Calculation of the relationship between BCD and CC for cranksets. In the past I've done this by holding the chainring up to the crankset - but this isn't possible when ordering online. I got the information I needed on Sheldon Brown's website, but was still curious where the 1.7 figure came from (to get from CC to BCD on a five bolt). Turns out it's 1/(sin(PI/5)). Not completely intuitive, and the 1.7 is good to 2 decimal places so go ahead and use it. Pretty fun formula really, if you plug in 2 bolts you get BCD=CC, they are on opposite sides of the bolt circle. Even better try to enter 1 bolt and it's undefined!