Physics question, I think

[Cruiser51]

and just to complicate this a bit more ... we quote 30 degrees for GE and 25 for bridge hammocks .... but those numbers are for a loaded hammock, the angles change when you get in. If I set initially set my bridge up at 19-20 degrees, it will actually be 25-26 when I actually get in

[offGridDownUnder]

Hi.

Take a cord, strong enough, and hang it from some support and let fall straight down. Now add a scale to it, and then a 10lb weight. The scale will register 10lbs.

Now, take that same cord, with the scale on the end, and fasten the other end of the scale to another support - so both ends are fastened to support, and the cord is horizontal. Don't make it taught, so the scale doesn't register any pre-load. Now, hang the 10lb weight in the middle of the cord, and read what the scale registers.

If the cord stretches, you'll see much deflection due to that. If the cord has very little stretch (like dyneema fibre or UHMWPE) all the resulting sag will be due to the scale deflecting. In either case, the scale will register more than 10lbs. You can actually do this, and that will show you why a hanging horizontal hammock will put significant stress on it's anchor points.

But the stress isn't downwards - it's horizontal. And the load on the poles is downward. They aren't the same.

[piscator]

This seems to be thread to ask ask a math question.
If you are say 200lbs weight what would be the expected compression force down each leg of a Tensa 4 frame?
Im curious as I stood one side of mine on some scales which fluctuated quite a lot between 91 to 112lbs

[Caconym]

I suck at actual engineering maths, but I'm pretty good at rough napkin force diagrams. I'll assume the Tensa legs are also at about 30º, that there is zero tension on the ridgeline, and that the stand is perfectly balanced so there’s no force on the foot end guy line.
In this case your 200lb per end is being split evenly into two legs for 100lb each up and down the leg.

That compression force is being opposed at the feet by a combination of the opposing leg's compression force and the tensile force of the ground strap, and the force of the ground pushing up against them. (Sorry if this bit makes no sense. Forces always need to equal out or Bad Things happen)

The weight you measured on the scales is merely half the downward force of gravity acting on the two points where the feet meet the (upward, equalising force) of the ground. Two things affect this measurement that I can think of;
As more tension is put on the foot end guy line, such as by shifting your weight towards the head end, the downward force at each foot will rise. You’re essentially turning the foot points into the fulcrum of a lever by doing this.
As the hammock swings slightly left to right, this weight will shift a little between the two points.

If you cut all four legs and inserted a compressive force measuring device in each, I'd expect each one to register about 100lb.

Now that I write this out I'm sure I've missed something. Someone better at it than me may come along to tell me why I'm off, but I don't think it would be by a huge amount.

[Sean McC]

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.

Specifically is is 1/2 your total weight divided by the sine of the hang angle. Sine 30 = 0.5. So 100/.5 = 200. Sine 10 = 0.17 making 100/sin(10) equal to 575.877.

If you want the maths.

And it is not so much shear force as bending moment that pulls things over. Bending moment being force * distance. Which is why levers work. Or in this case, pull things over on you. That moment has to be balanced at any individual point with a portion of the post/beam being in tension and some in compression (otherwise it will be in motion and spinning). And some things do not have good tensile strength. Like masonry. The tension side fails and then it comes over (same forces now have to be distributed across a now smaller beam which is weaker and, yeah, game over unless you have rebar in there as well)

I was just going to leave it as it 'good enough'. Though I am an engineer and engineering is the 'science of close enough'...

[piscator]

Thanks for the replies, I initially expected about 50lbs per leg, compressive force. Assuming it would be shared evenly through the four poles. So its the same each end (200lbs) but shared between the two poles = 100lbs approx each. Makes sense now.
Its interesting that a telescoping carbon fibre solution has not shown up, well that I know of. (Ignoring expense lol)

[Sean McC]

Well, actually...

If you want to get into it, as the load in the x-y plane has to be handled by cords with anchors, it might have a force pulling down on both sides of the pole if the anchors are in the ground. So double that. Though a carbon fiber pole is going to be strong in compression (slip joints aside). The primary risk would be the pole buckling. Which gets tricky.


Somewhat unrelated -

I am getting more interested in carrying a single pole. In higher elevations, finding one tree is frequently not that hard, but getting two the right distance is. Tensa being the best show in town. They have the trekking treez (currently out of stock), which is their carbon fiber option. The non-trekking pole option is quite a bit heavier.

Here is my hesitancy on the options.

The treez are light for a support but heavy for a trekking pole. Now if your objective is to minimize your base weight, then this is obviously a good option as the treea are 'worn weight' and if you use a trekking pole anyway (like all civilized folk), this appears to be a good option.

But... If your objective is reducing how hard you have to work to walk a mile. And if the weight of a trekking pole factors into total effort like the weight of shoes (I have read this is true). And a pound on your feet is ergonomically equivalent to 6lbs on your back (I have also read this, but I am not expert). Would it not better to use a lighter trekking pole and just carry the treez as they are 12oz? With the treez being better off if it did not have the various trekking pole bits and were just a pole (Tensa does not appear to sell this option).

I have GG poles. They are 5oz each. It seems like the pole would add 43oz effective (12oz-5oz X 6). Each. Better if you have heavy poles. Am I looking at this wrong?

[Pavel]

Attachment 190691

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...

So tell me how many courses in wood science you've taken? How much force does it take to damage the cambium layer and how many trees die when bent but not broken by the wind. You do know that cambium cells are constantly replaced, right? Math is a good idea, but wood science is in short supply it would seem.

Edit: I didn't mean to sound so condescending and harsh ... apologies.

[piscator]

Sean Mcc

I saw somewhere on the forum where a guy from Taiwan uses a tree for one end then his ordinary trekking poles and strap to form a triangle the other end to hang from. Pretty low to the ground but looked ok.
Said he had done it many times that way. They were not expensive poles either.

[JollyRoger70]

I like the idea of cobbling together something like a Yobo freedom from the pair of poles, but I don’t have so much trust in my poles’ section-locking mechanisms that I’d care to try it. I’m not sure what the total compression force would be on the individual poles, but it feels like it would be a lot for a friction lock. I could see it working with the kind of poles that nest in a socket (sort of like Ridgerunner spreader bars), or that come without sections like ski poles. I have and like a Tensa Solo, but have only used it bike touring and car camping.


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