Patent Publication Number: US-10316924-B2

Title: Front bicycle suspension assembly with inertia valve

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation application of and claims the benefit of co-pending U.S. patent application Ser. No. 14/294,113, filed Jun. 2, 2014, entitled, “FRONT BICYCLE SUSPENSION ASSEMBLY WITH INERTIA VALVE”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 14/294,113 is a continuation application of and claims the benefit of U.S. patent application Ser. No. 13/663,019, filed Oct. 29, 2012, now Issued U.S. Pat. No. 8,770,360, entitled, “FRONT BICYCLE SUSPENSION ASSEMBLY WITH INERTIA VALVE”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 13/663,019 is a continuation application of and claims the benefit of U.S. patent application Ser. No. 12/848,736, filed Aug. 2, 2010, now Issued U.S. Pat. No. 8,297,417, entitled, “FRONT BICYCLE SUSPENSION ASSEMBLY WITH INERTIA VALVE”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 12/848,736 is a continuation application of and claims the benefit of U.S. patent application Ser. No. 11/750,901, filed May 18, 2007, now Issued U.S. Pat. No. 7,766,135, entitled, “FRONT BICYCLE SUSPENSION ASSEMBLY WITH INERTIA VALVE”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 11/750,901 is a continuation application of and claims the benefit of U.S. patent application Ser. No. 11/259,629, filed Oct. 26, 2005, now Issued U.S. Pat. No. 7,273,137, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 11/259,629 is a continuation application of and claims the benefit of U.S. patent application Ser. No. 10/778,882, filed Feb. 13, 2004, now Issued U.S. Pat. No. 7,128,192, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/778,882 claims the benefit of U.S. Provisional patent application 60/451,303, filed Feb. 28, 2003, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/778,882 claims the benefit of U.S. Provisional patent application 60/451,318, filed Feb. 28, 2003, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/778,882 is a continuation-in-part application of and claims the benefit of U.S. patent application Ser. No. 10/378,091, filed Feb. 28, 2003, now abandoned, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/378,091 is a continuation-in-part application of and claims the benefit of U.S. patent application Ser. No. 10/043,079, filed Jan. 9, 2002, now Issued U.S. Pat. No. 6,581,948, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/043,079 claims the benefit of U.S. patent application 60/329,042, filed Oct. 12, 2001, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/043,079 claims the benefit of U.S. patent application 60/316,442, filed Aug. 30, 2001, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/378,091 is a continuation-in-part application of and claims the benefit of U.S. patent application Ser. No. 10/042,767, filed Jan. 9, 2002, now Issued U.S. Pat. No. 6,604,751, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/042,767 claims the benefit of U.S. patent application 60/329,042, filed Oct. 12, 2001, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
     The patent application Ser. No. 10/042,767 claims the benefit of U.S. patent application 60/316,442, filed Aug. 30, 2001, entitled, “INERTIA VALVE SHOCK ABSORBER”, by Robert C. Fox, assigned to the assignee of the present application, which is incorporated herein in its entirety by reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to vehicle suspensions systems. More particularly, the present invention relates to acceleration sensitive damping arrangements suitable for use in vehicle dampers (e.g., shock absorbers, struts, front forks). 
     Description of the Related Art 
     Inertia valves are utilized in vehicle shock absorbers in an attempt to sense instantaneous accelerations originating from a particular portion of the vehicle, or acting in a particular direction, and to alter the rate of damping accordingly. For example, the inertia valve may be configured to sense vertical accelerations originating at the sprung mass (e.g., the body of the vehicle) or at the unsprung mass (e.g., a wheel and associated linkage of the vehicle). Alternatively, the inertia valve may be configured to sense lateral accelerations of the vehicle. 
     Despite the apparent potential, and a long history of numerous attempts to utilize inertia valves in vehicle suspension, commercial inertia valve shock absorbers have enjoyed only limited success. Most attempted inertia valve shock absorbers have suffered from unresponsive or inconsistent operation due to undesired extraneous forces acting on the inertia valve. These extraneous forces may result from manufacturing limitations and/or external sources and often inhibit, or even prevent, operation of the inertia valve. 
     Further, there are currently no commercially available inertia valve shock absorbers for off-road bicycle, or mountain bike, applications. The problems associated with the use of inertia valves, mentioned above in relation to other vehicles, are magnified in the environment of lightweight vehicles and the relatively small size of mountain bike shock absorbers. Therefore, a need exists for an inertia valve shock absorber that can be commercially produced, and provides responsive, consistent performance without the problems associated with prior inertia valve designs. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment is a shock absorber comprising a first fluid chamber, a second fluid chamber and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass movable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position permits a second rate of fluid flow through the fluid circuit in the second position of the inertia mass. The second rate of fluid flow is non-equal to the first rate. A leading surface of the inertia mass when moving in a direction from the first position to the second position defines a leading surface area. A ratio of a mass of the inertia mass to the leading surface area is greater than about 130 grams per square inch. 
     A preferred embodiment is a shock absorber including a first fluid chamber, a second fluid chamber and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass movable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position and permits a second rate of fluid flow in the second position. The second rate of fluid flow is non-equal to the first rate. A ratio of a mass of the inertia mass to a volume of the inertia mass is greater than about 148 grams per cubic inch. 
     A preferred embodiment is a shock absorber including a first fluid chamber, a second fluid chamber, and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass movable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position of the inertia mass and a second rate of fluid flow in the second position of the inertia mass. The second rate of fluid flow is non-equal to the first rate. At least a portion of the inertia mass comprises tungsten. 
     A preferred embodiment is a shock absorber including a first fluid chamber, a second fluid chamber, and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass movable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position of the inertia mass and a second rate of fluid flow through the fluid circuit in the second position. The second rate of fluid flow is non-equal to the first rate. The inertia mass comprises a first portion and a second portion. The first portion is constructed from a first material having a first density and the second portion being constructed from a second material having a second density, the second density being greater than the first density. 
     A preferred embodiment is a shock absorber including a first fluid chamber, a second fluid chamber, and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass moveable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position of the inertia mass and a second rate of fluid flow in the second position. The second rate of fluid flow is non-equal to the first rate. The inertia mass includes a collapsible section defining at least a portion of an external surface of the inertia mass. The collapsible section has a first orientation when the inertia mass is moving in a first direction from the first position to the second position and a second orientation when the inertia mass is moving in a second direction from the second position to the first position. The inertia mass has a first flow resistance when the collapsible section is in the first orientation and a second flow resistance when the collapsible section is in the second orientation. The second flow resistance is greater than the first flow resistance. 
     A preferred embodiment is a shock absorber including a first fluid chamber, a second fluid chamber, and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass moveable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position of the inertia mass and a second rate of fluid flow in the second position. The second rate of fluid flow is non-equal to the first rate. The inertia mass includes first and second opposing end surfaces oriented generally normal to a direction of motion of the inertia mass and a side wall extending between the first and second end surfaces. The inertia mass additionally includes at least one movable, annular skirt extending from the side wall. At least an outer portion of the at least one skirt moves toward the side wall when the inertia mass moves in a first direction and moves away from the side wall when the inertia mass moves in a second direction opposite the first direction. The at least one skirt increases a fluid flow drag coefficient of the inertia mass when moving in the second direction compared to the drag coefficient of movement of the inertia mass in the first direction. 
     A preferred embodiment is a method of delaying an inertia valve within a shock absorber from returning to a closed position after an acceleration force acting on the inertia valve diminishes. The method includes providing an inertia mass movable in a first direction from a closed position toward an open position of the inertia valve in response to an acceleration force above a predetermined threshold and movable in a second direction from the open position toward the closed position of the inertia valve when the acceleration force is below the threshold. The method further includes configuring the inertia mass to have a first fluid flow drag coefficient when moving in the first direction. The method also includes providing the inertia mass with a drag member configured to increase the fluid flow drag coefficient when the inertia mass moves in the second direction to delay the inertia valve from returning to the closed position until a period of time after the acceleration force is reduced to, and remains, below the threshold. 
     A preferred embodiment is a shock absorber including a first fluid chamber, a second fluid chamber, and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass and a stop. The inertia mass is movable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position of the inertia mass and a second rate of fluid flow through the fluid circuit in the second position of the inertia mass. The second rate of fluid flow is non-equal to the first rate. One of the inertia mass and the stop defines a pocket for receiving the other of the inertia mass and the stop in the second position of the inertia mass. A first refill passage connects the second fluid chamber and the pocket and restricts fluid flow therethrough from the second fluid chamber to the pocket to provide a delay in movement of the inertia mass toward the first position. A second refill passage connects the second fluid chamber and the pocket and a pressure actuated valve substantially prevents fluid flow between the second fluid chamber and the pocket through the second refill passage below a predetermined threshold pressure differential between the second fluid chamber and the first fluid chamber. The pressure actuated valve permits fluid flow between the second fluid chamber and the pocket through the second refill passage at, or above, a predetermined threshold pressure differential between the second fluid chamber and the first fluid chamber, thereby reducing or eliminating the delay. 
     A preferred embodiment is a method of delaying an inertia valve within a shock absorber from returning to a closed position after an acceleration force acting on the inertia valve diminishes. The method includes providing an inertia mass movable in a first direction from a closed position toward an open position of the inertia valve in response to an acceleration force above a predetermined threshold and movable in a second direction from the open position toward the closed position of the inertia valve when the acceleration force is below the threshold. The method further includes providing a first delay force tending to resist movement of the inertia mass in the second direction when a fluid pressure differential between a first chamber and a second chamber within the shock absorber is below a predetermined threshold. The method also includes providing a second delay force, less than the first delay force, when the fluid pressure differential is at, or above, the predetermined threshold. 
     A preferred embodiment is a shock absorber including a first fluid chamber, a second fluid chamber and a fluid circuit connecting the first fluid chamber and the second fluid chamber. An inertia valve includes an inertia mass and a moveable stop. The inertia mass is movable between an open position and a closed position. The moveable stop is movable between a first position and a second position. The inertia mass is biased to move toward the closed position at substantially a first rate. The moveable stop and the inertia mass cooperate to define a pocket configured to receive the other of the moveable stop and the inertia mass in the open position of the inertia mass and the first position of the moveable stop. The movement of the inertia mass toward the closed position is restrained to a second rate less than the first rate. The moveable stop moves from the first position to the second position in response to a pressure within the second fluid chamber being greater than a pressure within the first fluid chamber by at least a predetermined pressure differential threshold, thereby permitting the inertia mass to return to the closed position at substantially the first rate. 
     A preferred embodiment is a damper including a first fluid chamber and a second fluid chamber. A fluid circuit connects the first fluid chamber and the second fluid chamber. An acceleration sensor is configured to produce a control signal in response to an acceleration force above a first predetermined threshold. The damper also has an inertia valve including an inertia mass that at least partially comprises a magnetic material and is movable between a first position and a second position. The inertia valve permits a first rate of fluid flow through the fluid circuit in the first position of the inertia mass and a second rate of fluid flow through the fluid circuit in the second position of the inertia mass. The second rate of fluid flow is non-equal to the first rate. The inertia mass moves in a direction from the first position to the second position in response to an acceleration force above a second predetermined threshold. An electromagnetic force generator is capable of retaining the inertia mass in the second position. A control system is configured to receive the control signal from the sensor and selectively activate the electromagnetic element in response to the control signal to retain the inertia mass in the second position for a predetermined period of time after the acceleration force diminishes below the first predetermined threshold. 
     A preferred embodiment is a bicycle including a front wheel defining a hub axis, a rear wheel, and a main frame. An acceleration sensor is mounted for movement with the hub axis of the front wheel and is configured to produce a control signal in response to sensing an acceleration above a predetermined threshold. A shock absorber is operably positioned between the rear wheel and the frame. The shock absorber includes a valve arrangement configured to receive the control signal from the sensor and to selectively alter a damping rate of the shock absorber in response to the control signal. 
     A preferred embodiment is a bicycle including a front wheel defining a hub axis, a rear wheel, and a main frame. An acceleration sensor is mounted for movement with the hub axis of the front wheel and is configured to produce a control signal in response to sensing an acceleration above a predetermined threshold. A shock absorber is operably positioned between the front wheel and the frame and includes a valve arrangement configured to receive the control signal from the sensor. The valve arrangement is configured to selectively alter a damping rate of the shock absorber in response to the control signal. 
     A preferred embodiment is a method of altering a rate of damping of a bicycle rear wheel shock absorber including sensing an acceleration force above a predetermined threshold acting on a hub axis of a front wheel of said bicycle. The method further includes providing a valve assembly within said rear wheel shock absorber configured to selectively alter a damping rate of said rear wheel shock absorber, and altering said damping rate of said rear wheel shock absorber in response to an acceleration force above said predetermined threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the damper will now be described with reference to drawings of preferred embodiments. The embodiments are illustrated in the context of use on an off-road bicycle, however, these embodiments are merely intended to illustrate, rather than limit, the present invention. The drawings contain the following figures: 
         FIG. 1  is a perspective view of a bicycle including preferred front and rear shock absorbers; 
         FIG. 2  is a cross-section of the rear shock absorber of  FIG. 1 ; 
         FIG. 3 a    is an enlarged cross-section of a main portion of the shock absorber of  FIG. 2  and  FIG. 3 b    is an enlarged cross-section of a reservoir of the shock absorber of  FIG. 2  showing an inertia valve in a closed position; 
         FIG. 4 a    is a top plan view of the inertia mass of the shock absorber of  FIG. 2 .  FIG. 4 b    is a side cross-section view of the inertia mass of  FIG. 2  taken along line  4   b - 4   b  in  FIG. 4 a   .  FIG. 4 c    is a bottom plan view of the inertia mass of  FIG. 2 ; 
         FIG. 5  is an enlarged cross-section of the reservoir of the shock absorber of  FIG. 2 , showing the inertia valve in an open position; 
         FIG. 6  is an enlarged cross-section of the inertia valve of the shock absorber of  FIG. 2 ; 
         FIG. 7 a    is an enlarged view of a portion of the inertia valve of  FIG. 6 .  FIG. 7 b    is an enlarged view of a portion of an alternative inertia valve; 
         FIG. 8  is a graph illustrating the relationship between position, velocity and acceleration for a simple mass; 
         FIG. 9  is a schematic illustration of an inertia valve in an off-center condition; 
         FIG. 10  is a schematic illustration of an inertia valve in a second off-center condition; 
         FIG. 11  is a cross-section view of the inertia valve of  FIG. 3 b    showing various zones of cross-sectional fluid flow areas; 
         FIG. 12  is a cross-section view of the inertia valve of  FIG. 3 b    in an off-center condition; 
         FIG. 13  is an enlarged view of an adjustable return fluid flow beneath the inertia mass; 
         FIG. 14  is the front shock absorber, or suspension fork, of  FIG. 1  as detached from the bicycle; 
         FIG. 15  is a cross-section view of the right leg of the fork of  FIG. 14 , illustrating various internal components; 
         FIG. 16  is an enlarged cross-section of a lower portion of the fork leg of  FIG. 15 , illustrating an inertia valve damping system; 
         FIG. 17  is an enlarged cross-section of a base valve assembly of the lower portion of the fork leg of  FIG. 16 ; 
         FIG. 18  is a cross-section view of the lower portion of the fork of  FIG. 15 , with the inertia valve in an open position; 
         FIG. 19  is the base valve assembly of  FIG. 17 , with the inertia valve in an open position; 
         FIG. 20  is a cross-section view of the lower portion of the fork of  FIG. 16  illustrating rebound fluid flow; 
         FIG. 21  is the base valve assembly of  FIG. 17  illustrating rebound fluid flow; 
         FIG. 22  is a cross-section view of a lower portion of an alternative embodiment of a suspension fork; 
         FIG. 23  is an enlarged view of the base valve assembly of the fork of  FIG. 22 , with the inertia valve in a closed position; 
         FIG. 24  is the lower portion of the fork of  FIG. 22 , with the inertia valve in an open position; 
         FIG. 25  is the base valve assembly of  FIG. 23 , with the inertia valve in an open position; 
         FIG. 26  is a graph of the pressure differential of fluid acting on the left and right sides of the inertia mass versus internal diameter of the inertia mass; 
         FIG. 27  is a graph of the pressure differential factor of fluid acting on the left and right sides of the inertia mass versus the internal diameter of the inertia mass for a radial gap between the inertia mass and shaft of 0.002 inches; and 
         FIG. 28  is a graph of the pressure differential factor of fluid acting on the left and right sides of the inertia mass versus the internal diameter of the inertia mass for a radial gap between the inertia mass and shaft of 0.001 inches. 
         FIG. 29  is an enlarged, cross-section view of an alternative inertia valve assembly comprising an inertia mass having increased density, in comparison to the embodiments of  FIGS. 1-28 , in order to provide increased responsiveness to acceleration forces. 
         FIG. 30  is an enlarged view of an alternative embodiment of an inertia mass including a plurality of drag members to increase the fluid drag on the inertia mass when moving in one direction in comparison with the drag on the inertia mass during movement in the opposite direction. 
         FIG. 31A  is a cross-section view of the inertia mass of  FIG. 30  illustrating an orientation of the drag members when the inertia mass is moving in a downward direction within a fluid-filled reservoir chamber.  FIG. 31B  is a cross-section view of the inertia mass of  FIG. 30  illustrating an orientation of the drag members when the inertia mass is moving in an upward direction within a fluid-filled reservoir chamber. 
         FIG. 32  is an enlarged, cross-section view of a pressure-responsive inertia valve assembly. 
         FIG. 33  is an enlarged, cross-section view of another embodiment of a pressure-responsive inertia valve assembly. 
         FIG. 34  is a side elevational view of bicycle employing yet another embodiment of an acceleration-sensitive shock absorber. 
         FIG. 35  is an enlarged, cross-section view of an acceleration-sensitive valve assembly within a shock absorber of the bicycle of  FIG. 35 . The inertia valve assembly of  FIG. 30  includes a valve body that is at least partially controlled by an electromagnetic system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an off-road bicycle, or mountain bike,  20  including a frame  22  which is comprised of a main frame portion  24  and a swing arm portion  26 . The swing arm portion  26  is pivotally attached to the main frame portion  24 . The bicycle  20  includes front and rear wheels  28 , connected to the main frame  24 . A seat  32  is connected to the main frame  24  and provides support for a rider of the bicycle  20 . 
     The front wheel  28  is supported by a preferred embodiment of a suspension fork  34  which, in turn, is secured to the main frame  24  by a handlebar assembly  36 . The rear wheel  30  is connected to the swing arm portion  26  of the frame  22 . A preferred embodiment of a rear shock  38  is operably positioned between the swing arm  26  and the main frame  24  to provide resistance to the pivoting motion of the swing arm  26 . Thus, the illustrated bicycle  20  includes suspension members  34 ,  38  between the front and rear wheels  28 ,  30  and the frame  22 , which operate to substantially reduce wheel impact forces from being transmitted to the rider of the bicycle  20 . The rear shock absorber  38  desirably includes a fluid reservoir  44  hydraulically connected to the main shock body by a hydraulic hose  46 . Preferably, the reservoir  44  is connected to the swingarm portion  26  of the bicycle  20  above the hub axis of the rear wheel  30 . 
     The suspension fork  34  and the rear shock  38  preferably include an acceleration-sensitive valve, commonly referred to as an inertia valve, which allows the damping rate to be varied depending upon the direction of an acceleration input. The inertia valve permits the suspension fork  34  and rear shock  38  to distinguish between accelerations originating at the sprung mass, or main frame  24  and rider of the bicycle  20 , from accelerations originating at the unsprung mass, or front wheel  28  and rear wheel  30 , and alter the damping rate accordingly. It is generally preferred to have a firm damping rate when accelerations originate at the sprung mass and a softer damping rate when the accelerations originate at the unsprung mass. On an automobile or other four-wheel vehicle, this helps to stabilize the body by reducing fore and aft pitching motions during acceleration and braking, as well as by reducing body roll during cornering. 
     In a similar manner, on two-wheel vehicles such as motorcycles and bicycles, vehicle stability is improved by reduction of fore and aft pitching motions. In addition, in the case of bicycles and other pedal-driven vehicles, this reduces or prevents suspension movement in response to rider-induced forces, such as pedaling forces, while allowing the suspension to absorb forces induced by the terrain on which the bicycle  20  is being ridden. As will be described in detail below, the inertia valving within the suspension fork  34  and rear shock  38  include features which permit responsive, consistent performance and allow such inertia valves to be manufactured in a cost effective manner. Preferably, the inertia valve is located within the reservoir  44 , which may be rotated relative to the swingarm portion  26  of the bicycle  20 . Rotating the reservoir  44  alters the component of an upward acceleration of the rear wheel  30  which acts along the axis of motion of the inertia valve and thereby influences the responsiveness of the inertia valve. 
       FIGS. 2-7  illustrate a preferred embodiment of the rear shock absorber  38 . A shock absorber  38  operates as both a suspension spring and as a damper. Preferably, the spring is an air spring arrangement, but coil springs and other suitable arrangements may also be used. The shock  38  is primarily comprised of an air sleeve  40 , a shock body  42  and a reservoir  44 . In the illustrated embodiment, a hydraulic hose  46  physically connects the main body of the shock  38  (air sleeve  40  and shock body  42 ) to the reservoir  44 . However, the reservoir  44  may also be directly connected to the main body of the shock absorber  38 , such as being integrally connected to, or monolithically formed with, the air sleeve  40 . 
     The air sleeve  40  is cylindrical in shape and includes an open end  48  and an end closed by a cap  50 . The cap  50  of the air sleeve  40  defines an eyelet  52  which is used for connection to the main frame  24  of the bicycle  20  of  FIG. 1 . The open end  48  of the air sleeve  40  slidingly receives the shock body  42 . 
     The shock body  42  is also cylindrical in shape and includes an open end  54  and a closed end  56 . The closed end  56  defines an eyelet  58  for connecting the shock  38  to the swing arm portion  26  of the bicycle  20  of  FIG. 1 . Thus, the air sleeve  40  and the shock body  42  are configured for telescopic movement between the main frame portion  24  and the swing arm portion  26  of the bicycle  20 . If desired, this arrangement may be reversed and the shock body  42  may be connected to the main frame  24  while the air sleeve  40  is connected to the swing arm  26 . 
     A seal assembly  60  is positioned at the open end  48  of the air sleeve  40  to provide a substantially airtight seal between the air sleeve  40  and the shock body  42 . The seal assembly  60  comprises a body seal  62  positioned between a pair of body bearings  64 . The illustrated body seal  62  is an annular seal having a substantially square cross-section. However, other suitable types of seals may also be used. A wiper  66  is positioned adjacent the open end  48  of the air sleeve  40  to remove foreign material from the outer surface of the shock body  42  as it moves into the air sleeve  40 . 
     A damper piston  68  is positioned in sliding engagement with the inner surface of the shock body  42 . A shock shaft  70  connects the piston  68  to the cap  50  of the air sleeve  40 . Thus, the damper piston  68  is fixed for motion with the air sleeve  40 . 
     A piston cap  72  is fixed to the open end  54  of the shock body  42  and is in sliding engagement with both the shock shaft  70  and the inner surface of the air sleeve  40 . The piston cap  72  supports a seal assembly  74  comprised of a seal member  76  positioned between a pair of bearings  78 . The seal assembly  74  is in a sealed, sliding engagement with the inner surface of the air sleeve  40 . A shaft seal arrangement  80  is positioned to create a seal between the cap  72  and the shock shaft  70 . The shaft seal arrangement  80  comprises a seal member  82  and a bushing  84 . The seal member  82  is an annular seal with a substantially square cross-section, similar to the body seal  62 . The shaft seal arrangement  80  creates a substantially airtight seal between the cap  72  and the shock shaft  70  while allowing relative sliding motion therebetween. 
     A positive air chamber  86  is defined between the closed end  50  of the air sleeve  40  in the cap  72 . Air held within the positive air chamber  86  exerts a biasing force to resist compression motion of the shock absorber  38 . Compression motion of the shock absorber  38  occurs when the closed ends  56  and  50  of the shock body  42  and air sleeve  40  (and thus the eyelets  52 ,  58 ) move closer to one another. 
     A negative air chamber  88  is defined between the cap  72  and the seal assembly  60 , which in combination with the shock body  42  closes the open end  48  of the air sleeve  40 . Air trapped within the negative air chamber  88  exerts a force which resists expansion, or rebound, motion of the shock absorber  38 . Rebound motion of the shock absorber  38  occurs when the closed ends  56  and  50  of the shock body  42  and air sleeve  40  (and thus the eyelets  52 ,  58 ) move farther apart from each other. Together, the positive air chamber  86  and the negative air chamber  88  function as the suspension spring portion of the shock absorber  38 . 
     An air valve  90  communicates with the positive air chamber  86  to allow the air pressure therein to be adjusted. In this manner, the spring rate of the shock absorber  38  may be easily adjusted. 
     A bypass valve  92  is provided to allow the pressure between the positive air chamber  86  and the negative air chamber  88  to be equalized. The bypass valve  92  is configured to allow brief communication between the positive air chamber  86  and the negative air chamber  88  when the air sleeve seal assembly  74  passes thereby. A bottom out bumper  94  is positioned near the closed end  50  of the air sleeve  40  to prevent direct metal to metal contact between the closed end  50  and the cap  72  of the shock body  42  upon full compression of the shock absorber  38 . 
     The shock absorber  38  also includes a damper assembly, which is arranged to provide a resistive force to both compression and rebound motion of the shock absorber  38 . Preferably, the shock absorber  38  provides modal response compression damping. That is, the shock absorber  38  preferably operates at a first damping rate until an appropriate acceleration input is sensed, then the shock absorber  38  operates at a second damping rate for a predetermined period thereafter, before returning the first damping rate. This is in opposition to a system that attempts to continually respond to instantaneous input. Such a modal system avoids the inherent delay associated with responding separately to each input event. 
     The piston  68  divides the interior chamber of the shock body  42  into a compression chamber  96  and a rebound chamber  98 . The compression chamber  96  is defined between the piston  68  and the closed end  56  of the shock body  42  and decreases in volume during compression motion of the shock absorber  38 . The rebound chamber  98  is defined between the piston  68  and the piston cap  72 , which is fixed to the open end  54  of the shock body  42 . The rebound chamber  98  decreases in volume upon rebound motion of the shock absorber  38 . 
     The piston  68  is fixed to the shock shaft  70  by a hollow threaded fastener  100 . A seal  102  is fixed for movement with the piston  68  and creates a seal with the inner surface of the shock body  42 . The illustrated seal  102  is of an annular type having a rectangular cross-section. However, other suitable types of seals may also be used. 
     The piston  68  includes one or more axial compression passages  104  that are covered on the rebound chamber  98  side by a shim stack  106 . As is known, the shim stack  106  is made up of one or more flexible shims and deflects to allow flow through the compression passages  104  during compression motion of the shock absorber  38  but prevents flow through the compression passages  104  upon rebound motion of the shock absorber  38 . Similarly, the piston  68  includes one or more rebound passages  108  extending axially therethrough. A rebound shim stack  110  is made up of one or more flexible shims, and deflects to allow flow through the rebound passages  108  upon rebound motion of the shock absorber  38  while preventing flow through the rebound passages  108  during compression motion of the shock absorber  38 . 
     A central passage  112  of the shock shaft  70  communicates with the compression chamber  96  through the hollow fastener  100 . The passage  112  also communicates with the interior chamber of the reservoir  44  through a passage  114  defined by the hydraulic hose  46 . Thus, the flow of hydraulic fluid is selectively permitted between the compression chamber  96  and the reservoir  44 . 
     A rebound adjustment rod  116  extends from the closed end  50  of the air sleeve  40  and is positioned concentrically within the passage  112  of the shock shaft  70 . The rebound adjustment rod  116  is configured to alter the amount of fluid flow upon rebound motion thereby altering the damping force produced. An adjustment knob  118  engages the rebound adjustment rod  116  and is accessible externally of the shock absorber  38  to allow a user to adjust the rebound damping rate. A ball detent mechanism  120  operates in a known manner to provide distinct adjustment positions of the rebound damping rate. 
     The reservoir  44  includes a reservoir tube  122  closed on either end. A floating piston  124  is in sliding engagement with the interior surface of a reservoir tube  122 . A seal member  126  provides a substantially fluid-tight seal between the piston  124  and the interior surface of the reservoir tube  122 . The seal member  126  is preferably an annular seal having a substantially square cross-section. However, other suitable seals may also be used. 
     The floating piston  124  divides the interior chamber of the reservoir tube  122  into a reservoir chamber  128  and a gas chamber  130 . The reservoir chamber  128  portion of the reservoir tube is closed by an end cap  132 . The end cap  132  additionally receives the end of the hydraulic hose  46  and supports a hollow reservoir shaft  134 . The central passage  136  of the reservoir shaft  134  is in fluid communication with the passages  114  and  112  and, ultimately, the compression chamber  96 . 
     The reservoir shaft  134  supports an inertia valve assembly  138  and a blowoff valve assembly  140 . Each of the inertia valve assembly  138  and the blowoff valve assembly  140  allows selective communication between the compression chamber  96 , via the passages  112 ,  114 ,  136 , and the reservoir chamber  128 . 
     The gas chamber  130  end of the reservoir tube  122  is closed by a cap  142  which includes a valve assembly  144  for allowing gas, such as nitrogen, for example, to be added or removed from the gas chamber  130 . The pressurized gas within the gas chamber  130  causes the floating piston  124  to exert a pressure on the hydraulic fluid within the reservoir chamber  128 . This arrangement prevents air from being drawn into the hydraulic fluid and assists in refilling fluid into the compression chamber  96  during rebound motion of the shock absorber  38 . 
     With reference to  FIG. 3 b   , the blowoff valve assembly  140  is supported by the reservoir shaft  134  and positioned above the inertia valve assembly  138 . The reservoir shaft  134  reduces in diameter to define a shoulder portion  154 . An annular washer  156  is supported by the shoulder  154  and the blowoff valve assembly  140  is supported by the washer  156 . The washer  156  also prevents direct contact between the inertia mass  150  and the blowoff valve assembly  140 . 
     The blowoff valve assembly  140  is primarily comprised of a cylindrical base  158  and the blowoff cap  160 . The base  158  is sealed to the reservoir shaft  134  by a shaft seal  162 . The illustrated seal  162  is an O-ring, however other suitable seals may also be used. The upper end of the base  158  is open and includes a counterbore which defines a shoulder  164 . The blowoff cap  160  is supported by the shoulder  164  and is sealed to the inner surface of the base  158  by a cap seal  166 . The cap seal  166  is preferably an O-ring, however other suitable seals may also be used. A threaded fastener  168  fixes the blowoff cap  160  and base  158  to the reservoir shaft  134 . 
     The blowoff cap  160  and base  158  define a blowoff chamber  170  therebetween. A plurality of radial fluid flow passages  172  are defined by the reservoir shaft  134  to allow fluid communication between the blowoff chamber  170  and the shaft passage  136 . 
     The blowoff cap  160  includes one or more axial blowoff passages  174  and one or more axial refill passages  176 . A blowoff shim stack  178  is positioned above the blowoff cap  160  and covers the blowoff passages  174 . The blowoff shim stack  178  is secured in place by the threaded fastener  168 . The individual shims of the shim stack  178  are capable of deflecting about the central axis of the fastener  168  to selectively open the blowoff passages  174  and allow fluid communication between the blowoff chamber  170  and the reservoir chamber  128 . The blowoff shim stack  178  is preferably configured to open in response to pressures within the blowoff chamber above a minimum threshold, such as approximately 800 psi, for example. 
     A refill shim stack  180  is positioned between the blowoff cap  160  and the reservoir shaft  134  and covers the refill ports  176 . The refill shim stack  180  is configured to prevent fluid from flowing from the blowoff chamber  170  through ports  176  to the reservoir  128  while offering little resistance to flow from the reservoir  128  into the blowoff chamber  170 . 
     The inertia valve assembly  138  includes a plurality of radially extending, generally cylindrical valve passages  148 , connecting the passage  136  to the reservoir chamber  128 . The inertia valve assembly  138  also includes a valve body, or inertia mass  150 , and a spring  152 . The spring  152  biases the inertia mass  150  into an upward, or closed, position wherein the inertia mass  150  covers the mouths of the valve passages  148  to substantially prevent fluid flow from the passage  136  to the reservoir chamber  128 . 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, the inertia mass  150  uncovers at least some of the valve passages  148  to allow fluid to flow therethrough. 
     The end cap  132 , which closes the lower end of the reservoir tube  122 , defines a cylindrical pocket, or socket,  182  which receives the inertia mass  150  in its lowermost or open position. The lowermost portion of the pocket  182  reduces in diameter to form a shoulder  184 . The shoulder  184  operates as the lowermost stop surface, which defines the open position of the inertia mass  150 , as illustrated in  FIG. 5 . 
     The inertia mass  150  includes a check plate  190  which allows fluid to be quickly displaced from the pocket  182  as the inertia mass  150  moves downward into the pocket  182 . The inertia mass  150  has a plurality of axial passages  188  extending therethrough. The check plate  190  rests on several projections, or standoff feet,  192  ( FIG. 6 ) slightly above the upper surface of the inertia mass  150  and substantially covers the passages  188 . A series of stop projections  193 , similar to the standoff feet, are formed or installed in the upper, necked portion of the inertia mass  150  to limit upward motion of the check plate  190 . 
     With reference to  FIG. 4 a   , a top plan view of the inertia mass  150  is shown. The axial passages  188  are preferably kidney-shaped, to allow the passages  188  to occupy a large portion of the transverse cross-sectional area of the inertia mass  150 . Desirably, the ratio of the passage  188  cross-sectional area to the inertia mass  150  cross-sectional area is greater than approximately 0.3. Preferably, the ratio of the passage  188  cross-sectional area to the inertia mass  150  cross-sectional area is greater than approximately 0.5, and more preferably greater than approximately 0.7. 
     The large area of the passages  188  provides a low-resistance flow path for hydraulic fluid exiting the pocket  182 . As a result, the flow rate of the fluid exiting the pocket  182  is high, and the inertia mass is able to move rapidly into the open position. In addition, the amount of fluid which must be displaced by the inertia mass  188  for it to move into the open position is reduced. Advantageously, such an arrangement allows the inertia mass  150  to respond rapidly to acceleration forces. 
     When the check plate  190  is resting against the standoff feet  192  on the upper surface of the inertia mass  150  it provides restricted fluid flow through the passages  188 . The check plate  190  also has an open position in which it moves upward relative to the inertia mass  150  until it contacts the stop projections  193 . When the check plate  190  is open, fluid is able to flow from the pocket  182  through the passages  188  and into the reservoir  128 , with desirably low resistance. 
     The inertia mass  150  also includes a third series of projections, or standoff feet,  194 . The standoff feet  194  are comprised of one or more projections located on the uppermost surface of the upper neck portion of the inertia mass  150 . The standoff feet  194  on the upper surface of the neck portion of the inertia mass  150  contact the washer  156  when the inertia mass  150  is in its uppermost or closed position. A fourth set of projections, or standoff feet,  195  are positioned on the lower surface of the inertia mass  150  ( FIG. 4 c   ) and contact the shoulder  184  when the inertia mass  150  is in its lower or open position. 
     In each set of stop projections, or standoff feet,  192 - 195 , preferably between three to five individual projections are disposed radially about the inertia mass  150 . However, other suitable numbers of feet may also be used. Desirably, the surface area of the stop projections, or standoff feet,  192 - 195  is relatively small. A small surface area of the standoff feet  194 ,  195  lowers the resistance to movement of the inertia mass  150  by reducing the overall surface contact area between the inertia mass  150  and the washer  156  or shoulder  184 , respectively. The small surface area of the standoff feet  192  and stop projections  193  lower the resistance to movement of the check plate  190  relative to the inertia mass  150 . Desirably, the projections  192 - 195  have dimensions of less than approximately 0.025″×0.025″. Preferably, the projections  192 - 195  have dimensions of less than approximately 0.020″×0.020″ and, more preferably, the projections  192 - 195  have dimensions of less than approximately 0.015″×0.015″. 
     When utilized with an inertia mass  150  having a mass (weight) of approximately 0.5 ounces, the preferred projections  192 - 195  provide a desirable ratio of the mass (weight) of the inertia valve mass  150  to the contact surface area of the projections  192 - 195 . Due to the vacuum effect between two surfaces, a force of approximately 14.7 lbs/in 2  (i.e., atmospheric pressure) is created when attempting to separate the inertia mass  150  from either the washer  156  or shoulder  184 , respectively. By lowering the contact surface area between the inertia mass  150  and either the washer  156  or shoulder  184 , the vacuum force tending to resist separation of the contact surfaces is desirably reduced. 
     Preferably, the contact surface area is small in comparison with the mass (weight) of the inertia mass  150  because the magnitude of the acceleration force acting on the inertia mass  150  is proportional to its mass (weight). Accordingly, a large ratio of the mass (weight) of the inertia valve mass  150  to the contact surface area of the projections  192 - 195  is desired. For example, for a set of three (3) standoff feet  194 ,  195  with dimensions of approximately 0.025″.×0.025″, the ratio is at least approximately 17 lbs/in 2 . A more desirable ratio is at least approximately 25 lbs/in 2 . Preferably, the ratio is at least 50 lbs/in 2  and more preferably is at least 75 lbs/in 2 . These ratios are desirable for an inertia mass utilized in the context of an off-road bicycle rear shock absorber and other ratios may be desirable for other applications and/or vehicles. Generally, however, higher ratios increase the sensitivity of the inertia mass  150  (i.e., allow the inertia mass  150  to be very responsive to acceleration forces). For example, with a ratio of 50 lbs/in 2  the sensitivity of the inertia mass  150  is about +/−⅓ G. Likewise, for a ratio of 147 lbs/in 2  the sensitivity of the inertia mass  150  is about +/− 1/10 G. 
     As illustrated in  FIG. 6 , the outside diameter of the lower portion of the inertia mass  150  is slightly smaller than the diameter of the pocket  182 . Therefore, an annular clearance space is defined between them when the inertia mass  150  is positioned within the pocket  182 . The clearance C restricts the rate with which fluid may pass to fill the pocket below the inertia mass  150 , to influence the rate at which the inertia mass  150  may exit the pocket  182 . Thus, in the illustrated shock absorber  38 , a fluid suction force is applied to the inertia mass  150  within the pocket  182  to delay the inertia mass  150  from returning to the closed position. 
     The interior surface of the inertia mass  150  includes an increased diameter central portion  195  which, together with the shaft  134 , defines an annular recess  196 . The annular recess  196  is preferably located adjacent to one or more of the ports  148  when the inertia mass  150  is in its closed position. Thus, fluid exiting from the shaft passage  136  through the passages  148  enters the annular recess  196  when the inertia mass  150  is its dosed position. 
     The interior surface of the inertia mass  150  decreases in diameter both above and below the central portion  195  to create an upper intermediate portion  197  and a lower intermediate portion  199 . The upper intermediate portion  197  and lower intermediate portion  199 , together with the shaft  134 , define an upper annular clearance  198  ( FIG. 7 a   ) and a lower annular clearance  200 , respectively. An upper lip  201  ( FIG. 7 a   ) is positioned above, and is of smaller diameter than, the upper intermediate portion  197 . A step  205  ( FIG. 7 a   ) is defined by the transition between the upper intermediate portion  197  and the upper lip  201 . Similarly, a lower lip  203  is positioned below, and has a smaller diameter than, the lower intermediate portion  199 . A step  205  is defined by the transition between the lower intermediate portion  199  and the lower lip  203 . The upper lip  201  and the lower lip  203 , together with the shaft  134 , define an upper exit clearance  202  ( FIG. 7 a   ) and a lower exit clearance  204 . 
     With reference to  FIG. 7 a   , the upper lip  201  preferably includes a labyrinth seal arrangement  206 . As is known, a labyrinth seal comprises a series of annular grooves formed into a sealing surface. Preferably, the lower lip  203  also includes a labyrinth seal arrangement substantially similar to the labyrinth seal  206  of the upper lip  201 . 
     Advantageously, the labyrinth seal arrangement  206  reduces fluid flow (bleed flow) between the reservoir shaft  134  and the upper lip  201  when the inertia mass  150  is in a closed position. Excessive bleed flow is undesired because it reduces the damping rate when the inertia valve  138  is closed. By utilizing a labyrinth seal  206 , the clearance between the inertia mass  150  and the shaft  134  may be increased, without permitting excessive bleed flow. The increased clearance is particularly beneficial to prevent foreign matter from becoming trapped between the inertia mass  150  and shaft  134  and thereby inhibiting operation of the inertia valve  138 . Thus, reliability of the shock absorber  38  is increased, while the need for routine maintenance, such as changing of the hydraulic fluid, is decreased. 
     With reference to  FIG. 7 b   , an alternative inertia mass  150  is illustrated. The upper intermediate portion  197  of the inner surface of the inertia mass  150  of  FIG. 7 b    is inclined with respect to the outer surface of the shaft  134 , rather than being substantially parallel to the outer surface of the shaft  134  as in the inertia mass of  FIG. 7 a   . Thus, in the inertia mass  150  of  FIG. 7 b   , the step  205  is effectively defined by the entire upper intermediate portion  197 . The inertia mass  150  configuration of  FIG. 7 b    theoretically provides approximately one-half the self-centering force of the inertia mass  150  of  FIG. 7 a   . In addition, other suitable configurations of the inner surface of the inertia mass  150  may be utilized to provide a suitable self-centering force, as will be apparent to one of skill in the art based on the disclosure herein. For example, the inclined surface may begin in an intermediate point of the upper intermediate portion  197 . Alternatively, the step  205  may be chamfered, rather than orthogonal. 
     With reference to  FIGS. 1-7 , the operation of the shock absorber  38  will now be described in detail. As described previously, the shock absorber  38  is operably mounted between the main frame  24  and the swing arm portion  26  of the bicycle  20  and is capable of both compression and rebound motion. Preferably, the shock body  42  portion of the shock absorber  38  is connected to the swing arm portion  26  and the air sleeve  40  is connected to the main frame  24 . The reservoir  44  is desirably connected to the swing arm portion  26  of the bicycle  20  preferably near the rear axle, and preferably approximately vertical as shown in  FIG. 1 . 
     When the rear wheel  30  of the bicycle  20  encounters a bump the swing arm portion  26  articulates with respect to the main frame  24 , tending to compress the shock absorber  38 . If the acceleration imparted along the longitudinal axis of the reservoir  44  is below a predetermined threshold, the inertia mass  150  will remain in its closed position, held by the biasing force of the spring  152 , as illustrated in  FIG. 3   b.    
     For the piston  68  to move relative to the shock body  42  (i.e., compression motion of the shock absorber  38 ) a volume of fluid equal to the displaced volume of the shock shaft  70  must be transferred into the reservoir  128 . With the inertia mass  150  closing the passages  148  and the blowoff valve  140  remaining in a closed position, fluid flow into the reservoir  128  is substantially impeded and the shock absorber  38  remains substantially rigid. 
     If the compressive force exerted on the rear wheel  30 , and thus the shock absorber  38 , attains a level sufficient to raise the fluid pressure within the blowoff chamber  170  above a predetermined threshold, such as 800 psi for example, the blowoff shims  178  open to allow fluid to flow from the blowoff chamber  170  through the blowoff ports  174  and into the reservoir  128 . As an example, if the diameter of the shock shaft  70  is ⅝″ (Area=0.31 square inches) and the predetermined blow-off threshold is 800 psi, then a compressive force at the shaft of at least 248 pounds is required to overcome the blowoff threshold and commence compression of the shock absorber. This required force, of course, is in addition to the forces required, as is known in the art, to overcome the basic spring force and the compression damping forces generated at the piston  68  of the shock absorber. In this situation, compression of the shock absorber is allowed against the spring force produced by the combination of the positive and negative air chambers  86 ,  88 . The damping rate is determined by the flow through the compression ports  104  of the piston  68  against the biasing force of the compression shim stack  106 . When the pressure within the blowoff chamber  170  falls below the predetermined threshold, the blowoff shim stack  178  closes the blowoff ports  174  and the shock absorber  38  again becomes substantially rigid, assuming the inertia mass  150  remains in the closed position. 
     If the upward acceleration imposed along the longitudinal axis of the reservoir  44  (i.e., the axis of travel of the inertia mass  150 ) exceeds the predetermined minimum threshold, the inertia mass  150 , which tends to remain at rest, will overcome the biasing force of the spring  152  as the reservoir  44  moves upward relative to the inertia mass  150 . If the upward distance of travel of the reservoir  44  is sufficient, the inertia mass will move into the pocket  182 . With the inertia mass  150  in the open position, fluid is able to be displaced from the compression chamber  96  through the passages  112 ,  114  and the shaft passage  136 , through the passages  148  and into the reservoir  128 . Thus, the shock  38  is able to compress with the compression damping force again being determined by flow through the compression ports  104  of the piston  68 . 
     The predetermined minimum threshold for the inertia mass  150  to overcome the biasing force of the spring  152  is determined primarily by the mass of the inertia mass  150 , the spring rate of the spring  152  and the preload on the spring  152 . Desirably, the mass of the inertia mass is approximately 0.5 ounces. However, for other applications, such as the front suspension fork  34  or vehicles other than off-road bicycles, the desired mass of the inertia mass  150  may vary. 
     The spring rate of the spring  152  and the preload on the spring  152  are preferably selected such that the spring  152  biases the inertia mass  150  into a closed position when no upward acceleration is imposed along the longitudinal axis of the reservoir  44 . However, in response to such an acceleration force the inertia mass  150  will desirably overcome the biasing force of the spring  152  upon experiencing an acceleration which 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 which is between 0.25 and 1.5 G&#39;s and more preferably upon experiencing an acceleration which is between 0.4 and 0.7 G&#39;s. For certain riding conditions or other applications, such as the front suspension fork  34 , or other applications besides off-road bicycles, however, the predetermined threshold may be varied from the values recited above. 
     The check plate  190  resting on the standoff feet  193  of the inertia mass  150  allows fluid to be easily displaced upward from the pocket  182  and thus allows the inertia mass  150  to move into the pocket  182  with little resistance. This permits the inertia mass  150  to be very responsive to acceleration inputs. As the inertia mass  150  moves into the pocket  182 , fluid within the pocket  182  flows through the passages  188  and lifts the check plate  190  against the stop projections  193 . 
     Once the inertia mass  150  is in its open position within the pocket  182 , as illustrated in  FIG. 5 , the spring  152  exerts a biasing force on the inertia mass  150  tending to move it from the pocket  182 . Fluid pressure above the inertia mass  150  causes the check plate  190  to engage the standoff feet  192  located on the upper surface of the inertia mass  150  restricting flow through the ports  188 . The height of the standoff feet  192  which the check plate  190  rests on is typically 0.003″ to 0.008″ above the exit surface of the passages  188  to provide an adequate level of flow restriction upon upward movement of the inertia mass  150 . Fluid may be substantially prevented from flowing through the passages  188  and into the pocket  182 , except for a small amount of bleed flow between the checkplate  190  and the upper surface of the inertia mass  150 . However, the height of the standoff feet  192  may be altered to influence the flow rate of the bleed flow and thereby influence the timer feature of the inertia mass  150 , as will be described below. 
     Fluid also enters the pocket  182  through the annular clearance, or primary fluid flow path, C ( FIG. 6 ) between the interior surface, or valve seat, of the pocket  182  and the exterior surface of the inertia mass  150 . Thus, the size of the clearance C also influences the rate at which fluid may enter the pocket  182  thereby allowing the inertia mass  150  to move upward out of the pocket  182 . 
     Advantageously, with such a construction, once the inertia mass  150  is moved into an open position within the pocket  182 , it remains open for a predetermined period of time in which it takes fluid to refill the pocket behind the inertia mass  150  through the clearance C. This is referred to as the “timer feature” of the inertia valve assembly  138 . Importantly, this period of time can be independent of fluid flow direction within the shock absorber  38 . Thus, the shock absorber  38  may obtain the benefits of a reduced compression damping rate throughout a series of compression and rebound cycles, referred to above as “modal response.” Desirably, the inertia mass  150  remains in an open position for a period between approximately 0.05 and 5 seconds, assuming no subsequent activating accelerations are encountered. Preferably, the inertia mass  150  remains in an open position for a period between about 0.1 and 2.5 seconds and more preferably for a period between about 0.2 and 1.5 seconds, again, assuming no subsequent accelerations are encountered which would tend to open the inertia mass  150 , thus lengthening or resetting the timer period. The above values are desirable for a rear shock absorber  38  for an off-road bicycle  20 . The recited values may vary in other applications, however, such as when adapted for use in the front suspension fork  34  or for use in other vehicles or non-vehicular applications. 
     In order to fully appreciate the advantages of the modal response inertia valve assembly  138  of the present shock absorber  38 , it is necessary to understand the operation of a bicycle having an acceleration-sensitive damping system utilizing an inertia valve. With reference to  FIG. 8 , the relationship between vertical position P, vertical velocity V and vertical acceleration A, over time T, for a simple mass traversing two sinusoidally-shaped bumps is illustrated.  FIG. 8  is based on a mass that travels horizontally at a constant velocity, while tracking vertically with the terrain contour. This physical model, somewhat simplified for clarity, correctly represents the essential arrangement utilized in inertia-valve shock absorbers wherein the inertial element is shaft-mounted and spring-biased within the unsprung mass. 
     The primary simplification inherent in this model, and in this analysis, is that the flexibility of an actual bicycle tire is ignored. The tire is assumed to be inflexible in its interaction with the terrain, offering no compliance. An actual tire, of course, will provide some compliance, which in turn produces some degree of influence on the position, velocity, and acceleration of the unsprung mass. The actual degree of influence in a given situation will depend on many variables, including the actual vehicle speed and the specific bump geometry, as well as the compliance parameters of the particular fire. However, the simplified analysis discussed here is a good first approximation which clearly illustrates the key operative physics principles, while avoiding these complications. The basic validity of this simplified analysis can be demonstrated by a sophisticated computer motion analysis that incorporates the effects of tire compliance and several other complicating factors. 
     Relating  FIG. 8  to the situation of a bicycle, the heavy solid line indicating position P represents both the trail surface and, assuming the wheel of the bicycle is rigid and remains in contact with the trail surface, the motion of any point on the unsprung portion of the bicycle, such as the hub axis of the front or rear wheel, for example. The lines representing velocity V and acceleration A thus correspond to the vertical velocity and acceleration of the hub axis. In  FIG. 8 , the trail surface (solid line indicating position P) includes a first bump  81  and a second bump  82 . In this example, as shown, each bump is preceded by a short section of smooth (flat) terrain. 
     As the wheel begins to traverse the first bump B 1 , the acceleration A of the hub axis H rises sharply to a maximum value and, accordingly, the velocity V of the hub axis H increases. Mathematically, of course, the acceleration as shown is calculated as the second derivative of the sinusoidal bump curve, and the velocity as the first derivative. At a point P 1 , approximately halfway up the first bump B 1 , the second derivative (acceleration A) becomes negative (changes direction) and the velocity begins to decrease from a maximum value. At a point P 2 , corresponding with the peak of the bump B 1 , the acceleration A is at a minimum value (i.e., large negative value) and the velocity V is at zero. At a point P 3 , corresponding with the mid-point of the downside of the first bump B 1 , the acceleration A has again changed direction and the velocity V is at a minimum value (i.e., large negative value). At a point P 4 , corresponding with the end of the first bump B 1 , the acceleration A has risen again to a momentary maximum value and the velocity V is zero. The second bump B 2  is assumed to be sinusoidally-shaped like the first bump B 1 , but, as shown, to have somewhat greater amplitude. Thus, the relationship between position P, velocity V and acceleration A are substantially identical to those of the first bump B 1 . 
     When a simple inertia valve is utilized in the suspension system of a bicycle and the acceleration A reaches a threshold value, the inertia mass overcomes the biasing force of the spring and begins moving relatively downward on the center shaft, which moves upward. Once the shaft has moved upward relative to the inertia mass a sufficient distance, the inertia valve passages are uncovered and a reduced compression damping rate is achieved. Although a compression inertia valve is discussed in this example, the same principles may be applied to an inertia valve which operates during rebound. 
     Before the inertia valve passages are open, the shock absorber operates at its initial, firm damping rate. This results in an undesirably firm damping rate, creating a “damping spike”, over the initial portion of the bump B 1 . The damping spike continues until the shaft has moved upward relative to the inertia mass a sufficient distance to open the valve passages. The amount of movement of the shaft relative to the inertia mass necessary to uncover the passages is determined primarily by the size of the passages and the position of the uppermost surface of the inertia mass relative to the passages when the mass is in its fully dosed position. This distance is referred to as the spike distance S D . The amount of time necessary for the inertia passages to be opened and to reduce the damping rate is dependent upon the shape of the bump and the spike distance S D , and is referred to as the spike time S T . The reduction of the damping rate is at least partially dependent upon the size of the passages and, therefore, it is difficult to reduce the spike time S T  without reducing the spike distance S D  which necessarily affects the achievable lowered damping rate. 
     The inertia mass begins to close (i.e., move relatively upward) when the acceleration acting upon it either ceases, changes direction, or becomes too small to overcome the biasing force of the spring. As shown graphically in  FIG. 8 , the acceleration A becomes zero at point P 1 , or at approximately the mid-point of the bump B 1 . Accordingly, a simple inertia valve begins to close at, or before, the middle of the bump B 1 . Therefore, utilizing a simple inertia valve tends to return the shock absorber to its initial, undesirably firm damping rate after only about one-half of the up-portion of bump B 1  has been traversed. The operating sequence of the inertia valve is similar for the second bump B 2  and each bump thereafter. 
     In actual practice, the specific point on a bump where a simple inertia valve will close will vary depending on bump configuration, vehicle speed, inertia valve size and geometry, spring bias force, compliance of the tire and other factors. Thus, it should be understood that the extent of mid-bump “spiking” produced by “premature closing” of a simple inertia valve will be greater for some bumps and situations than for others. 
     It is desirable to extend the amount of time the inertia valve stays open so that the reduced damping rate can be utilized beyond the first half of the up-portion of the bump. More complex inertia valve arrangements utilize the fluid flow during compression or rebound motion to hydraulically support the inertia valve in an open position once acceleration has ceased or diminished below the level necessary for the inertia valve to remain open from acceleration forces alone. However, these types of inertia valve arrangements are dependent upon fluid flow and allow the inertia valve to close when, or slightly before, the compression or rebound motion ceases. A shock absorber using this type of inertia valve in the compression circuit could experience a reduced damping rate from after the initial spike until compression motion ceases at, or near, the peak P 2  of the bump B 1 . This would represent an improvement over the simple inertia valve shock absorber described previously. However, the flow dependent inertia valve necessarily reacts to specific terrain conditions. That is, the inertia mass responds to each individual surface condition and generally must be reactivated upon encountering each bump that the bicycle traverses. Therefore, this type of shock absorber experiences an undesirably high damping rate “spike” as each new bump is encountered. 
     In contrast, the inertia valve arrangement  138  of the present shock absorber  38  is a modal response type. That is, the inertia valve  138  differentiates rough terrain conditions from smooth terrain conditions and alters the damping rate accordingly. During smooth terrain 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 bump B 1  is encountered, the inertia valve  138  opens to advantageously lower the damping rate so that the bump may be absorbed by the shock absorber  38 . The timer feature retains the inertia valve  138  in an open position for a predetermined period of time thereby allowing the shock absorber  38  to maintain the lowered damping rate for the entire bump (not just the first half of the up-portion), and to furthermore absorb the second bump B 2  and subsequent bumps possibly without incurring any additional “spikes.” Thus, in the preferred embodiment of the present shock absorber  38 , the timer feature is configured to delay the inertia mass  150  from closing until a period of time after completion of both the compression stroke and rebound stroke and, preferably, until after the beginning of the second compression stroke resulting from an adjacent bump. As discussed above, the timer period may be adjustable by altering the rate at which fluid may refill the timer pocket  182 . 
     Once the shock absorber  38  has been compressed, either by fluid flow through the blowoff valve  140  or the inertia valve  138 , the spring force generated by the combination of the positive air chamber  86  and the negative air chamber  88  tend to bias the shock body  42  away from the air sleeve  40 . In order for the shock absorber  38  to rebound, a volume of fluid equal to the displaced volume of the shock shaft  70  must be drawn from the reservoir  128  and into the compression chamber  96 . Fluid flow is allowed in this direction through the refill ports  176  in the blowoff valve  140  against a desirably light resistance offered by the refill shim stack  180 . Gas pressure within the gas chamber  130  exerting a force on the floating piston  124  may assist in this refill flow. Thus, the rebound damping rate is determined primarily by fluid flow through the rebound passages  108  against the biasing force of the rebound shim stack  110 . 
     With reference to  FIGS. 3 b    and  5 , the fluid flow path during compression or rebound motion of the shock absorber  38 , with the inertia mass  150  in either of an open or closed position, is above and away from the inertia mass  150  itself Advantageously, such an arrangement substantially isolates fluid flow from coming into contact with the inertia mass  150 , thereby inhibiting undesired movement of the inertia mass due to drag forces resulting from fluid flow. Thus, the inertia mass  150  advantageously responds to acceleration inputs and is substantially unaffected by the movement of hydraulic fluid during compression or rebound of the shock absorber  38 . 
     The present shock absorber  38  includes an inertia valve  138  comprising a self-centering valve body, or inertia mass  150 . In order to fully appreciate the advantages of the self-centering inertia mass  150  of the present inertia valve assembly  138 , it is necessary to describe the conditions which have prevented prior inertia valve designs from operating reliably, with acceptable sensitivity, and for a reasonable cost. 
     Each of  FIGS. 9 and 10  schematically illustrate an off-center condition of the inertia mass  150  relative to the shaft  134 . The off-center condition of the inertia mass  150  may cause it to contact the shaft  134  causing friction, which tends to impede motion of the inertia mass  150  on the 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 shaft  134  seriously impairs the performance of the inertia valve  138  and may render it entirely inoperable. Each of the off-center conditions illustrated in  FIGS. 8 and 9  may result from typical manufacturing processes. However, modifying the manufacturing process to avoid these conditions often results in a prohibitively high manufacturing cost. 
       FIG. 9  illustrates an inertia valve arrangement in which the inertia valve passages  148  are of slightly different diameter. Such a condition is often an unavoidable result of the typical manufacturing process of drilling in a radial direction through a tubular piece of material. Such a process may result in an entry diameter N created by the drilling tool being slightly larger than the exit diameter X created by the drilling tool. The resulting difference in area between the passages  148  causes the fluid pressure within the shaft passage  136  to exert an unequal force between the entry passage  148  having an entry diameter N and the exit passage  148  having an exit diameter X. 
     For example, a difference between the entry diameter N and the exit diameter X of only two thousandths of an inch (0.090″ exit diameter versus 0.092″ entry diameter) at a fluid pressure of 800 psi, results in a force differential of approximately 0.2 pounds, or 3.6 ounces, between the passages  148 . The inertia mass  150  itself may weigh only about one half of an ounce (0.5 oz.). Such a force differential will push the inertia mass  150  off-center and reduce the responsiveness of the inertia mass  150 , if not prevent it from moving entirely. 
       FIG. 10  illustrates an off-center condition of the inertia mass  150  caused by the inertia valve passages  148  being positioned off-center relative to the shaft  134 . A center axis AC of the inertia valve passages  148  is offset from the desired diametrical axis AD of the shaft  134  by a distance O. Therefore, the force resulting from fluid pressure within the shaft passage  136  does not act precisely on a diametrical axis AD of the inertia mass  150 , resulting in the inertia mass  150  being pushed off-center with respect to, and likely contacting, the shaft  134 . The offset condition of the center axis AC of the passages  148  is the result of inherent manufacturing imperfections and cannot easily be entirely avoided, at least without raising the cost of manufacturing to an unfeasible level. 
     Furthermore, even if manufacturing costs were not of concern and the passages  148  could be made with identical diameters and be positioned exactly along the diametrical axis AD of the shaft  134 , additional forces may tend to push the inertia mass  150  off-center. For example, if the reservoir  44  experiences an acceleration which is not exactly aligned with the axis of travel of the inertia mass  150  (such as braking or forward acceleration), the transverse component of the acceleration would create a force tending to move the inertia mass  150  off-center and against the shaft  134 . If the transverse component of the acceleration is large enough, the resulting frictional force between the inertia mass  150  and the reservoir shaft  134  will inhibit, or prevent, movement of the inertia mass  150 . Accordingly, it is highly desirable to compensate for factors which tend to push the inertia mass  150  off-center in order to ensure responsive action of the inertia valve  138 . This is especially important in off-road bicycle applications, where it is desirable for the inertia valve assembly  138  to respond to relatively small accelerations and the mass of the inertia mass  150  is also relatively small. 
     As described above, the inertia valve assembly  138  preferably includes a self-centering inertia mass  150 . With reference to  FIG. 11 , the inertia mass  150  of  FIG. 5  is shown without the fluid flow lines to more clearly depict the cross-sectional shape of its interior surface. The inertia mass  150  has a minimum internal diameter “D” while the shaft  134  has a constant external diameter “d,” which is smaller than the internal diameter D. The difference between the shaft diameter d and the inertia valve diameter D is desirably small. Otherwise, as described above, the bleed flow between the shaft  134  and the inertia mass  150  undesirably reduces the damping rate which may be achieved when the inertia mass  150  is in a closed position. Accordingly, for the rear shock  38  the difference between the shaft diameter d and the inertia mass diameter D is desirably less than 0.01 inches. Preferably, difference between the shaft diameter d and the inertia mass diameter D is less than 0.004 inches and more preferably is approximately 0.002 inches. For the front suspension fork  34 , the difference between the shaft diameter d and the inertia mass diameter D is desirably less than 0.02 inches. Preferably, difference between the shaft diameter d and the inertia mass diameter D is less than 0.008 inches and more preferably is approximately 0.004 inches. The recited values may vary in other applications, however, such as when adapted for vehicles other than off-road bicycles or non-vehicular applications. 
     The preferred differences between the shaft diameter d and the inertia mass diameter D recited above assume that a labyrinth seal arrangement  206  ( FIG. 7 ) is provided at the upper and lower portions of the internal surface of the inertia mass  150 , as described above. However, the bleed rate may be influenced by factors other than the difference between the shaft diameter d and the inertia mass diameter D. Accordingly, driven by a pressure differential of 400 psi, the bleed rate between the inertia mass  150  and the shaft  134 , for an off-road bicycle shock with a shaft diameter of ⅝ inches, is desirably less than 1.0 cubic inches/sec. Preferably, the bleed rate between the inertia mass  150  and the shaft  134  is less than 0.5 cubic inches/sec and more preferably is less than 0.3 cubic inches/sec. However, for applications other than off-road bicycle shock absorbers, the preferred bleed rates may vary. 
     As described, an annular recess  196  is defined between the interior surface of the inertia mass  150  and the shaft  134 . The annular recess  196  is preferably located in approximately the center of the inertia mass  150 . The annular recess  196  is referred to as zone 1 (Z 1 ) in the following description of the fluid flow between the shaft  134  and the self-centering inertia mass  150 . The upper annular clearance  198 , above the annular recess  196 , is referred to as zone 2 (Z 2 ) and the upper exit clearance  202  is referred to as zone 3 (Z 3 ). One half of the difference between the diameter of the upper annular clearance  198  and the diameter D at the upper exit clearance  202  defines a distance B, which is equivalent to the size of the step  205 . The size B of the step  205  (referred to as a “Bernoulli Step” in  FIGS. 26, 27 and 28 ) may be precisely manufactured by a computer controlled lathe operation, for example. Other suitable methods for creating a precisely sized step  205  may also be used. Thus, in the illustrated arrangement, the outer surface of the shaft  134  defines a first surface and the interior surface of the inertia mass  150  defines a second surface which faces the first surface. Preferably, a first annular passage is defined by the upper annular clearance  198  and the upper exit clearance  202 . A first portion of the first annular passage is defined by the upper exit clearance  202  and a second portion of the first annular passage is defined by the upper annular clearance  198 . Thus, in the illustrated embodiment, the first and second portions define first and second cross-sectional flow areas of the first annular passage. Preferably, a second annular passage is defined by the lower annular clearance  200  and the lower exit clearance  204 . A first portion of the second annular passage is defined by the lower exit clearance  204  and a second portion of the second annular passage is defined by the lower annular clearance  200 . Thus, in the illustrated embodiment, the first and second portions of the second annular passage also define first and second cross-sectional flow areas of the second annular passage. 
     Zone 1 Z 1  has a larger cross-sectional fluid flow area than zone 2 Z 2  which, in turn, has a larger cross-sectional flow area than zone 3 Z 3 . The cross-sectional area differential between the zones Z 1 , Z 2 , Z 3  causes the fluid within each zone Z 1 , Z 2 , Z 3  to vary in velocity, which causes a self-centering force to be exerted on the inertia mass  150  when it becomes off-center, as will be described below. Although the zones Z 1 , Z 2 , Z 3  are annular, the discussion below, for simplicity, is in the context of a two-dimensional structure having left and right sides. Accordingly, the zones Z 1 , Z 2 , Z 3  of the example will vary in cross-sectional distance, rather than in cross-sectional area. Although the example is simplified, it correctly describes the general self-centering action of the inertia mass  150 . 
     A rough approximation of the centering force developed by the self-centering inertia mass  150  can be estimated using Bernoulli&#39;s equation. This is a rough approximation only since Bernoulli&#39;s equation assumes perfect frictionless flow, which is not valid for real fluids. However, this is a useful starting point for understanding the general principles involved, and for estimating the forces that occur. Bernoulli&#39;s equation expresses the law of conservation of energy for the flow of an incompressible fluid. In estimating the centering force of the inertia mass  150 , the potential energy height) portion of Bernoulli&#39;s equation is not significant and may be ignored. Thus, for any two arbitrary points on a fluid streamline, Bernoulli&#39;s equation reduces to:
 
 P   1 +(ρ/2 g )( V   1 ) 2   =P   2 +(ρ/2 g )( V   2 ) 2  
 
where:
         P 1 =fluid pressure (psi) at point 1   P 2 =fluid pressure (psi) at point 2   V 1 =fluid velocity (in/sec) at point 1   V 2 =fluid velocity (in/sec) at point 2   p=fluid density   g=gravity constant       

     Using the values of 0.03125 lb/in 3  for fluid density p of typical hydraulic fluid and 386 in/sec2 for gravity constant g, the equation becomes:
 
 P   1 +(4.05×10 −5 )( V   1 ) 2   =P   2 +(4.05×10 −5 )( V   2 ) 2  
 
     For a simple example, assume that the fluid pressure P 1  in zone 1 is 400 psi, due to an external force tending to compress the shock absorber  38  and the fluid velocity V 1  is zero due to relatively little fluid exiting from zone 1. Also, for simplicity, assume that the floating piston  124  is absent or is not exerting a significant pressure on the fluid within the reservoir chamber  128 . Accordingly, the fluid pressure P 3  in zone 3 Z 3  is 0 psi. Insert these values into Bernoulli&#39;s equation to find the velocity in zone 3:
 
400+(4.05×10 −5 )(0) 2 =0+(4.05×10 −5 )( V   3 ) 3  
 
 V   3 =3,142 in/sec
 
     Therefore, as a first approximation (accurate to the degree that the assumptions Bernoulli&#39;s equation are based upon are valid here) the velocity V 3  of fluid exiting zone 3 is 3,142 in/sec. Assuming the validity of assumptions inherent in Bernoulli&#39;s equation here, this value is true for all exit points of zone 3 Z 3  regardless of their dimensions. Further, based on flow continuity, the change in velocity of the fluid between zone 2 Z 2  and zone 3 Z 3  is proportional to the change in the clearance, or gap G, between zone 2 Z 2  and zone 3 Z 3 . The gap G is the cross-sectional distance between the outer surface of the shaft  134  and the relevant inner surface of the inertia mass  150 . 
     The relationship between the change in the size of the gap G and the change in velocity allows solving of the velocity in zone 2 Z 2  for both the right and left sides. Assuming that D is 0.379 inches, d is 0.375 inches and B is 0.001 inches, then the gaps on both the right and left sides, with the inertia mass  150  centered are:
 
GAP Zone 2= B +( D−d )/2=0.003
 
GAP Zone 3=( D−d )/2=0.002
 
     Then, based on flow continuity, fluid velocity in Zone 2 is calculated as follows:
 
 V   3 [Gap Zone 3/Gap Zone 2]= V   2 =2,094 in/sec
 
     Therefore, the fluid velocity V 2  in zone 2 Z 2  for each of the right and left side is 2,094 in/sec. Using Bernoulli&#39;s equation to find the pressure P 2  in zone two gives:
 
400+(4.05×10 −5 )(0) 2 =( P   2 )+(4.05×10 −5 )(2,094) 2  
 
 P   2 =222 psi
 
Assuming that, for a particular inertia valve, the area in zone 2 Z 2  that the fluid pressure acts upon for each of the right and left side is 0.0375 in 2 , then the force F at both the left and right sides of the inertia mass  150  can be calculated as:
 
 F= 222 psi(0.0375 in 2 )=8.3 lbs.
 
     The force F acting on the inertia mass  150  in the above example is equal for the right and left side due to the velocity V 2  in zone 2 Z 2  being the same for each side. The velocity V 2  is the same because the ratio of gap 3 G 3  to gap 2 G 2  between the right side and the left side is equal due to the inertia mass  150  being centered relative to the shaft  134 . 
     With reference to  FIG. 12 , however, if the inertia mass  150  becomes off center relative to the shaft  134  by a distance x, for example 0.001 inches to the left, the ratio of gap 3 G 3  to gap 2 G 2  is different between the right and left sides. This results in the velocity V 2  being different between the right and left sides and, as a result, a force differential between the right side and left side is produced. These calculations are substantially similar to the previous calculations and are provided below (for an off-center condition 0.001 inches to the left:
 
 V   3 =3,142 in/sec
 
Left Side: GAP Zone 3( G   3L )=( D−d )/2+ x= 0.003
 
GAP Zone 2( G   2L )= B +( D−d )/2+ x= 0.004
 
 V   3 [Gap Zone 3/Gap Zone 2]= V   2 =2,356.5 in/sec
 
 P   2 =175 psi
 
 F =(175)(0.0375)=6.55 lbs.
 
Right Side: GAP Zone 3( G   3R )=( D−d )/2− x= 0.001
 
GAP Zone 2( G   2R )= B +( D−d )/2− x= 0.002
 
 V   3 [Gap Zone 3/Gap Zone 2]= V   2 =1571 in/sec
 
 P   2 =300 psi
 
 F =(300)(0.0375)=11.25 lbs.
 
 F   right   −F   left =4.7 lbs. pushing right
 
     As shown, a force differential of as much as 4.7 lbs, depending on the degree of validity of the Bernoulli assumption, pushes the inertia mass  150  to the right to correct for the off-center condition. As noted above, preferably the lower portion of the inertia mass  150  also includes a step  205  creating a lower zone 2 and zone 3 ( FIG. 12 ). Accordingly, a centering force acts on the lower portion of the inertia mass  150  when it is off-center from the shaft  134 . Therefore, in the example above, a force of as much as 4.7 lbs also acts on the lower portion of the inertia mass  150 , resulting in a total centering force of as much as 9.4 lbs acting to center the inertia mass  150  relative to the shaft  134 . 
     For a typical off-road bicycle application, with the inertial mass centered, the ratio of the velocity in zone 2 V 2  to the velocity in zone 3 V 3  (i.e. V 2 /V 3 ) is desirably between 0.9 and 0.2. Preferably, the ratio of the velocity in zone 2 V 2  to the velocity in zone 3 V 3  is desirably between 0.8 and 0.35 and more preferably the ratio of the velocity in zone 2 V 2  to the velocity in zone 3 V 3  is desirably between 0.75 and 0.5. 
     The ratio of the gap G between the shaft  134  and the inertia mass  150  in zone 3 Z 3  and in zone 2, Z 2  (i.e., G 3 /G 2 ), as demonstrated by the calculations above, influences the magnitude of the self-centering force produced by the inertia mass  150 . The ratio (G 3 /G 2 ) is desirably less than one. If the ratio (G 3 /G 2 ) is equal to one, then by definition there is no step  205  between zone 2 Z 2  and zone 3 Z 3 . 
     Based on flow continuity from Zone 2 to Zone 3, the ratio of the velocity V 2  in Zone 2 to the velocity V 3  in Zone 3 (V 2 N 3 ) is equal to the ratio of the Gap G 3  at Zone 3 to the Gap G 2  at Zone 2 (G 3 /G 2 ). In other words, based on flow continuity it follows that: (G 3 /G 2 )=(V 2 /V 3 ). 
     Thus, for a typical off-road bicycle application with the inertia mass centered, the ratio of the gap at Zone 3 to the gap at Zone 2 is desirable between 0.90 and 0.20. Preferably the ratio of the gap at Zone 3 to the gap at Zone 2 is desirably between 0.80 and 0.35 and more preferably the ratio of the gap at Zone 3 to the gap at Zone 2 is desirably between 0.75 and 0.50. 
     Advantageously, the self-centering inertia mass  150  is able to compensate for force differentials due to the manufacturing variations in the passage  148  size and position as well as transverse accelerations, all of which tend to push the inertia mass  150  off-center. This allows reliable, sensitive operation of the inertia valve assembly  140  while also permitting cost-effective manufacturing methods to be employed without compromising performance. 
     Although a fluid pressure in zone 1 Z 1  of 400 psi was used in the above example, the actual pressure may vary depending on the force exerted on the shock assembly  38 . The upper pressure limit in zone 1 Z 1  is typically determined by the predetermined blow off pressure of the blow off valve  140 . Desirably, for an off-road bicycle rear shock with a shaft diameter of ⅝ inches, the predetermined blow off pressure is approximately 400 psi. Preferably, the predetermined blow off pressure within zone 1 Z 1  is approximately 600 psi and more preferably is approximately 800 psi. These predetermined blow off pressures are provided in the context of an off-road bicycle rear shock application and may vary for other applications or vehicle types. 
       FIG. 13  illustrates an alternative arrangement for controlling the refill rate, or timer function, of fluid flow into the pocket  182  as the inertia mass  150  moves in an upward direction away from its closed position. The end cap  132  includes a channel  208  communicating with an orifice  209  connecting the reservoir chamber  128  and the pocket  182 . The orifice  209  permits fluid to flow between the reservoir chamber  128  and the pocket  182  in addition to the fluid flow through the clearance C and bleed flow between the check plate  190  and inertia mass  150 . The size of the orifice  209  may be varied to influence the overall rate of fluid flow into the pocket  182 . 
       FIG. 13  also illustrates an adjustable pocket refill arrangement  210 . The adjustable refill arrangement  210  allows external adjustment of the refill rate of fluid flow into the pocket  182 . The adjustable refill arrangement includes an inlet channel  212  connecting the reservoir chamber  128  to a valve seat chamber  213 . An outlet channel  214  connects the valve seat chamber  213  to the pocket  182 . 
     A needle  215  is positioned within the valve seat chamber  213  and includes a tapered end portion  216 , which extends into the outlet channel  214  to restrict the flow of fluid therethrough. External threads of the needle  215  engage internal threads of the end cap  132  to allow the needle  215  to move relative to the outlet channel  216 . The needle  215  includes a seal  217 , preferably an O-ring, which creates a fluid tight seal between the needle  215  and the end cap  132 . The exposed end of the needle  215  includes a hex-shaped cavity  218  for receiving a hex key to allow the needle  215  to be rotated. The exposed end of the needle  215  may alternatively include other suitable arrangements that permit the needle  215  to be rotated by a suitable tool, or by hand. For example, an adjustment knob may be connected to the needle  215  to allow a user to easily rotate the needle without the use of tools. 
     Rotation of the needle  215  results in corresponding translation of the needle  215  with respect to the end cap  132  (due to the threaded connection therebetween) and adjusts the position of the tapered end  216  relative to the outlet channel  214 . If the needle  215  is moved inward, the tapered end  216  blocks a larger portion of the outlet channel  214  and slows the fluid flow rate into the pocket  182 . If the needle  215  is moved outward, the tapered end  216  reduces its blockage of the outlet channel  214  and speeds the fluid flow rate into the pocket  182 . This permits user adjustment of the refill rate of the pocket  182  and, accordingly, adjustment of the period of time the inertia mass  150  is held in an open position. Advantageously, the adjustable refill arrangement  210  allows a user to alter the period of time the inertia valve  138  is open and thus, the period of lowered compression damping once the inertia valve  138  is opened. 
       FIG. 14  illustrates the suspension fork  34  detached from the bicycle  20  of  FIG. 1 . The suspension fork  34  includes right and left legs  220 ,  222 , as referenced by a person in a riding position on the bicycle  20 . The right leg  220  includes a right upper tube  224  telescoping received in a right lower tube  226 . Similarly, the left leg  222  includes a left upper tube  228  telescopingly received in a left lower tube  230 . A crown  232  connects the right upper tube  224  to the left upper tube  228  thereby connecting the right leg  220  to the left leg  222  of the suspension fork  34 . In addition, the crown  232  supports a steerer tube  234 , which passes through, and is rotatably supported by the frame  22  of the bicycle  20 . The steerer tube  234  provides a means for connection of the handlebar assembly  36  to the suspension fork  34 , as illustrated in  FIG. 1 . 
     Each of the right lower tube  226  and the left lower tube  230  includes a dropout  236  for connecting the front wheel  28  to the fork  34 . An arch  238  connects the right lower tube  226  and the left lower tube  230  to provide strength and minimize twisting of the tubes  226 ,  230 . Preferably, the right lower tube  226 , left lower tube  230 , and the arch  238  are formed as a unitary piece, however, the tubes  226 ,  230  and the arch  238  may be separate pieces and connected by a suitable fastening method. 
     The suspension fork  34  also includes a pair of rim brake bosses  240  to which a standard rim brake assembly may be mounted. In addition, the fork  34  may include a pair of disc brake bosses (not shown) to which a disc brake may be mounted. Of course, the suspension fork  34  may include only one or the other of the rim brake bosses  240  and disc brake bosses, depending on the type of brake systems desired. 
       FIG. 15  is a cross-section view of the right leg  220  of the suspension fork  34  having the front portion cutaway to illustrate the internal components of a damping assembly  244  of the fork  34 . Preferably, the left leg  222  of the suspension fork  34  houses any of a known suitable suspension spring assembly. For example, an air spring or coil spring arrangement may be used. In addition, a portion of the suspension spring assembly may be housed within the right fork leg  220  along with the damper assembly  244 . 
     As described previously, the upper tube  224  is capable of telescopic motion relative to the lower tube  226 . The fork leg  220  includes an upper bushing  246  and a lower bushing  248  positioned between the upper tube  224  and the lower tube  226 . The bushings  246 ,  248  inhibit wear of the upper tube  224  and the lower tube  226  by preventing direct contact between the tubes  224 ,  226 . Preferably, the bushings  246 ,  248  are affixed to the lower tube  226  and are made from a self-lubricating and wear-resistant material, as is known in the art. However, the bushings  246 ,  248  may be similarly affixed to the upper tube  224 . Preferably, the bushings  246 ,  248  include grooves (not shown) that allow a small amount of hydraulic fluid to pass between the bushings  246 ,  248  and the upper fork tube  224  to permit lubrication of the bushing  246  and seal, described below. 
     The lower tube  226  has a closed lower end and an open upper end. The upper tube  224  is received into the lower tube  226  through its open upper end. A seal  250  is provided at the location where the upper  224  enters the open end of the lower tube  226  and is preferably supported by the lower tube  226  and in sealing engagement with the upper tube  224  to substantially prevent oil from exiting, or a foreign material from entering the fork leg  220 . 
     The damping assembly  244  is operable to provide a damping force in both compression and a rebound direction to slow both compression and rebound motion of the fork  34 . The damper assembly  244  is preferably an open bath, cartridge-type damper assembly having a cartridge tube  252  fixed with respect to the closed end of the lower tube  226  and extending vertically upward. A damper shaft  254  extends vertically downward from a closed upper end of the upper tube  224  and supports a piston  258 . Thus, the piston  258  is fixed for movement with the upper tube  224  while the cartridge tube  252  is fixed for movement with the lower tube  226 . 
     The piston  258  is positioned within the cartridge tube  252  and is in telescoping engagement with the inner surface of the cartridge tube  252 . A cartridge tube cap  260  closes the upper end of the cartridge tube  252  and is sealing engagement with the damper shaft  254 . Thus, the cartridge tube  252  defines a substantially sealed internal chamber which contains the piston  258 . 
     The piston  258  divides the internal chamber of the cartridge tube  252  into a variable volume rebound chamber  262  and a variable volume compression chamber  264 . The rebound chamber  262  is positioned above the piston  258  and the compression chamber  264  is positioned below the piston  258 . A reservoir  266  is defined between the outer surface of the cartridge tube  252  and the inner surfaces of the upper and lower tubes  224 ,  226 . A base valve assembly  268  is operably positioned between the compression chamber  264  and the reservoir  266  and allows selective communication therebetween. 
       FIG. 16  is an enlarged cross section of the damping assembly  244 . As described above, a cartridge tube cap  260  closes the upper end of the cartridge tube  252 . An outer seal  270  creates a seal between the cartridge tube cap  260  and the cartridge tube  252  while an inner seal  272  creates a seal between the cartridge tube cap  260  and the damper shaft  254 . Accordingly, extension and retraction of the damper shaft  254  with respect to the cartridge tube  252  is permitted while maintaining the rebound chamber  262  in a substantially sealed condition. 
     The cartridge cap  260  includes a one-way refill valve  274  which, during inward motion of the damper shaft  254  with respect to the cartridge tube  252 , allows fluid flow from the reservoir  266  into the rebound chamber  262 . The refill valve  274  comprises one or more axial passages  276  through the cap  260  which are closed at their lower end by refill shim stack  278 . Thus, the shim stack  278  allows fluid flow from the reservoir  266  to the rebound chamber  262  with a relatively small amount of resistance. When the fluid pressure in the rebound chamber  262  is greater than the fluid pressure in the reservoir  266 , such as during retraction of the damper shaft  254 , the refill shim stack  278  engages the lower surface of the cartridge tube cap  260  to substantially seal the refill passages  276  and prevent fluid from flowing therethrough. 
     The piston  258  is fixed to the end of the damper shaft  254  by a threaded fastener  280 . The piston includes an outer seal  282  which engages the inner surface of the cartridge tube  252  to provide a sealing engagement between the piston  258  and the inner surface of the cartridge tube  252 . Thus, fluid flow around the piston is substantially eliminated. 
     The piston  258  includes a one-way rebound valve assembly  284  which permits fluid flow from the rebound chamber  262  to the compression chamber  264  while preventing flow from the compression chamber  264  to the rebound chamber  262 . The rebound valve assembly  284  comprises one or more axial passages  286  through the piston  258  dosed at their lower end by a rebound shim stack  288 . Fluid is able to flow from the rebound chamber  262  through the passages  286  and into the compression chamber  264  against the resistance offered by the shim stack  288 . When the pressure is greater in the compression chamber  264  than in the rebound chamber  262 , the shim stack  288  engages the lower surface of the piston  258  to substantially seal the passages  286  and prevent the flow of fluid therethrough. 
     In the illustrated embodiment, the cartridge tube  252  is split into an upper portion  290  and a lower portion  292 , which are each threadably engaged with a connector  294  to form the cartridge tube  252 . Optionally, a one-piece cartridge tube may be employed. A base member  296  is fixed to the closed end of the lower tube  226  and supports the cartridge  252 . The lower portion  292  of the cartridge tube  252  is threadably engaged with the base member  296 . 
       FIG. 17  is an enlarged cross-sectional view of the base valve assembly  268 . The base valve assembly  268  is housed within the lower portion  292  of the cartridge tube  252  and is supported by a shaft  298  which extends in an upward direction from the base member  296 . The entire base valve assembly  268  is secured onto the shaft  298  by a bolt  300  which threadably engages the upper end of the shaft  298 . 
     The base valve assembly  268  includes a compression valve  302 , a blowoff valve  304 , and an inertia valve  306 . The compression valve  302  is positioned on the upper portion of the shaft  298 . The blowoff valve  304  is positioned below the compression valve  302  and spaced therefrom. The compression valve  302  and the blowoff valve  304  define a blowoff chamber  308  therebetween. A plurality of passages  310  connect the blowoff chamber  308  to a central passage  312  of the base valve shaft  298 . 
     A snap ring  314 , which is held in an annular recess of the shaft  298 , supports the compression valve  302 . A washer  316  positioned underneath the bolt  300  holds the compression valve  302  onto the shaft  298 . The compression valve  302  includes a compression piston  318  sealingly engaged with the inner surface of the lower portion  292  of the cartridge tube  252  by a seal  320 . The compression piston  318  is spaced from both the snap ring  314  and the washer  316  by a pair of spacers  322 ,  324  respectively. 
     The compression piston  318  includes one or more compression passages  326  covered by a compression shim stack  328 . The compression shim stack  328  is secured to the lower surface of the compression piston  318  by the lower spacer  322 . The compression shim stack  328  deflects about the lower spacer  322  to selectively open the compression passages  326 . The compression shim stack  328  seals against the lower surface of the compression piston  318  to prevent unrestricted compression flow past the compression shim stack  328 . 
     As illustrated in  FIGS. 20 and 21 , which show fluid flows during the rebound stroke, the compression piston  318  also includes one or more refill passages  330  extending axially through the compression piston  318 . The refill passages  330  are covered at the upper surface of the compression piston  318  by a refill shim stack  332 . The refill shim stack  332  is held against the upper surface of the compression piston  318  by the upper spacer  324  and deflects to open the refill passages  330 . Thus, the refill shims  332  prevent fluid flow through the refill passages from the compression chamber  264  to the blowoff chamber  308 , but permit fluid flow from the blowoff chamber  308  through the refill passages  330  and into the compression chamber  264  against the slight resistance offered by the refill shim stack  332 . 
     As illustrated in  FIG. 17 , the blowoff valve  304  is positioned between a lower snap ring  334  and an upper snap ring  336 . A separator plate  338  is supported by the lower snap ring  334  and is sealingly engaged with the inner surface of the lower portion  292  of the cartridge tube  252  by a seal  340 . A lower spacer  342  spaces the blowoff piston  344  in an upward direction from the separator plate  338 . The blowoff piston  344  is also sealingly engaged with the inner surface of the lower portion  292  of the cartridge tube  252  by a seal  346 . An upper spacer  348  spaces the blowoff piston  344  from the upper snap ring  336 . A separator chamber  350  is defined between the blowoff piston  344  and the separator plate  338 . 
     As illustrated in  FIGS. 20 and 21 , the blowoff piston  344  includes one or more blowoff passages  352  covered on the lower surface of the blowoff piston  344  by a blowoff shim stack  354 . The blowoff shim stack  354  is positioned between the blowoff piston  344  and the lower spacer  342  to allow fluid flow from the blowoff chamber  308  into the separator chamber  350  at pressures above a predetermined threshold. The blowoff shim stack  354  seals passages  352  to prevent unrestricted (without blowoff) compression fluid flow from the blowoff chamber  308  to the separator chamber  350 . 
     The blowoff piston  344  also includes one or more refill passages  356  covered at the upper surface of the blowoff piston  344  by a refill shim stack  358 . The refill shim stack  358  is held against the upper surface of the blowoff piston  344  by the upper spacer  348  to seal the refill passages  356  and prevent fluid flow from the blowoff chamber  308  into the separator chamber  350 . However, the refill shims deflect about the upper spacer  348  to allow fluid flow from the separator chamber  350  into the blowoff chamber  308  through the refill passages  356  with relatively little resistance. One or more passages  360  are formed within the lower portion  292  of the cartridge tube  252  at a height between the separator plate  338  and the blowoff piston  344  to allow fluid communication between the separator chamber  350  and the reservoir  266 . 
     Preferably, the inertia valve  306  is substantially identical to the inertia valve previously described in relation to the shock absorber  38 . The inertia valve  306  includes an inertia mass  362  movable between a closed position, where the inertia mass  362  closes two or more passages  364 , and an open position, where the inertia mass  362  uncovers the two or more passages  364 . The uppermost or closed position of the inertia mass  362  is defined by the snap ring  334 , which supports the separator plate  338 . 
     The inertia mass  362  is biased into its closed position by a spring  366 . The lowermost or open position of the inertia mass  362  is defined when the lower surface of the inertia mass  362  engages the lower interior surface of a pocket  368 , defined by the base member  296 . The inertia mass  362  includes one or more axial passages  370  covered at the upper surface of the inertia mass  362  by a check plate  372  which is movable between a substantially dosed position against the standoff feet  394  at the upper surface of the inertia mass  362  and an open position against the stop projections  392  on the upper, necked portion of the inertia mass  362 . 
     The check plate  372  moves into an open position when the inertia mass  362  moves downward in relation to the base valve shaft  298  to allow fluid to flow from the pocket  368  into an inertia valve chamber  376  above the inertia mass  362  through the passages  370 . The check plate  372  moves into a substantially closed position upon upward movement of the inertia mass  362  relative to the base valve shaft  298  to restrict fluid flow through the passages  370 . One or more passages  378  are defined by the lower portion  292  of the cartridge tube  252  to allow fluid communication between the inertia valve chamber  376  and the reservoir  266 . 
     An annular clearance C is defined between the inertia mass  362  and the pocket  368  when the inertia mass  362  is in its open position. In a similar manner to the inertia valve described in relation to the shock absorber  38 , the clearance C restricts fluid flow from the inertia valve chamber  376  into the pocket  368 . The inertia valve  306  preferably includes other features described in relation to the inertia valve of the shock absorber  38 . For example, the inertia mass  362  preferably includes a plurality of standoff feet  394  at the locations discussed above in relation to the inertia mass of the shock absorber  38 . Additionally, the inertia mass  362  includes an annular recess  380  aligned with the passages  364  when the inertia mass  362  is in its closed position. The inertia mass  362  also includes a step preferably on each end of the interior surface of the inertia mass  362  which is sliding engagement with the base valve shaft  298 , as described above. As shown, the inertia mass  362  also includes a labyrinth seal arrangement substantially as described above. 
     When the front wheel  28  of the bicycle  20  of  FIG. 1  encounters a bump, a force is exerted on the fork  34 , which tends to compress the fork legs  224 ,  226  in relation to each other. If the upward acceleration of the lower fork tube  226  along its longitudinal axis (i.e., the axis of travel of the inertia mass  362 ) is below a predetermined threshold, the inertia mass  362  remains in its closed position. Pressure within the compression chamber  264  causes fluid to flow through the compression passages  326  and into the blowoff chamber  308 . If the pressure within the blowoff chamber  308  is below a predetermined threshold, the blowoff shims  354  remain closed and the suspension fork  34  remains substantially rigid. 
     If the pressure within the blowoff chamber  308  exceeds the predetermined threshold, the blowoff shim stack  354  deflects away from the blowoff piston  344  to allow fluid to flow through the blowoff passage  352  into the separator chamber  350  and into the reservoir through the passages  360 , as illustrated in  FIG. 17 . Thus, the fork  34  is able to compress with the compression damping rate being determined primarily by the shim stack  354  of the blowoff piston  344 . 
     As the upper fork leg  224  moves downward with respect to the lower fork leg  226 , and thus the piston  258  and damper shaft  254  move downward with respect to the cartridge  252 , fluid is drawn into the rebound chamber  262  through the refill valve  274 , as illustrated in  FIG. 16 . 
     When the upward acceleration of the lower fork leg  226  exceeds a predetermined threshold, the inertia mass  362  tends to stay at rest and overcomes the biasing force of the spring  366  to open the passages  364 . Thus, fluid flow is permitted from the central passage  312  of the base valve shaft  298  into the inertia chamber  376  through the passages  364  and from the inertia chamber  376  into the reservoir  266  through the passages  378 , as illustrated in  FIGS. 18 and 19 . Accordingly, at pressures lower than the predetermined blowoff pressure, when the inertia mass  362  is open (down) fluid is permitted to flow from the compression chamber  264  to the reservoir  266  and the suspension fork  244  is able to compress. 
     Upon rebound motion of the suspension fork  34 , the refill valve  274  closes and the fluid within the rebound chamber  262  is forced through the rebound passages  286  of the piston  258  against the resistive force of the rebound shim stack  288 , as illustrated in  FIG. 20 . A volume of fluid equal to the displaced volume of the damper shaft  254  is drawn into the compression chamber  264  from the reservoir chamber  266  via the passages  356  and  330  against the slight resistance offered by the refill shims  358  and  332 , as illustrated in  FIG. 21 . 
       FIGS. 22-25  illustrate an alternative embodiment of the suspension fork  34 . The embodiment of  FIGS. 22-25  operates in a substantially similar manner as the suspension fork  34  described in relation to  FIGS. 14-21  with the exception that the embodiment of  FIGS. 22-25  allows flow through a compression valve  382  in the piston  258  during compression motion. This is known as a shaft-displacement type damper, because a volume of fluid equal to the displaced volume of the shaft  254  is displaced to the reservoir  266  during compression motion of the fork  34 . For reference, this compares with the previously-described embodiment where the displaced fluid volume equals the displaced volume of the full diameter of the piston  258 . Flow through the piston  258  into the rebound chamber during compression eliminates the need for refill passages in the cartridge cap, and thus a solid cap  260  is utilized. 
     The compression valve  382  is a one-way valve, similar in construction to the one-way valves described above. The compression valve  382  comprises one or more valve passages  384  formed axially in the piston  258  and a shim stack  386  closing the valve passages  384 . As is known, the shim stack  386  may comprise one or more shims. The shims may be combined to provide a desired spring rate of the shim stack  386 . The shim stack  386  is deflected to allow fluid flow between the compression chamber  264  and the rebound chamber  262  during compression of the suspension fork  34 . Preferably, shim stack  386  is significantly “softer” than shim stack  328  in the base valve assembly  268 , in order to ensure sufficient pressure for upward flow through piston  258  into rebound chamber  262  during compression strokes. 
     The operation of the suspension fork  34  of  FIGS. 22-25  is substantially similar to the operation of the suspension fork  34  described in relation to  FIGS. 14-21 . However, during compression motion of the fork  34  of  FIGS. 22-25 , fluid flows from the compression chamber  264  to the rebound chamber  262 . This results in less fluid being displaced into the reservoir  266  than in the previous embodiment. As will be appreciated by one of skill in the art,  FIGS. 22  and  23  illustrate compression fluid flow when the blow off valve  304  is open.  FIGS. 24 and 25  illustrate compression fluid flow when the inertia valve  306  is open. 
     As will be appreciated by one of ordinary skill, the illustrated suspension fork and rear shock absorber arrangements advantageously minimize unintended movement of the inertia mass  150  due to normal compression and rebound fluid flow. With particular reference to  FIG. 3 b   , compression fluid flow (illustrated by the arrow in  FIG. 3 b   ) through the blow off valve  140  of the rear shock absorber  38  occurs through the passage  136  of the reservoir shaft  134  as it passes the inertia mass  150 . Accordingly, fluid moving with any substantial velocity does not directly contact the inertia mass  150 , thereby avoiding undesired movement of the inertia mass  150  due to forces from such a flow. Similarly, compression fluid flow through the passages  148  when the inertia mass  150  is in an open position ( FIG. 5 ) and refill fluid flow upon rebound of the shock absorber  38  are similarly insulated from the inertia mass  150 . With reference to  FIGS. 17, 19 and 21 , the inertia mass  150  is also insulated from contact with moving fluid in the suspension fork  34 .  FIGS. 23 and 25  illustrate similar flow paths for the second embodiment of the suspension fork  34 . 
       FIG. 26  is a graph illustrating the influence of a change in the internal diameter D of a specific inertia mass  150  on the pressure differential between the right and left side when the inertia mass  150  is off-center by a distance x of 0.001 inches. As described above in relation to  FIGS. 11 and 12 , the reservoir shaft  134 , which defines an axis of motion for the inertia mass  150 , has a diameter referred to by the reference character “d.” The reference character “B” refers to the size of the step  205 , or the difference in the radial dimensions of the inner surface of the inertia mass  150  between zone 2 Z 2  and zone 3 Z 3 . For the purposes of illustration in the graph of  FIG. 26 , the diameter d of the shaft  134  is given a value of 0.375 inches. The step size B is given a value of 0.001 inches. 
     In the graph of  FIG. 26 , the value of the minimum internal diameter of the inertia mass  150  (i.e., the diameter at zone 3 Z 3 ) is varied and the corresponding pressure differential between the left and right sides is illustrated by the line  388 , given the constants d, B and x. As described above, the self-centering force is proportional to the pressure differential produced by the design of zones 1, 2 and 3 of the self-centering inertia mass  150 . Thus, as the pressure differential increases, so does the ability of the inertia mass  150  to center itself with respect to the shaft  134 . As illustrated, the value of the pressure differential between the left and right sides varies greatly with relatively small changes in the internal diameter D of the inertia mass  150 . The pressure differential is at its maximum value on the graph when the difference between the inertia valve diameter D and the shaft diameter d is small. The pressure differential diminishes as the difference between the inertia valve diameter D and the shaft diameter d increases. 
     For example, when the inertia valve diameter D is equal to 0.400 inches, the pressure differential is equal to approximately 8 psi. With the inertia valve diameter D equal to 0.400 inches and the shaft diameter d equal to 0.375 inches, the total gap at zone 3 G 3  for both the left and right sides is equal to 0.025 inches (0.400-0.375), when the inertia mass  150  is centered. Accordingly, each gap at zone 3 for the left and right side, G 3L  and G 3R , is equal to 0.0125 inches (0.025/2), when the inertia mass  150  is centered ( FIG. 11 ). 
     The pressure differential has substantially increased at a point when the inertia valve diameter D is equal to 0.385. At this point, the resulting pressure differential is approximately 38 psi. Following the calculation above, each gap at zone 3 for the left and right side, G 3L  and G 3R , is equal to 0.005 inches, with a centered inertia mass  150 . 
     The pressure differential has again substantially increased, to approximately 78 psi, at a point when the inertia valve diameter D is equal to 0.381 inches. When the inertia diameter D is equal to 0.381 inches, each gap at zone 3 for the left and right side, G 3L  and G 3R , is equal to 0.003 inches, assuming the inertia mass  150  is centered about the shaft  134 . At a point when the inertia valve diameter D is equal to 0.379, the pressure differential has increased significantly to approximately 125 psi. At this point, the gap at zone 3 for the left and right side, G 3L  and G 3R , is 0.002 inches. 
     The illustrated pressure differential reaches a maximum when the inertia valve diameter D is equal to 0.377 inches. At this value of D, the pressure differential is approximately 180 psi and each gap at zone 3 for the left and right side, G 3L  and G 3R , is equal to 0.001 inches, again assuming a centered inertia mass  150  and the values of d, B and x as given above. Although the gap at zone 3 G 3  may be reduced further, resulting in theoretically greater self-centering forces, a gap in zone 3 G 3  of at least 0.001 inches is preferred to allow the inertia mass  150  to move freely on the shaft  134 . A gap G 3  below this value may allow particulate matter within the damping fluid to become trapped between the inertia mass  150  and shaft  134 , thereby inhibiting or preventing movement of the inertia mass  150 . 
       FIG. 27  is a graph illustrating the relationship between the size B of the “Bernoulli step”  205  and the resulting pressure differential percentage. A pressure differential of 0% indicates no pressure differential, and thus no self-centering force, is present (i.e., the pressure on the right and left sides of the inertia mass  150  are equal), while a pressure differential of 100% indicates a maximum pressure differential, and self-centering force, is present (i.e., zero pressure on one side of the inertia mass  150 ). The graph is based on a gap at zone 3 G 3  of 0.002 inches, with the inertia mass  150  centered. In other words, the inertia mass diameter D minus the shaft diameter d is equal to 0.004 inches, which results in a gap on each of the right and left sides, G 3R  and G 3L , of 0.002 inches. 
     The graph includes individual lines  390 ,  392 ,  394  and  396  representing different off-center values of the inertia valve. The values are given in terms of the percentage of the total gap G 3  (0.002″ in  FIG. 27 ) that the inertia mass  150  is off-center. For example, an off-center amount of 25% means that the center axis of the inertia mass  150  is offset 0.0005 inches to either the left or right from the center axis of the shaft  134 . Similarly, an off-center amount of 50% means that the center axis of the inertia mass  150  is offset 0.001 inches from the center axis of the shaft  134 . Line  390  represents an off-center amount of 25%, line  392  represents an off-center amount of 50%, line  394  represents an off-center amount of 75%, and line  396  represents an off-center amount of 99%. 
     The largest step size B illustrated on the graph of  FIG. 27  is 0.008 inches. A step  205  of a larger size B may be provided, however, as indicated by the graphs, theoretical self-centering effects have diminished significantly at this point. Accordingly, the step size is desirably less than 0.008 inches, at least for off-road bicycle applications based on these theoretical calculations. The ratio between the gap at zone 3 G 3  and the gap at zone 2 G 2  (i.e., G 3 /G 2 ) in this situation is ⅕, for a centered inertia mass  150  and a gap at zone 3 G 3  of 0.002 inches. 
     With continued reference to  FIG. 27 , lines  390 - 396  illustrate that the pressure differential has increased at a point when the step size B is equal to 0.006 inches in comparison to the pressure differential at a step size B of 0.008 inches. At this point, the ratio between the gap at zone 3 G 3  and the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is ¼. As a result, the self-centering effect is more substantial for ratios which are greater than ¼. The pressure differential again increases at a point when the step size B is equal to 0.004 inches. At this point, the ratio between the gap at zone 3 G 3  and the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is ⅓. As a result, the self-centering force for ratios above self-centering force ⅓ is increased over the self-centering force obtained with a larger step size B. 
     For at least a portion of the lines  390 - 396 , the pressure differential again increases for step sizes B less than 0.003. At this point, the ratio of the gap at zone 3 G 3  to the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is. Accordingly, the self-centering effect is more substantial for ratios which are greater than. Furthermore, at least a portion of the lines  390 - 396  illustrate an increase in the pressure differential at a point when the step size B is equal to 0.002 inches. At this point, the ratio of the gap at zone 3 G 3  to the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is ½. As a result, the self-centering effect is more substantial for ratios which are greater than ½. 
     The graph of  FIG. 27  illustrates a general trend that, up to a point, the pressure differential percentage (and self-centering force) increases as the step size B is reduced, especially for large off-center amounts. However, practical considerations also prevent the size B of the step  205  from becoming too small. For example, extremely small step sizes may be difficult to manufacture, or in the very least, difficult to manufacture for a reasonable cost. Accordingly, the size B of the step  205  (i.e., G 2 -G 3 ) is desirably greater than, or equal to, 0.0001 inches. Preferably, the size B of the step  205  is greater than or equal to 0.001 inches. Additionally, for the practical concerns described above, the effectiveness of the self-centering inertia mass  150 , at least theoretically, declines as the step sizes B become too large. Accordingly, the size B of the step  205  is preferably less than 0.002 inches. However, as mentioned above, the graph of  FIG. 27  is based on theoretical calculations using Bernoulli&#39;s equation, which assumes perfect fluid flow. For actual fluid flows, a much larger step size B may be desirable. For example, in actual applications, a step size B of 0.02 inches, 0.03 inches, or even up to 0.05 inches is believed to provide a beneficial self-centering effect. The effectiveness of larger step sizes B in actual applications is primarily a result of boundary layers of slow-moving, or non-moving fluid adjacent the inertia mass  150  and shaft  134  surfaces resulting in a lower actual flow rate than theoretically calculated using Bernoulli&#39;s equation. 
       FIG. 28  is a graph, similar to the graph of  FIG. 27 , illustrating the relationship between the size B of the step  205  and the resulting pressure differential percentage, except that the gap G 3  is 0.001 inches when the inertia valve  150  is centered. That is, the inertia mass diameter D minus the shaft diameter d is equal to 0.002 inches, which results in a gap on each of the right and left sides, G 3R  and G 3L , of 0.001 inches. 
     The graph includes individual lines representing inertia mass  150  off-center values of 25%, 50%, 75% and 99%. Line  400  represents an off-center amount of 25%, line  402  represents an off-center amount of 50%, line  404  represents an off-center amount of 75%, and line  406  represents an off-center amount of 99%. 
     The largest step size B illustrated on the graph of  FIG. 28  is 0.008 inches. The ratio between the gap at zone 3 G 3  and the gap at zone 2 G 2  (i.e., G 3 /G 2 ) in this situation is 1/9, for a centered inertia mass  150  and a gap at zone 3 G 3  of 0.001 inches. A step size B of greater than 0.008 inches is possible however, as discussed above, at least for off-road bicycle applications, the step size B is preferably less than 0.008 inches based on theoretical calculations. 
     For at least a portion of the illustrated off-center amounts, the pressure differential increases at a point when the step size B is equal to 0.003 inches. At this point, the ratio between the gap at zone 3 G 3  and the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is ¼. As a result, the centering effect is more substantial for ratios which are greater than ¼. The lines  400 - 406  illustrate that the pressure differential again increases at a point when the step size B is equal to 0.002 inches. At this point, the ratio between the gap at zone 3 G 3  and the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is ⅓. As a result, the self-centering effect is greater for ratios above ⅓. 
     The pressure differential again increases for step sizes B less than 0.0015. At this point, the ratio of the gap at zone 3 G 3  to the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is. Accordingly, the centering effect is more substantial for ratios which are greater than. Further, the pressure differential increases at a point when the step size B is equal to 0.001 inches. At this point, the ratio of the gap at zone 3 G 3  to the gap at zone 2 G 2  (i.e., G 3 /G 2 ), for a centered inertia mass  150 , is ½. As a result, the centering effect is more substantial for ratios which are greater than ½. 
     The design parameters of the self-centering inertia mass  150  described above, including the size of the gaps G in the different zones (Z 1 , Z 2 , Z 3 ) and the size B of the step  205 , for example, as well as other considerations, such as the length of time the inertia mass  150  stays open in response to an activating acceleration force, the spring rate of the biasing spring and the mass of the inertia mass  150 , for example, may each be varied to achieve a large number of possible combinations. More than one combination may produce suitable overall performance for a given application. In a common off-road bicycle application, the combination desirably provides a self-centering force of between 0 and 800 lbs. for an off-center amount of 25%. Preferably, a self-centering force of between 0 and 40 lbs. is produced and more preferably, a self-centering force of between 0 and 5 lbs. is produced for an off-center value of 25%. Desirably, the combination provides a self-centering force of at least 0.25 ounces for an off-center amount of 25%. Preferably, a self-centering force of at least 0.5 ounces is produced and more preferably, a self-centering force of at least 1 ounce is produced for an off-center value of 25%. Most preferably a self-centering force of at least 2 ounces is produced for an off-center value of 25%. The above values are desirable for a rear shock absorber  38  for an off-road bicycle  20 . The recited values may vary in other applications, such as when adapted for use in the front suspension fork  34  or for use in other vehicles or non-vehicular applications. 
       FIG. 29  illustrates an inertia valve assembly  410 , which is similar to the inertia valve assembly  138  of  FIG. 3B . The inertia valve assembly  410  of  FIG. 29  may be incorporated in a shock absorber, such as the shock absorber  38  of the bicycle  20  illustrated in  FIG. 1 . The inertia valve assembly  410  desirably includes an inertia mass  412 , which has an increased density in comparison to the inertia mass  150  of  FIG. 3B . As a result, the inertia mass  412  is more responsive to an acceleration force of a given magnitude. Preferably, the inertia valve assembly  410  operates in a substantially similar manner to the inertia valve arrangement  138  described above and, therefore, the inertia valve assembly  410  and associated shock absorber are described in limited detail. 
     Preferably, the inertia valve assembly  410  is disposed within a reservoir tube  414  and is operable to selectively permit fluid flow between a first fluid chamber  416  and a second fluid chamber  418 . In a preferred embodiment, the first fluid chamber  416  comprises a compression chamber of the shock absorber and the second fluid chamber  418  comprises a reservoir chamber of the shock absorber. Preferably, the inertia mass  412  is supported for axial movement on an axis A C , which is defined by a shaft  420 . The inertia mass  412  is biased in an upward direction (with respect to the orientation of the tube  414  illustrated in  FIG. 29 ) against an upper stop, defined by snap ring  422 , by a biasing member, such as coil spring  424 . In this position, the inertia mass  412  closes openings  434  in the shaft  420  to define a dosed position of the inertia valve assembly  410 . 
     A base  426  is coupled to a lower end of the reservoir tube  414  and, preferably, includes a cavity  428 , which defines a pocket  430  below the inertia mass  412 . The pocket  430  is sized and shaped to receive at least a lower portion of the inertia mass  412 . A bottom surface of the cavity  432  functions as a lower stop for the inertia mass  412 . As described in detail above, preferably, the inertia mass  412  is responsive to an appropriate acceleration force input above a predetermined threshold. Upon being subjected to such an acceleration force, the inertia mass  412  moves downwardly relative to the shaft  420 , against the biasing force of the spring  424 , and into the pocket  430 . In this position, the inertia mass  412  uncovers openings  434  to permit fluid flow from the first fluid chamber  416  to the second fluid chamber  418  and define an open position of the inertia valve assembly  410 . 
     The inertia valve assembly  410  also includes a refill valve assembly  436 , which preferably is configured to at least partially control a flow of fluid between the reservoir chamber  418  and the pocket  430 . In the illustrated embodiment, the valve assembly  436  includes a plurality of hooks  438  (only one shown) extending in an upward direction from the base  426 . Preferably, the hooks  438  are disposed around the periphery of the cavity  428  adjacent an inner surface of the reservoir tube  414 . In a preferred arrangement, four such hooks  438  are equally spaced around a periphery of the cavity  428 . 
     The hooks  438  define an upper stop surface  440  and an upper surface of the base  426  defines a corresponding lower stop surface  442 . A check plate  444  is retained for movement between the upper stop surface  438  and the lower stop surface  442 . Preferably, the check plate  444  is substantially annular in shape with an inner diameter which is slightly larger than an outer diameter of an adjacent portion of the inertia mass  412 , such that a clearance distance C is defined therebetween. 
     In a preferred arrangement, the check plate  444  is configured to restrict a flow of fluid from the reservoir chamber  418  into the pocket  430  at a first level and permit fluid flow from the pocket  430  to the reservoir  418  at a second level, which preferably is greater than the first level. In operation, when the inertia mass  412  is moving downward relative to the shaft  420 , such as due to an appropriate acceleration force, the movement of fluid out of the pocket  430  lifts the check plate  444  in an upward direction against the upper stop surface  440 , as illustrated in phantom. Accordingly, a large amount of fluid is permitted to be displaced from the pocket  430  to the reservoir chamber  418 , as illustrated by the phantom flow line  445 . 
     Conversely, when the inertia mass  412  is moving from a lower most position, within the pocket  430 , toward the upper stop  422 , fluid within the reservoir  418  attempts to fill the pocket  430  thereby urging the check plate  444  against the lower stop surface  442 , as illustrated by the solid line position of the check plate  444 . In the lower position of the check plate  444 , fluid is restricted to entering the pocket  430  by passing through the clearance distance C between an inner surface of the check plate  444  and an outer surface of the inertia mass  412 , as illustrated by the solid flow line  446 . Preferably, with such an arrangement, the flow into the pocket  430  is restricted to a rate that is lower than the rate in which fluid may exit the pocket  430 . Accordingly, the inertia mass  412  may move quickly in a downward direction into the pocket  430 , while movement in an upward direction is slowed to delay the closing of the inertia valve  410  in order to extend the reduced-damping mode of the shock absorber, as described in detail above. 
     Desirably, the inertia mass  412  is configured to have a relatively high density, and thus a high mass for a given volume, so that the inertia mass  412  moves more easily through the damping fluid within the chambers  418  and  430  to increase the responsiveness of the inertia valve  410  to acceleration force inputs. Preferably, the inertia mass  412  includes a first section, comprising a first material, and a second section, comprising a second material having a greater density than the first material. Desirably, the second material has a density greater than about 10 g/cm 3  and, preferably, greater than about 15 g/cm 3 . More preferably, the second material has a density of about 19 g/cm 3 . In the illustrated arrangement, the inertia mass  412  comprises a body portion  450 , which defines an annular cavity  452  filled with a high density material  454 , so as to increase the overall mass of the inertia mass  412  without increasing the volume that it occupies. A presently preferred high density material  454  is tungsten, preferably in a powdered form. 
     In addition, the ratio of the mass of the inertia mass  412  to the surface area of a lowermost surface  456  of the inertia mass  412 , normal to the axis A C , is also increased in comparison to the previously described inertia mass constructions. The surface  456  may be defined as a leading surface of the inertia mass  412  when the inertia mass  412  is moving in a downward direction (i.e., toward the open position). Accordingly, the leading surface area includes a surface  456   a  of standoff feet  455 , which is generally parallel with the surface  456  and perpendicular to the axis A C  of the shaft  420 . Due to the increased mass to volume, and mass to leading surface area ratios, the inertia mass  412  more easily displaces fluid from the pocket  430  to move more quickly toward the open position in response to suitable acceleration force inputs. 
     In a preferred arrangement, a threaded cap  458  closes an open, upper end of the cavity  452  to retain the tungsten  454  within the cavity. A peripheral edge of the cap  458  includes external threads  460 , which mate with internal threads  462  of the cavity  452 . Thus, the cavity  452  may be filled with tungsten  454 , or another high density material, and closed with the threaded cap  458 . 
     The embodiment illustrated in  FIG. 29  is preferred at least because the main body portion  450  of the inertia mass  412  may be made from a relatively dense, yet readily processable material, such as brass for example, while permitting a material with even higher density, such as tungsten powder, to be held within the cavity  452  without the need for it to be formed or otherwise processed. Alternatively, the entire inertia mass  412  may be made from a material having higher density than brass, such as solid tungsten for example. In a preferred embodiment, the cavity  452 , and thus the tungsten powder  454  or other high density material, occupies a significant portion of the total volume of the inertia mass  412 . For example, desirably the high density material occupies at least one-third volume of the inertia mass  412 . Preferably, the high density material occupies at least one-half and, more preferably, at least two-thirds of the volume of the inertia mass  412 . However, other ratios between the material comprising the main body  450  and the material within the cavity  452  may also be used. 
     An inertia mass configured substantially as described above provides advantages mass to surface area, or mass to volume, ratios so that the inertia mass is very responsive to acceleration force inputs. The tables below illustrate the change in mass to surface area and mass to volume ratios for a constant volume inertia mass and a constant mass inertia mass, respectively, having varying relative volumes of brass and tungsten. In generating the tables, the annular inertia mass was assumed to have a length of 0.875 inches, an inner diameter of 0.375 inches and, for the constant volume inertia mass, an outer diameter of one (1) inch. For the constant mass inertia mass, the outer diameter (and, thus, the leading surface area) varies. The density of brass was assumed to be 8.5539 g/cm 3  and the density of tungsten was assumed to be 19.3 g/cm 3 . The constant volume inertia mass was assumed to have a volume of 9.685 cm 3  and the constant mass inertia mass was assumed to have a mass of 83 grams. The ratios are provided in grams/cubic inch for mass to volume and grams/square inch for mass to surface area. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Constant Volume 
               
            
           
           
               
               
            
               
                   
                 % Tungsten 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 0 
                 10 
                 20 
                 30 
                 40 
                 50 
                 60 
                 70 
                 80 
                 90 
                 100 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Mass 
                 83 
                 93 
                 104 
                 114 
                 124 
                 135 
                 145 
                 156 
                 166 
                 177 
                 187 
               
               
                 Mass/Vol. 
                 140 
                 158 
                 175 
                 193 
                 211 
                 228 
                 246 
                 263 
                 281 
                 299 
                 316 
               
               
                 Mass/Surf. Area 
                 123 
                 138 
                 154 
                 169 
                 184 
                 200 
                 215 
                 231 
                 246 
                 262 
                 277 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Constant Mass 
               
            
           
           
               
               
            
               
                   
                 % Tungsten 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 0 
                 10 
                 20 
                 30 
                 40 
                 50 
                 60 
                 70 
                 80 
                 90 
                 100 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Mass 
                 9.7 
                 9.2 
                 8.6 
                 8.1 
                 7.5 
                 7.0 
                 6.5 
                 5.9 
                 5.4 
                 4.8 
                 4.3 
               
               
                 Mass/Vol. 
                 140 
                 148 
                 158 
                 168 
                 180 
                 194 
                 210 
                 230 
                 253 
                 281 
                 316 
               
               
                 Mass/Surf. Area 
                 123 
                 130 
                 138 
                 147 
                 158 
                 170 
                 184 
                 201 
                 221 
                 246 
                 277 
               
               
                   
               
            
           
         
       
     
       FIGS. 30, 31A and 31B  illustrate an alternative inertia mass  470 , which preferably is configured to provide increased flow resistance, or drag, when moving in a first direction compared to the flow resistance when moving in a second, or opposite direction. In a preferred arrangement, the inertia mass  470  includes one or more collapsible drag members  472 , which are configured to assume a first orientation when the inertia mass  470  is moving in a first direction and a second orientation when the inertia mass  470  is moving in the opposite direction. 
     As in the inertia valve assemblies described above, the inertia mass  470  is supported for axial movement on a shaft  474  within a reservoir chamber  476 . In the illustrated embodiment, the inertia mass  470  includes a body portion  478 , the outer surface of which defines a pair of annular grooves  480 . The annular grooves support the drag members  472 , which are also annular in shape. In a preferred arrangement, the drag members  472  are constructed from a flexible material, such as rubber or plastic, and extend upwardly and outwardly from the outer surface of the body portion  478  of the inertia mass. In addition, the drag members  472  may curve in an upward direction from an inner diameter to an outer diameter of the drag member  472 . Accordingly, a peripheral edge portion of each drag member  472  tends to be collapsible in an upward direction relative to the inner edge portion of the drag member  472 . 
     In operation, when the inertia mass  470  is moving in a downward direction relative to the shaft  474 , or toward an open position, fluid flow illustrated by the arrows  482  in  FIG. 31A  exerts an upward force on the drag members tending to collapse the drag members radially inward. Accordingly, a leading surface area of the inertia mass  470  is reduced and the fluid  482  flows past the drag members  472  with, preferably, little interruption. Thus, preferably, the drag members  472  exert little resistive force against the downward movement of the inertia mass  470  toward the open position. 
     Conversely, when the inertia mass  470  is moving in an upward direction relative to the shaft  474 , toward the closed position, fluid  484  flowing beside the inertia mass  470  tends to open the drag members  472  into their relaxed, or radially extended, orientation, as illustrated in  FIG. 31B . Thus, preferably, the drag members  472  cause turbulent flow of the fluid adjacent the body portion  478 . Such flow significantly increases the resistance to fluid  484  flowing past the inertia mass  470  and, thereby, slows the movement of the inertia mass  470  toward the closed position. Thus, the drag members  472  provide a delay, or timer function, to the inertia mass  470 , in a manner similar to the timer arrangements described above. 
     The drag members  472  may be used in addition, or in the alternative, to other delay producing devices, such as the valve  436  of  FIG. 29  or the clearance passage C illustrated in  FIG. 6 . Furthermore, although two drag members  472  are provided in the illustrated inertia valve assembly  470 , a greater or lesser number of drag members  472  may also be used. In addition, although the drag members  472  are illustrated as annular members extending outwardly from a side wall of the inertia mass  470 , other constructions are also possible. For example, collapsible drag members may be disposed above or below the main body  478  of the inertia mass  470  and be configured in a similar manner to achieve the same, or similar, effect. 
       FIG. 32  illustrates an alternative inertia valve assembly  490  in which the delay in closing of the inertia mass  492  is influenced by a pressure differential between the pressure of the fluid within the reservoir chamber  494  and the pressure of the fluid within the passage  526 . During a rebound stroke of the shock absorber, as fluid exits the reservoir chamber  494 , flowing downward (relative to the orientation shown in  FIG. 32 ) through the central shaft  496 , a pressure drop occurs. For a given flow rate, the magnitude of the pressure drop is influenced by the diameter of the flow passage in the shaft  496 . A smaller flow passage diameter creates a larger pressure drop top to bottom. 
     Similar to the previous embodiments, the inertia mass  492  is supported by a shaft  496  for axial movement about an axis A C . The inertia mass  492  is positioned within the reservoir chamber  494  defined by a reservoir tube  498 . A base  500  is connected to a lower end of the reservoir tube  498  and defines a recess  502  which, in turn, defines a pocket  504  for receiving at least a lower portion of the inertia mass  492  when the inertia mass  492  is in the open position. Thus, a bottom surface of the recess  502  functions as a lower stop for the inertia mass  492 . The inertia mass  492  is biased against an upper stop, defined by snap ring  506 , by a biasing member, such as coil spring  508 . 
     Preferably, the base  500  defines a first passage  510  that connects the reservoir chamber  494  and the pocket  504 . Desirably, the base  500  also defines a second passage  512  that connects the reservoir chamber  494  and the pocket  504 . A pressure actuated valve arrangement  514  selectively permits fluid communication through the second passage  512  when the pressure in the reservoir chamber is above a predetermined threshold. The valve assembly  514  includes a valve body  516  biased into a closed position by a biasing member, such as coil spring  518 . In the closed position, an enlarged diameter upper portion  517  of the valve body is arranged to block the second passage  512  to substantially prevent fluid flow therethrough. 
     Preferably, an upper stop for the valve body  516  is defined by a snap ring  520  and a lower stop is defined by a lower end of a valve seat  521 , which receives the upper portion  517  of the valve body  516 . Desirably, the valve body  516  includes an elongated lower end, or shaft portion  522 , which functions as a guide for the coil spring  518 . In addition, preferably a seal member  528  creates a seal between the valve body  516  and the base  500  to inhibit fluid from passing therebetween. Thus, the valve body  516  is normally biased into a closed position by the force of the biasing member  518 . If the pressure differential between the reservoir chamber  494  and the passage  526  exceeds a predetermined threshold, the valve body  516  moves toward the open position, against the biasing force of the spring  518 . In the illustrated arrangement, the predetermined threshold is determined primarily by the surface area of the upper end surface of the valve body  516  and the spring constant of the biasing member  518 . 
     As described above, when the inertia mass  492  moves into its open position, refilling of the pocket  504  is restricted to fluid flow between an outer surface of the inertia mass  492  and an inner surface of the cavity  502 . In addition, fluid may refill the pocket  504  by flowing through the passage  510 , if provided. Thus, the inertia mass  492  is delayed from moving toward its open position due to the restriction of the fluid from entering the pocket  504 . However, in the embodiment of  FIG. 32 , if the pressure differential between the reservoir chamber  494  and the passage  526  exceeds a predetermined threshold, the pressure actuated valve assembly  514  opens to permit fluid flow into the pocket  504  through the second passage  512 . Preferably, the second passage  512  is configured to permit a greater rate of flow into the pocket  504  in comparison to fluid flow through the clearance between the inertia mass  492  and the cavity  502  and fluid flow through the passage  510  (if provided). Accordingly, when the pressure actuated valve assembly  514  opens, the inertia mass  492  may return to its closed position more quickly. 
       FIG. 33  illustrates an alternative embodiment of a pressure activated inertia valve assembly  530 . In the embodiment of  FIG. 33 , an inertia mass  532  is configured for axial movement on a shaft  534  about an axis A C . Preferably, the inertia mass  532  is disposed within a reservoir chamber  536  defined at least partially by a reservoir tube  538  and a base  540 . A passage  542  extends through the base  540  and shaft  534  and is in fluid communication with the reservoir chamber  536  through openings  544 . Desirably, the passage  542  receives fluid from a compression chamber (not shown) of the shock absorber, as will be appreciated by one of skill in the art. Thus, the inertia mass  532  selectively permits fluid communication between the passage  542  and the reservoir chamber  536 . 
     In the embodiment of  FIG. 33 , a slide member  546  is interposed between the base  540  and the inertia mass  532 . The slide  546  includes a recess  548  that defines a pocket  550  for receiving the inertia mass  532 . The inertia mass  532  is biased into an uppermost, or closed, position (against stop  552 ) by a biasing member, such as coil spring  554 . The spring  554  is supported relative to the shaft  534  by a lower stop, defined by snap ring  556 . The snap ring  556  also defines an uppermost position of the slide  546 . The slide  546  is also axially moveably relative to the shaft  534  and is biased into its uppermost position by a biasing member, such as coil spring  558 . 
     The base  540  defines a cavity  560 , which receives a lower end of the slide  546  in a sealed arrangement. One of a lower surface  562  of the cavity  560  or an upper surface  564  of the base  540  function as a stop to define a lowermost position of the slide  546 . In addition, preferably one or more passages  566  permit fluid communication between the passage  542  and a pocket  568  defined by the cavity  560 . Preferably, the pocket  568  is substantially sealed, with the exception of the passages  566 , such that fluid within the pocket  568  is at substantially the same pressure as fluid within the passage  542  (and, thus, the compression chamber of the shock absorber). 
     In operation, the inertia mass  532 , upon receiving an appropriate acceleration force, moves in a downward direction relative to the shaft  534  and into the pocket  550 . Once in the pocket  550 , the inertia mass  532  is delayed in moving in an upward direction due to the restriction of fluid being permitted to refill the pocket  550 . Thus, the inertia mass  532 , when positioned within the pocket  550 , moves toward the closed position at a delayed rate. In the illustrated embodiment, fluid may pass from the reservoir chamber  536  into the pocket  550  through a clearance distance C between an outer diameter of the inertia mass  532  and an inner diameter of the cavity  548 . 
     When a difference in fluid pressure between the reservoir chamber  536  and the passage  542  (and, thus, the pressure within the compression chamber of the shock absorber) exceeds a predetermined threshold, the slide  546  moves downward relative to the shaft  534  and into the pocket  568 . In the illustrated embodiment, preferably, the predetermined threshold is determined primarily by a surface area of an end surface  569  the slide  546 , which is perpendicular to the center axis A C  of the shaft  534  and disposed within the pocket  568 , along with the spring rate of the biasing member  558 . 
     Thus, with the inertia mass  532  in its open position, the slide  546  moves in a downward direction away from the inertia mass  532 . When the slide  546  moves downwardly a sufficient distance, the inertia mass  532  is no longer present within the pocket  550  and fluid may refill the pocket  550  at a relatively high rate. Thus, the inertia mass  532  is no longer restricted from moving in an upward direction due to the restriction of fluid moving into the pocket  550  and, as a result, the biasing member  554  returns the inertia mass  532  to its closed position at a normal rate, determined primarily by the weight of the inertia mass  532  and the spring rate of the spring  554 . Accordingly, with such an arrangement, when the inertia mass  532  is in the open position and the pressure within the reservoir chamber  536  exceeds the pressure within the passage  542  by a predetermined threshold, the inertia mass  532  is permitted to return to the closed position without significant delay. 
       FIGS. 34 and 35  illustrate a bicycle that employs yet another alternative embodiment of an acceleration sensitive shock absorber. The bicycle  580  includes a main frame portion  582 , an articulating frame portion  584 , a front wheel  586 , and a rear wheel  588 . Preferably, a front suspension assembly  590  is operably positioned between the front wheel  586  and the main frame  582  and a rear suspension assembly, or shock absorber  592 , is operably positioned between the rear wheel  588  and the main frame  582 . Preferably, the articulating frame portion  584  carries the rear wheel  588  and the shock absorber  592  is connected to the articulating frame portion  584  to resist movement of the rear wheel  588  in an upward direction. Preferably, the shock absorber  592  is positioned on one lateral side of the rear wheel  588  and, desirably, on the left-hand side of the rear wheel  588 . 
     With reference to  FIG. 35 , desirably, the shock absorber  592  includes a reservoir chamber  594  at least partially defined by a reservoir tube  596  and a base  598 . Preferably, an acceleration sensitive valve assembly  600  is disposed within the reservoir chamber  594 . The valve assembly  600  preferably includes a valve body  602  biased into an uppermost, or open position, by a biasing member, such as coil spring  604 . The valve body  602  is supported for axial movement along an axis A C , which is defined by a shaft  606 . An uppermost position of the valve body  602  preferably is determined by a snap ring  608 . In the illustrated embodiment, the uppermost position defines a closed position of the valve  600 . 
     The base  598  preferably includes a cavity  610  that defines a pocket  612  in which the valve body  602  enters in its lowermost position. In a preferred arrangement, when the valve body  602  is in its lowermost position, fluid flow is permitted through openings  613  of the shaft  606 . A bottom surface  614  of the cavity  610  defines a lower stop for the valve body  602 . Preferably, as described above, a valve assembly  616  is provided to permit relatively free flow of fluid from the pocket  612  to the reservoir chamber  594  while permitting restricted flow of fluid from the reservoir chamber  594  into the pocket  612 . 
     Desirably, the valve assembly  600  includes a system for sensing acceleration force inputs and for moving the valve body  602  to an open position and/or retaining the valve body  602  in an open position. In the illustrated embodiment, preferably an electromagnetic system  618  is provided. The system  618  preferably includes an electromagnetic force generator  620  within the base  598  and positioned below the valve body  602 . A control assembly  622  is operably connected to the electromagnetic force generator  620 . Preferably, the valve body  602  includes a lower portion  624 , which is constructed from a magnetic material. The electromagnetic force generator  620  desirably is configured to selectively apply an attractive force to the magnetic portion  624  of the valve body  602 . Thus, the valve body  602  may be moved toward, or retained in, an open position by the electromagnetic force generator  620 . 
     With reference to  FIG. 34 , preferably, a sensor  626  is positioned on the front suspension assembly  590  for movement with a hub axis AH of the front wheel  586 . In addition, or in the alternative, a sensor  628  may be secured to the articulating frame portion  584  for movement with a hub axis A H  of the rear wheel  588 . Preferably, each of the sensors  626  and  628  are configured to sense substantially vertical acceleration force inputs to the front or rear wheels  586 ,  588 , respectively. 
     The sensors  626 ,  628  are configured to communicate with the control assembly  622  to provide a control signal indicative of the acceleration forces acting on the front or rear wheels  586 ,  588 . In a preferred embodiment, the sensors  626 ,  628  produce an electronic signal to communicate with the control assembly  622 . In such an embodiment, the sensors  626 ,  628  may communication with the control assembly  622  through a hardwired system or, preferably, over a wireless communication system. Furthermore, other suitable types of sensors and methods of communication between the sensors  626 ,  628  and the control assembly  622  may also be used, such as hydraulic or mechanical systems, for example. Thus, the control signal may include changes in hydraulic pressure, or movement of a mechanical linkage, for example. Other suitable systems apparent to one of skill in the art may also be used. 
     The control assembly  622  preferably includes a processor and a memory for storing a control algorithm, or protocol. The control assembly  622  uses the control signal provided by the sensors  626 ,  628  along with the control algorithm to determine whether to activate the electromagnetic force generator  620 . Thus, when an appropriate acceleration force input is detected, the control assembly  622  may activate the electromagnetic force generator  620  to move the valve body  602  from its closed position into an open position and, if desirable, retain the valve body  602  in an open position for a period of time, or a delay period. 
     Desirably, the control assembly  622  includes an adjustment mechanism, to permit adjustment of the delay period in which the valve body  602  is held in an open position and/or the acceleration force threshold above which the valve assembly  600  is opened. Preferably, the control assembly  622  includes a first adjustment knob  630 , to permit adjustment of the delay period, and a second adjustment knob  632 , to permit adjustment of the acceleration force threshold. 
     The valve body  602  may be fully controlled by the electromagnetic force generator  620  or may be configured to be self-responsive to acceleration force inputs due to the inertia of the valve body  602 . Furthermore, the valve  616  may be provided to determine a delay period of the valve body  602  or the electromagnetic force generator  620  may be relied on to provide the delay in the valve body  602  from returning to the closed position. In addition, a combination of inertia forces and electromagnetic forces may be utilized to open the valve body  602  and a combination of fluid restriction, or fluid suction, forces and electromagnetic forces may be utilized to provide the valve body  602  with a delay period in moving from an open position to a closed position. 
     Advantageously, by positioning the sensor  626  to sense acceleration force inputs of the front wheel  586 , the valve body  602  in the rear shock absorber  592  may be moved into its open position before the object (e.g., such as a bump, rock or other irregularity in the trail surface) which caused the acceleration force is encountered by the rear wheel  588 . Thus, there is no delay in the altered rate of damping of the rear shock absorber  592  due to the valve body  602  having to move from its closed position to its open position upon encountering the bump, or other obstacle, because the bump has been “anticipated” by the sensor  626  positioned to detect acceleration of the front wheel  586 . 
     As described above, preferably, the valve body  602  remains in an open position, or is delayed from returning to its closed position, so that the rear wheel  588  may absorb a series of bumps and the valve assembly  600  does not have to reactivate upon encountering each individual bump. Advantageously, by permitting the delay to be controlled by the adjustment mechanism  630 , a rider can tune the shock absorber  592  to suit anticipated trail conditions by providing a relatively short or a relatively long delay time. In addition, the acceleration threshold may also be adjusted such the size of bump necessary to open the valve assembly may be varied. 
     Furthermore, the front suspension assembly  590  may also be configured to include an acceleration sensitive valve assembly, similar to the valve assembly  600 . In addition, the various features illustrated in  FIGS. 1-35  may be used in combination with one another to provide a desired result, as may be determined by one of skill in the art. 
     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 an inertia valve 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 self-centering and timer features of the inertia valve assembly 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.