Rope Wrench feedback

[Ewan Murray]

Still don't get it, I never was much cop at applied mathematics and engineering....!

 

What I can't fathom is how one end of the rope being attached back to the climber makes any difference to the force applied to a limb when compared to one end of the rope attached to a static point.... I'm having a thick day!

 

Ref the tie in point, if a limb is suspended by natural rigging with a rope thrown over a branch, a small amount of friction is generated to aid controlled lowering. If the piece you're lowering is heavier, you take a couple of turns round it to reduce the amount of force needed in the tail of the rope to lower it, same principal as a capstan. So the force on the tail (or theoretical base anchor) is reduced, but the force on the limb you're rigging from remains the same.

 

So while i understand the reasoning, i can't understand the maths which mean a 100kg suspended on a loop from a point exerts 50kg down each leg, 1kn in total, but if you unclip it from itself and fix it back to a static point all of a sudden it generates 2kn of force on the limb...? (Have I got the units right there?)

 

This is why I use structural engineers when I'm designing a building!!

 

Imagine if you hung on one side of the rope weighing 100kg, what would take to stop you falling to the ground? another 100kg, so when its over the limb in base anchor set up in order to hold you 100 kg up the same force has to be exerted on the other side, resulting in doubling the force on you anchor point. If you choked your line e.g. running bowline that is only 1x your force as its is a straight pull down on the rope no other opposing force is required in order to keep you in place, the same princible is applied to Ddrt but the main difference is that the load is shared between two legs of line resulting in the load being split at your splice and friction cord.

 

In reference to your comment about friction in terms of lowering you are reffering to dynamic forces which will increase in some rigging situations i dont believe its a fair comparison compared to anchoring an srt system as that is static, but regardless, By adding wraps you are not changing the force applied to the limb the force is always there all you are doing is decelerating the limb as it falls the limb still weighs the same and is still applying the same force to the limb. Imagine if you were being lowered out on a belayable system, when lowering you down your weight its going to stay the same all that will change depending on the size of the limb is the friction required to slow you down.

[WorcsWuss]

By adding wraps you are not changing the force applied to the limb the force is always there all you are doing is decelerating the limb as it falls the limb still weighs the same and is still applying the same force to the limb. Imagine if you were being lowered out on a belayable system, when lowering you down your weight its going to stay the same all that will change depending on the size of the limb is the friction required to slow you down.

 

This is where I become confused, because if we're talking force on the end of the line, which is where a base anchor comes in, adding wraps round the tip reduces the force which must be exerted on the tail of the rope to support 100kg of weight by using friction, but the 100kg is still exerting 1kn of force on the anchor point......

So you could have 100kg suspended, but by taking 3 or 4 wraps round the limb, only have to exert 0.25kn on the tail of the rope to suspend it.

So, are you exerting 1kn, 2kn or 1.25kn of force on the anchor....?

[Ewan Murray]

The rope doesnt run in the system if you wrapped it around one branch 10 times you still going to have the same force applied to that point, if it was spread over 10 different branches the force would be dissipated over those anchor points, Adding wraps in a rigging situation doesnt change the weight of the limb it makes it easier for you to handle, In order to stop the limb from dropping you need to hold the line statically, yes the friction helps you but it doesnt change the weight of the limb if the line was no longer held static no matter what the wraps the limb will drop. The force will stay the same regardless of how many times you wrap it around the limb, all it changes is how easy it is to be held at the base of the tree the overall force at the top will still be double that of the climbers weight.

[WorcsWuss]

So the force applied to the tail makes no difference.

Which is where I get lost again..... If that follows then the force applied to the anchor should be double the climber's weight no matter where he's anchored.....

Hold on, is that where dynamic forces come in?

To just hang there ddrt it's 100kg. To start hauling yourself up ddrt you need to exert 100kg of force on the tail to lift the 100kg dead weight on the standing line which is the climber.

So when hanging ddrt the anchor is subject to 1kn of force, but when climbing it's subject to 2kn of force.... And a base anchor always exerts 2kn of force because the load is never split between the same point......

 

I'm going to just switch the lights off and lie here in the dark for a few minutes......

[Ewan Murray]

You are getting mixed up with Dynamic and static forces, In double rope you are hanging in the sytem with x1 at the anchor point you have mechanical advantage so your only pulling half your weight up with still only x1 at the anchor point, ignore double rope for now and rigging, if your on a base anchor its your weight either side to hold you up resulting in x2 at the anchor point if you have wraps around a capstan and are holding a it as the rope exits it will be easier to hold but that doesnt mean the force isnt there, imagine grabbing the single line going into the capstan from above its still going to be the same weight, the friction doesnt negate the force at the anchor point or the opposing leg. If you base anchored climbed up and went through 5 re directs they would be sharing the load but it would still be double and the load on the anchored side of the line would still be the same as the force you have applied to the line.

[WorcsWuss]

Ok, so here's what confuses me....

 

This bit I understand....

[WorcsWuss]

But this bit I don't....

 

Surely if you use friction around the tip to reduce the force needed on the tail to support it, then you reduce the overall force on the tip as well.....?

Ultimately (theoretically because otherwise it would defeat the object of base anchoring) the friction could be increased to such a point that no force is required to prevent the rope running back..... And you'd have the same force on the tip as a standard choked single line....?

 

I guess what I'm driving at is there are degrees of force between top anchor and base anchor....?

[Dan Curtis]

But this bit I don't....

 

Surely if you use friction around the tip to reduce the force needed on the tail to support it, then you reduce the overall force on the tip as well.....?

Ultimately (theoretically because otherwise it would defeat the object of base anchoring) the friction could be increased to such a point that no force is required to prevent the rope running back..... And you'd have the same force on the tip as a standard choked single line....?

 

I guess what I'm driving at is there are degrees of force between top anchor and base anchor....?

 

Yes mate, just a quick one...I'm at work:D

 

http://vtio.org.au/Content/wp-content/uploads/2010/07/Working-the-Angles-i.pdf

[alex_m]

That's a good little guide, Dan. Cheers.

[Dan Curtis]

That's a good little guide, Dan. Cheers.

 

Check out the other downloads on there, full of info