Thread: Bridge Design Principles View Single Post
05-08-2009, 07:36   #4

Join Date: Apr 2007
Location: Illinois
Hammock: DIY Bridge, v0.n, where n is large
Tarp: depends on season
Insulation: DIY UQ
Posts: 4,684
Quote:
 Originally Posted by I Splice TeeDee - Thanks for the design notes. I've been looking at your Awk script but I can't figure out what a "belly band" is. Google gives me a lot of hits for a pregnancy aid. Somehow I don't think that's the right meaning.
belly-band : piece of lightweight elastic that goes attaches to opposites of a bugnet or over-cover, and passes under the hammock to pull the sides of the bugnet/over-cover down and flush with the hammock body.

Quote:
 I also coded up some simple-minded asin and acos functions. I didn't even use Newton-Raphson. The version of gawk that I have (Cygwin) doesn't have those as built-ins. My versions almost certantly have some bugs but they give the same answers as your test output, so I hope they are good enough. I can post those if anyone is too lazy to do a good job on them.
I've done all the calculations I need with Perl scripts; also I always use parabolas and not cat curves (can't remember if the awk scripts are solving parameters for cat curves). For iterative calculations (e.g. find the parameter of a centered parabola that intersects vertical lines y +/= S/2, where S is the spreader bar width, and has a given arc length over that range) I just do a binary search over the parameter space. No inverse trig functions involved, just some piece-wise linear approximations to the arc length.

As long as I'm rambling here, I'd say that my own experiences with the bridge are congruent with TeeDee's. All of our early versions used the Australian model, with deep cuts and high walls. We all learned that less is more, and narrowed the design down.

I like center widths in the 32" - 36" range. Being a mere child of 52 years in reasonably good shape I have less concerns about getting in and out of one with the wider width.

I'd try to use lightweight polyester micro-fiber fabric if I could find some in muted colors. The only stuff I've seen is on the Rockywoods site, haven't pulled the trigger on any of it yet.

One point TeeDee did not address on the cord versus webbing for side suspension question is keeping the hammock body from slipping down the cord when you get in. I know a couple of people that have sewn the cord through to the fabric, we'll see how that solution holds up. I have another that I've posted on, TeeDee has one that has IP restrictions on it.

I like a narrow foot bridge. I cut fabric to give me this shape. When TeeDee uses a significantly shorter spreader bar at the foot than at the head then it seems to me he gets essentially the same effect, except that the curve of the hammock at the foot is deeper than the curve at the head. I cut mine so that the depth of the hammock at the foot and the head is the same, even as the hammock body narrows. I don't know that any of this makes a significant difference, except that cutting the hammock to be narrow saves a little weight.

Something else TeeDee did not touch on are bugnets. You can easily make a big bugnet cocoon like the ENO or other DIY versions people do. That's a lot of unnecessary netting though. TeeDee's system has a bugnet like an over-cover. The challenges there are fastening the thing down, hence the bellyband and something at the corners. A couple of times I've sewn in netting, which is a cleaner design I think, uses less netting, but adds the weight of a zipper, and leaves you with a bugnet all the time. The latter part doesn't bother me much because if I don't need a bugnet I can carry a hammock that doesn't have one. Takes more work to sew in a bugnet though. My current working version is of the over-cover design, with a belly-band. 'cause I'm in a hurry to have a working version for a hike I'm doing in a week!

Grizz