Patent Publication Number: US-10309515-B2

Title: Bicycle rear sprocket

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a bicycle rear sprocket. 
     Discussion of the Background 
     Bicycling is becoming an increasingly more popular form of recreation as well as a means of transportation. Moreover, bicycling has become a very popular competitive sport for both amateurs and professionals. Whether the bicycle is used for recreation, transportation or competition, the bicycle industry is constantly improving the various components of the bicycle. One bicycle component that has been extensively redesigned is a sprocket. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, a bicycle rear sprocket comprises a sprocket body, a plurality of sprocket teeth, a shifting facilitation recess, and an upshifting facilitation projection. The plurality of sprocket teeth extends radially outwardly from the sprocket body. The shifting facilitation recess is to facilitate a shifting operation in which a bicycle chain is shifted between the bicycle rear sprocket and a smaller rear sprocket adjacent to the bicycle rear sprocket without another sprocket between the bicycle rear sprocket and the smaller rear sprocket. The upshifting facilitation projection is provided in the shifting facilitation recess to support the bicycle chain in an upshifting operation in which the bicycle chain is shifted from the bicycle rear sprocket to the smaller rear sprocket. 
     With the bicycle rear sprocket according to the first aspect, the upshifting facilitation projection smoothly brings the bicycle chain into engagement with the smaller rear sprocket in the upshifting operation. 
     In accordance with a second aspect of the present invention, the bicycle rear sprocket according to the first aspect is configured so that the upshifting facilitation projection is integrally provided with the sprocket body as a one-piece unitary member. 
     With the bicycle rear sprocket according to the second aspect, it is possible to easily manufacture the bicycle rear sprocket. 
     In accordance with a third aspect of the present invention, the bicycle rear sprocket according to the first aspect is configured so that the upshifting facilitation projection is a separate member from the sprocket body. 
     With the bicycle rear sprocket according to the third aspect, it is possible to improve design freedom of the shape of the upshifting facilitation projection. 
     In accordance with a fourth aspect of the present invention, the bicycle rear sprocket according to any one of the first to third aspects is configured so that the upshifting facilitation projection includes a chain contact surface contactable with the bicycle chain in the upshifting operation. The chain contact surface is inclined relative to a sprocket center plane perpendicular to a rotational center axis of the bicycle rear sprocket. The sprocket center plane bisects the bicycle rear sprocket in an axial direction with respect to the rotational center axis. 
     With the bicycle rear sprocket according to the fourth aspect, the upshifting facilitation projection more smoothly brings the bicycle chain into engagement with the smaller rear sprocket in the upshifting operation. 
     In accordance with a fifth aspect of the present invention, the bicycle rear sprocket according to any one of the first to fourth aspects is configured so that the upshifting facilitation projection includes a chain contact surface contactable with the bicycle chain in the upshifting operation. The chain contact surface extends in an axial direction with respect to a rotational center axis of the bicycle rear sprocket. 
     With the bicycle rear sprocket according to the fifth aspect, the upshifting facilitation projection more smoothly brings the bicycle chain into engagement with the smaller rear sprocket in the upshifting operation. 
     In accordance with a sixth aspect of the present invention, the bicycle rear sprocket according to any one of the first to fifth aspects is configured so that the upshifting facilitation projection includes a chain contact surface contactable with the bicycle chain in the upshifting operation. The chain contact surface includes a flat surface. 
     With the bicycle rear sprocket according to the sixth aspect, the chain contact surface stably supports the bicycle chain in the upshifting operation. 
     In accordance with a seventh aspect of the present invention, the bicycle rear sprocket according to any one of the first to sixth aspects is configured so that the upshifting facilitation projection includes a chain contact surface contactable with the bicycle chain in the upshifting operation. The chain contact surface is inclined relative to a first radial direction perpendicular to a rotational center axis of the bicycle rear sprocket, the first radial direction extending from the rotational center axis to the upshifting facilitation projection. 
     With the bicycle rear sprocket according to the seventh aspect, it is possible to arrange the chain contact surface to extend along the bicycle chain in the upshifting operation. 
     In accordance with an eighth aspect of the present invention, the bicycle rear sprocket according to any one of the first to seventh aspects is configured so that the upshifting facilitation projection includes a chain contact surface contactable with the bicycle chain in the upshifting operation. The chain contact surface includes a first circumferential end and a second circumferential end. The chain contact surface extends between the first circumferential end and the second circumferential end. A first radial distance provided between a rotational center axis of the bicycle rear sprocket and the first circumferential end is different from a second radial distance provided between the rotational center axis and the second circumferential end. 
     With the bicycle rear sprocket according to the eighth aspect, it is possible to arrange the chain contact surface to extend along the bicycle chain in the upshifting operation. 
     In accordance with a ninth aspect of the present invention, the bicycle rear sprocket according to the eighth aspect is configured so that the first circumferential end is provided on an upstream side of the second circumferential end in a driving rotational direction in which the bicycle rear sprocket is rotated about the rotational center axis during pedaling. The first radial distance is smaller than the second radial distance. 
     With the bicycle rear sprocket according to the ninth aspect, it is possible to arrange the chain contact surface to extend along the bicycle chain in the upshifting operation. 
     In accordance with a tenth aspect of the present invention, the bicycle rear sprocket according to any one of the first to ninth aspects is configured so that the plurality of sprocket teeth includes a first tooth and a second tooth. The first tooth and the second tooth are provided in an angular range of the shifting facilitation recess. The upshifting facilitation projection is provided between the first tooth and the second tooth in a circumferential direction with respect to a rotational center axis of the bicycle rear sprocket. 
     With the bicycle rear sprocket according to the tenth aspect, the upshifting facilitation projection more smoothly brings the bicycle chain into engagement with the smaller rear sprocket in the upshifting operation. 
     In accordance with an eleventh aspect of the present invention, the bicycle rear sprocket according to the tenth aspect is configured so that the shifting facilitation recess is continuously provided between the first tooth and the second tooth through the upshifting facilitation projection. 
     With the bicycle rear sprocket according to the eleventh aspect, the upshifting facilitation projection more smoothly brings the bicycle chain into engagement with the smaller rear sprocket in the upshifting operation. 
     In accordance with a twelfth aspect of the present invention, the bicycle rear sprocket according to any one of the first to eleventh aspects is configured so that the plurality of sprocket teeth includes at least one driving tooth. 
     With the bicycle rear sprocket according to the twelfth aspect, the upshifting facilitation projection more smoothly brings the bicycle chain into engagement with the smaller rear sprocket in the upshifting operation. 
     In accordance with a thirteenth aspect of the present invention, the bicycle rear sprocket according to the first aspect is configured so that the plurality of sprocket teeth includes a first driving tooth and a second driving tooth. The first driving tooth has a first maximum axial width extending in an axial direction with respect to a rotational center axis of the bicycle rear sprocket. The second driving tooth has a second maximum axial width extending in the axial direction. The first maximum axial width is larger than the second maximum axial width. 
     With the bicycle rear sprocket according to the thirteenth aspect, it is possible to improve chain-holding performance of the bicycle rear sprocket with smoothening the upshifting operation. 
     In accordance with a fourteenth aspect of the present invention, the bicycle rear sprocket according to any one of the first to thirteenth aspects is configured so that the sprocket body includes a first axial surface and a second axial surface provided on a reverse side of the first axial surface in an axial direction with respect to a rotational center axis of the bicycle rear sprocket. An axial end of the upshifting facilitation projection is provided in an axial area extending from the first axial surface to the second axial surface. 
     With the bicycle rear sprocket according to the fourteenth aspect, the upshifting facilitation projection more smoothly brings the bicycle chain into engagement with the smaller rear sprocket in the upshifting operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
         FIG. 1  is a schematic diagram of a bicycle drive train including a bicycle rear sprocket in accordance with a first embodiment. 
         FIG. 2  is a side elevational view of a bicycle sprocket assembly of the bicycle drive train illustrated in  FIG. 1 . 
         FIG. 3  is a front view of the bicycle sprocket assembly illustrated in  FIG. 2 . 
         FIG. 4  is a side elevational view of the bicycle rear sprocket of the bicycle sprocket assembly illustrated in  FIG. 2 . 
         FIG. 5  is a partial side elevational view of the bicycle rear sprocket illustrated in  FIG. 4 . 
         FIG. 6  is a partial perspective view of the bicycle rear sprocket illustrated in  FIG. 4 . 
         FIG. 7  is a cross-sectional view of the bicycle rear sprocket taken along line VII-VII of  FIG. 5 . 
         FIG. 8  is a cross-sectional view of the bicycle rear sprocket taken along line VII-VII of  FIG. 5  (modification). 
         FIG. 9  is a cross-sectional view of the bicycle rear sprocket taken along line VII-VII of  FIG. 5  (another modification). 
         FIG. 10  is a cross-sectional view of the bicycle rear sprocket taken along line X-X of  FIG. 5 . 
         FIG. 11  is a cross-sectional view of the bicycle rear sprocket taken along line XI-XI of  FIG. 5 . 
         FIG. 12  is a cross-sectional view of the bicycle rear sprocket taken along line XII-XII of  FIG. 14 . 
         FIG. 13  is a cross-sectional view of the bicycle rear sprocket taken along line XII-XII of  FIG. 14  (modification). 
         FIG. 14  is an enlarged partial side elevational view of the bicycle rear sprocket illustrated in  FIG. 4 . 
         FIG. 15  is a partial perspective view of the bicycle rear sprocket illustrated in  FIG. 4 . 
         FIG. 16  is a partial perspective view of the bicycle rear sprocket illustrated in  FIG. 4 , with a bicycle chain. 
         FIG. 17  is a partial perspective view of the bicycle rear sprocket illustrated in  FIG. 4 , with the bicycle chain. 
         FIG. 18  is a side elevational view of a bicycle rear sprocket in accordance with a second embodiment. 
         FIG. 19  is a cross-sectional view of the bicycle rear sprocket taken along line XIX-XIX of  FIG. 18 . 
         FIG. 20  is a cross-sectional view of the bicycle rear sprocket taken along line XX-XX of  FIG. 18 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The embodiment(s) will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. 
     First Embodiment 
     Referring initially to  FIG. 1 , a bicycle drive train  10  comprises a bicycle sprocket assembly  12 , a bicycle front sprocket  14 , and a bicycle chain  16 . The bicycle sprocket assembly  12  is attached to a bicycle rear hub assembly  18 . The bicycle rear hub assembly  18  is mounted to a bicycle frame BF. The bicycle sprocket assembly  12  has a rotational center axis A 1  and is rotatably supported by the bicycle rear hub assembly  18  relative to the bicycle frame BF about the rotational center axis A 1 . The bicycle chain  16  is engaged with the bicycle front sprocket  14  and the bicycle sprocket assembly  12  to transmit a driving rotational force F 1  between the bicycle front sprocket  14  and the bicycle sprocket assembly  12 . The bicycle front sprocket  14  comprised a solitary bicycle front sprocket. In this embodiment, the bicycle sprocket assembly  12  is a bicycle rear sprocket assembly. However, the structure of the bicycle sprocket assembly  12  can be applied to a front sprocket assembly in a case where the front sprocket assembly includes at least two front sprockets. A total number of the bicycle front sprocket  14  is not limited to this embodiment. 
     In the present application, the following directional terms “front,” “rear,” “forward,” “rearward,” “left,” “right,” “transverse,” “upward” and “downward” as well as any other similar directional terms refer to those directions which are determined on the basis of a user (e.g., a rider) who sits on a saddle (not shown) of a bicycle with facing a handlebar (not shown). Accordingly, these terms, as utilized to describe the bicycle sprocket assembly  12 , should be interpreted relative to the bicycle equipped with the bicycle sprocket assembly  12  as used in an upright riding position on a horizontal surface. 
     As seen in  FIG. 2 , the bicycle sprocket assembly  12  is engaged with the bicycle chain  16  to transmit the driving rotational force F 1  between the bicycle sprocket assembly  12  and the bicycle front sprocket  14  ( FIG. 1 ). The bicycle sprocket assembly  12  is rotatable about the rotational center axis A 1  in a driving rotational direction D 11  during pedaling. A reverse rotational direction D 12  is opposite to the driving rotational direction D 11 . The driving rotational direction D 11  and the reverse rotational direction D 12  extend along a circumferential direction D 1  of the bicycle sprocket assembly  12 . 
     The bicycle sprocket assembly  12  comprises bicycle rear sprockets SS 1  to SS 12 . Each of the bicycle rear sprockets SS 1  to SS 12  is engageable with the bicycle chain  16 . The bicycle rear sprockets SS 1  to SS 12  respectively have first to twelfth speed stages of the bicycle drive train  10 . The bicycle rear sprocket SS 1  corresponds to the first speed stage (i.e., low gear) of the bicycle sprocket assembly  12  and has a largest pitch-circle diameter among the bicycle rear sprockets SS 1  to SS 12 . The bicycle rear sprocket SS 12  corresponds to the twelfth speed stage (i.e., top gear) of the bicycle sprocket assembly  12  and has a smallest pitch-circle diameter among the bicycle rear sprockets SS 1  to SS 12 . A total number of bicycle rear sprockets of the bicycle sprocket assembly  12  is not limited to this embodiment. 
     As seen in  FIG. 3 , the bicycle sprocket assembly  12  further comprises spacers SP 1  to SP 11 . The spacers SP 1  to SP 11  are respectively provided between adjacent two sprockets of the bicycle rear sprockets SS 1  to SS 12  in the axial direction D 2 . However, at least one of the spacers SP 1  to SP 11  can be omitted from the bicycle sprocket assembly  12 . 
     In the bicycle sprocket assembly  12 , an upshift occurs when the bicycle chain  16  is moved from a large sprocket to the next small sprocket by a derailleur DR in an upshifting direction D 31 . In the bicycle sprocket assembly  12 , a downshift occurs when the bicycle chain  16  is moved from a small sprocket to the next large sprocket in a downshifting direction D 32 . 
     The bicycle rear sprocket SS 8  will be described in detail below. In the following description, the bicycle rear sprocket SS 9  is referred to as a smaller rear sprocket SS 9 . The bicycle rear sprockets SS 1  to SS 7  and SS 9  to SS 12  have substantially the same structure as that of the bicycle rear sprocket SS 8 . Thus, they will not be described in detail here for the sake of brevity. 
     As seen in  FIG. 4 , the bicycle rear sprocket SS 8  comprises a sprocket body  20  and a plurality of sprocket teeth  22  extending radially outwardly from the sprocket body  20 . The sprocket body  20  includes a hub engagement structure  24  to engage with the bicycle rear hub assembly  18 . The plurality of sprocket teeth  22  includes at least one driving tooth  26 . In this embodiment, the plurality of sprocket teeth  22  includes a plurality of driving teeth  26 . The driving tooth  26  is engageable with the bicycle chain  16  to transmit the driving rotational force F 1  between the bicycle rear sprocket SS 8  and the bicycle chain  16 . A total number of the sprocket teeth  22  is 18. However, the total number of the sprocket teeth  22  is not limited to this embodiment. 
     As seen in  FIG. 4 , the bicycle rear sprocket SS 8  comprises a shifting facilitation recess  28  to facilitate a shifting operation in which the bicycle chain  16  ( FIG. 3 ) is shifted between the bicycle rear sprocket SS 8  and the smaller rear sprocket SS 9  ( FIG. 3 ) adjacent to the bicycle rear sprocket SS 8  without another sprocket between the bicycle rear sprocket SS 8  and the smaller rear sprocket SS 9  ( FIG. 3 ). In this embodiment, the bicycle rear sprocket SS 8  comprises a plurality of shifting facilitation recesses  28 . However, a total number of the shifting facilitation recesses  28  is not limited to this embodiment. The shifting facilitation recess  28  is provided on the sprocket body  20  to facilitate the shifting operation. 
     As seen in  FIG. 5 , the bicycle rear sprocket SS 8  comprises an upshifting facilitation projection  30  provided in the shifting facilitation recess  28  to support the bicycle chain  16  in an upshifting operation in which the bicycle chain  16  is shifted from the bicycle rear sprocket SS 8  to the smaller rear sprocket SS 9 . A total number of the upshifting facilitation projection  30  is not limited to this embodiment. The bicycle rear sprocket SS 8  can comprise a plurality of upshifting facilitation projections  30 . 
     The plurality of sprocket teeth  22  includes a first tooth  32  and a second tooth  34 . The first tooth  32  and the second tooth  34  are provided in an angular range RG 1  of the shifting facilitation recess  28 . The upshifting facilitation projection  30  is provided between the first tooth  32  and the second tooth  34  in the circumferential direction D 1  with respect to the rotational center axis A 1  of the bicycle rear sprocket SS 8 . 
     In this embodiment, the first tooth  32  is engageable with the bicycle chain  16  to transmit the driving rotational force F 1  between the bicycle rear sprocket SS 8  and the bicycle chain  16 . The second tooth  34  is engageable with the bicycle chain  16  to transmit the driving rotational force F 1  between the bicycle rear sprocket SS 8  and the bicycle chain  16 . The second tooth  34  is provided on an upstream side of the first tooth  32  in the driving rotational direction D 11 . The second tooth  34  is adjacent to the first tooth  32  in the circumferential direction D 1  without another tooth between the first tooth  32  and the second tooth  34 . 
     The upshifting facilitation projection  30  is provided radially inwardly of a tooth bottom TB 1  provided between the first tooth  32  and the second tooth  34 . The upshifting facilitation projection  30  is provided on a reference line RL when viewed along the rotational center axis A 1 . The reference line RL extends from the rotational center axis A 1  to the tooth bottom TB 1 . 
     As seen in  FIG. 5 , the plurality of sprocket teeth  22  includes a third tooth  36  and a fourth tooth  38 . The third tooth  36  and the fourth tooth  38  are provided in the angular range RG 1  of the shifting facilitation recess  28 . The third tooth  36  is engageable with the bicycle chain  16  to transmit the driving rotational force F 1  between the bicycle rear sprocket SS 8  and the bicycle chain  16 . The fourth tooth  38  is engageable with the bicycle chain  16  to transmit the driving rotational force F 1  between the bicycle rear sprocket SS 8  and the bicycle chain  16 . The fourth tooth  38  is provided on an upstream side of the third tooth  36  in the driving rotational direction D 11 . The fourth tooth  38  is adjacent to the third tooth  36  in the circumferential direction D 1  without another tooth between the third tooth  36  and the fourth tooth  38 . 
     The third tooth  36  is provided on an upstream side of the second tooth  34  in the driving rotational direction D 11 . The third tooth  36  is adjacent to the second tooth  34  in the circumferential direction D 1  without another tooth between the second tooth  34  and the third tooth  36 . At least one of the third tooth  36  and the fourth tooth  38  can be omitted from the sprocket teeth  22 . 
     As seen in  FIG. 6 , the shifting facilitation recess  28  is continuously provided between the first tooth  32  and the second tooth  34  through the upshifting facilitation projection  30 . In this embodiment, the shifting facilitation recess  28  is continuously provided between the first tooth  32  and the fourth tooth  38  through the upshifting facilitation projection  30  in the circumferential direction D 1 . 
     As seen in  FIG. 7 , the sprocket body  20  includes a first axial surface  20 A and a second axial surface  20 B provided on a reverse side of the first axial surface  20 A in the axial direction D 2  with respect to the rotational center axis A 1  of the bicycle rear sprocket SS 8 . The first axial surface  20 A faces toward the smaller rear sprocket SS 9  in the axial direction D 2 . The second axial surface  20 B faces toward an opposite side of the smaller rear sprocket SS 9  in the axial direction D 2 . The sprocket body  20  has a sprocket center plane CP 1  perpendicular to the rotational center axis A 1  of the bicycle rear sprocket SS 8 . The sprocket center plane CP 1  bisects the bicycle rear sprocket SS 8  in the axial direction D 2  with respect to the rotational center axis A 1 . The sprocket body  20  has a maximum axial width W 1  defined between the first axial surface  20 A and the second axial surface  20 B in the axial direction D 2 . The sprocket center plane CP 1  bisects the maximum axial width W 1 . 
     The shifting facilitation recess  28  is provided on the first axial surface  20 A to facilitate the shifting operation. The shifting facilitation recess  28  includes a side surface  28 A. The side surface  28 A faces toward the smaller rear sprocket SS 9  in the axial direction D 2 . The side surface  28 A is offset from the first axial surface  20 A toward the second axial surface  20 B in the axial direction D 2 . The side surface  28 A is provided between the first axial surface  20 A and the sprocket center plane CP 1  in the axial direction D 2 . 
     As seen in  FIG. 7 , the first tooth  32  includes a first surface  32 A and a first additional surface  32 B. The first surface  32 A faces toward the smaller rear sprocket SS 9  in the axial direction D 2 . The first surface  32 A is offset from the first axial surface  20 A toward the second axial surface  20 B in the axial direction D 2 . The first surface  32 A is provided at the same axial position as an axial position of the side surface  28 A. The first additional surface  32 B faces toward an opposite side of the smaller rear sprocket SS 9  in the axial direction D 2 . The first additional surface  32 B is provided at the same axial position as an axial position of the second axial surface  20 B. 
     The first tooth  32  has a first maximum axial width W 21 . The first maximum axial width W 21  is provided between the first surface  32 A and the first additional surface  32 B in the axial direction D 2 . The bicycle chain  16  includes an opposed pair of outer link plates  16 A and an opposed pair of inner link plates  16 B. The first tooth  32  is engageable in an outer link space  16 A 1  provided between the opposed pair of outer link plates  16 A. The first tooth  32  is engageable in an inner link space  16 B 1  provided between the opposed pair of inner link plates  16 B. 
     An axial end  30 A of the upshifting facilitation projection  30  is provided in an axial area AR 1  extending from the first axial surface  20 A to the second axial surface  20 B. In this embodiment, the axial end  30 A of the upshifting facilitation projection  30  is provided at the same axial position as the axial position of the first axial surface  20 A in the axial direction D 2 . As seen in  FIG. 8 , however, the axial end  30 A of the upshifting facilitation projection  30  can be provided closer to the second axial surface  20 B than the first axial surface  20 A in the axial direction D 2 . As seen in  FIG. 9 , the axial end  30 A of the upshifting facilitation projection  30  can be provided outside the axial area AR 1 . 
     As seen in  FIG. 10 , the second tooth  34  includes a second surface  34 A and a second additional surface  34 B. The second surface  34 A faces toward the smaller rear sprocket SS 9  in the axial direction D 2 . The second surface  34 A is offset from the first axial surface  20 A toward the second axial surface  20 B in the axial direction D 2 . The second surface  34 A is provided at the same axial position as an axial position of the side surface  28 A. The second additional surface  34 B faces toward an opposite side of the smaller rear sprocket SS 9  in the axial direction D 2 . The second additional surface  34 B is provided at the same axial position as an axial position of the second axial surface  20 B. 
     The second tooth  34  has a second maximum axial width W 22 . The second maximum axial width W 22  is provided between the second surface  34 A and the second additional surface  34 B in the axial direction D 2 . The second tooth  34  is engageable in the outer link space  16 A 1 . The second tooth  34  is engageable in the inner link space  16 B 1 . 
     As seen in  FIG. 11 , the driving tooth  26  includes a tooth surface  26 A and an additional tooth surface  26 B. The tooth surface  26 A faces toward the smaller rear sprocket SS 9  in the axial direction D 2 . The tooth surface  26 A is provided at the same axial position as an axial position of the first axial surface  20 A. The additional tooth surface  26 B faces toward an opposite side of the smaller rear sprocket SS 9  in the axial direction D 2 . The additional tooth surface  26 B is provided at the same axial position as an axial position of the second axial surface  20 B. Namely, the driving tooth  26  has a maximum axial width W 3  equal to the maximum axial width W 1  of the sprocket body  20 . 
     The third tooth  36  and the fourth tooth  38  have substantially the same structure as that of the first tooth  32  or the second tooth  34 . Thus, they will not be described in detail here for the sake of brevity. 
     As seen in  FIG. 4 , the bicycle rear sprocket SS 8  comprises a plurality of shifting facilitation recesses  28 . The plurality of sprocket teeth  22  includes a plurality of first teeth  32 , a plurality of second teeth  34 , and a plurality of third teeth  36 . The first tooth  32 , the second tooth  34 , and the third tooth  36  are provided in each of the shifting facilitation recesses  28 . The fourth tooth  38  is provided in one of the shifting facilitation recess  28  (the shifting facilitation recess  28 X). However, a total number of the shifting facilitation recesses  28  is not limited to this embodiment. A total number of the first teeth  32  is not limited to this embodiment. A total number of the second teeth  34  is not limited to this embodiment. A total number of the third teeth  36  is not limited to this embodiment. 
     In this embodiment, the upshifting facilitation projection  30  is provided between one of the first teeth  32  (the first tooth  32 X) and one of the second teeth  34  (the second tooth  34 X) in the circumferential direction D 1  with respect to the rotational center axis A 1  of the bicycle rear sprocket SS 8 . However, the bicycle rear sprocket SS 8  can comprises a plurality of upshifting facilitation projections  30 , and another of the upshifting facilitation projection  30  can be provided in another of the shifting facilitation recesses  28 . 
     As seen in  FIG. 12 , the upshifting facilitation projection  30  extends from the side surface  28 A of the shifting facilitation recess  28  in the axial direction D 2 . The upshifting facilitation projection  30  is integrally provided with the sprocket body  20  as a one-piece unitary member. As seen in  FIG. 13 , however, the upshifting facilitation projection  30  can be a separate member from the sprocket body  20 . In this embodiment, the sprocket body  20  and the upshifting facilitation projection  30  are made of a metallic material such as iron, titanium, and aluminum. 
     As seen in  FIG. 12 , the upshifting facilitation projection  30  includes a chain contact surface  30 B contactable with the bicycle chain  16  in the upshifting operation. The chain contact surface  30 B extends from the side surface  28 A of the shifting facilitation recess  28  in the axial direction D 2 . In this embodiment, the chain contact surface  30 B is inclined relative to the sprocket center plane CP 1  perpendicular to the rotational center axis A 1  of the bicycle rear sprocket SS 8 . However, the chain contact surface  30 B can be perpendicular to the sprocket center plane CP 1 . 
     As seen in  FIG. 14 , the chain contact surface  30 B is inclined relative to a first radial direction D 4  perpendicular to the rotational center axis A 1  of the bicycle rear sprocket SS 8 . The first radial direction D 4  extends from the rotational center axis A 1  to the upshifting facilitation projection  30 . In this embodiment, the first radial direction D 4  is parallel to the reference line RL. 
     The chain contact surface  30 B includes a first circumferential end  30 B 1  and a second circumferential end  30 B 2 . The chain contact surface  30 B extends between the first circumferential end  30 B 1  and the second circumferential end  30 B 2 . In this embodiment, the first circumferential end  30 B 1  is provided on an upstream side of the second circumferential end  30 B 2  in the driving rotational direction D 11  in which the bicycle rear sprocket SS 8  is rotated about the rotational center axis A 1  during pedaling. 
     As seen in  FIG. 14 , a first radial distance RD 1  provided between the rotational center axis A 1  of the bicycle rear sprocket SS 8  and the first circumferential end  30 B 1  is different from a second radial distance RD 2  provided between the rotational center axis A 1  and the second circumferential end  30 B 2 . The first radial distance RD 1  is smaller than the second radial distance RD 2 . However, the first radial distance RD 1  can be equal to or larger than the second radial distance RD 2 . 
     In this embodiment, as seen in  FIG. 15 , the chain contact surface  30 B includes a flat surface. The axial end  30 A includes a flat surface. However, at least one of the axial end  30 A and the chain contact surface  30 B can include a curved surface instead of or in addition to the flat surface. 
     As seen in  FIG. 16 , the bicycle chain  16  is shifted toward the smaller rear sprocket SS 9  in the axial direction D 2  by the derailleur DR in the upshifting operation. For example, the bicycle chain  16  (the opposed pair of inner link plates  16 B 2 ) is first derailed at the first tooth  32  from the bicycle rear sprocket SS 8 . 
     As seen in  FIG. 17 , the outer link plate  16 B 2  of the bicycle chain  16  is lifted by the upshifting facilitation projection  30  when the bicycle rear sprocket SS 8  further rotates in the driving rotational direction D 11  after the inner link plates  16 B 2  is first derailed at the first tooth  32 . The bicycle chain  16  extends along a route RT 1  in a state where the outer link plate  16 B 2  of the bicycle chain  16  is lifted by the upshifting facilitation projection  30 . The route RT 1  is longer than a route RT 2  along which the bicycle chain  16  extends without the upshifting facilitation projection  30 . Thus, the upshifting facilitation projection  30  smoothly brings the bicycle chain  16  into engagement with the smaller rear sprocket SS 9  in the upshifting operation in which the bicycle chain  16  is shifted from the bicycle rear sprocket SS 8  to the smaller rear sprocket SS 9 . 
     Second Embodiment 
     A bicycle rear sprocket SS 208  in accordance with a second embodiment will be described below referring to  FIGS. 18 to 20 . The bicycle rear sprocket SS 208  has the same structure and/or configuration as those of the bicycle rear sprocket SS 8  except for the driving tooth  26 . Thus, elements having substantially the same function as those in the first embodiment will be numbered the same here, and will not be described and/or illustrated again in detail here for the sake of brevity. 
     As seen in  FIG. 18 , in the bicycle rear sprocket SS 208 , the plurality of sprocket teeth  22  includes a first driving tooth  226  and a second driving tooth  227 . In this embodiment, the plurality of sprocket teeth  22  includes a plurality of first driving teeth  226  and a plurality of second driving teeth  227 . However, a total number of the first driving teeth  226  is not limited to this embodiment. A total number of the second driving teeth  227  is not limited to this embodiment. 
     As seen in  FIG. 19 , the first driving tooth  226  has a first maximum axial width W 231  extending in the axial direction D 2  with respect to the rotational center axis A 1  of the bicycle rear sprocket SS 208 . As seen in  FIG. 20 , the second driving tooth  227  has a second maximum axial width W 232  extending in the axial direction D 2 . The first maximum axial width W 231  is larger than the second maximum axial width W 232 . The first maximum axial width W 231  is larger than an axial width of the inner link space  16 B 1  of the bicycle chain  16 . The first maximum axial width W 231  is smaller than an axial width of the outer link space  16 A 1  of the bicycle chain  16 . 
     The structures of the first driving tooth  226  and the second driving tooth  227  can be applied to other sprockets SP 1  to SP 7  and SP 9  to SP 12 . 
     The term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. This concept also applies to words of similar meaning, for example, the terms “have,” “include” and their derivatives. 
     The terms “member,” “section,” “portion,” “part,” “element,” “body” and “structure” when used in the singular can have the dual meaning of a single part or a plurality of parts. 
     The ordinal numbers such as “first” and “second” recited in the present application are merely identifiers, but do not have any other meanings, for example, a particular order and the like. Moreover, for example, the term “first element” itself does not imply an existence of “second element,” and the term “second element” itself does not imply an existence of “first element.” 
     The term “pair of,” as used herein, can encompass the configuration in which the pair of elements have different shapes or structures from each other in addition to the configuration in which the pair of elements have the same shapes or structures as each other. 
     The terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. 
     Finally, teams of degree such as “substantially,” “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. All of numerical values described in the present application can be construed as including the terms such as “substantially,” “about” and “approximately.” 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.