Ascending with I'D

Hi all,

I've seen a couple of sources online claim that this method of ascending gives a 2:1 (or even a 3:1) advantage. I've made a little drawing below.

Ascending.

I'd like someone to correct me if I'm wrong. I don't believe this gives any advantage whatsoever because the pulley and ascender are attached together on the rope. In other words, standing up puts the full weight of the person on the ascender. It's still a good setup...I just don't see a mechanical advantage.

Thoughts? Did I miss something?

Yes, you missed something. Think about the scenario where the upper pulley was directly attached to an upper anchor. That is clearly a 3:1 (if the I'D works as an efficient pulley, but at worst 2:1 if it doesn't).

Now, consider that what you have done pictured above with the ascender is effectively the exact same thing. The upper pulley is connected directly, albeit via a rope, to an upper anchor.

Edit: The upper pulley is attached to a solid anchor, through an ascender solidly attached to a rope which is solidly attached to the upper anchor.

Edit edit: I think I better understand your confusion. You are pulling down on the free end (far left in your drawing) when you are ascend in this setup if you aren't using a foot loop. THAT generates the 3:1 advantage, though it isn't terribly efficient for your muscles. You aren't standing on the ascender when you are ascending this way. You aren't even pulling on it. You are just moving it up between strokes.

Step 1: Pull down on free end (left in your drawing), which in turn pulls you up the rope.
Step 2: Repeat from step 1 until your personal attachment point is very close to the upper pulley.
Step 3: Push the ascender up the rope.
Step 4: Repeat from step 1 until you are at the top anchor, or wherever you are going.

However, it's much easier and more efficient to add the foot loop attached to the ascender as I believe you are thinking. That turns this setup into a 1:1, but far more efficient for your muscles. Your legs are much better at pushing than your arms are at pulling.

Ahhh yes, that was my confusion. The foot loop is necessary to make it a 1:1. Thanks!!!

I'm relatively new to rappelling in general. Is this a fairly common setup for ascent/descent?

I've been using this set up since day one. If the wall is less than vertical, I may use a different set up.

grigri jug

For every unit length W (climber) moves up, 3 units of rope are pulled out relative to the position of W, thus a 3:1.

2:1 if a buddy pulls 3:1 if you pull yourself

batguano, why the difference based on who pulls?

If you are at w pulling on the rope, you move up as you pull, thus for every unit you go up three units of rope go past you. If someone else pulls, they are not moving, and it's only 2:1. The last pulley is only a redirect in that case and does not increase the advantage.

Jeremy Riesberg wrote:I've been using this set up since day one. If the wall is less than vertical, I may use a different set up.

+1

that is THE setup to use.

Agree with Larry - it is only a 2:1 pulling up the W.

An easy way to determine the mechanical advantage is what is called the "T-method." This is done by tracing a force through the system. The forces must balance through the system.

Start at the arrow on the left and "pull" with a force of one. Trace this force up and through the pulley so the force on the rope coming out of the pulley must also be 1. So there is a force of 1 on each side of the pulley, which means there is a force of 2 at the dotted line going to the ascender.

Trace the force of 1 down to the pulley attached to W, and out the other side going to the ascender. This must also be 1. So the pulley attached to W has a force of one acting on each side of the pulley, giving a total force of 2 pulling on W (hence the 2:1).

If you trace the force of 1 up to the ascender, it shows that there is a force of 3 right above the ascender - 2 from the dotted line plus 1 from the rope coming up from W.

If you label the diagram with 1's tracing the force through it will be easier to see.

Note that the real advantage is less than this due to friction in the system.

The T-method works for any pulley system.

T method mechanical advantage

Such setups aren't really 2 or 3 to ones. The idea is that you still drive from the foot, but the 2 to 1 of the grigri helps to raise your upper body a little.

Great on steep stuff after a long day.

Great stuff. Thanks.

This is a 2:1 system. Its really weird to use an ID, and Grigri is way lighter... The lower pully (The Petzl ID) gives the 2:1 mechanical advantage. The upper pully (attached to the ascender) acts only as a redirect, which is convenient so that the climber can pull the rope down instead of up. If the system has only the jumar and the ID, for each 2 feet of rope pulled upward, the climber rises 1 foot, when the upper pully is added, there is no additional mechanical advantage.

It is a 3:1 system since the pull is coming from the weight, and not from the outside world.

The losses are quite high due to the ~50% grigri efficiency and ~70% pulley efficiency, but in the big picture your foot is doing most of the work, so neither the fact that it is 3:1 or that it has lousy efficiency matter much. I find little difference in effort using a biner to redirect versus a small pulley simply because my arm is doing only ~10% of the work.