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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. :-( jeverett3ATearthlinkDOTnet http://home.earthlink.net/~jeverett3 |
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