Originally Posted by
TejanoTrackie
Well, possible, yes, but easier, no. First off, this cannot be done with a straight measure, or even a caliper. You need a machinist's pin tool, which centers in the crank arm's puller threads at one end and extends to the bolt hole at the other end. You would either have to remove the crank arm bolt or pull the crank. My case is even worse, since it's a cottered crank, and it would have to be removed to perform the measurement, and a new cotterpin installed. Doing a simple multiplication after measuring the bolt center distance is much simpler IMO.
If absolute precision is required, I agree with what you are saying. However, most of the BCD sites I have seen (including Sheldon's) measure bcc with a ruler and "estimate' to the nearest millimeter (or 10th).....since chainrings are only available in fairly standard sizes, I would argue the radius x 2 will yield a number that will tell you what you need....regardless, I was just checking that I wasn't missing some basic math - either way is very easy, and using bcc works with chainrings equally well.....
Mark