Carbon spreader bars

**Military hiker** · 11-02-2016, 10:36

I have been hammock camping a very long time. Most of my stuff is diy. I have recently been turned on to bridge hammocks, but don't want to carry around big aluminum spreader bars. I weigh 120, would a .346 carbon pole be strong enough to be a spreader bar?

**Boston** · 11-02-2016, 11:00

You'll need to provide more information than that. What, specifically, are you looking to buy? (link?) How are you wanting to configure them? (for instance, a 2 piece pole with the ferrule in the middle will be the weakest.

Also depends on the dimensions of your hammock. What's the hammock width to bar length ratio at the head end? How long s the suspension triangle?

My gut says "No", however.

And if you're not absolutely sure, because carbon fiber is more likely to break and splinter (vs just bending for Al), if you're not absolutely sure, I'd be disinclined to have that sitting next to your head, under tension.

FWIW I weigh 150/160 and used the Gossamer Gear LT4 poles for spreader bars fairly successfully. The locking mechanism did slip on occasion. They are also larger poles than what you are looking at.

Also, FWIW, the weight vs cost probably isn't going to be super favorable. You're better off trying to find a way to use something you already carry (like hiking poles).

At 120lbs you may be able to use the 0.49" easton poles, which are pretty freaking light. I used them on a hammock with a something like a 1.2-1.3 ratio, and a fairly long suspension triangle (at least the length of the bar, if not more). It curved sightly, but held me. Was a bit bouncy though. I wouldn't use them on a lower ratio hammock (like the ridge runner is)

**Carbon** · 11-02-2016, 11:11

Originally Posted by Military hiker

I have recently been turned on to bridge hammocks, but don't want to carry around big aluminum spreader bars.

I did not like the longish aluminum bars that came with my Ridgerunner so I made shorter aluminum pieces for compactability. The skinnier, foot end pieces all nest within the larger diameter head end pieces. No weight penalty either, compared to the stock Ridgerunner bars. I got the tubing and inserts from Quest Outfitters.

**Military hiker** · 11-02-2016, 13:11

Originally Posted by Boston

You'll need to provide more information than that. What, specifically, are you looking to buy? (link?) How are you wanting to configure them? (for instance, a 2 piece pole with the ferrule in the middle will be the weakest.

Also depends on the dimensions of your hammock. What's the hammock width to bar length ratio at the head end? How long s the suspension triangle?

My gut says "No", however.

And if you're not absolutely sure, because carbon fiber is more likely to break and splinter (vs just bending for Al), if you're not absolutely sure, I'd be disinclined to have that sitting next to your head, under tension.

FWIW I weigh 150/160 and used the Gossamer Gear LT4 poles for spreader bars fairly successfully. The locking mechanism did slip on occasion. They are also larger poles than what you are looking at.

Also, FWIW, the weight vs cost probably isn't going to be super favorable. You're better off trying to find a way to use something you already carry (like hiking poles).

At 120lbs you may be able to use the 0.49" easton poles, which are pretty freaking light. I used them on a hammock with a something like a 1.2-1.3 ratio, and a fairly long suspension triangle (at least the length of the bar, if not more). It curved sightly, but held me. Was a bit bouncy though. I wouldn't use them on a lower ratio hammock (like the ridge runner is)

Thank you, I might just use my hiking poles. I might even be getting some LT4's. Thank you again for responding quickly and with very helpful information. I will do a little more research.

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**FJRpilot** · 11-02-2016, 20:39

Originally Posted by Boston

I used them on a hammock with a something like a 1.2-1.3 ratio, and a fairly long suspension triangle (at least the length of the bar, if not more). It curved sightly, but held me.

Boston, can you explain how you are calculating both the ratio as well as the forces the ratio experts? I've heard Grizz talk about this as well but I'm still not really clear on how to insure my bridge suspensions are not exerting excessive pressure on my poles.

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**Boston** · 11-03-2016, 13:00

The ratio I'm talking about is a simple Width of hammock / Width of spreader bar. So for instance, then RR has a spreader bar length of 40", and the width of the hammock body at the head end is 44" IIRC. So it has a length ratio of 1.1. The closer this ratio is to 1, the higher the force on the spreader bar will be (think of it like the hammock suspension. The closer the suspension is to horizontal, the more force you impart on the tree/suspension components).

This ratio also has an impact on shoulder squeeze in a bridge hammock, and stability.

This is a pretty involved calculation, and I've never actually done it. But if you're pushing the limits in materials, I think it's a good idea. The easiest way to do it would be to make some simplifications and overestimate the force involved. My comments above are based on trial and error.

TeeDee did some work with it at the bottom of his post
here: http://www.hammockforums.net/forum/s...9664#post39664
continued here: http://www.hammockforums.net/forum/s...-System-Part-7
but it's pretty hard to follow, and I don't know how accurate it is - I haven't really worked through it.

You also need to understand that realistically there will be a bending component involved, so pure tensile strength isn't the only consideration.

**FJRpilot** · 11-03-2016, 16:29

Originally Posted by Boston

The ratio I'm talking about is a simple Width of hammock / Width of spreader bar. So for instance, then RR has a spreader bar length of 40", and the width of the hammock body at the head end is 44" IIRC. So it has a length ratio of 1.1. The closer this ratio is to 1, the higher the force on the spreader bar will be (think of it like the hammock suspension. The closer the suspension is to horizontal, the more force you impart on the tree/suspension components).

This ratio also has an impact on shoulder squeeze in a bridge hammock, and stability.

You also need to understand that realistically there will be a bending component involved, so pure tensile strength isn't the only consideration.

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?

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**GrizzlyAdams** · 11-03-2016, 18:58

Originally Posted by FJRpilot

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?

Yes, and yes. Thought experiment...imagine having a tree whose diameter is the same as the spreader bar length, and being able to run
parallel lines from the corners of the hammock to the tree. There is _no compression at all_ on the spreader bar due to those legs (there is of course compression 'from below' due to the fabric under load trying to bring those corners together).

The longer the suspension legs, the closer the angle between the leg and corner is to the thought experiment case

**FJRpilot** · 11-03-2016, 19:15

Originally Posted by GrizzlyAdams

Yes, and yes. Thought experiment...imagine having a tree whose diameter is the same as the spreader bar length, and being able to run
parallel lines from the corners of the hammock to the tree. There is _no compression at all_ on the spreader bar due to those legs (there is of course compression 'from below' due to the fabric under load trying to bring those corners together).

The longer the suspension legs, the closer the angle between the leg and corner is to the thought experiment case

Thanks Grizz... so, your latest Bridge, the "Ariel"... with a 46" fabric width, and a 36" spreader bar... I calculate the ratio to be about 1.555. If you increase the spreader bar to 40, the ratio decreases to 1.15. Seems like it's getting pretty close to the 1.0 that Boston mentions in a prior post as being the ratio you don't want to exceed. Can you calculate the increase in compressive force that adding length to a spreader would cause?

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**GrizzlyAdams** · 11-04-2016, 06:59

Originally Posted by FJRpilot

Thanks Grizz... so, your latest Bridge, the "Ariel"... with a 46" fabric width, and a 36" spreader bar... I calculate the ratio to be about 1.555. If you increase the spreader bar to 40, the ratio decreases to 1.15. Seems like it's getting pretty close to the 1.0 that Boston mentions in a prior post as being the ratio you don't want to exceed. Can you calculate the increase in compressive force that adding length to a spreader would cause?

Not reliably. The force transfer to the pole tip looks complex to me. You've got force carried to it by the suspension on the sides, the way the cables in a suspension bridge do. But the curve isn't perfect nor is the weight distribution uniform across the hammock body.
I do know the principle that the smaller this ratio is, the more compression. And believe that the compression grows approximately as
W/sine(theta) where theta is an angle that gets smaller and smaller as the ratio decreases. But knowing just what theta ought to be is
the question.

I wouldn't try any ratio smaller than what the RidgeRunner uses, and for the Ariel would back off a bit from that even because the spreader bars are closer to the source of the force, the 'm' part of F = m*a, where 'a' is gravity.