Bridge Hammock - Thoughts and questions

Printable View

01-29-2008, 13:38
dog8mycode

Bridge Hammock - Thoughts and questions

So, I've made my speer hammock, 8x10 tarp (gonna make a bigger one soon), and my hammock sock. I've taken them on 2 trips to date, and had great success with the system down to 26°. I consider myself a fully converted hanger at this point.

Now, I've decided the time has come to try out the bridge hammock to see if I prefer it to the speer style. Plus, I also have a dad that if he could lay flat, could probably be converted to see the light.

I've read through Griz's Article on making a bridge hammock and have found it extremely informative. I am pretty sure I have the basic's down, but I do have a couple of questions that I'd like some clarity on.

First is compression. I'm a pretty heavy fella, so I'm trying to understand this to avoid disaster (trying to go down, but around 275 right now). The formula noted states that an alpha of 45° results a multiple of 1 x body weight. Is this technically divided by 2 because there are actually 2 spreader bar's being used over the length of the hammock, or is that already figured into the equasion? I suspose given that a second spreader bar would create additional pressure that didn't exist before, but it should relieve some of the pressure from the first bar as the hammock side alpha would be very close to 90°.

Second is the use of a catanary curve in place of the parabolic curve for the suspension. I ask because in terms of simplicity, it's easier for me to mark and cut a cat curve vs taking the time to draw up the parabolic curve. I'm not opposed to the work, I just want to validate that I am in fact gaining something for the extra effort.

I'm sure I'll have more come up, but that's my start. Thanks in advance for any help/advice.
01-29-2008, 16:58
FanaticFringer

I just used a tent pole and cut a 6" deep cat cut on my DIY bridge.
I'm at a standstill on it because I cant figure out a good way yet to attach my hiking poles as spreader bars. I would have to be sure they would stay on. For obvious reasons. Good thing I really like my JRB bridge so there is no rush.
01-29-2008, 17:51
Walking Bear

I think that any of the ways that have been used to mark and determine the cut on the bridge hammock will work. I'm not sure what the difference would be. That would be a good experiment to try at some point to see if there is much of a difference.
I'll defer the compression math to Grizz. However, the lengths of the suspenson triangle, width, and angle of the hang all will enter into the equation as you know. I'm at about 240 and have the compression down so that the poles work. I have had the poles work with just the locking devices. However, I now have some inserts in the poles with the wider spread that I am working with.
A bridge hammock can be designed lots of ways. I'm sure you can find a combination that will work.
01-29-2008, 22:51
GrizzlyAdams

Quote:

Originally Posted by dog8mycode

So, I've made my speer hammock, 8x10 tarp (gonna make a bigger one soon), and my hammock sock. I've taken them on 2 trips to date, and had great success with the system down to 26°. I consider myself a fully converted hanger at this point.

and there's no turning back now...

Quote:

Now, I've decided the time has come to try out the bridge hammock...but I do have a couple of questions that I'd like some clarity on.

First is compression. I'm a pretty heavy fella, so I'm trying to understand this to avoid disaster (trying to go down, but around 275 right now). The formula noted states that an alpha of 45° results a multiple of 1 x body weight. Is this technically divided by 2 because there are actually 2 spreader bar's being used over the length of the hammock, or is that already figured into the equasion?

That figure is the total compression force on one pole from the suspension lines, and assumes that half your body weight is exerting force down there. There is another component of compression due to the suspension webbing, but that is usually small by comparison and so I left it out of the description for the sake of simplicity.

Quote:

I suspose given that a second spreader bar would create additional pressure that didn't exist before, but it should relieve some of the pressure from the first bar as the hammock side alpha would be very close to 90°.

Not sure what you're saying here, sorry.

Quote:

Second is the use of a catanary curve in place of the parabolic curve for the suspension. I ask because in terms of simplicity, it's easier for me to mark and cut a cat curve vs taking the time to draw up the parabolic curve. I'm not opposed to the work, I just want to validate that I am in fact gaining something for the extra effort.

I'm convinced it doesn't matter. The difference between a true catanary curve and parabola at this shallow a bend is within the error you introduce sewing. Well, error I introduce sewing, anyway.

You must have a program or scripted spreadsheet to do this, because computation of a parabola is trivial compared with computation of a cat curve.

Grizz
01-30-2008, 15:56
dog8mycode

Quote:

Originally Posted by GrizzlyAdams

and there's no turning back now...

I'm convinced it doesn't matter. The difference between a true catanary curve and parabola at this shallow a bend is within the error you introduce sewing. Well, error I introduce sewing, anyway.

Ya, I introduce my fair share of error in sewing. :D I was very tempted to graph the 2 out to see the difference at that shallow of a depth, or better yet, find a depth and starting point that caused them to nearly overlap. You know, for the geek in me.

Quote:

You must have a program or scripted spreadsheet to do this, because computation of a parabola is trivial compared with computation of a cat curve.

Wouldn't hanging a chain (preferably one with small links) from 2 level points with the prefered depth give me a perfect cat curve? As I understand it, that's the basis for the physics of the curve and it's value. Ultimately, this is really what I meant by it being easier for me to mark and cut a cat curve vs dealing with graphing out the points.

Thanks to all for the replies.
01-30-2008, 16:09
GrizzlyAdams

Quote:

Originally Posted by dog8mycode

Ya, I introduce my fair share of error in sewing. :D I was very tempted to graph the 2 out to see the difference at that shallow of a depth, or better yet, find a depth and starting point that caused them to nearly overlap. You know, for the geek in me.

If I had time, the geek in me would do the same thing. I don't have time though even to find Ivelph's spreadsheet and hack it to do bridge dimensions.

Quote:

Wouldn't hanging a chain (preferably one with small links) from 2 level points with the prefered depth give me a perfect cat curve? As I understand it, that's the basis for the physics of the curve and it's value. Ultimately, this is really what I meant by it being easier for me to mark and cut a cat curve vs dealing with graphing out the points.

emphasis mine. Perfect if your chain is really a cable with perfectly uniformly cross-section and density throughout, perfect if the attachment points have perfect freedom of movement in the cable's plane.

You're right, that the catenary curve is what you get from a mathematically perfect abstraction of a hanging cable. (It is also true that if you hang load uniformly on that perfect cable you get a perfect parabola.)

What you figure on doing will work just fine. Me, I'd bump the cable for sure sketching it out....and it still wouldn't matter much to the end result.

Grizz
01-30-2008, 16:33
dog8mycode

That makes sense to me. Thanks again for the help. Now to just find time and finish the prototype with some $1 wally wonder cloth and see if it's worth investing in some better materials.