Re: [CR]Sheldon Brown method of measuring crank BCD


Example: Component Manufacturers
Date: Wed, 03 Dec 2008 10:46:14 -0500
To: classicrendezvous@bikelist.org
From: "John Betmanis" <johnb@oxford.net>
Subject: Re: [CR]Sheldon Brown method of measuring crank BCD
In-Reply-To: <168d349907cc03d4cd8b48dc0681a1c2@comcast.net>


At 07:48 PM 02/12/2008 -0500, Bianca Pratorius wrote:
>According to the Sheldon Brown method of measuring adjacent bolt holes
>for getting BCD, my Dura Ace track crankset would be the larger Campy
>BCD and not the standard 144 BCD I was thinking it was. I've seen the
>plastic template for measuring this at my LBS but I'm sure that someone
>could tell me without checking. The old fluted Dura Ace track crank was
>144 wasn't it? My calipering isn't an exact science but I think the
>adjacent measurement I got was closer to 88 mm than 84.6. Maybe the
>info on the Sheldon Brown website was a misprint.

Sheldon's chart http://www.sheldonbrown.com/gloss_bo-z.html is correct. Just work it out using triganomometry. The formula will be b=c/sin36 where "b" is the bcd and "c" is the centre to centre distance of adjacent bolt holes in a 5-bolt circle.

The easiest way to measure the c-c distance with a dial or digital caliper is to measure the inside diameter of the hole, zero the caliper, then measure the distance from the outside of one hole to the outside of the next hole using the inside measurement jaws. Your reading will be the exact c-c distance between the holes.

If you only have a tape measure or ruler handy, there is another method to determine an approximate bcd which is used for 5-bolt car rims. Measure from the outside edge of one hole to the centre of the one of the opposite holes. This may not be as accurate for chaiwheels as car rims because of the relative diameter of the holes and the bcd, but it sould be in the ballpark. http://www.mrt-wheels.com/baseimages/5lug.jpg

John Betmanis
Woodstock, Ontario
Canada