Re: [CR]Why 70mm for Italian BB?


Example: Racing:Beryl Burton
Date: Mon, 04 Feb 2002 20:42:50 -0800
From: "Jim Allen" <jimallen@nctimes.net>
To: Tom Dalton <tom_s_dalton@yahoo.com>
Cc: LouDeeter@aol.com, classicrendezvous@bikelist.org
Subject: Re: [CR]Why 70mm for Italian BB?
References: <20020204170641.94022.qmail@web10908.mail.yahoo.com>


Tom,

Even stranger, the thread profile is 55 degrees.

So we have 36mm metric, 24 threads per inch (American?) 55 degree thread profile (English Whitworth?).

Still puzzled after all these years.

Jim Allen

Tom Dalton wrote:
>> Anybody know why they differ in the first place and
>> why the threading is 1.37 inches for English and 36mm
>> for Italian?
>
>
> I don't know why there are two different standards in
> widespread use, let alone all the others, but I do
> have one piece of info that may or may not already be
> obvious. For some time is wondered where an archane
> standard like 1.370" came from. When I realized that
> this is simply 1 3/8" and it suddenly seemed a bit
> more reasonable. Don't know of many machinists who
> work in fractions, but maybe back in the day...
>
> What IS whacky is the mixing of, metric and "English"
> units on Italian thread specs. BBs are 36 millimeters
> at 24 threads per inch???? (Campy axles and hanger
> bolts are similarly confused). I think this is
> because many machine tools can cut an infinite range
> of diameters, but threading options are restricted by
> the lead screw pitch.
>
> Tom Dalton
> Lehigh Valley, PA
>
> --- LouDeeter@aol.com wrote:
>
>> Does the 70mm for Italian bb shell, vs 68mm for
>> English offer any advantages?
>> As I was riding today, I was noticing how the
>> chainstays fit into the bb
>> shell and wondering whether an extra mm width would
>> make for a stronger rear
>> triangle. Seems like it is too small to make a
>> difference to me. Anybody
>> know why they differ in the first place and why the
>> threading is 1.37 inches
>> for English and 36mm for Italian? Lou Deeter,
>> Orlando FL. Obviously with
>> way too much time on my hands on a Stupor Bowl
>> Sunday.