Bolt Hole Circles

On Mon, 27 Dec 2004 22:15:10 GMT, Joe Riel wrote:

writes:

From:
Subject: Bolt Hole Circles
Date: Mon, 27 Dec 2004 19:52:38 GMT

John Everett writes:

specify the radius and # of teeth, correct?

Nope. BCD or bolthole chainring diameter is taken from a straight
line of two adjacent holes and a line perpendicular to that line to
the opposite bolt hole.

Peter, Peter, Peter... wrong! I've never actually measured a
chainring before, but your posting had me reaching for a chainring
(Shimano SG 42), and a ruler. Turns out your method measures 118mm,
but the true bolthole diameter measures 130mm.

I think this works better:

http://tinyurl.com/699er

It does, but I think that it's less error prone to skip one
hole by 1.05 (1/cos(18 degrees)).

Not being confident in my ability to still apply high school level
trig, I used the "brute force and ignorance" method. I traced the IDs
of the five bolt holes onto a piece of paper, eyeballed the centers of
each circle, and drew two lines connecting three adjacent circles. I
then bisected each of these lines and drew a line from each center to
the opposite bolt hole center. Where those lines crossed is the center
of the bolt hole circle.

Measuring from one of the adjacent bolt hole connecting lines to the
opposite bolt hole (the Peter Chisolm method) gives 118mm. Measuring
from the center of the bolt hole circle to one of the bolt hole
centers gives 65mm, half the bolt hole diameter.

Whew! ;-)

BTW, I have to confess that I used the little scale that is in every
Nashbar catalog to verify my results. :-(


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