Bridge Hammock

[GrizzlyAdams]

Wow, this is quite interesting Grizz. First, thanks for taking the time and effort to explore the possibilities. I know sewing in all that webbing was a pain....

I'm pretty sure I've followed your analysis but have a few questions.
Are we getting a lower spreader bar?

The spreader bar is definitely lower relative to the occupant, or the bottom of the hammock if you prefer.

It seems it's just like your first version.

The webbing is like it, only with this one it is even more agressively shallower. v0.0 had a depth of 9", this has a depth of 6".

Or are we getting it lower by having longer suspension lines?

The impact of long suspension lines is equivalent for tall and shallow hammocks.

Is there indeed less force on the spreader?

This is a little slippery. In the diagram laden post of early this afternoon, I noted that one angle (gamma) was larger and so the compression would be lower for a given spreader bar. I can't help suspect that there is another source of compression---from the fabric---that increases as the spread increases. As the fabric gets flatter (just like a suspension rope getting flatter) it would seem to reflect more compression. But I haven't sorted that one out. Can't with any confidence because I'm not well trained in physics. Aside from these things, for a given spreader bar the height of it in the hammock doesn't matter to the compression. The subletly is that that spreader bar does a better job of separating the hammock body when placed lower than when placed higher. This means that for a given compression force budget you get away with a shorter bar when placed lower, and for a given bar you get more spread when it's placed lower.

It sounds pretty cool to have that lower cut in the middle. That could add a little more cross ventilation. In colder temps an adjustable under layer could have full sized walls to do the opposite.

On your suspension, I read the discussion on the other thread about tying the webbing. Well the sewing is no different than what you've done on the end of you hammock webbing. I also have some tied webbing but the sewn loops save weight because you use less. Other than that, 1 lb sounded kinda high for suspension. I need to see what mine weighs right now...

I'll have pics later today on my latest version.

I figure I need about a 9 inch bight to make the figure-8 on a bight work. I need 12 inches to make a bowline, so that's a win. I could probably get away with 3" of overlapped webbing to sew in a loop. Savings of 1' for any particular hugger.

As I flagged from my smartphone, the first figures I posted on weights were significantly off because I mistook the per-yard weight of webbing for the per-foot weight, and then doubled that in the table before doubling again. Comes from posting at a late hour!

...

looks like you could make the fabric from the lower arc up out of a completely different piece of fabric.

yes I think so too. I've pondering what I'd want to do with that ability...

is there more tension anywhere due to the flatter arc?

The strain within the webbing itself is higher. I read about this when coming up to speed on suspension bridges. Look at the depth of those cable curves....much deeper than what we're doing here. Spec'd to cable strength.

is the less compression on the poles due only to the longer suspension lines, or is there another factor? if the lines were the same length as the upper lines were, would there be the same amount of force applied?

see above.

when are you going to try your hiking poles with this?

What's fighting the compression inside a pole is a plastic expander piece. TeeDee had the excellent idea of fitting a dowel inside the pole to jam up against the end of this piece, so that the force is on the dowel pushing on the center of the piece rather than the piece's side pressure holding it off. Because of the way one uses Pacer Poles, I figure I could carry dowel to hold off compression for a 28" bar. I want more than that for the head, and so need to get a pair of tent pole segments like Scott was talking about and see if they fit inside my hiking poles also. My hope is to get the dowels inside of the tent poles, inside of the hiking poles. Then I'll be set. And after a good day's hike I'll have massive biceps.

maybe two stakes per corner if one is not enough. for those who don't carry poles, 8 extra stakes may still be lighter than 2 hp's.

The problem left is to determine how much body separation one can get using stakes. I doubt very very much you can get to the separation I get with a 36" spreader. Don't know about the 30" spreader...I didn't think I could easily move the webbing much. But as I saw with v0.0, you can get quite a lot of force using a block and tackle approach. The question is how massive the stakes need to be to hold it.

Grizz

[warbonnetguy]

why do you think you need a dowel AND tent pole inside the hp's? couldn't you just use tent poles?

yeah, it's just like a regular hammock, the less sag, in the case of the bridge side to side sag, the more force on the fabric and for the webbing, the same is true for a deep curve vs. a shallow one. it seems like this factor is true in all it's forms.

what wt. fabric did you use?

did it seem like the fabric and the webbing can take these higher forces over time?

[GrizzlyAdams]

why do you think you need a dowel AND tent pole inside the hp's? couldn't you just use tent poles?

I use the hiking poles at a height that, with my present set of hitches, makes 31" the longest spread they'd provide. If I put dowels enough to use the pole at 36" in, the poles in use would be too long. But 31" is just fine for spreading at the foot end. So the idea is to pack in two 18" segments that can be taken out and assembled into a spreader for the head, and the dowels can be put in one pole that would serve at the foot.

I just now had another look at the poles and realized that to get 18" tent poles inside I would have to remove the expander between two segments. I'm not wanting to do that, so Plan B is to work on shortening the hitch arrangement to lengthen the span a pole can take. 36" looks possible, maybe, just. I'd take 34" as a consolation prize I think.

yeah, it's just like a regular hammock, the less sag, in the case of the bridge side to side sag, the more force on the fabric and for the webbing, the same is true for a deep curve vs. a shallow one. it seems like this factor is true in all it's forms.

Seems like the known equations work to describe what kind of force is on the fabric, at least assuming that the weight carried by (for example) a one inch strip is the body weight under that one inch strip. Not at all sure how that translates into compression at one point of the webbing---the spreader bar.

what wt. fabric did you use?

This is 1.9 oz ripstop.

did it seem like the fabric and the webbing can take these higher forces over time?

my first bridge was made of taffeta, and I could see stress along the seam threads pretty quickly. Due in part to my inexperienced sewing I'm sure. But in any case I don't see that here. But don't have but a few hours in the yard experience with it.

Grizz

[GrizzlyAdams]

Report on the afternoon experiences with the lowered spreader version of the Bridge hammock.

I drafted my 15-year-old son, a.k.a. Hebephrenic Lab Hamster (HLH) to get in the hammock for some photos. In order to visually demonstrate how the length of the spreader bar affects squeeze, I took pictures at 30" and at 36" peg settings at the head spreader bar. As you can see if you look closely at the photograph, there is a gap between the peg and webbing that is about twice the length of the Quick Link in use---about 2.5". That translates effectively into 31" and 25" separations.

Here are shots at 25", with HLH laying on his stomach, and then on his side.



That's looking tight-ish in there. From my own experience at that level, it is uncomfortably tight.

Here are corresponding shots at an effective separation of 31".



This is much much better for the shoulders and hips. Hardly any squeeze to speak of. There is however a down-side. I haven't thought through the issue of tippy-ness well enough. I thought (and have said several times) that the distance between the body and the height of the attachment point on the ring or tree determined tippy-ness. That's still true I think, but for tippy-ness along the longitudinal axis of the hammock. Side-to-side tippy-ness is determined by---you guessed it---the difference in the position of the body and the height of the suspension at the side. So by lowering the suspension curve we are directly causing the side-to-side movement to be less stable. You can see that in the photos above, how easily the hammock tilts to one side or the other, depending on the weight distribution.

As turkeyboy pointed out yesterday, we can mitigate the lateral tippy-ness using stake-outs. Shy of revisiting the dual suspension line approach, I'm not seeing other solutions. You want a side-to-side lay that is that flat, it comes with a stability cost.

I also changed the suspension to straddle the tree, as described earlier to increase the angle between spreader bar and suspension line. Although most of my hangs I've had the lines come to the tree at about a 5' level, in this shot I pushed it up to about 7'. This further reduces the compression force. HLH (who is just shy of 6') likes the OWF camo webbing.



The suspension on a line uses some ideas being batted about over at Sgt. Rock's site, applied here to the straddle suspension. I have a 12' line from the hammock to the tree. At each end of the tree hugger I have a nanowire biner (optional, I think) in the loop. At the end of the line I have put a small sized figure-9. So I run the line through the biner and back towards the hammock, then fastening off temporarily with the figure-9. This is just to make the rope work to suspend the hammock. Then I choose a point mid-way between hammock and tree, and use a lark's head to attach an SMC descending ring onto the rope. I undo the figure-9, bring the line back to the ring, through it, and out to the biner. Back and forth once or twice to do the block and tackle thing again. The loose end of the rope is tied off with a half hitch on a bight, and then the loose end (with the figure-9 still hanging there) placed loosely through the bight in case of slippage. That's all there is too it.

Another shot, this time without using biners on the loop. I just read that narrow rope may cut webbing, so I'll go back to putting something there. Over at Sgt Rock's site TeeDee suggested slipping an SMC ring onto a tree hugger loop using a lark's head. I can do that for each hugger---makes for a lot of SMC rings!---indeed since I've tied the loops onto the tree huggers rather than, I can untie the loops, put the rings and, and retie. I'd better tell Slowhike about this.



The hugger in the picture is 12', clearly 10' would have been long enough to use with one wrap. For a pitch that high, it seemed prudent to use the extra time around the tree. The weight budget for this set up is then

10 ft. OWF 1" polyester webbing 2.8 oz
24 ft. 2.8mm Spyderline 1.7 oz
4 SMC descending rings 1.6oz


The total suspension cost at one end is thus 6.1 oz. I could lose an ounce by carrying only 6' tree huggers, and not wrap on a big tree as I have done here. Since the use of figure-9's is temporary, I can use the same ones I use for the tarp suspension (although moving them around I'm in danger of dropping them!)

Using a straddle suspension I would run a tree-to-tree ridgeline for the tarp, and use my v0.1 trick of a secondary ridgeline for bugnet, etc. I don't have a place for a structural ridgeline. If I'm using corner tie-outs (to the tarp corner stakes) for stability, then these can be used to set levelness as well (again, as pointed out by turkeyboy).

Stakes on the menu for Labor Day.

Grizz

[GrizzlyAdams]

Where is these .

Couldn't find them.

Thanks.

Discussion on another site is one place.
http://hikinghq.net/forum/showthread.php?t=2378

At this site it is the thread on the figure-9's, which I see by your posts that you have found.

Grizz

[dblhmmck]

I got photos today!

In the first you see the hammock profile without anybody in it. You can see the slight inverse banana and the lowered foot segment.empty_w_bugcanopy.jpg

The head end is closed off with two hooks and loops attached to the bug netting, while the foot section is permanently attached to the netting. empty_w_bugcanopy_headend.jpg

Foot end has extension collar which contains the bundled hammock when packing it away.empty_footend.jpg

Entry is through the side with bug net retracted.side_entry.jpg

Inside is small, but not too constricted. Knees are allowed to flex slightly- VERY comfortable. Amity_headdend.jpg

Seems fine for side sleepers, and my girlfriend said that sleeping on her stomache was the most comfortable of all positions for her ( hard to make it out, but the second photo below shows her lying on her stomache. Amity_sidesleep.jpg
top_stomachesleeper.jpg

I was pleased that it received high kid ratings also!above_kid_approved.jpg

[GrizzlyAdams]

[QUOTE=dblhmmck;26922]I got photos today!

great pictures!

I was trying to puzzle out the suspension. One shot suggests that there are two suspension lines at the head end heading off to different branches, that right? Could you hang this by attaching to the trunks only of two trees? The separation between sides on the interior shot looks remarkable if the end lines are going to the same attachment point.

Grizz

[warbonnetguy]

looks interesting, there's alot to look at there.

what is the shape of the bed fabric when it's fully opened? is it like the hourglass shape of the bridge or different?

[dblhmmck]

I was trying to puzzle out the suspension. One shot suggests that there are two suspension lines at the head end heading off to different branches, that right? Could you hang this by attaching to the trunks only of two trees? The separation between sides on the interior shot looks remarkable if the end lines are going to the same attachment point.

Yes Grizz, this is meant to hang from only two tree trunks (although in this case it is two branches of the same tree). It's range is 12 - 20 feet, the closer to 12 the better.

There is a single attachment at the foot end. At the head, in addition to my carabiner attachment, I have a small guy line attached about two feet higher in the tree. My rain fly attaches to the upper line. A dangling piece of webbing tightens the ridgeline (when there is no fly), and simultaneously, with a swith of position of a mini-carabiner, tightens the fly- if sagging has occurred. I actually need to make an adjustment to the length of that line so that I will be able to comfortably make that adjustment from inside the hammock.

Oh, explanation contributing to your comment on the spread...I had the head end hiking pole adjusted to 41" and the foot end hiking pole at it's minimum- 21". I weigh around #150, my girlfriend far less. When a #190 guy tried it out, the hiking poles shortened rather abruptly.

I guess I might get a hose clamp and permanent adjust the middle section to the 135 cm mark (I leave the smallest pole section completly retracted.) It would mean that the pole would need to be extended 6 1/2 inches to make it comfortable walking height for me. Then the rest of the time it would stay at 41". A very slight inconvenience that I could accept...I think, I'll see how it goes.

[dblhmmck]

what is the shape of the bed fabric when it's fully opened? is it like the hourglass shape of the bridge or different?

It's a little different.

Imagine an hour glass then make it less thin by half. Then widen the head out and begin the curves from an angle 20 percent wider at the top than the bottom. Then instead of keeping the curve consistant, flare at the knees increasing the curvature. When the curve becomes parrralel to the ends at the level of the knee. Make an intersecting cut in a straight line perpendicular to the ends, and running from knees to the end of the fabric (foot). So the stocky top heavy hourglass sits on a square pedestal, see? I don't know if that explanation does it for you. I can't explain stuff like grizz.