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


Example: Bike Shops
Date: Mon, 4 Feb 2002 10:17:12 -0800 (PST)
From: "Tom Dalton" <tom_s_dalton@yahoo.com>
Subject: Re: [CR]Why 70mm for Italian BB?
To: dave <kawika@austin.rr.com>
Cc: classicrendezvous@bikelist.org
In-Reply-To: <p0433011cb884772704a1@[24.160.3.91]>


Good point... I had once assumed that the English spec. was just rounded to the hundredth, but then I guess it would be 1.37" rather than 1.370". The newer ISO freewheel and BB thread spec is 1.375". If there was reason to believe that machinine tools couldn't hold thousandths "back in the old days", I doubt there is, then the rounding theory would make sense. More likely the "0" in the older spec. is significant and the spec was simply chosen because it was round. The ISO spec may differ because it is an attempt to get the best compromise between English and Italian freewheels. The English and Ital. freewheel theads only differ in thread profile, so perhaps the extra 0.005" leaves more material to mush around with different freewheel bodies.

Tom Dalton


--- dave wrote:


> > >Anybody know why they differ in the first place

\r?\n> and

\r?\n> >>why the threading is 1.37 inches for English and

\r?\n> 36mm

\r?\n> >>for Italian?

\r?\n> >

\r?\n> >I don't know why there are two different standards

\r?\n> in

\r?\n> >widespread use, let alone all the others, but I do

\r?\n> >have one piece of info that may or may not already

\r?\n> be

\r?\n> >obvious. For some time is wondered where an

\r?\n> archane

\r?\n> >standard like 1.370" came from. When I realized

\r?\n> that

\r?\n> >this is simply 1 3/8" and it suddenly seemed a bit

\r?\n> >more reasonable. Don't know of many machinists who

\r?\n> >work in fractions, but maybe back in the day...

\r?\n>

\r?\n> math not strong but: 1 divided by 8 = .125 (or

\r?\n> 1/8th) thus 3/8ths =

\r?\n> .125 x 3 or .375

\r?\n>

\r?\n> is it close enough? or some rounding downwards

\r?\n> going on?

\r?\n>

\r?\n> d dickerson, austin, tx