Abstract:
An air spring is optionally integrated with a shock absorber or other damping unit, and has an air cylinder closed at one end, and in axially-slidable engagement with a sealing head at the other end. A sealed partitioning plate is fixed in the cylinder, and divides the cylinder into a primary air chamber and a secondary air chamber. A sealed air passage connects the primary and secondary air chambers, and can be opened or closed by a simple one-quarter turn rotation of an external knob to instantly switch the air spring between two different spring rate curves.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    A preferred embodiment relates to air springs for vehicle suspensions and preferably two-wheeled vehicle suspensions, such as the suspensions of bicycles, which are typically mounted between the chassis of the vehicle and a wheel. In particular, a preferred embodiment relates to an air spring, optionally combined with a shock absorber or other damper to form an integrated suspension unit, which permits instant selection between two different spring rate curves by simply rotating an external knob one-quarter turn.  
           [0003]    2. Description of the Related Art  
           [0004]    One advantage of air springs is the ability to change spring rate (“softer” or “stiffer”) simply by adjusting the internal air pressure. Such a method permits changing the spring rate curve of an air spring, and is available for essentially all air springs. On many air springs this is accomplished relatively easily by pressurizing or de-pressurizing the air spring with a hand pump and gauge, or by using an air pressure hose at an automobile service station (“gas station”). Most air springs are purposely designed with a standardized Schrader air valve (similar to those in automobile tires) to make this relatively easy and convenient.  
           [0005]    As is known in the art, spring rate characteristics can also be changed by altering the initial air chamber volume. Increasing or decreasing initial air chamber volume softens or stiffens, respectively, the air spring curve.  
           [0006]    Conventional prior-art air springs for bicycles and motorcycles have been known to provide features for altering air chamber volume. The most common method, used especially in air-sprung mountain bike front forks, is to increase or decrease air chamber volume by adding or removing hydraulic fluid (“changing the oil level”). In other cases, air spring suspension components have been provided with solid, light-weight, “volume plugs”. The air spring is opened and the “volume plugs” are added or removed from the air chamber. Both methods, of course, generally require depressurization and opening of the hydraulic unit and/or the air spring.  
           [0007]    Another feature known in the art for altering the air chamber volume on certain bicycle and motorcycle air springs has been an adjustable-position threaded cap closing off the air chamber. Threading this cap in or out, which can require a fair amount of torque to overcome frictional forces resulting from the internal pressure, changes the air chamber volume. For example, U.S. Pat. No. 5,346,236 teaches this for a bicycle front fork. Also known in the art is a threaded-cap adjustable-volume external air reservoir which can be added to a basic shock absorber or fork air spring. Changes of this type can typically be accomplished faster than adding or removing hydraulic fluid, and may be accomplished in about 1 minute.  
           [0008]    Motorcycles having air suspension with an on-board pressurization system including an on-board air compressor to monitor and regulate air pressure on demand, are also known in the art.  
           [0009]    There is a need in bicycles and motorcycles which incorporate air spring suspension for a quick, easy way to alter the air spring curve “on-the-fly”. All the prior-art methods noted above suffer from various limitations, including time and effort required, weight, bulk, complexity, and cost.  
         SUMMARY OF THE INVENTION  
         [0010]    One aspect of a preferred embodiment is to provide a suspension air spring, optionally integrated with a shock absorber or other damper, that permits instant selection between “soft” and “firm” spring rate curves by simply turning an external knob one-quarter turn. This is much quicker and easier than other methods provided by conventional prior-art designs. The illustrated embodiments are particularly applicable to bicycles.  
           [0011]    In the context of real-world mountain biking, all prior-art methods of changing air spring rates create a significant interruption in the ride, and thus typically are done infrequently, or not at all, during a ride. In contrast, turning an external knob as described according to a preferred embodiment is so quick and simple that it can be done in a routine “on-the-fly” manner dozens of times as desired during a typical ride. Since terrain and trail conditions constantly change, this greatly benefits the rider by enabling him/her to continuously select the best spring rate for the current situation.  
           [0012]    The illustrated embodiments achieve this result by partitioning the air spring (more generally, “gas spring”) into two separate partial volumes. The two partial volumes are connected by a sealed passage which is selectively opened or closed by turning an external knob. Turning the knob rotates a cam which desirably is in contact with a cam follower. The cam follower then preferably moves a check ball up or down, causing the ball to either seat on or unseat from, respectively, a seal in the connecting passage.  
           [0013]    When the check ball is seated, the passage is closed and air flow from the first partial volume to the second partial volume is blocked. This isolates the second partial volume and prevents it from physically participating as a part of the air spring upon compression of the suspension. As is well-known in the art, air spring characteristics depend upon the initial pressure and volume characteristics of the air spring. When the total initial volume is effectively reduced, as occurs here when the passage leading to the second partial volume is blocked, the air spring characteristic (“spring curve”) becomes firmer.  
           [0014]    When the check ball is unseated, the passage is open and air flow between the two partial volumes is unrestricted. This, of course, makes both partial volumes physically available to the air spring, and results in greater total initial volume and a softer air spring characteristic.  
           [0015]    A preferred embodiment is an air spring for a two wheeled vehicle. The air spring being positionable between a vehicle sprung mass and a vehicle wheel. The air spring includes an air cylinder closed at one end and connectable to one of a vehicle sprung mass and a vehicle wheel. A piston is in axially-slidable engagement with the air cylinder and is connectable to the other of a vehicle sprung mass and a vehicle wheel. A partitioning member is positioned within the cylinder and at least partially divides the cylinder into a primary air chamber and a secondary air chamber. A passage connects the primary air chamber and the secondary air chamber and a valve assembly is configured to selectively permit air flow through the passage.  
           [0016]    Another preferred embodiment is a gas spring assembly including a body portion and a shaft portion. The shaft portion is telescopingly engaged with the body portion. A piston is carried by the shaft portion and cooperates with the body portion to define a variable volume first gas chamber. One of the shaft portion and the body portion at least partially defines a second gas chamber. A valve assembly is positionable in a first position and a second position. In the first position, the valve assembly substantially prevents communication between the first gas chamber and the second gas chamber and in the second position, the valve assembly permits communication between the first gas chamber and the second gas chamber.  
           [0017]    Another aspect of the present invention involves a front suspension fork assembly positionable on a bicycle. The front fork assembly includes a gas spring assembly having a body portion and a shaft portion. The shaft portion is telescopingly engaged with the body portion. A piston is carried by the shaft portion and cooperates with the body portion to define a variable volume first gas chamber. One of the shaft portion and the body portion at least partially defines a second gas chamber. A valve assembly is positionable in a first position and a second position. In the first position, the valve assembly substantially prevents communication between the first gas chamber and the second gas chamber and in the second position, the valve assembly permits communication between the first gas chamber and the second gas chamber. The valve assembly is movable between the first position and the second position by an actuator positioned to be accessible to a hand of a rider of the bicycle while riding. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is an overall front view of a prior-art embodiment of a suspension unit consisting of a shock absorber (“damper”) integrated with an air spring.  
         [0019]    [0019]FIG. 2 is a partial sectional front view of the prior-art suspension unit of FIG. 1.  
         [0020]    [0020]FIG. 3 illustrates the effect of the negative chamber by comparing the spring curve of the prior-art suspension unit of FIG. 2 with the spring curve it would have if the negative chamber was eliminated.  
         [0021]    [0021]FIG. 4 is an overall front view of a preferred embodiment of a suspension unit.  
         [0022]    [0022]FIG. 5 is an overall side view of the suspension unit of FIG.4.  
         [0023]    [0023]FIG. 6 is a partial sectional front view of the suspension unit of FIG. 4.  
         [0024]    [0024]FIG. 7 is a partial sectional top view of the suspension unit of FIG. 6.  
         [0025]    [0025]FIG. 8 is an enlarged partial sectional view of the suspension unit of FIG. 6.  
         [0026]    [0026]FIG. 9 is an isometric view of the upper eyelet housing of the suspension unit of FIG. 6.  
         [0027]    [0027]FIG. 10 is an isometric view of the air sleeve partition of the suspension unit of FIG. 6.  
         [0028]    [0028]FIG. 11 is an enlarged partial sectional view of the switching mechanism of the suspension unit of FIG. 6, with the adjusting lever set so the air passage is open.  
         [0029]    [0029]FIG. 12 is an enlarged partial sectional view of the switching mechanism of the suspension unit of FIG. 6, with the adjusting lever set so the air passage is closed.  
         [0030]    [0030]FIG. 13 is a partial sectional view of the suspension unit of FIG. 6, showing the approximate full travel position with the adjusting lever set in the open position.  
         [0031]    [0031]FIG. 14 is a partial sectional view of the suspension unit of FIG. 6, showing the approximate full travel position with the adjusting lever set in the closed position.  
         [0032]    [0032]FIG. 15 illustrates the effect of the lever adjustment by comparing the spring curves of the suspension unit of FIG. 6 with the lever in the closed position, and with it in the open position.  
         [0033]    [0033]FIG. 16 shows an alternate embodiment of a suspension unit, where the connecting passageway occurs through the center shaft. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    The prior-art integrated suspension unit  100  of FIGS. 1 and 2 will be described first, in order to provide a point of departure for better understanding the improvements of the preferred embodiments, which will be described further on.  
         [0035]    The typical prior-art integrated suspension unit  100  as shown in FIGS. 1 and 2 is manufactured by Fox Racing Shox. It is to be understood, of course, that this specific prior-art embodiment is representative only, and that the present air spring arrangement can be applied to other types of suspension units. Additionally, the present air spring arrangement can be applied as a separate air spring unit, not integrated with a damper.  
         [0036]    In FIGS. 1 and 2 the integrated suspension unit  100  is comprised of an air spring assembly  110  and a damper assembly  190 . The integration is seamless, with several of the components such as an upper eyelet housing  116  and seal head  194  shared by both assemblies and performing dual functional roles. For example, as part of the damper assembly  190 , the seal head  194  closes and seals off one end of the shock body  197 . At the same time, as part of the air spring assembly  110 , the seal head  194  also seals off the open end of the air cylinder  126 .  
         [0037]    Still referring to FIGS. 1 and 2, the ends of the integrated suspension unit  100 , the upper eyelet  114  and the lower eyelet  198 , are connected to the sprung and unsprung portions of the vehicle (not shown) in a conventional manner. The air pressure in the positive air chamber  128  creates a force tending to lengthen the suspension unit  100 , while pressure in the negative air chamber  136  tends to shorten it. As is well-known in the art, the net effect of these opposing forces is to create a desirable air spring curve (“force vs. travel curve”), especially in that portion of the travel regime where the suspension unit  100  is near full extension.  
         [0038]    In particular, it is well-known that without the counteracting force produced by the negative air chamber  136 , which rapidly increases as the shock absorber approaches full extension and the volume of the negative air chamber  136  rapidly decreases, the initial portion of the spring curve (“spring preload”) would be quite stiff. Thus, an undesirably large beginning force would be required to initiate the first portion of travel from full extension.  
         [0039]    Typical spring curves produced with and without the negative air chamber  136  are illustrated in FIG. 3. Curve “A” shows a force versus travel spring curve that would be produced by the embodiment of FIG. 2, which includes the negative air chamber  136 . In contrast, Curve “B” shows the spring curve that would result if the negative air chamber  136  was removed (not shown). On a bicycle, or other vehicle, spring curve “B” would generally produce an undesirably harsh ride due to the large initial force required to initiate travel from full extension.  
         [0040]    The positive air chamber  128  is pressurized via the air valve  112 . As is typical, an air passage (not shown) is drilled in the upper eyelet housing  116 , and leads from the air valve  112  to the positive air chamber  128 .  
         [0041]    The negative air chamber  136  is pressurized via a transfer port  132 . Transfer occurs at that pre-determined point near the beginning of suspension travel where the transfer port  132  bridges the positive/negative seal assembly  130 , as depicted in FIG. 2. This air transfer feature provides an effective and simple means for properly balancing the pressures of the positive air chamber  128  and the negative air chamber  136 , and is more fully described in U.S. Pat. No. 6,135,434.  
         [0042]    The positive/negative seal assembly  130  provides a moving seal between the positive air chamber and the negative air chamber and seals at all times except when bridged by the transfer port  132 . The inside bore of the air cylinder  126  is burnished or otherwise finished to provide a smooth, low-friction surface which seals well.  
         [0043]    The negative chamber seal assembly  140  seals the lower side of the negative chamber on the outside of the shock body  197 , which is burnished or otherwise finished to provide a smooth, low-friction surface which seals well.  
         [0044]    The prior-art integrated suspension unit  100  of FIGS. 1 and 2 includes provisions for adjusting the internal damping by rotating a damping adjuster knob  191  which, in turn, rotates the damping adjuster rod  192  which extends down the shaft  193  into the piston assembly  195 . This basic construction, available in many conventional high-performance shock absorbers and well-known to those skilled in the art, enables external adjustability of compression damping, rebound damping, or both.  
         [0045]    Although this damper construction feature is not required for application of the preferred embodiments, it is illustrated here in the prior-art and it is also included in the illustrated embodiment shown in FIG. 6. If this adjustable damping feature is not included, a somewhat simplified and less costly preferred embodiment, as described later and illustrated in FIG. 16, is made possible.  
         [0046]    The rest of the prior-art integrated suspension unit  100 , including the piston assembly  195  which creates damping as it moves thru the damping fluid  196 , are not illustrated or described in further detail since they are conventional features well-known to those skilled in the art, and are not required for an understanding of the preferred embodiments.  
         [0047]    External views of a preferred embodiment are shown in FIGS. 4 and 5. Suspension unit  200  comprises a damper assembly  190  identical to that of FIG. 1, and an adjustable air spring assembly  210 . A manually-operable travel adjust lever  252  which extends from the upper portion of suspension unit  200 . The travel adjust lever  252  can be rotated 90-degrees clockwise or counterclockwise between the two positions shown, the “long-travel mode” and the “short-travel mode”, as will be described more fully further on.  
         [0048]    [0048]FIG. 6 shows a partial sectional view of the suspension unit  200  of FIG. 4. In comparison to the prior-art device of FIGS. 1 and 2, the damper assembly  190  of FIG. 6 is identical to the damper assembly  190  of FIGS. 1 and 2; however, the adjustable air spring assembly  210  of FIG. 6 contains additional structure and modified structure as compared with air spring assembly  110  of FIGS. 1 and 2. The additional and modified structure comprises an air cylinder partition  272  sealed within the air cylinder  126  which separates the divided positive air chamber  228  into a first partial volume  227  and a second partial volume  229 , and a travel adjust assembly  250  which enables these two partial volumes to be either connected or separated by rotation of the external travel adjust lever  252 .  
         [0049]    [0049]FIGS. 7 and 8 show enlarged views illustrating this additional structure and modified structure, which will now be described in detail. In FIG. 8 and other drawings, various seals (such as a conventional O-ring seal between the air cylinder  126  and the air cylinder partition  272 ) are included in the drawing, but are not numbered or described, since they are conventional features well-known to those skilled in the art.  
         [0050]    The detent ball assembly  260  provides a detenting effect such that, after adjustment, the travel adjust lever  252  is held in the selected position. It also provides tactile feedback to the operator to indicate attainment of a new position upon rotation. The travel adjust lever  252  is incorporated into the upper eyelet housing  216  and is secured to an actuating cam shaft  254  by a retaining screw  256 . A surface of the actuating cam shaft  254  has a ball indent  255  spaced every 90-degrees on its outer surface near one end. A surface of a detent ball  262 , urged by a detent spring  264  which is secured by a detent set screw  266 , engages the ball indent  255 . Thus, in an engaged position, the detent ball  262  engages one of the ball indents  255  and a first level of resistance to rotation of the travel adjust lever  252  is provided that, desirably, inhibits unintentional rotation of the lever  252 , while still allowing the lever  252  to be rotated by hand. In an unengaged position, the detent ball  262  contacts a surface of the cam shaft  254  between the indents  255  and, desirably, provides little or no resistance to rotation of the travel adjust lever  252 .  
         [0051]    In FIG. 8, the retaining ring  278  serves to secure the axial location of the air cylinder partition  272  on the shaft  193 .  
         [0052]    In order to facilitate clear visualization of the interface between the upper eyelet housing  216  and the air cylinder partition  272 , FIG. 9 shows an isometric view of the upper eyelet housing  216 , and FIG. 10 shows an isometric view of the air cylinder partition  272 . As shown, the underside of the upper eyelet housing  216  includes a downwardly-projecting upper passage port coupler  217  which engages the upwardly-projecting lower passage port boss  273  thru which the lower passage port  274  passes. This connection is sealed by a lower passage port seal  276  as shown in FIG. 11.  
         [0053]    [0053]FIG. 11 shows an enlarged partial sectional view of the travel adjust assembly  250 , which is now described in detail. As previously described, the travel adjust lever  252  is secured to the actuating cam shaft  254  by a retaining screw  256 . The actuating cam shaft  254  is retained in the upper eyelet housing  216  by a retaining screw  253 . The actuating cam shaft  254  includes a cam profile  259 . This cam profile  259  consists of 2 flats  259 A 180-degrees apart as shown here in FIG. 11, and 2 deeper flats  259 B as shown in FIG. 12, which also are 180-degrees apart and are at 90-degrees from flats  259 A. These flats control the position of the cam follower  258 , as determined by the setting of the travel adjust lever  252 . Cam follower  258  is sealed by cam follower seal  257 .  
         [0054]    With the travel adjust lever  252  in the position shown in FIG. 11, the cam follower  258  is in contact with the check ball  282  and maintains it in a position out of contact with the check ball seal  283 . As shown by the heavy flow lines drawn, this enables air flow from the first partial volume  227  (not shown in this view) thru the lower passage port  274 , past the check ball  282 , thru the upper passage port  219 , and into the second partial volume  229  (not shown in this view). This is one direction of air flow. The opposite direction of air flow is also enabled. These flows, of course, provide open communication between the first partial volume  227  and the second partial volume  229  such that their combined volume is available during compression of the suspension unit  200 .  
         [0055]    [0055]FIG. 12 shows the travel adjust lever  252  in the closed position. The cam follower  258 , urged upward by internal air pressure, engages cam profile  259 B and, as shown, moves away from check ball  282  by a distance “X”, which is desirably 0.040″ or more. The check ball  282 , urged upward by the check ball spring  284  engages check ball seal  283 . This seals off any upward air flow from first partial volume  227  to second partial volume  229 .  
         [0056]    However, this does not seal off flow in the opposite direction, since check ball spring  284  is specified to produce only a small spring force, for example about 0.03 to 0.05 pounds, with the check ball  282  in the sealed position. Accordingly, if the pressure from the second partial volume  229  above the check ball  282  exceeds the pressure below it from first partial volume  227  by approximately 3 to 5 psi, then this pressure differential will overcome the force of check ball spring  284  and check ball  282  will move downward away from sealing contact with check ball seal  283 . In this event, air will flow from second partial volume  229  to first partial volume  227 .  
         [0057]    This characteristic is desirable in order to prevent unintended entrapment of excess air and pressure in the second partial volume  229 . For correct function of the adjustable air spring assembly  210 , it is preferred that the pressure in second partial volume  229  does not become significantly greater than the pressure in first partial volume  227 . Such a situation would result in the pressure within the first partial volume  227  being reduced from its initial, preset level, due to the finite quantity of air within the suspension unit  200 . As a result, the spring rate of the air spring  200  in its short travel mode (i.e., only utilizing the first partial volume  227 ) would be undesirably reduced from its initial setting. Rather, according to the preferred embodiments, the pressure in the second partial volume  229  preferably remains approximately equal to or less than the pressure in first partial volume  227 , since the check ball spring  284  creates only a small preload force.  
         [0058]    Although the above-described valve assembly is preferred for its simplicity, reliability and low manufacturing cost, other valve arrangements may also be employed. For example, a needle-type valve body may be used in place of the check ball  282 . In an alternative arrangement, the cam surface  259  may directly contact the valve body (e.g., the check ball  282 ) and the cam follower  258  may be omitted. Further, the above-described functions of the valve assembly do not necessarily have to be performed by a single valve arrangement. For example, a first valve arrangement may selectively connect and disconnect the first partial volume  227  and second partial volume  229 , while another valve arrangement provides the check valve function of preventing the pressure of the second partial volume  229  from becoming substantially greater than the pressure of the first partial volume  227 .  
         [0059]    [0059]FIG. 13 illustrates a typical full-travel position of suspension unit  200  when travel adjust lever  252  is set in the long-travel mode, such that first partial volume  227  and second partial volume  229  are in full communication.  
         [0060]    Similarly, FIG. 14 illustrates a typical full-travel position of suspension unit  200  when travel adjust lever  252  is set in the short-travel mode, such that first partial volume  227  and second partial volume  229  are in not in communication.  
         [0061]    Note that the overall compressed lengths of suspension unit  200  are different, with the length L 1  in FIG. 13 being shorter than the length L 2  in FIG. 14. This will be explained with reference to FIG. 15.  
         [0062]    [0062]FIG. 15 illustrates an example of the force-versus-travel relationships provided by suspension unit  200  in the two different selectable modes: the short-travel mode and the long-travel mode. In the long-travel mode, as shown by curve “B”, the force rises more gradually and reaches, in this example, a value of 750 pounds at a stroke distance of about 1.75 inches. In the short-travel mode, as shown by curve “A”, the force rises more rapidly and reaches a value of 750 pounds at a stroke distance of only about 1.27 inches, almost ½ inch less than the value for curve “B”. This relationship, of course, is the basis for describing the two modes as “long-travel mode” and “short-travel mode”.  
         [0063]    It should be explained that, although for simplicity in the above example a final external compression force of 750 pounds on the suspension unit  200  is assumed for both cases, this is only an approximation. A rigorous computer motion analysis of a specific situation, centering on the basic equation of motion F=ma (force equals mass times acceleration), would show some difference, but this analysis is generally quite complicated and the difference would generally be relatively small. Thus, the above is a reasonably close approximation assuming that in both cases the vehicle upon which the suspension unit  200  is mounted is subjected to the same bump (or other terrain feature) and other conditions.  
         [0064]    Additionally, it should be noted that at 1.27 inches of travel curve “A” is rising steeply. Thus, even if the final force that occurs in the short-travel mode is somewhat greater than the 750 pounds used in the above example, final travel would still be significantly less than curve “B”. For example, even if the final force reached 1000 pounds, final travel would still only be slightly more than 1.40 inches. As a preferred embodiment of the present invention is as a shock absorber for a mountain bike, it is desirable that the final force is less than 3000 pounds, desirably, less than 2000 pounds and, more desirably, less than 1000 pounds. Such an arrangement allows the air spring to withstand the impact forces resulting from traversing rough terrain with suspension arrangements presently incorporated on mountain bikes (e.g., wheel travel/shock travel ratio). As will be appreciated by one of skill in the art, for other applications or suspension arrangements, the preferred final force may vary from the values recited above.  
         [0065]    In the context of mountain bike suspension assemblies, preferably, the first partial volume  227  is between about 1 and 8 cubic inches. Desirably, the first partial volume  227  is between about 1.5 and 6 cubic inches and, more desirably, between about 2 and 4 cubic inches. Preferably, the second partial volume  229  is between about 0.3 and 4 cubic inches. Desirably, the second partial volume  229  is between about 0.4 and 3 cubic inches and, more desirably, between about 0.5 and 2 cubic inches. Such an arrangement provides a desirable spring rate of the suspension unit  200  when utilizing only the first partial volume  227 , as well as when both the first partial volume  227  and second partial volume  299  are used to provide a spring force, for a substantial number of mountain bike applications.  
         [0066]    In at least a significant portion of mountain bike suspension applications, it is preferable that the suspension unit  200  provides between about 0.5 and 3 inches of suspension travel in the short travel mode (i.e., utilizing only the first partial volume  227 ). Desirably, the suspension unit  200  provides between about 0.6 and 2.5 inches of travel and, more desirably, between about 0.75 and 2 inches of suspension travel in the short travel mode. Further, preferably the suspension unit provides between about 0.6 and 5 inches of suspension travel in the long travel mode (i.e., utilizing both the first partial volume  227  and the second partial volume  229 ). Desirably, the suspension unit  200  provides between about 0.8 and 4 inches of travel and, more desirably, between about 1 and 3 inches of suspension travel in the long travel mode. The range of values set forth above pertains to the relative movement between the two portions of the suspension unit  200  and the actual travel of the suspended bicycle wheel may vary from the travel of the suspension unit  200 .  
         [0067]    As described earlier, the differences between curve “A” and curve “B” result from the differences in initial chamber volume available during compression of the suspension unit  200 . With the travel adjust lever  252  set as in FIG. 13, the total volume of both the first partial volume  227  and the second partial volume  229  are available. With the travel adjust lever  252  set as in FIG. 14, only the volume of first partial volume  227  is available.  
         [0068]    These following example calculations will serve to clarify these concepts.  
         [0069]    These calculations are based on the well-known Ideal Gas Law for isothermal processes, which is a good first approximation for illustrating the basic principles of the preferred embodiments. This law states that for an enclosed variable volume the internal pressure will vary with volume according to the equation: 
         ( P 1)*( V 1)=( P 2)*( V 2) 
         [0070]    where:  
         [0071]    P1=initial gas (air) pressure  
         [0072]    P2=second gas (air) pressure  
         [0073]    V1=initial volume  
         [0074]    V2=second volume  
         [0075]    Here is a simple example of this relationship. Assuming the initial conditions of a sealed, variable chamber are 10 cubic inches of air at 100 psi, if the volume is then reduced to 5 cubic inches the pressure will increase to 200 psi. Considered from another point of view, initial volume divided by final volume equals “compression ratio”. In this example the compression ratio is 10 divided by 5, or a compression ratio of 2. Final pressure can be calculated by multiplying initial pressure times compression ratio: 100 psi times 2=200 psi.  
         [0076]    In the example of FIGS. 13, 14, and  15 , the initial first partial volume  227  of suspension unit  200  is 3.08 cubic inches, and the second partial volume  229  is 1.15 cubic inches. Thus, their combined volume is 4.23 cubic inches, and the volume of first partial volume  227  alone is just 3.08 cubic inches. For the configuration of this example, volume displaced by the seal head  194  per inch of stroke is 1.65 cubic inches per inch. The following sample calculations are made using these values:  
         [0077]    For the configuration of FIG. 13, a compression ratio of 3.16 is reached at 1.75 inches of travel:  
         [0078]    initial total chamber volume=4.23 cubic inches  
         [0079]    reduced volume at 1.75 inches travel=1.75×1.65=2.89 cubic inches  
         [0080]    chamber volume at 1.75 inches travel=4.23−2.89=1.34 cubic inches  
         [0081]    Thus:  
         [0082]    compression ratio at 1.75 inches travel=(4.23)/(1.)=3.16  
         [0083]    For the configuration of FIG. 14, an almost identical compression ratio of 3.14 is reached at 1.27 inches of travel:  
         [0084]    initial total chamber volume=3.08 cubic inches  
         [0085]    reduced volume at 1.27 inches travel=1.27×1.65=2.10 cubic inches  
         [0086]    chamber volume at 1.27 inches travel=3.08−2.10=0.98 cubic inches  
         [0087]    Thus:  
         [0088]    compression ratio at 1.27 inches travel=(3.08)/(0.98)=3.14  
         [0089]    For the configuration used in this example for suspension unit  200 , and assuming an initial pressure of 150 psi, these compression ratios translate to an air spring force in both cases of about 750 pounds. However, the actual air spring force may vary depending on the specific application. Preferably, as described above, in the context of mountain bike suspension assemblies, the spring force is less than approximately 3000 pounds at a substantially fully compressed position of the air spring.  
         [0090]    This example, of course, is by way of illustration only, and a wide spectrum of desired relationships between compression ratio and travel, and of the ratio of travel achieved in the short travel mode with that achieved in the long travel mode, can be attained with the illustrated embodiments by designing a particular variable air spring with appropriate dimensional relationships. Preferably, the percentage of travel achieved in the short travel mode with that achieved in the long travel mode is between about 40 and 90 percent. Desirably, the percentage of travel achieved in the short travel mode with that achieved in the long travel mode is between about 50 and 85 percent and, more desirably, between about 60 and 80 percent. Such a change in travel provides desirable suspension performance in both the short travel and long travel modes for at least a significant portion of typical suspension arrangements presently incorporated on mountain bikes.  
         [0091]    [0091]FIG. 16 shows an alternate preferred embodiment. As discussed previously, this embodiment is somewhat simplified and less costly than the embodiment of FIG. 6. The embodiment of FIG. 16 is possible for suspension units which are generally similar to that of FIG. 6, but provided that no thru-shaft damping adjustment feature, such as shown in FIG. 6, is required. As shown in FIG. 16, when a thru-shaft damping adjustment feature is not required, then the upper end of the shaft  393  becomes available for incorporation of the travel adjust feature. Thus, the travel adjust valve in the embodiment illustrated in FIG. 16 generally extends along a central axis A of the shock shaft  393 , which allows a simpler and more cost-effective structure.  
         [0092]    In this embodiment, the travel adjust assembly  350  uses the same travel adjust lever  252  as utilized previously. The actuating cam shaft  354  is similar to the previous actuating cam shaft  254 , but is somewhat longer. The upper eyelet housing  316  is similar to the previous upper eyelet housing  216 , but is somewhat simpler and less costly to produce due to elimination of the previously-required off-center upper passage port coupler  217  which was depicted in FIG. 9. The air cylinder partition  372  is similar to the previous air cylinder partition  272 , but it also is somewhat simpler and less costly to produce due in this case to elimination of the previously-required off-center lower passage port boss  273  which was depicted in FIG. 10. The lower passage port  374  and the upper passage port  319 , as shown, both consist of a cross-holes drilled in the shaft  393 . The upper passage port  319  further consists of drilled or milled passageways in the lower portion of the upper eyelet housing  316  which communicate with the drilled passageways in the shaft  393 .  
         [0093]    The other elements of the travel adjust assembly  350  as shown in FIG. 16 are neither numbered nor described here since they are essentially identical to the elements numbered and described in the embodiment of FIG. 6.  
         [0094]    The present invention is not limited to the above embodiments and various changes may be made within the technical scope of the invention as understood by a person skilled in the art without departing from the spirit and scope thereof.