How to minimize distortion on "lowered top tube" bikes- Mtbr.com
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  1. #1
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    How to minimize distortion on "lowered top tube" bikes

    A few frames ago, I built myself a mountain bike that looks like this:

    As you can see, I dropped the top tube down for better stand-over clearance.
    The problem is that when I welded the top tube to the seat tube, the seat tube curved forward significantly from the heat of welding, enough to make the seatpost bind in the seat tube when I put it down all the way. Not a huge deal, but I would like to prevent it from happening in the future. Does anyone have any suggestions on how to mitigate this distortion?
    My welding has gotten better since then, so I think I could put a little less heat into the tube than previous. I don't think I can heat-sink it, because it's below the seat tube collar, which has a smaller ID than the rest of the tube.
    Should I build a fixture to hold the seat tube straight while I weld it?

    Should I tack weld it first, and then weld it at the same time that I weld the seatstays so that the pull opposite each other?

    As always, yall's advice and criticism is appreciated.
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  2. #2
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    What seattube did you use? Was it bent forward before you put the brace in?

  3. #3
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    I was using straight gauge 1-3/8" .030 wall for the seat tube and a topper/collar thingy welded on top.
    I'm confident that the tube was relatively straight to begin with. And to clarify, I don't believe that it was the brace itself that caused this distortion, but the location of the top tube seat tube junction.

  4. #4
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    The miters on TT or brace could have had gaps that closed up when welded or it could have been just too hot as you mention or any number of things.

    I don't think adding seatstays at the same time will solve the problem.

  5. #5
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    I would put a sleeve over the top tube/seat tube/seatstay junction and braze it after to the seat tube. Thatíll be beefy and prevent distortion and give the frame a bit of flair. Welding that much to 0.035Ē wall tubing is too much for a MTB and a new builder (IMO). That is the most common crack location on traditional frames even with ext butt seat tubes (0.047Ē at the top).
    Otherwise you could slide in a huge heat sink I guess. But Iíd do the sleeve (one diameter up and 0.058 wall).


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  6. #6
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    You can weld the ST/TT junction with the gusset already brazed in there. Also you will want to make sure that your TT and seat stays are welded to a tube that is supported from the inside by the seatpost, or they will pierce the seat tube. Heat sinks help too.

  7. #7
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    My first thought was to alter the design so you don't need a buttress. Those are unfashionable. Then i saw you're 6'8 and leggy.

    You can probably still get away with a shorter seat tube than you could a couple years ago because dropper posts have gotten a lot longer.

    Another option would be to use an oversized seat tube, along with a topper and possibly a shim. This has the benefit of adding some extra stiffness to the frame- less flexy when you pedal standing. Also an alu shim is kinder to your seatpost.
    "Things that are complex are not useful, Things that are useful are simple."
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  8. #8
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    Iím not a super experienced frame builder, but donít plenty roll cage welding, so this may be a load of rubbish!

    First check seattube is straight, if not give it a tweak, you want the best possible starting point. I would use a topper that overlaps and drops into the seattube, before welding double check it is a very good straight fit.

    Second I would weld TT and Seatstays (maybe with a heat sink, if not now is the chance to grind off and high spots that may hinder seatpost), making sure mitres are perfect. At this point check for bend, if it has, get a bar in there and get cold working until itís straight.

    Weld in seatube topper thing, again cold work if necessary to get it straight again.

    Now for the brace thing, again mitres need to be perfect, weld the top joint first, this will now probably pull Seatube forward, either go for more cold working, or put a tack or 2 at the back of the brace where it joins top tube, as this weld cools it should hopefully pull the brace backwards, and help remove any forwards bend in the seatube. Now think carefully about what parts of this weld you are going to finish, it will have an effect and pull top of seatube in a certain direction.

    Very long winded, hope it helps. And more importantly, I hope you understood!!!!m not a super experienced frame builder, but donít plenty roll cage welding, so this may be a load of rubbish!

    First check seattube is straight, if not give it a tweak, you want the best possible starting point. I would use a topper that overlaps and drops into the sea tube, before welding double check it is a very good straight fit.

    Second I would weld TT and Seatstays, making sure mitres are perfect. At this point check for bend, if it has, get a bar in there and get cold working until itís straight.

    Weld in seatube topper thing, again cold work if necessary to get it straight again.

    Now for the brace thing, again mitres need to be perfect, weld the top joint first, this will now probably pull Seatube forward, either go for more cold working, or put a tack or 2 at the back of the brace, as this weld cools it should hopefully pull the brace backwards, and help remove any forwards bend. Now think carefully about what parts of this weld you are going to finish, it will have an effect and pull top of seatube in a certain direction.

    Very long winded, hope it helps. And more importantly, I hope you understood!!!! (Main thing is that mitres are perfect, the less gap, the less pull)

  9. #9
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    Quote Originally Posted by Meriwether View Post
    ......I would put a sleeve over the top tube/seat tube/seatstay junction and braze it after to the seat tube....
    Agree.

  10. #10
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    Cool, thanks for all the suggestions!

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