Patent Publication Number: US-11649875-B2

Title: Fluid damper for a bicycle component

Description:
RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of the filing date of U.S. Utility application Ser. No. 15/702,161, filed Sep. 12, 2017, which claims the benefit of the filing date of U.S. Utility application Ser. No. 15/470,357, filed Mar. 27, 2017, now U.S. Pat. No. 10,435,111, issued Oct. 8, 2019, the entirety of which is incorporated by reference herein and relied upon. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure is generally directed to bicycle chain tensioner, and more particularly to a fluid damper for a bicycle rear derailleur. 
     DESCRIPTION OF RELATED ART 
     Bicycle rear derailleurs are well known in the art as a part of a drivetrain of a bicycle. The typical drivetrain also includes a crank assembly that is coupled to one or more sprockets. The crank assembly is operable to drive a chain that is routed or wrapped around one of the sprockets. The chain is also routed to the rear wheel of the bicycle. 
     Rear derailleurs are provided as a part of the drivetrain to perform two basic functions. The primary function of the rear derailleur is to selectively shift a bicycle chain of the drivetrain among a series of different diameter cogs that are attached to the rear wheel. Shifting of the bicycle chain from one cog to another at the rear wheel is done in order to change the gear ratio of the drivetrain. The secondary function of the rear derailleur is to apply a tension to the chain to take-up slack, as well as to maintain the desired tension, in the chain on the non-drive-side of the drivetrain. 
     The rear derailleur accomplishes the secondary tensioning function by employing a chain tensioning mechanism known as a cage assembly, or, more commonly, a cage. The cage typically has one or two rotatable cogs or pulleys and the chain is routed or wrapped around the pulleys. The cage is connected to the main body of the rear derailleur in a manner that allows the cage to pivot relative to the main body. The cage is also biased to pivot or rotate in a direction that tensions or applies a tensioning force to the chain. 
     The cage may also have a first guide member and a second guide member. The first guide member is commonly configured to connect the cage assembly to other components of the rear derailleur. The second guide member is commonly configured so that the one or two rotatable pulleys are sandwiched between the first and second guide members. In this way, the chain may pass between the first and second guide members of the cage while engaging teeth of the pulleys. 
     When a bicycle travels over smooth terrain, the standard rear derailleur and cage are often sufficient to maintain enough tension in the chain so that the chain does not fall off the sprockets or cogs. However, when a bicycle travels over rough terrain, the forces transmitted to the rear derailleur can cause the cage to undesirably rotate in the chain slackening direction against the biasing force applied to the cage. This creates a slack condition in the chain. A slack chain can lead to the chain slapping against the frame of the bicycle. A slack chain can also lead to the chain falling off the sprockets or cogs. 
     A solution to this problem is to incorporate a damping system into the chain tensioning part of the derailleur. A damping system is designed to resist cage rotation, particularly in the chain slackening direction. A one-way damping system is configured to resist cage rotation in the chain slackening direction while still allowing free cage rotation in the chain tensioning direction. The typically one-way damping systems work by using a frictional element to provide a damping force in the chain slackening direction of cage rotation. Some of these types of damping systems employ a one-way roller clutch to prevent the frictional element from engaging in the chain tensioning direction. 
     A problem with these friction type damping systems is that the friction created in the chain slackening direction also makes it more difficult to shift the rear derailleur from a smaller cog to a larger cog at the rear wheel. This difficulty arises because shifting from a smaller cog to a larger cog requires that the cage rotate against the frictional forces of the damper or damping element of the damping system. In the case of a cable actuated rear derailleur, this problem results in a ride experiencing a high or higher shift effort required to change gears. In the case of an electrically actuated rear derailleur, this problem can result in shortening the battery life of the rear derailleur. This is because the actuator or motor of the rear derailleur works harder to overcome the frictional forces of the damping system. 
     Another problem with friction type damping systems is that the system parts are relatively heavy, which runs counter to a common performance goal of reducing bicycle weight. Still further, friction type damping systems may be rather complicated in construction, requiring multiple parts and numerous manufacturing steps. One result of the complicated nature of friction type damping systems is that the parts are relatively expensive, which increases the cost of the rear derailleurs. Another problem with friction type damping systems is that the friction force is difficult to control resulting in inconsistency across multiple rear derailleur components. Because the friction force is difficult to control, and because of the complicated nature of friction type damping systems, the systems are also difficult to properly assemble and the friction force is difficult to properly establish at the factory. 
     SUMMARY 
     In one example, according to the teachings of the present disclosure, a bicycle rear derailleur has a base member mountable to a bicycle frame, a movable member movably coupled to the base member, a chain guide assembly rotatably connected to the movable member for rotation about a rotational axis, a guide wheel rotatable about the rotational axis, a biasing element configured and arranged to bias the chain guide assembly for rotation in a first rotational direction about the rotational axis with respect to the movable member and a fluid damper having a fluid cavity containing a volume of fluid. In this example, the fluid damper is disposed at a connection of the chain guide assembly and the movable member and has a shaft rotatable about the rotational axis and configured to apply a damping force to the chain guide assembly when the chain guide assembly rotates in a second rotational direction opposite to the first rotational direction. 
     In another example, a bicycle rear derailleur has a base member movably coupled to the base member, the movable member having a fluid cavity configured to contain a volume of fluid. In this example, the bicycle rear derailleur also has a closure mechanism configured to create a seal along a wall of the fluid cavity, a shaft configured to create a seal with an opening of the closure mechanism, a vane disposed on the shaft and configured to interact with the volume of fluid, a bias device configured to bias the shaft in a first rotational direction, a first guide member rotationally fixed to the shaft disposed between the movable member and a freely rotatable guide wheel, a second guide member rotationally fixed to the first guide member such that the guide wheel is disposed between the first and second guide member, and a guide fixing member configured to affix the second guide member to the shaft, the guide fixing member contacting both the second guide member and the shaft. 
     In yet another example, a fluid damper for a bicycle chain tensioner has a housing couplable to a first portion of a bicycle, the housing defining a fluid cavity therein including a damping chamber and a return chamber and the fluid cavity containing a volume of fluid. In this example, the fluid damper also has a rotational shaft having a first axial end and a second axial end, the rotational shaft supported at the first axial end for rotation about a first rotation axis, and a vane radially extending from the rotational shaft and movable within the fluid cavity about the first rotation axis to define a vane sweep region and dividing the fluid cavity into the damping chamber and the return chamber on opposite sides of the vane. In this example, the fluid damper also has a guide wheel configured to rotatably interact with a bicycle chain, the guide wheel having a second rotation axis, wherein the second rotation axis is within the vane sweep region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Objects, features, and advantages of the present invention will become apparent upon reading the following description in conjunction with the drawing figures, in which: 
         FIG.  1    is a side view of a bicycle, which may be constructed to utilize a fluid damper on the rear derailleur. 
         FIG.  2    is a close-up side view of one example of an electronic rear derailleur mounted to a bicycle, the electronic rear derailleur including a fluid damper in accordance with the teachings of the present disclosure. 
         FIG.  3    is a close-up side view of one example of a manually actuated rear derailleur mounted to a bicycle, the manually actuated rear derailleur including a fluid damper in accordance with the teachings of the present disclosure. 
         FIG.  4    is a close-up side view of one example of an electronic rear derailleur mounted to a bicycle, the electronic rear derailleur including a fluid damper in accordance with the teachings of the present disclosure. 
         FIG.  5    is the manually actuated rear derailleur of  FIG.  3   , but not mounted to a bicycle. 
         FIG.  6    is an exploded perspective view of the fluid damper of  FIGS.  2 - 5   . 
         FIG.  7    is an assembled perspective view of the fluid damper of  FIG.  6   . 
         FIG.  8    is a cross-section view taken along line  8 - 8  and through the rotation axis of the manually actuated rear derailleur of  FIG.  5   . 
         FIG.  9    is a perspective view of a housing of the fluid damper shown in  FIG.  7    and depicting aspects of a compensation device of the fluid damper. 
         FIG.  10 A  is a cross-section view taken along line  10 A- 10 A of the fluid damper of  FIG.  8    and depicting portions of the fluid damper and a check valve in one position during use. 
         FIG.  10 B  is a cross-section view taken along line  10 B- 10 B of a compensation device of the fluid damper of  FIG.  10 A  and in a normal state. 
         FIG.  10 C  is the compensation device depicted in  FIG.  10 B  but in a compressed state. 
         FIG.  11    is a perspective view of the assembled fluid damper of  FIG.  7    and in an exploded view of portions of the rear derailleur of  FIGS.  3  and  5   . 
         FIG.  12    is a perspective view of portions of the rear derailleur of  FIG.  4   . 
         FIG.  13    is a perspective view of the assembled fluid damper of  FIG.  12    and an exploded view of portions of the rear derailleur of  FIG.  4   . 
         FIG.  14    is a bisected cross-sectional view of the assembled rear derailleur of  FIG.  4   . 
         FIGS.  15  and  16    are cross-sections, like the cross-section of  FIG.  10 A , but depicting the portions of the fluid damper and the check valve in two other positions during use. 
         FIG.  17    is a cross-section view, which is like  FIG.  8   , but that shows an alternate example of a fluid damper in accordance with the teachings of the present disclosure. 
         FIGS.  18 - 22    are cross-section views, which are like  FIG.  10 A , but that show further alternate examples of fluid dampers in accordance with the teachings of the present disclosure. 
         FIG.  23    is a cross-section view, which is like  FIG.  8   , but that shows yet another alternate example of a fluid damper in accordance with the teachings of the present disclosure. 
         FIG.  24    is a perspective view of a housing of the fluid damper shown in  FIG.  23   . 
         FIGS.  25 - 27    are cross-section views, which are like  FIG.  8   , but that show still further alternate examples fluid dampers in accordance with the teachings of the present disclosure. 
         FIGS.  28  and  29 A  are cross-sections, like the cross-section of  FIG.  10 A , but depicting a representation of a vane boundary, the movement of which defines a vane sweep region. 
         FIG.  29 B  is a cross-section view, which is like  FIG.  8   , of the embodiment of the fluid damper as illustrated in  FIG.  29 A . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The present disclosure provides examples of fluid dampers and bicycle rear derailleurs that solve or improve upon one or more of the above-noted and/or other problems and disadvantages with prior known dampers and derailleurs. The disclosed fluid dampers can replace the traditional friction type, i.e., friction and roller clutch dampers with a fluid damper and one-way valve (i.e. check valve) arrangement. A significant advantage of the disclosed fluid dampers is that the damping forces will be low while shifting, which occurs at low rotational speeds of the cage, and that the damping forces will be high only when the derailleur is subjected to an impact force (for example while riding over rough terrain), which causes high rotational speeds of the cage. Another advantage of the disclosed fluid dampers is that the fluid damper is lighter than a traditional friction-based or friction type damper. Yet another advantage of the disclosed fluid dampers is that the shape and size of the fluid damper may be such that additional clearance is created between the rear derailleur and the heel of a rider&#39;s foot, which can reducing the likelihood of the rider&#39;s foot colliding with the derailleur. 
     Examples of one-way fluid dampers and bicycle rear derailleurs that employ such fluid dampers are disclosed and described herein. The disclosed fluid dampers resist cage motion in the chain slackening direction of cage rotation. The disclosed fluid dampers also allow for free rotation of the cage in the chain tensioning direction. Fluid damping torque in the disclosed fluid dampers is proportional to cage rotation velocity. As a result, damping torque applied by the disclosed fluid dampers is low when a rider is shifting gears, and is high only when needed (i.e., during an impact or vibration situation). The disclosed fluid dampers thus can reduce shift effort of a manually operated or actuated rear derailleur and can extend battery life of an electronic rear derailleur. This solves or improves upon the above-noted problem is reduced battery life of higher shifting forces when employing a friction type damper. 
     The disclosed fluid dampers can reduce the weight of a rear derailleur equipped with such a damper. In one example, the disclosed fluid dampers may reduce the weight of a rear derailleur by about 18 grams in comparison to a mechanical and/or roller clutch based damped equivalent, i.e., a derailleur with a friction type damper. The disclosed fluid dampers can also be relatively small and may be cylindrical in shape, which, when mounted on a rear derailleur, reduces the overall size of the rear derailleur or cage. A damper of smaller size allows for or produces more heel clearance for a rider during use when compared to existing derailleurs with friction type dampers. The foregoing solves or improves upon the problems of excessive weight and heel-to-derailleur contact when employing a friction type damper. 
     These and other objects, features, and advantages of the disclosed fluid dampers will become apparent to those having ordinary skill in the art upon reading this disclosure. Throughout the drawing figures, where like reference numbers are used, the like reference numbers represent the same or substantially similar parts among the various disclosed examples. Also, specific examples are disclosed and described herein that utilize specific combinations of the disclosed aspects, features, and components of the disclosure. However, it is possible that each disclosed aspect, feature, and/or component of the disclosure may, in other examples not disclosed or described herein, be used independent of or in different combinations with other of the aspects, features, and components of the disclosure. 
     Turning now to the drawings,  FIG.  1    generally illustrates a bicycle  50 , which employs a rear derailleur and a fluid damper constructed in accordance with the teachings of the present disclosure. The bicycle  50  includes a frame  52 , a front wheel  54  and a rear wheel  56  each rotatably attached to the frame, and a drivetrain  58 . A front brake  60  is provided for braking the front wheel  54  and a rear brake  62  is provided for braking the rear wheel  56 . The bicycle  50  also generally has a seat  64  near a rear end of the frame  52  and carried on an end of a seat tube  66  connected to the frame. The bicycle also has handlebars  68  near a forward end of the frame  52 . A brake lever  70  is carried on the handlebars  68  for actuating one of the front brake  60  or rear brake  62 , or both. If one, a second brake lever (not shown) may also be provided to actuate the other brake. A front and/or forward riding direction or orientation of the bicycle  50  is indicated by the direction of the arrow A in  FIG.  1   . As such, a forward direction of movement for the bicycle  50  is indicated by the direction of arrow A. 
     While the illustrated bicycle  50  depicted in  FIG.  1    is a road bike having drop-style handlebars  68 , the present disclosure may be applicable to bicycles of any type, including mountain bikes with full or partial suspension, as well as bicycles with mechanically controlled (e.g. cable, hydraulic, pneumatic) and non-mechanical controlled (e.g. wired, wireless) drive systems. 
     The drivetrain  58  has a chain C and a front sprocket assembly  72 , which is coaxially mounted with a crank assembly  74  having pedals  76 . The drivetrain  58  also includes a rear sprocket assembly  78  coaxially mounted with the rear wheel  56  and a rear gear change mechanism, such as a rear derailleur  80 . 
     As is illustrated in  FIG.  1   , the front sprocket assembly  72  may include one or more coaxially mounted chain rings, gears, or sprockets. In this example, the front sprocket assembly  72  has two such sprockets, F 1  and F 2 , each having teeth  82  around its respective circumference. As shown in  FIGS.  1  and  2   , the rear sprocket assembly  78  may include a plurality of, such as eleven, coaxially mounted gears, cogs, or sprockets G 1 -G 11 . Each sprocket G 1 -G 11  also has teeth  84  arranged around its respective circumference. The number of teeth  82  on the smaller diameter front sprocket F 2  is preferably less than the number of teeth on the larger diameter sprocket F 1 . The numbers of teeth  84  on the rear sprockets G 1 -G 11  typically gradually decrease from the largest diameter rear sprocket G 1  to the smallest diameter sprocket G 11 . Though not described in any detail herein, a front gear changer  85  may be operated to move from a first operating position to a second operating position to move the chain C between the front sprockets F 1  and F 2 . Likewise, the rear derailleur  80  may be operable to move between eleven different operating positions to switch the chain C to a selected one of the rear sprockets G 1 -G 11 . In another embodiment, the bicycle may include twelve or thirteen sprockets, and the rear derailleur  80  may be operable to move between any of the twelve or thirteen operating positions, respectively, to switch the chain C to a selected one of the rear sprockets. 
     Referring to  FIG.  2   , the rear derailleur  80  is depicted in one example as a wireless, electrically actuated rear derailleur mounted to the frame  52  of the bicycle  50 . The electric rear derailleur  80  has a base  86  that is mounted to the bicycle frame  52 . A linkage  88  has two links L (one is hidden behind the other in  FIG.  2   ) that are pivotally connected to the base  86 . A fluid damper  90 , constructed in accordance with the teachings of the present disclosure, is connected to the linkage  88 . A chain guide  92  has a cage  93  with a proximal end  91  that is pivotally connected to a part of the fluid damper  90 , as described further below. The cage  93  can rotate or pivot about a cage rotation axis R in a damping direction D and a chain tensioning direction T. 
     The cage  93  may also include a first guide member  99  and a second guide member  97 . For instance, the first guide member  99  may be pivotally connected to a part of the fluid damper  90  and the second guide member  97  may be fixed relative to the first guide member  99 . The second guide member  97  may also be removable independently from the first guide member  99 , for instance to facilitate servicing. 
     A motor module  94  is carried on the electric rear derailleur  80  and has a battery  96 , which supplies power to the motor module. In one example, the motor module  94  is located in the base  86 . However, the motor module  94  can instead be located elsewhere, such as in one of the links L of the linkage  88  or in the fluid damper  90 . The motor module  94  may include, though not shown herein, a gear mechanism or transmission. As is known in the art, the motor module  94  and gear mechanism may be coupled with the linkage  88  to laterally move the cage  93  and thus switch the chain C among the rear sprockets G 1 -G 11  on the rear sprocket assembly  78 . 
     The cage  93  also has a distal end  98  that carries a tensioner cog or wheel  100 . The wheel  100  also has teeth  102  around its circumference. The cage  93  is biased in the chain tensioning direction T to maintain tension in the chain C. The chain guide  92  may also include a second cog or wheel, such as a guide wheel  104  disposed nearer the proximal end of the cage  93  and the fluid damper  90 . In operation, the chain C is routed around one of the rear sprockets G 1 -G 11 . An upper segment of the chain extends forward to the front sprocket assembly  72  and is routed around one of the front sprockets F 1  or F 2 . A lower segment of the chain C returns from the front sprocket assembly  72  to the tensioner wheel  100  and is then routed forward to the guide wheel  104 . The guide wheel  104  directs the chain C to the rear sprockets G 1 -G 11 . Lateral movement of the cage  93 , tensioner wheel  100 , and guide wheel  104  may determine the lateral position of the chain C for alignment with a selected one of the rear sprockets G 1 -G 11 . 
     One or both of the guide wheel  104  and the tensioner wheel  100  may be located between the first guide member  99  and the second guide member  97 . The guide wheel  104  may be connected to at least one of the first guide member  99  and the second guide member  97  such that the guide wheel  104  is freely rotatable. The tensioner wheel  100  may be similarly connected such that it is also freely rotatable. 
     Though not shown herein, a control unit may be mounted to the handlebars  68  for actuating the motor module  94  and operating the rear derailleur  80  for executing gear changes and gear selection. The control unit, however, may be located anywhere on the bicycle  50  or, alternatively, may be distributed among various components of the bicycle, with routing of a communication link to accommodate necessary signal and power paths. The control unit may also be located other than on the bicycle  50 , such as, for example, on a rider&#39;s wrist or in a jersey pocket. The communication link may include wires, may be wireless, or may be a combination thereof. In one example, the control unit may be integrated with the rear derailleur  80  to communicate control commands between components. The control unit may include a processor, a memory, and one or more communication interfaces. 
     The battery  96  may instead be an alternate power supply or power source and may operate other electric components of the bicycle  50  within a linked system. The battery  96  or other power supply may also be located in other positions, such as attached to the frame  52 . Further, multiple power supplies may be provided, which may collectively or individually power the electric components of the system, including the rear derailleur  80 , such as a drive motor for an embodiment involving an electrically powered bicycle. In this example, however, the battery  96  is configured to be attached directly to the rear derailleur  80 , and to provide power only to the components of the derailleur. 
     Referring to  FIG.  3   , a cable actuated or manual rear derailleur  80  is shown mounted to the frame  52  of the bicycle  50 .  FIG.  5    shows the manual rear derailleur  80  but not attached to the bicycle. The manual rear derailleur  80  is substantially the same as the electric rear derailleur and operates in a similar manner, as described above, except for the difference noted below. Thus, the manual rear derailleur  80  includes the base  86  mounted to the bicycle frame  52 . The linkage  88  and its two links L is pivotally connected to the base  86 . The fluid damper  90  is connected to the links L of the linkage  88 . The cage  93  is pivotally connected to the fluid damper  90  and is rotatable about the cage rotation axis R in a damping direction D and a chain tensioning direction T. In this example, an actuator cable  110  is connected to a gear shifter (not shown) that is carried on the handlebars  68  or another part of the bicycle  50 . The actuator cable  110  is routed around a cable guide wheel  112  carried by the base  86  and is coupled to the linkage  88 . A rider operates the gear shifter to move the linkage laterally to shift the chain C among the rear sprockets G 1 -G 11 , as is known in the art. 
     Referring to  FIG.  4   , a manual or electric configuration of the rear derailleur  80  may be otherwise configured. The rear derailleur  80  is shown as one of an electric configuration. In the embodiment shown in  FIG.  4    the guide wheel  104  is rotatable about the cage rotation axis R. In such a configuration, rotation about the cage rotation axis R will not change the relative position of the guide wheel  104  and the teeth  84  of the rear sprockets G 1 -G 11 . For instance, a change between front sprockets F 1  and F 2  resulting in rotational movement of the tensioner wheel  100  about the cage rotation axis R will not change the relative positions of the guide wheel  104  and the rear sprockets G 1 -G 11 . The portion of the chain C that the guide wheel  104  directs to the rear sprockets G 1 -G 11  may thus be tunable to have the same position relative to one of the rear sprockets G 1 -G 11  regardless of which of the front sprockets F 1  or F 2  the chain C engages. 
     The fluid damper  90 , hereinafter identified as the “damper  90 ” to simplify the description, is now described referring first to  FIGS.  6 - 8   . Though discussed herein as a part of a rear derailleur of a bicycle, the fluid damper  90  may be incorporated onto a chain tensioner of a bicycle, where the chain tensioner is not a part of a front or rear derailleur. In accordance with the teachings of the present disclosure, the damper  90  generally has a housing  120  that defines a fluid cavity  122  therein. An access opening  124  in the housing provides access into the fluid cavity  122  (see  FIG.  6   ). A rotational shaft or shaft  126  has a first axial end  128  received in the fluid cavity  122  and a second axial end  130 , a portion of which is exposed to the outside of the housing  120  with the damper  90  assembled (see  FIG.  7   ). The damper  90  also has a one-way valve or check valve  132  disposed within the fluid cavity  122  when the damper is assembled (see  FIGS.  6  and  8   ). As discussed in more detail below, the check valve  132 , and other check valve examples described herein, are movable between an open position and a closed position. The damper further has a ring-shaped bearing  134  positioned within the fluid cavity  122  in the assembled damper (see  FIGS.  6  and  8   ). The damper  90  further has a closure or cap  136  that is configured to close off the access opening  124  into the cavity while still exposing a portion of the second axial end  130  of the shaft. 
     Referring to  FIGS.  8  and  9   , the fluid cavity  122  has a closed end wall  138  defined by the housing  120  and disposed opposite the access opening  124 . A blind bore  140  is formed into the end wall  138  and is positioned to define the cage rotation axis R. The fluid cavity  122  also has a side wall  142  extending between the end wall  138  and the access opening  124 . The side wall  142  surrounds and defines the boundaries of the fluid cavity  122  in combination with the end wall  138  and the cap  136 , when assembled. Details of the fluid cavity  122  are described in further detail below. 
     Referring to  FIGS.  6  and  8   , the shaft  126  has a main section  144  that is cylindrical and has a first diameter. The first axial end  128  is also cylindrical and protrudes from one end of the main section  144 . The first axial end  128  has a second diameter that is smaller than the first diameter of the main section  144 . The first axial end  128  of the shaft  126  is received in the blind bore  140  such that the shaft can rotate about the cage rotation axis R. The shaft  126  also has a bearing section  146  disposed at an end of the main section  144  opposite the first axial end  128 . The bearing section  146  is also cylindrical and defines a circumferential bearing surface  148 . The bearing section  146  has a third diameter that is larger than the first diameter of the main section  144  of the shaft  126 . The second axial end  130  protrudes from the bearing section  146  of the shaft  126 . The second axial end  130  has a double-D shape in cross section. The second axial end  130  thus has two curved portions  150  disposed opposite one another and two flat portions  152  opposite one another around the circumference of the second axial end. 
     Referring to  FIGS.  6  and  8   , the bearing  134  is circular and ring-shaped, as noted above. The bearing  134  has a static seal, such as an O-ring  154 , that is seated in a groove  156  in an outer circumferential surface  158  of the bearing. The bearing  134  also has a dynamic seal, such as an O-ring  160 , that is seated in a recess  162  in an inner circumferential surface  164  of the bearing. The inner circumferential surface  164  defines a hole through the bearing. 
     As shown in  FIGS.  6  and  9   , the side wall  142  of the fluid cavity  122  has several different segments. A first or outermost segment is circular and is disposed adjacent the outside of the housing  120 . This outermost segment defines the access opening  124  into the fluid cavity. Female mechanical threads  166  are provided around the circumference of the access opening  124 . A second or intermediate segment of the side wall  142  is disposed internally (relative to the fluid cavity  122 ) and directly adjacent the access opening  124 . This intermediate segment is also circular and defines a sealing surface  168 . The diameter of the sealing surface  168  is slightly smaller than the diameter of the threaded access opening  124 , creating an outward facing (relative to the fluid cavity  122 ) step or shoulder, hereinafter a bearing stop  170 . When assembled, the outer static O-ring  154  seats and seals against a portion of the side wall  142 , i.e., the sealing surface  168  within the fluid cavity  122 . An edge of the inner surface  172  of the bearing  134  is also borne against the bearing stop  170 , as depicted in  FIG.  8   . Also, the shaft  126  is received through the central hole defined by the inner surface  164  of the bearing  134  when the damper  90  is assembled. As shown in  FIG.  8   , the bearing surface  148  on the bearing section  146  of the shaft  126  is rotatably received in the hole through the bearing. The dynamic O-ring  160  on the inner surface  164  of the bearing  134  seats and seals between the bearing surface  148  of the shaft  126 . 
     Referring to  FIGS.  6 ,  8 , and  10   , an isolator or divider feature is movably disposed within the fluid cavity to divide the cavity into two variable volume compartments on either side of the isolator or divider feature. The isolator or divider feature may move linearly, rotationally, or in some other manner, if desired. In this example, the isolator or divider feature is a vane  180  that projects radially outward from the main section  144  of the shaft  126 . As the shaft  126  rotates, the vane  180  will move rotationally therewith, as described below. In this example, the vane  180  is a generally rectangular wall that is integrally formed as a part of the shaft  126 . The vane  180  essentially divides the fluid cavity  122  within the housing  120  into a fluid return chamber  182  and a damping chamber  184 . When the fluid damper  90  is assembled, the return chamber  182  and damping chamber  184  both are filled to contain a fluid, such as a hydraulic liquid, as described in greater detail below. The fluid in one example may be a viscous damping fluid, such as a silicone oil. The fluid may have a viscosity of between about 1,000 and about 100,000 centistokes. 
     Referring to  FIG.  28   , movement of the vane  180 , represented as a vane boundary  540 , defines a vane sweep region  532 . The vane sweep region  532  is a space through which the vane rotates. For example, the movement of the vane  180  and associated vane boundary  540  has a first position  534  to divide the fluid cavity  122  to have a minimally-sized damping chamber  184  and a second position  536  to divide the fluid cavity  122  to have a maximally-sized damping chamber  184 . Movement between the first and second positions  534 ,  536  defines the vane sweep region  532 . 
     The relative sizes of the damping chamber  184  and the return chamber  182  may be defined as proportional to a damping angle  584  and a return angle  582 . The damping angle  584  is defined as an angle between the vane boundary  540  and the second extreme position  536 . The return angle  582  is defined as an angle between the vane boundary  540  and the first extreme position  534 . The size of the damping chamber  182  may thus be directly proportional to a magnitude of the damping angle  582  and the size of the return chamber  184  may thus be directly proportional to a magnitude of the return angle  582 . 
     As the vane boundary  540  moves from the first extreme position  534  to the second extreme position  536  in the chain tensioning direction T, the damping angle  584  and the damping chamber  184  increase in magnitude and size while the return angle  582  and the return chamber  182  decrease in magnitude and size. As the vane boundary  540  moves from the second extreme position  536  to the first extreme position in the damping direction D, the return angle  582  and the return chamber  182  increase in magnitude and size while the damping angle  584  and the damping chamber  184  decrease in magnitude and size. 
     Referring now to  FIGS.  29 A and  29 B , regardless of the position of the vane boundary  540 , a combination of the damping angle  584  and the return angle  582  will yields the vane sweep angle. The addition of the damping angle  584  and the return angle  582  may yield the vane sweep angle  586 . The vane sweep angle  586  also may be defined as an angle defining the range of motion of the vane boundary  540  between the first extreme position  534  and the second extreme position  536 . The range of motion of the vane boundary  540  is shown in a plane P of  FIG.  29 B  bisecting the vane  180  perpendicular to the rotation axis R. The vane sweep region  532  in the plane P may be described as a vane sweep area  538 . 
     As shown in  FIG.  29 B , the vane  180 , and associated vane boundary  540  as illustrated in  FIG.  28 B , extends axially beyond the plane P in at least one direction. Thus the vane sweep region  532  is a three dimensional region having a volume. In the illustrated embodiment, the vane sweep region  532  is defined as a cylindrical sector with an area of the vane sweep area  538  and a height of a vane height H correlating to the length of the vane along the rotation axis R. The rotation axis R will be the axis of the cylindrical sector defining the vane sweep region  532 . In such a way, the rotation axis R will intersect the vane sweep region  532  and the vane sweep area  538 . In another embodiment, the vane sweep region may not be a cylindrical region as formed by a rectangular vane. In such an embodiment, for example an embodiment including a triangular or other shaped vane, a vane sweep area may be defined as the largest area of the vane sweep region contained in a plane perpendicular to the rotation axis. 
     Referring again to  FIGS.  6 ,  8 , and  10   , the check valve  132  in this example has a valve body or poppet  186  including a head  188  disposed at one end of a valve stem  190 . The valve stem  190  is slidably received in a stem hole  192  in the vane  180 . The head  188  is sized and shaped to selectively block a first, free, main, or primary flow path within the fluid cavity  122 . In this example, a series of flow holes  194  that pass through the vane  180  define the first flow path. In this example, the flow holes  194  are spaced apart from one another around the stem hole  192 . The head  188  is thus a circular shape so that the head may selectively cover and block each of the flow holes  194 . A valve spring  195  is received over the valve stem  190  on the opposite side of the vane  180  from the head  188 . Likewise, a spring support member or spring stop  196  may also be slidably received on the valve stem  190 , after the valve spring  195  is installed on the same side of the vane  180 . A spring retainer, such as a snap ring  198 , is attached to the valve stem  190  to retain the valve spring  195  and the spring stop  196  on the stem. In this example, the snap ring  198  has a C shape and is snapped onto the free end of the valve stem  190  and seats in a groove  200  thereon. The valve spring  195  is thus captured between the vane  180  and the snap ring  198  and/or the spring stop  196  to retain the valve spring on the poppet  186 . The valve spring  195  is configured and arranged to bias the poppet  186  toward a closed position, as depicted in  FIG.  10 A . In the closed position, the head  188  is biased against the vane  180  by the bias force of the valve spring  195  to close of the first flow path, i.e., each of the flow holes  194  in this example. In the open position, as noted above and as described further below, the head  188  is forced by fluid pressure away from the vane  180  against the bias force of the valve spring  195  to open the first flow path, i.e., each of the flow holes  194 . 
     Referring to  FIGS.  6 - 9   , the cap  136  in this example is disc shaped and has an outer circumferential surface with male mechanical threads  202 . The male threads  202  of the cap  136  are configured to threadably engage the female threads  166  within the access opening  124  to the fluid cavity  122 . When installed, the cap  136  closes off the access opening  124  on the housing  120 . The installed cap  136  also firmly clamps the bearing  134  against the bearing stop  170 , as shown in  FIG.  8   . 
     Referring to  FIG.  12   , the cap  136  may be otherwise configured to engage with the access opening  124 . For example, the cap  136  may be press fit into the access opening  124 . The cap  136  may also be adhesively attached or bonded to the access opening  124 . 
     Referring to  FIGS.  8 ,  11 ,  12  and  13   , the rear derailleur  80  includes a tensioning spring  204  for tensioning the chain C of the drivetrain  58 . The tensioning spring  204  in this example is a torsion spring that is received in an annular recess or channel  206  on the housing  120 . In this example, the annular channel  206  is formed in the material of the housing  120  and surrounds the fluid cavity  122 . The tensioning spring  204  has a first prong or leg  229  that protrudes angularly from one end of the spring and toward a bottom surface of the annular channel  206 . The first prong or leg  229  engages a hole  208  (see  FIG.  9   ) in the bottom surface of the annular channel  206 . The first prong  229  and hole  208  retain the one end of the tensioning spring  204  in a fixed position within the annular channel  206  and relative to the housing  120 . The tensioner spring  204  is disposed radially outward of the fluid damper  90  on the housing  120 . 
     Referring to  FIGS.  8  and  11    and as noted above, the proximal end  91  of the cage  93  is coupled to the damper  90 . In this example, a disc-shaped torque carrier  210  has a double-D shaped central hole  212 . The torque carrier  210  resides in a recess  214  in the exterior side of the cap  136  and the hole  212  is received over and engages with the correspondingly double-D shaped second axial end  130  of the shaft  126 . Thus, the torque carrier  210  and the shaft  126  are rotationally fixed to each other. A female threaded bore  216  is formed into the second axial end  130  of the shaft. A chamfered hole  218  is formed through the proximal end  91  of the cage  93  and aligns with both the central hole  212  in the torque carrier  210  and the blind threaded bore. A male threaded bolt  220  is passed through the chamfered hole  218  and the central hole  212  and screws into the threaded bore  216 . The bolt  220  clamps and attaches the cage  93  to the damper  90  with the torque carrier  210  sandwiched between the cage  93  and a shoulder  222  that is formed on the free end of the bearing section  146  of the shaft  126  facing the second axial end  130 . 
     The torque carrier  210  has two threaded fastener blind holes  224  disposed on opposite sides of the central hole  212 . Likewise, the cage  93  has two through holes  226  disposed on opposite sides of the chamfered hole  218 . The through holes  226  are aligned with the blind holes  224  to properly orient the cage  93  relative to the damper  90 . Two screws  228  are used to further secure and fix the torque carrier  210  and the cage  93  to one another. This structure results in the cage  93 , the torque carrier  210 , and the shaft  126  all being fixed to one another to rotate as a unit or in concert with one another about the rotation axis R. 
     Alternatively, as in  FIG.  12   , the torque carrier  210  may be omitted. For instance, a torque member  512  may be attached to or integrally formed with the cage  93 . For instance, the torque member  512  may be comolded or overmolded with the first guide member  99  of the cage  93 . The torque member  512  may include an insert hole  512  with which the shaft  126  may interact. For instance, the torque member  512  may be received by and engage with the correspondingly double-D shaped second axial end  130  of the shaft  126 . Thus, the torque member  510  and the shaft  126  may be rotationally fixed to each other. 
     The torque member  512  may be secured to the shaft  126  with the use of an outer guide fixing member  520 . For instance, the outer guide fixing member  520  may have outer guide fixing member threads  522 , such as female outer guide fixing member threads  522  in  FIG.  12   . The threads  522  may be configured to engage with male threads of the second axial end  130  of the shaft  126 . The outer guide fixing member  520  may thus rotationally secure the cage  93  to the shaft  126  and axially secure the cap  136  to the access opening  124 , for example in the case of a press fit interface. Alternatively, threaded engagement of the cap  136  and the access opening  124  may close off the fluid cavity  122 . The outer guide fixing member  520  may secure the tensioner spring  204  between the first guide member  99  of the cage  93  and the housing  120  within the annular channel  206 . 
     The outer guide fixing member  520  may include a fixing feature  528 . For instance, the fixing feature  528  may be one or more notches with which an installation tool may engage. Because a fluid damper  90  may be designed to not require internal service or repair, the fixing feature  528  may be designed to facilitate installation but not removal. For instance, the fixing feature  528  may be a pair of notches each having an orthogonal configuration to facilitate torque transfer from an installation tool and an opposing obtuse configuration so that reverse rotation of the installation tool forces the installation tool axially out of engagement. 
       FIGS.  12 - 14    show an embodiment in which the guide wheel  104  rotates about a guide wheel axis G that intersects the fluid chamber within the vane sweep region  532  of the vane  180 . More specifically, the embodiment of  FIGS.  12 - 14    has an inner guide fixing member  524  fixing the guide wheel through interaction with the shaft  126 . The inner guide fixing member  524  may be a male threaded bolt for engagement with the female threaded bore  216  of the shaft  126 . As shown in  FIG.  14   , the inner guide fixing member  524  may fix the guide wheel  104  between the second guide member  97  and the first guide member  99 . The inner guide fixing member  524  may have threaded engagement with the threaded bore  216  about the cage rotation axis R. Additionally, the guide wheel  104  may rotate about the cage rotation axis R with this attachment. 
     The guide wheel  104  may include a central portion  105 . The central portion  105  of the guide wheel  104  may be configured to interact with at least one of the first and second guide members  99 ,  97 . For instance, the guide wheel  104  may be used to fix the second guide member  97  relative to the first guide member  99  through fixing of the central portion  105  between the first guide member  99  and the second guide member  97 . The central portion  105  may be rotationally fixed to one or both of the guide members  99 ,  97 . Other components of the guide wheel  104  may be rotationally attached to the central portion  105 . For instance, the guide wheel  104  may have an outer portion  103  rotatable about the central portion  105 . 
     The guide wheel  104  may have a guide wheel face  107 . The guide wheel face  107  may be located on the central portion  105 . The guide wheel face  107  may be configured to locate the guide wheel  104  and associated components to other components of the rear derailleur  80 . For instance, the guide wheel face  107  may abut a shaft face  127  on the second axial end  130  of the shaft  126  in order to locate the guide wheel  104  and the second guide member  97 . In such a way, the central portion  105  of the guide wheel  104  may be configured to transmit torque between the shaft  126  and the cage  93 . 
     The guide wheel axis G may be the same as the rotational axis R as shown, for example, in  FIG.  14   . Alternatively, the guide wheel axis G may be distinct from the rotational axis R, for example as illustrated in  FIGS.  29 A and  29 B . Certain objectives may be achieved by locating the guide wheel axis G at a specific radial distance relative to the rotational axis R. For instance, locating the guide wheel axis G relatively close to the rotational axis R may be used to limit changes in position of the rear derailleur  80  responsive to position of the chain C as determined by the front gear changer  85 . In such a configuration, rotation about the cage rotation axis R will change the relative position of the guide wheel  104  and the teeth  84  of the rear sprockets G 1 -G 11  relatively little as compared with a configuration having a guide wheel axis G at a longer distance from the rotational axis R. 
     The tensioning spring  204  also has a second prong or leg  230  that protrudes angularly from the other end of the spring. The second prong  230  is received in or engages a receiving hole  232  in the cage  93 . This fixes the second prong  230 , and thus the other end of the tensioning spring  204 , to the cage  93 . During assembly, the tensioning spring  204  is preloaded to a predetermined torque value. The spring  204  thus acts to bias the cage  93  in the chain tensioning direction T, as depicted in  FIGS.  5  and  10   . 
     Referring to  FIGS.  6 ,  9 , and  10   , in this example, the side wall  142  has a third or innermost segment within the fluid cavity  122  and adjacent the end wall  138 . The innermost segment includes a circular portion  234  that is concentric with and spaced a distance radially from the cage rotation axis R. The innermost segment of the side wall  142  also has a non-circular portion  236  that is curved toward and spaced closer to the cage rotation axis R. The non-circular portion  236  of the side wall  142  thus creates a shelf  238  within the fluid cavity  122 . The shelf  238  protrudes radially inward toward the cage rotation axis R in comparison to the circular portion  234 . The shelf  238  also protrudes relative to the end wall  138  with the cavity toward the access opening. When the damper  90  is assembled, the bearing  134  lies in contact with a surface  239  of the shelf  238 . 
     In this example, a compensation device is provided in fluid communication with the fluid cavity  122 , either directly or more indirectly through various fluid flow paths. The compensation device is disposed within the return chamber  182  of the cavity  122  in this example for purposes described in further detail below. In general, the compensation device allows for some volume expansion, either directly or indirectly, of the return chamber  182  during use. Further, any of the disclosed compensation devices, as described below, can compensate for a change in fluid volume caused by a fluid temperature change, a fluid leak from the fluid cavity, or both, and perhaps more so if the compensation device is pressurized upon assembly, also as described below. Alternate examples of compensation devices are disclosed and described herein. 
     Referring again to  FIGS.  6 ,  9 , and  10 A , the compensation device in this example includes a movable body in the form of a resilient body  240  that is positioned within a blind auxiliary bore  242  formed into the surface  239  of the shelf  238 . In one example, the resilient body  240  is formed of a closed-cell foam and is loosely received in the blind bore  242  in the shelf  238 . The shape of the resilient body  240  may mirror the shape of the auxiliary bore  242 . In this example, the resilient body  240  is a cylinder and the auxiliary bore  242  has a cylindrical shape. As shown in  FIG.  10 B , the height of the resilient body  240  may be slightly less than a depth or height of the auxiliary bore  242  so that a small, variable volume space or variable volume expansion chamber S may be left above and between the end of the resilient body and the inner surface  172  (i.e., the surface facing the cavity  122 ) of the bearing  134 . Further, a shallow flow channel  246  is formed along the surface  239  of the shelf  238  and facing the inner surface  172  of the bearing  134 . The flow channel  246  extends between the auxiliary bore  242  and the return chamber  182 . The flow channel  246  allows fluid to flow between the return chamber  182  and the auxiliary bore  242  and thus exposes the end of the resilient body  240  within the variable volume expansion chamber S to fluid from the return chamber  182 . 
     Referring to  FIGS.  6 ,  8 , and  10 A , the shape of the damping chamber  184  within the fluid cavity  122  is a segment of an annular ring. The damping chamber  184  has i) a flat “bottom” defined by the end wall  138 , ii) a flat “top” defined by the inner surface  172  of the bearing  134 , iii) a concave outer boundary defined by the circular portion  234  of the side wall  142 , and iv) a convex inner boundary defined by the cylindrical main section  144  of the shaft  126 . One boundary of the circumferential boundaries of the damping chamber  184  is movable and is defined by the position of the vane  180 . Referring to  FIG.  10 A , as described in more detail below, the vane  180  moves circumferentially relative to the cage rotation axis R. The other boundary of the circumferential boundaries of the damping chamber  184  is fixed and is defined by a face  248  of the shelf  238 . The face  248  is a part of the non-circular portion  236  of the side wall  142 , as best illustrated in  FIGS.  9  and  10 A . Still referring to  FIG.  10 A , the damping chamber  184  has a generally rectangular cross-section shape between the shaft  126  and the side wall  142 . Thus, the vane  180  also has the same cross-section shape. 
     Referring to  FIGS.  3 ,  5 , and  10 A , during use and operation of the bicycle  50  and the rear derailleur  80 , the shaft  126 , which is rotationally fixed to the cage  93 , can only rotate in the damping direction D by forcing fluid in the damping chamber  184  into the return chamber  182 . This is because the vane  180  will attempt to move in the damping direction D about the cage rotation axis R, which results in the vane  180  attempting to reduce the volume of the damping chamber  184 . In this example, the incompressible nature of the hydraulic fluid within the damping chamber  184  will prevent this from occurring. Further, the check valve  132  will remain closed and thus prevent fluid from flowing via the first flow path, i.e., the flow holes  194  from the damping chamber  184  to the return chamber  182 . 
     Thus, for fluid to flow from the damping chamber  184  to the return chamber  182  to permit rotation of the shaft  126  and cage  93  in the damping direction D, one or more second, secondary, or auxiliary flow paths must be provided. In this example, as depicted in  FIGS.  8  and  10   , a plurality of second flow paths is defined by relatively small or limited clearance gaps between parts within the fluid cavity  122 . For example, one clearance gap or second flow path  250  may be provided or defined between an axial edge of the vane  180  and the end wall  138  of the cavity (see  FIG.  7   ). Another clearance gap or second flow path  252  may be provided or defined between a radially distant edge of the vane end  180  and the circular portion  234  of the side wall  142  of the fluid cavity  122  (see  FIGS.  8  and  10 A ). Yet another clearance gap or second flow path  254  may be provided or defined between the main section  144  of the shaft  126  and the end of the shelf  238  adjacent the shaft (see  FIG.  10 A ). Though not shown herein as a second flow path, another limited clearance gap may be provided or defined between the opposite axial surface of the vane  180  and the inner surface  172  of the bearing  134 . Any one or more of these or other clearance gaps may be used to create a desired second flow path for fluid from the damping chamber  184  to the return chamber  182 . These second flow paths or gaps can be provided between portions of the rotatable shaft  126  and the housing  120  defining the fluid cavity  122 . 
     Due in large part to the viscous nature of the fluid held in the damping chamber  184 , the process of forcing fluid to bypass the shaft  126  via the one or more second flow paths  250 - 254  will dissipate a considerable amount of energy. The damping force will also be determined in part by the number and overall size of the one or more second flow paths, the size of the damping chamber  184 , and the like. In any case, a damping force is exerted through the vane  180  on the shaft  126  and thus the cage  93 , which is coupled to the shaft. The fluid damping force applied to the shaft  126  and thus the cage  93  is proportional to the rotational velocity of the shaft. Thus, slow cage rotational speeds (for example, when shifting gears) will provide little resistance to cage rotation in the damping direction D. In contrast, fast cage rotational speeds (for example, during an impact situation when riding over rough terrain), will provide high or significant resistance to cage rotation in the damping direction D. In the case of an electric derailleur (see  FIG.  2   ), the low damping force when shifting will extend battery life. In the case of a cable actuated derailleur (see  FIGS.  3  and  5   ), the low damping force when shifting will result in reduced shift effort to the rider. 
       FIG.  10 A  shows the rear derailleur  80 , including the shaft  126  in a first rotational position, such as a normal operating position with the damper  90  in a normal state.  FIG.  15    shows the shaft  126  after the shaft and the cage  93  have rotated about the cage rotation axis R in the damping direction D. In this example, the vane  180  and shaft have rotated in a direction, which reduces the volume of the damping chamber  184 . Thus, fluid will have been pushed into the return chamber  182  via the second flow paths  250 ,  252 , and  254 . Movement from the position in  FIG.  10 A  to the position in  FIG.  15    may have occurred because of a lower velocity gear shift operation or a higher velocity impact situation experienced by the derailleur  80  and cage  93 . The amount of damping force exerted in the damping direction D by the damper  90  would be proportional to the rotational speed of the shaft  126 . Further, the amount of damping force varies according to the rotational position of the cage  93 , which determines the volume of the damping chamber  184 . 
     The damping force can be varied in the damper  90  by changing the viscosity of the fluid in the fluid cavity  122 . A higher viscosity fluid will result in higher damping forces, while a lower viscosity fluid will result in lower damping forces. The fluid cavity  122  may be filled with an appropriate fluid having a desired viscosity at the factory where the damper  90  is assembled. In one example, the original fluid may remain in the damper  90  for the life of the rear derailleur  80 . Alternatively, a user could disassemble the rear derailleur  80  to replace the factory installed fluid with a fluid of different viscosity. In this manner, a user may change or vary the damping characteristics of the damper  90  without changing any other component of the rear derailleur. At the design or manufacturing stage, the damping force can be varied in the damper  90  by altering the volume of the cavity, and/or by altering the number and/or size of the second flow paths, and/or the like. 
     When the cage  93  and shaft  126  are rotated in the damping direction D, tension in the chain C is reduced. It is thus desirable for the cage to quickly rotate in the opposite chain tensioning direction T, as shown in  FIGS.  3 ,  5 , and  10 A . For this to occur, fluid must flow back from the return chamber  182  to the damping chamber  184 . However, if the only flow paths were the second flow paths  250 ,  252 , and  254  for the fluid to return, the return rotation would be slow and delayed. Slow return rotation would result in a slack chain, which can inhibit performance of the drivetrain  58 , permit the chain C to jump gears, or even permit the chain to fall off entirely. 
       FIG.  16    illustrates how the shaft  126  and thus the cage  93  rotates in the chain tensioning direction T. In order for the shaft  126  and cage  93  to rotate in the chain tensioning direction T, fluid must flow from the return chamber  182  to the damping chamber  184  quickly and freely. The check valve  132  provides this function. The fluid pressure in the return chamber must be sufficient to overcome the force of the valve spring  195 . When this occurs, fluid pressure in the flow holes  194  will push the head  188  of the poppet  186  away from the surface of the vane  180  against the bias force of the valve spring  195 . The fluid can then easily and freely pass through flow holes  194  in the vane  180 . Thus, fluid can freely pass from the return chamber  182  to the damping chamber  184  without being forced through the second flow paths  250 ,  252 , and  254 . The vane  180  and the shaft  126  can thus easily move through the fluid in the chain tensioning direction T. Therefore, very little, if any, damping force is exerted on the cage  93  when rotating in the chain tensioning direction T. 
     According to one aspect of the disclosed damper  90 , referring to  FIGS.  6 ,  9   , and  10 A- 10 C, the closed-cell foam resilient body  240  is disposed in the auxiliary bore  242  in the housing  120  within the cavity  122 . The resilient body  240  is in fluid communication with the return chamber  182  via a shallow restrictive or constrictive flow channel  246  along the shelf  238  in the housing. The auxiliary bore is rather isolated from the damping chamber by the check valve  132 , the limited second flow paths  250 ,  252 ,  254 , and the restrictive nature of the flow channel  246 . This isolates or protects the movable body, i.e., the resilient body  240  in this example, during dynamic operation of the fluid damper  90  when damping force is required and generated. The function of the resilient body  240  is to compensate for fluid expansion within the enclosed fluid cavity  122  caused by temperature increase in the fluid. The resilient body  240  does so by moving or changing state, i.e., compressing or expanding as fluid volume changes occur. The fluid temperature may increase due to the ambient temperature of the surroundings to which the bicycle  50  is exposed. The fluid temperature may also increase from heat or energy dissipated to the fluid from damping action exerted by the damper  90  from repeated motion of the cage  93 . 
     As the temperature of a fluid increases, the fluid will expand. Without the compensation device, such as the resilient body  240 , fluid expansion would exert a large internal pressure within the fluid cavity  122  on the housing  120 . Such a pressure increase may potentially cause damage to the damper  90  or limit or inhibit its function. By employing a compensation device, such as the resilient body  240 , the volume of the fluid cavity  122  may increase to compensate for the expanding fluid. In this example, the foam material of the resilient body  240  will be compressed down into the auxiliary bore  242  as expanding fluid is pushed through the flow channel  246  into the expansion chamber S (see  FIGS.  10 B and  10 C ). Compressing the resilient body  240  to a compressed state reduces the volume of the foam material, which increases the size of the expansion chamber S, as depicted in  FIG.  10 C . This increases the overall volume of the return chamber  182  and thus the fluid cavity  122  to compensate for the increasing volume of the fluid. Thus, the resilient body  240  prevents the expanding fluid from exerting an excessive pressure within the fluid cavity  122  on the housing  120 . As the temperature of the fluid returns to its original level or normal state, the resilient body  240  can expand to its original volume within the auxiliary bore  242 . This decreases the size of the expansion chamber S, which reduces the volume of the return chamber  182  and thus the cavity  122 , as depicted in  FIG.  10 B . 
     It is important that the compensation device not be in direct communication with the damping chamber  184 . In this example, if the resilient body  240  were in direct communication with the damping chamber  184 , then the pressure increase in the damping chamber caused by movement in the damping direction D of the shaft  126  and vane  180  would compress the resilient body  240 . This would result in a reduction or complete loss of any damping force until the resilient body  240  became fully compressed. In other words, compression of the resilient body  240  and volume increase of the expansion chamber S would compensate for the pressure increase in the damping chamber instead of the increased fluid pressure creating the desired damping force or action against rotation of the shaft  126  and vane  180 . 
     Another aspect of the disclosed damper  90  is described referring to  FIG.  8   . In this example, the gland or annular space, within which the dynamic O-ring  160  is received, is formed by surfaces within the recess  162  of the bearing  134  and by an interior facing  260  of the cap  136 . Thus, the dynamic O-ring  160  tends to remain stationary relative to the bearing  134  and the cap  136  when the shaft  126  rotates about the cage rotation axis R. If, alternatively, the gland was formed within the outer circumference shaft  126  (for example, by a groove in the bearing section  146  of the shaft), then the dynamic O-ring  160  would tend to rotate along with the shaft. Experience has shown that if the O-ring  160  were to rotate along with the shaft  126 , this would tend to cause leakage of fluid from the system. Furthermore, in this disclosed example, the exposed and normal inner diameter of the dynamic O-ring  160  should preferably not be less than the corresponding diameter of the bearing section  146  of the shaft  126 . This geometry further aids in preventing the dynamic O-ring  160  from “grabbing onto” the shaft  126  and rotating relative to the bearing  134  and the cap  136 . Experience has again shown that optimal sealing is achieved when relative rotation occurs between the shaft  126  and the dynamic O-ring  160 ; not between the dynamic O-ring and the bearing  134  and the cap  136 . 
     The dynamic seal, such as the O-ring  160 , may also be configured so that the bearing  134  fixes the dynamic O-ring  160  in both axial directions as shown in the example of  FIG.  14   . For example, the dynamic O-ring  160  may be configured to contact the recess  162  on the inner circumferential surface  164 , a first axial surface  163 , and a second axial surface  165 . Thus, in alternative embodiments, the dynamic O-ring  160  may contact the second axial surface  165  or the cap  136 . The O-ring may be configured to simultaneously contact the first and second axial surfaces  163 ,  165 . Alternatively, the O-ring  160  may be configured to contact only one of the first and second axial surface  163 ,  165 , for instance to facilitate relative movement between the shaft  126  and the bearing  134  and/or to limit wear on the O-ring  160 . In an embodiment, the O-ring  160  only contacts the bearing  134  and the shaft  126  without contacting the cap  136 . 
     Yet another aspect of the disclosed damper  90  is described referring to  FIG.  10 A . The return chamber  182  may be arranged such that it is substantially above the damping chamber  184 . Herein, ‘substantially above’ means that all of the return chamber  182  need not be above the damping chamber  184 , but instead only that substantially all, most, or a majority should be above the damping chamber. In the event that fluid leaks out of the fluid cavity  122 , and air (or a vacuum) is then introduced into the fluid cavity, the air will tend to slowly migrate upwards to the return chamber  182  due to its buoyancy. If, alternatively, air or a vacuum remained trapped in the damping chamber  184 , the air or vacuum would cause an undesirable reduction in damping force due to the motion of the shaft  126  and vane  180  in the damping direction D compressing the air or vacuum. Adequate damping would be achieved only after the air or vacuum in the damping chamber  184  had been compressed. Therefore, it is desirable to configure the fluid cavity  122 , as shown in  FIG.  9   , such that air tends to migrate into the low pressure chamber (L), where it will not adversely affect damping performance. 
     Still another aspect of the disclosed damper  90  resides in the size and shape of the damper, and more particularly in the ratio of the width or depth of the damper to the diameter of the damper. Because of the arrangement of the various parts relative to each other, the damper  90  may an overall depth or width, as measured along the cage rotation axis R, that is less than the overall diameter, centered about the axis R. For example, the damper  90  may have an overall depth or width of about 29 mm and an overall diameter of about 40 mm. Advantageously, this ratio may create or provide additional clearance between the foot of a rider and the rear derailleur  80 , minimizing the chance of a collision between the rider&#39;s foot and part of the derailleur. 
     In order to fill the fluid cavity  122  of the housing  120  with fluid such as oil, as noted above, the following assembly procedure may be performed. Referring to  FIGS.  6  and  8   , the check valve  132  is assembled to the vane  180  on the shaft  126 . The stem  190  of the poppet  186  is inserted through the hole  192  in the vane  180 . The valve spring  195  is then installed over the stem  190 . The spring stop  196  is positioned on the free end of the stem  190  against the end of the valve spring  195  and pushed onto the stem to compress the spring. The snap ring  198  is then snapped into the groove  200  on the end of the stem  190  to secure the poppet. The shaft  126  is then positioned within the fluid cavity  122  in the housing  120  with the first axial end  128  received in the blind bore  140  in the end wall  138 , as shown in  FIGS.  6  and  8   . With the housing  120  oriented with the access opening  124  to the fluid cavity  122  facing up, as shown in  FIGS.  6  and  8   , fluid or oil is introduced into the fluid cavity. The fluid cavity  122  is filled until the oil level reaches approximately the level of the female threads  166  within the access opening  124 . 
     The static O-ring  154  is installed in the groove  156  on the outer surface  158  of the bearing  134 . The bearing  134  is inserted into the access opening  124  of the housing  120 . As the bearing  134  is pushed down through the fluid or oil, excess fluid is displaced upward and escapes through the radial clearance gap between the inner surface  164  of the bearing and the corresponding outer diameter of the bearing surface  148  on the bearing section  146  of the shaft  126 . The bearing  134  is pushed downward until contacting the bearing stop  170  within the fluid cavity  122  of the housing  120 . At this stage, the damping chamber  184  and the return chamber  182  are completely full with fluid or oil and are each free of air, whereas any excess fluid has escaped upward, as previously described. The dynamic O-ring  160  is then installed in the recess  162  by pushing the O-ring down onto the bearing section  146  and into the recess, which is open at the exterior side of the bearing  134  (see  FIG.  8   ). The excess fluid from within the access opening  124  that had previously escaped the fluid cavity  122  can then be removed. The cap  136  is then threaded into the access opening  124  on the housing  120  until the interior facing side  260  of the cap contacts the corresponding side of the bearing  134  and clamps the bearing against the bearing stop  170 . The damper  90  is completed at this stage. 
     The assembled damper  90  can then be assembled with the rear derailleur  80 . As shown in  FIGS.  3 ,  5 ,  8 ,  11  and  12   , the tensioner spring  204  can be installed in the channel  206  with the first prong  229  of the tensioner spring received in the hole  212  in the channel. The torque carrier  210  can be inserted into the recess  214  and onto the double-D shaped second axial end  130  of the shaft. The cage  93  can then be placed over the damper  90  with the chamfered hole  218  aligned with the double-D hole  212 . The bolt  220  can be inserted into the chamfered hole  218  in the torque carrier and loosely threaded into the shaft bore  216  in the second axial end  130  of the shaft  126 . The second prong  230  of the tensioner spring  204  can then be engaged with the receiving hole  232  in the cage  93 . The cage can then be rotated to load the tensioner spring  204  until the through holes  226  in the cage  93  are aligned with the blind holes  224  in the torque carrier  20 . The screws  228  can then be inserted and threaded into the blind holes  224  of the torque carrier  210  and the screws and the bolt  220  can be tightened. To complete assembly of the cage  93  and the damper  90 . The cage and damper can then be assembled to the other parts of the derailleur  80  as is known in the art. 
     In the example of  FIGS.  1 - 16   , the components of the fluid damper  90  may be fabricated from any suitable materials and material combinations. In one example, the housing  120  may be made from aluminum, glass-filled nylon or other suitable metal, plastic, or composite materials. The bearing  134  may be made from a suitable metallic material or from a non-metallic material, such as a thermoplastic. In one example, the bearing may be made from an acetal resin or compound such as Delrin®, nylon, or the like. Likewise, the cap  136  may be made from similar materials, as desired. The shaft  126  may also be made from a metallic material or non-metallic material having sufficient strength in torsion. In one example, the shaft  126  may be made from aluminum or steel. Both the tensioner spring  204  and valve spring  195  can be made from spring steel or the like and each may be tuned to provide a specific desired load or force during use. The valve spring  195  may be tuned to provide a relatively light spring load so that the check valve freely opens when needed during use. However, the valve spring  95  should by stiff enough so as to avoid ‘valve float’ during use. In other words, the valve spring  195  should be strong enough so as to be capable of very quickly closing the poppet  186  through the highly viscous fluid or oil during use. Any delay in the check valve  132  closing can cause undesirable fluid transfer from the damping chamber  184  to the return chamber  182 , which would reduce the damping effects of the damper  90 . 
     Many modifications and changes may be made to the disclosed fluid damper without deviating from the intended function. The housing  120  may vary in size and shape, as can the fluid cavity  122  therein. The location, number, arrangement, and features of the chambers  182 ,  184 , the check valve  132 , the first flow path(s), and the second flow path(s) may also vary from the damper  90  as described above. The configuration of the shaft  126 , bearing  134 , and/or cap  136  can also vary from the above described fluid damper example. However, the disclosed fluid damper  90  has a fairly simple construction and yet is very effective in operation. The fluid damper generally has a housing with a main cavity open at one end, a seal closing off the opening, and a threaded cap to retain the assembly in the assembled and fluid filled condition. 
       FIG.  17    shows one alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  270  is much the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In this example, the bearing  134  and the cap  136  have been replaced by a single component, i.e., a seal head  272 . In this example, the seal head  272  has a ring shape with an outer surface  274  that carries a seal groove  276  for the static O-ring  154  and an inner surface  278  that carries another seal groove  280  for the dynamic O-ring  160 . The seal head  272  also external male threads  282  on the outer surface (below the static O-ring  154  in the view of  FIG.  17   ). These threads are configured to engage the female threads  166  within the access opening  124  (though the female threads are positioned deeper into the fluid cavity in this example). 
     The seal head  272  has a pair of bleed ports  284 , which are threaded, and which communicate between the outside of the housing  120  and the fluid cavity  122  when the seal head  272  is installed in the housing. Two sealing or bleed screws  286  are provided and each has an O-ring  288  or seal that is integrated under the head of the screw. The screws  286  can be threaded into the bleed holes  284  to seal the bleed ports  284  in the seal head  272 . The screws  286  in one example can seal the fluid cavity  122  up to 6,000 psi of pressure. 
     In this example, the fluid cavity  122  of the damper  270  can be filled via the following assembly procedure. The check valve  132  is pre-assembled to the shaft  126  and the shaft is positioned in the fluid cavity as described above for the damper  90 . Fluid is then introduced into the fluid cavity  122  in the housing  120  until it covers all of the threads  166  in the access opening  124  of the housing  120 . The static O-ring  154  and the dynamic O-ring  160  are both pre-assembled to the seal head  272  in this example. The seal head  272  is then threaded into the access opening  124  of the housing  120 . As the seal head  272  moves downward through the fluid or oil, the excess fluid is forced upwards through the bleed ports  284  in the seal head. The seal head  272  is threaded into the access opening  124  of the housing  120  until the seal head abuts the bearing stop  170 . At this stage, any excess fluid will have escaped through the bleed ports  284  in the seal head  272  and the return chamber  182  and damping chamber  184  are completely filled with the fluid or oil. The bleed screws  286  are then threaded into the bleed ports  284  in the seal head  272  with the heads and O-rings  288  seated tightly against the seal head. The bleed screws  286  thus seal the bleed ports  284  to prevent any further fluid or oil from escaping the fluid cavity  122 . The damper  270  in this example is now assembled at this stage and ready to be attached to the cage, as described above. 
       FIG.  18    shows another alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  290  is much the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In this example, rather than being incorporated into the shaft, the check valve  132  is incorporated into a portion of a modified housing  292 . The check valve  132  is still configured and arranged to block fluid flow from the damping chamber  184  to the return chamber  182  and to still allow free flow of fluid from the return chamber  182  to the damping chamber  184 . In this example, the check valve  132  is positioned in an opening and recess  294  in a shelf  296  or wall within the cavity that replaces the earlier described shelf  238 . 
     Also in this example, a shaft  298 , which is modified from the earlier described shaft  126 , also includes a vane  300  extending radially from the shaft. The shelf  296 , in part, and the vane  300 , in combination with the shelf  298 , separate the damping chamber  184  from the return chamber  182  in this example. The compensation device in this example is carried on the movable vane  300  of the shaft  298  since the check valve  132  is carried on the shelf  298 , which is a fixed portion of the housing  292 . The compensation device in this example, however, is otherwise identical that that of the damper  90 . More specifically, the compensation device includes a blind auxiliary bore  242  in communication with the return chamber via a flow channel  246  formed in a top surface of the vane  300 . A resilient body  240  of a closed cell foam material is received in the auxiliary bore  242  and defines an expansion chamber S between the body and the inner side or underside  172  of the bearing  134 . The expansion chamber S is again in fluid communication with the return chamber  182  via the flow channel  246 . 
       FIG.  19    shows another alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  310  is much the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In this example, the damper  310  has a shaft  312  with a vane  314  that, rather than being integrally formed with the shaft as are the shaft  126  and vane  180 , is a separate part. The vane  314  may be attached or fastened to the shaft  312 , such as by one or more screws  316 . Such a construction may allow the shaft  312  to have a less complex configuration to make the shaft simpler, easier, or less complicated to manufacture. This construction further allows the vane  314  and the shaft  312  to be made from different materials, if desired. 
     The damper  310  also includes a modified housing  318  with a fluid cavity  320  having a different configuration. This example indicates that the housing  318  and fluid cavity  320  can vary in shape, size, and configuration. The damper  310  also includes a modified compensation device. The compensation device in this example includes a blind auxiliary bore  322  that is formed less than a radial distance from a wall  324  of the fluid cavity  320 . Thus, a portion of the wall  324  is open to the return chamber  182  over a height or depth of the auxiliary bore  322 . As a result, the expansion chamber S is disposed at the top of the resilient body  240  and directly communicates with the return chamber. Further, the resilient body  240  may compress along the wall  324  where the body is exposed directly to the return chamber  182 , to permit direct volume expansion of the return chamber as well. 
       FIG.  20    shows another alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  330  is much the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In this example, the damper  330  has a shaft  332  with a separate vane  334  that is attached or connected to the shaft, such as by one or more screws  336 . The end of the vane  334  terminates short of the circular portion  234  of a surface such as the side wall  142  within a fluid cavity  338  in the housing  339 . A gap  340  remains between the side wall  142  and the end of the vane  334 . The gap  340  defines the first flow path between the return chamber  182  and the damping chamber  184 . A sealing member, such as a resilient or flexible wiper seal  342 , is coupled to, i.e., fixed to and extends from the vane  334 . The wiper seal  342  may be made from any suitable material, such as an oil safe elastomeric material. 
     The wiper seal  342  in a natural or static state is disposed such that its free end  344  wipes along and is biased against the side wall  142  within the fluid cavity  338  of the housing  120 . However, the wiper seal  342  is also angled in a specific way so as to act as a form of a check valve, replacing the check valve  132  of the prior examples. Specifically, when the shaft  332  rotates in the damping direction D, the resulting fluid pressure in the damping chamber  184  biases a free end of the wiper seal  342  in a closed position firmly against the side wall  142  of the fluid cavity  338 , providing a high resistance to fluid flow and essentially closing the gap  340 , i.e., the first flow path. Fluid will then primarily flow from the damping chamber  184  to the return chamber  182  via the second flow paths, such as the flow paths  250  above and/or below the vane  334  and wiper seal  342  and the flow path  254  between the shaft  332  and a wall of the fluid cavity  338 . When the shaft  332  rotates in the opposite chain tensioning direction T, the resulting higher fluid pressure in the return chamber  182  deflects or bends the free end of the wiper seal  342  away from the side wall  142  of the fluid cavity  338  in an open position. This opens the gap  340  and thus the first flow path, allowing a free flow of fluid from the return chamber to the damping chamber  184 . 
       FIG.  21    shows another alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  350  is much the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In this example, damper  350  has a shaft  352  with a vane  354  formed as a separate part from the shaft. The vane  354  is again attached or connected to the shaft  352 , such as by one or more screws  356 . The end of the vane  354  terminates short of the circular portion  234  of the side wall  142  within a fluid cavity  358  in the housing  360 . A gap  362  remains between the side wall  142  and the end of the vane  354 . The gap  362  defines the first flow path between the return chamber  182  and the damping chamber  184 . A rigid wiping member  364  may be made of a rigid material such as plastic. The wiping member  364  is rotatably received in a recess in the vane  354 . 
     A biasing element such as a spring  366  has one end held or retained to the shaft  352  by one of the screws  356 . The other end of the spring  366  contacts a free end of the wiping member  364  and biases the free end of the wiping member against a surface such as the side wall  142  of the fluid cavity  358  within the housing  360 . When the shaft  352  rotates in the damping direction D, fluid pressure in the damping chamber  184  further biases the free end of the wiping member  364  against the side wall  142  of the fluid cavity  358  in the housing  360  in a closed position. This results in a high resistance to fluid flow through the gap  362 . When the shaft  352  rotates in the chain tensioning direction T, fluid pressure in the return chamber  182  deflects the free end of wiping member  364  away from the side wall  142  against the biasing force of the spring  366  to an open position. This allows free flow of fluid via the gap  362  from the return chamber  182  to the damping chamber  184 . 
       FIG.  22    shows another alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  370  is much the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . The damper  370  is similar to the dampers  330  and  350  described above. In this example, the damper  370  has a shaft  372  but does not have a separate vane or an integral vane. Instead, a C-shaped sealing member  374  has one end attached or connected to the shaft  372 , such as by one or more screws  376 . The sealing member  374  may be made of a flexible or resilient material. In a static state, the sealing member  374  may have a free end or other end  377  that is biased against a surface such as the wall  142  of a fluid chamber  378 . When the shaft  372  rotates in the damping direction D, the resulting pressure in the damping chamber  184  energizes or biases the other end  377  of the sealing member  374  firmly against the side wall  142  of the fluid chamber  378  in a housing  380  in a closed position. This results in a high resistance to fluid flow from the damping chamber  184  to the return chamber  182 . When the shaft  372  rotates in the chain tensioning direction T, the resulting pressure in the return chamber  182  deflects the free end  377  of the sealing member  374  away from the side wall  142  in an open position. This allows free fluid flow from the return chamber  182  to the damping chamber  184  via the first flow path between the side wall  142  and the other end  377  of the sealing member  374 . 
       FIGS.  23  and  24    show another alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  390  is substantially the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In this example, as depicted in  FIG.  23   , the damper  390  includes a shallow recess  392  in the end wall  394  of the fluid cavity  122  in the housing  120 . As shown in  FIG.  24   , the shallow recess  392  curves about the cage rotation axis R and has a variable width. The recess  392  provides an additional or modified second flow path in combination with or as a replacement for the second flow path  250  noted above below the vane  180 . The recess  392  provides a flow path for fluid traveling from the damping chamber  184  to the return chamber  182  that varies depending on the rotational position of the shaft  126  and vane  180 . 
     More specifically, as the shaft  126  rotates in the damping direction D, the resistance to fluid flow at any instant is inversely proportional to the width of the portion of the recess that is adjacent the axial edge of the vane  180 . The recess  392  can become narrower as the shaft  126  rotates in the direction D to increasingly resist flow from the damping chamber  184  to the return chamber  182  to retain sufficient damping force. As the shaft rotates in the chain tensioning direction T, the recess can become wider to more quickly allow fluid to flow from the return chamber  182  to the damping chamber  184 . The magnitude of the fluid damping force in the damping direction D will vary as a function of the angular position of the shaft  126 . The specific shape of the recess  392  shown in  FIG.  24    is only meant to be illustrative and not limiting. The shape of the recess  392  may be varied as needed to provide the desired damping characteristics as a function of the angular or rotational position of the shaft  126 . 
       FIG.  25    shows another alternative example of a modified fluid damper constructed in accordance with the teachings of the present disclosure. In this example, a damper  400  is substantially the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In this example, the damper  400  has a cap  402  with an annular shoulder or rib  404  protruding from the interior facing side  260 . The recess  162  in the bearing  134  for the dynamic O-ring  160  is deeper in in this example. The annular rib or shoulder  404  is received in the recess  162  and resides adjacent the O-ring  160 . During assembly, fluid in the recess  162 , which holds the dynamic O-ring  160 , is displaced by the annular rib or shoulder  404  and is forced into the damping chamber  184 . This fluid increases fluid pressure in the damping chamber  184 , which in turn causes the resilient body  240  or other compensation device to compress by an amount equal to the displaced fluid. The damping chamber  184  is thus in a pressurized state because the resilient body  240  exerts pressure on the fluid while trying to expand to it natural expanded state or size. This yields an advantage in that, in the event of a fluid temperature decrease causing a fluid volume reduction, the resilient body  240  can expand to compensate for the decrease in fluid volume. Without this feature, a vacuum would otherwise be created by the fluid volume reduction, resulting in a loss of damping force. Another advantage of pressurizing the damping chamber  184  in this manner is that any air bubbles or pockets trapped inadvertently in the fluid cavity  122  will also be compressed to a small volume, which will minimize any loss of damping force otherwise caused by such trapped air. 
       FIGS.  26  and  27    show additional alternative examples of modified fluid dampers constructed in accordance with the teachings of the present disclosure. In the examples of  FIGS.  26  and  27   , dampers  410  and  430  are much the same as the above-described damper  90 . Thus, like components are not further described in detailed below but have been given the same reference numbers as the corresponding components of the above-described damper  90 . In these examples, the dampers  410  and  430  each include an alternative compensation device that can be used in addition to the auxiliary bore and resilient body examples described above or as a replacement for the earlier described compensation device examples. 
     Referring to  FIG.  26   , the damper  410  has a compensation device with a bore  412  formed through a housing  414  of the damper. The bore  412  opens directly into the end wall  142  of the return chamber  182  within the fluid cavity  122  through the housing  414 . A movable body in the form of a piston  416  with an O-ring seal  418  is slidably received in the bore  412 . A spring  420  biases the piston  416  toward the return chamber  182 . A surface of the piston  316  is exposed directly to the return chamber  182  in the fluid cavity  122 . An adjustment screw  422  includes mechanical threads that engage threads in the opening of the bore  412 . The end  424  of the screw  422  contacts the spring  420  to act as a spring stop. The screw  422  may also be adjusted to adjust the spring load, if desired. In use, the bore  412  defines an expansion chamber S on the side of the piston  416  facing the return chamber  182 . The piston  416  can move according to and against the bias force of the spring  420  to change the size of the expansion chamber S, which is effectively a part of the return chamber  182  in this example. The expansion chamber S in this example can accommodate changes in fluid volume within the fluid cavity  122  caused by fluid temperature changes. However, the expansion chamber S can also accommodate changes in volume, to a degree, caused by leakage of fluid from the fluid cavity  122 . 
     Referring to  FIG.  27   , the damper  430  also has a compensation device with a bore  432  formed by a housing  434 . However, the bore  432  is a blind bore in the end wall  142  within the return chamber  182  in the fluid cavity  122  of the housing  434 . A movable body in the form of a floating piston  436  with an O-ring seal  440  is slidably received in the bore  432 . The expansion chamber S is formed within the bore  412  adjacent the piston  436  on the side of the piston facing the return chamber  182 . Again, a surface of the piston  436  is exposed directly to the return chamber  182  in this example. Thus, the expansion chamber S is effectively a part of the return chamber  182  in this example as well. A closed air pocket  438  is formed within the bore  412  on the opposite side of the piston  436 . The air pocket  438  may be filled with air, which biases the piston  436  toward the return chamber  182 . The air pocket may be created when the piston  436  is installed from within the fluid cavity  122  during assembly of the damper  430 . Alternatively, though not shown herein, the blind terminal end  424  of the bore  432  may include an optional valve for adding or adjusting the air pressure within the air pocket  438 . In use, the piston  416  can move according to and against the bias force of the air pocket  438  to change the size of the expansion chamber S. The expansion chamber S in this example, can thus accommodate changes in fluid volume within the fluid cavity  122  caused by fluid temperature changes. However, the expansion chamber S can also accommodate changes in volume, to a degree, caused by leakage of fluid from the fluid cavity  122 . 
     Each of the above-described examples of a fluid damper illustrates that the configuration and construction of the dampers can be varied in different ways. However, other examples are also certainly possible, different from those disclosed and described herein. The invention and the disclosure are not intended to be limited to only the examples of  FIGS.  1 - 27   . 
     Although certain fluid dampers, bicycle derailleurs, and have been described herein in accordance with the teachings of the present disclosure, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the disclosure that fairly fall within the scope of permissible equivalents. 
     The illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The illustrations are not intended to serve as a complete description of all of the elements and features of apparatus and systems that utilize the structures or methods described herein. Many other embodiments may be apparent to those of skill in the art upon reviewing the disclosure. Other embodiments may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. Additionally, the illustrations are merely representational and may not be drawn to scale. Certain proportions within the illustrations may be exaggerated, while other proportions may be minimized. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive. 
     While this specification contains many specifics, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Similarly, while operations and/or acts are depicted in the drawings and described herein in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that any described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     One or more embodiments of the disclosure may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any particular invention or inventive concept. Moreover, although specific embodiments have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all subsequent adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, are apparent to those of skill in the art upon reviewing the description. 
     The Abstract of the Disclosure is provided to comply with 37 C.F.R. § 1.72(b) and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, various features may be grouped together or described in a single embodiment for the purpose of streamlining the disclosure. This disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter may be directed to less than all of the features of any of the disclosed embodiments. Thus, the following claims are incorporated into the Detailed Description, with each claim standing on its own as defining separately claimed subject matter. 
     It is intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it is understood that the following claims including all equivalents are intended to define the scope of the invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.