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  1. #1
    krshome's Avatar
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    parabola formula question

    If some one can help me, I need the Parabola for a bridge hammock. I'm looking for 85" length and 6"-6.5" depth. Grizz is busy traveling! Thanks

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    Ramblinrev's Avatar
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    I have never used a formula. I mark the end points of the arc, take a fiberglass tent pole (they gotta be good for something) and bend it to fit the depth I want at the center and trace the resulting curve. Has worked for me.
    I may be slow... But I sure am gimpy.

    "Bless you child, when you set out to thread a needle don't hold the thread still and fetch the needle up to it; hold the needle still and poke the thread at it; that's the way a woman most always does, but a man always does t'other way."
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  3. #3
    gargoyle's Avatar
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    Another trick is to tack the fabric to a wall. Keep it level and straight. Now let a string sag between the tacks to your desired 6" cat cut.

    Mark the line the string makes. Remove the fabric from the wall and cut.
    Remember to leave any seam allowance.

  4. #4
    BrianWillan's Avatar
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    Quote Originally Posted by krshome View Post
    If some one can help me, I need the Parabola for a bridge hammock. I'm looking for 85" length and 6"-6.5" depth. Grizz is busy traveling! Thanks
    You need to pin down your depth of parabola you are going to need before you start.

    I made a spreadsheet to do the calculation, however if you want to know the formula that Grizz gave me when I made my bridge, it is as follows. I used a 7.5" depth of curve on a 80" long piece of fabric that was 54" wide.

    y=d - ax^2, where d is the depth of curve and 'a' is (d / (L/2)^2) of course L is the length of fabric you're using.

    Cheers

    Brian
    Good judgment comes from experience, and often experience comes from bad judgment. - Unknown

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  5. #5
    Senior Member DemostiX's Avatar
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    Ramblinrev and Gargoyle have given you the catenary curve that is theoretically ideal. Also the shape and method (RR's) of the oak window stools leading edges I thought and were more attractive than straight ones.

    The parabola is hardly distinguishable from it to the untrained eye, and in practice is good (enough). There's no closed form formula for it, though, as there is for the parabola.

  6. #6
    krshome's Avatar
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    re

    Thanks all. I found the Grizz man between trips and got my numbers. That Grizz man is the man.

  7. #7
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    I like gargoyles explanation - my walls are much longer than my floors.
    I got in a fight one time with a really big guy, and he said, "I'm going to mop the floor with your face." I said, "You'll be sorry." He said, "Oh, yeah? Why?" I said, "Well, you won't be able to get into the corners very well."


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  8. #8
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    Quote Originally Posted by BrianWillan View Post
    You need to pin down your depth of parabola you are going to need before you start.

    I made a spreadsheet to do the calculation, however if you want to know the formula that Grizz gave me when I made my bridge, it is as follows. I used a 7.5" depth of curve on a 80" long piece of fabric that was 54" wide.

    y=d - ax^2, where d is the depth of curve and 'a' is (d / (L/2)^2) of course L is the length of fabric you're using.

    Cheers

    Brian
    I have been looking everywhere for that equation, but I didn't want to bother Grizz. So, since you've already talked to Grizz about this, is that curve structurally beneficial or just aesthetic? And was this parabola determined from a mathematical model or from empirical data?

  9. #9
    Senior Member DemostiX's Avatar
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    Parabola:

    If you look up the catenary curve you will see this is no closed form solution for it. You can estimate coordinates and then estimate further coordinates conditional on the previous estimates.

    Said Griz, somewhere (paraphrasing) : "While mathematically the parabola is a different beast, in the real world, with fabric, our bodies, and what my eye tells me about how little the catenary curve differs from a simple parabola, the difference may be no greater than a DIY'ers cutting and sewing error."

    And, so you have the parabola as an alternative.

    If I recall correctly, a catenary curve solved with load varying with the mass of your body along the length would be different. It might be "better", but comparison to the simple curve does not, again, show greater difference than cutting and sewing variation or error.

    In summary: Close enough to not delay or disrupt sleep in a hammock.

  10. #10
    BrianWillan's Avatar
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    Quote Originally Posted by thematt View Post
    I have been looking everywhere for that equation, but I didn't want to bother Grizz. So, since you've already talked to Grizz about this, is that curve structurally beneficial or just aesthetic? And was this parabola determined from a mathematical model or from empirical data?
    I have no idea how the equation was derived. You'll have to ask the Professor that question. It is the curve that he used in his series of videos on the construction of the Rhino bridge hammock. I used it in my recent Martian bridge hammock project. It holds up my portly self quite nicely. I would lean more towards the curve being structurally beneficial than aesthetic.

    Cheers

    Brian
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