Brazing to Thin Sections of Main Tubes

[cblandford]

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

[e-RICHIE]

Add the braze-on to the tube before adding the tube to the frame assembly.
Do otherwise at your own peril.

[Eric Estlund]

Is the concern, in this case, distortion and insertion of the seat post, or general strength of the tube itself?

...yes.

Add beef and you should be ok.

[cblandford]

Eric - thanks. Add beef to the joint at the top of both pairs of seatstays? Or only at the top of the usual pair (where reaming through distortion would cause issues)?

Chris Blandford

[cblandford]

Add the braze-on to the tube before adding the tube to the frame assembly.
Do otherwise at your own peril.

Thanks Richard - I appreciate the advice!

-Chris

Chris Blandford

[Eric Estlund]

I'd be hesitant about any stays brazed on a .6mm seat tube wall.

[cblandford]

I'd be hesitant about any stays brazed on a .6mm seat tube wall.

Great - that helps a lot. Thank you for taking the time!

Chris Blandford

[steve garro]

IMG_3285.JPG


Here is my solution


- Garro.

[cblandford]

Here is my solution

- Garro.

Steve - That's both helpful and awesome. Thanks for sharing!

-Chris

[steve garro]

Steve - That's both helpful and awesome. Thanks for sharing!

-Chris

Finished

IMG_3316.JPG




- Garro.

[cblandford]

Finished

- Garro.

Steve - If you don't mind my asking: what is the round hole in the upper sleeve for? I'd love to see a shot of the entire frame if/when you have one.

-Chris

[steve garro]

IMG_3442.JPGIMG_3445.JPG

Steve - If you don't mind my asking: what is the round hole in the upper sleeve for? I'd love to see a shot of the entire frame if/when you have one.

-Chris

It's the seat tube slot pucker hole

- Garro.

[cblandford]

It's the seat tube slot pucker hole

- Garro.

Well, sure. Now I feel dumb - makes perfect sense to do it A. Ahead of brazing and B. on the front side. Thank you for posting.

I'll post back with my completed frame down the road - thanks again for all the help!

-Chris

[cblandford]

All - Here's what I came up with:








Single-butted seat tube, mixte-style seat lug/binder, and an .058" sleeve for the extra set of stays. I decided to go with side-tacked stays instead of fastback, because the tire clearances and brake posts asked me to. I debated adding a bridge/brace -- that would also attach to the seat tube -- to those stays but decided against it because I liked the aesthetic without. I have slight concerns (not sure if they're warranted or not) about that bridge-less junction -- especially given the brake setup -- but I figure this bike is going to live in my garage and be ridden pretty lightly, so I can keep an eye on it. Feel free to advise if you see anything amiss, though.

Thank you again for the replies and direction. This will be the 3rd frame that I feel ok enough about to send off for paint... I think my wife is going to dig it!

(By the way, Steve... while your example was awesome, my wife isn't really one for hearts. I'm not going to tell her that her new bike is hiding one!)

-Chris

-
Chris Blandford

[steve garro]

All - Here's what I came up with:



Chris Blandford

It came out sweet, man!

- Garro.

[cblandford]

It came out sweet, man!

- Garro.

Thank you!