Patent Publication Number: US-8540322-B2

Title: Rear wheel assembly for a bicycle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/551,505, filed Oct. 18, 2006, which issued as U.S. Pat. No. 7,708,297 on May 4, 2010, and is based on and claims priority to International Application No. PCT/GB04/000941, filed Mar. 5, 2004 and Great Britain Application No. 0307429.1, filed Mar. 31, 2003. Each of these documents is incorporated by reference herein in their entireties for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to bicycles, and is particularly concerned with bicycles having a planetary gear-change mechanism contained in the hub of the driven wheel. 
     Hub mounted gearboxes are well known in the bicycle art, and are conventionally based on the widely understood, planetary gearbox principle. Planetary-geared hubs for bicycles having two, three, four, five, seven, eight, and even fourteen speed ratios are currently available commercially. 
     In a conventional three-speed hub-mounted gearbox, a central spindle is provided which is rigidly attachable at both its ends to the bicycle frame. A sun gear is non-rotatably mounted to the spindle, and engages planet gears mounted in a planet carrier rotatably carried by the spindle. A drive sprocket, to engage the bicycle chain, rotates coaxially with the spindle. 
     An annulus gear is engaged with the planet gears, and is movable to engage and rotate with either the drive sprocket or with the outer casing of the hub, to which the spokes of the wheel are attached. The planet carrier is likewise movable to engage and rotate with either the drive sprocket or the hub. 
     In the “low” gear position, the drive sprocket and annulus rotate together, and the planet carrier rotates with the hub, so that the wheel rotates more slowly than the drive sprocket. 
     In the “middle” gear position, the drive sprocket engages the annulus and the hub likewise engages the annulus, so that the wheel rotates at the same speed as the drive sprocket. 
     In the “high” gear position, the drive sprocket engages the planet carrier and the annulus engages the hub, so that the wheel rotates faster than the drive sprocket. 
     The present invention does not concern the inner workings of the hub gearbox, beyond the considerations that the bicycle wheel is mounted to the outer casing of the gearbox, and the central spindle must be held against rotation relative to the bicycle frame in order to transmit torque from the drive sprocket to the outer casing of the gearbox and thence to the wheel. 
     The main advantage of a planetary-geared hub is that the gears are compactly sealed within the hub outer casing. Because the gears are protected from water and debris, planetary-geared hubs require low maintenance. 
     Another advantage is that there is no requirement for a rear derailleur mechanism to shift gears. This simplifies the bicycle chain drive because the chain path does not move relative to the bicycle frame as the rider changes gear, as is the case with conventional multi-sprocket derailleur arrangements. Since there is a fixed chain path, a chain cover can be fitted to the bicycle to further protect the bicycle chain drive from dirt. This chain cover also protects the rider&#39;s leg or clothing from soiling by chain lubricants. 
     There are, however, drawbacks to the current state of the art in planetary-geared hubs. Because planetary-geared hubs are usually designed with a long central spindle whose ends are adapted to be mounted to the bicycle frame on either side of the rear bicycle wheel, then in order to remove the tyre from the wheel, for example if the tyre is punctured, the entire rear wheel—including the planetary-geared hub—must be removed from the bicycle frame in order to repair the flat tyre. This can be extremely time consuming. 
     If the bicycle has been fitted with a rear chain drive cover, even more time is required to remove the rear wheel. First the cover must be removed, and then the planetary-geared hub axle nuts must be loosened. The bicycle drive chain can then be removed and consequently, the planetary-geared hub, including the rear bicycle wheel, can be removed and the tyre can then be repaired. 
     Another drawback to current planetary-geared hubs is that if the rear wheel needs to be removed for transporting the bicycle in a car or for storing the bicycle in a compact space, the same time-consuming procedure must be followed. 
     It has been proposed, in a bicycle referred to as an “Entertainment Vehicle” and exhibited by Sony Corporation in Japan in 2000, to mount a planetary-geared hub to a bicycle frame having a single rear arm extending on only one side of the rear bicycle wheel. Such a mounting allows the tyre to be removed without the need to remove either the planetary geared hub or the wheel from the bicycle frame. The Entertainment Vehicle has a solid rear arm, in order to achieve the required rigidity and strength, and an entirely exposed rear chain drive comprising a drive sprocket attached to pedals mounted to the bicycle frame, a driven sprocket at the rear hub, and a drive chain linking the sprockets. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a compact bicycle in which the rear wheel is provided with a planetary transmission in the hub, the rear wheel is mounted from one side only, and wherein the drive train from the pedals to the rear wheel is enclosed. 
     A further objective is to provide a bicycle having a rear wheel mounted to the bicycle frame on one side only and provided with a hub gearbox, wherein the rear wheel is removably mounted to the hub gearbox, so that the rear wheel may be removed without removing the gearbox from the bicycle. 
     One aspect of the present invention provides a bicycle having front and rear wheels mounted to a frame, wherein the rear wheel comprises a hub-mounted planetary gearbox having an outer casing, a spindle and a driven wheel, and wherein the rear wheel is mounted to the frame at one side of the wheel only by means of a rear arm having a transversely-extending mounting portion at its rear end adapted to engage a bearing mounted to the casing of the gearbox, and the rear arm being substantially tubular in form with a central cavity adapted to receive a driving wheel, the driven wheel, and an endless tension element for transmitting driving force from the driving wheel to the driven wheel. 
     The endless tension element is advantageously a chain, and the driving and driven wheels are sprockets. Alternatively the endless tension element may be a toothed or plain belt, and the driving and driven wheels may be corresponding pulleys. 
     The rear arm may be a cantilever arm fixedly mounted at its forward end to the bicycle frame, or the rear arm may be braced by a strut extending from a point at or near its rear end to the frame. 
     The bicycle frame may incorporate rear suspension by pivotally mounting the front end of the rear arm to the bicycle frame to form a swinging arm, and mounting the wheel to the rear end of the swinging arm. In this case a spring and damper assembly will be provided, acting between the swinging arm and the frame. 
     A second aspect of the invention provides a frame for a bicycle, to which a rear wheel having a hub gearbox having an outer casing, a spindle and a driven wheel may be mounted with one side of the wheel unobstructed by frame elements, the frame comprising a rear arm having a transversely-extending mounting portion at its rear end adapted to engage a bearing mounted to the casing of the gearbox, and the rear arm being substantially tubular in form with a central cavity adapted to receive a driving wheel, the driven wheel, and an endless tension element for transmitting driving force from the driving wheel to the driven wheel. 
     In one embodiment the frame comprises a single rear arm, to the rear end of which the rear wheel is mountable, with the wheel positioned to one side of the rear arm. Alternatively, the rear arm may be braced by a strut extending from a point at or near its rear end to the frame. 
     A third aspect of the invention provides a rear arm for a bicycle frame, to which a rear wheel having a hub gearbox having an outer casing, a spindle and a driven wheel may be mounted with one side of the wheel unobstructed, the rear arm having a transversely-extending mounting portion at its rear end adapted to engage a bearing mounted to the casing of the gearbox, and the rear arm being substantially tubular in form with a central cavity adapted to receive a driving wheel, the driven wheel, and an endless tension element for transmitting driving force from the driving wheel to the driven wheel. 
     The driving wheel may be mounted to a pedal axle supported in bearings mounted in the rear arm. 
     In one embodiment, the rear arm comprises a pair of concave half-shells having internal ribs extending between internal studs integrally formed with the ribs and the half-shells respectively. Preferably the studs  115  of one half-shell are formed with blind threaded holes, while the studs of the other half-shell are formed with through holes to accept threaded fasteners for securing the half-shells together. The ribs and studs may be positioned, in relation to a chain loop running within the rear arm, so as to minimise the overall size of the rear arm. All or some of the studs may be positioned within an area bounded by the chain loop. 
     A further aspect of the invention provides a hub for a bicycle wheel comprising a generally cylindrical casing for a planetary gearbox, the casing being adapted to receive an external bearing intermediate the axial extent of the casing for mounting the hub to a bicycle frame, and the hub further comprising releasable mounting elements cooperable with mounting formations formed on a bicycle wheel. The wheel may be a conventional spoked wheel, or a moulded wheel. 
     A yet further aspect of the invention provides a hub-mountable planetary gearbox for a bicycle, comprising a central spindle attachable at one end to a bicycle frame, and an axisymmetric outer casing having a circumferential bearing surface intermediate its axial extent, the bearing surface being engageable with a supporting bearing for mounting the gearbox to a bicycle frame, and means for releasably attaching a bicycle wheel to the gearbox casing. A pair of spaced bearing surfaces may be provided instead of a single bearing surface. 
     The gearbox is preferably provided with the means for attaching the bicycle wheel to the gearbox casing at its axial end remote from said one end. Most preferably the bearing surface is positioned to be substantially central, in the axial direction, relative to the wheel in its mounted position. 
     Preferably the outer hub casing is made of steel or aluminium similar to current planetary-geared hub outer casings. The rotatable bearing may be of the ball, roller, or tapered roller type. The means for attaching the bicycle wheel may comprise a screw thread, or fixing elements cooperable with mounting elements of a mating hub of a bicycle wheel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a rigid-frame bicycle incorporating the hub gear; 
         FIG. 2  is a perspective view of a full-suspension bicycle incorporating the hub gear; 
         FIG. 3  is a cutaway perspective view of the rear arm and hub gear; 
         FIG. 4  is a perspective view of the hub gear housing and supporting bearing of  FIG. 3 ; 
         FIG. 5  is a perspective view of the hub gearbox, supporting bearing, drive sprocket and housing; 
         FIG. 6  is a perspective view of the hub gearbox housing, showing an alternative arrangement for attaching a bicycle wheel; 
         FIG. 7  is a view similar to  FIG. 6 , illustrating a second alternative attachment arrangement; 
         FIG. 8  is a view similar to  FIGS. 6 and 7 , illustrating a third attachment arrangement; 
         FIGS. 9 and 10  are sectional and perspective views, respectively, showing a fourth wheel attachment arrangement; 
         FIG. 11  is an exploded perspective view from the off side of a rear arm for a bicycle, in accordance with a preferred embodiment of the invention; 
         FIG. 12  is an exploded perspective view from the near side of the rear arm of  FIG. 11 ; and 
         FIG. 13  is a schematic cutaway side view from the off side of a rear arm of the rear arm of  FIGS. 11 and 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings,  FIG. 1  shows a rigid-frame bicycle  1 , having both its front  2  and rear  3  wheels mounted to the frame  4  at only one end of their respective axles. 
     The bicycle frame  4  comprises a rigid lower tube  5 , to the approximate centre of which the pedals  6  are mounted. The forward end of the lower tube  5  incorporates a steering tube  7 , to which the front fork assembly  8  and the handlebars  9  and handlebar stem  10  are mounted for rotation as a single unit. 
     Extending upwardly from the approximate centre of the lower tube  5  is the saddle stem  11 , to the top of which the saddle  12  is mounted. 
     The part of the lower tube  5  extending rearwardly from the pedals  6  constitutes a rear arm  13 , to one side of which the rear wheel  3  is mounted. The frame  4  is so arranged that the mid-plane of the rear wheel  3  passes through the steering tube  7  and through the centre of the saddle  12 . The rear arm  13  may be laterally offset from the forward part of the lower tube  5  to achieve this geometrical arrangement. 
     An optional reinforcing strut  4   a  may extend from the rear arm  13  to the saddle stem  11 , to reduce stresses at the forward end of the rear arm  13 . 
       FIG. 2  is a view similar to  FIG. 1 , showing a bicycle having rear suspension. In this embodiment, the rear arm  13  is pivotally mounted at its forward end to the lower tube  5  of the frame  4 . A spring and damper assembly  14  extends between the rear arm  13  and the lower part of the saddle stem  11 . In other respects, the frame  4  is similar to that described in relation to  FIG. 1 . 
     The rear wheel  3  is mounted to the rear arm  13  in a similar manner in both of the arrangements shown in  FIGS. 1 and 2 .  FIG. 3  is a cut-away perspective view of the rear arm  13  and rear wheel  3 , to show the hub-mounted gearbox arrangement. 
     Referring now to  FIG. 3 , the rear arm  13  is tubular in form, and the pedals  6  are mounted to a pedal spindle supported in bearings in the rear arm  13 . A driving sprocket is mounted to the pedal spindle to rotate with the pedals, and a chain  15  passes round this driving sprocket and round a driven sprocket  16  associated with a hub-mounted gearbox  17 . 
     As can be seen in  FIG. 3 , the rear arm  13  is tubular in form and has an internal cavity. The chain  15  is enclosed within the cavity and is thus protected from dirt and moisture. Likewise, the rider is kept out of contact with chain lubricants. The tubular rear arm  13  may be rigidly attached to the bicycle frame, as seen in  FIG. 1 , or may be pivotally attached as seen in  FIG. 2  to provide suspension for the rear wheel. In the latter case, the pivot axis between the rear arm  13  and the bicycle frame  5  may coincide with the axis of rotation of the pedals  6 , or may be offset therefrom. In the latter case, the pedals  6  may be mounted to the rear arm  13  or to the frame  5  if sufficient clearance is provided within the cavity to accommodate relative movement of the driving sprocket and the rear arm. 
     As stated above, the inner workings of the hub-mounted gearbox are not relevant to the present invention, in that the number of speed ratios provided by the gearbox and the means in which gear selection is effected may be entirely conventional. The hub gearbox  17  includes a central spindle  18  and an outer casing  19 . In this example, the outer casing  19  has a pair of spaced external flanges  20  and  21  to receive spokes  22  connecting the hub to the rim  23  of the rear wheel  3 . The outer casing  19  comprises an inner cylindrical shell  24  which surrounds the transmission components of the gearbox, and an outer shell  25  to which the flanges  20  and  21  are mounted. An annular space  26  between the inner and outer shells  24  and  25  accommodates a bearing  27 , preferably arranged to be positioned centrally between the flanges  20  and  21  in the direction of the wheel axis  28 . 
     The rear arm  13  has a lateral tubular extension  29 , open at its end to closely receive the bearing  27 . The rear wheel  3  is thus mounted to the arm  13  by means of the bearing  27 , which supports the outer casing  19  of the hub gearbox  17 . The arm  13  is provided with a bore through which the central spindle  18  of the hub gearbox  17  can extend, the central spindle  18  being secured to the rear arm  13  by a clamping nut  30 . The clamping nut  30  may serve to immobilise the central spindle  18  relative to the rear arm  13 , or alternatively the central spindle  18  may have a non-circular cross-section which is received in a complementary opening in the rear arm  13  to prevent relative rotation of the spindle  18  and the rear arm  13  about the rear wheel axis  28 . 
       FIG. 4  shows, to an enlarged scale, the outer casing  19  and the bearing  27  of the assembly shown in  FIG. 3 . The transmission components of the hub gearbox and the rear arm  13 , are omitted for clarity.  FIG. 5  is a view similar to  FIG. 4 , showing the transmission components of the hub gearbox  17  in schematic form, the driven sprocket  16  and the central spindle  18 . The bearing  27  is shown in position on the inner shell  24  of the outer casing  19  of the hub gearbox  17 . As can be clearly seen in  FIG. 5 , the central spindle  18  is formed with a flat  18   a  to give the central spindle a non-circular cross-section. The central spindle  18  in this embodiment will be received in a “D” shaped opening in the rear arm  13 , in order to prevent any rotation of the central spindle  18  about the rear wheel axis  28  relative to the rear arm  13 . 
     This ensures that the sun gear of the planetary hub gear box remains fixed, and torque can be transmitted through the gearbox from the drive sprocket  16  to the outer casing  19 . Two or more flats  18   a  may be formed on the spindle  18 , or the spindle may have a polygonal cross-section. The opening in the rear arm  13  will be shaped to receive the spindle  18  but to prevent rotation of the spindle in the opening. 
     The preferred location of bearing  27  is such that rotatable bearing  27  lies in the central plane of rear wheel  3 . With this preferred location, forces acting substantially perpendicular to the axis of rotation  28  of rear wheel  3  will be transferred to the rear arm  13  of bicycle frame  4  through outer hub casing  19  and rotatable bearing  27  thus substantially reducing or eliminating bending moment forces acting on central spindle  18 . 
     It is however foreseen that the bearing  27  may be fitted externally to the lateral extension  29  of the rear arm  13 , with the bearing supporting the outer shell  25  of the hub casing  19  directly by abutting the radially outer wall of the annular space  26 . Furthermore, two or more bearings  27  may be mounted either internally in the extension  29  to engage the outer surface of the inner shell  24 , or may be mounted externally of the extension  29  to engage the inner face of the outer shell  25 . 
     In the previously-described example, the flanges  20  and  21  and the outer shell  25  are integrally formed as part of the outer casing  19  of the hub gearbox  17 . While the tyre may be removed from the wheel  3  without the need to dismount the wheel  3  from the rear arm  13 , removal of the wheel from the rear arm  13  requires removal of the gearbox  17  and disconnection of the chain  15  from the driven sprocket  16 . There will now be described embodiments of the invention, which provide for simple removal of the rear wheel  3  from the bicycle frame, without disturbing the hub gearbox  17  or its connection to the chain  15 . 
       FIGS. 6 to 10  illustrate embodiments of the invention wherein the outer casing  19  of the hub gearbox  17  is provided with a mounting arrangement to which the wheel is removably attachable. 
       FIG. 6  shows a first alternative arrangement for attaching a bicycle wheel to the hub gearbox. Outer hub casing  19  with rotatable bearing  27  has a threaded section  31  at its free end. Threaded section  31  cooperable with a mating threaded bore  32  in a bicycle wheel  33 . This alternative means for attaching a bicycle wheel allows removal of the bicycle wheel by unscrewing mating threaded bicycle wheel  33  from outer hub casing  19 . A locking arrangement such as a locknut (not shown) or an axially-extending locking pin such as that shown in  FIG. 8  may be provided on the threaded section  31  to secure the wheel against inadvertent loosening. 
       FIG. 7  shows a second alternative arrangement for attaching the bicycle wheel to the hub gearbox. Outer hub casing  19  with rotatable bearing  27  has a tapered section  34  at its free end and drive pins  35  extending axially from the end of tapered section  34 . Tapered section  34  cooperates with a mating tapered bore  36  in a bicycle wheel  37 . Drive pin holes  38  are provided in the bicycle wheel  37  to accept the drive pins  35  as the tapered bore  36  is placed over the tapered section  34  of the outer casing  19  of the hub gearbox  17 . Drive pins  35  prevent bicycle wheel  37  from rotating independently of outer hub casing  19 . A retaining means such as an axially extending locking pin such as that shown in  FIG. 8  may be provided to retain the wheel on the tapered section  34 . Alternatively, pins  35  may be threaded to receive threaded nuts to retain the wheel. 
       FIG. 8  shows a third alternative arrangement, in which outer hub casing  19  with rotatable bearing  27  has a splined section  38  at its free end. Splined section  38  is cooperable with a mating splined bore  39  in bicycle wheel  40 . Splined section  38  cooperates with splined bore  39  to prevent bicycle wheel  40  from rotating independently of outer hub casing  19 . Locking means, such as an axially extending threaded fastener  41  extending through a clearance hole  42  in the wheel  40  to engage a threaded bore  43  in the hub casing  19 , may be provided to retain the wheel on the hub casing. The threaded fastener  41  may have a hand operable “butterfly” head  44 , or may be a tool-operated fastener such as a hexagon head, slotted head, or socket screw. 
       FIGS. 9 and 10  are sectional and perspective views, respectively, of a fourth alternative arrangement for mounting a bicycle wheel to the hub gearbox. In the arrangement shown, the outer casing  19  of the hub gearbox  17  is formed at its free end with a protruding cylindrical section  44  having a circumferential external groove  45 . The bicycle wheel  46  is formed with a central bore  47  which closely engages the outer surface of the cylindrical section  44  of the hub. Adjacent the bore  47 , the wheel  46  is provided with three threaded holes  48 , through which threaded fasteners  49  extend. The threaded fasteners  49  have an enlarged head  50  at one end, to which an operating lever  51  is attached. The end of the fastener  49  remote from the head  50  protrudes beyond the threaded bore  48 . 
     The operating levers  51  have a grip portion  52  at one end, and an engaging lobe  53  adjacent the head  50  of the fastener  49 . 
     To mount the wheel to the hub gearbox, the wheel is offered up so that the protruding cylindrical section  44  enters the bore  47  of the wheel  46 . The wheel is then aligned with the outer casing  19 , so that the protruding ends of the fasteners  49  enter clearance openings  54  in a flange  55  surrounding the base of the cylindrical protruding section  44  of the outer casing  19  of the gearbox. The operating levers  51  are then rotated by means of the gripping portions  52 , so that the engagement lobes  53  of the levers  51  engage the circumferential groove  45 . As the levers  51  are rotated with the engaging lobes  53  in the groove  45 , the threaded engagement between the fasteners and the threaded bores  48  causes the wheel  46  to be clamped securely onto the flange  55 , as the fasteners  49  are withdrawn from the threaded bores  48  with the engagement lobes  53  in contact with the wall of the groove  45 . 
     In addition to the mounting arrangements described in the Figures, other arrangements for releasably mounting a wheel to the end of the outer casing  19  of the hub gearbox remote from the rear arm  13  are possible and are to be considered as included in the scope of the present application. 
     A preferred embodiment of the rear arm structure is shown in  FIGS. 11 to 13 . Referring to these Figures, the rear arm  13  is formed from a pair of concave half-shells  13   a  and  13   b . The half-shells  13   a  and  13   b  have internal ribs  113 ,  114  extending between studs  115 ,  116  integrally formed with the ribs  113 ,  114  and the half-shells  13   a ,  13   b  respectively. The studs  115  of half-shell  13   b  are formed with blind threaded holes  117 , while the studs  116  of half-shell  13   a  are formed with through holes  118 . Threaded fasteners  119  are passed through the holes  118  to engage with the threaded holes  117 , and tightened to secure the half-shells  13   a ,  13   b  together. The ribs  113  of half-shell  13   a  are cutaway, as at  113   a , to provide clearance for the internal components to be received within the rear arm. 
     Half-shell  13   b  is formed at its front end with a cylindrical projection  120 , which encloses a bearing to mount a pedal axle  121  to the half-shell  13   b . Half-shell  13   a  has an opening  122  at its forward end, to allow pedal axle  120  to pass through and offside crank  123  to engage the exposed end of axle  121 . A nearside crank  124  is mounted to the protruding end of the axle  121 , at the free end of the projection  120 . The rear arm shown in  FIGS. 11 and 12  is intended for a bicycle with rear suspension, the rear arm being mounted to the bicycle frame in bearings (not shown) surrounding the projection  120  and coaxial with the pedal axle  121 . 
     Positioned intermediate the length of the half-shell  13   b  is a suspension mounting lug  13   d , to which one end of a spring/damper assembly  14  may be attached. The other end of the spring/damper assembly  14  is attached to the frame of the bicycle, as is seen in  FIG. 2 . 
     At the rear end of half-shell  13   b , a second tubular projection  129  extends parallel to the projection  120 . The projection  129  corresponds in function to the lateral tubular extension  29  shown in  FIG. 3 , and is open at its free end to closely receive a bearing  127  mounted on the casing of a hub gearbox  130 . The rear wheel  3  is mounted to the rear arm  13  by means of the bearing  130 , which supports the outer casing  131  of the hub gearbox  130 . The half-shell  13   a  is provided with a bore  132  through which a central spindle  133  of the hub gearbox  130  can extend, the central spindle  133  being secured to the half-shell  13   a  by a sleeve  134  which engages a “D” section end of the spindle  133  and secures the spindle against rotation relative to the rear arm by means of a lug  135  on the sleeve  134  engaging in a slot  136  in the half-shell  13   b . Alternatively the spindle  133  may have a non-circular cross-section which is received in a complementary opening  132  in the half-shell  13   a  to prevent relative rotation of the spindle  133  and the rear arm  13 . 
     When fixed together with fasteners  119 , the half-shells  13   a ,  13   b  define a substantially closed cavity to contain a driving sprocket  137  mounted on the pedal axle  121 , a driven sprocket  138  coaxial with the spindle  133 , and a chain loop  139  for transmitting drive from sprocket  137  to sprocket  138 . The chain loop comprises an upper chain run  140  and a lower chain run  141 , the lower chain run  141  being engaged by a jockey  142  mounted to a swing arm  143  biased to urge the jockey toward the upper chain run  140 . The swing arm biasing is arranged to ensure the correct tension in the chain  139 , and to compensate for chain length variations, since the distance between centres of the pedal axle  121  and spindle  133  is fixed. Other chain tensioning arrangements may be used, such as a pair of jockeys engaging opposite sides of the lower chain run  141  and urged in opposite directions. 
     To provide the smallest possible overall size to the rear arm  13 , the ribs  113 ,  114  and the studs  115 ,  116  are positioned such that the region of the rear arm  13  above the upper chain run  140  is free of studs  115 ,  116 . Most preferably, a region bounded by vertical planes AA and SS, containing the axes of the pedal axle  121  and the spindle  133  respectively, and above the common tangent TT to the sprockets  138  and  139 , is free of ribs  113 ,  114  and studs  115 ,  116 . The structure of the assembled rear arm is such that the loads imposed by the wheel, suspension, and pedals are transmitted predominantly by the ribs  113   114  forming a box-like structure within the cavity in the rear arm. The outline of the rear arm is smooth, without irregular excrescences for positioning fixing screws to hold the half-shells together. If all the ribs and studs are positioned within a region bounded by the two common tangents to the sprockets  138  and  139 , a rear arm of reduced outline can be effectively achieved. Due to the re-entrant shape of the chain loop caused by the tensioning jockey  142 , the studs  115 ,  116  may not all be within the chain loop in such an embodiment 
     As an alternative to the projection  120  and suspension lug  13   d , the front end of the half-shell  13   b  may include alternative formations for fixed mounting of the rear arm  13  to a bicycle frame, as seen in  FIG. 1 . 
     The half-shells are preferably formed from metal or metal alloy, and most preferably by moulding methods such as casting or die-casting. The half-shells may alternatively be formed by fabrication from metals and/or from plastics or composite materials.