Does wrapping a tree strap twice equal using a wider strap?

[gmcttr]

How many times have you heard it said that "if you wrap a (pick a width) 1/2" strap around a tree twice, it is the same a using a 1" strap"?

I have heard this repeated on HF many times and I do not believe it to be true. If the strap is wrapped around the tree twice, the pounds per square inch imparted to the tree from the center back forward are exactly the same as if it was passed around the tree once except there are now girdling forces added to the mix.

With a 200 lb load and the strap around the tree once, there is 100 lbs on each leg from the attachment point to center back of the tree imparting 'X' PSI.

IMG_5113 (Medium)noted.jpg

With the same 200 lb load and the strap wrapped around the tree twice, there is still 100 lbs on each leg from the attachment point to center back of the tree imparting 'X' PSI.

IMG_5110 (Medium) noted.jpg

The only way to spread the load is to add straps. For example, if you used a continuous loop or two straps wrapped once around the tree, the forces on each leg would be cut in half and they would impart 'X/2' PSI the same as a strap twice the width.

IMG_5103 (Medium) noted.jpg

Discuss......

[pgibson]

Well put!

And I like those black straps with Orange stitching, Very sharp.

[DrPhun]

You are confusing the tension in the strap, which runs parallel to the bark of the tree inline with the strap, with the force that is perpendicular with the strap, which crushes the bark. That perpendicular force is constant, related to the weight in the hammock and the hanging angle, and it is divided across the surface area of the strap, so more strap is the same force spread across more area.

A couple other ways to think about it
If more wraps, don't divide the girdling force but instead multiply it, then if you do enough wraps the tree should be crushed like a paper cup. (Get up to around 2000 lbs/sq inch and you can crush concrete, so it should go right through wood.) If you didn't use straps and instead used amsteel, due to the smaller surface area, would cut through the tree like butter with not too many wraps i.e. 7/64" with 100lbs tension would be over 800 lbs/sq in with just the first wrap by that math.

Another way to think about it is what is the difference between one 2" strap and two 1" straps? What if you sew the edges of the 1" straps together to make it a 2" strap? That shouldn't change the force on the tree.

Think of a whipped end hammock. Why wrap the suspension multiple times around the fabric if not to keep the cloth from tearing? If multiple turns around the tree don't reduce the compression on the tree, the same would go for the hammock and one turn would be enough, since extra turns would've help.

[Gresh]

The math's solid, but it doesn't change the fact that the jury is out on whether or not it's actually an issue in the short term.

[DrPhun]

Looking at your picture 1, consider that, aside from the impact of friction, the tension in the black strap has to be consistent throughout its length. The tension is your 100 or 200lbs you show. The compressive force on the bark of the tree varies, though. It is at its greatest on the right side of the tree and becomes less and less as you move to the left, until the strap doesn't even want to touch the tree anymore. On the right side it would be difficult to slide your finger between the strap and the tree wile on the left close to where the strap leaves the tree it would be easy.

The average actual compression on the tree is the 200 lbs divided by the total area of strap touching the tree, so more strap touching the tree is less avg psi. Because there is more area touching the tree, multiple wraps are actually better for the tree than multiple straps. Picture 2 is better than picture 3.

[gmcttr]

You are confusing the tension in the strap, which runs parallel to the bark of the tree inline with the strap, with the force that is perpendicular with the strap, which crushes the bark. That perpendicular force is constant, related to the weight in the hammock and the hanging angle, and it is divided across the surface area of the strap, so more strap is the same force spread across more area.

No, I'm not confusing the tension in the strap with the perpendicular force on the tree or with the PSI the strap is imparting on the tree.

A couple other ways to think about it
If more wraps, don't divide the girdling force but instead multiply it, then if you do enough wraps the tree should be crushed like a paper cup. (Get up to around 2000 lbs/sq inch and you can crush concrete, so it should go right through wood.) If you didn't use straps and instead used amsteel, due to the smaller surface area, would cut through the tree like butter with not too many wraps i.e. 7/64" with 100lbs tension would be over 800 lbs/sq in with just the first wrap by that math.

I mentioned girdling forces but I didn't make any reference to the effect additional wraps would have on it. Those forces have no effect on the PSI imparted from the first half wrap of each leg.

Another way to think about it is what is the difference between one 2" strap and two 1" straps? What if you sew the edges of the 1" straps together to make it a 2" strap? That shouldn't change the force on the tree.

Correct, it would not change the perpendicular force on the tree in the direction of the hammock. However, it would reduce the PSI imparted to the tree by 50% which is the topic of discussion. In this case, you seem to be confusing perpendicular force and PSI.

Think of a whipped end hammock. Why wrap the suspension multiple times around the fabric if not to keep the cloth from tearing? If multiple turns around the tree don't reduce the compression on the tree, the same would go for the hammock and one turn would be enough, since extra turns would've help.

The suspension has no need to be wrapped more than once either around the whipped end or if run through a channel. If the fabric were going to tear, it would do so where it exits the wrap, not in the middle of it.

Looking at your picture 1, consider that, aside from the impact of friction, the tension in the black strap has to be consistent throughout its length. The tension is your 100 or 200lbs you show. The compressive force on the bark of the tree varies, though. It is at its greatest on the right side of the tree and becomes less and less as you move to the left, until the strap doesn't even want to touch the tree anymore. On the right side it would be difficult to slide your finger between the strap and the tree wile on the left close to where the strap leaves the tree it would be easy.

This is correct and my point is that the compressive force on the bark (PSI) would be exactly the same in picture 2 at any given point as in picture 1.....almost nothing where the legs of the strap first touch the tree and steadily increasing to the backside center (ignoring friction). This does not change simply because the strap continues around the tree in one case and not the other, just as the perpendicular force on the tree toward the hammock does not change in either case.

The average actual compression on the tree is the 200 lbs divided by the total area of strap touching the tree, so more strap touching the tree is less avg psi. Because there is more area touching the tree, multiple wraps are actually better for the tree than multiple straps. Picture 2 is better than picture 3.

I do not agree with your premise in this case. Things are not always as they seem.

For example, in the following sketch, how much downward force is being imparted to the suspending eye?

Basil Anchor Forces Question.jpg

[DuctTape]

While one can calculate the force on the outside of a trees bark, the pressure exerted on the cambium layer has yet to be calculated for any tree species. Not to mention the wide variety of species, some of which the outer bark is so thick that it defies belief that any considerable pressure is exerted on the cambium layer. Finally, the short term length of one sleeping in the hammock even with 100% force on the cambium layer has yet to be shown to have any long term effect of the health of the tree. All that said, I use the same 1-in straps I have always used not to protect the bark of tbe tree, but for the friction the surface area provides to minimize slippage.

[infamous1]

So what if you tied the 3/4 strap back to itself like two loops and tied to both? Wouldn't that split the load in half effectively?

[gmcttr]

While one can calculate the force on the outside of a trees bark, the pressure exerted on the cambium layer has yet to be calculated for any tree species. Not to mention the wide variety of species, some of which the outer bark is so thick that it defies belief that any considerable pressure is exerted on the cambium layer. Finally, the short term length of one sleeping in the hammock even with 100% force on the cambium layer has yet to be shown to have any long term effect of the health of the tree....

Agreed and I doubt any studies will ever be done. It would be expensive and take a few decades for something only of concern to a small market. Nope...no funding for that.

Thank goodness our topic is whether wrapping the strap twice makes a difference in PSI to the backside of the tree and not if or how much it takes to damage the cambium. Hmmmm.......I better edit the title of this thread.

So what if you tied the 3/4 strap back to itself like two loops and tied to both? Wouldn't that split the load in half effectively?

If I'm understanding you correctly, yes by my understanding. Something like this...connect end of strap to biner, wrap around tree, back through the biner, wrap the tree again and connect to the biner....

IMG_5119 copy pe (Medium).jpg

[DanglingModifier]

IMO, the easiest way to make a case for multiple wraps is not to fumble with our rusty physics or math skills and expect to convince other equally rusty people, but to use some hypothetical real world situations that are easier to wrap our heads around.

We are focusing on two wraps versus one, but let's briefly consider a larger number of wraps to really help illuminate the situation.

Consider the following two scenarios where great perpendicular compressive force is being applied to a small area of an organism:

• Lift a 53lb kettlebell up by a 1" tree strap that wraps around your forearm ONCE. The strap is very, very uncomfortable. It cuts in to your arm pretty hard over the whole length that is in contact with your arm.
• Lift a 53lb kettlebell up by a 1" tree strap that is wrapped around your forearm multiple times. The strap will still be uncomfortable, but not "very, very uncomfortable" like it was before. It no longer feels like it is actually cutting into your arm. Where the strap begins its wrap, the "entry" and "exit" points of the strap that first touch your arm is still considerably more uncomfortable than in the middle of the wraps.

I just did this, and it was enough to convince me that multiple wraps does reduce the pressure of the straps against soft tissue. More wraps around my arm would definitely reduce or delay the compressive damage caused by the straps and weight versus having only a single wrap.

My conclusion is that additional wraps helps prevent damage to the tree, but I'm not sure by how large a margin. I do not think two 1/2" wraps directly equals one 1" wrap.

Keep in mind just because the average PSI might be less with more wraps doesn't mean that PSI is evenly distributed. Based on the kettlebell experiment the entry and exit points felt like they still took a disproportionate share of the load. That's where the discomfort was still focused when I tried it with more wraps. For that reason I wouldn't dare to guess how much extra protection a second wrap provides. I'm pretty confident it's going to be more than one wrap, but I'm not sure by how much.

If you don't like this example consider if I had tried it with dental floss or fishing line. Few would argue that a single loop of fishing line attached to a 53lb weight might actually garrotte my arm down to the bone. Lots and lots and lots of loops of fishing line would likely still lead to an injury, but it's harder to imagine they might actually need to reattach my hand if I tried it.

Thinking about it this way should make you realize it's probably something you've personally experienced. Have you ever tried to pull hard on a thin string? It hurts! So you wind up wrapping more loops around your finger before you try pulling again. Guess what? It still hurts! But it hurts less. More wraps spreads that force out a bit more across your finger meat so it doesn't hurt or dig in quite as much. Ditto for the tree with the straps.

Sure, bark is more hearty than my arm meat, and the thin cambium layer of a tree doesn't directly compare to a human arm. It may not be apples to apples, but it doesn't need to be.

Let me know if you think I'm making any mistakes in my assumptions or conclusions, and try out a similar experiment for yourself if you think I'm off base.

The finger pulling string analogy makes me realize part of what lessens the pain in that scenario is the ability to layer the string over itself. String-on-string where the entry/exit string is concerned really cuts down on how much the leading string cuts into your fingers when it is loaded. For that reason I'm inclined to think if you layered the leading strap over other layers of lower strap you might provide considerably more protection to the tree overall.