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  1. #1
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    Bottom Bracket cluster weld sequence

    I am a welder by trade, and am going to attempt my first frame. I have a couple tube sets, and I assume my first couple are going to be simply learning how/where everything moves with each weld pass.

    My question: The ST will be mitered directly to the BB, while the DT will be mitered to both the BB and ST. Are you TIG welders welding the ST completely out before fitting up the DT, or are you fitting it all up together, and welding it while leaving the portion of the ST that will be covered by the DT unwelded? I would assume weld the ST out, but think it will twist without additional reinforcement.

    For reference, this is the cluster in question...
    Bottom Bracket cluster weld sequence-bb.jpg

  2. #2
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    Yes the first thing I do is miter and weld seat tube to the bottom brakes shell.

    Read my post in this thread to see how I build frames. Primum Multorum

  3. #3
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    Weld in the entire seat tube first, then weld in the compound-miter downtube (you can also do the downtube first and miter the seat tube to it if you want)

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  4. #4
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    Yep;

    Miter and fully weld the ST first. Square ("cold set") the ST/BB shell. All other tubes mitered and tacked/welded in after that. Incidentally, I tack the "peaks" of the compound mitered DT first to draw it tight. The "peaks" are the points created between the miters. The idea is to draw the center point of the miter cuts as tight as possible, as this is the plane in which the critical geometry lies. It's a theory.

    After these peaks, I tack at the bottom of the DT. I think this method draws it together most square and tight.
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

  5. #5
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    Just as a matter of discussion: I believe Merlin did some testing regarding attaching the ST to the BB and then dealing with the DT versus attaching the DT to the BB and then dealing with the ST.

    Conclusion was having the ST attached to the BB ended up creating a better overall build in terms of strength, 'stiffness', and longevity of the frame.

  6. #6
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    One thing that I would add is that that you may want to attach the drop outs to the chainstay before you attach the stays to the bottom bracket. I have done most of my frames attaching the drop outs first then fine tuning the miters on the BB side and it seems to work well. On one frame I tried to attach at the BB side first then do the dropouts later and it was much harder to maintain phase and alignment in the dropouts.

    I have not been doing the little bridge on the chainstays and for the most part that seems to be an optional bridge. Many builders do the little bridge while many others avoid it.

    Welding the entire seat tube seems to be the normal practice. It seems that it can be made to work with doing down tube first but for me doing the complete rear triangle before working on the front triangle makes sense.
    Mark Farnsworth, Raleigh, NC
    http://farnsworthbikes.com

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