shock absorber  42  and a linkage  44  for coupling rear suspension  40  to shock absorber  42 . Linkage  44  enables transfer of forces around seat tube  26  between suspension  40  and shock absorber  42  as discussed in detail below.  
         [0039]     In the illustrated embodiment, rear suspension  40  is configured in a conventional low linkage four bar arrangement. Such an arrangement is described in detail in U.S. Pat. Nos. 5,509,679, 5,678,837 and 5,899,480 which are incorporated herein by reference, as indicated above. In this arrangement, rear suspension  40  includes a chain stay link  46 , a seat stay link  48  and a rocker arm link  50  which, together with frame  18 , define a four bar linkage. More particularly, chain stay link  46  includes spaced-apart first and second arms  47  which straddle rear wheel  14  when bicycle  10  is assembled ( FIG. 3 ). A yolk-shaped first end  52  of chain stay arms  47  is pivotably coupled to bottom bracket block  37  of frame  18  by means of a pivot  56 . A second end  58  of each chain stay arm  47  is pivotably coupled to a drop-out bracket  60  by means of a pivot  62 . Drop-out bracket  60  supports axle  64  and hub  65  of rear wheel  14  (FIGS.  5 ( a ) and  5 ( b )). In the embodiment of  FIG. 3-5  pivot  62  is therefore spaced a short distance fowardly of the axis of axle  64  and hence the center of rear wheel  14  (in the schematic views of  FIGS. 6-10  reference  64  denotes the center of wheel  14 ).  
         [0040]     Seat stay link  48  includes spaced-apart arms  49  which also straddle rear wheel  14  when bicycle  10  is assembled. A first end  66  of each seat stay arm  49  is coupled to a corresponding drop-out bracket  60  and a second end  68  of each seat stay arm  49  is coupled to rocker arm link  50  by means of a pivot  70 . In the illustrated embodiment rocker arm link  50  includes spaced-apart arm elements  51  which are connected together at a first end  72  thereof by a seat stay arch  53 . Seat stay arch  53  extends transversely between arm elements  51  in the vicinity of pivots  70 . Each rocker element  51  also has a second end  74  pivotably coupled to bottom block  37  of frame  18  by means of a pivot  76 . As shown best in FIGS.  5 ( a ) and  5 ( b ), pivot  76  is located proximate seat tube  26  and above a pivot  56 .  
         [0041]     Rear suspension  40  illustrated in  FIGS. 3-5  thus includes four separate pivot points, namely pivots  56 ,  62 ,  70  and  76 . This arrangement enables rear wheel  14  to pivot relative to bicycle frame  18  between a rear position ( FIG. 5 ( a )) when traveling over flat terrain and a raised forward position ( FIG. 5 ( b )) to accommodate uneven terrain. The maximum amount of travel of rear wheel  14  permitted by rear suspension  40  may vary, but is typically in the range of 5-10 inches measured vertically. Unlike conventional single pivot suspension designs, rear suspension  40  has multiple pivot points and is therefore not constrained to follow a constant arc pivoting wheel path. Rather, as will be appreciated by a person skilled in the art, suspension  40  exhibits the advantages of other known four bar suspension geometries in terms of pedal-induced suspension movement, acceleration, pedal feedback and braking characteristics.  
         [0042]     As will also be appreciated by a person skilled in the art, the drawings illustrate only one example of a rear suspension  40 , namely a low linkage four bar configuration, and many other types of suspension systems could be substituted without departing from the invention, including walking beam four bar linkages, virtual pivot point designs and single pivot swing arm designs.  
         [0043]     As indicated above, rear suspension system  12  also includes a rear shock absorber  42  for absorbing forces received from rear suspension  40  via linkage  44 . In the Applicant&#39;s invention shock absorber  42  is positioned entirely forwardly of seat tube  26 . In the illustrated embodiment, shock absorber  42  extends along an axis substantially parallel to top tube  22 . This orientation is sometimes referred to in the bicycle industry as a “horizontal” orientation as opposed to a “vertical” orientation as exemplified by the walking beam design shown in  FIG. 1 . However, neither orientation is precisely “horizontal” or “vertical” and many variations are possible. One advantage of orientating shock absorber  42  parallel to top tube  22  is that the length of shock absorber  42  may be maximized (i.e. there is sufficient available space to accommodate relatively large, long stroke length shock absorbers  42 ). There are also advantages to isolating shock absorber  42  from the effects of side loading of rear suspension  40  and protecting shock absorber  42  from “tire roost” (i.e. protecting shock absorber  40  from being exposed to soil and water flung upward by the bicycle tires).  
         [0044]     In the illustrated embodiment a first end  80  of shock absorber  42  is coupled by means of a pivot  82  to a first support bracket  84  mounted on an upper, forward portion of frame down tube  24 . A second end  86  of shock absorber  42  is coupled by means of a pivoting swing link  88  to a second support bracket  90  mounted on an upper lower portion of frame down tube  24 . More particularly, pivoting swing link  88  has one end coupled to second support bracket  90  by means of a pivot  92  and another end coupled to second end  86  of shock absorber  42 .  
         [0045]     Linkage  44  couples rear suspension  40  to shock absorber  42 . In the embodiment illustrated in  FIGS. 3-5 , linkage  44  couples second end  86  of shock absorber  42  to a central portion of rocker arm  50 . However, many other arrangements for operatively coupling rear suspension  40  and shock absorber  42  may be envisioned, as described below. In the embodiment of  FIGS. 3-5 , linkage  44  includes a pair of spaced-apart push linkages  96  which straddle seat tube  26 . A first end of  98  of each push linkage  96  is pivotably connected to second end  86  of shock absorber  42  by means of a pivot  101  and a second end  100  of each push linkage  96  is pivotably connected to a midportion of a corresponding rocker arm element  51  by means of a pivot  102 . In operation, forward pivoting motion of rocker arm  50  toward seat tube  26  causes a transfer of forces through push linkages  96  to shock absorber  42  ( FIG. 5 ( b )). Thus linkage  44  transfers suspension forces received by rear suspension  40  at a location rearward of seat tube  26  to shock absorber  42  located forward of seat tube  26 . In order to accomplish this function linkage  44  extends in a plane intersecting the longitudinal axis of seat tube  26 .  
         [0046]     In an alternative embodiment of the invention linkage  44  may include a single push linkage  96  (i.e. it is not a requirement of the invention that two separate push linkages  96  straddle seat tube  26 ).  
         [0047]     Linkage  44  is sometimes referred to herein as Four×4™ or 4×4™ linkage. This is because each push linkage  96  includes four separate pivots to transfer forces from rear suspension  40  around seat tube  26  to shock absorber  42 , namely pivots  76 , 92 ,  101  and  102 . As described above, rear suspension  40  also employs four different pivot points, namely pivots  56 , 62 , 70  and  76 . Thus rear suspension system  12  may be considered do be a “dual four bar” suspension system. As a result of the relative positions and spacing of the aforesaid pivots, the spacial relationship between the various linkages of rear suspension system  12  changes significantly between the uncompressed riding configuration of  FIG. 5 ( a ) and the compressed configuration of  FIG. 5 ( b ). For example, as rocker arm link  50  moves forwardly to a more upright orientation extending generally parallel to seat tube  26  as shown in  FIG. 5 ( b ), swing link  88  is also caused, via push linkages  96 , to pivot to a more upright orientation, thereby causing compression of shock absorber  42 .  
         [0048]     One notable difference between the present invention and some prior art configurations is that in the applicant&#39;s design the most forward pivot point of rear suspension  40  is located rearward of the longitudinal axis of seat tube  26  rather than on or in front of seat tube  26 . One advantage of this design is that the length of chain stay link  46 , seat stay link  48  and rocker arm link  50  can be maintained at a normal length (i.e. the length of one or more of such linkages does not need to be increased in order to connect directly to shock absorber  42  or to support structures located forwardly of seat tube  26 ). By contrast, some prior art designs have comparatively long linkages or swing arms which results in a sacrifice of lateral rigidity.  
         [0049]     One other important advantage of the present invention is that a continuous seat tube  26  may be employed. This ensures that seat post has a sufficient height adjustment range. As used in this patent application a “continuous” seat tube  26  refers to a seat tube that extends upwardly from a lower portion of frame  18 , such as down tube  24 , and is straight and uninterrupted between frame  18  and its upper end. By way of contrasting example,  FIG. 2  shows an arrangement of the prior art where the seat tube is discontinuous (i.e. it is interrupted at its lower end to accommodate a rear shock absorber underneath the seat tube). As explained above, such an arrangement restricts the adjustability of the seat post slidable within the seat tube and hence the available range of seat positions. By way of another example, a seat tube having a lower portion which is bent to avoid contact with the rear suspension would also considered to be “interrupted” and not continuous since the bend in the tube would impede a full range of adjustment of the seat post slidable therein.  
         [0050]     Further, as shown best in  FIG. 6 , in the applicant&#39;s invention the position and angle of continuous seat tube  26  has been engineered to ensure that a rider seated on seat  34  is in an optimum biomechancial position. In the following discussion seat tube  26  is assumed to extend in the same plane as the remainder of bicycle frame  18  (i.e. in a substantially vertical plane when bicycle  10  is in a normal riding orientation). The position and angle of seat tube  26  is determined by two primary constraints. First, seat tube  26  is positioned so that its longitudinal axis extends tangentially relative to the most forwardly intrusive position of rear wheel  14  and rear suspension  40  (FIGS.  5 ( b ) and  6 ). As used herein “forwardly intrusive” refers to the most forward position of rear wheel  14  and/or rear suspension  40  when rear suspension system  12  is under full compression. In this case, rear wheel  14  is in a raised and forward position and seat stay link  48  and rocker arm link  50  are pivoted forwarded ( FIG. 5 ( b )). For example, comparing FIGS.  5 ( a ) and  5 ( b ), seat stay arch  53  and pivots  70  have moved forwardly and upwardly to a position closely proximate to seat tube  26 .  
         [0051]     Secondly, the longitudinal axis of seat tube  26  is such that when seat  34  is raised or lowered to the desired height the rider is positioned in a biomechanically efficient position. As discussed above, seat  34  ordinarily needs to be raised to allow for effective pedaling performance when climbing hills and traversing non-technical terrain. Seat  34  also needs to be lowered substantially (usually by 4-8 inches or more depending on the rider&#39;s height and body proportions) so that the rider can safely and effectively traverse difficult or challenging terrain and obstacles, for example during aggressive freeriding. In particular, it is desirable to move seat  34  to a lower forward position so that it does not interfere with preferred downhill riding positions (when traversing steep downhill slopes or other challenging terrain, it is often desirable for the rider&#39;s “bottom” to be lowered and moved rearwardly to alter the rider&#39;s center of gravity).  
         [0052]     As shown in  FIG. 6 , a “region of biomechanical efficiency” is shown in hatched lines which denotes a preferred range of positions of seat tube  26  (and hence seat  34 ). In this example, the bottom boundary of the region is the longitudinal axis of frame top tube  22 ; the front boundary of the region is a line that is 5° from a vertical plane passing through the axis of bottom bracket  35  and 85° from a horizontal plane centered on the bottom bracket axis; the rear boundary is of the region is a line that is 30° from a vertical plane passing through the axis of bottom bracket 35° and 60° from a horizontal plane centered on the bottom bracket axis; and the top boundary is a line formed by the intersection of the plane of the bicycle front frame and a horizontal plane which includes a point defined by the intersection of the longitudinal axes of top tube  22  and head tube  28 .  
         [0053]     As should be apparent from  FIG. 6 , it is important that the longitudinal axis of seat tube  26  extend within the region of biomechanical efficiency at a location that does not interfere with rear wheel  14  or rear suspension  40  when such components are in their most forward position (i.e. when rear shock absorber  42  is under full compression). As explained above, the position of seat  34  is largely determined by the angle of seat tube  26  (although seat  34  may be adjustable forwardly and rearwardly to a limited extent on rails (not shown) mounted on the top of seat post  32 ). Further, the actual position of seat  34  may be above the above-defined region of biomechanical efficiency, especially when seat  34  is in the most raised position for hill climbing, but the longitudinal axis of seat tube  26  preferably extends through this region. In the illustrated embodiment the longitudinal axis of seat tube  26  extends at an angle of about 58° relative to a horizontal plane (as defined above). Since the longitudinal axis of seat tube  26  is not coincident with the axis of bottom bracket  35 , but is instead offset forwardly as described above, this corresponds to an effective seat tube angle of about 72° relative to the horizontal plane as measured from bottom bracket  35  (which support&#39;s the bicycle pedals). In one embodiment of the invention, the preferred actual angular range of seat tube  26  as measured above is between about 50-70° which corresponds to a preferred effective angular range of between about 60-85° as measured above. If the longitudinal axis of seat tube  26  is too sharply inclined (e.g. having an actual angle less than about 45° relative to a horizontal plane), this may result in rear suspension  40  contacting seat tube  26 , seat post  32  or seat  34  when rear suspension  40  is in its most forwardly intrusive position, particularly in the case of long travel rear suspension systems. Further, such a sharply inclined seat tube  26  would restrict the amount of space available forward of seat tube  26  to accommodate rear shock absorber  42 . Conversely, if the orientation is overly upright (e.g. if the longitudinal axis of seat tube  26  extends at an actual angle more than about 75° degrees relative to a horizontal plane), then the rider will not be an optimum biomechanical position for uphill pedaling.  
         [0054]     As explained above, the principles of the present invention may apply to many alternative bicycle configurations, including bicycles having different frame  18  and/or rear suspension  40  configurations. Some examples of alternative arrangements are shown schematically in  FIGS. 6-10 .  FIG. 6-7  illustrates schematically the embodiment of  FIGS. 3-5  where linkage  44  is driven by rocker arm link  50  ( FIG. 6  showing this configuration in a compressed configuration and  FIG. 7  showing the same configuration in an uncompressed configuration).  
         [0055]      FIG. 8  illustrates an alternative arrangement where linkage  44  is driven by chain stay link  46 . In particular, instead of coupling linkage  44  to rocker arm  50 , pivot  102  is located rearwardly of rocker arm  50  and is coupled by means of linkages  110  and  112  to pivots  56  and  62  located on opposite ends of chain stay link  
         [0056]      FIG. 9  illustrates a further alternative arrangement where linkage  44  is driven by a single pivot suspension system employing a longer swing arm (i.e. chain stay link  46 ). In this arrangement rear wheel  14  is mounted at a fixed location on chain stay link  46  which is in turn connected directly to pivot  54 . In this embodiment pivot  62  is positioned at one end of seat stay link  48  and linkage  44  is driven by a combination of rocker link  50  and seat stay link  48 .  
         [0057]      FIG. 10  illustrates a walking beam four bar linkage utilizing the invention. In this case a modified walking beam linkage  114  extends between an upper end of seat stay link  48  and a pivot  116  mounted on seat tube  26 . Linkage  44  employs a push linkage  96  having one end pivotably coupled to walking beam linkage  114  by means of a pivot  115  and another end coupled to triangular a A-links  116  fixed to frame  18 . At least one segment of A-links  116  intersects a plane parallel to the longitudinal axis of seat tube  26  to couple push linkage  96  to shock absorber  42 . As in other embodiments of the invention, linkage  44  functions as an intermediate coupling for transferring rear suspension forces around seat tube  26  to shock absorber  42  located at a position forwardly of seat tube  26 .  
         [0058]     As will be appreciated by a person skilled in the art, many other alternative variations of linkage  44  may be envisioned for use in association with different rear suspension designs.  
         [0059]     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.