Abstract:
A shift control apparatus for a hub transmission includes an axle defining an axle axis; a pawl support rotating member rotatably supported around the axle axis; a pawl rotatably supported to the pawl support rotating member; a biasing mechanism for biasing the pawl in a radial direction; and a pawl control rotating member for controlling a radial position of the pawl. One of the pawl support rotating member and the pawl control rotating member includes a location for coupling to a shift control mechanism, and the other one of the pawl support rotating member and the pawl control rotating member includes a location for coupling to an actuating member. A spring is provided for biasing the pawl support rotating member and the pawl control rotating member in a rotational direction relative to each other, and a stopper is provided for holding the pawl support rotating member and the pawl control rotating member in a rotational position relative to each other. The actuating member rotates the pawl support rotating member and the pawl control rotating member in an opposite rotational direction relative to each other when resistance from the shift control mechanism overcomes a biasing force of the spring.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of copending application no. 09/522,703, filed Mar. 10, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed to bicycle transmissions and, more particularly, to an internally mounted multi-speed hub transmission for a bicycle. 
     An internally-mounted multi-speed hub transmission sometimes is mounted to the rear wheel of a bicycle so that the rider can select different gear ratios to vary the pedaling effort. A typical hub transmission includes a hub axle that is mounted to the bicycle frame, a driver rotatably supported to the hub axle for receiving the pedaling force through a sprocket and chain, and a hub shell rotatably supported to the hub axle. A power transmitting mechanism is disposed between the driver and the hub shell for communicating rotational power from the driver to the hub shell through a plurality of power transmission paths, wherein each power transmission path typically produces a unique gear ratio. The power transmitting mechanism ordinarily comprises a planetary gear mechanism including one or more sun gears rotatably supported around the hub axle, a ring gear rotatably supported around the hub axle, a planet gear carrier rotatably supported around the hub axle, and a plurality of planet gears rotatably supported to the planet gear carrier and meshing with the sun gear and the ring gear. The plurality of power transmission paths and the corresponding gear ratios are selected by selectively nonrotatably coupling the various components to each other. For example, one gear ratio may be selected by nonrotatably coupling a sun gear to the hub axle, another gear ratio may be selected by nonrotatably coupling the driver relative to the planet gear carrier, and another gear ratio may be selected by nonrotatably coupling the driver relative to the ring gear. Many such coupling relationships often are possible in a typical hub transmission, thus resulting in a relatively large number of possible gear ratios. 
     When a gear ratio is selected by nonrotatably coupling the sun gear to the axle, the coupling may be accomplished by a ratchet and pawl mechanism disposed between an inner peripheral surface of the sun gear and the hub axle. More specifically, a plurality of pawls may be mounted to the inner peripheral surface of the sun gear such that an end of each pawl is biased radially inwardly by a spring. The outer peripheral surface of the hub axle typically forms a plurality of ratchet teeth or abutments which engage the ends of the pawls to nonrotatably couple the sun gear to the hub axle, and a control sleeve is rotatably supported to the hub axle to selectively expose the abutments. As a result, the sun gear is free to rotate relative to the hub axle when the abutments are covered by the control sleeve, and the sun gear is nonrotatably coupled to the hub axle when the abutments are exposed. 
     The sun gears usually are supported to the hub axle through the plurality of pawls. As a result, often there is some looseness in the support of the sun gear on the hub axle which decreases the precision of the ratchet and pawl mechanism. Such looseness can be compensated for by increasing the number of pawls, but that increases the cost and complexity of the transmission, not to mention the risk of malfunction. Additionally, the circumferential distance between successive ratchet teeth or abutments on the hub axle ordinarily is relatively large. As a result, the sun gear ordinarily must rotate a substantial distance before the pawls engage the ratchet teeth or abutments. This causes undesirable delay in the gear switching operation. 
     Another type of hub transmission includes a sun gear rotatably mounted around the axle, wherein an inner peripheral surface of the sun gear defines a plurality of ratchet teeth. One or more pawls may be disposed in an aperture formed in a hollow axle to selectively engage the plurality of ratchet teeth. However, a hollow axle is not very strong and is not suitable for severe operating conditions. Yet another type of hub transmission also uses pawls mounted to the axle, but the pawls are operated by a control sleeve that directly supports the sun gears. Such a configuration causes excessive friction on the control sleeve. 
     Another disadvantage of conventional hub transmissions is that, when switching from one gear ratio to another gear ratio, the transmission sometimes must pass temporarily through another gear ratio that is not near the destination gear ratio as the various components change their coupling relationships. This phenomenon is discussed more fully in the detailed description below. For example, when shifting from a small gear ratio, wherein the hub shell rotates around the axle at a relatively slow rate relative to the driver, to a higher gear ratio, wherein the hub shell rotates around the axle at a larger rate relative to the driver (such as occurs when the bicycle is decelerating), the transmission may temporarily switch into a gear ratio that is lower than the original gear ratio. This causes the pedals to speed up temporarily, which is opposite the desired effect and can be very disconcerting to the rider. 
     Another disadvantage of conventional hub transmissions is that the sun gear ratchet and pawl mechanisms ordinarily are controlled by a relatively thin sleeve that is rotatably supported on the hub axle. As noted above, such a sleeve often is used to selectively expose the abutments on the hub axle for engaging the pawls on the sun gears. The sleeve typically is relatively long and is operated from outside the hub shell, thus creating significant torsional stresses on the sleeve. Such forces create a risk of bending or twisting the sleeve. 
     Another disadvantage of conventional hub transmissions is that the sleeve that controls the ratchet and pawl mechanisms (and any other desired coupling mechanisms) is sometimes coupled to an external actuating member such as an actuating ring through one or more return springs that bias the actuating ring to a start position. Such a biasing force is used not only to provide proper tensioning of the components during the switching operations but also to help control a shift assist function. Such a shift assist function uses the force of the rotating driver to help overcome resistance to the shift operation such as occurs when significant pedaling force is applied to the hub. More specifically, a coupling mechanism that is normally biased to an inoperative state is activated to couple the sleeve to the driver so that the force from the driver overcomes the excessive resistance. In any event, when such a biased actuating ring is operated by a battery-operated motor, the motor must overcome the biasing force of the return spring. This typically requires a relatively large motor that consumes a substantial amount of power, thus significantly reducing battery life. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a bicycle hub transmission wherein the sun gears are stably supported on the hub axle, wherein the gear switching operation is performed with precision and minimal delay and with minimal effect on the rider, wherein components such as the control sleeve used to control a ratchet and pawl mechanism is stably supported to minimize the risk of bending or other damage, and wherein the actuating member used for the gear switching operation does not cause excessive power consumption when driven by a battery-operated motor. 
     In one embodiment of the present invention directed to a basic sun gear apparatus, the sun gear apparatus includes an axle, a sun gear rotatably supported around the axle, and a sun gear guide ring disposed between an inner peripheral surface of the sun gear and the axle. The sun gear guide ring minimizes or eliminates looseness in the coupling between the sun gear and the axle. One or more such guide rings may be used to support a single sun gear, or one guide ring may be used to support multiple sun gears. 
     In a more specific embodiment of the present invention wherein a pawl is disposed between an inner peripheral surface of the sun gear and the axle for moving between an engaged position (wherein the sun gear is nonrotatably coupled to the axle) and a disengaged position (wherein the sun gear rotates relative to the axle), a pawl is retained to the axle such that an end of the pawl is biased radially outwardly to engage one of a plurality of ratchet teeth on the sun gear. To minimize the delay when switching the sun gear from the engaged state to the disengaged state, only one such pawl is provided, an the sun gear includes more than ten ratchet teeth (e.g., twelve) to ensure quick engagement between the pawl and one of the ratchet teeth. If the apparatus is used in a hub transmission of the type having a driver and a hub shell rotatably supported to the hub axle, wherein the sun gear mechanism is part of a planetary gear mechanism of the type described above, a roller clutch may be disposed between the ring gear and the hub shell to further reduce the delay when switching from one gear ratio to another gear ratio. 
     In another more specific embodiment of the present invention, a pawl control member may be provided for moving the pawl between the engaged position and the disengaged position. If the pawl control member is an elongated member disposed between the sun gear guide ring and the axle, then the sun gear guide ring not only stably supports the sun gear on the axle but also provides reinforcement to the pawl control member to minimize or eliminate the risk of bending or other damage to the pawl control member. 
     In another feature of the present invention directed to how the transmission is shifted from one gear to another gear, a clutch is provided for selecting the plurality of power transmission paths such that, when the clutch changes the power transmitting mechanism from a first intermediate speed transmission path having a first intermediate gear ratio to a second intermediate speed transmission path having a second intermediate gear ratio lower than the first intermediate gear ratio and adjacent to the first intermediate gear ratio, the clutch switches the power transmitting mechanism from the first intermediate speed transmission path to a third intermediate speed transmission path having a third intermediate gear ratio higher than the first intermediate gear ratio and less than a high speed gear ratio before switching the power transmitting mechanism to the second intermediate speed transmission path. Conversely, the clutch may be provided such that, when the clutch changes the power transmitting mechanism from a first intermediate speed transmission path having a first intermediate gear ratio to a second intermediate speed transmission path having a second intermediate gear ratio higher than the first intermediate gear ratio and adjacent to the first intermediate gear ratio, the clutch switches the power transmitting mechanism from the first intermediate speed transmission path to a third intermediate speed transmission path having a third intermediate gear ratio lower than the first intermediate gear ratio and higher than the low speed gear ratio before switching the power transmitting mechanism to the second intermediate speed transmission path. 
     In another feature of the present invention, an unbiased actuating member is used to operate the clutch while still providing the shift assist function noted above. In general, a shift control apparatus for a hub transmission includes an axle defining an axle axis; a pawl support rotating member rotatably supported around the axle axis; a pawl rotatably supported to the pawl support rotating member; a biasing mechanism for biasing the pawl in a radial direction; and a pawl control rotating member for controlling a radial position of the pawl. One of the pawl support rotating member and the pawl control rotating member includes a location for coupling to a shift control mechanism, and the other one of the pawl support rotating member and the pawl control rotating member includes a location for coupling to an actuating member. A spring is provided for biasing the pawl support rotating member and the pawl control rotating member in a rotational direction relative to each other, and a stopper is provided for holding the pawl support rotating member and the pawl control rotating member in a rotational position relative to each other. The actuating member rotates the pawl support rotating member and the pawl control rotating member in an opposite rotational direction relative to each other when resistance from the shift control mechanism overcomes a biasing force of the spring. More specifically, the axle defines an axle axis; a first rotating member is rotatably supported around the axle axis, wherein the first rotating member includes a location for coupling to a shift control mechanism; a second rotating member is rotatably supported around the axle axis; and a first spring is coupled between the first rotating member and the second rotating member for biasing the first rotating member and the second rotating member in a predetermined rotational direction relative to each other. A pawl support rotating member is rotatably supported around the axle axis, wherein the second rotating member is disposed between the first rotating member and the pawl support rotating member; a pawl is rotatably supported to the pawl support rotating member; a biasing mechanism biases the pawl in a radial direction; a first coupling member couples the first rotating member to the pawl support rotating member; and a pawl control rotating member controls a radial position of the pawl. An actuating member is rotatably supported around the axle axis for rotating the first rotating member; and a second coupling member couples the second rotating member, the pawl control rotating member and the actuating member for rotating the pawl control rotating member relative to the pawl support rotating member. This structure provides the shift assist function while providing no net bias to the actuating member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a rear of a bicycle that includes a hub transmission that incorporates many features of the present invention; 
     FIG. 2 is cross sectional view of a particular embodiment of a hub transmission according to the present invention; 
     FIG. 3 is an exploded view of an axle and sun gear apparatus used in the hub transmission; 
     FIG. 4 is an assembled view of the axle and sun gear apparatus; 
     FIG. 5 is a side view showing how a sun gear guide ring used for either the second or third sun gear is fitted on the axle; 
     FIG. 6 is a side view showing how a sun gear guide ring used for the fourth sun gear is fitted on the axle; 
     FIG. 7 is a side view of a shift control sleeve used in the hub transmission; 
     FIG. 8 is a view taken along line VIII—VIII in FIG. 7; 
     FIG. 9 is a view taken along line IX—IX in FIG. 7; 
     FIG. 10 is a view taken along line X—X in FIG. 7; 
     FIG. 11 is a view taken along line XI—XI in FIG. 2 showing the shift control sleeve in a first position; 
     FIG. 12 is a view taken along line XI—XI in FIG. 2 showing the shift control sleeve in a second position; 
     FIG. 13 is a view taken along line XI—XI in FIG. 2 showing the shift control sleeve in a third position; 
     FIG. 14 is a view taken along line XI—XI in FIG. 2 showing the shift control sleeve in a fourth position; 
     FIG. 15 is an exploded view of a shift/assist mechanism used in the hub transmission; 
     FIG. 16 is an oblique view showing the sun gear apparatus and the shift/assist mechanism in an assembled state; 
     FIG. 17 is a detailed view showing the shift/assist mechanism when the driver is coupled to the planet gear carrier; 
     FIG. 18 is a detailed view showing the shift/assist mechanism when the driver is uncoupled from the planet gear carrier; 
     FIG. 19 is a schematic view illustrating the biasing of the shift/assist mechanism; 
     FIG. 20 is a view taken along line XX—XX in FIG. 17 showing the shift/assist mechanism in an inoperative state; 
     FIG. 21 is a view taken along line XX—XX in FIG. 17 showing the shift/assist mechanism in an operative state; 
     FIG. 22 is a view taken along line XX—XX in FIG. 17 showing the shift/assist mechanism moving back toward the inoperative state; 
     FIG. 23 is a cross-sectional view of an alternative embodiment of a hub transmission according to the present invention; 
     FIG. 24 is an exploded view of the shift/assist mechanism used in the hub transmission shown in FIG. 23; and 
     FIG. 25 is a schematic view illustrating the biasing of the shift/assist mechanism. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is a side view of a rear of a bicycle  10  that includes a particular embodiment of a hub transmission  14  that incorporates many features of the present invention. The rear portion of bicycle  10  includes a frame  18  with a seat tube  22  supporting a saddle  24  a pair of conventional chain stays  26  and a pair of conventional seat stays  30 . A wheel  34  is rotatably supported by a frame end  35  around an axle  36  of hub transmission  14  at the intersection of chain stays  26  and seat stays  30 , and a crank assembly  38  having pedals  42  and a chain ring  46  is rotatably supported at the intersection of seat tube  22  and chain stays  26 . A chain  50  engages chain ring  46  and wraps around a sprocket  54  that rotatably drives hub transmission  14 . A Bowden-type control cable  62  is used to change gear ratios in hub transmission  14  in a manner described more fully below. 
     FIG. 2 is cross sectional view of a particular embodiment of hub transmission  14 . As shown in FIG. 2, hub transmission  14  includes axle  36 , a driver  70  rotatably supported to axle  36 , a hub shell  74  including spoke flanges  78  rotatably supported to axle  36 , a power transmitting mechanism  82  disposed between driver  70  and hub shell  74  for communicating rotational power from driver  70  to hub shell  74  through a plurality of power transmission paths, a coaster brake  86  for stopping the rotation of hub shell  74  relative to axle  36 , and a shift/assist mechanism  90  for controlling the selection of the plurality of power transmission paths and for using the rotational power of driver  70  to help change the power transmission paths in power transmitting mechanism  82 . 
     Sprocket  54  is coupled to driver  70  in a splined manner using a snap ring  94 , and driver  70  is rotatably supported on axle  36  through ball bearings  98  and a bearing cone  102 . Bearing cone  102  is maintained in place by an actuator plate  104 , a spacer  108 , a washer  112 , a nonrotatable lock washer  113  and a lock nut  114 . 
     A right cup  120  is nonrotatably fitted at the right side inner peripheral surface of hub shell  74 , and a left cup  124  is nonrotatably fitted at the left side inner peripheral surface of hub shell  74 . Right cup  120  rotatably supports hub shell  74  to driver  70  through ball bearings  128 , and the internal components at the right side of hub transmission  14  are protected from external contaminants by a sealing cup  132  fitted over right cup  120 . Left cup  124  rotatably supports hub shell  74  on axle  36  through ball bearings  136  and a brake cone  138 . Brake cone  138  is maintained on axle  36  by a stop nut  142  and a lock nut  146 . A brake arm  150  is nonrotatably coupled to brake cone  138  and is fastened to chain stay  26  in a well known manner. The internal components at the left side of hub transmission  14  are protected from external contaminants by a sealing cup  152 . 
     As shown in FIGS. 2-6, power transmitting mechanism  82  includes a first sun gear  160 , a separate second sun gear  164 , a separate third sun gear  168 , and a separate fourth sun gear  172 . First sun gear  160  is nonrotatably supported to axle  36 , and it includes a clutch cam portion  176 , a plurality of first sun gear teeth  178  (e.g., 48T) formed on an outer peripheral surface thereof, and an outer peripheral second sun gear contact surface  180 . Second sun gear  164  is rotatably supported around axle  36  adjacent to first sun gear  160 , and, as shown more clearly in FIG. 3, it includes an inner peripheral first sun gear contact surface  192  for slidably contacting second sun gear contact surface  180  on first sun gear  160 , a plurality of second sun gear ratchet teeth  206  (e.g., 12T) formed on an inner peripheral surface thereof for engaging a second sun gear pawl  207 , an inner peripheral guide ring contact surface  208  for slidably contacting an outer peripheral surface  209  of a sun gear guide ring  210 , an inner peripheral third sun gear contact surface  200 , and a plurality of second sun gear teeth 198 (e.g., 48T) formed on an outer peripheral surface thereof. Third sun gear  168  is rotatably supported around axle  36  adjacent to second sun gear  164 , and it includes an inner peripheral first guide ring contact surface  220  for slidably contacting the outer peripheral surface  209  of sun gear guide ring  210 , a plurality of third sun gear ratchet teeth  224  (e.g., 12T) formed on an inner peripheral surface thereof for engaging a third sun gear pawl  226 , an inner peripheral second guide ring contact surface  228  for contacting an outer peripheral surface  232  of a sun gear guide ring  234 , an outer peripheral second sun gear contact surface  235  for slidably contacting third sun gear contact surface  200  on second sun gear  164 , and a plurality of third sun gear teeth  236  (e.g., 42T) formed on an outer peripheral surface thereof. Fourth sun gear  172  includes a plurality of fourth sun gear teeth  244  (e.g., 36T) formed on an outer peripheral surface thereof, a plurality of fourth sun gear ratchet teeth  248  (e.g., 12T) formed on an inner peripheral surface thereof for engaging a fourth sun gear pawl  250 , and an inner peripheral guide ring contact surface  252  for slidably contacting the outer peripheral surface  254  of a guide ring  258 . 
     Pawl  207  includes a pawl seat  260  that is pivotably seated in a pawl receiving groove  264  formed in axle  36 , a spring receiving groove  268  for receiving a spring  272  mounted in a spring receiving groove  276  formed in axle  36 , a pawl control surface  280  for contacting an inner peripheral surface  282  of a pawl control arm  284  of a pawl control sleeve  288 , and a pawl tooth  289  for engaging second sun gear ratchet teeth  206 . Similarly, pawl  226  includes a pawl seat  290  that is pivotably seated in a pawl receiving groove  294  formed in axle  36 , a spring receiving groove  298  for receiving a spring  302  mounted in a spring receiving groove  306  formed in axle  36 , a pawl control surface  310  (FIG. 2) for contacting an inner peripheral surface  312  of a pawl control arm  314  of pawl control sleeve  288 , and a pawl tooth  316  for engaging third sun gear ratchet teeth  224 . Finally, pawl  250  includes a pawl seat  320  that is pivotably seated in a pawl receiving groove  324  formed in axle  36 , a spring receiving groove  328  for receiving a spring  332  mounted in a spring receiving groove  336  formed in axle  36 , a pawl control surface  340  (FIG. 2) for contacting an inner peripheral surface  342  of a pawl control arm  344  of pawl control sleeve  288 , and a pawl tooth  346  for engaging fourth sun gear ratchet teeth  248 . Pawl teeth  289 ,  316  and  346  of pawls  207 ,  226  and  250  are biased radially outwardly by their respective springs  272 ,  302  and  332  in a well own manner. 
     In this embodiment, half of sun gear guide ring  210  is fitted between guide ring contact surface  208  of second sun gear  164  and axle  36 , and the other half of sun gear guide ring  210  is fitted between first guide ring contact surface  220  of third sun gear  168  and axle  36 . As shown in FIGS. 3 and 5, in addition to outer peripheral surface  209 , sun gear guide ring  210  includes a locking recess  360  for engaging a locking ridge  364  formed intermittently in the direction of axis X on axle  36 , a locking projection  368  for engaging a locking groove  372  formed intermittently in the direction of axis X on axle  36 , a locking recess  376  for engaging a locking ridge  380  formed intermittently in the direction of axis X on axle  36 , a locking projection  384  for engaging a locking recess  388  formed intermittently in the direction of axis X on axle  36 , a locking groove  392  for engaging a locking ridge  396  formed intermittently in the direction of axis X on axle  36 , and a control sleeve support surface  404  for supporting a base sleeve  408  of pawl control sleeve  288  between sun gear guide ring  210  and axle  36 . 
     Sun gear guide ring  234  is fitted between second guide ring contact surface  228  of third sun gear  168  and axle  36 . As shown in FIGS. 3 and 5, in addition to outer peripheral surface  232 , sun gear guide ring  234  includes a locking recess  420  for engaging locking ridge  364  formed on axle  36 , a locking projection  424  for engaging locking groove  372  formed on axle  36 , a locking recess  428  for engaging locking ridge  380  formed on axle  36 , a locking projection  432  for engaging locking recess  388  formed on axle  36 , a locking groove  436  for engaging locking ridge  396  formed on axle  36 , and a control sleeve support surface  440  for supporting base sleeve  408  of pawl control sleeve  288  between sun gear guide ring  234  and axle  36 . 
     Sun gear guide ring  254  is fitted between guide ring contact surface  252  of fourth sun gear  172  and axle  36 . As shown in FIGS. 3 and 6 unlike sun gear guide rings  210  and  234 , sun gear guide ring  254  has a circular inner peripheral surface  444  that is fitted around locking ridges  364 ,  380  and  396  on axle  36 . A portion of inner peripheral surface  444  forms a control sleeve support surface  448  for supporting an end  452  of base sleeve  408  between sun gear guide ring  258  and axle  36 . End  452  of base sleeve  408  terminates in a groove  454  in a washer  456 . 
     Base sleeve  408  of pawl control sleeve  288  is rotatably fitted withing a control sleeve groove  460  formed in the direction of axis X on axle  36  and is supported radially outwardly by sun gear guide rings  210 ,  234  and  258 . Pawl control arms  284 ,  314  and  344  are slidably disposed in control arm grooves  464 ,  468  and  472 , respectively, formed circumferentially in axle  36 . As shown in FIG. 8, pawl control arm  344  includes a recess  480  having tapered side surfaces  484  and  486 , and a recess  490  having tapered side surfaces  492  and  494 . As noted above, inner peripheral surface  342  of pawl control arm  344  contacts pawl control surface  340  of pawl  250 . Thus, since pawl  250  is biased radially outwardly by spring  332 , pawl  250  is held radially inwardly and disengaged from fourth sun gear ratchet teeth  248  whenever inner peripheral surface  342  of pawl control arm  344  contacts pawl control surface  340  except when recess  480  or  490  is aligned with pawl control surface  340 . In that case pawl control surface  340  rises into recess  480  or  490 , and pawl tooth  346  engages one of the fourth sun gear ratchet teeth  248  to nonrotatably couple fourth sun gear  172  to axle  36 . Tapered surfaces  484 ,  486 ,  492  and  494  facilitate the entry and exit of pawl control surface  340  into recesses  480  and  490  as pawl control sleeve  288  rotates around axle  36 . 
     Similarly, as shown in FIG. 9, pawl control arm  314  includes a recess  500  having tapered side surfaces  502  and  504 , and a recess  510  having tapered side surfaces  512  and  514 . As noted above, inner peripheral surface  312  of pawl control arm  314  contacts pawl control surface  310  of pawl  226 . Thus, since pawl  226  is biased radially outwardly by spring  302 , pawl  226  is held radially inwardly and disengaged from third sun gear ratchet teeth  224  whenever inner peripheral surface  312  of pawl control arm  314  contacts pawl control surface  310  except when recess  500  or  510  is aligned with pawl control surface  310 . In that case pawl control surface  310  rises into recess  500  or  510 , and pawl tooth  316  engages one of the third sun gear ratchet teeth  224  to nonrotatably couple third sun gear  168  to axle  36 . Tapered surfaces  502 ,  504 ,  512  and  514  facilitate the entry and exit of pawl control surface  310  into recesses  500  and  510  as pawl control sleeve  288  rotates around axle  36 . 
     Finally, as shown in FIG. 10, pawl control arm  284  includes a recess  520  having tapered side surfaces  522  and  524 . A tapered surface  526  also is disposed at the end  527  of pawl control arm  284 . As noted above, inner peripheral surface  282  of pawl control arm  284  contacts pawl control surface  280  of pawl  207 . Thus, since pawl  207  is biased radially outwardly by spring  272 , pawl  207  is held radially inwardly and disengaged from second sun gear ratchet teeth  206  whenever inner peripheral surface  282  of pawl control arm  284  contacts pawl control surface  280  except when recess  520  is aligned with pawl control surface  280 . In that case pawl control surface  280  rises into recess  520 , and pawl tooth  289  engages one of the second sun gear ratchet teeth  206  to nonrotatably couple second sun gear  164  to axle  36 . Tapered surfaces  522  and  524  facilitate the entry and exit of pawl control surface  280  into recess  520 , and tapered surface  526  facilitates the entry of pawl control surface  280  beneath pawl control arm  284  as pawl control sleeve  288  rotates around axle  36 . 
     FIGS. 11-14 illustrate the operation of pawl control sleeve  288  using second sun gear  164  as an example. In the position shown in FIG. 11, pawl control sleeve  288  is at its most counterclockwise position, pawl  207  is outside pawl control arm  284 , pawl tooth  289  is engaged with second sun gear ratchet teeth  206 , and second sun gear  164  is nonrotatably coupled to axle  36 . 
     When pawl control arm  284  rotates clockwise to the position shown in FIG. 12, pawl control surface  280  slides beneath tapered surface  526  and abuts against inner peripheral surface  282  of pawl control arm  284 . As a result, pawl tooth  289  is held radially inwardly and disengaged from second sun gear ratchet teeth  206 . In this state second sun gear  164  is free to rotate relative to axle  36 . Since base sleeve  408  of pawl control sleeve  288  is fitted between control sleeve support surface  404  of sun gear guide ring  210  and control sleeve groove  460  on axle  36 , torsional and other forces applied to base sleeve  408  do not cause undesirable bending of pawl control sleeve  288 . 
     When pawl control arm  284  rotates further clockwise to the position shown in FIG. 13, pawl control surface  280  enters recess  520 , thus allowing pawl tooth  289  to move radially outwardly into engagement with one of second sun gear ratchet teeth  206 , thus once again nonrotatably coupling second sun gear  164  to axle  36 . Since second sun gear  164  has twelve second sun gear ratchet teeth  206  and only one pawl  207 , the nonrotatable engagement between second sun gear  164  and axle  36  occurs very quickly. In prior art systems that have more than one pawl (typically mounted to the inner peripheral surface of the sun gear) and fewer ratchet teeth (typically formed on the axle), the circumferential distance of the ratchet teeth and the requirement that all of the pawls align with the ratchet teeth causes great delay in the coupling operation. 
     When pawl control arm  284  rotates further clockwise to the position shown in FIG. 14, pawl control surface  280  slides out of recess  520  along tapered surface  522  until pawl control surface  280  is supported radially inwardly by inner peripheral surface  282  of pawl control arm  284 . As a result, pawl tooth  289  is disengaged from second sun gear ratchet teeth  206 , and second sun gear  164  is free to rotate relative to axle  36 . 
     As shown in FIG. 2, power transmitting mechanism  82  further comprises a first planet gear carrier  550 , a first ring gear  551 , a second planet gear carrier  552  and a second ring gear  553 , all of which are rotatably mounted around axle  36 . Planet gear carrier  550  includes a plurality (e.g., 12) of circumferentially disposed clutch engaging splines  554  on its right side for engaging a complementary plurality of planet gear carrier engaging splines  558  formed on a clutch ring  562 , a plurality (e.g., 12) of circumferentially disposed second planet gear carrier engaging splines  564  formed on its left side for engaging complementary circumferentially disposed first planet carrier engaging splines  568  formed on the right side of second planet gear carrier  552 , and a plurality of circumferentially disposed planet gear supporting pins  572 , each of which rotatably supports a planet gear  576 . Each planet gear  576  has a small diameter gear portion  580  (e.g., 14T) engaging the plurality of first sun gear teeth  178  on first sun gear  160 , and a large diameter gear portion  584  (e.g., 22T) engaging a first inner peripheral gear portion  585  (e.g., 84T) of first ring gear  551 . 
     In addition to the inner peripheral gear portion  585 , first ring gear  551  includes a second inner peripheral gear portion  586  (e.g., 36T) for engaging a plurality of circumferentially disposed pawls  587  that are mounted on a corresponding plurality of circumferentially disposed pawl pins  588  that are fixed to driver  70 . Pawls  587  are biased radially outwardly by a pawl spring  589  and thus function as a one-way clutch between driver  70  and first ring gear  551 . A plurality of pawls  590  also are circumferentially disposed on pawl pins  588  for driving first ring gear  551  in the rearward direction in a well known manner. 
     In addition to the plurality of circumferentially disposed first planet gear carrier engaging splines  568  which nonrotatably couple second planet gear carrier  552  to the plurality of second planet gear carrier engaging splines  564  on first planet gear carrier  550 , second planet gear carrier  52  includes a plurality of circumferentially disposed brake roller cage engaging splines  592  for engaging a complementary plurality of second planet carrier engaging splines  596  formed on a brake roller cage  597 , and a plurality (e.g., 18) of circumferentially disposed outer peripheral cam surfaces  904  that engage a corresponding plurality of rollers  900  in coaster brake  86 . 
     Second planet gear carrier  552  also includes a plurality of circumferentially disposed planet gear supporting pins  604 , each of which rotatably supports a planet gear  608 . Each planet gear  608  includes a large diameter gear portion  612  (e.g., 29T) engaging the plurality of fourth sun gear teeth  244  on fourth sun gear  172 , an intermediate diameter gear portion  616  (e.g., 18T) engaging the plurality of third sun gear teeth  236  on third sun gear  168 , and a small diameter gear portion  620  (e.g., 14T) engaging the plurality of second sun gear teeth  198  on second sun gear  164  as well as an inner peripheral gear portion  624  (e.g., 78T) of second ring gear  553 . Second ring gear  553  is coupled to right cup  120 , and hence to hub shell  74 , through a one-way clutch in the form of a roller clutch  628  having, e.g., 18 rollers and cam surfaces. 
     FIG. 15 is an exploded view of shift/assist mechanism  90 , FIG. 16 is an oblique view showing shift/assist mechanism  90  assembled to axle  36 , FIG. 17 is a detailed view showing shift/assist mechanism  90  when clutch ring  562  is coupled to first planet gear carrier  550 , and FIG. 18 is a detailed view showing shift/assist mechanism  90  when clutch ring  562  is uncoupled from first planet gear carrier  550 . As shown in those Figures, shift/assist mechanism  90  includes a ring-shaped shift key member  700 , a shift key member guide  704 , a saver spring  708 , a spring washer  712 , a return spring  716 , a shift sleeve  720 , a spring washer  724 , a pawl support  728 , a shift sleeve  732 , and a pawl control washer  736 . Shift key member  700  includes radially inwardly extending cam followers  740  that extend through clutch cam portion  176  of first sun gear  160  (FIG. 17) into axially extending grooves  744  formed in a side wall  748  of shift key member guide  704 . As shown in FIGS. 17 and 18, clutch cam portion  176  of first sun gear  160  includes a cam surface  749  defining a first cam step  750  and a second cam step  751 . Also, clutch ring  562 . is biased to the left by a clutch bias spring  747 . Thus, when shift key member  700  is in the position shown in FIG. 17, planet gear carrier engaging splines  558  on clutch ring  562  engage clutch engaging splines  554  on first planet gear carrier  550 , and a plurality of circumferentially disposed driver engaging splines  753  on clutch ring  562  nonrotatably engage a complementary plurality of clutch engaging splines  754  on driver  70  so that driver  70 , clutch ring  562  and first planet gear carrier  550  rotate as a unit. However, when shift key member  700  is rotated, cam followers  740  on shift key member  700  move to second cam step  751  on first sun gear  160  as shown in FIG.  18 . In this position planet gear carrier engaging splines  558  on clutch ring  562  disengage from clutch engaging splines  554  on first planet gear carrier  550  so that planet gear carrier  550  no longer is coupled directly to driver  70 . 
     Shift key member guide  704  also includes a shift control sleeve coupling opening  752  for coupling to an end  756  of shift control sleeve  288 . Saver spring  708  and spring washer  712  both are disposed radially inwardly within side wall  748  of shift key member guide  704 , wherein a first end  756  of saver spring  708  is retained to an axially extending spring ledge  760  formed on shift key member guide  704 , and a second end  764  of saver spring  708  is retained to a side edge of a sleeve coupling ledge  768  formed on spring washer  712 . 
     A first end  772  of return spring  716  is retained to a spring ledge  776  formed on spring washer  712 , and a second end  780  of return spring  716  is retained to a spring ledge  784  on spring washer  724 . Spring washer  724  includes radially inwardly extending and diametrically opposed axle engaging projections  792  that are fitted within diametrically opposed axle grooves  796  formed in axle  36  (only one such groove  796  is shown in FIG. 15) so that spring washer  724  is nonrotatably coupled to axle  36 . As a result of the nonrotatable coupling of spring washer  724  on axle  36 , return spring  716  biases spring washer  712  clockwise relative to spring washer  724 . 
     Diametrically opposed left side coupling legs  800  on shift sleeve  720  nonrotatably engage complementary recesses  804  in sleeve coupling ledges  768  on spring washer  712  (only one such sleeve coupling ledge  768  is shown in FIG.  15 ), and diametrically opposed right side coupling legs  808  on shift sleeve  720  extend through central opening  812  in spring washer  724  and nonrotatably engage complementary shift sleeve coupling recesses  816  in pawl support  728 . Thus, spring washer  712 , shift sleeve  720  and pawl support  728  rotate as a unit. 
     Diametrically opposed pawls  820  are rotatably mounted through C-clips  822  to pawl support pins  824  which, in turn, are mounted to spring washer  728 . Pawl bias springs  828  each having one end  832  engaging a spring retaining ledge  836  on pawl support  728  and another end  840  engaging its respective pawl  820  to bias pawl ends  844  radially outwardly. Pawl control washer  736  includes diametrically opposed and axially extending pawl control ledges  850  that ordinarily press pawls  820  radially inwardly. When pawl control ledges  850  move away from pawls  820  as described in more detail below, pawls  820  swing radially outwardly and engage with shift assist teeth  854  (FIG. 17) formed on the inner peripheral surface of driver  70 . 
     Diametrically opposed left side coupling legs  860  on shift sleeve  732  contact the diametrically opposed right side coupling legs  808  on shift sleeve  720  (as shown in FIG.  20 ), and diametrically opposed right side coupling legs  868  on shift sleeve  732  nonrotatably extend through coupling recesses  872  in pawl control washer  736  and though opening  876  in bearing cone  102  and nonrotatably engage complementary shift sleeve coupling recesses  880  in actuator plate  104 . Thus, shift sleeve  732 , pawl control washer  736  and actuator plate  104  rotate as a unit. However, shift sleeve  732  can rotate clockwise relative to shift sleeve  720  and pawl support  728  as discussed more fully below. Since return spring  716  biases spring washer  712  clockwise relative to spring washer  724 , since spring washer  712  is coupled to pawl support  728  through shift sleeve  720 , and since pawl support  728  is coupled to actuator plate  104  through shift sleeve  732 , actuator plate  104  also has a net clockwise bias as shown schematically in FIG.  19 . Given the initial clockwise start position of actuator plate  104 , the transmission paths in power transmitting mechanism  82  are subsequently selected by rotating actuator plate  104  counterclockwise. 
     The coupling of the various components for each speed stage is shown in Table 1, and the power transmission path for each speed stage is shown in Table 2: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Speed 
                 Clutch Ring 
                 Sun Gear 
                 Sun Gear 
                 Sun Gear 
                   
               
               
                 Stage 
                 562 
                 164 
                 168 
                 172 
                 Gear Ratio 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 (Low) 
                 Disengaged 
                 Free 
                 Free 
                 Free 
                 0.53 
               
               
                 2 
                 Disengaged 
                 Free 
                 Free 
                 Locked 
                 0.64 
               
               
                 3 
                 Disengaged 
                 Free 
                 Locked 
                 Free 
                 0.74 
               
               
                 4 
                 Disengaged 
                 Locked 
                 Locked 
                 Free 
                 0.85 
               
               
                 5 
                 Engaged 
                 Free 
                 Free 
                 Free 
                 1.0 
               
               
                 6 
                 Engaged 
                 Free 
                 Free 
                 Locked 
                 1.22 
               
               
                 7 
                 Engaged 
                 Free 
                 Locked 
                 Free 
                 1.42 
               
               
                 8 (High) 
                 Engaged 
                 Locked 
                 Free 
                 Free 
                 1.62 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Speed Stage 
                 Power Transmission Path 
               
               
                   
               
             
             
               
                 1 
                 Driver 70 → Pawl 587 → First Ring Gear 551 → First Planet Gear Carrier 550 
               
               
                   
                 (planet gear 576 rotates around first sun gear 160) → Second Planet Gear 
               
               
                   
                 Carrier 552 → Pawl 908 → Hub Shell 74 
               
               
                 2 
                 Driver 70 → Pawl 587 → First Ring Gear 551 → First Planet Gear Carrier 550 
               
               
                   
                 (planet gear 576 rotates around first sun gear 160) → Second Planet Gear 
               
               
                   
                 Carrier 552 (planet gear 608 rotates around fourth sun gear 172) → Second 
               
               
                   
                 Ring Gear 553 → Roller Clutch 628 → Hub Shell 74 
               
               
                 3 
                 Driver 70 → Pawl 587 → First Ring Gear 551 → First Planet Gear Carrier 550 
               
               
                   
                 (planet gear 576 rotates around first sun gear 160) → Second Planet Gear 
               
               
                   
                 Carrier 552 (planet gear 608 rotates around third sun gear 168) → Second 
               
               
                   
                 Ring Gear 553 → Roller Clutch 628 → Hub Shell 74 
               
               
                 4 
                 Driver 70 → Pawl 587 → First Ring Gear 551 → First Planet Gear Carrier 550 
               
               
                   
                 (planet gear 576 rotates around first sun gear 160) → Second Planet Gear 
               
               
                   
                 Carrier 552 (planet gear 608 rotates around second sun gear 164) → Second 
               
               
                   
                 Ring Gear 553 → Roller Clutch 628 → Hub Shell 74 
               
               
                 5 
                 Driver 70 → Clutch Ring 562 → First Planet Gear Carrier 550 → Second 
               
               
                   
                 Planet Gear Carrier 552 → Pawl 908 → Hub Shell 74 
               
               
                 6 
                 Driver 70 → Clutch Ring 562 → First Planet Gear Carrier 550 → Second 
               
               
                   
                 Planet Gear Carrier 552 (planet Gear 608 rotates around fourth sun gear 
               
               
                   
                 172) → Second Ring Gear 553 → Roller Clutch 628 → Hub Shell 74 
               
               
                 7 
                 Driver 70 → Clutch Ring 562 → First Planet Gear Carrier 550 → Second 
               
               
                   
                 Planet Gear Carrier 552 (planet gear 608 rotates around third sun gear 
               
               
                   
                 168) → Second Ring Gear 553 → Roller Clutch 628 → Hub Shell 74 
               
               
                 8 
                 Driver 70 → Clutch Ring 562 → First Planet Gear Carrier 550 → Second 
               
               
                   
                 Planet Gear Carrier 552 (planet Gear 608 rotates around second sun gear 
               
               
                   
                 164) → Second Ring Gear 553 → Roller Clutch 628 → Hub Shell 74 
               
               
                   
               
             
          
         
       
     
     When shifting from speed stage  4  to speed stage  5 , such as when the bicycle is accelerating, the timing of the coupling mechanisms are set to follow the following sequence: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Clutch Ring 
                 Sun Gear 
                 Sun Gear 
                 Sun Gear 
               
               
                 Speed Stage 
                 562 
                 164 
                 168 
                 172 
               
               
                   
               
             
             
               
                 4 
                 Disengaged 
                 Locked 
                 Locked 
                 Free 
               
               
                 (same as 3) 
                 Disengaged 
                 Free 
                 Locked 
                 Free 
               
               
                 (same as 7) 
                 Engaged 
                 Free 
                 Locked 
                 Free 
               
               
                 5 
                 Engaged 
                 Free 
                 Free 
                 Free 
               
               
                   
               
             
          
         
       
     
     Thus, when the bicycle is accelerating and the cyclist shifts from speed stage  4  to speed stage  5 , the second sun gear  164  is released first to create the same state as speed stage  3 . The rider would perceive this as a slight acceleration of the pedals, but that is expected when the bicycle is accelerating. Then, clutch ring  562  is engaged with first planet gear carrier  550  to create the same state as speed stage  7 . The rider would perceive this as a deceleration of the pedals, which is expected when the transmission shifts to a higher gear ratio. Thereafter, third sun gear  168  is released to produce the desired speed stage  4 . If the sun gears were released before clutch ring  562  were engaged, then the transmission would be in the same state as speed stage  1 , which would produce very undesirable rapid acceleration of the pedals and a significant shock when the transmission completes the shift to speed stage  5 . 
     This sequence also has particular benefit when the bicycle is decelerating and the rider wishes to shift from speed stage  5  to speed stage  4 . In this case, the temporary transition from speed stage  5  to speed stage  7  causes a deceleration of the pedals, but that is far preferable than if clutch ring  562  were disengaged first. If clutch ring  562  were disengaged first, then the transmission would be in the same state as speed stage  1 , with rapid acceleration of the pedals. Such rapid acceleration of the pedals is undesirable when climbing a hill, for example. Thereafter, the transmission makes a temporary transition from speed stage  7  to speed stage  3 . This produces an acceleration of the pedals, but since speed stage  3  is adjacent to speed stage  4  which the cyclist was just in, the transition is much more acceptable. Thereafter, the transmission makes the transition to the desired speed stage  4 . The overall shift from speed stage  5  to speed stage  4  thus avoids excessive acceleration or deceleration of the pedals under conditions when such rapid acceleration or deceleration is least desired. 
     As noted previously, hub transmission  14  also includes a coaster brake  86  for stopping the rotation of hub shell  74  relative to axle  36  when sprocket  54  (and hence driver  70 ) is rotated in a rearward direction. As shown in FIG. 2, coaster brake  86  includes left cup  124  that is nonrotatably supported to hub shell  74  and defines a circumferentially disposed braking surface  890 , a plurality of circumferentially disposed arcuate brake shoes  894  biased radially inwardly from braking surface  890  by a brake spring  898 , roller cage  597  supporting the plurality of circumferentially disposed rollers  900 , and the plurality of cam surfaces  904  disposed circumferentially on the outer peripheral surface of second planet gear carrier  552 . A plurality of circumferentially disposed pawls  908  are mounted to roller cage  597  and are biased radially outwardly by pawl springs  912  for engaging an inner peripheral gear  916  formed on the right side of left cup  124 . Pawls  908  communicate forward rotation of second planet gear carrier  552  to left cup  124  and hence to hub shell  74 . 
     Coaster brake  86  is operated by rotating sprocket  54  in reverse. Planet gear carrier splines  558  on clutch ring  562  and clutch engaging splines  554  on first planet gear carrier  550  are tapered such that, when clutch ring  562  is engaged with first planet gear carrier  550  and sprocket  54  is rotated in reverse to activate coaster brake  86 , clutch ring  562  disengages from first planet gear carrier  550 . Thus, regardless of what speed stage hub transmission  14  is in at the time, the rotational power of driver  70  is communicated along the following path: Driver  70 →Pawls  590 →First Ring Gear  551 →First Planet Gear Carrier  550 →Second Planet Gear Carrier  552 →Brake Roller  900 →Brake Shoe  898 →Braking Surface  890 →Hub Shell  74 . This path produces 1.3 times more braking power than when the transmission path goes through clutch ring  562 . 
     As noted previously, shift/assist mechanism  90  also uses the rotational power of driver  70  to help change the power transmission paths in power transmitting mechanism  82 . This is desirable when significant drive force is applied to sprocket  54  and causes great resistance to the coupling or uncoupling of the various components. During normal operation, actuator plate  104 , shift sleeve  732 , pawl control washer  736 , pawl support  728 , shift sleeve  720 , spring washer  712 , shift key member guide  704  and shift control sleeve  288  rotate as a unit to couple and uncouple the various components. As a result, the positions of shift sleeve  732 , pawl control washer  736 , pawls  820  and shift sleeve  720  are as shown in FIG.  20 . In this state pawls  820  are disengaged from shift assist teeth  854  on driver  70 . However, when significant drive force is applied to sprocket  54  and causes significant resistance to the operation of shift control sleeve  288 , shift control sleeve  288  tends to remain stationary despite rotation of actuator plate  104 . In this case shift sleeve  732  rotates relative to shift sleeve  720 , thus causing pawl control washer  736  to rotate relative to pawl support  728  so that pawl control ledges  850  move away from pawls  820  as shown in FIG.  21 . As a result, pawls  820  rotate radially outwardly and engage shift assist teeth  854  on driver  70  so that pawl support  728  rotates together with driver  70 . This, in turn, provides an assisting force to rotate shift sleeve  720 , shift key member guide  704  and shift control sleeve  288  to complete the shifting operation. When the resistance from shift control sleeve  288  is overcome, pawl support  728  rotates clockwise relative to pawl control washer  736  as shown in FIG. 22 until the shift operation is complete and the state resumes to that shown in FIG.  20 . 
     FIG. 23 is a cross-sectional view of a hub transmission  14 ′ which represents an alternative embodiment of the present invention. Hub transmission  14 ′ is constructed substantially the same as hub transmission  14 , so the identical components are numbered the same. This embodiment differs in that coaster brake  86  and pawls  590  are omitted (a brake disk rotor is mounted to hub shell  74  at mounting holes  950 ), a roller clutch  952  is substituted for pawls  908 , and the structure of a shift/assist mechanism  90 ′ is slightly different from shift/assist mechanism  90  in the first embodiment. FIG. 24 is an exploded view of the relevant portions of shift/assist mechanism  90 ′. In this embodiment, a spring washer  724 ′ is rotatably mounted around axle  36 , and left side legs  860 ′ of shift sleeve  732 ′ engage spring washer  724 ′. As a result, spring  716  causes pawl support  728  and pawl control washer  736  to be biased in opposite directions relative to each other to provide the assist finction, but pawl control ledges  850  abut against pawls  820  to prevent further rotation of pawl control washer  736  relative to pawl support  728 . As a result, there is no net return spring biasing force applied to actuator plate  104 . This structure is useful when actuator plate  104  is rotated by a battery-powered motor, for the unbiased actuator plate  104  does not create significant power drain on the batteries during operation of the motor. 
     While the above is a description of various embodiments of the present invention, further modifications may be employed without departing from the spirit and scope of the present invention. For example, the size, shape, location or orientation of the various components may be changed as desired. The functions of one element may be performed by two, and vice versa. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the scope of the invention should not be limited by the specific structures disclosed or the apparent initial focus on a particular structure or feature.