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  1. #71
    @dejoha I figured out the issue. I was hitting your older links from the beginning of the thread.

  2. #72
    Member mp3prozac's Avatar
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    Dejoha- just picked up the book on itunes... will be a great resource to have with my ipad on hangs. It looks great so far, thanks for putting all that great info into one place. I also like the blog.

  3. #73
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    Hammock Hang Calcs

    Sorry to resurrect an old thread, but I have noticed a lot of confusion, both here and in the comments on the Hammock Hang Calculator page, regarding how the calculator actually works. I worked through the trig in Excel and compared the results from the web page and realized that there is a problem with the way the web page works. The calculations on the web page seem to assume that there is a structural ridgeline, but then work out the dimensions of the hammock itself as if there were no structural ridgeline.

    The calculator takes as one of its inputs the length of the hammock ridgeline. But the calculator does not distinguish between the hang angle, which is the angle of the suspension line running from the tree, and the sag angle, which is the angle of the sides of the hammock as compared to the hammock ridgeline. A structural ridgeline will maintain the desired 30* sag angle for a comfortable lay, even if the hang angle is shallower, say 20*. In the current Hammock Hang Calculator, the hammock sag angle always equals the hang angle, which would only be true if there were no structural ridgeline (or if the hang angle were greater than the sag angle set by the structural ridgeline). Because the distance calculations operate as if there were no structural ridgeline, a decrease in the hang angle means a decrease in the sag of the hammock, which means that you need to increase the value for the "ridgeline" to account for the spreading of the actual hammock that would occur in the absence of a structural ridgeline. If you don't make this adjustment, then the measurements for the suspension length and the hang point will be slightly off. In effect, reducing the hang angle without increasing the "ridgeline" figure treats your hammock as if it shrunk in order to keep the ridgeline length constant.

    I apologize if the above explanation is confusing. To make matters worse, I have included below my understanding of what the math here should be for a hammock with a structural ridgeline. I believe that the missing part of the equation in the current calculator is variable "F" for the sag angle. If you use variable "E" (the hang angle) to calculate "J" (the amount of hammock sag), and E is actually less than F, then you will get the wrong answer for J, which causes you to get the wrong value for K, the hang point.

    By the way, my formulas below won't work properly for a hammock without a structural ridgeline. If the hammock has no ridgeline, then it would be better to specify the overall length of the hammock instead, since the length of the notional "ridgeline" will change as the hang angle changes.

    The math below also suggests a possible new feature for the Hammock Hang Calculator. Variable "P" is the total tension on the structural ridgeline. If the hang angle and the sag angle are equal, then P equals zero. If the hang angle is shallower than the sag angle, then P equals the tension on the ridgeline as it prevents the hammock from spreading further. As the hang angle gets shallower and shallower, the tension on the structural ridgeline can actually exceed the tension on the hammock suspension lines. Understanding this dynamic makes it clear why you should not use a very shallow hang angle with a structural ridgeline made from Zingit.

    Code:
    //These are the default inputs to the Hammock Hang Calculator
    A = 180  // Distance between trees in inches
    B = 108  // Ridgeline length in inches
    C = 18   // Seating height in inches
    D = 200  // Weight in hammock in lbs
    E = 30   // Hang angle in degrees
    
    // One more input is needed for the hammock sag angle
    F = 30   // Hammock sag angle in degrees; currently F = E, which is not always true
    
    // Calculate distances
    G = (A-B) / 2          // Horizontal distance from end of hammock to tree
    H = G / cos(E)         // Suspension length
    I = G * tan(E)         // Vertical distance from ridgeline to anchor point
    J = (B / 2) * tan(F)   // Hammock sag below ridgeline
    K = C + I + J          // Hang point on tree
    
    // Calculate forces
    L = D / 2              // Vertical load on each tree
    M = L / sin(E)         // Suspension cord tension
    N = L / tan(E)         // Horizontal shear force on the tree
    O = L / tan(F)         // Horizontal shear due to hammock sag
    P = 2 * (N-O)          // Tension(compression) on structural ridgeline

  4. #74
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    Q = Hammock Length
    F = arccos ( (B/2) / (Q/2) )

  5. #75
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    For a hammock without a structural ridgeline, F (the sag angle) equals E (the hang angle). Since the notional "ridgeline" length varies with the hang angle, and the sag angle equals the hang angle, the ridgeline length of such a hammock can be calculated from Q (the overall hammock length) and the hang angle as follows:

    B = cos(E) * Q

  6. #76
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    I have been tinkering a bit more, and now I realize that the Hammock Hang Calculator is, in fact, working correctly for a hammock with no structural ridgeline. I had missed the drop-down that lets you input the hammock length rather than the ridgeline length. If I input a hammock length of 124.7 inches, then the "ridgeline" at a 30* hang angle is 108 inches, and increases to 117 inches at a 20* hang angle, exactly as expected. My apologies for missing this earlier. Am I correct, though, that the calculator does not work for a hammock with a structural ridgeline unless the hang angle and sag angle are the same?

  7. #77
    I do own the apple app version of the hammock hang calculator. Thanks for putting this out there, it's very helpful.

    I was wondering how to use the calculator for a bridge hammock with spreader bars? I have WBRR on the way and not sure if the calculator will work, and where I should be taking my measurements from.
    Thanks for any insight you helpful hammockers can provide!!

  8. #78
    Senior Member dejoha's Avatar
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    The calculator doesn't (yet) work with bridge hammocks, but the only real difference is that the hang point is _lower_ than a regular hammock because the spreader bars raise the hammock higher. I would start with hanging about 12 inches lower than the calculator indicates and adjust from there.

  9. #79
    Thanks so much for the reply. My WBRR showed up today so I'll play around with it.

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