Re: Bolt Hole Circles

On 27 Dec 2004 14:04:44 GMT, [email hidden] (Qui si parla

Campagnolo ) said:

jones-<< you
specify the radius and # of teeth, correct? >><BR><BR>

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.

jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3

Peter Chisholm said:

Nope. BCD or bolthole chainring diameter is taken from a straight

line of two

Quoted message said:

adjacent holes and a line perpendicular to that line to the opposite

bolt hole.

That method doesn't give the bolt circle diameter.

Tom Ace

A usually-reliable source wrote:

BCD or bolthole chainring diameter is taken from a straight line of two

Quoted message said:

adjacent holes and a line perpendicular to that line to the opposite bolt hole.

Nope, that's not correct. It is "measured" the cirucular arc, not from
a straignt line.

The easiest way to calculate this is to measure between two adjacent
bolts, then multiply that dimension by 1.7. and round up.

I've got a chart on a couple of different places on my Website that
lists this information:

http://sheldonbrown.com/gloss_bo-z.html#bcd

and:

http://harriscyclery.com/chainrings

Sheldon "Insert Nickname Here" Brown
+----------------------------------------------------+
| A little inaccuracy sometimes saves tons of |
| explanation. --H.H.Munro ("Saki"😉(1870-1916) |
+----------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

John Everett said:

Quoted message said:

Quoted message said:

specify the radius and # of teeth, correct?

Quoted message said:

Quoted message said:

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.

Quoted message said:

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

Jobst Brandt
[email hidden]

Quoted message said:

From: [email hidden]
Subject: Re: Bolt Hole Circles
Date: Mon, 27 Dec 2004 19:52:38 GMT

John Everett said:

Quoted message said:

> specify the radius and # of teeth, correct?

Quoted message said:

Quoted message said:

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.

Quoted message said:

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)).

Joe

Joe Riel said:

Quoted message said:

From: [email hidden]
Subject: Re: Bolt Hole Circles
Date: Mon, 27 Dec 2004 19:52:38 GMT

John Everett said:

>> specify the radius and # of teeth, correct?

Quoted message said:

> 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.

Quoted message said:

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. :-(

jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3