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  1. #11
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    Thanks for the responses.

    My math says for an equilateral suspension triangle, the pole only sees ~60lbs for a 200lb occupant. Each side of the hammock gets half the weight (100lbs), and the lateral force applied to each end of the rod is 100lbs * tan(30d) = 58 lbs. Please correct me if I've got this wrong.

    At 6" diameter, the end of the cylinder is 28sq.in. 58lbs over that area is ~2psi. At 8" diameter, that drops to 1.15psi. A cursory web search says human lung power can be as high as 2psi, and Jeff-oh quoted 1.3psi. 2psi in a 6" cylinder generates a tension around the circumference of just 6 lb/in - well under the strength of 1osy nylon; ditto the 8lbs for an 8" cylinder. This would be a great application for 0.5oz DCF - very low stretch equates to rigidity, and the small area means affordable - one yard would do it.

    Either way, I figured the pressure would rise with the load, just like an air mattress. And varying the diameter (football shaped) did cross my mind.

    HandyRandy - I was thinking permanently attached, basically as partial end caps.

    Grizz - not long after posting (as it so often happens) I saw the problem you describe w/ relative tension on a lower suspension/spreader. With out any lower suspension, the lower spreader isn't really good for much at all. It seems a fixed ridge line should let you fix the upper/lower suspension lengths, no?

    Regardless, the original motivation was weight savings, not changing how the structure works.

    I suppose you could control the buckling by putting a vertical crease in the middle of the tube so it only wants to deform horizontally (towards/away from your head, not the ground), and then tethering the crease to the suspension and cat curves to stabilize it in that plane. That effectively cuts the spread span in two, which makes each half of the cylinder much more resistant to buckling. Harder to do w/ an inflation bladder than heavier heat sealable fabric, but still doable.

  2. #12
    Phantom Grappler's Avatar
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    Inflatable spreader bars?

    autox, my high school math has left me long ago, there is one error in calculations that needs correction. Weight on EACH end of your hammock suspension is roughly equal to your body weight—not half your weight.
    This is only if your suspension is around 30* angle, if your suspension is less than 30* , then the forces on suspension increase not by simple addition but by geometric progression. A flat hang can approach infinite forces. Suspension can snap (tree can snap) and spreader bars would be subject to greater forces than originally calculated.
    A common cause for flat line suspension is tree spacing farther apart than optimal. The average hammock camper does not place straps high enough up tree trunk to reach 30* angle of hang when trees are spaced far apart.
    I don’t remember my high school math and am parroting what others have said, and have shown charts and graphs and formulas.
    Building prototypes can be fun and ideas can be tested. A test build can try a new idea—proof is in the pudding, good luck

  3. #13
    cmoulder's Avatar
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    Indeed, compression forces are so high that by the time you make something inflatable with a large enough diameter that it will not buckle, the weight (and probably the bulk as well) of the inflatable is likely going to significantly exceed that of carbon fiber. CF is going to be a whole bunch more practical.

    A pair of 36" CF spreaders weigh about 6 oz.
    UL, because nobody ever asks "How can I make my pack heavier?"

  4. #14
    Senior Member jeff-oh's Avatar
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    The thought process of you are suggesting is there, however, nature rarely complies with our simplification of how we think things it will react. What I mean is the loads will never be applied uniformly as assumed. The off angle loading will put bending load into the column and buckling load conditions must be considered. in a transverse loading condition this column is very weak. As a simple test blow up a 6" air mat and observe its longitudinal strength.
    Last edited by jeff-oh; 02-11-2019 at 06:24.

  5. #15
    I dont really have an opinion on inflatable spreader bars for a bridge hammock, but it could be interesting to have one attached to or incorporated in a gathered end around the shoulder area. Could give a little spread but still be comfortable. That would be a place that you could not use a solid bar without it feeling like one of the old horrifically uncomfortable pull-out couch beds.

    And inflatable channel on a tarp instead of spreader bar could be an idea too.

    Interesting thought.
    "We've all heard that a million monkeys banging on a million typewriters will eventually reproduce the entire works of Shakespeare. Now, thanks to the Internet, we know this is not true." Robert Wilensky

  6. #16
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    I know there are tents available that use inflatable tubes instead of poles. However, knowing that here have been multiple failures with spreader bars on bridge hammocks due to undersized or otherwise inadequate tubing, I'd say that inflatable tubes are probably not a viable solution for this application.

  7. #17

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    36" two piece carbon fiber poles can go as light as 5.25 ounces. 5.5-6 ounces is more accurate though.

    If you were willing to go with single piece poles you can go even lighter.
    My micro bridge used .490 easton aluminum poles in single 26" pieces.
    I've been debating ordering a CF version but it's not a big issue... and most folks generally prefer a breakdown set even at the expense of an ounce or two.

    The lightest air pads I know of (to take the welded/glued seams) are 15d. So I'd look at that as a starting point as those take body weight without failing... since seams would be fairly critical.

    If you think you can do it with an yard of fabric... then even with 3 ounce per yard fabric you're still doing well.
    I don't know the rest of the math to even contemplate inflatable spreader bars... though I appreciate the creativity.
    In real life... unless you work for themarest or a pad company I'm not sure you'd have access to the equipment you would need to build them right?
    So the heavier fabric that you'd heat seal or glue weld makes more sense to me. And I don't see how you'd realistically build that at home in a perfect cylinder.
    You also have to interface with the suspension some how.

    What i would picture personally- is a pack of hot dogs. 3 or 4 small tubes, bound with one outer wrap. The pointed ends of the tubes would be easier to close up I'd think... while also providing a notch between them to shove between the suspension legs a bit like a knock on an arrow.

    As Tom in minnessota mentioned though... not the first piece of inflatable gear around. The inflatable tent pole thing never quite worked right or came in lighter than a pole with tents.
    As mentioned- sleeping pads have come a long way and hold up pretty well... so if you think you can sort out the math on the front end... I'd look to that technology and material to execute it.

  8. #18
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    Yes, I forgot a 30d hang doubles your weight on the suspension.

    12" diameter means 4x the cross section, so even at twice the load cuts the pressure in half, and gives us a 3:1 aspect ratio on the beam, and comes in around 2.5oz for a pair in 0.5oz DCF.

    At 18", we only need a half psi and the pair weigh 3oz. That's a 2:1 aspect ratio and you can double the required pressure with your lungs.

    No way to know but try, I suppose.

  9. #19
    GrizzlyAdams's Avatar
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    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>
    Grizz
    (alias ProfessorHammock on youtube)

  10. #20
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    Interesting that it seems no one has tried to actually measure the load on a spreader. Some plywood and a bathroom scale should do the trick.

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