First Bridge Hammock Build, Question(s)

[MiteyF]

If it's too snug a fit, the angle of the suspension triangle imparts a bending force on the pole tip

Perhaps my brain is working backwards, or I don't fully understand how hiking poles would be used as spreader bars, but this doesn't make sense to me. Assuming you're using proper spreader bars instead of hiking poles, if your hardware piece sits nice and flat against the pole, there should be no bending force on the pole tip. Remember, ropes are single force members. They can only transmit force in one direction, directly along their axis. The sides of a properly built bridge hammock will act in the same way as the dogbones. The hardware should direct all of that force onto the "face" of the end of the pole, and only a slight amount of shear force on the tip resulting from the differences of force between the dogbone and the hammock edge (AKA, if the pole were free to slide along the hammock body/dogbone, it would migrate to the middle of the hammock). The forces of the hammock edge and dogbone should be simply squeezing the pole. In fact, in this instance, a larger hole for the pole tip would allow the hardware piece to slant slightly, and THAT would result in a bending force of the pole tip.

[Just Bill]

Perhaps my brain is working backwards, or I don't fully understand how hiking poles would be used as spreader bars, but this doesn't make sense to me. Assuming you're using proper spreader bars instead of hiking poles, if your hardware piece sits nice and flat against the pole, there should be no bending force on the pole tip. Remember, ropes are single force members. They can only transmit force in one direction, directly along their axis. The sides of a properly built bridge hammock will act in the same way as the dogbones. The hardware should direct all of that force onto the "face" of the end of the pole, and only a slight amount of shear force on the tip resulting from the differences of force between the dogbone and the hammock edge (AKA, if the pole were free to slide along the hammock body/dogbone, it would migrate to the middle of the hammock). The forces of the hammock edge and dogbone should be simply squeezing the pole. In fact, in this instance, a larger hole for the pole tip would allow the hardware piece to slant slightly, and THAT would result in a bending force of the pole tip.

That's where it falls apart... that assumption.
Hiking poles don't have proper bridge tips, which increases the odds you will not cleanly seat the entire surface of the face of the pole tips.
Or more accurately- no force will be on the 'spike', but will all rest cleanly upon the part of the tip that contacts the hardware face.

But back to my favorite tool the bottle opener.

If you set the beer bottle into a keyhole style opener... and tie a string to it and pull... there is compression force on the beer bottle, but the bottle cap will shear as the flat lever of the opener is pulled as that's the weaker item.
That said... the hole on the opener needs to be around the size of the beer cap... or it won't work right.

This is potentially something that has happened to CF poles. Especially if the dogbones were too short.

Remember this is a triangle. The rope/webbing is under tension, but the bar is changing that from inline (tugging a rope) to an angle (rope being pulled over an edge).

The tip of the trekking poles are good in compression, but even a little bending and they can bend easily... even while under compression.

If you were to increase the distance from your hole for the pole tip, and the hole for the dogbone... you could probably imagine the leverage being put by the dogbone onto your metal hardware.

Go the other way. Assume the pole tip was indestructible... single force or not, eventually the load from the dogbone would shear your hardware between the pole tip and the dogbone hole. The pole becomes the immovable force and the dogbone eventually exerts enough leverage on the hardware as that is the only piece preventing the force from returning to inline, rather than spread.

The change in direction is the key and what turns your hardware into a bottle opener.
Force is inline from webbing to hardware, then change of angle from hardware to apex is where the rotational force comes in. (moment if I recall engineer terms properly).
The hardware and all the forces want to 'collapse' so the full load is in it's natural state- inline.
It is only the 'square face' of the pole pushing on the hardware that keeps it from rolling to match the angle of the dogbone.

They do not maintain a perfect 90* angle here. So then the flat plate of hardware rolls, or rounds the corner of the triangle if you like. If the hole for the pole tip exactly matches the size of the pole tip, it begins to add pry the top of the ol beer bottle (shear).
If the hole for the pole tip is sloppy, the hardware can roll off 90* but the oversized hole won't begin to torque the tip until it reaches some critical angle... say 5, 10, or 15* out of square.
Basically the bottle opener is too big and can't quite grab the cap right... so you reduce the chance of 'popping the top'

Where this may have caused CF pole failure, is as that hardware rolls from perfectly 90* and equal distribution of force onto the pole shaft.. into a slightly off skew contact. Even a degree or two is enough to load one side of the pole shaft more than another. The poles are the weak points... the webbing, rope, and hardware the unbreakable items.

Hole tip too close to the diameter of the spike, break the tip (or bend the trekking pole).
Hole tip is loose, tip doesn't break, but the plate of the hardware loads the pole unevenly.
In AL poles... the base plate of .75" pole tips are like 1/8"+ thick... so that's a sufficient 'distribution member' to spread the load.

In CF poles... the tips have a thinner plate about 1/16" thick... possibly not enough to deflect an uneven load by the hardware and overloading one side of the 'column' and creating a scenario where one side is under compression and the far side is under tension. Carbon fiber doesn't like that from what I understand so it snaps under load.



I probably should just sketch something up...

[Just Bill]

A rough visual aide
6463AA42-DEF6-4E0D-8A2D-046C9DCC22D4.jpg

[MiteyF]

It seems you're really focusing on the force put on the bar and hardware from the dogbone. What is equally important is the force from the hammock, since you'll never have any load on the bar/hardware unless you're IN the hammock. This force can essentially be considered another "rope", as the tension is primarily held by the seams down the edge which stretch less than the fabric. Further, it will basically be another triangle, thanks to the majority of the weight being in the middle of the hammock, and the cat cut exacerbating the effect. The angle and "prying" force you have denoted in your drawing will essentially be replicated on the opposite side (although the angle and forces will be slightly different, but not substantially). Because of this, you won't have a huge moment (yup, right word!) about the edge of the pole nearest the dogbone, as it will be more or less negated by the opposite and not-quite-but-nearly equal moment on the hammock side. This is what forces the hardware "flat" against the end of the pole, and keeps the hardware from prying against the pole tip.

The problem with your bottle opener analogy is that the only force resisting the torque from the bottle opener (bridge hardware) is the bottle cap (pole tip), where in reality there's another force which will be nearly equal to that applied by the opener, and in essentially the opposite direction. It's more like trying to open a bottle with your left hand, but the harder you pry up with your left hand, you also press down on your left hand with your right. The forces on the hardware (and thus the pole tip) will always have a linear relationship, as it's due almost entirely (with a small change from your dogbone length) to your own body weight. More weight in the hammock will increase BOTH forces, and BOTH moments.

I see your paper drawing and raise you an even *more* ghetto MS Paint drawing, where the red in the left "image" is the spreader bar, with the dogbones to the right, and the hammock (with its exaggerated triangle shape) to the left.

Now, if you make the hole for the pole tip larger like you're suggesting, the small difference in forces will be more easily realized, as the hardware will tilt toward the dogbone (slightly higher force thanks to the angle you mentioned), creating a slight effect as you have illustrated. This will have the added effect of placing the force against the pole tip by the hardware further away from the face of the pole end, which will make the effect worse still. If the pole is a very tight fit, the hardware has no ability to rotate, and since the torque we're interested in is against the pole tip, and about the edge of the pole, you will have next to no force on the pole tip itself.

[hangnout]

The young man is referring to trekking poles.

The most common cause (location) of failure when using those is the pole tip. (the item added on the end of the pole itself)
.

Here is one way of reducing the failure point on a trekking pole tip. Pole tips are flexible by design and this cuts out the flex. I drilled the aluminum insert to form a pocket for the pole tip also.

20180317_113802.jpg20180317_113809.jpg20180317_113835.jpg

This all made from .625 tent poles and insert.

I used the Dutch bridge hardware but agree just larksheading to the webbing is best.

Been awhile since I made a bridge but just finished a new one .................

[WV]

... and hangnout has experience to back up his engineering. What do you think led him to make this improvement in the first place?

[hangnout]

... and hangnout has experience to back up his engineering. What do you think led him to make this improvement in the first place?

When was that? 8-9 years ago? Good memory! Yeah when your hiking pole goes flying 30-40 feet in the air you look for another option. Back then I used my lathe to make custom parts but the ones above are simpler and better. Experience= learning from mistakes

[Banjoman]

It seems you're really focusing on the force put on the bar and hardware from the dogbone. What is equally important is the force from the hammock, since you'll never have any load on the bar/hardware unless you're IN the hammock. This force can essentially be considered another "rope", as the tension is primarily held by the seams down the edge which stretch less than the fabric. Further, it will basically be another triangle, thanks to the majority of the weight being in the middle of the hammock, and the cat cut exacerbating the effect. The angle and "prying" force you have denoted in your drawing will essentially be replicated on the opposite side (although the angle and forces will be slightly different, but not substantially). Because of this, you won't have a huge moment (yup, right word!) about the edge of the pole nearest the dogbone, as it will be more or less negated by the opposite and not-quite-but-nearly equal moment on the hammock side. This is what forces the hardware "flat" against the end of the pole, and keeps the hardware from prying against the pole tip.

The problem with your bottle opener analogy is that the only force resisting the torque from the bottle opener (bridge hardware) is the bottle cap (pole tip), where in reality there's another force which will be nearly equal to that applied by the opener, and in essentially the opposite direction. It's more like trying to open a bottle with your left hand, but the harder you pry up with your left hand, you also press down on your left hand with your right. The forces on the hardware (and thus the pole tip) will always have a linear relationship, as it's due almost entirely (with a small change from your dogbone length) to your own body weight. More weight in the hammock will increase BOTH forces, and BOTH moments.

I see your paper drawing and raise you an even *more* ghetto MS Paint drawing, where the red in the left "image" is the spreader bar, with the dogbones to the right, and the hammock (with its exaggerated triangle shape) to the left.

Now, if you make the hole for the pole tip larger like you're suggesting, the small difference in forces will be more easily realized, as the hardware will tilt toward the dogbone (slightly higher force thanks to the angle you mentioned), creating a slight effect as you have illustrated. This will have the added effect of placing the force against the pole tip by the hardware further away from the face of the pole end, which will make the effect worse still. If the pole is a very tight fit, the hardware has no ability to rotate, and since the torque we're interested in is against the pole tip, and about the edge of the pole, you will have next to no force on the pole tip itself.

MiteyF, you might be right. For me, I am not convinced (yet) that the angles of force on either side of the pole tip are similar enough to not cause a decent amount of torque.

Another issue could be the distances from the pole tip to the points of attachment where the hardware connects to the dogbone and hammock. If these distances are not equal, then the longer distance will have greater leverage.

Sent from my Moto Z (2) using Tapatalk

[Just Bill]

Bridge LC.jpg

MiteyF... you are of course correct. But if we are going to stick with my quasi engineering talk (I'm a carpenter with a HS diploma) then let me raise you one more...

Load Case 1- is what you accurately and correctly describe. You also accurately and beautifully drew WV's People's Bridge Hammock
Though I could niggle over the details of exactly what angle the hammock side of your pole tip diagram actually has... close enough. Even if not at an exact equal angle on each side, the forces balance and the pole contacts the hardware squarely. Load Case 1 is basically ideal... and why folks love bridge hammocks. Once you are 'in' a bridge distributes your load quite efficiently and smoothly back through the system. It is a very stable, self leveling, self-balancing structure.

Unfortunately this load case only occurs once the occupant is inside and happy. It's the old 'in and out' where all the trouble in life comes from.

Load Case 2 is not as extreme as one might think... basically- full load of occupant on one edge at the middle of the bridge body arc.
Now to be fair... the poles also rotate as shown in the diagram. But there are portions of this load case where the pole has not quite caught up to the occupant.
So for a period of time the pole tip and hardware interact in the manner shown... with the load actually coming from the opposite direction (outside the edge of the bridge beyond the pole tips)
Basically- the outer edge of the bridge turns into a long crescent shape rather than a simple rectangle with triangles on each end.

As to Banjoman's point- See load case 3.
While most hardware is not this unbalanced (x and 2x) I exaggerated it to demonstrate the point he was making.

One thing to consider as well... we are sketching these load cases in 2 dimensions.
Bridges operate in 3 dimensions if speaking geometry and engineering... though I hear physicists are up to 10 dimensions or so.
And of course spiritualists or 'Hammock Yogi' will speak of the fourth dimension known as Nirvana when describing the depth of their hammock pleasures beyond the mere mortal plane of existence. While many claim bridges excel in this dimension, I'm not sure that is relevant here.

Point being- if you introduced a few planes and dimensions you'd find that the dogbone triangle is on one plane, but the occupant and area of the bridges forms a pretty complicated 3d shape that changes quite a bit.
So besides 'turning the corner' from bridge body to Apex, you're also turning a corner from the relatively flat (level to ground) plane of the bridge area to the roughly 45* plane the suspension triangle sits on. This can introduce another direction of moment for the hardware to handle... and ultimately imposed on the bridge pole tip.

And if Grizz was here it would be time for bowling balls, greek letters and calcumalations galore.
I'm not quite smart enough for that so fer a minor real world demonstration of load case 2...
If you skip past the fat dancing tablecloth walking around to the end of the video... you'll see said feller standing on the edge demonstrating this in real life.

[Just Bill]

Here is one way of reducing the failure point on a trekking pole tip. Pole tips are flexible by design and this cuts out the flex. I drilled the aluminum insert to form a pocket for the pole tip also.

20180317_113802.jpg20180317_113809.jpg20180317_113835.jpg

This all made from .625 tent poles and insert.

I used the Dutch bridge hardware but agree just larksheading to the webbing is best.

Been awhile since I made a bridge but just finished a new one .................

I'm not a trekking pole guy, but I've been working with Josh at Ruta Locura who does something similar with his CF trekking poles.
He's got a near perfect fit on his... it take a bit of pressure but the piece he has basically clicks on like it was made to fit the pole tip... which it basically is. Super solid.

More an more I'm convinced that this is the best solution to resolve or even completely eliminate this issue.

So if the original idea was your's... Trekking pole users owe you a huge thanks!

On the other side... my contribution to our collaboration on this pole set we are working on...

His solution was to pop a pole tip into his trekking pole shaft. Basically you have access to the tube itself at the handle. Not bad with a hardware based bridge, but with a recessed bar... the foam handle can more easily dislodge the pole tip as it gets in the way.

So I have him making up a shaft the size of the main tube with a ferrule... basically turns the handle side into a bridge pole (and increases the tip size used too) while keeping the handle about 6" away from the suspension.
This has the added advantage of allowing the pole to be completely collapsed so you are in full compression and not relying on the adjustment mechanism or an internal dowel. (The ferrule is the dowel basically).

I'll have to get some pics at somepoint, but we're on round three of them and initial testing. So didn't want to share until I knew it was holding up.
9.75 ounces for the pair including the adapters for the bridge.