need help measuring chainring
 

I'm trying to determine the BCD of my 2009 Schwinn Madison chainring

http://img51.imageshack.us/img51/4301/bcdchainring.jpg
note the ruler starts at a half centimeter, so the measurement is 7.5 cm.

75 mm x 1.701 = 127.575

looking at http://www.sheldonbrown.com/gloss_bo-z.html#bcd it appears this is an obsolete size. but that can't be right, it's a 2009

what did I do wrong?

A FSA Vero Track crank is what you have, yes? It's a 130mm BCD crank.

Precise measuring of the BCD is difficult.

d = 2r. Diameter is 2 times the radius.
Try measuring from the center of a crank bolt to the center of the spindle. This cannot be done with accuracy because you can't measure to the axis of the spindle in the same plane as the bolt, but you should be able to get close enough. Looks like a 130 to me.

You likely got messed up with the parallax issues during taking the measurement. When you're looking down a hole like that to try to center the ruler it's best to shift your eye about the same distance to mostly take the parallax into account. Then round as required to what makes sense. And as Garthr points out it makes sense to jump to the 130mm option since you already know that it's not the obsolete Nevar size.

as i just discovered

thanks everyone