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Thread: longjohn mk8

  1. #1
    faster than a glacier...
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    longjohn mk8

    So I am working on a design for a 26" wheeled longjohn cargo bike. I've got a few questions that I need some help with. First, I am thinking of using 1.5 x 0.083" for the DT and 1.75 x 0.083" for the boom. Is 0.083 sufficient given the long lever I'm dealing with? My other question centers around the durability of the boom. If I put two 1.5" holes in it so I can braze in 1.5 x 0.058 sleeves that will be a slip fit for 1 3/8" tubing, will the boom hold up given the high leverage forces a loaded cargo bike will experience?

    Thanks,
    Chris

    longjohn mk8-bicycle-forge-longjohn-mk8.jpg

  2. #2
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    I've only built one, so take this for what it's worth:
    -I used 1 5/8x.083" for the keel/boom and the same for the downtube. This has been fine with loads up to about 120 pounds. I haven't tried anything heavier. 1.75 x .083 should be fine, I think.
    -I used 1 1/4x.035 for my rear head tube and used a 1" headset to minimize the size of the pierced hole through the keel, because that also worried me. I don't know why you would need to sleeve that pierced joint area but maybe I'm not understanding you correctly. I would be a bit sketched about piercing 1.75" with a 1.5" hole even with something welded into there.

    As I said, though, I have pretty limited experience. There is a forum member (who helped me design mine) that has done a bunch, maybe he will chime in.

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

  3. #3
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    Thanks for the advice Walt. I was thinking of the two 1.5" to serve as sleeves for the cargo rack, but now I'm rethinking having them pierce the keel tube. It will be more tricky to align the pieces externally, but it shouldn't compromise the strength of the keel.

    -Chris

  4. #4
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    Would it be possible to drop the wall thickness to 0.065" for the keel and down tube if the design is changed to having a top tube that runs the whole length of the bike?

    longjohn mk8-bicycle-forge-longjohn-mk8.jpg

  5. #5
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    Sure, but why? You'll ruin your ability to carry big/bulky loads.

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

  6. #6
    Randomhead
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    Walt,
    You just have the one 1.75x___ tube carrying the whole load, correct? The platform doesn't really keep that keel tube from carrying the whole load?

  7. #7
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    The platform on mine is just sitting on the keel. It doesn't help anything structurally. So yes, the keel takes the whole load. Is that what you meant?

    I imagine you could get a little bit of triangulation by using putting the keel lower and making the platform a structural element of the frame. That seemed overly complex to me for negligible gains so I just used a really beefy keel.

    Oh, btw, use a 20" front wheel, it will help a ton with making space for the cargo.

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

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