Patent Publication Number: US-7914031-B2

Title: Bicycle damper

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of, and claims priority to, U.S. Utility patent application Ser. No. 11/500,036, filed Aug. 7, 2006 now U.S. Pat. No. 7,690,666. 
    
    
     INCORPORATION BY REFERENCE 
     The entirety of U.S. Utility patent application Ser. No. 11/500,036, filed Aug. 7, 2006, is expressly incorporated by reference herein and made a part of the present specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to vehicle suspension systems. More specifically, the present invention relates to an improved shock absorber system to be incorporated into the suspension system of a bicycle. 
     2. Description of the Related Art 
     Bicycles intended for off-road use, i.e., mountain bikes, commonly include a suspension assembly operably positioned between the rear wheel of the bicycle and the frame of the bicycle. The suspension assembly typically includes a shock absorber configured to absorb forces imparted to the bicycle by bumps or other irregularities of the surface on which the bicycle is being ridden. However, an undesirable consequence of incorporating a suspension assembly in a bicycle is the tendency for the shock absorber to absorb a portion of the power output of a rider of the bicycle. In some instances, i.e. when the rider is standing, the proportion of power absorbed by the shock absorber may be substantial and may drastically reduce the efficiency of the bicycle. 
     Vehicle shock absorbers utilize inertia valves to sense rapid accelerations generated from a particular part of the vehicle. Inertia valves are also used to change the rate of damping in the shock absorber depending on the magnitude of the acceleration. As an example, the inertia valve assembly may be arranged to adjust the damping of the rear shock in accordance with accelerations that are generated by the body of the vehicle differently than it would adjust the damping of the rear shock for accelerations that are generated by the rear wheel of the vehicle. 
     One example of the type of shock absorber that utilizes an inertia valve to distinguish rider-induced forces from terrain-induced forces and is described in U.S. Pat. No. 6,604,751 B2. According to U.S. Pat. No. 6,604,751, the shock absorber of U.S. Pat. No. 6,604,751 is positioned between the swing arm and the main frame to provide resistance to the pivoting motion of the swing arm. The rear shock absorber includes a peripherally located fluid reservoir that is connected to the swing arm at a distance away from the shock body, and is hydraulically connected to the main shock body by a hydraulic hose. In one embodiment, the reservoir of U.S. Pat. No. 6,604,751 is connected to the swing arm portion of the bicycle above the hub axis of the rear wheel. 
     The inertia valve assembly of U.S. Pat. No. 6,604,751 discloses an inertia valve attempting to overcome the effects of external forces and manufacturing defects that inhibit the motion of the inertia valve with the use of a labyrinth seal having a series of “Bernoulli Steps” on an interior surface of the inertia mass. Also, the peripherally located reservoir of U.S. Pat. No. 6,604,751 discloses a blowoff valve that allows for an increased flow rate after a minimum threshold pressure is exceeded inside the blowoff chamber. Typically, this will occur when the bicycle hits a severe bump. Further, the refill ports and the axial blowoff passages of the shock absorber of U.S. Pat. No. 6,604,751 are located on the top surface of the reservoir. 
     However, the need exists for an improved, lightweight rear inertia valve shock. The availability of lightweight, high performance inertia valve shocks are critical to competition cyclists, where a reduction of even a few ounces can greatly benefit the cyclist, and significantly impact the desirability of the shock. 
     SUMMARY OF THE INVENTION 
     An aspect of one embodiment is a shock absorber for a bicycle comprising a primary unit, a remote unit that is substantially entirely outside of the primary unit, and an inertial valve within the remote unit. The primary unit comprises a damper tube, a spring chamber, and a piston rod that supports a main piston. The main piston is movable within the damper chamber of the primary unit. The main piston and the damper tube at least partially define a compression chamber. The remote unit comprises a remote fluid chamber. The inertial valve is preferably responsive to terrain-induced forces and preferably not responsive to rider-induced forces when the shock absorber is assembled to the bicycle. The shock absorber comprises a flow path separated from the piston rod that connects the remote fluid chamber and the compression chamber of the damper tube. 
     An aspect of one embodiment is a damper for a bicycle, comprising a primary unit comprising a damper tube, a piston rod that supports a main piston, a reservoir tube that is outside of compression chamber of the primary tube, and an inertia valve within the reservoir tube. The damper also comprises a flow path connecting the reservoir fluid chamber and the compression chamber of the primary tube. The main piston is movable within the damper chamber of the primary unit. The main piston and the damper tube at least partially define a compression chamber and a rebound chamber. The reservoir tube comprises a reservoir fluid chamber. At a piston speed of approximately 4 meters/second, at least 40% of the compression damping in the reservoir tube occurs in a circuit which is not closable by the inertia valve. 
     An aspect of one embodiment is a damper for a bicycle, comprising a primary unit comprising a damper tube, a piston rod that supports a main piston, a reservoir tube that is outside of the compression chamber of the primary tube, and an inertial valve within the reservoir tube. The damper also comprises a flow path connecting the reservoir fluid chamber and the compression chamber of the primary tube. The damper also comprises a damping valve in the reservoir tube. When the inertia valve is open, the damping valve opens before flow through the inertia valve is maximized. The main piston and the damper tube at least partially define a compression chamber and a rebound chamber. The main piston is movable within the damper chamber of the primary unit. The reservoir tube comprises a reservoir fluid chamber. The inertial valve is responsive to terrain-induced forces and not responsive to rider-induced forces when the shock absorber is assembled to the bicycle. 
     An aspect of one embodiment is a damper for a bicycle, comprising a primary unit comprising a damper tube, a piston rod that supports a main piston, a reservoir tube that is outside of compression chamber of the primary tube, an inertial valve within the reservoir tube, a flow housing within the reservoir tube, and a flow path connecting the reservoir fluid chamber and the compression chamber of the primary tube. The main piston is movable within the damper chamber of the primary unit. The main piston and the damper tube at least partially define a compression chamber and a rebound chamber. The reservoir tube comprises a reservoir fluid chamber. The flow housing defines a first end and a second end, a first one way valve positioned at the first end, and a second one way valve positioned at the second end. The inertia valve has an open position and a closed position. The inertial valve permits a flow of the fluid from the compression chamber of the primary tube to the reservoir fluid chamber of the reservoir tube when the inertial valve is in the open position and the flow through the inertia valve is reduced when the inertia valve is in the closed position. In one embodiment, the damping valve opens when there is 25 pounds of force on the damping valve. 
     An aspect of one embodiment is a shock absorber for a bicycle comprising a primary tube comprising a compression chamber and a spring chamber, a piston rod that supports a main piston, a remote tube that is separate from the primary tube, an inertial valve within the remote tube, a flow housing, and a flow path connecting the remote fluid chamber and the compression chamber of the primary tube. The main piston is movable within the compression chamber of the primary tube. The remote tube comprises a remote fluid chamber. The flow housing defines a first end and a second end, a first one way valve positioned at the first end, and a second one way valve positioned at the second end. The inertial valve is responsive to terrain-induced forces and not responsive to rider-induced forces when the shock absorber is assembled to the bicycle. The inertia valve has an open position and a closed position and permits a flow of the fluid from the compression chamber of the primary tube to the remote fluid chamber of the remote tube when the inertial valve is open and the flow through the inertia valve is reduced when the inertia valve is in the closed position. 
     An aspect of one embodiment is a shock absorber for a bicycle comprising a primary tube comprising a compression chamber and a spring chamber, a piston rod that supports a main piston, a remote tube that is separate from the primary tube, an inertial valve within the remote tube, a shaft within the remote tube defining a plurality of flow ports and an outer annular groove connecting the plurality of flow ports, and a flow path connecting the remote fluid chamber and the compression chamber of the primary tube. The main piston is movable within the compression chamber of the primary tube. The remote tube comprises a remote fluid chamber. The inertial valve is responsive to terrain-induced forces and not responsive to rider-induced forces when the shock absorber is assembled to the bicycle. The inertia valve has an open position and a closed position. The inertial valve permits a flow of the fluid from the compression chamber of the primary tube to the remote fluid chamber of the remote tube when the inertial valve is open and the flow through the inertia valve is reduced when the inertia valve is in the closed position. 
     An aspect of one embodiment is an inertia valve for a bicycle damper comprising a reservoir shaft defining a first inside surface and an outside surface, a groove formed in the outside surface of the reservoir shaft, a plurality of openings formed in the reservoir shaft between the inside surface and the outside surface, an inertia mass defining a second inside surface that faces the outside surface of the reservoir shaft, and a spring. The inertia valve defines a closed position wherein the second inside surface of the inertia mass substantially completely prevents fluid from flowing through the plurality of openings. The inertia mass also defines an open position wherein the fluid is permitted to flow through any of the plurality of openings. The fluid flowing in an outward direction through any of the plurality of openings flows into the groove. The inertia mass is biased toward the closed position by the spring. The second inside surface of the inertia mass is preferably spaced apart from the outside surface of the reservoir shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present bicycle shock absorber are described below with reference to drawings of preferred embodiments, which are intended to illustrate, but not to limit, the present invention. The drawings contain sixteen (16) figures. Sixteen figures are described herein. 
         FIG. 1  is a perspective view of a bicycle including a preferred rear shock absorber; 
         FIG. 2  is a cross-section of the rear shock absorber of  FIG. 1 ; 
         FIG. 3  is an exploded perspective view of the components of the rear shock absorber of  FIG. 1 ; 
         FIG. 4  is an enlarged cross-section of a main portion of the shock absorber of  FIG. 2 , showing the piston in an uncompressed position; 
         FIG. 5  is an enlarged cross-section of a main portion of the shock absorber of  FIG. 2 , showing the piston in a partially compressed position; 
         FIG. 6  is perspective view of the rebound side of a preferred piston component of the rear shock absorber of  FIG. 1 ; 
         FIG. 7  is perspective view of the compression side of a preferred piston component of the rear shock absorber of  FIG. 1 ; 
         FIG. 8  is an enlarged cross-section of a main portion of the shock absorber of  FIG. 1 , showing the flow path of hydraulic fluid through the piston during the compression motion of the rear shock; 
         FIG. 9  is an enlarged cross-section of a main portion of the shock absorber of  FIG. 1 , showing the flow path of hydraulic fluid through the piston during the rebound motion of the rear shock; 
         FIG. 10  is an enlarged cross-section of the reservoir of the shock absorber of  FIG. 1  showing an inertia valve in a closed position; 
         FIG. 11  is an exploded perspective view of the components of the reservoir of  FIG. 1 ; 
         FIG. 12  is an enlarged cross-section of the reservoir of  FIG. 1  showing the inertia valve being in a closed position; 
         FIG. 13  is an enlarged cross-section of the reservoir of  FIG. 1  showing the flow path of hydraulic fluid through the primary valve during the compression motion of the rear shock, the inertia valve being in a closed position; 
         FIG. 14  is an enlarged cross-section of the reservoir of  FIG. 1  showing the flow path of hydraulic fluid through the primary valve during the rebound motion of the rear shock, the inertia valve being in a closed position; 
         FIG. 15  is an enlarged cross-section of the reservoir of  FIG. 1  showing the inertia valve being in an open position; 
         FIG. 16  is an enlarged cross-section of the reservoir of  FIG. 1  showing the flow path of hydraulic fluid through the inertia valve during the compression motion of the rear shock, the inertia valve accordingly being in an open position; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a bicycle  20  (e.g., a mountain bike) having a preferred embodiment of a rear suspension assembly, or shock absorber, is illustrated. The bicycle  20  includes a frame  22 , preferably comprised of a generally triangular main frame portion  24  and an articulating frame portion, or subframe  26 , which is preferably pivotally connected to the seat post tube  25  of the main frame portion  24 . The bicycle  20  also includes a front wheel  28  and rear wheel  30 . The rear wheel  30  is connected to the subframe portion  26 . A seat  32 , to provide support to a rider in a sitting position, is connected to the seat post tube  25 . It is understood that in some embodiments, main frame portion  24  may not be generally triangular or have a seat tube which extends uninterrupted to the bottom bracket. 
     Positioned between the subframe  26  and the seat post tube  25  is a preferred embodiment of a rear shock  38 . It is noted that, while the shock  38  disclosed herein is described in the context of its use as a rear shock absorber for an off-road bicycle, the applicability of the invention is not so limited. Aspects of the invention can be utilized in bicycle forks. 
     The rear shock  38  provides resistance to the pivoting motion of the subframe  26 , providing a suspension spring and damping to the motion of the subframe  26 . Preferably, the spring is an air spring arrangement, but coil springs and other suitable arrangements may also be used. Thus, the bicycle  20  illustrated in  FIG. 1  includes a rear shock  38  between the rear wheel  30  and the frame  22 . In this configuration, the rear shock  38  substantially reduces the magnitude of the impact forces imparted on the rear wheel  30  by the terrain and felt by the operator of the bicycle. Referring to  FIG. 2  the rear shock  38  desirably includes a primary unit or main body portion  39  and a remote unit or secondary or reservoir body portion  44 . Note that the reservoir body portion  44  may be located adjacent to, or otherwise remote with respect to, the main body portion. However, in another embodiment, the reservoir body portion may be located within the main body portion. In some embodiments, the fluid reservoir body portion  44  is directly connected to the main body portion  39  external to the main body portion  39 . 
     As is discussed in detail below, the inertia valve described herein may advantageously be configured to be highly responsive to changes in the acceleration of the rear shock  38 . Further, in some embodiments, the inertia valve components described herein are relatively easy and cost effective to produce, resulting in low manufacturing costs and few production errors. As discussed, the rear shock  38  preferably includes an inertia valve  138  that varies the damping rate of the rear shock  38  depending upon the direction of an acceleration of the inertia valve  138 . In this configuration, the inertia valve  138  can distinguish between forces imparted on the rear wheel  30  originating from the rider of bicycle from forces imparted on the rear wheel  30  by bumps in the path of travel. Performance of the bicycle is improved when forces generated by the rider are more firmly damped and forces imparted on the rear wheel  20  by bumps in the road are damped more softly. This reduces or prevents shock absorber movement resulting from rider-induced forces, such as by pedaling, while allowing the shock absorber to compensate for forces imparted on the rear wheel  20  by uneven terrain. It is understood that in some embodiments, the shock absorber will move very little in response to rider induced pedal forces. 
     A preferred embodiment of the rear shock  38  is illustrated in  FIGS. 2-16 . Generally, the rear shock  38  comprises a spring, a main piston assembly, and a reservoir. In one embodiment, the spring comprises an air spring formed by an air tube  40  and a spring piston comprising a seal formed on the exterior of a hydraulic fluid body portion  42 . In the illustrated embodiment, reservoir body portion  44  is external to the main body portion  39  but is directly connected to the hydraulic fluid body portion  42  without long external passages, hydraulic hoses, or the like. The connection between the reservoir body portion  44  and the main body of the shock  38  can be achieved by any suitable means, such as by, but not limited to, threading or press-fitting the reservoir body portion  44  into the hydraulic fluid body portion  42 . Alternatively, the reservoir body portion  44  can be monolithically formed with the hydraulic fluid body portion  42 . 
       FIG. 3  is an exploded perspective view of the components that comprise the main body portion  39  of the rear shock  38 . Preferably, the main body portion  39  is generally comprised of a main piston or hydraulic fluid body portion  42 , a spring or air tube  40  closed by an upper cap  50 , a piston  68 , and a hydraulic fluid body portion cap  72 . The hydraulic fluid body portion  42  may be cylindrical in shape and includes an open end portion  54  and a lower closed end portion  56 . The lower closed end portion  56  has a lower eyelet  58  that is used for connecting the shock  38  to the subframe portion  26  of the bicycle  20  of  FIG. 1 . 
       FIG. 1  illustrates an embodiment of the rear shock  38  mounted in its preferred configuration to the main frame portion  24  (using upper eyelet  52 ) and the subframe portion  26  (using lower eyelet  58 ) of the bicycle  20 . With reference to  FIGS. 1 and 3 , it can be seen that the mounting planes of the upper eyelet  52  and the lower eyelet  58 , respectively, are not coplanar. The mounting plane of the lower eyelet  58  is clocked at a different orientation with respect to the mounting plane of the upper eyelet  52  because, as illustrated in  FIG. 1 , the subframe mounting tab  26   a  is positioned at a different orientation as compared to the mounting plane on the main frame portion  24 . However, while the orientation of the mounting plane of the lower eyelet  58  is not coplanar with the orientation of the mounting plane of the upper eyelet  52  in the embodiment illustrated in  FIGS. 1-3 , the respective orientations of the eyelets  52 ,  58  is not so limited. The mounting planes of the eyelet  52 ,  58  can be clocked at any orientation suitable for the frame to which the rear shock  38  is mounted. 
     The air tube  40  may also be cylindrical in shape. The air tube  40  includes an open end  48 . The opposite end is closed by an upper cap  50 . The upper cap  50  of the air tube  40  has an elongated portion  51  and an upper eyelet  52 . The upper eyelet  52  is used to connect the rear shock  38  to the seat post tube  25  of the bicycle  20 . The open end  48  of the air tube  40  slidingly receives the hydraulic fluid body portion  42 . In this configuration, the air tube  40  and the hydraulic fluid body portion  42  are configured for telescopic movement between the main frame portion  24  and the subframe portion  26  of the bicycle  20 . 
     In another embodiment, the orientation of the rear shock  38  may be changed such that the hydraulic fluid body portion  42  is attached to the seat post tube  25  (at the lower eyelet  58 ) while the air tube  40  is attached to the subframe  26  (at the upper eyelet  52 ). However, this is not preferred. 
     The air tube  40  has a seal assembly  60  positioned at the open end  48  thereof, forming a substantially airtight seal between the hydraulic fluid body portion  42  and the air tube  40 . In the illustrated embodiment, the seal assembly  60  is comprised of an annular seal body seal  62  having a substantially square cross-section that is located between a pair of bearings  64 . A wiper  66  is located adjacent the open end  48  of the air tube  40  to prevent dust, dirt, rocks, and other potentially damaging debris from entering into the air tube  40  as the hydraulic fluid body portion  42  moves into the air tube  40 . A piston member  68  is positioned within and slides relative to the inner surface of the hydraulic fluid body portion  42 . The piston member  68  is connected to the upper cap  50  by a shock shaft  70 , fixing the piston member  68  for motion within the air tube  40 . 
     As most clearly illustrated in  FIG. 4 , a hydraulic fluid body portion cap  72  is fixed to the open end portion  54  of the hydraulic fluid body portion  42  and is configured to allow the shock shaft  70  to slide within a central opening in the hydraulic fluid body portion cap  72 . The hydraulic fluid body portion cap  72  accordingly slides within the inner surface of the air tube  40 . Because the hydraulic fluid body portion cap  72  is easier to manufacture in two portions, the hydraulic fluid body portion cap  72  is preferably comprised of an upper cap portion  72   a  and a lower cap portion  72   b . After the lower cap portion  72   b  is inserted over the end of the hydraulic fluid body portion  42 , the upper cap portion  72   a  is preferably fixed to the hydraulic fluid body portion  42  by threading the upper cap portion  72   a  into threads formed on the inside surface of the hydraulic fluid body portion  42 . The upper cap portion  72   a  and lower cap portion  72   b  are configured such that, when the upper cap portion  72   a  is attached to the hydraulic fluid body portion  42  as described above, the lower cap portion  72   b  will also be firmly attached to the hydraulic fluid body portion  42 . Annular seals  82 ,  83  are preferably used to prevent hydraulic oil from leaking into the primary air chamber  86  and, similarly, to prevent the gas located in the primary air chamber  86  from leaking into the compression chamber  96 . 
     A seal assembly  74  is preferably positioned on the hydraulic fluid body portion cap  72 . The seal assembly  74  is preferably comprised of a seal member  76 , which is preferably an annular seal having a substantially round cross-section and is positioned between a pair of bearings  78 , and a bushing  84 . Together, the seal member  76  and the bushing  84  create a seal between the hydraulic fluid body portion cap  72  and the shock shaft  70 , while allowing the shock shaft  70  to translate within the hydraulic fluid body portion cap  72 . Note that the cross-section of the seal member  76  may be any suitable shape, such as square or rectangular. 
     A bottom out bumper  92  is desirably positioned near the closed end portion  50  of the air tube  40  to prevent direct metal to metal contact between the closed end portion  50  and the hydraulic fluid body portion cap  72  of the hydraulic fluid body portion  42  upon full compression of the rear shock  38 . The bottom out bumper  92  is preferably formed from a soft, pliable, and resilient material, such as rubber. The bottom out bumper  92  is positioned between two washers  94   a ,  94   b , which hold the bottom out bumper  92  in position next to the closed end portion  50 . Washers  94   a ,  94   b  can also be formed from a soft, pliable, and resilient material, such as rubber. Similarly, an annular rebound bumper  89  is preferably positioned around the outside of the hydraulic fluid body portion  42  below the hydraulic fluid body portion cap  72 , but above the bearings  64 . The rebound bumper  89  prevents metal to metal contact between the bottom portion of the hydraulic fluid body portion cap  72  and the constricted portion of the air tube  40 , and buffers the magnitude of the impact between the two components, at the end of the rebound motion of the rear shock  38 . 
     The space between the hydraulic fluid body portion cap  72  and the seal assembly  60  defines a second air chamber  88 . Air chamber  88  is most clearly illustrated in  FIG. 5 , which illustrates the main body of the rear shock  38  in a partially compressed state. Air that fills the second air chamber  88  exerts a pressure that resists the rebound motion of the rear shock  38 . Rebound motion is defined as the motion of the rear shock  38  that occurs when the shock  38  extends axially such that the closed ends  56  and  50  of the hydraulic fluid body portion  42  move away from each other. In conjunction, the primary air chamber  86  and the second air chamber  88  form the suspension spring portion of the rear shock  38 . An air valve  90  (see  FIGS. 2-3 ) communicates with the primary air chamber  86  to allow the air pressure therein to be adjusted. In this manner, the spring rate of the rear shock  38  may be easily adjusted. 
     The primary air chamber  86  is defined as the space between the closed end portion  50  of the air tube  40  and the hydraulic fluid body portion cap  72 . Air held within the primary air chamber  86  exerts a biasing force to resist compression motion of the rear shock  38 . Compression motion is defined as the motion of the rear shock  38  that occurs when the closed ends  56  and  50  of the hydraulic fluid body portion  42  and air tube  40  (and thus the eyelets  52 ,  58 ) move closer to one another. 
     The hydraulic fluid body portion  42  of the rear shock  38  will now be described in detail. The interior chamber of the hydraulic fluid body portion  42  is divided by the piston member  68  into two portions. The first portion is the compression chamber  96 . The second portion is the rebound chamber  98 . The rebound chamber  98  is defined to be the space between the piston member  68  and the hydraulic fluid body portion cap  72 . The rebound chamber  98  increases in volume during the compression motion of the rear shock  38 , and decreases in volume during the rebound motion of the rear shock  38 . The compression chamber  96  is defined as the space between the piston member  68  and the closed end portion  56  of the hydraulic fluid body portion  42 . The compression chamber  96  decreases in volume during compression motion of the rear shock  38 , and decreases in volume during the rebound motion of the rear shock  38 . As is stated above,  FIG. 4  illustrates an embodiment of the rear shock  38  wherein the piston  68  is in an uncompressed state.  FIGS. 5 ,  8 , and  9  illustrate an embodiment of the rear shock  38  wherein the piston  68  is in a partially compressed state. 
     As most clearly seen in  FIG. 4 , a hollow threaded fastener  100  fixes the piston member  68  to the shock shaft  70 . A seal  102 , of an annular type having a rectangular cross-section, is attached to the piston member  68  and seals the piston  68  with the inner surface of the hydraulic fluid body portion  42 . 
     In the illustrated embodiment, the piston member  68  preferably includes a plurality of compression flow passages  104 , each compression flow passage  104  preferably having an elongated shape. The plurality of compression flow passages  104  are most clearly seen in  FIGS. 6 and 7 . In various embodiments, the compression flow passages  104  may cumulatively perforate and, hence, allow the passage of hydraulic fluid through 10% to 60%, 15% to 40%, or 20% to 35% of included cross-sectional area of the piston  68 . As used herein, “included cross-sectional area” means the cross-sectional within the periphery of the piston member  68  in a plane perpendicular to the axis. In the case of the piston member  68 , the axis is aligned with the shock shaft  68 ). The compression flow passages  104  may cumulatively perforate and allow the passage of hydraulic fluid through at least 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% and 60% of the included cross-sectional area. 
     The compression flow passages  104  are covered on the rebound chamber  98  side of the piston member  68  by a shim stack  106 . The shim stack  106  can be made up of one or more flexible, preferably annular, shims. The shim stack  106  preferably operates as a one-way check valve—deflecting to allow a flow path of minimal restriction through the compression flow passages  104  during compression motion of the rear shock  38 , while preventing flow through the compression flow passages  104  during the rebound motion of the rear shock  38 . In the illustrate configuration, the shim stack  106  is preferably made up of multiple shims having a range of thicknesses, stiffnesses, and diameters that are preferably easily deflected to allow hydraulic fluid to flow with minimal restriction through compression flow passages  104  during compression motion of the rear shock  38 . The substantially unrestricted flow path of hydraulic fluid (represented by arrows) through the compression flow passages  104  and the deflection of the shim stack  106  during the compression motion of the rear shock  38  are illustrated in  FIG. 8 .  FIG. 8  also illustrates the flow of hydraulic fluid out of the secondary passage  113  into the rebound chamber  98 . For this flow path, the hydraulic fluid flows from the compression chamber  96  through the hollow pin  100  and the central passage  112  before flowing out of the secondary passage  113  and into the rebound chamber  98 . 
     As most clearly seen in  FIGS. 6 and 7 , the piston member  68  shown in the illustrated embodiment also comprises a plurality of rebound flow passages  108 , preferably three, through the piston member  68 . The rebound flow passages  108  preferably have axial through holes  108   a  and planar channels  108   b . The planar channels  108   b  are formed on the rebound side of the piston member  68  and permit fluid to bypass the compression shim stack  106  during the rebound motion of the rear shock  38 . As such, the hydraulic oil flows through both the planar channels  108   b  and the axial through holes  108   a  during the rebound motion of the rear shock  38 . A notable advantage of this configuration is that the size of the compression flow passages  104  can be increased to permit a very high flow rate of hydraulic fluid through the piston  68  during the compression motion without otherwise limiting the size of the, and, hence, the amount of fluid that can flow through the, rebound flow passages  108  that may otherwise be required if the planar channels  108   b  were not present. This also permits the piston member  68  to be formed from a single piece of material, instead of a multi-piece or cup design. 
     In certain embodiments, the rebound flow passages  108  may cumulatively perforate and, hence, allow the passage of hydraulic fluid through 2% to 25%, 5% to 15% to 5% to 10% of the included cross-sectional area. The rebound flow passages  108  may cumulatively perforate and, hence, allow the passage of hydraulic fluid through no more than 2%, 5%, 10% or 15% of the included cross-sectional area. 
     A rebound shim stack  110 , which can be made up of one or more flexible shims, is preferably positioned on the compression side of the piston member  68  adjacent to the planar channels  108   b . The rebound shim stack  110  deflects to allow, but to control the amount of, flow through the rebound flow passages  108  during the rebound motion of the rear shock  38 . The rebound shim stack  110  prevents flow through the rebound flow passages  108  during the compression motion of the rear shock  38 , but is preferably configured to not obstruct the flow of hydraulic oil through the more outwardly located compression flow passages  104  during compression motion. As such, the rebound shim stack  110  provides damping to the flow of hydraulic fluid through the piston  68  during the rebound motion of the rear shock  38 . 
       FIG. 9  illustrates the damped flow path of hydraulic fluid (represented by arrows) from the rebound chamber  98  through the rebound flow passages  108 , as well as the deflection of the shim stack  110 , during the rebound motion of the rear shock  38 .  FIG. 9  also illustrates the flow of hydraulic fluid from the rebound chamber  98 , through the secondary passage  113 , the central passage  112 , and the hollow pin  100  into the compression chamber  96 . 
     The shock shaft  70  defines a central passage  112  therethrough. The central passage  112  is in communication with the compression chamber  96  through the hollow pin  100 . The interior chamber of the reservoir body portion  44  also communicates with the compression chamber  96  through a passage  114  that goes through the closed end portion  56  of the hydraulic fluid body portion  42  of the main body portion  39 . This permits hydraulic fluid to flow between the reservoir body portion  44  and the compression chamber  96 . 
     As seen most clearly in  FIGS. 8 and 9 , a secondary passage  113  through the shock shaft  70  provides a port through which hydraulic fluid may flow between the central passage  112  and the compression chamber  96  when the shock is partially to fully compressed. When the rear shock  38  is in its substantially uncompressed state, as illustrated in  FIG. 4 , the bushing  84  and plate  115  substantially prevent the hydraulic fluid from flowing through the secondary passage  113  into the rebound chamber  98 . 
     An adjustment rod  116  is positioned concentrically within the central passage  112  of the shock shaft  70 , extending from the closed end portion  50  of the air tube  40 . The adjustment rod  116  is preferably configured to alter the damping force in the rear shock  38  by altering the amount of fluid that can flow through the secondary passage  113  upon compression motion and rebound motion. This is achieved by adjusting the adjustment rod  116  such that the annular ring  116   a  partially or fully blocks the secondary passage  113 , thus partially or fully preventing fluid from flowing through the secondary passage  113 . However, because in the configuration of the main body portion  39  illustrated in  FIGS. 2-9 , the compression flow passages  104  allow significantly more flow volume therethrough as compared to the rebound flow passages  108 , the additional volume of fluid that is permitted to flow through secondary passage  113  more significantly affects the rebound motion than the compression motion of the rear shock  38 . 
     Thus, while adjustment of the adjustment rod  116  alters fluid flow from the compression chamber  96  to the rebound chamber  98  during both compression motion and rebound motion, the adjustment rod  116  more significantly adjusts the fluid flow from the compression chamber  96  to the rebound chamber  98  during the rebound motion of the rear shock  38 . The rebound damping, as compared to the compression damping, is more greatly affected by the adjustment of the adjustment rod  116  for the following reason. Barring from consideration the flow restriction provided by the various shim stacks, as discussed above, the compression flow passages  104  are desirably configured to allow a greater flow rate therethrough as compared to the rebound flow passages  108 . This is because, as discussed above, the cumulative size of the openings comprising the compression flow passages  104  is desirably significantly greater than the cumulative size of the openings comprising the rebound flow passages  108 . 
     Further, the size of the opening comprising the secondary passage  113  is preferably much less than the cumulative size of the openings comprising the compression flow passages  104 . In certain embodiments, the size of the opening comprising the secondary passage  113  can be 2% to 30%, 5% to 25%, 10% to 20% of the cumulative cross-sectional area of the openings comprising the compression flow passages  104 . In certain embodiments, the size of the opening comprising the secondary passage  113  no more than 30%, 25%, 15%, 10%, 5% of the cumulative cross-sectional area of the openings comprising the compression flow passages  104 . Thus, the additional flow through the secondary passage  113  does not significantly increase the flow from the compression chamber  96  to the rebound chamber  98  during the compression motion of the rear shock  38 . 
     Similarly, the size of the opening comprising the secondary passage  113  is preferably less than the cumulative cross-sectional area of the openings comprising the rebound flow passages  108 . In certain embodiments, the cross-sectional area of the opening comprising the secondary passage  113  can be approximately 15% to approximately 35% of the cumulative cross-sectional area of the openings comprising the rebound flow passages  108 . In certain embodiments, the cross-sectional area of the opening comprising the secondary passage  113  is no more than 25% of the cumulative cross-sectional area of the openings comprising the rebound flow passages  108 . In sum, because the ratio of the size of the secondary passage  113  to the size of the openings comprising the rebound flow passages  108  is greater than the ratio of the size of the secondary passage  113  to the size of the openings comprising the compression flow passages  104 , allowing flow through the secondary passage  113  will more significantly affect the net overall flow during the rebound motion of the rear shock  38  as compared to the compression motion of the rear shock  38 . Therefore, adjustments to the adjustment rod  116  will preferably have a greater effect on rebound damping as compared to compression damping of the rear shock  38 . 
     As such, the adjustment rod  116  provides the user of the rear shock  38  with the ability to adjust the rebound damping of the rear shock  38 . An adjustment dial  118 , which is attached to the end of the rebound adjustment rod  116 , allows a user to adjust the adjustment rod  116  and, hence, the rebound damping rate of the rear shock  38 . The adjustment dial  118  is located on the outside of the rear shock  38 . Thus, it is easily accessible by the user. A ball detent mechanism  120  provides distinct adjustment positions of the adjustment dial  118 . 
     It is noted that, while the central passage  112  may be described as having a secondary passage  113 , the annular ring  116   a  of the adjustment rod  116  desirably does not completely prevent flow through the secondary passage  113  even in the fully blocked or closed position. That is, a fluid-tight seal is not typically created between the annular ring  116   a  of the adjustment rod  116  and the secondary passage  113  even in the fully blocked or closed position. Thus, some fluid may flow through the secondary passage  113  in its closed position. Such fluid flow is often referred to as “bleed flow” and, preferably, is limited to a relatively small flow rate. To create a fluid-tight seal between the above-referenced components would require precise dimensional tolerances, which would be expensive to manufacture, and may also inhibit movement of the adjustment rod  116  in the central passage  112 . 
     With reference to  FIGS. 10 through 16 , the components of the reservoir body portion  44  will now be described.  FIG. 11  is an exploded perspective view of the components that comprise the reservoir body portion  44  of the rear shock  38 . As most clearly shown in  FIG. 10 , the reservoir body portion  44  includes a reservoir tube  122 . The reservoir tube  122  is closed on both ends thereof. A floating reservoir piston  124  is positioned inside of the reservoir tube  122  and is in sliding communication with an inside surface of the reservoir tube  122 . A substantially fluid-tight seal between the interior surface of the reservoir tube  122  and the reservoir piston  124  is provided by the seal member  126 . Although other suitable seals may also be used, the seal member  126  is preferably a substantially round cross-section, annular seal. A low friction bushing  123  helps align the reservoir piston  124  on a reservoir adjustment rod  184 . 
     The interior space of the reservoir tube  122  is divided into a reservoir chamber  128  and a gas chamber  130  by the floating reservoir piston  124 . An end cap  132  closes the reservoir chamber  128  portion of the reservoir tube. A connector  133  attached to the end cap  132  allows the reservoir body portion  44  to interface with the closed end portion  56  of the hydraulic fluid body portion  42  so that hydraulic fluid can flow from the passage  114  in the closed end portion  56  of the hydraulic fluid body portion  42  to the reservoir chamber  128  of the reservoir body portion  44 . In this configuration, the passages  112  and  114  are in fluid communication with the central passage  136  of the reservoir shaft  134 , as well as with the compression chamber  96 . 
     An inertia valve assembly  138  is also supported by the reservoir shaft  134 . When in the open configuration, as illustrated in  FIGS. 15 and 16 , the inertia valve assembly  138  permits communication between the reservoir chamber  128  and the compression chamber  96  via the passages  114  and  136 . Stated another way, when the inertia valve assembly  138  is in the open configuration, hydraulic fluid is permitted to flow from the compression chamber  96  through the passage  114  and passage  136 , and out through reservoir shaft fluid ports  148  into the reservoir chamber  128 . 
     Cap  142  closes the gas chamber  130  end of the reservoir tube  122 . The cap  142  includes a valve assembly  144  to add or remove gas, such as nitrogen, for example, to or from the gas chamber  130 . The positive pressure exerted on the floating reservoir piston  124  by the pressurized gas within the gas chamber  130  causes the floating reservoir piston  124  to exert a pressure on the hydraulic fluid in the reservoir chamber  128 . In this configuration, the positive pressure causes the gas chamber  130  to expand to include any space made available when hydraulic fluid flows from the reservoir chamber into the compression chamber. It also improves the flow of fluid from the reservoir body portion  44  into the into the compression chamber  96  during the rebound motion of the rear shock  38 . 
     Referring to  FIGS. 10 and 11 , a primary valve assembly  140  is positioned above the inertia valve assembly  138  and is carried by the reservoir shaft  134 . A shoulder portion  154  is defined where the reservoir shaft  134  reduces in diameter. The shoulder  154  supports an annular washer  156 . The annular washer  156  supports the primary valve assembly  140 . The washer  156  also provides a buffer between the inertia mass  150  and the primary valve assembly  140 . 
     As is clearly illustrated in  FIG. 12 , the primary valve assembly  140  is generally comprised of a cylindrical base  158  and a cap  160 . The cap  160  is preferably threadably engaged with the base  158  and supported by an upper surface  164  of the base  158 . A cap seal  166  seals the cap  160  to the inner surface of the base  158 . The cap seal  166  is preferably an annular ring with a round cross-section, but the cap seal  166  can have any suitable configuration. The cap  160  is preferably threadably fastened to the base  158 . The base  158  is attached to the reservoir shaft  134  by a threaded fastener  168 . A primary valve chamber  170  is defined as the space between the cap  160  and the base  158 . The reservoir shaft  134  partially extends into the primary valve chamber  170  and has an open end such that the passage  136  is in communication with the primary valve chamber  170 . 
     The cap  160  has one or more axial compression flow passages  174 . The base  158  has one or more axial refill ports  176 . Because the axial refill passages are located in the base  158  and not in the cap  160  (where the compression flow passages  174  are located), the geometric configuration of the cap  160  is advantageously simplified. A further advantage of having the refill ports  176  in the base  158  as opposed to having them in the cap  160  along with the compression flow passages  174  is that the size of either the refill ports  176  or the compression flow passages  174  will not be constrained by the size limitations of the cap  160 . A compression flow shim stack  178 , which covers the compression flow passages  174 , is located above the cap  160 . A threaded fastener  169  secures the compression flow shim stack  178  in place. Once the threaded fastener  169  is threaded into the cap  160 , it can be held in place with an adhesive or other suitable material to prevent it from loosening. As will be discussed below, the threaded fastener  169  also comprises a bleed valve port  171  which adjustably provides another flow path for hydraulic fluid to flow from the primary valve chamber  170  to the reservoir chamber  128 . As discussed below, adjustment of the bleed valve port  171  adjusts the stiffness of the rear shock  38 . 
     As stated above, the illustrated embodiment preferably comprises a compression flow shim stack  178  to regulate the flow rate of hydraulic fluid through the compression flow passages  174 . In one embodiment, between 50 lbs and 75 lbs of force is required to be exerted on the compression flow shim stack  178  in order to deflect the compression flow shim stack  178  enough to allow the hydraulic fluid to flow through the compression flow passages  174  at a rate that allows the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 0.05 m/s. In another embodiment, between 25 lbs and 50 lbs of force is required to be exerted on the compression flow shim stack  178  in order to allow the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 0.05 m/s. 
     In certain embodiments, when there is 25 lbs, 35 lbs, 45 lbs, 55 lbs, 65 lbs or 75 lbs of force exerted on the compression flow shim stack  178 , the shim stack  178  deflects thereby opening the damping valve. Specifically, the compression flow shim stack  178  deflects enough to allow the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 0.05 meters/sec. 
     However, to regulate the flow rate of hydraulic fluid through the compression flow passages  174 , a flow element having a series of ports may be substituted for the shim stack  178 . In general, any of the shim stacks described herein may be replaced or augmented with a flow element having a series of ports for the purpose of regulating the flow rate of hydraulic fluid through the various components comprising the rear shock  38 . 
     The axial compression flow passages  174  may cumulatively perforate and, hence, allow the passage of hydraulic fluid through, 10% to 50%, or 25% to 35%, of the included surface area of the cap  160 . The axial refill ports  176  may cumulatively perforate and, hence, allow the passage of hydraulic fluid through, 10% to 50% or more of the included surface area of the base  158 . The axial refill ports  176  may cumulatively perforate and allow the passage of hydraulic fluid through 2% to 25% of the included surface area of the base  158 . The axial refill ports  176  may cumulatively perforate and allow the passage of hydraulic fluid through the base  158  at a flow rate approximately equal to the amount of flow of hydraulic fluid that is flowing through passage  114 , i.e., approximately equal to the amount of flow of hydraulic fluid that is flowing from the reservoir body portion  44  to the main body portion  39 . 
     In one embodiment, the compression flow shim stack  178  is configured to deflect to allow, but damp the flow rate of, hydraulic fluid through the compression flow passages  174  at normal operating pressures of the rear shock  38 . In certain embodiments, each of the shims comprising the compression flow shim stack  178  is preferably a bendable disc made from a metallic alloy. In one embodiment, five shims that are approximately 16 mm in diameter and 0.15 mm thick, stacked together, would produce a compression damping force of approximately 75-80 lbs at a rate of fluid flow that allows the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 0.05 m/s. In another embodiment, four shims that are approximately 16 mm in diameter and 0.15 mm thick, stacked together, would produce a compression damping force of approximately 65-70 lbs at a rate of fluid flow that allows the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 0.05 m/s. In another embodiment, three shims that are approximately 16 mm in diameter and 0.15 mm thick, stacked together, would produce a compression damping force of approximately 55-60 lbs at a rate of fluid flow that allows the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 0.05 m/s. In another embodiment, two shims that are approximately 16 mm in diameter and 0.15 mm thick, stacked together, would produce a compression damping force of approximately 45-50 lbs at a rate of fluid flow that allows the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 0.05 m/s, and so on. 
     The compression flow shim stack  178  of the present invention operates to damp the compression motion of the rear shock  38  and, accordingly, can be configured to deflect to allow hydraulic fluid to flow through the compression flow passages  174  at low or regular operating pressures within the primary valve chamber  170 . In one embodiment, approximately 90% or more of the compression motion damping of the rear shock is accomplished by the compression flow shim stack  178  located in the reservoir body portion  44 , whereas the remainder of the compression motion damping of the rear shock is accomplished by other components of the rear shock (e.g., the compression shim stack  106  located in the main body portion  39 ). In another embodiment, approximately 80% or more of the compression motion damping of the rear shock is accomplished by the compression flow shim stack  178  located in the reservoir body portion  44 . In yet another embodiment, approximately 70% or more of the compression motion damping of the rear shock is accomplished by the compression flow shim stack  178  located in the reservoir body portion  44 . In yet another embodiment, approximately 50% or more of the compression motion damping of the rear shock is accomplished by the compression flow shim stack  178  located in the reservoir body portion  44 . 
     As illustrated in  FIGS. 10 and 12 , a bleed valve plug  182  extends downwardly from below the reservoir piston  124 , and threads into a cylindrical interior threaded surface of the threaded fastener  169 . The reservoir adjustment rod  184  preferably inserts into the bleed valve plug  182  such that the bleed valve plug  182  is in rotational communication with the reservoir adjustment rod  184 . On its other end, the reservoir adjustment rod  184  is preferably attached to a reservoir adjustment dial  185 . The reservoir adjustment dial  185  is in communication with, but is free to rotate relative to, the cap  142 . In particular, a clip  189  inserted into a circumferential groove in the valve post  191  holds the reservoir adjustment dial  185  in communication with the cap  142 . A ball detent mechanism  187  provides distinct adjustment positions of the reservoir adjustment dial  185 . 
     Further, the bleed valve plug  182  defines a tip  182   a  that preferably adjustably regulates the flow of hydraulic fluid through a metering rod flow port  186  located in the end of the threaded fastener  169 . The tip  182   a  preferably defines a conically shaped surface that tapers to a smaller cross-sectional diameter toward the bottom end of the tip  182   a . The largest diameter of the conical portion is greater than the diameter of the cylindrical metering rod flow port  186 , and the smallest diameter of the conical portion is smaller than the diameter of the cylindrical metering rod flow port  186 . In this configuration, the flow of hydraulic oil through the metering rod flow port  186  can be reduced by engaging the tip  182   a  of the bleed valve plug  182  into the metering rod flow port  186 . Accordingly, the flow of hydraulic oil through the metering rod flow port  186  can be substantially prevented by fully engaging the tip  182   a  of the bleed valve plug  182  into the metering rod flow port  186 . However, some amount of flow may occur through a clearance space between the tip  182   a  and the metering rod flow port  186 , which may occur due to normal manufacturing variations. 
     As most clearly illustrated in  FIG. 12 , in this configuration, as the reservoir adjustment dial  185  is turned either clockwise or counter-clockwise, the axial position of the bleed valve plug  182  is preferably moved either up or down relative to the threaded fastener  169 , respectively, within the interior threaded surface of the threaded fastener  169 . As the bleed valve plug  182  is moved down relative to the threaded fastener  169 , the bleed valve plug  182  progressively blocks the bleed valve port  171  and metering rod flow port  186 , though not necessarily simultaneously. Thus, as the bleed valve plug  182  is rotated further into the threaded fastener  169 , the flow of hydraulic fluid through the bleed valve port  171  is substantially cut off. Because the bleed valve port  171  provides another, albeit more constricted, flow path for hydraulic fluid to flow from the primary valve chamber  170  into the reservoir chamber  128 , cutting off the flow of hydraulic fluid through the bleed valve port  171  effectively makes the rear shock  38  stiffer during the compression motion of the rear shock  38 . 
     In the illustrated embodiment, a single shim comprising the rebound flow shim stack  180  is preferably located between an annular ring  179  and the base  158 . However, the rebound flow shim stack  180  is not so limited. The rebound flow shim stack  180  can be comprised of multiple shims, similar to the compression flow shim stack  178  described above, and the reservoir body portion  44  may or may not have the annular ring  179 . The rebound flow shim stack  180  covers the refill ports  176 . The rebound flow shim stack  180  substantially prevents fluid from flowing from the primary valve chamber  170  to the reservoir chamber  128  through refill ports  176 , while not significantly affecting the rate of fluid flow from the reservoir chamber  128  into the primary valve chamber  170 . I.e., the rebound flow shim stack  180  prevents hydraulic fluid flow through refill ports  176  during the compression motion of the rear shock  38 , but does not substantially affect the flow rate of hydraulic fluid through the refill ports  176  during the rebound motion of the rear shock  38 . 
     In the illustrated embodiment, the damping control of the rebound motion of the rear shock  38  is advantageously located in the main shock body of the rear shock  38 , as opposed to being located in the reservoir body portion  44  as in other, conventional designs. Because the flow restriction, or damping, is located in the main shock body of the rear shock  38 , the flow of hydraulic fluid into the compression chamber  96  is not disturbed by cavitation or other flow disrupting effects that often result when the hydraulic fluid is sucked or pulled through the flow restriction devices or shim stacks that are located in the reservoirs of other, conventional designs. In the illustrated embodiment, during the rebound motion of the rear shock, a compressive force pushes the hydraulic fluid located in the rebound chamber  98  through the rebound flow passages  108 , thus avoiding cavitation and other flow efficiency effects that may otherwise result. 
     In certain embodiments, at least 90%, at least 80%, at least 70%, at least 60% or at least 50% of the rebound motion damping of the rear shock  38  is accomplished in the main body portion  39 , whereas the remainder of the rebound damping of the rear shock is accomplished by other components of the rear shock (preferably in the reservoir body portion  44 ). In one embodiment, this rebound damping in the main body portion  39  can be substantially accomplished by the rebound shim stack  110  located in the main body portion  39 . 
       FIG. 14  illustrates the flow of hydraulic fluid from the reservoir chamber  128 , around the cap  160  and the base  158  and through the rebound flow passages  176  and into the passage  136 , as well as the corresponding preferred deflection of the rebound flow shim stack  180 , when the inertia valve  138  is in the closed position. 
     As most clearly illustrated in  FIG. 16 , a plurality of radially extending reservoir shaft fluid ports  148 , each having a generally cylindrical geometry, extend through the reservoir shaft  134 . The reservoir shaft fluid ports  148  connect the passage  136  to the reservoir chamber  128 . As mentioned above, the inertia valve assembly  138  also includes an inertia mass  150  that is disposed in an upward position by a spring  152 , as is shown in FIGS.  10  and  12 - 14 . 
     The diameter of each reservoir shaft fluid port  148  may be between 0.5 mm and 5.0 mm. As illustrated, the reservoir shaft  134  preferably has a total of four equally spaced reservoir shaft fluid ports  148 , each with a diameter equal to approximately 1.0 mm. In another embodiment, the diameter of each reservoir shaft fluid port  148  is approximately 1.5 mm or more. In another embodiment, the diameter of each reservoir shaft fluid port  148  is approximately 2.0 mm or more. In another embodiment, the diameter of each reservoir shaft fluid port  148  is approximately 3.0 mm or more. In yet another embodiment, the diameter of each reservoir shaft fluid port  148  is approximately 4.0 mm or more. In another embodiment, the diameter of each reservoir shaft fluid port  148  is approximately 5.0 mm or more. In another embodiment, the reservoir shaft  134  may have six or more reservoir shaft fluid ports  148 , regardless of the diameter of the reservoir shaft fluid ports  148 . In certain embodiments, the total cross-sectional area of the reservoir shaft fluid ports  148  is 2 square millimeters to 100 square millimeters, 2 square millimeters to 80 square millimeters, 2 square millimeters to 60 square millimeters, 2 square millimeters to 40 square millimeters, 2 square millimeters to 20 square millimeters, 2 square millimeters to 10 square millimeters, or 2 square millimeters to 5 square millimeters. In certain embodiments, the total cross-sectional area of the reservoir shaft fluid ports  148  is no more than 12 square millimeters, no more than 10 square millimeters, no more than 8 square millimeters, no more than 6 square millimeters, or no more than 5 square millimeters. 
     Furthermore, in one embodiment, when the rear shock  38  encounters a bump that causes the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 1.0 m/s, the components comprising the inertia valve  136  will preferably be configured such that virtually all of the hydraulic fluid flows into the reservoir chamber  128  via the reservoir shaft fluid ports  148  and, accordingly, such that only a small volume of hydraulic fluid flows through the compression flow passages  174  at that rate of piston  68  movement. However, the inertia valve  136  of that same embodiment will preferably be configured such that, when the rear shock  38  encounters a more severe bump that causes the piston  68  to move within the hydraulic fluid body portion  42  at a rate of approximately 4.0 m/s, the components comprising the inertia valve  136  will preferably be configured such that approximately 20% or more of the total flow of hydraulic fluid flowing into the reservoir chamber  128  will flow through the reservoir shaft fluid ports  148  and approximately 80% or less of the total flow of hydraulic fluid flowing into the reservoir chamber  128  will flow through the compression flow passages  174 . 
     In certain embodiments, when the rear shock  38  encounters a more severe bump that causes the piston  68  to move at a rate of approximately 4.0 m/s, the components comprising the inertia valve  136  will preferably be configured such that at least 80%, at least 70%, at least 60%, at least 50%, at least 40%, or at least 35% of the total flow of hydraulic fluid flowing into the reservoir chamber  128  will flow through passages other than passages closable by the inertia mass  150  (in the illustrated embodiment, the compression flow passages  174  and the bleed valve port  171 ). 
     In certain embodiments, the inertia valve  136  will preferably be configured such that, when the rear shock  38  encounters a more severe bump that causes the piston  68  to move at a rate of approximately 4.0 m/s, the components comprising the inertia valve  136  will preferably be configured such that no more than 10%, no more than 20%, no more than 30%, no more than 40%, no more than 50% or no more than 60% of the total flow of hydraulic fluid flowing into the reservoir chamber  128  will flow through the passages closable by the inertia mass (in the illustrated embodiment, the reservoir shaft fluid ports  148 ). 
     The inertia mass  150  is preferably made from brass and preferably has a mass less than approximately two ounces. In another embodiment, the inertia mass  150  preferably has a mass less than approximately one and one-half ounces. In another embodiment, the inertia mass  150  has a weight of approximately 32 grams, or 1.13 ounces. In another embodiment, the inertia mass  150  preferably has a mass less than approximately one ounce. In yet another embodiment, the inertia mass  150  preferably has a mass less than or equal to approximately one-half ounce. The inertia mass  150  preferably is free of any axial passages or other sophisticated internal features or contours other than the main, cylindrical passage through the longitudinal center of the inertia mass  150 , and also the annular groove  151  on the inside surface of the inertia mass  150 . Without such passages and sophisticated internal features and contours, the inertia mass  150  is advantageously easier to manufacture, does not require substantial deburring on the internal surfaces, and is less likely to bind or stick to the reservoir shaft  134  as compared to other, conventional designs. Preferably, the inertia mass  150  has a streamlined geometric configuration such that the mass to fluid resistance ratio is increased. The annular groove  151  is preferably formed on the inside surface of the inertia mass  150  to limit the amount of surface area on the inside surface of the inertia mass  150  that may come into contact with the outer surface of the reservoir shaft  134  and, hence, limit the amount of drag between the two components. The inertia mass  150  may also have an annular groove  153  around the exterior of the inertia mass  150 . 
     As mentioned above, the spring  152  biases the inertia mass  150  into an upward, or closed, position wherein the inertia mass  150  covers the openings of the reservoir shaft fluid ports  148  to substantially prevent fluid flow from the passage  136  to the reservoir chamber  128 . Preferably, when the inertia mass  150  is in a closed (upward) position, flow to the reservoir chamber  128  primarily occurs through the compression flow passages  174  in the cap  160 .  FIG. 13  illustrates the flow of hydraulic fluid from the passage  136  through the compression flow passages  174  in the cap  160  and into the reservoir chamber  128 , as well as the corresponding preferred deflection of the compression flow shim stack  178 , when the inertia valve  138  is in the closed position. However, the flow path, but not necessarily the flow volume, of hydraulic fluid through the compression flow passages  174  in the cap  160  and into the reservoir chamber  128  may be as illustrated in  FIG. 13  even if the inertia valve  138  were in an open position. 
     The inertia mass  150  is also movable into a downward, or open, position against the biasing force of the spring  152 . In the open position, which is illustrated in  FIGS. 15 and 16 , the inertia mass  150  uncovers at least some of the reservoir shaft fluid ports  148  to allow fluid to flow therethrough, and a reduced compression damping rate is achieved. As illustrated in  FIG. 10 , the end cap  132  preferably operates as the lowermost stop surface for the inertia mass  150 .  FIG. 16  illustrates the flow of hydraulic fluid through the inertia valve  138  during the compression motion of the rear shock  38  while the inertia mass  150  is in the open position. In this configuration, hydraulic fluid flows from the passage  136  through the reservoir shaft fluid ports  148 , around the base  158  and cap  160  and into the reservoir chamber  128 . Note that, while the inertia mass  150  is in the open position, hydraulic fluid may still flow from the passage  136  through the compression flow passages  174  in the cap  160  and into the reservoir chamber  128 , as illustrated in  FIG. 13 , in addition to flowing through inertia valve. 
     It is noted that, while the inertia mass  150  may be described as having an open and a closed position, the inertia mass  150  likely does not completely prevent flow through the reservoir shaft fluid ports  148  in the closed position. That is, a fluid-tight seal is not typically created between the inertia mass  150  and the reservoir shaft  134  on which it slides. Thus, some fluid may flow through the inertia valve  138  in its closed position. Such fluid flow is often referred to as “bleed flow” and, preferably, is limited to a relatively small flow rate. To create a fluid-tight seal between the inertia mass  150  and the reservoir shaft  134  would require precise dimensional tolerances, which would be expensive to manufacture, and may also inhibit movement of the inertia mass  150  on the reservoir shaft  134  in response to relatively small acceleration forces. 
     With reference to  FIGS. 12-16 , another advantageous feature of the illustrated inertia valve  138  is a circumferential groove  188  around the exterior of the reservoir shaft  134 . The center plane of the groove  188  preferably aligns with the axial centerlines of each of the reservoir shaft fluid ports  148 . The groove  188  functions as a flow accumulator, equalizing the pressure of the hydraulic fluid emanating from the reservoir shaft fluid ports  148 . 
     As most clearly illustrated in  FIG. 16 , the groove  188  preferably comprises an upper chamfer portion  188   a , an arcuate portion  188   b , and a lower chamfer portion  188   c . The width of the groove  188  (i.e., the combined width of the upper chamfer portion  188   a , the arcuate portion  188   b , and the lower chamfer portion  188   c ) is preferably greater than the diameter of each of the reservoir shaft fluid ports  148  such the groove  188  extends both above and below each of the reservoir shaft fluid ports  148  and such that a significant amount of fluid can accumulate in the groove  188 . In another embodiment, the groove  188  could be smaller than the diameter of the ports  148 . The groove  188  allows the fluid pressure to be distributed evenly over the inner circumference of the inertia mass  150 . The even distribution of fluid pressure preferably creates a force tending to center the inertia mass  150  around the reservoir shaft  134 , thus partially or fully compensating for any inconsistencies in fluid pressure that would otherwise occur due to the locations or orientations of, or variations in size between, the reservoir shaft fluid ports  148 . Such a feature helps to prevent binding of the inertia mass  150  on the reservoir shaft  134 . The prevention of binding of the inertia mass  150  on the reservoir shaft  134  is beneficial in a bicycle application because it is desirable that the inertia valve be very sensitive to any terrain features which may only transmit relatively small acceleration forces to the inertia valve  138 . 
     The preferred configuration of the groove  188  illustrated in  FIG. 16  provides a nearly uniform (i.e., simultaneous) cutoff of hydraulic fluid flow emanating from each of the reservoir shaft fluid ports  148  as the inertia mass  150  reverts to its closed position. This is beneficial to ensuring that the inertia mass is not pushed off-center by the reservoir shaft fluid ports  148 . As discussed, the preferred configuration of the groove  188  also advantageously ensures that the inertia mass  150  is not pushed off-center by a non-uniform flow of hydraulic fluid through the reservoir shaft fluid ports  148 , or by non-uniform forces exerted by the hydraulic fluid flowing through the reservoir shaft fluid ports  148 , during the compression motion of the rear shock  38 . 
     Additionally, the chamfers  188   a  advantageously provide for a progressive shut off of hydraulic fluid flow through the reservoir shaft fluid ports  148  as the inertia mass  150  reverts to its closed position. In particular, as the acceleration causing the inertia mass  150  to move downward relative to the reservoir shaft fluid ports  148  is reduced, causing the inertia mass  150  to move upward, the inertia mass  150  first blocks the flow of hydraulic fluid flowing away from the lower chamfer portion  188   c , thus blocking only a portion of the hydraulic fluid flow going through the reservoir shaft fluid ports  148  in this position. The hydraulic fluid flowing from the lowest portion of the lower chamfer portion  188   c  is less than the hydraulic fluid flowing from the upper portion of the lower chamfer portion  188   c . Thus, as the hydraulic mass  150  continues to move upward, it progressively blocks a greater amount of the hydraulic fluid flowing away from the lower chamfer portion  188   c . As the hydraulic mass  150  continues to move upward, it progressively blocks a greater portion of the arcuate portion  188   b  and, finally, the upper chamfer portion  188   a , until substantially all of the hydraulic fluid flowing through the reservoir shaft fluid ports  148  is stopped. 
     Although the illustrated reservoir body portion  44  includes an inertia valve  138 , in other arrangements, the inertia valve  138  may be omitted or may be replaced with, or supplemented with, other compression or rebound fluid flow valves. However, the inertia valve  138  is preferred because it operates to distinguish terrain-induced forces from rider-induced forces. Terrain-induced forces are generally upwardly directed (compression) forces caused by the vehicle (such as a bicycle) encountering a bump. Rider-induced forces, in the case of a bicycle application, typically are short duration, relatively large amplitude forces generated from the pedaling action of the rider. The inertia valve may alternatively be configured to operate in response to rebound forces, rather than compression forces. 
     The operation of the rear shock  38  is now discussed in detail, with reference to  FIGS. 1-16 . As discussed above, the rear shock  38  is preferably mounted between the seat post tube  25  and the subframe portion  26  of the bicycle  20 . Preferably, the hydraulic fluid body portion  42  portion of the rear shock  38  is connected to the subframe portion  26  and the air tube  40  is connected to the seat post tube  25 . As shown in  FIG. 1 , the reservoir body portion  44  is preferably connected to the subframe portion  26  of the bicycle  20  near the rear axle. The rear shock  38  is capable of both compression and rebound motion. 
     When the rear wheel  30  of the bicycle  20  is impacted by a bump, the subframe portion  26  rotates with respect to the main frame portion  24 , tending to compress the rear shock  38 . The inertia mass  150  is biased by the force of the spring  152  to remain in the closed position. The closed position of the inertia valve  138  is illustrated in  FIGS. 10 , and  12 - 14 . In order for the inertia mass  150  to overcome the force of the spring  152  and move to an open position such that fluid flows from the passage  136  through the reservoir shaft fluid ports  148  and into the reservoir chamber  128 , the inertia mass  150  must be in an open position. The open position of the inertia mass  150  is shown in  FIGS. 15 and 16 . The inertia mass  150  translates to the open position if the acceleration experienced by the reservoir body portion  44  along its longitudinal axis exceeds a predetermined threshold value. 
     For compression motion of the rear shock  38  (i.e., for the piston member  68  to move into the hydraulic fluid body portion  42 ), the fluid that is displaced from the shock shaft  70  must flow into the reservoir chamber  128 . However, when the inertia mass  150  is in a closed position with respect to the reservoir shaft fluid ports  148 , fluid flow into the reservoir chamber  128  is preferably substantially impeded. When the inertia valve  138  is in the closed position, the rear shock  38  preferably remains substantially rigid. 
     However, even if the inertia valve  138  remains in the closed position, fluid can still transfer from the compression chamber  96  into the reservoir chamber  128  if the compressive force exerted on the rear shock  38  is of a magnitude sufficient to increase the fluid pressure within the primary valve chamber  170  to an amount that will cause the compression flow shim stack  178  to open and allow fluid to flow from the primary valve chamber  170  through the compression flow passages  174  and into the reservoir chamber  128 . 
     In the configurations described herein, the spring force of the rear shock  38  is produced by the pressure of the gas in the primary air chamber  86 . The damping rate in compression is determined mainly by the flow through the compression flow passages  174  in the reservoir body portion  44 , as well as the less significant damping effects produced by the compression shim stack  106  in the main body portion  39 . 
     If a sufficient magnitude of acceleration is imposed along the longitudinal axis of the reservoir body portion  44  (i.e., the axis of travel of the inertia mass  150 ), the inertia mass  150  will overcome the biasing force of the spring  152  and move downward relative to the reservoir shaft  134  into an open position. The open position of the inertia mass is illustrated in  FIGS. 15 and 16 . With the inertia valve  138  in the open position, hydraulic fluid is able to be displaced from the compression chamber  96  through the passages  112 ,  114  and the shaft passage  136 , through the reservoir shaft fluid ports  148  and into the reservoir chamber  128 . Thus, the rear shock  38  is able to be compressed and the compression damping is preferably determined primarily by flow through the compression flow passages  174  in the reservoir body portion  44  as well as the reservoir shaft fluid ports  148 . 
     The mass of the inertia mass  150 , the spring rate of the spring  152 , and the preload on the spring  152  determine the minimum threshold for the inertia mass  150  to overcome the biasing force of the spring  152  and move to the open position. The spring rate of the spring  152  and the preload on the spring  152  are preferably selected such that the inertia mass  150  is biased by the spring  152  into a closed position when no upward acceleration is imparted in the axial direction of the reservoir body portion  44 . However, the inertia mass  150  will preferably overcome the biasing force of the spring  152  when subject to an acceleration that is between 0.1 and 3 times the force of gravity (G&#39;s). Preferably, the inertia mass  150  will overcome the biasing force of the spring  152  upon experiencing an acceleration that is between 0.25 and 1.5 G&#39;s. However, the predetermined threshold may be varied from the values recited above. 
     With reference to  FIGS. 15 and 16 , when the inertia mass  150  is in the open position, the spring  152  exerts a biasing force on the inertia mass  150  which tends to move the inertia mass  150  toward the closed position. Advantageously, with the exception of the spring biasing force and fluid resistance, the inertia mass  150  moves freely within the body of fluid contained in the reservoir chamber  128  to increase the responsiveness of the inertia valve  138  and, hence, the rear shock  38  to forces exerted on the rear wheel  30 . The inertia valve  138  differentiates between bumpy surface conditions and smooth surface conditions, and alters the damping rate accordingly. During smooth surface conditions, the inertia valve  138  remains in a closed position and the damping rate is desirably firm, thereby inhibiting suspension motion due to the movement of the rider of the bicycle  20 . When the first significant bump is encountered, the inertia valve  138  opens to advantageously lower the damping rate so that the bump may be absorbed by the rear shock  38 . 
     Once the rear shock  38  has been compressed, either by fluid flow through the primary valve assembly  140  or the inertia valve  138 , the spring force generated by the combination of the primary air chamber  86  and the second air chamber  88  tend to bias the hydraulic fluid body portion  42  away from the air tube  40 . In order for the rear shock  38  to rebound, a volume of fluid equal to the displaced volume of the shock shaft  70  must be drawn from the reservoir chamber  128  and into the compression chamber  96 . Fluid flow is allowed in this direction through the refill ports  176  in the primary valve assembly  140  against a desirably light resistance offered by the rebound flow shim stack  180 . Gas pressure within the gas chamber  130  exerting a force on the floating reservoir piston  124  may assist in this refill flow. Thus, the rebound damping rate is determined primarily by fluid flow through the rebound flow passages  108  against the biasing force of the rebound shim stack  110 . 
     As discussed, the present rear shock  38  includes an inertia valve  138  comprising an inertia mass  150  and a reservoir shaft  134  having a circumferential groove  188  in the reservoir shaft  134  aligned with the reservoir shaft fluid ports  148  to create an even distribution of fluid pressure on the inertia mass  150  and, hence, prevent the inertia mass  150  from binding on the reservoir shaft  134 . The off-center condition of the inertia mass  150  may cause it to contact the reservoir shaft  134  causing friction, which tends to impede motion of the inertia mass  150  on the reservoir shaft  134 . Due to the relatively small mass of the inertia mass  150  and the desirability of having the inertia mass  150  respond to small accelerations, any friction between the inertia mass  150  and the reservoir shaft  134  seriously impairs the performance of the inertia valve  138  and may render it entirely inoperable. The off-center condition may result from typical errors associated with the manufacturing processes needed to produce the components of the inertia valve  138 . Further, the binding effect of the inertia mass  150  may result from burrs located on the inner surface of the inertia mass  150  or the outer surface of the reservoir shaft  134 . Because the inertia mass  150  advantageously has a generally smooth inner surface, the deburring operations on the inside surface of the inertia mass  150  are substantially simplified and the risk of binding is substantially reduced. 
     As the accompanying figures show, the rear shock  38  has other features and components such as seals which will are shown but not described herein that are obvious to one of ordinary skill in the art. Accordingly, a discussion of these features has been omitted. 
     Although the present invention has been explained in the context of several preferred embodiments, minor modifications and rearrangements of the illustrated embodiments may be made without departing from the scope of the invention. For example, but without limitation, although the preferred embodiments described the bicycle damper for altering the rate of compression damping, the principles taught may also be utilized in damper embodiments for altering rebound damping, or for responding to lateral acceleration forces, rather than vertical acceleration forces. In addition, although the preferred embodiments were described in the context of an off-road bicycle application, the present damper may be modified for use in a variety of vehicles, or in non-vehicular applications where dampers may be utilized. Furthermore, the pressure and flow equalization features of the inertia valve components may be applied to other types of valves, which may be actuated by acceleration forces or by means other than acceleration forces. Accordingly, the scope of the present invention is to be defined only by the appended claims.