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  1. #1

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    diy bridge hammock questions

    we are looking to make a couple bridge hammocks. one for me and one for each of my two kids (age 9 and 6). i am generally following grizzlyadams and his stuff on youtube. it seems like there has been some evolution in his designs over the years. i'm mostly interested in the design in the video "conting the grams". mainly i'm looking at that design because i like the spreader bars being in further for a couple of reason: 1. the comfort factor grizz mentioned, 2. it makes the overall minimum hang distance lower, 3. it is going to be easier for the kids to "grow into". i'm gonna make the ones for the kids full sized (or at least bigger than they need). with the spreader bars pulled in a bit, it will give them a guide for getting into the middle of the hammock but will also allow them room to grow.

    anyway, i have a couple questions before i get started:

    1. does a bridge hammock need a structural ridge line? it seems there is one mentioned in the write up grizz did on here, but i don't see one in the "counting the grams" video. are they necessary and if so, what do they do for a bridge hammock?

    2. the hammock in "counting the grams" had 48" material and 36" spreader bars... as i understand, the closer those two numbers, the more tippy the thing will be (we have a flat rope hammock in our back yard that is a head injury waiting to happen). i definitely don't want a tippy hammock for the kids who toss and turn all night while sleeping.

    3. the suspension triangle affects the tippyness too as i understand it. it also affects compression on the spreader bar and affects minimum hang distance. if i work for an angle of 50 - 60 degrees, that should be a fairly stable bridge, right?

    4. i want to allow one of these to be setup as a tent (or a bivy sack -- see this thread: https://www.hammockforums.net/forum/...son-tent-combo)... for this, i'm thinking of putting a small aluminum piece on the ridgeline similar to the one for the spreader bars then connect the ridge line to the suspension with a buried loop over a diamond knot. then when in tent mode, the suspension triangle, etc just sits idle on the ground (rolled up) and the ridge line is by itself with guy lines. does this seem like it would work? what kind of pressure (other than the weight of the tarp and the bugnet) is on the ridge line?.

    5. i was planning on making the side suspension out of amsteel so i need a channel just like the ariel grizz bridge and the one in the "counting the grams". in the ariel, he used an extra strip of the same material to create a type of bias tape and made the channel out of that. in the "counting the grams" he used 1.9 silnylon as the extra reinforcement for the channel and did some folding. in the comments of the ariel video, someone said the bias tape approach wouldn't work if one didn't seal the edge of the hammock body from fraying. would a rolled hem on the hammock body before sewing in the bias tape do the trick or do i need a hot knife as he has.

    6. with bridge hammocks do you still shoot for a 30 degree angle off the tree like in other hammocks or do you hang them flatter?

    thanks all for the help.

    john

  2. #2
    IrishSitter's Avatar
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    I'll leave others to answer the Grizz-specific questions, but I like having a ridgeline in my bridge, although a structural ridgeline is not needed since minimum sag applies less to bridges.
    Last edited by IrishSitter; 01-04-2016 at 12:18. Reason: bad grammar

  3. #3

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    yes... i'll have a ridge line for the tarp and the bugnet, but my son wants to sleep under the same tarp (at least until he gets used to it) as me so we are likely gonna setup in a bunk hammock setup so i wouldn't need a ridge line on mine (lower bunk) and so i could hang his lower (less distance to fall on me in the middle of the night).

  4. #4
    Senior Member GrizzlyAdams's Avatar
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    Quote Originally Posted by johne View Post
    we are looking to make a couple bridge hammocks. one for me and one for each of my two kids (age 9 and 6). i am generally following grizzlyadams and his stuff on youtube. it seems like there has been some evolution in his designs over the years. i'm mostly interested in the design in the video "conting the grams". mainly i'm looking at that design because i like the spreader bars being in further for a couple of reason: 1. the comfort factor grizz mentioned, 2. it makes the overall minimum hang distance lower, 3. it is going to be easier for the kids to "grow into". i'm gonna make the ones for the kids full sized (or at least bigger than they need). with the spreader bars pulled in a bit, it will give them a guide for getting into the middle of the hammock but will also allow them room to grow.

    anyway, i have a couple questions before i get started:

    1. does a bridge hammock need a structural ridge line? it seems there is one mentioned in the write up grizz did on here, but i don't see one in the "counting the grams" video. are they necessary and if so, what do they do for a bridge hammock?
    doesn't have to have one, but a ridgeline is useful for setting the distance between triangle apex ends the same every time (same as 'setting the sag') and useful
    for hanging things from.

    Quote Originally Posted by johne View Post
    2. the hammock in "counting the grams" had 48" material and 36" spreader bars... as i understand, the closer those two numbers, the more tippy the thing will be (we have a flat rope hammock in our back yard that is a head injury waiting to happen). i definitely don't want a tippy hammock for the kids who toss and turn all night while sleeping.
    They "feel" tippier than they are when inside. Adult widths (say 34" at the waist) is going to be plenty deep to keep the kiddos in.
    The tippiness is a function too of the vertical distance between the center of mass and the points where the hammock rotates. With a true structural ridgeline that would be
    the suspension triangle apex, but with a looser ridgeline or none at all it will be the height of the attachment to the trees.

    Quote Originally Posted by johne View Post
    3. the suspension triangle affects the tippyness too as i understand it. it also affects compression on the spreader bar and affects minimum hang distance. if i work for an angle of 50 - 60 degrees, that should be a fairly stable bridge, right?
    I'm comfortable using triangle side of 30" used with a 36" spreader bar. That makes the angle between spreader bar and suspension triangle side 53 degrees.

    Quote Originally Posted by johne View Post
    4. i want to allow one of these to be setup as a tent (or a bivy sack -- see this thread: https://www.hammockforums.net/forum/...son-tent-combo)... for this, i'm thinking of putting a small aluminum piece on the ridgeline similar to the one for the spreader bars then connect the ridge line to the suspension with a buried loop over a diamond knot. then when in tent mode, the suspension triangle, etc just sits idle on the ground (rolled up) and the ridge line is by itself with guy lines. does this seem like it would work? what kind of pressure (other than the weight of the tarp and the bugnet) is on the ridge line?.
    I've given some thought to the hammock-cum-bivy idea but have never gotten to the point of actually trying. Some of my most compelling ideas don't work in practice...
    the challenge is to do this in a way that keeps the bugnet lifted up without stress. Will need two lines and two stakes at each end, lines going out from ridgeline do-dad at about 45 degress on either side. Did I mention that some of my most compelling ideas don't work out in practice?

    Quote Originally Posted by johne View Post
    5. i was planning on making the side suspension out of amsteel so i need a channel just like the ariel grizz bridge and the one in the "counting the grams". in the ariel, he used an extra strip of the same material to create a type of bias tape and made the channel out of that. in the "counting the grams" he used 1.9 silnylon as the extra reinforcement for the channel and did some folding. in the comments of the ariel video, someone said the bias tape approach wouldn't work if one didn't seal the edge of the hammock body from fraying. would a rolled hem on the hammock body before sewing in the bias tape do the trick or do i need a hot knife as he has.
    Love my hot knife. Rolling that edge will give you a world of complications up at the corner where the cut of the body makes a turn. Sans hot knife I'd seriously consider covering that edge with ordinary bias tape before putting on the channel. Bias tape 'bends' around shallow corners. Rolling does not.

    The sil thing was cautious engineering but was a Major Hassle to get pinned in and fold in properly. Major Hassle. The approach of the Ariel is an experiment, but one that so far seems to be working. Time will tell.

    Quote Originally Posted by johne View Post
    6. with bridge hammocks do you still shoot for a 30 degree angle off the tree like in other hammocks or do you hang them flatter?
    You get compression on the spreader bars with the flatter angle too...30 is a good target, but there's a balancing act between pulling the
    ends of the hammock far enough apart and getting a good angle. You end up pushing the straps up the trees a bit.

    good luck.
    Grizz
    (alias ProfessorHammock on youtube)

  5. #5

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    Quote Originally Posted by GrizzlyAdams View Post
    Rolling that edge will give you a world of complications up at the corner where the cut of the body makes a turn. Sans hot knife I'd seriously consider covering that edge with ordinary bias tape before putting on the channel. Bias tape 'bends' around shallow corners. Rolling does not.
    thank you so much for your response!

    i want to make sure i understand the layout of the fabric used to create the ariel... here is a very rough diagram (not to scale) that i made of what i figure you did to cut the single piece of body fabric of the ariel for a total hammock length of 84" with a curve depth of 6", a foot on each end beyond the spreader bars and 48" of material under the spreader bars (please excuse my poor drawing skills).

    ariel-fabric-cut.jpg

    does that look about right?

  6. #6
    Senior Member GrizzlyAdams's Avatar
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    I've got 46" under the spreader bar. I've got 66" between spreader bars on the side, and 9" beyond the corner at each, which brings one to 84" but a little differently. My curve is 7.5" deep. What you don't have on your diagram but need is the width of the fabric at each end. Given 46" between the corners and 9" straight out (not along the angled edge, but distance between fabric end and line between corners) 43" at the ends do nicely.

    I'm slowing putting together a video on the details, but I bet you'll have yours built before the vid is done.
    Grizz
    (alias ProfessorHammock on youtube)

  7. #7

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    Perfect! I was just going to use the alpha angle and a trapezoid calculator to figure that out. Figured if I tapered down at the same angle as the suspension triangle I would get a flat piece of material relative to the body. The line at the end would be the "top" of the trapezoid.

    Any particular reason you changed from 12" at each end to 9"?

  8. #8

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    I am likely to start soon. I'm kinda impatient that way. Especially since we intend on starting with summer backpacking which is quite a ways off.

    My daughter will request I build hers last. She knows that I figure stuff out as I go and the last one is likely to turn out best.

    We'll see after the fact how closely my finished product matches your video. I'll let you all judge me based on the third one I create.

  9. #9

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    i see after trying it out in a trapezoid calculator you did not use the same angle as the suspension triangle because when i put a trapezoid of 46" long base and 43" short base and 9" height i get an 80.5 degree angle which is quite a bit higher than the 50 to 60 degree angle you likely used for your suspension triangle.

    so, now i went totally math crazy with spreadsheets and a parabola arc calculator (http://www.had2know.com/academics/pa...ngth-area.html). i (maybe) figured out that you got 43" at the end this way:

    the height at the spread bar for 36" spreader bar and 46" of fabric is about 13" (the fabric follows a parabola -- i figured this out by just working the numbers backward in the calculator above... trial and error wise)
    the height under your end of fabric goin up at 30 degrees (angle on the tree) would be 17.5" (right triangle with 30 degree angle and a 9" hypotenuse)
    the distance between the points on your suspension triangle at the end would be about 22.5" (the short base of the trapezoid with a 9" altitude, 53 degree segments and long base of 36" spreader bar)
    the length of a parabolic arc under the end: 43.4" (the arc with segment width 22.5" and height of 17.5" -- excel formula: =0.5*SQRT(16*J113^2+I113^2)+(I113^2/(8*J113))*(LN(4*J113+SQRT(16*J113^2+I113^2)) - LN(I113)) where J113 is height and I113 is width)

    so for me, if i want 12" extensions and 46" of fabric under the spreader bar, my fabric at the end would be 43.59" wide... so it doesn't change much, but if i wanted 48" under the spreader bar (like your other bridges) i would need 46" of fabric at the end.

    i suppose, in theory, one could extend the ends all the way out the height of the suspension triangle and just sew them together at the end which would eliminate a need for a separate end cap... it would give you a bit of a storage area, but would then increase the required length of your tarp or you'd get a hammock full of rain.

  10. #10
    Senior Member GrizzlyAdams's Avatar
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    12" -> 9" at the same time that distance between spreader bars went from 60" -> 66". The lightweight hammock
    in the earlier video was made for a guy who is maybe 5' 7". I'm 6' 1"... made the Ariel for ME! Didn't like the rod of
    aluminum under compression sitting directly above my eyes, so spread them out a little more.

    On where the 43" came from, you've got exactly the right idea.
    The the side suspension is rising, while the distance between the sides is decreasing,
    and you want to keep the bottom of the hammock at the same level. Fun with math!

    43" came from trying to keep the hammock flat under load, extending beyond the spreader bars.
    But I don't use the parabola, what actually happens there is load pulls the sides straight to the edge of the load, and then bends around it.
    So I approximate the cross section of the head with a circle, and assume the edge (fabric) of the cross-section has a circle at the bottom,
    then straight lines tangent to that circle up to the suspension sides. If there is a on-line program out there to help, it would be a root-finding program with user-defined objective function. I do a binary search on the angle from the vertical of the circle to the tangency point. Given an angle for the tangent line you can figure the distance from the bottom of the circle to the line that connects the two
    tangent lines with the known required distance between suspension sides at that point. You can compare that height with the known
    target height, adjust the angle and try again until you converge on the angle that works.

    Now if you plot the fabric width as a function of the distance from the corners it is not a straight line. I got 43" at 9" by
    fitting the straight line whose maximum deviation from the model curve is least,
    assuming that the endpoint of the line is rooted in the corner as it physically must be.
    Locally, for this line out past 9", it deviates quite a lot from the straight line. The data
    below assumes a head radius of 3.75", a rise in the suspension triangle of 25 degrees, a fabric width of 46" between corners, a 36" spreader bar, and length of triangle side suspension cord 30".
    First number is distance away from corner, 2nd number is width of fabric.
    Now the degree to which this matters will depend I think on whether there is actually load those last 3". If not then
    these details are less likely to be important. I put a parabola endcap on even though the computed shape is different because I find the parabola more pleasing to the eye.

    # 25.0 3.75 14.0129348245
    0.00, 46.01
    0.50, 45.71
    1.00, 45.43
    1.50, 45.17
    2.00, 44.93
    2.50, 44.72
    3.00, 44.49
    3.50, 44.32
    4.00, 44.16
    4.50, 44.00
    5.00, 43.85
    5.50, 43.74
    6.00, 43.67
    6.50, 43.57
    7.00, 43.52
    7.50, 43.48
    8.00, 43.50
    8.50, 43.50
    9.00, 43.52
    9.50, 43.58
    10.00, 43.66
    10.50, 43.76
    11.00, 43.85
    11.50, 44.00
    12.00, 44.1


    My first time out on this I assumed parabolas as you have done, but noticed that in practice the floor of the hammock dipped a little down
    rather than be flat. I realized that for a given target height the parabola gives you more fabric on the sides because of the
    continuous bending, hence when the sides under load are actually straight it drops down a little.

    Same graph as above assuming 48" between corners

    # 25.0 3.75 15.5104548855
    0.00, 48.00
    0.50, 47.76
    1.00, 47.52
    1.50, 47.29
    2.00, 47.09
    2.50, 46.93
    3.00, 46.74
    3.50, 46.59
    4.00, 46.48
    4.50, 46.38
    5.00, 46.29
    5.50, 46.21
    6.00, 46.15
    6.50, 46.10
    7.00, 46.09
    7.50, 46.09
    8.00, 46.12
    8.50, 46.17
    9.00, 46.22
    9.50, 46.30
    10.00, 46.41
    10.50, 46.54
    11.00, 46.66
    11.50, 46.85
    12.00, 47.02

    I've got an animation of a CAD model of this endbody design at this link . It uses the 'true' height,
    not the straight-line approximation.

    cheers
    Grizz
    (alias ProfessorHammock on youtube)

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