Inflatable spreader bars?

[autox]

Experiment 1.

I made a (rather sloppy) prototype inflatable spreader bar.

9.5" radius x ~36" length.

IMG_20190216_225411.jpg

The results are promising, but there are some design problems to be solved before it's practical.

It's basically a giant stuff sack with three sealed garbage bags full of air inside. Starting at zero relative pressure, I leaned my torso on it until almost all of my weight (190#) was on it. This pressurized it to about 0.6psi, calculated. The compression left a little vertical slack in the fabric which allowed some lateral instability if I pressed on it sideways, but it wouldn't actually buckle. Raising the pressure to the working pressure before loading would maintain good skin tension and prevent this.

My current design target is a cross section / pressure product of 400lbs, which is 2x the expected load on the spreader (basically body weight). A slightly larger radius of 11.3" will achieve this at 1psi. If a zero pressure fill is achieved, an additional 16L of air need to be added to reach 1psi (remember atmospheric pressure is 15psi, so 1psi relative pressure is only a 6.7% change). For reference, a Thermarest NeoAir Xlite sleeping pad holds around 53L. A typical latex party balloon inflates at around 0.6psi and bursts slightly above 1psi. Reaching 1psi should be both possible and practical by blowing.

Reaching a full zero pressure fill is a bit of a challenge. At 238L, you really don't want to do that with your lungs. For two spreaders, that's the equivalent of nine NeoAirs. Opening up one end and waving it around the way you open up a plastic produce or trash bag is one approach. It requires the ability to open and re-close the fabric skin, as well as a large opening in the bladder which can be sealed air tight. Another option is a pump bag, as used for down air mattresses. Best option here is probably a 40L pack liner.

There's still the simple awkwardness of a 2' diameter spreader bar. Reducing the diameter requires raising the pressure. At 1psi, there's still plenty of excess strength in 0.5osy DCF to handle higher pressure. A large volume pump bag can only go so high in pressure before we have pump weight eating in to our budget.

I've found a few papers on buckling in inflated beams, but haven't been able to get any useful info out of them yet. I don't have an understanding of how pressure relates to span, cross section and load.

Fitting a compression sack style lid to my prototype would be another way of raising the pressure before applying a load. From there I could iterate on reducing the diameter until buckling occurs.

[MiteyF]

Have you given any thought to what happens if one of your spreaders springs a leak on the trail? Other inflatables losing air on the trail may make you uncomfortable, but a bridge hammock without a spreader bar is useless. I'm no gram weenie like yourself, but it seems to me rule #1 of the ultralight game is "skip the backups". I personally don't put much trust in ANYTHING inflatable in the outdoors. In an emergency I suppose you could always whittle yourself a new spreader bar, but I personally can't think of anything much worse than waking up mid-taco at dark-o'clock, and having to go find and fab a new structural bit for my sleeping arrangement.

[TrailSlug]

Would anyone this concerned with weight even consider a bridge hammock in the first place? I backpack with my Warbonnet Ridgerunner but I'm not all that weight conscious until about the 6th mile :0

[cmoulder]

Would anyone this concerned with weight even consider a bridge hammock in the first place? I backpack with my Warbonnet Ridgerunner but I'm not all that weight conscious until about the 6th mile :0

I like innovation as much as the next person, but the downsides seem to far outnumber the potential benefits for a projected weight savings of less than 50 grams (and even that is very optimistic, IMO).

[Just Bill]

Would anyone this concerned with weight even consider a bridge hammock in the first place?

Yes. In some situations it's lighter than a ground setup.

Also depends on how you want to count things... but I have bridges lighter than several gathered ends if you use your trekking poles. Several on par with UL gathered ends.
I'm not a fan of 'spreadsheet magic' though... so I still consider the weight of the spreader bars as part of the package.

The lightest full bridge I had was 9.25 ounces, including the spreader bars. Course I have some 5 ounce M10 Gathered ends if you want to look at that extreme.

In a more realistic world though-
The lightest GE I can use is a 10'6" Hybrid 1.2. About 8.25 ounces with loops and a RL on it.
I've got a 1.2 bridge I'm testing that is about 12 ounces including the spreader bars.

Many folks like an 11' XL in a 1.6+ as a baseline. Those run 12.25 ounces with loops and RL.

Those numbers are without suspension as that's the same across the board.
So if the general question is more accurately stated- Why would anyone concerned about weight choose a HAMMOCK in the first place... well even SUL folks understand the power of good sleep.
So if you've already chosen air over ground... seems a fair question to consider what is the lightest setup that realistically meets all your needs.
In some cases... a bridge can be the correct answer.

To MiteyF's comment... "skip the backups"
I believe good UL gear adds redundancy and bonus functions if done well.
You could go nuts and cut out the 5.5 ounces of poles and swap it for my Harmony Trekking poles (2 ounces for the bridge components, 7.625 ounces for the trekking poles).

But a more realistic plan that builds luxury and redundancy is to carry one bridge spreader for the head end of the bridge. If you happen to snap a trekking pole... you've got a backup.
If you don't snap a pole... you have a pole available for a pole mod on your tarp or for porch mode. So 90% of the time you have more luxury.

It's about a wash (you save 1/2-3/4 ounces) with this hybrid system. But you actually get more for less in the process.

If you were on an FKT and went full blown 'stupid light'... you could trim that bridge down to about 8.5 ounces.
Of course even in that precariously risky position you may consider that one reason you choose a bridge is that it works well with a pad, which you happened to pick over an UQ and even if every component of your bridge explodes you still have a fairly comfortable way to sleep. Course not the end of the world to build a debris bed either if you'd like a third layer of safety.

Also okay to simply 'call it'. SUL extremo nutjobs understand they occasionally take high risk high reward trips that may require you to abort mission if conditions or gear failure occur.
Sometimes simply having the sense to quit is it's own backup. With that final layer of protection in place what seems reckless is often much more carefully planned and layered in safety than most conventional trips.

Is Autox on the right track?
Probably not. But so what? It's not our time or money so who cares.
Thankfully there are a few of us who keep flailing around on the wrong track hoping to break through the wilderness and forge a new path.
Bushwhacking isn't always fun, nor is it really all that productive... but sometimes you learn something.

You never know.

I've been told a lot of things about bridge hammocks. Many of them quite accurate and most of them absolutely true. I probably should have listened to all those facts.

Some inventions come from genius level leaps of intelligence, but most of them come from people who are do something dumb, accidentally, or while blundering along in ignorance of the facts.
One trick to inventing things is to lie to yourself.
'Bridges can be lighter than gathered ends', for example, is a very good lie to tell.

Lying is tricky and generally a poor trail to follow for most... mostly you are just a liar and mostly folks will quickly point out all the lies you're telling as soon as they come out of your mouth.

If you're a good liar or preferably just a lucky one, as the difference isn't that important; you can find a way to lie about the impossibilities just long enough to discover a true path around them.

Just because one lie doesn't pan out, doesn't mean you can't tell another.
Maybe 'Inflatable spreader bars will work' turns into 'I can turn a sleeping pad into a bridge' or 'laminating fabric to create an inflatable gathered end'.

Perhaps a real good whopper like, " I will build a bridge hammock out of Cuben Fiber" doesn't pan out... but the fine adventure had while bushwacking in that wilderness leads to discovering how to tell another fine lie.
And the best DIY bridge hammock to date is introduced to the world. A bridge named after a spirit pulled from the mind of one of the best liars the planet has ever known.

Every DIY person who sits down at a sewing machine who doesn't know how to sew is a liar until the gear they make works.
Every vendor here at one point lied and said to themselves they could build something better than the vendor before them.
Every successful vendor who told that lie well is eventually considered a purveyor of fine goods and an honest and trusted merchant.

[OutandBack]

I like innovation as much as the next person, but the downsides seem to far outnumber the potential benefits for a projected weight savings of less than 50 grams (and even that is very optimistic, IMO).

I also enjoy innovation and reading the brain storming we get in these kinda of threads.
That said and I don't want to derail this thread but wondered why no ones has made grosgrain spreader bar baskets, for the lack of a better term, so one could use small dead branches(3"-4" x 2.5-3') cut with a folding saw for spreader bars. I can't think of anything lighter in the backpack than 4 grosgrain spreader bar baskets. Wood this size is very easy to find in my hammock camping areas.

[BananaHammock]

Most likely because they are afraid of the injuries that would follow. The right density wood with no imperfections that would cause breakage in the middle is not something the every-man can do.

[JoeBobJr]

Just Bill: YES TO ALL OF THAT! Well said on all fronts

[Just Bill]

I also enjoy innovation and reading the brain storming we get in these kinda of threads.
That said and I don't want to derail this thread but wondered why no ones has made grosgrain spreader bar baskets, for the lack of a better term, so one could use small dead branches(3"-4" x 2.5-3') cut with a folding saw for spreader bars. I can't think of anything lighter in the backpack than 4 grosgrain spreader bar baskets. Wood this size is very easy to find in my hammock camping areas.

BananaHammock is on the right track, especially in terms of selling something. That's asking a lot of a customer to take on that task...

Grizz, WV and others I'm sure have made a few different baskets for both trekking poles and harvested wood. I believe WV has a favorite tree on his property he uses for spreaders actually. So it's not a far fetched idea at all...

Really though it's one of those items where if you haven't figured out how to do it yourself... you probably shouldn't be doing.
Perhaps the more useful trick I've seen is using a continuous loop to girth hitch onto a stick. I believe I've seen a post or two to that effect, especially for an end bar bridge like the RR. I'm sure Brandon doesn't endorse it either but that's a safer way to do it in my opinion, with the advantage being that you don't need an exact fit (or adjustable basket) to use the stick. You can just girth hitch onto the hardware and slide the girth hitch on the stick out until you get it close.


While others have uses smaller... I think the minimum I used was a 1" pine dowel, oak was better of course but I was being cheap. Usually that's been kiln dried, run through a machine or two, bounced on a truck, etc. So if it was going to 'go' it probably would have already. You're probably about right with your 3-4" found stick... but of course then you've got to find a way to get it in place. You're also typically loading it less than ideally so it will take more off axis force than a spreader bar would... letting you know pretty quick if it has a structural issue.

I definitely wouldn't suggest it on my recessed bar bridges.

[autox]

Thanks for the words, JustBill.

I got curious about this in pursuit of the question, can a hammock compare to ground gear in weight? I'd always assumed it was no, until I took a closer look.

Considering that I use a 12oz insulated mattress (NeoAir X-Lite), and any durable ground cloth will weight around 2oz, one metric to judge a hommock by is 14oz for hammock + suspension + UQ. The other is based on a 1lb ground shelter (full bug/rain protection), plus that mattress, so 28oz, all in.

Setting aside the spreaders, I figured bridge hammocks could get lighter since they use less fabric. Thus the focus on spreader weight. But again, I'm new to the hammock world. And I am curious about the comfort factor - this isn't entirely in pursuit of shaving a few ounces.

JustBill - can you offer any details of your 9oz bridge?

I'm also wondering how this would fare scaled down to a spreader, and if carbon tent poles would be strong enough: https://backpackinglight.com/forums/topic/106769/ That could be in the realm of 3oz/pr.