Load sharing SRT TIP for double stem

Jan_ said:
Just wanted to say that the maximum theoretical force pulling the stems together is 4x your weight, but only if you ignore the substantial friction and assume you are climbing at the worst angle to the TIP and create a 2:1 MA. All of this wont happen though, so there should really not be much force there at all.
Click to expand...

I don't follow how it is only 4x.

If you string a hammock between two trees tight enough that a 10' line sags 1" - all non-idealities aside (friction, rope weight, rope stretch, etc.) - then my 200lbs of body weight is applying 6 tons of weight laterally to each tree trunk.

Someone did an experiment with some load cells and posted it to YouTube (it was in a cinderblock room with anchors in the walls, can't find the video) where they were tensioning a horizontal line up and then applying weight to the center of the line to see how much force the anchor points experienced. It was much less then what you'd think, due to non-idealities that the back-of-the-napkin stuff doesn't account for, but you can still put some serious multiplicative loading on anchor points with this arrangement.
 
LordFarkwad said:
I don't follow how it is only 4x.

If you string a hammock between two trees tight enough that a 10' line sags 1" - all non-idealities aside (friction, rope weight, rope stretch, etc.) - then my 200lbs of body weight is applying 6 tons of weight laterally to each tree trunk.

Someone did an experiment with some load cells and posted it to YouTube (it was in a cinderblock room with anchors in the walls, can't find the video) where they were tensioning a horizontal line up and then applying weight to the center of the line to see how much force the anchor points experienced. It was much less then what you'd think, due to non-idealities that the back-of-the-napkin stuff doesn't account for, but you can still put some serious multiplicative loading on anchor points with this arrangement.
Click to expand...
I do not believe 6 tons. However, assuming you are using a rope with some stretch, as all ropes have, it would have to be very tight in the first place to only sag 1 inch with a 200 lb load.
 
RBJtree said:
I do not believe 6 tons.
Click to expand...

You are correct, and neither do I. It won't be anywhere near that in reality, due to things that simple little trig calcs don't account for. But, it will be more than something like a simple 2:1 or whatever would apply, simply due to it being a different arrangement of angles and forces.

It's more like a branch that deflects your basal tie by 10 degrees versus 180 degrees; it barely has any load on it at all, right? Now imagine that the force the branch experiences in that scenario is your weight - how much line tension would it require along the length of rope to apply your body weight's worth of force to a branch that barely deflects the rope? A. Lot.
 
LordFarkwad said:
... If you string a hammock between two trees tight enough that a 10' line sags 1" - all non-idealities aside (friction, rope weight, rope stretch, etc.) - then my 200lbs of body weight is applying 6 tons of weight laterally to each tree trunk...
Click to expand...

Do you know how much force it would take to get a climbing line that tight? Not gonna happen so why go there.

Co-doms have increased risk of failure in an outward movement not an inward movement.
 
DSMc said:
Do you know how much force it would take to get a climbing line that tight? Not gonna happen so why go there.

Co-doms have increased risk of failure in an outward movement not an inward movement.
Click to expand...

Ah man, good info. Did not know that. Thank you.

Went there just to try and illuminate that there's potentially much more force being applied in a situation like that, than would be from a simple 180 degree line direction change. No one is going to be applying that much weight in the real world because of real rope properties, but as that experiment showed, the principle still applies. I was being cautious; didn't want to have my climb line cinched straight across the gap between those two stems and be applying much more weight than I would be with something like a Y type of arrangement that allowed sag at the point my weight was being applied. But I also didn't know that about the common failure mode of a codom, as you just pointed out.
 
But it's all moot anyway, because y'all have given me ideas as to how to avoid bridging those two stems with a cinching PSP, if I ever end up with a situation where I feel like having the support of both stems.

Kinda want to try cinching on the left stem and wrapping the other with a bight and biner back to the center, as someone above suggested.
 
I dont belive this can be compared to a hammock since you are cinching the rope with your weight, not tensioning it in the middle. This would only be the case if your biner was in the middle between the two stems, avoid that and you should be fine.
 
The example I gave was in a silver maple, way spread out. Advancing was out of the question as both sides of the fork where under 3". This situation is more of a maximum height anchor in weak branches.
If the spread is not too great, rbj made the most practical suggestion. Rope in one side, lanyard in the other.
 
Hmm
Jan_ said:
I dont belive this can be compared to a hammock since you are cinching the rope with your weight, not tensioning it in the middle. This would only be the case if your biner was in the middle between the two stems, avoid that and you should be fine.
Click to expand...

Hmmm, I'ma have to think about this.
 
Jan_ said:
I dont belive this can be compared to a hammock since you are cinching the rope with your weight, not tensioning it in the middle. This would only be the case if your biner was in the middle between the two stems, avoid that and you should be fine.
Click to expand...
You're right, however if the angle in the middle is pretty low (lots of sag) there is nothing to worry about. If I had a second piece of rope with me in that tree I would cinch both ends of that rope and clip into the middle, creating an equalizing anchor.
 
climbstihl said:
You're right, however if the angle in the middle is pretty low (lots of sag) there is nothing to worry about. If I had a second piece of rope with me in that tree I would cinch both ends of that rope and clip into the middle, creating an equalizing anchor.
Click to expand...

I agree...multi point, equalized anchor is definitely the theoretical way to go. Practically, I am thinking it probably wouldn't go down that way unless I felt it was absolutely necessary
 
DSMc said:
An M rig is not a good choice in any situation that has a weakness, and potentially have one of the tip fail. With so much rope in the system, one side failing could be a major problem. It would be better to just use two separate systems so there is redundancy in the support.
Click to expand...
Hadn't thought about that, makes complete sense though. The tree is pretty low, and I need to do some far limbwalks on it, so I would definitely hit the ground if one side failed, although there are lots of tall bushes to catch me.
 
Could set a u saver manually to capture both stems. I can think of a way to set this in srt mode and have it retrievable, but I’m not feeling very articulate at the moment. Watch some of old farts videos and you’ll come up with a way.
I’ve used spare lanyards and various rings for floating anchors between co-dom stems. Works a treat!
 
Jan_ said:
I dont belive this can be compared to a hammock since you are cinching the rope with your weight, not tensioning it in the middle. This would only be the case if your biner was in the middle between the two stems, avoid that and you should be fine.
Click to expand...

Ah man, I totally forgot to mention, I did have the cinching element - in this case, a quickie - in the middle, between the stems. So it was indeed more like a hammock. But, if I understand you correctly, I could lessen stresses by placing the quickie against one of the stems. True, man.
 
.,
Jan_ said:
I dont belive this can be compared to a hammock since you are cinching the rope with your weight, not tensioning it in the middle. This would only be the case if your biner was in the middle between the two stems, avoid that and you should be fine.
Click to expand...

But keep concept in mind for speedlining...
 
You must log in or register to reply here.

New threads New posts

Back
Top Bottom