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Brompton gearing4

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Brompton Gear Ratios - Gear Inches                                                                                            (please email Colin Stroud <colin@cjstroud.wanadoo.co.uk> with requests for improvements or corrections)                               December 12, 2004

5-speed Sturmey Archer Sprinter                                       Sprockets                                  1st gear        2nd gear        3rd gear        4th gear          5th gear                              Ratio      Ratio       Chain
? to April 2001                                       Front                Rear                                   0.667           0.789             1             1.266               1.5                               Change      Range       Links
                                                       54                   13                                      46             54               69              87               103                                  8%        225%         100
                                                       52                   13                                      44             52               66              84                99                                  4%        225%         100
                     Standard before March 2000        50                   13                                      42             50               64              81                95                                  0%        225%         98
                                                       52                   14                                      41             48               61              78                92                                  -3%       225%        100?
                                                       48                   13                                      41             48               61              77                92                                  -4%       225%         98
                                    Not-approved ?     50                   14                                      39             47               59              75                89                                  -7%       225%         98?
                                                       46                   13                                      39             46               59              74                88                                  -8%       225%         96
                       Standard from March 2000        44                   13                                      37             44               56              71                84                                 -12%       225%         96        = my blue Brompton
                                                       46                   14                                      36             43               54              69                81                                 -15%       225%          ?
                                                       42                   13                                      36             42               53              68                80                                 -16%       225%         94?
                                                       44                   14                                      35             41               52              66                78                                 -18%       225%         96
                                                       42                   14                                      33             39               50              63                74                                 -22%       225%         94?


3-speed Sturmey Archer                                                Sprockets                                  1st gear                        2nd gear                          3rd gear                              Ratio      Ratio       Chain
( ? to April 2001)                                    Front                Rear                                    0.75                             1                               1.333                               Change      Range       Links
                                                       54                   13                                      52                             69                                 92                                  8%        178%         100
                                                       52                   13                                      50                             66                                 88                                  4%        178%         100
                                         Standard      50                   13                                      48                             64                                 85                                  0%        178%         98        = my yellow Brompton
                                                       52                   14                                      46                             61                                 82                                  -3%       178%        100?
                                                       48                   13                                      46                             61                                 81                                  -4%       178%         98
                                    Not-approved ?     50                   14                                      44                             59                                 79                                  -7%       178%         98?
                                                       46                   13                                      44                             59                                 78                                  -8%       178%         96
                                                       44                   13                                      42                             56                                 75                                 -12%       178%         96
                                                       42                   13                                      40                             53                                 71                                 -16%       178%         94?
                                                       44                   14                                      39                             52                                 69                                 -18%       178%         96
                                                       42                   14                                      37                             50                                 66                                 -22%       178%         94?


6 speed (3 speed SRAM + 2-speed drailer)                                                                       1st sprocket 2nd sprocket 1st sprocket 2nd sprocket 1st sprocket 2nd sprocket
(introduced April 2001)                                               Sprockets                                  1st gear     1st gear    2nd gear      2nd gear     3rd gear     3rd gear                               Ratio      Ratio       Chain
                                                      Front              Rear 1st   Rear 2nd                      0.734        0.734          1            1          1.362        1.362                                Change      Range       Links
                                                       52                   15        13                            42           49           57           66           78           90                                   4%        214%          ?
                                         Standard      50                   15        13                            40           47           55           64           75           87                                   0%        214%         100
                                                       48                   15        13                            39           45           53           61           72           83                                   -4%       214%        100?
                                                       46                   15        13                            37           43           51           59           69           80                                   -8%       214%         98?
                                                       44                   15        13                            36           41           49           56           66           76                                  -12%       214%         98
                                                       42                   15        13                            34           39           46           53           63           73                                  -16%       214%          ?


3 speed SRAM                                                          Sprockets                                  1st gear      MD 1st gear       2nd gear      MD 2nd gear         3rd gear    MD 3rd gear               Ratio      Ratio       Chain
(April 2001 to December 2004)                         Front                Rear                                   0.734                             1                               1.362                               Change      Range       Links
                                                       56                   13                                      52               21            71               28                97            39                   12%        186%         104
                                                       54                   13        13                            50               20            69               27                94            37                    8%        186%         102
                                                       52                   13                                      49               19            66               26                90            36                    4%        186%         100
                         Standard on the L3 and T3     50                   13                                      47               19            64               25                87            35                    0%        186%         98
                                                       52                   14                                      45               18            61               25                84            33                    -3%       186%         98?
                                                       48                   13                                      45               18            61               24                83            33                    -4%       186%         98
                                                       50                   14                                      43               17            59               24                80            32                    -7%       186%         98?
                               Standard on the C3      46                   13                                      43               17            59               23                80            32                    -8%       186%         96
                                                       48                   14                                      42               17            57               23                77            31                   -11%       186%         96?
                                                       44                   13                                      41               16            56               22                76            30                   -12%       186%         96
                                                       46                   14                                      40               16            54               22                74            30                   -15%       186%         96?
                                                       44                   14                                      38               15            52               21                71            28                   -18%       186%         96

3 speed SunRace Sturmey Archer                                        Sprockets                                  1st gear                        2nd gear                          3rd gear                              Ratio      Ratio       Chain
(Introduced January 2005)                             Front                Rear                                    0.73                             1                                1.36                               Change      Range       Links
                                                       52                   13                                      48                             66                                 90                                  4%        186%         100
                                          standard     50                   13                                      46                             64                                 86                                  0%        186%         98
                                                       52                   14                                      45                             61                                 84                                  -3%       186%         98?
                                                       48                   13                                      45                             61                                 83                                  -4%       186%         98
                                                       50                   14                                      43                             59                                 80                                  -7%       186%         98?
                                                       46                   13                                      43                             59                                 80                                  -8%       186%         96
                                                       48                   14                                      41                             57                                 77                                 -11%       186%         96?
                                                       44                   13                                      41                             56                                 76                                 -12%       186%         96
                                                       46                   14                                      40                             54                                 74                                 -15%       186%         96?
                                                       44                   14                                      38                             52                                 71                                 -18%       186%         96


2 speed                                                               Sprockets                                                                1st sprocket 2nd sprocket                                                 Ratio      Ratio       Chain
(introduced June 2005)                                Front              Rear 1st   Rear 2nd                                                                                                                            Change      Range       Links
                                           standard    54                   16        12                                                            56              74                                                    0%        133%          ?
(This system has no hub gears, just uses the two-      52                   16        12                                                            54              72                                                    -4%       133%          ?
speed drailer as fitted to the 6-speed Brompton)       50                   16        12                                                            52              69                                                    -7%       133%          ?
                                                       48                   16        12                                                            50              66                                                   -11%       133%          ?
                                                       46                   16        12                                                            48              63                                                   -15%       133%          ?
                                                       44                   16        12                                                            45              61                                                   -19%       133%          ?

8-speed SunRace (Sturmey Archer)                                      Sprockets                  1st gear        2nd gear        3rd gear        4th gear        5th gear          6th gear      7th gear    8th gear    Ratio      Ratio       Chain
                                                      Front                Rear                      1             1.28            1.45            1.64            1.85              2.10          2.38        3.05     Change      Range       Links
                                                       34                   23                      24              31              35              40              45                51            58          75                  305%
(manufactured by Sunrace from 2004                     36                   25                      24              30              35              39              44                50            57          73                  305%
but not yet fitted to production Bromptons.            38                   25                      25              32              36              41              46                53            60          77                  305%
Potential problems with space for large                36                   23                      26              33              38              42              48                54            62          79                  305%
 rear sprockets currently offered by Sunrace)          38                   23                      27              35              40              45              51                57            65          83                  305%
                                                       42                   25                      28              36              40              46              51                58            66          85                  305%
                                                       44                   25                      29              37              42              48              54                61            69          89                  305%
                                                       42                   23                      30              39              44              50              56                63            72          92                  305%
                                                       46                   25                      30              39              44              50              56                64            72          93                  305%
                                                       44                   23                      32              40              46              52              59                66            75          96                  305%                   my prefered ratios (just dreaming)
                                                       48                   25                      32              41              46              52              59                67            76          97                  305%
                                                       46                   23                      33              42              48              54              61                69            79         101                  305%
                                                       50                   25                      33              42              48              54              61                69            79         101                  305%
                                                       48                   23                      35              44              50              57              64                72            82         105                  305%
                                                       50                   23                      36              46              52              59              67                75            86         110                  305%

Rohloff 14-speed                                                      Sprockets                  1st gear        2nd gear        3rd gear        4th gear        5th gear          6th gear      7th gear    8th gear   9th gear   10th gear   11th gear    12th gear     13th gear      14th gear    Ratio   Ratio   Chain
(not fitted as standard by Brompton)                  Front                Rear                   0.279           0.316            0.36           0.409           0.464             0.528           0.6       0.682      0.774       0.881         1          1.135         1.292          1.467     Change   Range   Links
Rear triangle needs modifying to fit                   44                   13                      16             18               20              23              26                30            34          38         43         49          56           64            72             82                526%     96
54 is the largest chainwheel and 13 the only           46                   13                      16             18               21              24              27                31            35          40         45         52          59           66            76             86                526%     96
sprocket that can be fitted                            48                   13                      17             19               22              25              28                32            37          42         47         54          61           69            79             90                526%     98
                                                       50                   13                      18             20               23              26              30                34            38          43         49         56          64           72            82             93                526%     98
                                                       52                   13                      18             21               24              27              31                35            40          45         51         58          66           75            85             97                526%     100
                                                       53                   13                      19             21               24              28              31                36            40          46         52         59          67           77            87             99                526%     100
                                                       54                   13                      19             22               25              28              32                36            41          47         53         61          69           78            89             101               526%     100



Standard front chainwheel (teeth)                                50                            Mountain drive: Multiply the computed developments for the mountain mode by 0,4.
Standard rear sprocket (teeth)                                   13                            Available chainwheels: 27, 34 to 52 in two-steps, 53 and 54. Larger chainwheels do not fit to the Brompton.
Standard ratio                                                 3.85                            Speed drive: Multiply the computed developments for the high-speed mode by 1,65.
Wheel rolling-radius (inches)                                 8.268                            Available chainwheels: 27, 34 to 52 in two-steps, 53 and 54. Larger chainwheels do not fit to the Brompton.
                                                                                               Highspeed drive: Multiply the computed developments for the high-speed mode by 2,5.
Max front chainwheel size that can be fitted                    54                             The Highspeed drive available at present only with 27 chainwheel


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Brompton Gear Ratios - Progression Metres                                                                      (please email Colin Stroud <colin@cjstroud.wanadoo.co.uk> with requests for improvements or corrections)                                 December 12, 2004

5-speed Sturmey Archer Sprinter                            Sprockets                              1st gear        2nd gear        3rd gear        4th gear        5th gear                                Ratio        Ratio       Chain
? to April 2001                                   Front       Rear                                 0.667           0.789             1             1.266             1.5                                 Change        Range       Links
                                                   54          13                                   3.66            4.32            5.48            6.94            8.22                                   8%          225%         100
                                                   52          13                                   3.52            4.16            5.28            6.68            7.92                                   4%          225%         100
Standard before March 2000                         50          13                                   3.38            4.00            5.07            6.42            7.61                                   0%          225%         98
                                                   52          14                                   3.27            3.87            4.90            6.20            7.35                                   -3%         225%        100?
                                                   48          13                                   3.25            3.84            4.87            6.17            7.31                                   -4%         225%         98
Not-approved ?                                     50          14                                   3.14            3.72            4.71            5.97            7.07                                   -7%         225%         98?
                                                   46          13                                   3.11            3.68            4.67            5.91            7.00                                   -8%         225%         96
Standard from March 2000                           44          13                                   2.98            3.52            4.47            5.65            6.70                                  -12%         225%         96        = my blue Brompton
                                                   46          14                                   2.89            3.42            4.34            5.49            6.50                                  -15%         225%          ?
                                                   42          13                                   2.84            3.36            4.26            5.40            6.39                                  -16%         225%         94?
                                                   44          14                                   2.77            3.27            4.15            5.25            6.22                                  -18%         225%         96
                                                   42          14                                   2.64            3.12            3.96            5.01            5.94                                  -22%         225%         94?


3-speed Sturmey Archer                                     Sprockets                              1st gear                        2nd gear                        3rd gear                                Ratio        Ratio       Chain
( ? to April 2001)                                Front       Rear                                  0.75                             1                             1.333                                 Change        Range       Links
                                                   54          13                                   4.11                            5.48                            7.31                                   8%          178%         100
                                                   52          13                                   3.96                            5.28                            7.04                                   4%          178%         100
Standard                                           50          13                                   3.81                            5.07                            6.76                                   0%          178%         98        = my yellow Brompton
                                                   52          14                                   3.68                            4.90                            6.53                                   -3%         178%        100?
                                                   48          13                                   3.65                            4.87                            6.49                                   -4%         178%         98
Not-approved ?                                     50          14                                   3.53                            4.71                            6.28                                   -7%         178%         98?
                                                   46          13                                   3.50                            4.67                            6.22                                   -8%         178%         96
                                                   44          13                                   3.35                            4.47                            5.95                                  -12%         178%         96
                                                   42          13                                   3.20                            4.26                            5.68                                  -16%         178%         94?
                                                   44          14                                   3.11                            4.15                            5.53                                  -18%         178%         96
                                                   42          14                                   2.97                            3.96                            5.28                                  -22%         178%         94?


6 speed (3 speed SRAM + 2-speed drailer)                                                        1st sprocket 2nd sprocket 1st sprocket 2nd sprocket 1st sprocket 2nd sprocket
(introduced April 2001)                                    Sprockets                              1st gear     1st gear    2nd gear      2nd gear     3rd gear     3rd gear                               Ratio        Ratio       Chain
                                                  Front     Rear 1st Rear 2nd                      0.734        0.734          1            1          1.362        1.362                                Change        Range       Links
                                                   52          15      13                           3.36         3.36         4.57         4.57         6.23         6.23                                  4%          186%          ?
Standard                                           50          15      13                           3.23         3.23         4.40         4.40         5.99         5.99                                  0%          186%         100
                                                   48          15      13                           3.10         3.10         4.22         4.22         5.75         5.75                                  -4%         186%        100?
                                                   46          15      13                           2.97         2.97         4.05         4.05         5.51         5.51                                  -8%         186%         98?
                                                   44          15      13                           2.84         2.84         3.87         3.87         5.27         5.27                                 -12%         186%         98
                                                   42          15      13                           2.71         2.71         3.69         3.69         5.03         5.03                                 -16%         186%          ?


3 speed SRAM                                               Sprockets                              1st gear                        2nd gear                        3rd gear                                Ratio        Ratio       Chain
(introduced April 2001)                           Front       Rear                                 0.734                             1                             1.362                                 Change        Range       Links
                                                   52          13                                   3.87                            5.28                            7.19                                   4%          186%         100
Standard on the L3 and T3                          50          13                                   3.72                            5.07                            6.91                                   0%          186%         98
                                                   52          14                                   3.60                            4.90                            6.68                                   -3%         186%         98?
                                                   48          13                                   3.58                            4.87                            6.64                                   -4%         186%         98
                                                   50          14                                   3.46                            4.71                            6.42                                   -7%         186%         98?
Standard on the C3                                 46          13                                   3.43                            4.67                            6.36                                   -8%         186%         96
                                                   48          14                                   3.32                            4.52                            6.16                                  -11%         186%         96?
                                                   44          13                                   3.28                            4.47                            6.08                                  -12%         186%         96
                                                   46          14                                   3.18                            4.34                            5.90                                  -15%         186%         96?
                                                   44          14                                   3.04                            4.15                            5.65                                  -18%         186%         96

3 speed SunRace Sturmey Archer                             Sprockets                              1st gear                        2nd gear                        3rd gear                                Ratio        Ratio       Chain
(Introduced January 2005)                         Front       Rear                                  0.73                             1                              1.36                                 Change        Range       Links
                                                   52          13                                   3.85                            5.28                            7.18                                   4%          186%         100
                                      standard     50          13                                   3.70                            5.07                            6.90                                   0%          186%         98
                                                   52          14                                   3.58                            4.90                            6.67                                   -3%         186%         98?
                                                   48          13                                   3.56                            4.87                            6.63                                   -4%         186%         98
                                                   50          14                                   3.44                            4.71                            6.41                                   -7%         186%         98?
                                                   46          13                                   3.41                            4.67                            6.35                                   -8%         186%         96
                                                   48          14                                   3.30                            4.52                            6.15                                  -11%         186%         96?
                                                   44          13                                   3.26                            4.47                            6.07                                  -12%         186%         96
                                                   46          14                                   3.16                            4.34                            5.90                                  -15%         186%         96?
                                                   44          14                                   3.03                            4.15                            5.64                                  -18%         186%         96

2 speed                                                    Sprockets                                                            1st sprocket 2nd sprocket                                                 Ratio        Ratio       Chain
(introduced June 2005)                            Front     Rear 1st Rear 2nd                                                                                                                            Change        Range       Links
                                       standard    54          16      12                                                           4.45            5.94                                                   0%          133%          ?
(This system has no hub gears, just uses the       52          16      12                                                           4.29            5.72                                                   -4%         133%          ?
two-speed drailer as fitted to the 6-speed         50          16      12                                                           4.12            5.50                                                   -7%         133%          ?
Brompton)                                          48          16      12                                                           3.96            5.28                                                  -11%         133%          ?
                                                   46          16      12                                                           3.79            5.06                                                  -15%         133%          ?
                                                   44          16      12                                                           3.63            4.84                                                  -19%         133%          ?

8-speed SunRace (Sturmey Archer)                           Sprockets              1st gear        2nd gear        3rd gear        4th gear        5th gear        6th gear        7th gear    8th gear    Ratio        Ratio       Chain
                                                  Front       Rear                    1             1.28            1.45            1.64            1.85            2.10            2.38        3.05     Change        Range       Links
                                                   34          23                   1.95            2.50            2.83            3.20            3.61            4.10            4.64        5.95                   305%
(manufactured by Sunrace from 2004                 36          25                   1.90            2.43            2.76            3.12            3.52            3.99            4.52        5.80                   305%
but not yet fitted to production Bromptons.        38          25                   2.01            2.57            2.91            3.29            3.71            4.21            4.77        6.12                   305%
Potential problems with space for large            36          23                   2.07            2.64            2.99            3.39            3.82            4.34            4.92        6.30                   305%
 rear sprockets currently offered by Sunrace)      38          23                   2.18            2.79            3.16            3.58            4.03            4.58            5.19        6.65                   305%
                                                   42          25                   2.22            2.84            3.21            3.64            4.10            4.66            5.28        6.76                   305%
                                                   44          25                   2.32            2.97            3.37            3.81            4.30            4.88            5.53        7.08                   305%
                                                   42          23                   2.41            3.08            3.49            3.95            4.46            5.06            5.73        7.35                   305%
                                                   46          25                   2.43            3.11            3.52            3.98            4.49            5.10            5.78        7.40                   305%
                                                   44          23                   2.52            3.23            3.66            4.14            4.67            5.30            6.01        7.70                   305%                   my prefered ratios (just dreaming)
                                                   48          25                   2.53            3.24            3.67            4.15            4.69            5.32            6.03        7.73                   305%
                                                   46          23                   2.64            3.38            3.83            4.33            4.88            5.54            6.28        8.05                   305%
                                                   50          25                   2.64            3.38            3.83            4.33            4.88            5.54            6.28        8.05                   305%
                                                   48          23                   2.75            3.52            3.99            4.52            5.09            5.78            6.55        8.40                   305%
                                                   50          23                   2.87            3.67            4.16            4.70            5.31            6.02            6.83        8.75                   305%

Rohloff 14-speed                                           Sprockets              1st gear        2nd gear        3rd gear        4th gear        5th gear        6th gear        7th gear    8th gear   9th gear     10th gear   11th gear    12th gear     13th gear      14th gear    Ratio   Ratio   Chain
(not fitted as standard by Brompton)              Front       Rear                 0.279           0.316            0.36           0.409           0.464           0.528             0.6       0.682      0.774         0.881         1          1.135         1.292          1.467     Change   Range   Links
Rear triangle needs modifying to fit               44          13                   1.25            1.41            1.61            1.83            2.07            2.36            2.68        3.05       3.46          3.93       4.47          5.07          5.77           6.55              526%     96
54 is the largest chainwheel and 13 the only       46          13                   1.30            1.48            1.68            1.91            2.17            2.47            2.80        3.18       3.61          4.11       4.67          5.30          6.03           6.85              526%     96
sprocket that can be fitted                        48          13                   1.36            1.54            1.75            1.99            2.26            2.57            2.92        3.32       3.77          4.29       4.87          5.53          6.29           7.15              526%     98
                                                   50          13                   1.42            1.60            1.83            2.08            2.35            2.68            3.04        3.46       3.93          4.47       5.07          5.76          6.56           7.44              526%     98
                                                   52          13                   1.47            1.67            1.90            2.16            2.45            2.79            3.17        3.60       4.09          4.65       5.28          5.99          6.82           7.74              526%     100
                                                   53          13                   1.50            1.70            1.94            2.20            2.50            2.84            3.23        3.67       4.16          4.74       5.38          6.11          6.95           7.89              526%     100
                                                   54          13                   1.53            1.73            1.97            2.24            2.54            2.89            3.29        3.74       4.24          4.83       5.48          6.22          7.08           8.04              526%     100



Standard front chainwheel (teeth)                      50                       Mountain drive: Multiply the computed developments for the mountain mode by 0,4.
Standard rear sprocket (teeth)                         13                       Available chainwheels: 27, 34 to 52 in two-steps, 53 and 54. Larger chainwheels do not fit to the Brompton.
Standard ratio                                       3.85                       Speed drive: Multiply the computed developments for the high-speed mode by 1,65.
Wheel rolling-radius (metres)                       0.210                       Available chainwheels: 27, 34 to 52 in two-steps, 53 and 54. Larger chainwheels do not fit to the Brompton.
                                                                                Highspeed drive: Multiply the computed developments for the high-speed mode by 2,5.
Max front chainwheel size that can be fitted              54                    The Highspeed drive available at present only with 27 chainwheel

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