Hello everyone -
Long time lurker, first time poster. I'm currently drawing up frame number... 6-ish... and have run into a question. I've read nearly every thread on this forum over the last couple of years, but I do apologize in advance if this has been covered previously. My bookmarks didn't reveal much.
This project is a dropped top-tube, mixte-style frame for my wife. All fillet brazed. The top tube will connect to the middle of the seat tube. There will be two sets of seat stays - one attaching to the seat tube directly behind the top tube and one in the traditional place. All fastback style, fillet brazed again.
I've built one frame previously in this style, for myself. In that case, I used a straight gauge 1.25"/.035" seat tube, with a reinforcement sleeve only at the top-most junction. (Thanks to Steve Garro and John Norstog, who discussed this in a thread long ago.)
This time, I'd like to use a 28.6 seat tube, if I can. It seems my options are 1. An externally butted tube, everything brazed directly to it, or 2. A single-butted seat tube with sleeves at (both? or just the top-most?) brazing points.
Are there any rules-of-thumb you've come to accept when brazing to the thin part of butted tubes? I know there's been discussion of braze-on placement (like shifter bosses on downtubes) landing in the thin section of a tube, but anything specific to seat tubes? Is the concern, in this case, distortion and insertion of the seat post, or general strength of the tube itself?
Thank you for any replies. I appreciate your willingness to share more than I could state here!
(Also, the photos and ramblings of a hack-hobbyist, if you're interested: Instagram @theblandbicycle )
-Chris
Chris Blandford
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