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  1. #1
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    Help with butted seattube lengths and cuts

    First time builder, so excuse what may be an ignorant question.

    I purchased a 33mm seat post from Henry James. It is triple butted .9/.8/1mm. I want to preserve as much of the thicker parts at the top and bottom but still get it to the correct length I need, 420mm.

    If I cut 170mm off the top, that would leave me with 130 mm of the 1mm section. This is assuming that the ID doesn't change at the first taper. The dropper post I'm using is 190mm below the collar. That leaves 60mm being supported by the thinner section. Anyone see that being a problem?

    Also with the above numbers I would cut off 60mm from the bottom giving me 40mm of the .9mm at the bottom of the seat tube to braze onto. Does that seem okay to all you experts?

    Thank you for any insight.
    Cycle tracks will abound in Utopia. ~H.G. Wells

  2. #2
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    Will you be brazing on a front derailleur hanger?

  3. #3
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    Not sure what seat tube you're using. I'm assuming it's one not listed on the site. In general, butted seat tubes are made in a way that you don't cut the top off at all and just trim the bottom. If trimming just the bottom puts you too close to the butt, then you need a smaller sized seat tube.

    I'd say if you've got 40mm left of the thick portion on the bottom, you're probably fine, but the amount you're trimming off the top has me wondering if you've got the measurements of the seat tube right or if you might be using it upside down???

    Is this an externally butted seat tube?

  4. #4
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    I will not be using a front derailleur.

    And. Here are the specs.

    Help with butted seattube lengths and cuts-screen-shot-2017-07-18-1.25.21-pm.png

    It seems that it is partially externally butted and partially internally butted?

    Thank you both for responding.
    Cycle tracks will abound in Utopia. ~H.G. Wells

  5. #5
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    If you're planning on cutting 170mm off the "top" and still having 130mm left. I think you're thinking of this tube upside down. The top is the externally butted section on the right in the drawing.

    Think of this another way, the ID of the right section is 1.248" = 31.7mm i.e. slightly oversized (which is to say just right after joining) for a 31.6 seat post. The left side has an ID of 1.225" = 31.2mm i.e. not sized for any seat post.

    The entire tube is 650mm long, you need it to be 420mm. So you need to cut off ~230mm or 9".

    (I don't know if that's you're center of BB to top of ST distance (i.e. you still need to miter out of that) or that's you're crotch of the miter to top of the ST. If you're using a 38mm BB shell, there's only a 19mm difference between those two numbers, which I think it turns out won't be a deal breaker either way.)

    If you need to cut off ~230mm or 9", you've got 300mm/11.8" at the bottom. So if you just cut off the bottom, you'll still have around 70mm of butted section left and will still be fine. No need to cut off the top. And if you don't cut off the top you'll have 300mm of straight wall tubing at the top or plenty of room for your dropper.

  6. #6
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    Oh my. Yes I drew the tube upside down in my notebook with the lengths not matched to the correct end. Thank you.
    Cycle tracks will abound in Utopia. ~H.G. Wells

  7. #7
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    To explain the external butt on the seat tube. Its purpose is to allow for a variety of scenarios. One is to have enough wall thickness so as not to distort too much when heated. With butt joining methods this replaces the traditional seat lug which beefed this area up. It gives greater wall strength for the cantilever loads of you sitting on your saddle or bouncing on and off it when traversing bumpy terrain. And thirdly, there are 3 tubes to be welded to the wall here and you need some extra thickness for the attachment. I might also add that you need a reasonable amount of wall thickness for your seat clamp.

    This tube is of a larger OD, so controlling BB sway is not an issue with 70-80mm of thicker butt down there. At 420mm overall length, you have a small triangle working in your favour, so don't feel bad about cutting that much off.

    Eric
    If I don't make an attempt, how will I know if it will work?

  8. #8
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    Thanks Eric. I made it all work. Now working on figuring out my McGyver jig for cutting and brazing the chainstays.
    Cycle tracks will abound in Utopia. ~H.G. Wells

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