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Thread: Seat tube to down tube angle

  1. #1
    Join Date
    Jun 2012
    Carrollton TX
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    Default Seat tube to down tube angle

    For my last two frames, I changed the way I braze the BB (lugged).
    I used to braze the seat and down tubes in at the same time.
    Now, I braze the seat tube in first, clean it up, then braze the down tube.
    I use a piece of square aluminum bolted onto a BB face to place the tube parallel to the BB face.
    Then, I pin the joint with 4 pins.
    Then, I take off the aluminum and braze free standing.

    The seat to down tube angle ends up being a little more acute (just a bit smaller) than I had when I pinned the joint before brazing.
    So, frame drop was a bit more than I originally planned. (The ride is great, but I do strike a pedal now and then.)

    So, is this common?
    Do you set the joint up a bit wider to compensate for this?

    Thanks for any advice.
    Mark Walberg
    Building bike frames for fun since 1973.

  2. #2
    Join Date
    Jun 2009
    Happy Valley, PA
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    Default Re: Seat tube to down tube angle

    I have done it that way in the past. I made a practice front triangle without a rear triangle, then decided to finish it. I had the same problem as you did, the chainstay sockets are at too close of an angle to the seat tube. I am pretty sure it all would have been ok had I put the rear triangle on at the same time. This was with round-oval-round stays, I think the sockets for oval chainstays are a lot more adaptable to different angles.

    The only bike I did where I brazed the seat tube first worked out fine. I expect it depends a lot on what angles your bb shell has. I think doing it tube by tube is fraught with danger on this front though, the advantage of being able to take advantage of slop to move the lug angle a little is pretty useful.

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