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Gear Cog, derailleur jockey wheel or sprocket radius is measured from the centre of the sprocket to the centre of a chain pin or rivet. Below is a table of radius values in inches for different cog sizes for the standard 1/2 inch pitch chain used with bicycles. The Machinehead chain length calculator does all this stuff internally so you don't have to!
| Cog Teeth | Radius Inches | Cog Teeth | Radius Inches |
| 10 | 0.809027886318775 | 33 | 2.63009311340335 |
| 11 | 0.887379767661787 | 34 | 2.70955591216871 |
| 12 | 0.965941669640332 | 35 | 2.78902541710941 |
| 13 | 1.04466364585876 | 36 | 2.86850106731321 |
| 14 | 1.1235104986384 | 37 | 2.94798236277534 |
| 15 | 1.2024566882985 | 38 | 3.02746885634353 |
| 16 | 1.28148322154136 | 39 | 3.1069601469094 |
| 17 | 1.36057567355722 | 40 | 3.18645587362662 |
| 18 | 1.43972288832893 | 41 | 3.26595571097952 |
| 19 | 1.518916099869 | 42 | 3.34545936455968 |
| 20 | 1.59814832314133 | 43 | 3.42496656743452 |
| 21 | 1.67741392261326 | 44 | 3.50447707701329 |
| 22 | 1.75670830068737 | 45 | 3.58399067233274 |
| 23 | 1.83602766880394 | 46 | 3.6635071516982 |
| 24 | 1.91536887666401 | 47 | 3.7430263306270 |
| 25 | 1.99472928302824 | 48 | 3.82254804004996 |
| 26 | 2.0741066567262 | 49 | 3.90207212473375 |
| 27 | 2.15349909993283 | 50 | 3.9815984418933 |
| 28 | 2.23290498807221 | 51 | 4.06112685996837 |
| 29 | 2.31232292228593 | 52 | 4.14065725754055 |
| 30 | 2.39175169150125 | 53 | 4.22018952237484 |
| 31 | 2.47119024190823 | 54 | 4.2997235505667 |
| 32 | 2.55063765220899 | 55 | 4.37925924578353 |
Bicycle Software | Miscellaneous Downloads
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