Carbon spreader bars

[Boston]

If I understand you, then the ratio would be the same if the hammock body was 54" and the bar length was 50" (1.4)?
Is there any compressive force being exerted on the poles due to the length of the suspension legs? Would lengthening the legs reduce the compression?

An exact comparison would be a 49.1" spreader bar vs 54" fabric width, but yes, basically. (not sure what you mean by 1.4. 54/50 = 1.08)

And yes, as Griz mentioned, this is only one factor in determining the loading. Lengthening the suspension triangle will also reduce the load, as will various other design decisions.

[Just Bill]

I don't follow all the force calculations and body sizing (fabric to bar ratio) as part of the design...

I've found that:
As low as 30" of fabric on a 26" bar (So 1.15? fabric to bar) will give you a flat lay when the bar is past your head.
This also flattens and pops up the head end some (which is nice for sleep apnea) but not as comfortable for multiple positions of sleep.

When recessing bars you need to go with higher ratios and I also arrived at the same number as Grizz did in the Ariel independently- 45" of fabric under a 36" bar (1.25 fabric over bar)
I found this makes a very nice shape that is deep enough to get under the bar without being so deep as to be confining. It makes a soft pillow when recessing the bars, but not a hard ridge.

Though cannot comment on exactly how that affects compression of the bars mathematically.

I do recall Grizz mentioning roughly .8 as a good number for dogbone length relative to pole size. (If I am remembering that correctly).
So 36" bar x.8 is 28.8" or basically 30" pole tip to apex.
Below that was a no-no, above that is safer.

What I can say:
The best I can do on bar size is a .490 pole, single segment (no joints) at 26" which supports up to 230lbs reliably.
In order to make that work I use at minimum 30" for the dogbone dimension (pole tip to Apex)

So to answer the OP... that's the lightest pole size I can think of using and how I use it. (at least 90 nights of use) though I couldn't do the math to prove it or sort out exactly why it works.

[Just Bill]

oh... .490 aluminum at 26" long with grommet tips...
one pair weighs 113g.

Just to give you an idea. Don't think you can go much lower than that?

Haven't posted it yet but as far as I know I have the lightest bridge in the world.

The poles are 113g, Grizz ariel style suspension in Dynaglide 21g, body (Robic XL or HyperD 1.0) 108g. So bridge itself is 242g.

Add two soft shackles and two dynaglide 6' whoopies and it's 269g. A pair of Dutch's 2.2 Kevlar 4' straps add 22g and that's 291g or 10.21 oz tree to tree including the spreader bars.

No cuben or carbon involved

[FJRpilot]

An exact comparison would be a 49.1" spreader bar vs 54" fabric width, but yes, basically. (not sure what you mean by 1.4. 54/50 = 1.08

Thanks guys,

Boston you caught my typo... I was inferring that with the RR having a 44/40 ratio 1.1... a 54/50 would be close to a 1.1 as well (1.08 to be exact).

Would it be accurate to state that the lower the ratio, the flatter the lay?

My assumption would also be that the suspension arc exerts more compressive force then the force exerted by the weight of the person within the hammock... so by lengthening the suspension legs ( or widening their angle), you would reduce the compression on the spreader bars...




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[Country Roads]

Have subscribed to this thread. Good information. I have a test hammock that is 46 material width on each end. I can come up with a couple of DIY spreader bars: one would be 36" and the other would be 43 inches (not sure which I will use for each end). Have been planning 30 to 36" dog bones. I would like to use 36" at the head end and some JRB bars (32") at the foot end. Any reason why I might hit the ground with this set up? Depending on how the test goes, I plan to build a personalized prototype.

[Snowball]

Some years back I made kites and used carbon fiber tubes. They are very strong but there are limitations.
I found an 8mm (5/16) and made a small test. IMO its too thin for hammock use. It was 1 meter long and when placed on a scale it started to bend when loaded with 14,5kilo (load from end to end, compression). It’s the thickest I have. However when I compare it with a 6mm there is a big difference. The 6mm started to bend with 5kilo load. These tubes cannot be compared directly because it’s not the same brand but it gives a good indication. I am fairly sure it would work with a <11mm (7/16)tube depending on the persons weight.
However there is a downside to carbon fiber tubes and that’s if two prices are joined like a tent pole. Then some forces are directed outwards when its bend and in my experience its carbon fiber tubes weak point. Depending on how the joint its made the tube will crack or more likely splinter if overloaded. You don’t want you face near the tubes if that happens!
For the most part aluminum poles are joined with a smaller male part but when using carbon fiber tubes its better the other way (bigger female part) and a short tube made from aluminum. If the diameter is sufficient big you can overcome this problem by adding a short thin metal reinforcement ring on the female carbon fiber tubes, then they won’t be able to split (crack).
I have broken a few tubes in my kite flying days and all of them was around the joints so if in doubt use a bigger tube.
Joining two tubes is not the only problem we face. There will be similar challenges in the pole ends where they have to be attached to the hammock sides (tips) but not as big as the pole joints.
Depending on the source most carbon fiber tubes have a wall thickness of 1mm and not all tubes fit into each other although the measurements say they will. Ask your source before you buy.

[Snowball]

A way to overcome the pole tip problem is to make 4 pockets from Grosgrain and sew them onto the hammock body. If you do it like this you don’t have to do any precision splicing (for the pole tips).
This method is used for kites and I can’t see why it couldn’t be used for hammocks.
Just an idea.
Pockets.png

[Military hiker]

Some years back I made kites and used carbon fiber tubes. They are very strong but there are limitations.
I found an 8mm (5/16) and made a small test. IMO its too thin for hammock use. It was 1 meter long and when placed on a scale it started to bend when loaded with 14,5kilo (load from end to end, compression). It’s the thickest I have. However when I compare it with a 6mm there is a big difference. The 6mm started to bend with 5kilo load. These tubes cannot be compared directly because it’s not the same brand but it gives a good indication. I am fairly sure it would work with a <11mm (7/16)tube depending on the persons weight.
However there is a downside to carbon fiber tubes and that’s if two prices are joined like a tent pole. Then some forces are directed outwards when its bend and in my experience its carbon fiber tubes weak point. Depending on how the joint its made the tube will crack or more likely splinter if overloaded. You don’t want you face near the tubes if that happens!
For the most part aluminum poles are joined with a smaller male part but when using carbon fiber tubes its better the other way (bigger female part) and a short tube made from aluminum. If the diameter is sufficient big you can overcome this problem by adding a short thin metal reinforcement ring on the female carbon fiber tubes, then they won’t be able to split (crack).
I have broken a few tubes in my kite flying days and all of them was around the joints so if in doubt use a bigger tube.
Joining two tubes is not the only problem we face. There will be similar challenges in the pole ends where they have to be attached to the hammock sides (tips) but not as big as the pole joints.
Depending on the source most carbon fiber tubes have a wall thickness of 1mm and not all tubes fit into each other although the measurements say they will. Ask your source before you buy.

Very helpful info, thank you very much!!


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[Military hiker]

A way to overcome the pole tip problem is to make 4 pockets from Grosgrain and sew them onto the hammock body. If you do it like this you don’t have to do any precision splicing (for the pole tips).
This method is used for kites and I can’t see why it couldn’t be used for hammocks.
Just an idea.
Pockets.png

That is genius, I've never thought of that before!!


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