Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=framebuilders.10905.0051.eml
Date: Mon, 4 May 2009 00:01:17 -0700
From: Mark Bulgier <Mark(AT)bulgier.net>
Subject: Re: [Frame] seat tube sleeve on a fillet brazed bike, how thick?


Alex wrote: 

> I just made a top of seat tube sleeve for this frame
> that I'm building.
> My stupid question is what wall thickness should I use?
> 
> I started with 1/4" x 0.058".  

May I suggest a completely different approach?  I don't like "doubling"
a tube like that (tube over a tube) over a long distance.  Getting braze
penetration takes a significant time at high heat and increases warping.
Better to use a single thickness of tube that's thick enough without
doubling.  My preference is to make your sleeve starting with 1-1/4" x
.090, which has a 27.2 mm ID or just a little under - you will have to
bore it or ream it a scoshe for proper fit on a 27.2 seatpost (unless
your post is undersized as they often are...)  But you'll have the
sleeve piece in the lathe anyway because you're going to bore a 1-1/8"
step (plus a little for braze clearance), about 1 to 2 cm up from the
bottom.  Probably 5 mm of overlap between the seat tube and sleeve would
be enough for strength, but I like a bit more because it makes the
assembly accurately self-aligning, and because I usually sculpted the
bottom edge to a pleasing shape.

Turn the OD down a bit too, for light weight (or more realistically, for
the light weight LOOK -- the chips you make won't actually weigh that
much!)  If you wanted a 1mm thick "lug" edge, you'd turn the OD to 30.6
mm or around 1.200".  This has the added advantage of taking the black
oxide finish off leaving nice shiny metal for brazing. Use a 1-3/16"
hole saw to make the top tube miter, it's close enough, especially if
you have one that cuts a tad oversize as they so often do...  Or just
turn the OD of the sleeve to 1-3/16" (1.188) and have the lug edge a tad
thinner.

The shelf in the sleeve, that determines the top of the ST, must be
fully below the top tube if you braze the sleeve then TIG weld the TT to
it.  But for a fully brazed frame, the seat tube shelf can be well into
the TT fillet.  In my opinion, there is nothing to worry about if the
braze between the ST and sleeve gets remelted a bit, later when
attaching the TT (within reason).  Just have a care that your shorelines
are still clean.

Mark Bulgier
Seattle WA USA
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