How to measure B.C.D.?

[Cyclopathic]

When faced with a naked chain ring what is the best and most accurate way to measure the BCD? On asymetric rings you can't simply measure across from one to another and divide by two and I can't work out a way to quickly locate the very centre of the ring.
I'm looking for something as accurate as possible because some rings from different manufacturers can seem to vary by quite small amounts.

 

[black'n'yellow]

it's usually stamped on it somewhere.

 

[Biker Joe]

Is this any help?
http://www.hostelshoppe.com/tech_bolt_circle_diameter.php

 

[Poacher]

Very useful link, Biker Joe. One reasonably common size missing from that list: x=50.5 mm, BCD 86 mm. This is for various old Stronglight cranksets, and one I still use on my commuter. Chainrings for this are now scarce!

 

[Biker Joe]

I can't be much help there except for this:-
http://uk.picclick.com/Stronglight-Alloy-Chainring-5-Arm-190752625484.html

 

[Poacher]

Thanks for that - I've increased my stocks lately with a few off ebay.

 

[Pumpkin the robot]

Do you remember when you were at school learning trigonometry? Well here is a use for it!
If you have digital calipers you can easily measure the distance between the holes, if not use a rule and measure from the edge of the hole to the same edge on the other hole which is easier than trying to guesstimate the centres.
Why is it referred to as a BCD? At work it is always pcd, pitch circle diameter.

 

[Cyclopathic]

Is this any help?
http://www.hostelshoppe.com/tech_bolt_circle_diameter.php

Yes. Thanks. Exactly what I was after.

 

[Rob3rt]

Do you remember when you were at school learning trigonometry? Well here is a use for it!
If you have digital calipers you can easily measure the distance between the holes, if not use a rule and measure from the edge of the hole to the same edge on the other hole which is easier than trying to guesstimate the centres.
Why is it referred to as a BCD? At work it is always pcd, pitch circle diameter.

BCD = Bolt Circle Diameter. PCD is the same thing. Probably just an adaption to make it more intuitive.

 

[Pumpkin the robot]

I think PCD is the proper term and BCD is maybe cycling specific?

 

[Rob3rt]

I think PCD is the proper term and BCD is maybe cycling specific?

Yes. That is basically what I said above ^^