seat tube sleeve - brazing clearance?

[Andrew Drummond]

Hi all. I'm putting a seat tube sleeve on my daughter's frame (to reinforce where the twin top tube/seat stays are attached, because it's not an externally butted tube) and I'm wondering about how much clearance I should allow for brass brazing (I'm machining the tube). The tube will be fairly long, tapering to a point at the bottom - maybe three inches overall. Is the necessary clearance a function of joint length, or is it more or less constant regardless of length? 90% of my experience so far is with fillet brazing, the rest being braze-ons/fork crowns/dropouts - so I'm a little hesitant to tackle the long sleeve without some practice and a little guidance if I can get it. I'd be more comfortable with silver for this joint, but I'm fillet brazing some tubes to the outside of the sleeve... so brass it is. And I thought about brazing the tubes on first then silvering the sleeve, but that seems like a bad idea (whether it actually is or not, I'll leave to the experts).

thanks,
Andrew

[Mark Bulgier]

I recommend having the “sleeve” overlap the seat tube by only say 5 to 20mm.  The then rest of the “sleeve” should be full thickness, with the seatpost diameter as its ID.  I think having the sleeve be a doubler for the seat tube, brazed over its entire 3” length, is a bad idea. 

 

Assuming you’re talking about a 1-1/8” seat tube:  Start with a 1-1/4 x .090” tube, which naturally takes about a 27.0 seat post. Bore it from one end no deeper than 20 mm, to an ID about .008 larger than the actual (not nominal) OD of the seat tube.  That should be loose enough for brass but tight enough to be “self jigging” (it’ll hold itself in alignment while brazing).  Bore the rest of its length to a little larger than 27.2 mm, for a slip fit on that size seatpost.  This will take the mill scale off so you have shiny steel all the way up, and will very much reduce the wear on your reamer after final assembly. 

 

Finally (or do this part first, doesn’t matter much), turn the OD of the sleeve down to taste, so the shoreline of the “lug” at the bottom is not so thick.  For looks, light weight, stress-relief, or easier brazing, however you justify it to yourself, but I used to go for a 1.0 mm lug edge or less, so about 1.190- 1.200”.  Again, this takes the mill scale off so it’s all shiny and ready for brazing.  But really that step is optional and can be skipped if you don’t mind a somewhat thick (~.058” or so) lug edge and a fairly heavy-looking part.

 

The following is not specific to a twin-TT design, but refers to a trad bike with a regular TT:  Yeah it’s a little funny mitering your top tube to fit up against this 1.200” seat tube extension, if you use holesaws for mitering, since there’s no real common hole-saw in that size, but 1-3/16” saws do exist, which is nominal 1.1875”, or “close enough”. (Or just turn the OD of your ST extender to exactly match whatever radius your holesaw actually cuts.  Most cheap holesaws cut a little oversized anyway.)  Plus, adjusting the radius of the miter is fairly simple, by squishing the end of the tube a tiny bit oval – squish one way to increase the miter size, squish the other way to decrease it.  Ask me for tips if this is not obvious.  If I didn’t have a 1-3/16” holesaw, I’d just miter the TT with a 1-1/8” or 1-1/4” holesaw and ovalize the end of the TT a little as necessary to make the miter fit.  Such a small amount of ovalization is not noticeable.

 

Sculpt this 20mm deep socket to the points you like, so the depth of socket is maybe 5 mm or less at the minimum where it’s scooped out, and 20 mm at the points.  Brazing the seat tube into that socket is easy.

 

Now when you braze (or TIG weld?) the top tube and seat stays to the “sleeve”, you’re brazing on the solid one-piece part, not disturbing the earlier brazing of the tube to the sleeve.  In fact, if the brass (or TIG) fillet outside is far enough away from the socket for the ST, you can silver the socket without worrying about re-melting the sliver later.  Takes some heat control though…

 

Did I explain that OK?

 

Mark Bulgier

Seattle

[Andrew Drummond]

Thanks for the thorough response! I'm with you, Mark, except that this is a mixte-style frame and there's four inches or so of seat tube above the top tubes/seat stays, so I think I'm stuck with a sleeve brazed full length. I can make it shorter, though - the length is really an aesthetic thing (I like the look of a long point, but can live without it if my skills aren't up to it!). I suppose I could also splice the heavier tube (machined a bit thinner) onto the top as you suggest; I'll take a look and see how that'll work. I'm a little leery of a spliced seat tube with that much extension above the tt/st joint, but with a long enough seat post I think it'll be ok.

Andrew

[jon norstog]

Andrew, list:

I've learned the hard way that it helps to have a little more beef in the ST where a mixte's twin tubes join it.  I use a 28.6 mm st on all my city bikes and use 1.25" x .058 cro-mo tubing to make the reinforcing sleeves at that joint, and at the seat cluster. 

I clean inside the sleeve(s) with a carbide burr and flux it up before sliding it (them) over the ST, then braze as if they were a lug.  I don't do over-long sleeves, maybe2 1/2" max at the seat "lug." and in most cases they are pretty inconspicuous.    The brass goes right in, no muss no fuss.

Good luck!

jn

"Thursday"

On Mon, Jun 10, 2013 at 8:25 AM, Andrew Drummond <andrew.d...@gmail.com> wrote:

Thanks for the thorough response! I'm with you, Mark, except that this is a mixte-style frame and there's four inches or so of seat tube above the top tubes/seat stays, so I think I'm stuck with a sleeve brazed full length. I can make it shorter, though - the length is really an aesthetic thing (I like the look of a long point, but can live without it if my skills aren't up to it!). I suppose I could also splice the heavier tube (machined a bit thinner) onto the top as you suggest; I'll take a look and see how that'll work. I'm a little leery of a spliced seat tube with that much extension above the tt/st joint, but with a long enough seat post I think it'll be ok.

Andrew

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