Frame Build #2 with CAD drawings!- Mtbr.com
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  1. #1
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    Frame Build #2 with CAD drawings!

    Hi Everybody!
    Lots of time off work right now, and this is prep to help a student of mine build a frame. BungedUp has been super helpful to get this going, and I'd like to be able to hit the ground running as soon as we go back...
    Here we go!
    Second frame build, a dirt jumper for a #150 smooth riding friend. Despite that, I want this to be pretty tough and strong. Geometry is conservative on purpose, 26" wheels, and exactly what he likes. Goin straight gauge .9mm 4130 tubing throughout. 38mm DT, 35mm TT, 35mm seat tube, round SS and CS. Trying the sleeve for a 30.9 ST that Walt has advocated for as something new.
    Frame Build #2 with CAD drawings!-bike.jpg

    First question: Do you guys think I need a brace between SS and CS for this dropout I cut? These are very overbuilt at 1/4" thick 4130 lol. 160mm rotor drawn for reference against the 19mm stays.
    Frame Build #2 with CAD drawings!-dropout.jpg

    Frame Build #2 with CAD drawings!-parts.jpg

    Second question: with my 38x.9 DT, and 35x.9 TT, and 120mm paragon HT, do I need to consider any type of gusset at the HT junction? I'd like to go without, if possible...
    Frame Build #2 with CAD drawings!-headtube.jpg
    Thanks guys, looking forward to getting started!
    Scott

  2. #2
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    It will be tough and strong but it won't be light with the tube and drop out selection.

    You shouldn't need any brace under the brake tab or gusset at the head tube with good fit up and welds. I think head tube gussets especially can make the frame weaker if they aren't executed correctly.

  3. #3
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    Thanks for the input!
    Ok that was my thought on gussets... arenít dirt jumpers typically kind of heavy for strength/durability? I found a 38x9/6/9 downtube I could buy, and I have a 35x9/6/9 nova tube with bend. My concern with using it as the top tube is that I donít know where the butt at the ďbent endď terminates, if that makes sense. Can I assume itís after the bend?
    If I go to all this trouble for these two tubes, am I going to save much weight?

    I am using 19mm straight gauge in hopes that I can do my own bends here at home. My last frame I did bends in the 5/8 stays and it went well.

    I designed and cut the dropouts out of 1/4Ē 4130 since that was all I had, and figure it was better than mild steel.

  4. #4
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    Older dirt jumpers were definitely tanks for that very reason but a butted tube frame can be just as strong as straight gauge. My thought is the lighter the bike, the easier it is to maneuver especially mid air. And your point is well taken, those two tubes won't give you a huge weight savings but i think a few ounces on a 4-5 pound frame can be a big performance jump. However, Id also consider the parts build before I put too much work into those tubes.

    As far as the bent Nova tube, I doubt they provide a butting profile online so we won't know for sure unless maybe one of these forum guys has that info!

    And for the dropouts, 1/4" is pretty thick but send it if you like it. I would be tempted to go on Aircraft Spruce and Specialty and get a thinner 4130 plate. I think a 9"x9" of .190 is like $20.

  5. #5
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    Im currently mocking up a dirt jumper frame here. Bikefabsupply has a cool 1-.7-1 38mm heat treated downtube from TANGE for 20 bucks. Sounds overkill but the butts fall just into the right spot for a large-ish dirt jumper.

  6. #6
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    I saw that one! I decided to just stick with all aircraft spruce tubes since Iím not familiar with tange or the heat treated tubes. Looks really nice! My tiny bit of rewatch seemed that those tube sets were for lugged frames? Thanks for the heads up! Post up some pics of your DJ when itís done

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