Originally Posted by
schnurrp
That helps. I don't know why, though, you can find for sale a ring described as 24t five bolt 74mm bcd with bolt holes actually about 43mm apart.
Those are two different measurement points that describe the same thing.
BCD is Bolt Circle Diameter. So if you drew a circle through each of the bolt holes, it would be 74mm in diameter.
You're measuring the distance between two adjacent holes. Geometrically, a circle that's 74mm in diameter, with 5 holes equally spaced around the circumference, will have a 43.5mm center-to-center measurement between each of those holes.
Both measurements describe the same circle. Sheldon lists the "distance between adjacent holes" measurement because that's the easiest way to measure a 5-bolt crankset or chain ring You can directly measure the BCD of a traditional 4-bolt crankset...it's the distance between two opposite holes. That's the bolt circle diameter. But you can't do that easily with a 5-bolt crankset. The "distance between adjacent holes" measurement is quick way to gauge the diameter of the circle (the BCD) in this case.
This is why any chain ring sold with a 74mm BCD will have adjacent holes 43.5mm apart. Geometry dictates that.