I've been toying with the idea of tree camping for a little while now (shout-out to www.treefool.com). But I want to do it with my existing hammock gear. Turns out finding some anchor points far enough apart in the canopy can be kinda difficult. Wouldn't it be nice to only need one tie off point?
The ridgeline's job is to keep the hammock ends the correct distance apart, but it only works in tension. If I swap out the ridgeline for a rigid spreader bar it should work in compression, too, which means the ropes can come back toward the center of the hammock and join ends, which then can rise vertically to tie off on a branch.
The concept is simple enough, but the problem is the potential for the spreader bar to buckle under load. The bar can have plenty of compressive strength but if it's able to bend to the side it could collapse on itself.
WARNING: Nerdy math follows.
I looked up Euler's column formula, which deals with buckling, and determined n=1 (ends allowed to pivot). Then, using the free body diagram below, I calculated the compressive force in the spreader bar. From there the required EI (modulus of elasticity (material) * area moment of inertia (shape)) of the spreader bar can be calculated. I made a list of the EU of easily obtainable bars and tried each of them until I found a bar that offered a reasonable safety factor.
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All these calculations I built into this spreadsheet so I could play with different values. Some things to note:
- The force to buckle is inversely proportional to the (length of the bar)^2. That means increasing the length of the bar by 10% will decrease the force to buckle to 1/(1.1^2)=0.83 , or 83% of the force of the unlengthened bar.
- The height above the bar where the lines join together will influence how much force is on the bar. Increasing the height will decrease the force on the bar.
- A larger diameter hollow bar is much lighter than a solid, smaller diameter bar with the same EI.
END OF NERDY MATH
For my setup I didn't want a huge height between the spreader bar and where the ropes tied together so that I could hang high up more easily. With my weight and preferred ridge line length a 3/4" steel conduit pipe turned out to be strong enough and very cheap ($3.40 at Lowe's!).
Currently this is not a backpacking friendly setup. The bar weighs just under 4 pounds and is one continuous piece that's hard to fit anywhere. But I've got some ideas of how to use multiple tent poles attached together in a way to reduce their effective buckling length (remember that L^2 from Euler?). I'll come back with more details once I try it out.
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