Anchor forces from lowering
The illustrations in this article (shared with permission) come from the excellent website RopeLab, run by Australian rigging expert Richard Delaney. RopeLab has a ton of great material for anyone who wants to dive into ropes, rigging, and mechanical advantage, check it out! There's a fair amount of quality free information, but getting an annual subscription unlocks the entire website. You can also connect with Richard on Instagram and his YouTube channel, where he has loads of concise, informative videos.
These diagrams come from a RopeLab online mechanical advantage quiz, which you can find here.
You’re top rope climbing with your partner. They’re climbing, and you’re on the ground belaying. The rope goes from your harness, up to the anchor master point, and then back down to your partner.
In this common configuration, the load on the anchor can change depending on:
who is holding the rope
if the climber is resting and not being lowered
if the climber is being lowered, through typically a carabiner or two
For example:
Your partner finishes the climb, and calls for take and lower. You lock off your belay device, they lean back, weighting the rope, and you lower them to the ground.
What’s the force on the anchor when you're lowering them? Many people think it has to be twice the weight of the climber. Say the climber weighs 100 kg. You, the belayer, need to counterbalance that force with 100 kg of your own. So that means a 200 kg load on the anchor, right?
Well, turns out it's not quite that simple. Let's look at a few examples to see how this works!
Question 1 - A person (weight 1 kN) is holding their own weight, on a rope that goes through a 90% efficient pulley on an overhead anchor. What is the theoretical force on Anchor A?
IMAGE: ROPELAB.COM.AU
Answer: 1.0 kN
If you’re statically holding your own weight like the diagram, the anchor sees the force of your body weight. It doesn't matter if it goes through a 10% efficient pulley or 50% efficient carabiner, the force on the anchor is going to be the same.
(To add another interesting variable to this, if there's any sort of ledge or friction between you and the anchor, that will further reduce load on the anchor. But for now let's assume the climber is free hanging.)
Question 2 - A climber (weight 1 kN) is LOWERED with the rope running through a 90% efficient pulley. What’s the theoretical (including friction) force on Anchor B?
IMAGE: ROPELAB.COM.AU
Answer: 1.9 kN
The anchor sees the force of the climber, plus the force of the belayer to hold the rope, minus the 10% friction at the pulley. So, the force on the anchor is not two times the weight of the climber. (Yes, top roping through a pulley doesn’t happen in climbing very often, and it can actually be a bad idea if your belayer weighs much less than you do, but we’re using it here as an example.)
Question 3 - A climber (weight 1 kN) is LOWERED with the rope running through a 50% efficient carabiner. What’s the theoretical (including friction) force on Anchor C?
IMAGE: ROPELAB.COM.AU
Answer: 1.5 kN
When lowering, the anchor sees the force of the climber, plus the force of the belayer to hold the rope, MINUS the 50% friction at the carabiner. The friction from the carabiner significantly reduces the load on the anchor. This is why a belayer who’s a lot lighter than their partner (usually) doesn’t get lifted off the ground when they lower their partner from a toprope.
Here's another way to think about it:
When you’re raising something, friction can be your enemy.
When you're lowering something, friction can be your friend.
Here, friction at the anchor reduces the load on the anchor AND transfers less weight to the belayer for them to manage. That’s a good thing!
What are some practical uses for this in the real climbing world?
Rappelling, and lowering yourself, put the same load onto the anchor. If you top out on a 1 pitch climb and find yourself at an anchor you think is sketchy, rappelling might be less risky than being lowered.
Being lowered by your partner will always put more force on the anchor then rappelling or lowering yourself.
Any additional friction in the system, such as the rope running over rocks or ledges, will further decrease force on the anchor.
Top roping through a pulley increases load on the anchor, and can make it more difficult to catch a fall because of the reduced fraction. Don't do this.
It's easy to test systems like this yourself. Just get a barbell plate of a standard amount (a round number like 10 lbs/kg helps) and an inexpensive digital scale like the one below. This scale is about $10.
