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BCD for chainring(11 posts)

BCD for chainringthe bull
Sep 23, 2003 8:07 PM
Sheldon says current campy rings are 135
I am measuring 130
Am I doing it at this right?
re: HahaHaRusty Coggs
Sep 23, 2003 8:21 PM
That is not the true diameter.Trust him.
Where is the measurement taken from? nmthe bull
Sep 23, 2003 8:24 PM
Where is the measurement taken from? nmAllUpHill
Sep 23, 2003 8:35 PM
(i'm assuming you're not joking around, forgive me if I'm wrong.) It's the diameter of the circle described by the centers of the five bolt openings. Tricky to measure directly or indirectly due to the odd number of bolt openings. Try it. The best way is probably to use a little basic geometry to derive a formula for getting the BDC based on the straight-line distance between two bolt openings. Sorry, to lazy to do it for you tonight.
Feeling stupid, but now I understand. nmthe bull
Sep 23, 2003 8:41 PM
(67.5mm from bb center to middle of bolt on chain ring) nmthe bull
Sep 23, 2003 8:45 PM
Yep, (67.5)(2)=135. Seems easier to me. (nm)Chen2
Sep 24, 2003 8:58 AM
Never mind. EasyAllUpHill
Sep 23, 2003 8:44 PM
BCD = d / sin(36*) ~= d / 0.58778

where d is the on-center distance between two adjacent bolt holes. In general (in case you have a 9-hole chainring) it's d/sin(180/n) for n bolt holes.
Im right here laughing with you now!the bull
Sep 23, 2003 8:51 PM
At first I was thinking what the hell is so funny?
easy answer here..Qubeley
Sep 23, 2003 10:30 PM
The actual diameter of chainrings are hard to measure directly. No need to measure it anyway.
What you can measure though, is the distance between the center of two adjacent holes. There are charts available telling you this distance for different chainrings.
Assuming 5-holes pattern, here's the distance:
BCD -- Distance

Don't do it the hard way(e.g.Trignomentry), or you will be riding "622" wheels. They don't sell that at bike shops, you know?
This is what I was looking for. Thanks! nmthe bull
Sep 24, 2003 2:27 PM