Patent Publication Number: US-2016236750-A1

Title: Bicycle drive system

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention generally relates to a bicycle drive system. More specifically, the present invention relates to a sprocket assembly for a bicycle drive system. 
     2. Background Information 
     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 area that has been extensively redesigned over the years is the bicycle drive system. Specifically, manufacturers of bicycle components have been continually improving shifting performance of the various shifting components, such as shifters, derailleurs, chain and sprockets. 
     One particular component of the bicycle drive system that has been extensively redesigned in the past years is the sprocket assembly. Specifically, sprocket assemblies have been designed with improved sprockets to provide smoother shifting. 
     In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved sprocket assembly of a bicycle drive system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
     SUMMARY 
     Generally, the present disclosure is directed to various features of a bicycle drive system. An object of the present invention is to provide a bicycle drive system having a sprocket assembly to facilitate smooth, reliable shifting performance. 
     In view of the state of the know technology and in accordance with a first aspect of the present invention, a bicycle drive system is provided including a front sprocket assembly having a first front sprocket and a second front-sprocket. The first front-sprocket has a first front-tooth number that is the largest tooth number in the front sprocket assembly. The second front-sprocket has a second front-tooth number that is less than or equal to the first front-tooth number. The second front-sprocket is adjacent to the first front-sprocket without another sprocket intervening therebetween in an axial direction. The first front-tooth number is less than or equal to 40. 
     In accordance with a second aspect of the present invention, the bicycle drive system according to the first aspect is configured such that the first front-tooth number divided by the second front-tooth number is less than or equal to 1.4. 
     In accordance with a third aspect of the present invention, the bicycle drive system according to the first aspect is configured such that the first front-tooth number divided by the second front-tooth number is less than or equal to 1.2. 
     In accordance with a fourth aspect of the present invention, the bicycle drive system according to the first aspect is configured such that the first front-tooth number is less than or equal to 34. 
     In accordance with a fifth aspect of the present invention, the bicycle drive system according to the fourth aspect is configured such that the second front-tooth number is less than or equal to 30. 
     In accordance with a sixth aspect of the present invention, the bicycle drive system according to the first aspect is configured such to include a rear sprocket assembly including a first rear-sprocket having a first rear-tooth number less than or equal to 10 and a second rear-sprocket having a second rear-tooth number greater than or equal to 44. 
     In accordance with a seventh aspect of the present invention, the bicycle drive system according to the first aspect is configured such that the rear sprocket assembly includes at least 5 additional rear-sprockets positioned between the first rear-sprocket and the second rear-sprocket in an axial direction parallel to a rotational central axis of the rear sprocket assembly. 
     In accordance with an eighth aspect of the present invention, the bicycle drive system according to the seventh aspect is configured such that the rear sprocket assembly has a total of seven rear-sprockets. 
     In accordance with a ninth aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that the first rear-tooth number is 10, and the second rear-tooth number is 46. 
     In accordance with a tenth aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that the first rear-tooth number is 10, and the second rear-tooth number is 50. 
     In accordance with an eleventh aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that the first front-tooth number is 34, and the second front-tooth number is 30. 
     In accordance with a twelfth aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that the first front-tooth number is 32, and the second front-tooth number is 28. 
     In accordance with a thirteenth aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that the second rear-sprocket includes at least one shift assist projection. 
     In accordance with a fourteenth aspect of the present invention, the bicycle drive system according to the thirteenth aspect is configured such that the second rear-sprocket includes a plurality of shift assist projections. 
     In accordance with a fifteenth aspect of the present invention, the bicycle drive system according to the sixth aspect is configured to include a front shifting device disposed adjacent to the front sprocket assembly and a rear shifting device disposed adjacent to the rear sprocket assembly. 
     In accordance with a sixteenth aspect of the present invention, the bicycle drive system according to the fifteenth aspect is configured such that at least one of the front shifting device and the rear shifting device is configured to be operated by rotation of a bicycle crank arm. 
     In accordance with a seventeenth aspect of the present invention, the bicycle drive system according to the fifteenth aspect is configured such that at least one of the front shifting device and the rear shifting device is configured to be operated by movement of a bicycle control cable. 
     In accordance with an eighteenth aspect of the present invention, the bicycle drive system according to the fifteenth aspect is configured such that at least one of the front shifting device and the rear shifting device is configured to be operated in accordance with a prescribed shifting route. 
     In accordance with a nineteenth aspect of the present invention, the bicycle drive system according to the eighteenth aspect is configured such that the prescribed shifting route includes at least one of a prescribed up-shifting route and a prescribed down-shifting route. 
     In accordance with a twentieth aspect of the present invention, the bicycle drive system according to the nineteenth aspect is configured such that the prescribed shifting route includes the prescribed up-shifting route and the prescribed down-shifting route. 
     In accordance with a twenty-first aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that the first front-tooth number divided by the second front-tooth number is less than or equal to a large rear-sprocket tooth number of a large rear sprocket divided by a small rear-sprocket number of a small rear sprocket adjacent to the large rear sprocket without another rear sprocket intervening therebetween in the axial direction. 
     In accordance with a twenty-second aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that the first front-tooth number divided by the second front-tooth number is less than a large rear-sprocket tooth number of a large rear sprocket divided by a small rear-sprocket number of a small rear sprocket adjacent to the large rear sprocket without another rear sprocket intervening therebetween in the axial direction. 
     In accordance with a twenty-third aspect of the present invention, the bicycle drive system according to the sixth aspect is configured such that a large rear-sprocket tooth number of a large rear sprocket divided by a small rear-sprocket number of a small rear sprocket adjacent to the large rear sprocket without another rear sprocket intervening therebetween in the axial direction is greater than 1.27. 
     These and other objects, features, aspects and advantages will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses selected embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings that form a part of this original disclosure: 
         FIG. 1  is a partial side elevational view of a bicycle including a bicycle drive system in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a partial side elevational view of the bicycle drive system of  FIG. 1  in which the bicycle chain is shown in phantom lines for clarity and in which a first front-sprocket has forty teeth and a second front-sprocket has thirty teeth; 
         FIG. 3  is a partial side elevational view of the bicycle drive system of  FIG. 2  in which a first front-sprocket has thirty-four teeth and a second front-sprocket has thirty teeth; 
         FIG. 4  is a partial side elevational view of the bicycle drive system of  FIG. 2  in which a first rear-sprocket has ten teeth and a second rear-sprocket has forty-six teeth; 
         FIG. 5  is a partial side elevational view of the bicycle drive system of  FIG. 2  in which a first rear-sprocket has ten teeth and a second rear-sprocket has fifty teeth; 
         FIG. 6  is a partial side elevational view of the bicycle drive system of  FIG. 5  in which a first front-sprocket has thirty-four teeth and a second front-sprocket has thirty teeth; 
         FIG. 7  is a partial side elevational view of the bicycle drive system of  FIG. 5  in which a first front-sprocket has thirty-two teeth and a second front-sprocket has twenty-eight teeth; 
         FIG. 8  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has thirty-four teeth and a second front-sprocket has thirty teeth for a first configuration of a rear sprocket assembly; 
         FIG. 9  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has thirty-two teeth and a second front-sprocket has twenty-eight teeth for a first configuration of a rear sprocket assembly; 
         FIG. 10  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has thirty-four teeth and a second front-sprocket has thirty teeth for a second configuration of a rear sprocket assembly; 
         FIG. 11  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has thirty-four teeth and a second front-sprocket has thirty teeth for a third configuration of a rear sprocket assembly; 
         FIG. 12  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has thirty-two teeth and a second front-sprocket has twenty-eight teeth for a fourth configuration of a rear sprocket assembly; 
         FIG. 13  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has thirty teeth and a second front-sprocket has twenty-six teeth for a fourth configuration of a rear sprocket assembly; 
         FIG. 14  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has thirty teeth and a second front-sprocket has twenty-six teeth for a fifth configuration or a rear sprocket assembly; 
         FIG. 15  is a table of teeth ratios for a bicycle drive system in which a first front-sprocket has twenty-eight teeth and a second front-sprocket has twenty-four teeth for a sixth configuration of a sprocket assembly; 
         FIG. 16  is a partial side elevational view of the bicycle drive system of  FIG. 1  including front and rear electrical shifting devices; 
         FIG. 17A  is side elevational view of a conventional sprocket, and  FIG. 17B  is a side elevational view of a sprocket of  FIG. 1  including a shift assist projection; 
         FIG. 18  is a crank arm assembly of  FIG. 1  configured to initiate operation of an assist device; and 
         FIG. 19  is an exploded view of an assist device configured to operate a shifting device. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Selected exemplary embodiments will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the exemplary embodiments are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     Referring initially to  FIG. 1 , a bicycle  10  is illustrated that is equipped with, among other things, a front crankset  14 , a rear gear cassette  16 , a drive chain  18  (e.g., a chain) that connects the front crankset  14  to the rear gear cassette  16 . The front crankset  14  is rotatably supported to a bicycle frame  20  by a conventional bottom bracket in a conventional manner. The bicycle  10  further includes a front shifting device, such as a front derailleur  22 , and a rear shifting device, such as a rear derailleur  24 , for shifting the chain  18  laterally to change gears. The front and rear derailleurs  22  and  24  are operatively connected to shifters (not shown) by cables  26  and  28  for operating the front and rear derailleurs  22  and  24  in a conventional manner. 
     The bicycle  10  is conventional, except for the front crankset  14  and rear gear cassette  16  as discussed below. Thus, the bicycle  10  will not be discussed and/or illustrated in detail herein, except as related to the present invention. Rather, it will be apparent to those skilled in the art from this disclosure that the bicycle  10  includes various conventional bicycle components such as wheels, shifters, a handle etc. coupled to the bicycle frame  20  in a conventional manner. Moreover, it will be apparent to those skilled in the art from this disclosure that various modifications can be made to the bicycle  10  and its various components without departing from the present invention, as described and illustrated herein. Finally, it will be apparent to those skilled in the art from this disclosure that the bicycle  10  can be used on various types of bicycle such as road or mountain bicycles as needed and/or desired. 
     As illustrated in  FIG. 1 , the front crankset  14 , the rear gear cassette  16 , and the rear derailleur  24  engage with the chain  18  to form a bicycle drive system of the bicycle  10  that transmits a rotation of pedals  30  and  32  of the front crankset  14  to the rear gear cassette  16  and a rear wheel (not shown). The rear derailleur  24  is fixedly coupled to a chain stay of the bicycle frame  20 . In the illustrated embodiment, the front and rear derailleurs  22  and  24  are conventional front and rear derailleur devices. Thus, a detailed description of the front and rear derailleurs  22  and  24  will be omitted for the sake of brevity. Of course, the front and rear derailleurs  22  and  24  can be different types of front and rear derailleurs, as needed or desired. 
     As illustrated in  FIGS. 1 and 2 , the bicycle drive system in accordance with an exemplary embodiment of the present invention includes a front crankset  14 . The front crankset  14  includes a front sprocket assembly  34  having any suitable number of sprockets. In the illustrated embodiment, the front sprocket assembly  34  includes a first front-sprocket  36  and a second front-sprocket  38 . 
     The first front-sprocket  36  includes a body  40 , which is rotatable around a center rotational axis A, and a plurality of teeth  42  provided along a peripheral portion of the body  40 , as shown in  FIGS. 1 and 2 . The body  40  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The first front-sprocket  36  has a first front-tooth number that is the largest tooth number in the front sprocket assembly  34 . The first front-tooth number is less than or equal to forty (40). In the exemplary embodiment illustrated in  FIGS. 1 and 2 , the first front-sprocket  36  has forty (40) teeth  42 . 
     The second front-sprocket  38  includes a body  44 , which is rotatable around the center rotational axis A, and a plurality of teeth  46  provided along a peripheral portion of the body  44 , as shown in  FIGS. 1 and 2 . The second front-sprocket  38  is disposed adjacent to the first front-sprocket  36  without another sprocket being disposed or intervening therebetween in the axial direction, i.e., in the direction of the rotational axis A. The body  44  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The second front-sprocket has a second front-tooth number that is less than or equal to the first front-tooth number. The first front-tooth number divided by the second front-tooth number is preferably less than or equal to 1.4. More preferably, the first front-tooth number divided by the second front-tooth number is preferably less than or equal to 1.2. In the exemplary embodiment illustrated in  FIGS. 1 and 2 , the second front-sprocket  38  has thirty (30) teeth  44 . 
     In another exemplary embodiment illustrated in  FIGS. 1 to 3 , the bicycle drive system includes the rear gear cassette  16 , which includes a rear sprocket assembly  48  having a first rear-sprocket  50  and a second rear-sprocket  52 . 
     The first rear-sprocket  50  includes a body  54 , which is rotatable around a center rotational axis B, and a plurality of teeth  56  provided along a peripheral portion of the body  54 , as shown in  FIGS. 2 and 3 . The body  54  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The first rear-sprocket  50  has a first rear-tooth number that is less than or equal to ten (10). In the exemplary embodiment illustrated in  FIGS. 2 and 3 , the first rear-sprocket  50  has ten (10) teeth  56 . 
     The second rear-sprocket  52  includes a body  58 , which is rotatable around the center rotational axis B, and a plurality of teeth  60  provided along a peripheral portion of the body  58 , as shown in  FIG. 3 . At least five sprockets are disposed between the first rear-sprocket  50  and the second rear-sprocket  52  in an axial direction parallel to the center rotational axis B of the rear sprocket assembly  16 . As illustrated in  FIG. 3 , five sprockets are disposed between the first and second rear-sprockets  50  and  52  such that the rear sprocket assembly  48  has a total of seven rear sprockets. The body  58  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The second rear-sprocket  52  has a second rear-tooth number greater than or equal to forty-four (44). In the exemplary embodiment illustrated in  FIG. 3 , the second rear-sprocket  52  has forty-four (44) teeth  60 . 
     In another exemplary embodiment illustrated in  FIG. 3 , a front crankset  114  includes a front sprocket assembly  134  having a first front-sprocket  136  and a second front-sprocket  138 . 
     The first front-sprocket  136  includes a body  140 , which is rotatable around a center rotational axis A, and a plurality of teeth  142  provided along a peripheral portion of the body  140 , as shown in  FIG. 3 . The body  140  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The first front-sprocket  136  has a first front-tooth number that is the largest tooth number of the sprockets in the front sprocket assembly  134 . The first front-tooth number is less than or equal to forty (40), and more preferably the first front-tooth number is less than or equal to thirty-four (34). In the exemplary embodiment illustrated in  FIG. 3 , the first front-sprocket  136  has thirty-four (34) teeth  142 . 
     The second front-sprocket  138  includes a body  144 , which is rotatable around the center rotational axis A, and a plurality of teeth  146  provided along a peripheral portion of the body  144 , as shown in  FIG. 3 . The second front-sprocket  138  is disposed adjacent to the first front-sprocket  136  without another sprocket being disposed or intervening therebetween in the axial direction, i.e., in the direction of the rotational axis A. The body  144  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. In the exemplary embodiment illustrated in  FIG. 3 , the second front-sprocket  138  has thirty (30) teeth  144 . 
     In another exemplary embodiment illustrated in  FIG. 4 , the bicycle drive system includes a rear gear cassette  116 , which includes a rear sprocket assembly  148  having a first rear-sprocket  150  and a second rear-sprocket  152 . The front sprocket assembly  14  is described above with reference to  FIGS. 1 and 2 . 
     The first rear-sprocket  150  includes a body  154 , which is rotatable around a center rotational axis B, and a plurality of teeth  156  provided along a peripheral portion of the body  154 , as shown in  FIG. 4 . The body  154  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The first rear-sprocket  150  has a first rear-tooth number that is less than or equal to ten (10). In the exemplary embodiment illustrated in  FIG. 4 , the first rear-sprocket  150  has ten (10) teeth  156 . 
     The second rear-sprocket  152  includes a body  158 , which is rotatable around the center rotational axis B, and a plurality of teeth  160  provided along a peripheral portion of the body  158 , as shown in  FIG. 4 . At least five sprockets are disposed between the first rear-sprocket  150  and the second rear-sprocket  152  in an axial direction parallel to the center rotational axis B of the rear sprocket assembly  116 . As illustrated in  FIG. 4 , five sprockets are disposed between the first and second rear sprockets  150  and  152  such that the rear sprocket assembly  148  has a total of seven rear sprockets. The body  158  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The second rear-sprocket  152  has a second-rear tooth number greater than or equal to forty-four (44). In the exemplary embodiment illustrated in  FIG. 4 , the second rear-sprocket  152  has forty-six (46) teeth  160 . 
     In another exemplary embodiment illustrated in  FIGS. 5 to 7 , the bicycle drive system includes a rear gear cassette  216 , which includes a rear sprocket assembly  248  having a first rear-sprocket  250  and a second rear-sprocket  252 . 
     The first rear-sprocket  250  includes a body  254 , which is rotatable around a center rotational axis B, and a plurality of teeth  256  provided along a peripheral portion of the body  254 , as shown in  FIGS. 5 to 7 . The body  254  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The first rear-sprocket  250  has a first rear-tooth number that is less than or equal to ten (10). In the exemplary embodiment illustrated in  FIGS. 5 to 7 , the first rear-sprocket  250  has ten (10) teeth  256 . 
     The second rear-sprocket  252  includes a body  258 , which is rotatable around the center rotational axis B, and a plurality of teeth  260  provided along a peripheral portion of the body  258 , as shown in  FIGS. 5 to 7 . At least five sprockets are disposed between the first rear-sprocket  250  and the second rear-sprocket  252  in an axial direction parallel to the center rotational axis B of the rear sprocket assembly  216 . As illustrated in  FIGS. 5 to 7 , five sprockets are disposed between the first and second rear sprockets  250  and  252  such that the rear sprocket assembly  248  has a total of seven rear sprockets. The body  258  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The second rear-sprocket  252  has a second rear-tooth number greater than or equal to forty-four (44). In the exemplary embodiment illustrated in  FIGS. 5 to 7 , the second rear-sprocket  252  has fifty (50) teeth  260 . 
     With reference to the exemplary embodiment illustrated in  FIG. 5 , the front sprocket assembly  34  of the front crankset  14  is configured substantially similarly as described above with reference to  FIGS. 1 and 2 . 
     With reference to  FIG. 6 , the front sprocket assembly  134  of the front crankset  114  is configured substantially similarly as described above with reference to  FIG. 3 . 
     In another exemplary embodiment illustrated in  FIG. 7 , a front crankset  214  includes a front sprocket assembly  234  having a first front-sprocket  236  and a second front-sprocket  238 . 
     The first front-sprocket  236  includes a body  240 , which is rotatable around a center rotational axis A, and a plurality of teeth  242  provided along a peripheral portion of the body  240 , as shown in  FIG. 7 . The body  240  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. The first front-sprocket  236  has a first front-tooth number that is the largest tooth number of the sprockets in the front sprocket assembly  234 . The first front-tooth number is less than or equal to forty (40), and more preferably the first front-tooth number is less than or equal to thirty-four (34). In the exemplary embodiment illustrated in  FIG. 7 , the first front-sprocket  236  has thirty-two (32) teeth  242 . 
     The second front-sprocket  238  includes a body  244 , which is rotatable around the center rotational axis A, and a plurality of teeth  246  provided along a peripheral portion of the body  244 , as shown in  FIG. 7 . The second front-sprocket  238  is disposed adjacent to the first front-sprocket  236  without another sprocket being disposed or intervening therebetween in the axial direction, i.e., in the direction of the rotational axis A. The body  244  is formed of an annular plate-shaped member made, for example, of iron, aluminum, titanium, or any other metal or a carbon fiber reinforced material or any other non-metallic material. In the exemplary embodiment illustrated in  FIG. 7 , the second front-sprocket  238  has twenty-eight (28) teeth  246 . 
       FIGS. 8 to 15  are tables of teeth ratios for various exemplary front and rear sprocket assembly configurations. The top row of the upper portion of each of the tables is the tooth number of each front sprocket of the front sprocket assembly. The left column of the upper portion of each of the tables is the tooth number of each rear sprocket of the rear sprocket assembly. The corresponding table entries are the ratio of the number of teeth of the front sprocket to the number of teeth of the rear sprocket, i.e., the number of teeth of the front sprocket divided by the number of teeth of the rear sprocket. 
       FIG. 8  illustrates a front sprocket assembly having a first front-sprocket having thirty-four teeth and a second front-sprocket having thirty teeth. The rear sprocket assembly includes a first rear-sprocket having ten teeth and a second rear-sprocket having forty-six teeth. The intervening rear sprockets between the first and second rear-sprockets have thirteen, seventeen, twenty-two, twenty-eight and thirty-six teeth, respectively. 
       FIG. 9  illustrates a front sprocket assembly having a first front-sprocket having thirty-two teeth and a second front-sprocket having twenty-eight teeth. The rear sprocket assembly includes a first rear-sprocket having ten teeth and a second rear-sprocket having forty-six teeth. The intervening rear sprockets between the first and second rear-sprockets have thirteen, seventeen, twenty-two, twenty-eight and thirty-six teeth, respectively. 
       FIG. 10  illustrates a front sprocket assembly having a first front-sprocket having thirty-four teeth and a second front-sprocket having thirty teeth. The rear sprocket assembly includes a first rear-sprocket having ten teeth and a second rear-sprocket having fifty teeth. The intervening rear sprockets between the first and second rear-sprockets have thirteen, seventeen, twenty-two, twenty-nine and thirty-eight teeth, respectively. 
       FIG. 11  illustrates a front sprocket assembly having a first front-sprocket having thirty-four teeth and a second front-sprocket having thirty teeth. The rear sprocket assembly includes a first rear-sprocket having ten teeth and a second rear-sprocket having forty-eight teeth. The intervening rear sprockets between the first and second rear-sprockets have thirteen, seventeen, twenty-two, twenty-eight and thirty-six teeth, respectively. 
       FIG. 12  illustrates a front sprocket assembly having a first front-sprocket having thirty-two teeth and a second front-sprocket having twenty-eight teeth. The rear sprocket assembly includes a first rear-sprocket having nine teeth and a second rear-sprocket having forty-five teeth. The intervening rear sprockets between the first and second rear-sprockets have twelve, sixteen, twenty-one, twenty-seven and thirty-five teeth, respectively. 
       FIG. 13  illustrates a front sprocket assembly having a first front-sprocket having thirty teeth and a second front-sprocket having twenty-six teeth. The rear sprocket assembly includes a first rear-sprocket having nine teeth and a second rear-sprocket having forty-five teeth. The intervening rear sprockets between the first and second rear-sprockets have twelve, sixteen, twenty-one, twenty-seven and thirty-five teeth, respectively. 
       FIG. 14  illustrates a front sprocket assembly having a first front-sprocket having thirty teeth and a second front-sprocket having twenty-six teeth. The rear sprocket assembly includes a first rear-sprocket having eight teeth and a second rear-sprocket having forty-six teeth. The intervening rear sprockets between the first and second rear-sprockets have eleven, fifteen, twenty, twenty-six and thirty-four teeth, respectively. 
       FIG. 15  illustrates a front sprocket assembly having a first front-sprocket having twenty-eight teeth and a second front-sprocket having twenty-four teeth. The rear sprocket assembly includes a first rear-sprocket having eight teeth and a second rear-sprocket having forty-four teeth. The intervening rear sprockets between the first and second rear-sprockets have eleven, fifteen, twenty, twenty-six and thirty-four teeth, respectively. 
     The lower portion of each of the tables of  FIGS. 8 to 15  includes an “a” column corresponding to the number of teeth of the larger front sprocket and a “b” column corresponding to the number of teeth of the smaller front sprocket. The left column of the lower portion of each of the tables is the sprocket number from the sprocket with the fewest teeth to the sprocket with the largest number of teeth. Accordingly, the “top” row corresponds to the sprocket having the fewest teeth, with the “6” row being the next sprocket adjacent to the “top” sprocket in an axial direction. The “low” sprocket corresponds to the sprocket having the greatest number of teeth and having the largest axial spacing from the “top” sprocket. 
     The upper portion of the table of  FIG. 8  includes a column “34” corresponding to the first front-tooth number and a column “30” corresponding to the second front-tooth number. The row “10” corresponds to the first rear-tooth number and the row “46” corresponds to the second rear-tooth number. The rows “13”, “17”, “22”, “28” and “36” correspond to the five sprockets disposed between the first and second rear-sprockets. The entries “3.400” and “3.000” correspond to the ratio of the first front-tooth number to the first rear tooth number (34/10=3.400) and the ratio of the second front tooth number to the first rear-tooth number (30/10=3.000), respectively. The next row is the ratio of the first front-tooth number to the number of teeth of the next adjacent rear sprocket, i.e., 34/13=2.615 and 30/13=2.308, respectively. The remaining entries are determined in a similar manner. 
     The “a” column of  FIG. 8  is the ratio of the ratio of the second front tooth number to the first rear-tooth number of the preceding smaller rear sprocket to the ratio of the first front tooth-number to the tooth number of the current rear sprocket. Accordingly, there is no “a” entry for the top (or smallest) rear sprocket as there is no smaller or preceding rear sprocket. The “a” entry for the 6 (or 13 tooth) rear sprocket is 1.15, i.e., 3.000 (30/10) divided by 2.615 (34/13). The “a” entry for the 5 (or 17 tooth) rear sprocket is 1.15, i.e., 2.308 (30/13) divided by 2.000 (34/17). The remaining “a” column entries are determined in a similar manner. 
     The “b” column of  FIG. 8  is the ratio of the ratio of the first front-tooth number to the rear-tooth number of the current rear sprocket to the ratio of the second rear-tooth number to the tooth number of the current rear sprocket. The “b” entry for the “top” (10 tooth) rear sprocket is 1.13, i.e., 3.400 (34/10) divided by 3.000 (30/10). The “b” entry for the “6” (13 tooth) rear sprocket is 1.13, i.e., 2.615 (34/13) divided by 2.308 (30/13). The remaining “b” column entries are determined in a similar manner. 
     The first front-tooth number divided by the second front-tooth number is preferably less than or equal to a large rear-sprocket tooth number of a large rear sprocket divided by a small rear-sprocket number of a small rear sprocket adjacent to the large rear sprocket without another rear sprocket intervening therebetween in the axial direction. As illustrated in  FIG. 10 , for example, the first front-tooth number is 34 and the second front-tooth number is 30. The first front-tooth number divided by the second front-tooth number is 1.13 (34/30). The large rear-sprocket tooth number is 50 and the small rear-sprocket tooth number of the adjacent sprocket without any intervening sprockets is 38. The large rear-sprocket tooth number divided by the small rear-sprocket tooth number of the adjacent sprocket without any intervening sprockets therebetween in the axial direction is 1.31 (50/38). Thus, 1.13 is less than or equal to 1.31. 
     The first front-tooth number divided by the second front-tooth number is preferably less than a large rear-sprocket tooth number of a large rear sprocket divided by a small rear-sprocket number of a small rear sprocket adjacent to the large rear sprocket without another rear sprocket intervening therebetween in the axial direction. As illustrated in  FIG. 11 , for example, the first front-tooth number is 34 and the second front-tooth number is 30. The first front-tooth number divided by the second front-tooth number is 1.13 (34/30). The large rear-sprocket tooth number is 48 and the small rear-sprocket tooth number of the adjacent sprocket without any intervening sprockets is 36. The large rear-sprocket tooth number divided by the small rear-sprocket tooth number of the adjacent sprocket without any intervening sprockets therebetween in the axial direction is 1.33 (48/36). Thus, 1.13 is less than 1.33. 
     A large rear-sprocket tooth number of a large rear sprocket divided by a small rear-sprocket number of a small rear sprocket adjacent to the large rear sprocket without another rear sprocket intervening therebetween in the axial direction is preferably greater than 1.27. As illustrated in  FIG. 14 , for example, the large rear-sprocket tooth number is 46 and the small rear-sprocket tooth number of the adjacent sprocket without any intervening sprockets therebetween in the axial direction is 34. The large rear-sprocket tooth number divided by the small rear-sprocket tooth number of the adjacent sprocket without any intervening sprockets therebetween in the axial direction is 1.35 (46/34). Thus, 1.35 is greater than 1.27. 
     The upper and lower portions of the tables of  FIGS. 9 to 15  are determined in a similar manner to  FIG. 8  as described above. Additionally, the upper portions of  FIGS. 10 and 11  illustrate a prescribed shifting route. More specifically, the arrows indicate a prescribed down-shifting route. At least one of the front shifting device  22  and the rear shifting device  24  is configured to be operated in accordance with a prescribed shifting route. The prescribed shifting route includes at least one of a prescribed up-shifting route and a prescribed down-shifting route. Preferably, the prescribed shifting route includes the prescribed up-shifting route and the prescribed down-shifting route. For clarity, a prescribed up-shifting route is not shown, although the prescribed up-shifting route is the reverse of the illustrated prescribed down-shifting route. For example, as illustrated in  FIG. 10 , the prescribed down-shifting route is to shift the chain from the first front-sprocket to the second front-sprocket while retaining the chain on the same rear sprocket. The next shift results in the chain shifting from the second front-sprocket to the first front-sprocket and from the rear sprocket to the adjacent larger rear sprocket. For example, when the current chain position is on the 5 gear (17 tooth) rear sprocket and the first (34 tooth) front-sprocket, the prescribed down-shift is to the second (30 tooth) front-sprocket while remaining on the 5 gear (17 tooth) rear sprocket. The next prescribed down-shift is back to the first (34 tooth) front sprocket and to the 6 gear (22 tooth) rear sprocket. The prescribed down-shifting route follows the arrows illustrated in the upper portion of  FIG. 10 . The prescribed up-shifting route is the reverse of the arrows illustrated in the upper portion of  FIG. 10 . 
     The front and rear sprockets rotate in a rotational direction (clockwise as illustrated in  FIGS. 1 to 8 and 16 ) to drive a bicycle chain  18  in a drive direction. During a chain shifting process on the front sprocket assembly, the chain  18  is shifted from one of the front sprockets to the next adjacent one of the front sprockets by the front derailleur  22  moving the chain laterally in an axial direction relative to the center rotational axis A of the front sprocket assembly. During a chain shifting process on the rear sprocket assembly, the chain  18  is shifted from one of the rear sprockets to the next adjacent one of the rear sprockets by the rear derailleur  24 , thereby moving the chain in an axial direction relative to the center rotational axis B of the rear sprocket assembly. Bicycle chains are well known, and thus, a bicycle chain will not be illustrated in detail herein. Of course, the bicycle chain is a continuous loop that has a plurality of inner link plates and a plurality of outer link plates that are pivotally connected to each other by articulation chain pins and chain rollers. From the center of each of the chain rollers to the center of the next chain roller is about one-half inch (12.7 mm). This dimension is known as the “pitch” of the chain. The bicycle chain can be any chain that is used with a bicycle sprocket. Thus, the chain will not be described in further detail herein. 
     A front shifting device is disposed adjacent to the front sprocket assembly  34  and a rear shifting device is disposed adjacent to the rear sprocket assembly  48 , as shown in  FIG. 1 . The front derailleur  22  and the rear derailleur  24  are examples of front and rear shifting devices, respectively. Any suitable shifting device can be used to facilitate shifting, such as mechanical or electrical shifting devices. For example, as illustrated in  FIG. 1 , the front and rear derailleurs  22  and  24  are mechanical shifting devices configured to be operated by movement of the bicycle control cables  26  and  28 , respectively. At least one of the front shifting device and the rear shifting device is configured to be operated by movement of a bicycle control cable. Alternatively, as illustrated in  FIG. 16 , electrical switches  68  (SW 1 ) and  72  (SW 2 ) send signals through control cables  66  and  70  to motors  62  and  64  to operate the rear and front shifting devices  24  and  22 , respectively. Alternatively, the front shifting device can be a power change mechanism. Alternatively, the rear shifting device can be an internal transmission hub. 
     As illustrated in  FIG. 17B , a second rear-sprocket  74  includes at least one shift assist projection  82 A. Preferably, the second rear-sprocket  74  includes a plurality of shift assist projections  82 A and  82 B. The shift assist projections  82 A and  82 B are connected to a face of the second-rear sprocket  74  facing the adjacent smaller sprocket  78 . The shift assist projections  82 A and  82 B facilitate down-shifting from the smaller sprocket  78  to the larger sprocket  74 . As the chain  18  is released from the teeth  80  of the smaller sprocket  78 , the chain  18  engages one of the shift assist projections  82 A and  82 B, as illustrated in  FIG. 17B , thereby facilitating engagement of the chain  18  with teeth  76  of the larger sprocket  74 . The shift assist projections  82 A and  82 B engage the chain  18  to reduce the angle with which the chain approaches the teeth  76  of the larger sprocket  74  during down-shifting, thereby facilitating engagement of the chain  18  with the teeth  76 . As illustrated in  FIG. 17A , the chain  18  approaches the teeth  76  of the larger sprocket  74  at a greater angle, such that the down-shifting process is less smooth. 
     With reference to  FIGS. 18 and 19 , at least one of the front shifting device and the rear shifting device is configured to be operated by rotation of a bicycle crank arm. A left side crank arm assembly  84  is illustrated in  FIG. 18 , and includes an elongated crank arm body  86 , a crank arm mounting boss  88  at a first end having an inner peripheral surface defining the crank axle mounting hole  90  and splines  92  and a threaded pedal mounting hole  94  on a second end. An assisting apparatus  81  described below is disposed on a left side of a bottom bracket shell. Alternatively, the assisting apparatus can be disposed on a right side of the bottom bracket shell. 
     An annular drive ring  96  has an inner peripheral surface non-rotatably engaging an outer peripheral surface of a crank axle mounting boss  88 . Preferably, the annular drive ring  96  is press-fit onto the crank axle. Projections  98 A and  98 B are preferably not greater than, and preferably less than, an outer diameter of the crank arm mounting boss  88  transverse to a longitudinal median axis L of the crank arm  86 . 
     An oblique view of an exemplary embodiment of an assisting apparatus  81  is illustrated in  FIG. 19 . The assisting apparatus  81  includes mounting member  83 , a cam member (derailleur positioning cam)  85  with a cam surface  87  coupled to a mounting member  83  for rotation around a cam axis Y (which is usually but not necessarily coincident with the rotation axis A of crank arm assembly  42 ), a cam follower  89  cooperating with the cam surface  87  for moving in response to rotation of the cam member  85 , a transmission actuating element coupling member  91  for communicating movement of the cam follower  89  to a transmission actuating element  93 , a first coupling member  95  coupled for rotation of cam member  85 , wherein the first coupling member  95  moves between a first engaged position and a first disengaged position, a second coupling member  97  coupled for rotation of the cam member  85 , wherein the second coupling member  97  moves between a second engaged position and a second disengaged position, and an operating member  99  for moving the first coupling member  95  to the first engaged position. 
     The cam follower  89  includes a cam follower lever  61 , wherein an intermediate portion of cam follower lever  61  is pivotably mounted to the mounting member  83  through a pivot shaft  63 . A first end of the cam follower lever  61  includes a roller  65  for engaging the cam surface  87 , and a second end of the cam follower lever  61  contains the transmission actuating element coupling member  91 . The transmission actuating element  93  includes a Bowden cable wherein transmission actuating wire  67  slides within an outer casing  69 . Thus, the transmission actuating element coupling member  91  has the form of a wire connector, wherein a wire fastening screw  71  screws into the second end of the cam follower lever  61 . A mounting member  83  has a transmission actuating element coupling arm  73  for terminating the outer casing  69  of the transmission actuating element in a known manner. For example, the transmission actuating element coupling arm  73  may have a threaded opening for engaging a threaded portion  75  of an adjustment barrel  77  used to terminate the outer casing  69  and to adjust the position of the outer casing  69  relative to the transmission actuating wire  67 . 
     The operating member  99  has the shape of an operating lever  41 , wherein an intermediate portion of the operating leer  41  is pivotably mounted to the mounting member  83  through a pivot shaft  43  for pivoting around an operating lever axis Z. A first end of the operating lever  41  has the shape of a hook with a control surface  45  for supporting either a first pawl control abutment  47  of the first pawl  49  or a second pawl control abutment  51  of a second pawl  53 . A second end of the operating lever  41  includes an operating element coupling member  55 . The illustrated operating element is in the form of an operating wire  57  coupled between a shift operating device mounted to the bicycle handlebar (not shown) and the operating element coupling member  55 . Thus, the operating coupling member  55  has the form of a wire connector, wherein a wire fastening screw  59  screws into the second end of the operating lever  41 . An operating member biasing spring  79  is connected between the mounting member  83  and the operating lever  41  for biasing the operating lever  41  counterclockwise.  FIGS. 18 and 19  illustrate an exemplary embodiment in which at least one of the front and rear shifting devices is configured to be operated by rotation of a bicycle crank arm. Alternatively, any suitable configuration of bicycle crank arm can be used to operate at least one of the front and rear shifting devices. 
     In understanding the scope of the present invention, 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. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. The term “attached” or “attaching”, as used herein, encompasses configurations in which an element directly secured to another element by affixing the element is directly to the other element; configurations in which the element is indirectly secured to the other element by affixing the element to the intermediate member(s) which in turn are affixed to the other element; and configurations in which one element is integral with another element, i.e. one element is essentially part of the other element. This definition also applies to words of similar meaning, for example, “joined”, “connected”, “coupled”, “mounted”, “bonded”, “fixed” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Also it will be understood that although the terms “first” and “second” may be used herein to describe various components these components should not be limited by these terms. These terms are only used to distinguish one component from another. Thus, for example, a first component discussed above could be termed a second component and vice-a-versa without departing from the teachings of the present invention. Finally, terms of degree such as “substantially”, “about” and “approximately” as used herein mean an amount of deviation of the modified term such that the end result is not significantly changed. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Moreover, the size, shape, location or orientation of the various components can be changed as needed and/or desired so long as they do not substantially their intended function. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them unless specifically stated otherwise. The functions of one element can be performed by two, and vice versa unless specifically stated otherwise. The structures and functions of one embodiment can be adopted in another embodiment. 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 foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.