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  1. #21
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    Quote Originally Posted by GrizzlyAdams View Post
    I think forces are multiplied not only by the angle of the suspension triangle relative to the ground, but by the angle (under load) of the webbing or cord coming from the hammock body. Depending on the architecture, that's going to be in the 15-30 degree range as well.

    I've seen enough poles of different materials flex under load (sometimes alarmingly) to know that compression force just does not want to conveniently stay centered. The walls of the tube have to resist.

    Both Just Bill and I have special trash bins for failed bridge hammocks whose corners blew out because we didn't strengthen them enough. A testament to the force coming off the suspension just there. Just to prove that he is smarter than me, his blow-outs are made of various forms of relatively inexpensive fabric, and mine of the fabric formerly known as Cuben Fiber.

    But don't believe any of us just because we say so. There is no teacher like experience and if you find a way to pull this lightweight inflatable spreader bar thing off because of something clever that's been overlooked, that would be an accomplishment.
    Like a bridge hammock made entirely of Cuben Fiber ..... <grin>
    Thanks for the laugh!

    me poor nose.JPG
    Still got a small scar on my nose from getting punched in the face with a spreader bar during one of those special trash bin contributions.

  2. #22
    Senior Member Floridahanger's Avatar
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    Ooooh. This thread is getting back to the old days of calculating loads and compression and angles. Something the newer hangers never got to experience to see how the current batch of hammock gear came about.

    Thanks for bringing it back. Love the old feeling of ingenuity. Smells great with a cup of coffee.
    Enjoy and have fun with your family, before they have fun without you

  3. #23
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    When I first saw this thread my thought was "Beach ball!"; Phantom's idea of football makes even more sense to me.

  4. #24
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    In the name of thoroughness, last night I I verified that a 3' sphere would, in fact, approach too heavy: pi square yards, which puts a pair in DCF over 4oz. Also, it takes up 18" at each end of your hammock.

    Air pressure in an ovoid will be determined by the diameter at the ends.

    Nice to get a little love from the peanut gallery

  5. #25
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    Quote Originally Posted by autox View Post
    In the name of thoroughness, last night I I verified that a 3' sphere would, in fact, approach too heavy: pi square yards, which puts a pair in DCF over 4oz. Also, it takes up 18" at each end of your hammock.

    Air pressure in an ovoid will be determined by the diameter at the ends.

    Nice to get a little love from the peanut gallery
    A = 4 * pi * 0.5yd^2 = 4 * pi * 0.25sqyd = pi sqyd
    3.1416 sqyd * 0.5 oz / sqyd = 1.5708 oz + 0.5 oz valve = 2.0708 oz per beach ball = 4.1416 oz per pair

    Math confirmed.

  6. #26
    Recalc's Avatar
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    Quote Originally Posted by GreatBigDave View Post
    A = 4 * pi * 0.5yd^2 = 4 * pi * 0.25sqyd = pi sqyd
    3.1416 sqyd * 0.5 oz / sqyd = 1.5708 oz + 0.5 oz valve = 2.0708 oz per beach ball = 4.1416 oz per pair

    Math confirmed.
    One of the reasons I enjoy this forum. Thank you!

  7. #27
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    Quote Originally Posted by Floridahanger View Post
    Ooooh. This thread is getting back to the old days of calculating loads and compression and angles. Something the newer hangers never got to experience to see how the current batch of hammock gear came about.

    Thanks for bringing it back. Love the old feeling of ingenuity. Smells great with a cup of coffee.
    Quote Originally Posted by autox View Post
    In the name of thoroughness, last night I I verified that a 3' sphere would, in fact, approach too heavy: pi square yards, which puts a pair in DCF over 4oz. Also, it takes up 18" at each end of your hammock.

    Air pressure in an ovoid will be determined by the diameter at the ends.

    Nice to get a little love from the peanut gallery
    TL;DR- bridges are more complicated than they appear.


    https://hammockforums.net/forum/show...ll=1#post21456

    I'm a carpenter. So the math was never my thing per say and the great Grizz and TeeDee debate wasn't something I could ever quite quantify qualitatively.
    Though I could quantify the quandry Grizz got into quite quickly when he did his spreader bar free bridge... a project that I suspected he thought would be substantially less complicated that it turned out to be.


    I understand some engineering but I'm not the guy who crunches the complicated numbers. Course I also understand enough engineering to understand that many folks crunching numbers don't understand the numbers they are crunching or how those might come together in the real world either. So it all gets a little crunchy really and ultimately for me has always come down to the most proven engineering principal of all time- if you can build it and it works guess you were right.

    My rough understanding was that Grizz had a rough understanding of how to attempt to calculate the compression forces with a simple 2d force diagram.
    Eventually that led to a usable rule of thumb in that the Length of the Dogbone should be at least 80% of the Spreader to keep things reasonable. (L) >/= .8*S.
    That works out well enough if you are under 200lbs and use a 5/8" diameter Easton Aluminum Pole.
    If you are heavier, you should go to 100% and use a 3/4" pole.

    In theory you could take the compression values (if you could find them) of those two aluminum poles and reverse engineer a compression resistance value based upon those conditions.

    Where I do okay... being a blueprint guy... is with 3d spatial intelligence. Which is a bit of why I never really quite followed things when I tried to read up on the wise elders past efforts at crunching numbers.

    Maybe one of you can attempt to apply some mathemagic to this mess- but I do have a fairly hopeless outlook on it simply because of the scale of the issue as I see it.

    Bridge Forces.png
    Bridge Forces.pdf

    Posted two versions just so it hopefully shows up right (computers also being among the many specializations I lack).

    If you expand Grizz's basic 2d Diagram into the top view shown in the bottom right corner:
    In theory the bridge itself is balanced out better than it might appear. (Mean to talk to Grizz privately bout this more one day).
    I'm sure there are reasons as I can think of several... but simple start is that the occupant's weight (W) should in theory be split between each end, and split 4 ways to each corner of the bridge.

    While great attention was paid to angle 1 in the original diagrams, it seems that angle 2 turned out to be a source of frustration (or simply beyond me to follow).
    Though as a carpenter this seems simple enough as you simply have zero control over this angle in real life. The depth of cut you apply to the bridge gets you started...A back sleeper vs side sleeper and a heavy vs light user could both help set ranges of deflection from there. But if a user sits on one side, or god forbid walks around in there... that angle at best is at a range.

    I only point it out as- in theory and in practice- That yellow line of force pushing towards the end is real. It represents the primary direction of buckling and failure if you don't violate Grizz's 80% rule. If you do make the dogbones too short the pole is most likely to fail towards the interior of the bridge. I suspect what helps make that force real is the differing angles of the body

    To me though- you'd have to at minimum run the top view full force diagram all the way through as there is likely some balancing of loads going in both directions and not just at the triangle formed by dogbone and spreader. (I'm pretty sure there are at minimum 4 triangles, but as many as 8)

    Now; if you look at the side view... for my Luxury/Big Guy at least... you can see that the Grizz's original 2d diagram isn't really all that far off when loaded as far as actually sitting in a 2d plane rather than occuring over a curved surface. This plane is running dead flat from Apex to spreader, and relatively flat for at least the first few inches past the spreader.

    Though what always bothered me- specially in a recessed bar bridge- is that you have a line of force directly under the bar from the fabric. (FLg in light blue).
    Overall there is a gravity load on the whole system... though in theory the bulk of that should be dissipated through the fabric and already expressed.

    That said- in real life you do have excess tension at the point the fabric meets the bar in any model. This makes the spreader bar a bow (as in bow and arrow) in that you (gravity) is adding another line of force to the spreader bar completely separate from the other calculations when viewed from the top. The fabric is pulled like a bowstring creating another buckling force (lines in light blue) pushing both directly upwards and slightly inwards towards the center of the bridge.

    Go one step further in the side view... and look at the orange ridgeline. Which is a good bridge should be under some tension as that helps keep the dogbone angle stable in my opinion.
    Other than stabilization I don't know if it has any other serious value from an engineering perspective... however if one were attempting to run a full system calculation I can easily physically see the value.
    If nothing else it allows you to mainly fix the 30* suspension angle to run numbers.

    End of the day... I suppose I can think of the shaped sail cloths they build for racing. Maybe the canopies they are doing for outdoor spaces.
    A real suspension bridge has a rigid deck with some fairly straight forward surfaces and right angles to work with.
    I cannot think of another project that functions in shape or appearance to a bridge hammock.
    I don't think anyone is around who specializes in the modeling, math, and engineering who has taken this up.

    Not that critical though really.
    I'm a blunt force engineer. Get it close and try until it breaks.
    I have my own rules of thumb and even some formulas, ratios, and pretty diagrams to build these... and I even have some suspicions on why exactly what I'm doing is working.
    But I have a feeling it will be a long time before anyone could quantify that... might make a fun dissertation subject I suppose... or maybe it's a simple enough thing to do with the right software for all I know.

    I do recall it takes a full sleeping pad rolled up in that compression sack looking thing to support the seated load of a 200lb person... thermarest came up with that when the Neo-Air came out.
    I also recall that they don't sell it no more. Engineering was sound, but busting your $200 pad in real life turned out to not be too good for the bottom line.
    They still sell the same crazy creek style chair they've been selling for years... but that compression based one didn't last long.

    Maybe one day I'll sort out the math... or one of you will. Grizz did do some pretty neat modeling for the Ariel video.

    For now though... I'll stick with engineering 101. If you can build it and it works...


  8. #28
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    At first I thought this was the silliest post but, not to be judgmental, I gave it some thought.

    One of the things I thought of was the Thermarest NeoAir Jembe Seat Kit (https://www.amazon.com/gp/product/B0...83f86169797d49) which will hold a person's weight. Obviously bigger than a 6" tube but I would never have though the Thermarest would work so I'm not going to be quick to judge if this can't be made to work in some fashion.

    Love to see innovation so good luck with it.

  9. #29
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    Quote Originally Posted by scrope View Post
    At first I thought this was the silliest post but, not to be judgmental, I gave it some thought.

    One of the things I thought of was the Thermarest NeoAir Jembe Seat Kit (https://www.amazon.com/gp/product/B0...83f86169797d49) which will hold a person's weight. Obviously bigger than a 6" tube but I would never have though the Thermarest would work so I'm not going to be quick to judge if this can't be made to work in some fashion.

    Love to see innovation so good luck with it.
    That's the one I was thinking of!
    Discontinued as far as I know of however... not so much for blowing out the pads (though a few did) but because it wasn't a great chair unless you got the deluxe version...

  10. #30
    Senior Member rais'n hammock's Avatar
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    When I first thought of this idea, my mind went to a dropstitch technology that would allow for a very strong structure. It could easily be inflated to a dual purpose inflation bag/drip water filter container.

    The outer material would need to be much lighter than the standard PVC but the idea and material may be available with some development in the future.
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