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  1. #1
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    Physics question, I think

    I keep reading about how much stress is placed on buildings and trees when we attach a hammock. But then, I see people using what appears to be a hiking pole to hold one, or both ends of a hammock up. I’m afraid I’m going to get a bunch of math, but… why will a hammock put so much stress on a solid structure, but still allow people to use just a seemingly flimsy pole for a set up?

    I ask because I hike in a place that has lots of trees that are just a bit too far apart, and think I might be fine with a single pole on one end, and a tree on the other. Thanks.

  2. #2
    Phantom Grappler's Avatar
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    Yes, one tree and one pole works fine, if done right
    And I’ve seen no tree-two pole setups work too.
    Again, it has to be set up correctly, including stakes that are securely in ground.
    I’ve never used any of them, but I’ve seen them used on multi-night camping trips, with no reported failures.
    This is no guarantee, your setup will be without mishap.

  3. #3
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    W/o getting complicated I'll just say because it's two different directions of force.
    W/ a vertical post the force (us in a hammock) is trying to pull it over (towards the center of the hammock).
    W/ a "one pole" stand we get to put that pole at an angle. The closer we get to matching the angle of the hammock suspension (30°) the more the force is pulling the pole into the ground, straight down the length of the pole.
    So by using anything that can handle the end to end compression force and putting it at the best angle we're not counting on needing that bar across the top to hold the two poles apart. Then add in the ground lines to keep it in place and take any of the force that spills over from the poles not being at the "perfect" angle.
    Also why the closer we get the poles to that perfect angle (30°) the less force there is trying to pull the ground stakes out.

    "Sent w/o me knowing"

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    LowTech's Avatar
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    In addition, even a vertical pole, or tree, that is not sufficiently stable can be backed up by guylines and ground stakes.
    For example using a tree that is only the diameter of your hand width, something that is common in some places in the desert, can be done by guying the tree out like a one pole stand.
    I've done that a few times.

    "Sent w/o me knowing"

  5. #5
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    When people hang on trekking poles, they are tensa tracking poles which are heavy duty ones made to hold that weight and anchored in with very deep stakes that also have a 90 degree anchor attached to them.

    When people warn about a lot of building is that piece is anchored only on top and bottom where it is designed for only forces from above and not from the sides. When you pull from the side then that support can fall and brings down what it is holding up and then people get seriously hurt and sometimes die.

    Masonry is often a cause of that, because it looks thick and strong, but it's not built for lateral forces and then 100s if not 1000s of pounds or brick/rock fall in someone.

  6. #6
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    ATPringles, here's the school experiment: Put a small kid on the end of a rope about 5 -10 ft long (longer the better), and an adult on the other end. Have the kid try to pull the rope to move the adult. The kid/child is usually unsuccessful. Next, keep the adult on one end of the rope, tie the other end to a fixed, solid object, and have the adult positioned so the rope is tight/taut. Have the kid push on the center of the rope. Much more force is applied to the adult. The kid can usually move the adult. Finally, have the adult stand closer to the fixed anchor so there is some slack in the line. This time, when the kid pushes on the rope, she can't move the adult as easily, if at all.

    So this is what happens when you string your hammock up with no, or very little (like 5° - 10°) sag in the line. When you get in the hammock, that's like the kid pushing on the middle of the taut rope. At 10° hang angle, 200 lbs in the hammock will translate to 576 lbs on the tree and hammock suspension.

    Another way to think of it is a right triangle, with angles, 10°, 90°, 80°. The baseline is the distance between trees. the vertical iine represents the weight in the hammock. See that long hypotenuse line - that's the resulting force given the body weight in the hammock, the hang angle, and the distance between trees.

    So that's why we have the suggested angle of hang is 30°. At 30°, 200 lbs in the hammock results in 200 lbs of force on the suspension.
    Playing with the hammock hang calculator is fun: https://theultimatehang.com/calculator/index.php

    That covers the forces. Now - the difference between a sear force and a compression force. A shear force is pulling across something and a compression force is pulling down on something. When you are connected to a weak tree, a 2x4 on an interior wall, a masonry pillar by a swimming pool, etc. the suspension force is shear - it's pulling across and those structures are not designed for that. The wall of your house is designed to hold the floor above it up - weight is pushing down on the 2 x 4's, not pulling them inward.

    When you attach the hammock suspension to the top of a Solo pole or a Trekking pole, then attach the guylines AT THE SAME POINT, and approximate the same 30° hang angle, the force on the pole is down - a compression - which they are designed to endure.

    But imagine attaching the hammock suspension to the top of the pole and the guylines much further down on the pole. Now you've created a shear force as the guylines pull one way and the suspension at the top pulls another - break/bend city.

    Using trees, you have to adjust your suspension to give that approximate 30° angle. But using poles, you can angle the pole so the lines coming off it, though visible closer to level, are close to that magic 30°.
    Last edited by cougarmeat; 05-22-2023 at 12:12.
    In order to see what few have seen, you must go where few have gone. And DO what few have done.

  7. #7
    joe_guilbeau's Avatar
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    Quote Originally Posted by ATPringles View Post
    I’m afraid I’m going to get a bunch of math, but… why will a hammock put so much stress on a solid structure, but still allow people to use just a seemingly flimsy pole for a set up?
    tree_anatomy.jpg

    Trees sustain damage, hiking poles and aluminum poles etc... do not react the same way as a tree does. Minimum diameter for trees to hang from should be at least 8-inch diameter. And, yes, smaller diameters of trees might hold up for infrequent hangs, but with the numbers of hangers these days, in a State Park (for instance) the risk of damage to those trees grow exponentially with the numbers of hangers each week.

    No math required...

  8. #8
    Senior Member gargoyle's Avatar
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    Great picture Low Tech!
    Ambulo tua ambulo.

  9. #9
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    Thank you for the answers. The examples of pressure and vectors helped a lot. I can see how a wall is vulnerable and a heavy duty pole might not be. I really appreciate your responses.

  10. #10
    Senior Member Caconym's Avatar
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    Cougarmeat came close to the math bit, and it's not directly related to your question, but I think it's important for all hammockers to understand. At a 30º hang with 200lb in the hammock, each end of the suspension is experiencing 200lb of force. Slacken that down to the point where you're dangling from a roof beam with the suspension going straight up, and your 200lb is split evenly for 100lb per line. Tighten it up past 30º and the force starts rising. If you were able to get your suspension to be a perfectly straight line and put 200lb on it, the force on each end would be infinite! This is only possible in the hypothetical world of maths, but it teaches an important lesson about not hanging too tight.
    Corvis natum est.

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