...used a knot like a hangman's noose (though very sloppy). The rope over the branch and down was then basically a big circle and it would travel over the wood. It frayed and broke much more easily than I thought it would. So I switched to this stuff. I was planning on slipping a couple over and cinching them around the branch so that it bends more below the wood, rather than transiting over it, as the swing moves.
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This was my experience with making a swing in a tree:
http://igkt.net/sm/index.php?topic=4198.msg26316#msg26316 . I used 1/2' laid nylon line. The swing is still there and the branch is still healthy, even after having 'swallowed' the hitch.
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Basically just wonder what is the best way to create a loop at the bottom of the rope to attach the (pre-strung) swing to it.
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So, if I understand correctly, you want to tie off to the branch with short lines -- which have loops in the standing end -- and then attach the swing (and its long line/lines) to this loop? I wish I had done that. Part of the reason the swing is still there is that it was a major operation to remove it. This would have made it a very simple operation.
If this is, indeed, what you were thinking, there are many ways of attaching the "swing lines" to the "branch lines", incuding hitches to the loops in the branch lines that I think you have mentioned, and bends (tying two ropes end-to-end). If I had it to do again, I would use a loop-to-loop bend, e.g. two figure 8 loops with the loops attached, which I have found (in some limited, controlled, experimentation) to be very strong.
-- J:P