Rear transmission device of bicycle

Abstract

The utility model discloses a rear transmission device of a bicycle. The rear transmission device comprises a hub, a middle shaft and a transmission. The transmission is provided with a driving piece which drives a planetary gear set to rotate through a tower base. The planetary gear set is provided with a plurality of planetary gears, and each planetary gear is provided with a first planetary gear and a second planetary gear. The first planetary gears are meshed with a first sun gear; and the second planetary gears are meshed with a second sun gear. And the transmission piece is meshed with the first planetary gears so as to drive the hub to rotate. A switching member can move between a first position and a second position. When the switching piece moves to the first position, the switching piece fixes the first sun gear; and when the switching piece moves to the second position, the switching piece fixes the second sun gear.

Description

Technical Field

[0001] The utility model relates to a bicycle, in particular to a rear transmission device of a bicycle which provides an internal speed change function. Background Art

[0002] The general way of changing the speed of a bicycle is usually to adjust the chain and combine different sprockets of the front and rear flywheels, using different gear ratios to produce different transmission ratios. The so-called gear ratio is the ratio of the number of teeth on the front sprocket to the number of teeth on the rear sprocket. For example, if the number of teeth on the front sprocket is 36 and the number of teeth on the rear sprocket is 18, then the gear ratio is 36 / 18=2. A gear ratio of 2 means that when the front sprocket rotates 1 circle, it will drive the rear sprocket to rotate 2 circles. Since the front sprocket is driven by the crank, the rear sprocket will drive the rear wheel to rotate. Generally, when there is no transmission 14, the gear ratio is equal to the transmission ratio. Therefore, in the above example, it can be seen that when the crank rotates 1 circle, it drives the rear wheel to rotate 2 circles.

[0003] In order to allow riders to adjust the transmission efficiency of bicycles according to different riding conditions, most bicycles are equipped with a transmission. The traditional derailleur can drive the chain to move to different toothed discs by changing its position. Due to the different sizes of toothed discs, the weight of pedaling will change, thus achieving the effect of speed change.

[0004] Among them, the known rear sprocket transmission usually needs to be used with multiple sprockets. Through the actuation of the rear sprocket transmission, the chain can be moved between front sprockets with different numbers of teeth. However, if it is a belt drive, it is impossible to set multiple pulleys at the same time. Therefore, how to develop a transmission that can be used with a single pulley / sprocket and does not take up too much space is one of the goals that developers in the relevant field need to solve urgently. Utility Model Content

[0005] The main purpose of the utility model is to provide a rear transmission device of a bicycle, which can be used for a single pulley / toothed disc and has the function of speed change.

[0006] To achieve the above-mentioned purpose of the utility model, the utility model discloses a rear transmission device of a bicycle, comprising a hub having a shell, connected to a wheel, and rotating together with the wheel; a middle shaft, mounted on a frame; the middle shaft passes through the shell of the hub, so that the hub can rotate relative to the middle shaft; a transmission, connected to the shell of the hub, comprising a shell, a freewheel base, a driving member, a planetary gear set, a first sun gear, a second sun gear, a transmission member, and a switching member, wherein one end of the shell is fixed to the hub and covers the hub ratchet teeth; the freewheel base is sleeved on the middle shaft and can rotate relative to the middle shaft. The shaft rotates; the tower base has a tube and a connecting portion; the connecting portion is located in the shell, and the tube has at least a portion extending out of the shell; the driving member is located outside the shell and connected to the tube of the tower base to drive the tower base to rotate; the planetary gear set is arranged in the shell and has a plurality of planetary gears; each planetary gear has a first planetary gear and a second planetary gear connected to each other, and the number of teeth of the first planetary gear is greater than that of the second planetary gear; the first sun gear is arranged in the shell and is sleeved on the central shaft and can rotate relative to the central shaft; the first sun gear meshes with the The first planetary gear; the first sun gear has a first docking portion; the second sun gear is arranged in the housing and is sleeved on the central axis and can rotate relative to the central axis; the second sun gear meshes with the second planetary gear of the planetary gears; the second sun gear has a second docking portion; wherein the number of teeth of the first sun gear is different from that of the second sun gear; the transmission member is arranged in the housing and has a plurality of inner ring teeth to mesh with the first planetary gear of the planetary gears; the transmission member is connected to the outer shell of the hub; and the switching member has a first combining portion and a second combining portion; the switching member is arranged on the central axis, It can move along the central axis between a first position and a second position, but cannot rotate relative to the central axis; when the switching member moves to the first position, the first combining portion is combined with the first docking portion of the first sun gear to fix the first sun gear so that it cannot rotate, and the second combining portion is not combined with the second sun gear, so that the second sun gear can rotate freely; when the switching member moves to the second position, the second combining portion is combined with the second docking portion of the second sun gear to fix the second sun gear so that it cannot rotate, and the first combining portion is not combined with the first sun gear, so that the first sun gear can rotate freely.

[0007] In one embodiment, the surface of the hub shell is provided with a plurality of hub ratchets, and the transmission member is provided with a plurality of ratchets that mesh with the hub ratchets, so that the transmission member can only drive the hub to rotate when rotating in a specific direction.

[0008] In one embodiment, the outer shell of the hub has a groove, and the hub ratchet teeth are disposed on the annular side surface of the groove.

[0009] In one embodiment, a locking ring is further included, which is fixed to the tube portion of the tower base, is located outside the driving member and presses against the driving member, so that the driving member cannot be separated from the tower base.

[0010] In one embodiment, the planetary gear set further has a gear rack, and the planetary gears are pivotally mounted on the gear rack; and the connecting portion of the tower base is connected to the gear rack.

[0011] In one embodiment, the gear rack has an annular wall on which a plurality of gear seats are provided for the planetary gears to be pivotally mounted; in addition, a plurality of gear openings are provided on the ring arm so that a portion of the first planetary gear of the planetary gears extends out of the gear rack from the corresponding gear openings.

[0012] In one embodiment, the gear rack has an annular wall on which a plurality of gear seats are provided for the planetary gears to be pivotally mounted; the connecting portion of the tower base has a plurality of extension arms, each of which is located between two adjacent gear seats and connected to the annular wall.

[0013] In one embodiment, a protruding shaft is provided on the shell of the hub and extends into the gear frame to serve as a rotation pivot of the gear frame.

[0014] In one embodiment, the first docking portion of the first sun gear has a plurality of ratchet teeth, and the first combining portion of the switching member has a plurality of ratchet teeth; when the switching member moves to the first position, the ratchet teeth of the first combining portion mesh with the ratchet teeth of the first docking portion.

[0015] In one embodiment, the second docking portion of the second sun gear has a plurality of ratchet teeth, and the second combining portion of the switching member has a plurality of ratchet teeth; when the switching member moves to the second position, the ratchet teeth of the second combining portion mesh with the ratchet teeth of the second docking portion.

[0016] In one embodiment, the first sun gear is a predetermined distance away from the second sun gear, and the first connecting portion and the second connecting portion of the switching member are both located between the first sun gear and the second sun gear, so that the first connecting portion faces the first docking portion of the first sun gear, and the second connecting portion faces the second docking portion of the second sun gear.

[0017] In one embodiment, the switching member has a tube body, and the tube body is sleeved on the central axis; an annular protrusion is provided on the outer surface of the tube body, and the first connecting portion and the second connecting portion are respectively provided on the opposite end surfaces thereof.

[0018] In one embodiment, the switching member has a tube body and a bolt; the central axis is provided with an elongated hole; the tube body is sleeved on the central axis, so that the bolt passes through the tube body and the elongated hole, and the switching member moves between the first position and the second position.

[0019] In one embodiment, the switch member further has an elastic member, one end of which abuts against the tube body, so that the switch member is normally located at the first position.

[0020] In one embodiment, a stopper is disposed on the central axis and is located outside the tube body, and two ends of the elastic member are respectively against the tube body and the stopper.

[0021] Beneficial effects of the utility model: The rear drive device of the bicycle provided by the utility model adjusts the actuation mode of the first sun gear and the second sun gear by moving the switching member to achieve a multi-speed shifting effect. Compared with the existing rear transmission that requires multiple toothed discs and cannot be used for belt-driven pulleys, the utility model can be applied to a single toothed disc / pulley transmission structure, saving costs and increasing the scope of use. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The figure is a schematic diagram of the appearance of a preferred embodiment of the utility model installed on a bicycle.

[0023] Figure 2 It is a three-dimensional diagram of a preferred embodiment of the utility model.

[0024] Figure 3 This is an exploded view of a preferred embodiment of the utility model.

[0025] Figure 4 It is a partial cross-sectional view of a preferred embodiment of the utility model, showing that the switching member moves to the first position.

[0026] Figure 5 For along Figure 2 The sectional view along the section line 5-5 shows the transmission mode of the switching member moving to the first position.

[0027] Figure 6 It is a partial cross-sectional view of a preferred embodiment of the utility model, showing that the switching member moves to the second position.

[0028] Figure 7 For along Figure 2 The sectional view along line 7-7 shows the transmission mode of the switching member moving to the second position.

[0029] 1 Bicycle

[0030] 2 Frame

[0031] 3 wheels

[0032] 4 Crank Device

[0033] 5. Rear transmission

[0034] 6. Transmission belt

[0035] 10 hubs

[0036] 12-axis

[0037] 14 Transmission

[0038] 16 Shell

[0039] 18 Box

[0040] 20 End cap

[0041] 22 Flange

[0042] 24 Storage Space

[0043] 26 grooves

[0044] 28 shaft hole

[0045] 30 cam shaft

[0046] 32 hub ratchet

[0047] 34 Shell

[0048] 36 Tower Base

[0049] 38 driving parts

[0050] 40 planetary gear set

[0051] 42First sun gear

[0052] 44 Second sun gear

[0053] 46 Transmission parts

[0054] 48 Switching parts

[0055] 50 First box

[0056] 52 second box

[0057] 54 Open

[0058] 56 lugs

[0059] 58 Pipeline

[0060] 60 connection part

[0061] 62 notches

[0062] 64 extension arm

[0063] 66 center hole

[0064] 68 convex part

[0065] 70 Lock Ring

[0066] 72 holes

[0067] 76 gear rack

[0068] 78 planetary gear

[0069] 80 Ring Wall

[0070] 82 gear seat

[0071] 84 gear mouth

[0072] 86First planetary gear

[0073] 88 Second planetary gear

[0074] 90 center opening

[0075] 92 first docking part

[0076] 94 center opening

[0077] 96 Second docking part

[0078] 98 inner ring gear

[0079] 100 recesses

[0080] 102 ratchet

[0081] 104 tube body

[0082] 106 bolts

[0083] 108 elastic parts

[0084] 110 ring convex

[0085] 112 first joint

[0086] 114 Second joint

[0087] 116 Long hole

[0088] 118 stopper DETAILED DESCRIPTION

[0089] See also Figure 1 As shown, a preferred embodiment of the utility model provides a bicycle 1 including a frame 2, two wheels 3, a crank device 4, a rear transmission device 5 and a transmission belt 6. The two wheels 3 are pivotally mounted on the front and rear sides of the frame 2. The crank device 4 is pivotally mounted on the frame 2 and can be operated to generate a torque. The rear transmission device 5 is disposed at the center of the wheel 3 located on the rear side of the frame 2 and can rotate together with the wheel 3 on the rear side. The transmission belt 6 connects the crank device 4 and the rear transmission device 5. Thereby, the torque generated by the crank device 4 can be transmitted to the rear transmission device 5 through the transmission belt 6, so that it generates a corresponding driving torque, so that the wheel 3 rotates by the driving torque, and the bicycle 1 moves forward.

[0090] See also Figures 2 to 4 As shown, the rear transmission device 5 provided by a preferred embodiment of the present invention mainly includes: a hub 10, a middle shaft 12 and a transmission 14. Among them:

[0091] The hub 10 has a shell 16, which is composed of a box body 18 and an end cover 20. The outer side of the box body 18 has two annular flanges 22 for fixing the spokes of the wheel 3. The end cover 20 is a disc fixed to the opening of the box body 18 to close the opening of the box body 18 so that a receiving space 24 is provided in the shell 16. The central axis 12 passes through the shell 16, and a plurality of bearings (not shown) are provided therein so that the shell 16 can rotate relative to the central axis 12. Both ends of the central axis 12 are fixed to the frame 2. In the present embodiment, the hub 10 is a motor hub, that is, a motor (not shown) is provided in the receiving space 24 in the shell 16. The above-mentioned components of the hub 10 are all known, and their detailed structures are not repeated here. The new design of the preferred embodiment of the utility model includes: the outer side of the end cover 20 has a circular groove 26, the bottom center of the groove 26 has an axial hole 28 for the middle shaft 12 to pass through, and a protruding shaft 30 surrounding the axial hole 28. A plurality of hub ratchet teeth 32 are provided on the annular side of the groove 26.

[0092] The transmission 14 has a housing 34 , a freehub 36 , a driving member 38 , a planetary gear set 40 , a first sun gear 42 , a second sun gear 44 , a transmission member 46 , and a switching member 48 .

[0093] The housing 34 is a two-stage shell, having a first box body 50 and a second box body 52. ​​The first box body 50 and the second box body 52 are both circular, and the diameter of the first box body 50 is larger than that of the second box body 52. ​​The open end of the second box body 52 is connected to the bottom end of the first box body 50, and the bottom center of the second box body 52 has an opening 54. The periphery of the first box body 50 is provided with a plurality of lugs 56 near the open end for fixing the housing 34 to the end cover 20 of the hub 10, so that the groove 26 of the end cover 20 is covered by the housing 34.

[0094] The freewheel base 36 has a tube portion 58 and a connecting portion 60. The outer side of the tube portion 58 has a plurality of notches 62. The connecting portion 60 is connected to one end of the tube portion 58 and has five extension arms 64 extending radially outward. The connecting portion 60 is located in the housing 34, and the tube portion 58 extends out of the opening 54 of the housing 34. The central shaft 12 passes through the tube portion 58, and a bearing is disposed therebetween, so that the freewheel base 36 can rotate relative to the central shaft 12.

[0095] The driving member 38 is a pulley in this embodiment, which is driven to rotate in conjunction with the transmission belt (belt). In another embodiment, the driving member is a gear (not shown), which is driven to rotate in conjunction with the transmission belt (chain). A central hole 66 is provided in the center of the driving member 38, and a plurality of protrusions 68 are provided on the periphery of the central hole 66. The driving member 38 is sleeved on the tube portion 58 of the tower base 36, so that the protrusions 68 are combined with the recesses 62 to connect the tower base 36, so that the driving member 38 can drive the tower base 36 to rotate. A locking ring 70 has a central hole 72. The locking ring 70 is sleeved on the tube portion 58 of the tower base 36 in a locking manner, and is located on the outer side of the driving member 38, so that the locking ring 70 is pressed against the driving member 38. In this way, the driving member 38 will not be able to detach from the tower base 36.

[0096] The planetary gear set 40 has a gear carrier 76 and five planetary gears 78. The gear carrier 76 has an annular wall 80, on which five pairs of gear seats 82 are provided, extending from two ends of the annular wall 80 toward the center, and the annular wall 80 is provided with a gear opening 84 between two pairs of the gear seats 82. Each of the planetary gears 78 has a first planetary gear 86 and a second planetary gear 88 connected to each other. The diameter and number of teeth of the first planetary gear 86 are greater than those of the second planetary gear 88. The planetary gears 78 are pivotally mounted on the gear seats 82 through five gear shafts, so that the planetary gears 78 can rotate relative to the gear seats 82. And the first planetary gears 86 of the planetary gears 78 have a portion extending out of the gear carrier 76 from the gear openings 84. The planetary gear set 40 is disposed in the housing 34 and connected to the connecting portion 60 of the freewheeling base 36, so that the freewheeling base 36 can drive the planetary gear set 40 to rotate. In this embodiment, the extension arms 64 of the freewheeling base 36 are respectively located between the two gear seats 82 and are connected to the annular wall 80 by bolts. The first planetary gears 86 are closer to the end cover 20 than the second planetary gears 88. In addition, the convex shaft 30 of the end cover 20 is located between the five gear seats 82 at the same end, forming a rotation pivot of the gear rack 76.

[0097] The first sun gear 42 has a central opening 90, and a first docking portion 92 is provided on one side thereof, surrounding the central opening 90. In the present embodiment, the first docking portion 92 has a plurality of ratchet teeth. The first sun gear 42 is disposed in the housing 34, the central shaft 12 passes through the central opening, and the first sun gear 42 is connected to the central shaft 12 and can rotate freely by means of the setting of the bearing. At this time, the first sun gear 42 is meshed with the first planetary gear 86 of the planetary gears 78.

[0098] The second sun gear 44 has a central opening 94, and a second docking portion 96 is provided on one side thereof, surrounding the central opening 94. In the present embodiment, the second docking portion 96 has a plurality of ratchet teeth. The second sun gear 44 is disposed in the housing 34, the central axis 12 passes through the central opening 94, and the second sun gear 44 is connected to the central axis 12 and can rotate freely by means of the arrangement of the bearing. At this time, the second sun gear 44 is meshed with the second planetary gears 88 of the planetary gears 78. The diameter and number of teeth of the second sun gear 44 are both greater than those of the first sun gear 42, and the two are separated by a predetermined distance, and the second docking portion 96 faces the first docking portion 92. The second sun gear 44 is away from the end cover 20 relative to the first sun gear 42.

[0099] The transmission member 46 is a ring body, and its inner side surface has a plurality of inner ring teeth 98. The transmission member 46 is disposed in the groove 26 of the end cover 20, so that the inner ring teeth 98 mesh with the first planetary gears 86 of the planetary gears 78. The outer side surface of the transmission member 46 is provided with a plurality of recesses 100, in which a plurality of ratchets 102 are disposed to engage with the hub ratchets 32 on the end cover 20. In this way, when the transmission member 46 is driven by the planetary gears 78 to move in a specific direction, the hub 10 can be driven to rotate through the action of the ratchets 102 and the hub ratchets 32, and if the transmission member 46 is driven to rotate in the opposite direction, the transmission member 46 cannot drive the hub 10 to rotate.

[0100] The switching member 48 has a tube body 104, a bolt 106 and an elastic member 108. The outer side surface of the tube body 104 has an annular protrusion 110, and a first coupling portion 112 and a second coupling portion 114 are respectively provided on the opposite end surfaces of the annular protrusion 110. In this embodiment, the first and second coupling portions 112, 114 respectively have a plurality of ratchet teeth. The switching member 48 is disposed in the housing 34, and the tube body 104 passes through the central opening 90 of the first sun gear 42, so that the convex shaft 30 is located between the first sun gear 42 and the second sun gear 44, and further the first coupling portion 112 corresponds to the first docking portion 92, and the second coupling portion 114 corresponds to the second docking portion 96. The central shaft 12 passes through the tube body 104. The central axis 12 is provided with an elongated hole 116, and the bolt 106 passes through the tube 104 and the elongated hole 116, so that the tube 104 can move between a first position and a second position. A stopper 118 is fixed to the central axis 12 and is located outside the tube 104. The elastic member 108 is arranged in the tube 104, and its two ends are respectively against the tube 104 and the stopper 118. The elastic member 108 is used to make the tube 104 normally located in the first position. The tube 104 is connected to a driving member (not shown in the figure, it can be a steel cable) to drive the tube 104 to move from the first position to the second position. When the driving member no longer moves the tube 104, the elastic member 108 will restore the tube 104 to the first position.

[0101] See also Figure 4 and Figure 5 As shown, when the switching member 48 moves to the first position, the first coupling portion 112 will be coupled with the first docking portion 92 of the first sun gear 42, so that the first sun gear 42 is fixed by the switching member 48 and cannot rotate. At this time, when the driving member 38 drives the planetary gear set 40 to rotate through the tower base 36, the planetary gears 78 will rotate around the first sun gear 42, and drive the transmission member 46 to rotate through the first planetary gears 86, thereby driving the hub 10 to rotate. The second sun gear 44 will only be driven to rotate by the second planetary gears 88 and will not participate in the transmission. In this state, the transmission path of the transmission 14 is the first sun gear 42 (fixed end), the first planetary gears 86 (input end) to the transmission member 46 (output end), and finally drives the hub 10 to rotate. The transmission ratio is affected by the number of teeth of each gear involved in the transmission.

[0102] See also Figure 6 and Figure 7As shown, when the switching member 48 moves to the second position, the second coupling portion 114 will be coupled with the second docking portion 96 of the second sun gear 44, so that the second sun gear 44 is fixed by the switching member 48 and cannot rotate. At this time, when the driving member 38 drives the planetary gear set 40 to rotate through the freewheel base 36, the planetary gears 78 will rotate around the second sun gear 44, and drive the transmission member 46 to rotate through the first planetary gears 86, thereby driving the hub 10 to rotate. The first sun gear 42 will only be driven to rotate by the first planetary gears 86 and will not participate in the transmission. In this state, the transmission path of the transmission 14 is the second sun gear 44 (fixed end), the second planetary gears 88 (first input end), the first planetary gears 86 (second input end) to the transmission member 46 (output end), and finally drives the hub 10 to rotate. The transmission ratio is affected by the number of teeth of each gear involved in the transmission.

[0103] In summary, the rear transmission device of the bicycle provided by the utility model adjusts the actuation mode of the first sun gear 42 and the second sun gear 44 through the movement of the switching member 48 to achieve a multi-speed shifting effect. Compared with the conventional rear transmission that requires multiple toothed discs and cannot be used for belt-driven pulleys, the utility model can be applied to a transmission structure of a single rear toothed disc and pulley, saving costs and increasing the scope of use.

[0104] The above are the preferred embodiments and design drawings of the utility model, but the preferred embodiments and design drawings are only examples and are not used to limit the scope of rights of the technical solution of the utility model. Any implementation with equal technical means or within the scope of rights covered by the following "patent application scope" does not depart from the scope of the utility model and is the applicant's scope of rights.

Claims

1. A rear transmission device for a bicycle, characterized in that: Contains: A hub having a housing, connected to a wheel and rotating together with the wheel; A middle shaft is mounted on a bicycle frame; the middle shaft passes through the hub so that the hub can rotate relative to the middle shaft; A transmission connected to the shell of the hub includes a housing, a freehub, a driving member, a planetary gear set, a first sun gear, a second sun gear, a transmission member, and a switching member, wherein One end of the housing is fixed to the hub; The tower base is sleeved on the central axis and can rotate relative to the central axis; the tower base has a tube portion and a connecting portion; the connecting portion is located in the shell, and the tube portion has at least a portion extending out of the shell; The driving member is located outside the housing and connected to the pipe portion of the tower base to drive the tower base to rotate; The planetary gear set is disposed in the housing and has a plurality of planetary gears; each planetary gear has a first planetary gear and a second planetary gear connected to each other, and the number of teeth of the first planetary gear is greater than that of the second planetary gear; The first sun gear is disposed in the housing and sleeved on the central shaft, and can rotate relative to the central shaft; the first sun gear meshes with the first planetary gear of the plurality of planetary gears; the first sun gear has a first docking portion; The second sun gear is disposed in the housing and sleeved on the central shaft, and can rotate relative to the central shaft; the second sun gear meshes with the second planetary gear of the plurality of planetary gears; the second sun gear has a second docking portion; wherein the number of teeth of the first sun gear is different from that of the second sun gear; The transmission member is disposed in the housing and connected to the outer shell of the hub; the transmission member has a plurality of inner ring teeth to mesh with the first planetary gear of the plurality of planetary gears; and The switching member has a first coupling portion and a second coupling portion; the switching member is arranged on the central axis and can move along the central axis between a first position and a second position, but cannot rotate relative to the central axis; when the switching member moves to the first position, the first coupling portion is coupled to the first docking portion of the first sun gear to fix the first sun gear so that it cannot rotate, and the second coupling portion is not coupled to the second sun gear, so that the second sun gear can rotate freely; when the switching member moves to the second position, the second coupling portion is coupled to the second docking portion of the second sun gear to fix the second sun gear so that it cannot rotate, and the first coupling portion is not coupled to the first sun gear, so that the first sun gear can rotate freely.

2. The rear transmission device of a bicycle according to claim 1, characterized in that: The surface of the shell of the hub is provided with a plurality of hub ratchets, and the transmission member is provided with a plurality of ratchets meshing with the plurality of hub ratchets, so that the transmission member can only drive the hub to rotate when rotating in a specific direction.

3. The rear transmission device of a bicycle according to claim 2, characterized in that: The shell of the hub is provided with a groove, and the plurality of hub ratchet teeth are arranged on the annular side surface of the groove.

4. The rear transmission device for a bicycle according to claim 1, characterized in that: The utility model further comprises a locking ring which is fixed to the tube part of the tower base, is located outside the driving member and presses against the driving member so that the driving member cannot be separated from the tower base.

5. The rear transmission device for a bicycle according to claim 1, characterized in that: The planetary gear set further has a gear frame, and the planetary gear is pivotally arranged on the gear frame; the connecting portion of the tower base is connected to the gear frame.

6. The rear transmission device for a bicycle according to claim 5, characterized in that: The gear rack has an annular wall on which a plurality of gear seats are arranged for the planetary gear to be pivotally mounted; in addition, a plurality of gear openings are arranged on the annular wall so that a portion of the first planetary gear of the planetary gear extends out of the gear rack from the corresponding gear openings.

7. The rear transmission device for a bicycle according to claim 5, characterized in that: The gear rack has an annular wall on which a plurality of gear seats are arranged for the planetary gear to be pivotally mounted; the connecting portion of the tower base has a plurality of extension arms, each of which is located between two adjacent gear seats and connected to the annular wall.

8. The rear transmission device for a bicycle according to claim 5, characterized in that: The shell of the hub is provided with a convex shaft which extends into the gear frame to serve as a rotating pivot of the gear frame.

9. The rear transmission device for a bicycle according to claim 5, characterized in that: The shell of the hub is provided with a convex shaft which extends into the gear frame to serve as a rotating pivot of the gear frame.

10. The rear transmission device for a bicycle according to claim 1, characterized in that: The second docking portion of the second sun gear has a plurality of ratchet teeth, and the second combining portion of the switching member has a plurality of ratchet teeth; when the switching member moves to the second position, the ratchet teeth of the second combining portion mesh with the ratchet teeth of the second docking portion.

11. The rear transmission device for a bicycle according to claim 1, characterized in that: The first sun gear is a predetermined distance away from the second sun gear, and the first combining portion and the second combining portion of the switching member are both located between the first sun gear and the second sun gear, so that the first combining portion faces the first docking portion of the first sun gear, and the second combining portion faces the second docking portion of the second sun gear.

12. The rear transmission device of a bicycle as claimed in claim 11, wherein the switching member comprises a tube body, the tube body is sleeved on the central axis; an annular protrusion is provided on the outer surface of the tube body, and the first connecting portion and the second connecting portion are respectively provided on the opposite end surfaces thereof.

13. The rear transmission device of a bicycle as described in claim 1, wherein the switching member has a tube body and a bolt; the middle shaft is provided with an elongated hole; the tube body is sleeved on the middle shaft, so that the bolt passes through the tube body and the elongated hole, thereby allowing the switching member to move between the first position and the second position.

14. The rear transmission device of a bicycle as claimed in claim 13, wherein the switch member further comprises an elastic member, one end of which abuts against the tube body, so that the switch member is normally located at the first position.

15. The rear transmission device of a bicycle as claimed in claim 14, wherein a stopper is disposed on the middle shaft and is located outside the tube body, and two ends of the elastic member are respectively against the tube body and the stopper.

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