If a (double-butted) 531 down tube is used as a seat tube (either intentionally or unintentionally), the seat post subsequently required can be a 26.6 or 26.8 instead of a 27.2 mm one.
Also, if the single-butted 531 seat tube is inserted upside-down, the same thing will usually happen.
Regarding SP vs. SL tubing in larger framesets, often only the 'hull' is SP (down tube, chainstays, fork blades), and the rest remains SL, so the seat post would still be a 27.2 mm one in those cases.
Sometimes they also used whatever was lying around...... :-)
Greg Parker Ann Arbor, Michigan http://www.bicyclassics.com
Date: Wed, 5 Mar 2008 13:26:44 -0800 (PST) From: jeffrey piwonka <jmpiwonka@yahoo.com> To: Richard Robinson <robin.richard@gmail.com>, classicrendezvous@bikelist.org Subject: Re: [CR] Seat Post for Colnago Message: 3
they also could've used a mixed tubeset. i have a gazelle with a reynolds 531c sticker on it, it takes a 26.8mm seatpost (much to my surprise, i expected 27.2).
Jeff Piwonka
austin, texas usa
> Hello,
\r?\n>
\r?\n> As far as large frames using Columbus tubing go
\r?\n> they normally take a
\r?\n> smaller diameter seat post, normally 27.0 (Columbus
\r?\n> SP/SPX) or smaller due
\r?\n> to the increased wall thickness of the tubes used to
\r?\n> keep the frames stiff.
\r?\n> Or at least this is what I assume. I was shocked to
\r?\n> see my 78-79 Colnago
\r?\n> Super take a 27.2 seat post; come to think of it my
\r?\n> Basso Gap also takes a
\r?\n> 27.2 seat post, but my Mondonico and Bottecchia both
\r?\n> take a 27.0. I guess
\r?\n> this is the difference in tubing thickness between
\r?\n> SL/SLX and SP/SPX. BTW,
\r?\n> both my Reynolds 531 Schwinn Paramounts take 27.2.
\r?\n>
\r?\n> This leads to a question about Colnagos, how were
\r?\n> their frames measured?
\r?\n> I have a 62cm C to C frame and the one being sold by
\r?\n> David White is a
\r?\n> 62.5cmC to C. Did Colnago make half sizes? The
\r?\n> reason I ask is that
\r?\n> all my other
\r?\n> 62cm c to c frames (both American, French and
\r?\n> Italian) fit perfect with a
\r?\n> Brooks saddle and a campy Record seat post at or
\r?\n> close to the max limit
\r?\n> extension, that is all but my colnago super. for
\r?\n> some reason the Colnago is
\r?\n> still more than .5cm short. Does have to do with
\r?\n> the seat tube angle?
\r?\n>
\r?\n> Rich, who now believes he should use a larger
\r?\n> Colnago frame, Robinson
\r?\n> Jacksonville NC