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Thread: First Build with a bent seat tube

  1. #1
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    Default First Build with a bent seat tube

    Heya,

    About to start my first build with a bent seat tube. Everything seemed fine + dandy until I started poking around the interwebs for some info.

    I think I'm confused but maybe I'm not. Would love a bit of clarification.

    1. Seat tube angle is 71-degrees
    2. Effective seat tube angle is 74.4-degrees
    3. My fixture does not have a seat tube offset thingy (a la Cobra's fixture)

    My assumption is that the seat tube angle is meaningless and that I should be using the effective seat tube angle to set up my fixture. Is that correct?

    The seat tube offset addition is that something necessary (i.e. I should stop right now and fab one up) or just a nice-to-have for builds like this?
    How does it simplify the build process?
    What is that offset dimension pulled from in bikeCAD?

    Thanks
    elysian
    Tom Tolhurst

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    Default Re: First Build with a bent seat tube

    Set your HT, place your TT, adjust STA accordingly. None of this stuff exists in isolation.
    Last edited by Eric Estlund; 09-28-2021 at 01:21 PM.

  3. #3
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    Default Re: First Build with a bent seat tube

    Quote Originally Posted by false_aesthetic View Post
    Heya,

    About to start my first build with a bent seat tube. Everything seemed fine + dandy until I started poking around the interwebs for some info.

    I think I'm confused but maybe I'm not. Would love a bit of clarification.

    1. Seat tube angle is 71-degrees
    2. Effective seat tube angle is 74.4-degrees
    3. My fixture does not have a seat tube offset thingy (a la Cobra's fixture)

    My assumption is that the seat tube angle is meaningless and that I should be using the effective seat tube angle to set up my fixture. Is that correct?

    The seat tube offset addition is that something necessary (i.e. I should stop right now and fab one up) or just a nice-to-have for builds like this?
    How does it simplify the build process?
    What is that offset dimension pulled from in bikeCAD?

    Thanks
    Seat tube angle doesn't matter as it's a means to an end. The end is where the saddle is (placed.)
    If you know the linear setback measurement you can fixture to it. The angle is what it is.

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    Default Re: First Build with a bent seat tube

    Quote Originally Posted by false_aesthetic View Post
    Heya,

    About to start my first build with a bent seat tube. Everything seemed fine + dandy until I started poking around the interwebs for some info.

    I think I'm confused but maybe I'm not. Would love a bit of clarification.

    1. Seat tube angle is 71-degrees
    2. Effective seat tube angle is 74.4-degrees
    3. My fixture does not have a seat tube offset thingy (a la Cobra's fixture)

    My assumption is that the seat tube angle is meaningless and that I should be using the effective seat tube angle to set up my fixture. Is that correct?

    The seat tube offset addition — is that something necessary (i.e. I should stop right now and fab one up) or just a nice-to-have for builds like this?
    How does it simplify the build process?
    What is that offset dimension pulled from in bikeCAD?

    Thanks
    The offset thingy is nice, but not necessary depending on how fudge-tolerant your mindset is. You definitely don't need it to make a bike.

    The seat tube angle is meaningless in the context of what the rider feels. As long as the saddle is in the right spot, they're good to go and it doesn't much matter how you get there. For building the bike though, it depends on your workflow. The first thing you need to know is how does your actual bent seat tube compare with how you modeled it in BikeCAD? That's where knowing your offset helps. It'll help BikeCAD spit out an accurate TT/ST miter angle. If you're a "get it close and file it" kind of person, get it close and file it.

    The offset dimension pulled from BikeCAD is the distance from the center of the BB to the centerline of the top section of tube, measured perpendicular to the centerline of the tube. If you know that, your bend can have any radius to it, or even just land on the down tube and BikeCAD is going to give you a good miter angle. If you don't know it and can't figure it out, you're going to spend a little time filing.
    Sean Chaney
    www.vertigocycles.com
    a peek behind the curtain

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    Default Re: First Build with a bent seat tube

    Sean,

    Thanks. That makes sense. Your summary confirms some of what I was thinking and helps clarify my path forward.
    elysian
    Tom Tolhurst

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