tech
Chainrings: Understanding BCD
Bolt Circle Diameter (BCD) is the diameter of an imaginary circle running through the center of the chainring mounting holes. It is always defined in millimeters. See measurement "A" in the below diagram.
Measuring along this imaginary line can sometimes be tricky. Luckily, it's possible to calculate the BCD of a chainring by simply measuring the center-to-center distance of two adjacent bolt holes. With this measurement ("B") you can refer to the table below to identify the corresponding BCD, or use one of the following equations:
- For 3-bolt chainrings: "A" = "B" x 1.155
- For 4-bolt chainrings: "A" = "B" x 1.414
- For 5-bolt chainrings: "A" = "B" x 1.701
| "A" = BCD (mm) | Number of bolts | "B" = Bolt-to-bolt measurement (mm) | Common Application |
|---|---|---|---|
| * Campy 110 BCD rings have a single offset bolt. They are not interchangeable with other 110 BCD rings and cranks. | |||
| 58 | 5 | 34.3 | Compact MTB triple - inner |
| 64 | 4 | 45.3 | Standard MTB - inner |
| 74 | 5 | 43.5 | Compact Road double/triple - inner |
| 80 | 4 | 56.6 | SRAM MTB double - inner |
| 86 | 3 | 74.5 | FSA 3-bolt |
| 88 | 4 | 62.2 | XTR M985 - inner/outer |
| 92 | 5 | 53.3 | Dura Ace 7800/Ultegra 6700 triple - inner |
| 94 | 5 | 55.4 | Compact MTB triple - middle/outer |
| 102 | 4 | 72.1 | XTR M960 - inner |
| 104 | 4 | 73.6 | Standard MTB - middle/outer |
| 110 | 5 | 64.7 | Compact Road double/triple - middle/outer |
| 110 (Campy) * | 5 | 64.7 | Campagnolo Compact/CX - inner/outer |
| 120 | 4 | 84.9 | SRAM MTB double - outer |
| 130 | 5 | 76.4 | Standard Road double - inner/outer |
| 135 | 5 | 79.5 | Campagnolo Road - inner/outer |
| 144 | 5 | 84.6 | Track |
| 146 | 4 | 103.3 | XTR M960 - outer |