Re: [CR]Measuring BCD - - Bolt Circle Diameter

(Example: Component Manufacturers:Chater-Lea)

In-Reply-To: <011301c3fad4$70ae7cc0$e64efea9@oemcomputer>
References: <011301c3fad4$70ae7cc0$e64efea9@oemcomputer>
Date: Tue, 24 Feb 2004 13:36:17 -0500
To: "Raoul Delmare" <Raoul.L.Delmare@worldnet.att.net>, "C.R. List" <classicrendezvous@bikelist.org>, "Bruce C." <BruceCumberland@comcast.net>
From: "Sheldon Brown" <CaptBike@sheldonbrown.com>
Subject: Re: [CR]Measuring BCD - - Bolt Circle Diameter


Raoul Delmare wrote:
>
> In the past , looking at charts , the procedure was to measure the
>distance , center-to-center , or edge-to-edge , between any two bolt
>holes on your chainring ( chainwheel ) .
>
> BUT , you have to know which one your chart is looking for ,
>center-to-center , or edge-to-edge .

My chart, at http://sheldonbrown.com/gloss_bo-z.html#bcd uses the standard center-to-center procedure. This is the logical way to measure such things.

Since chainrings come in descrete specific BCDs, you don't need any more precision than you get by holding a ruler up against the chainring or the stack bolts and approximating where the centers are.
> Then just look up that number on your little chart ! Et voila !
>Instant Bolt Circle Diameter ( " B. C. D. " ) information about your
>chainrings ! Personally , I'd like to see SOMEBODY post this
>information , in a big way , SOMEWHERE ! Why not , at some public
>location , post three ( 3 ) easy-to-use conversion charts :
>
>1.) B.C.D. derived from center-to-center distance between any two
>bolts , or bolt holes .

See http://sheldonbrown.com/gloss_bo-z.html#bcd
>2.) B.C.D. derived from edge-to-edge distance between any two bolt
>HOLES - but the chainring must be OFF the crank to accurately measure
>this .

Not necessary, just subtract the bolt hole diameter (usually 10 mm) from the center-to-center dimension.
>3.) B.C.D. derived from edge-to-edge distance between any two BOLT
>HEADS - but the chainring must have the bolts fitted to accurately measure
>this .

Just subtract the bolt head diameter (usually 12 mm) from the center-to-center dimension.

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