Why do we tie in through two loops of a climbing harness, but belay through only one loop?
This is a great question, and one we are asked almost every time we teach beginner climbing classes. We see other climbers setting their belays up incorrecty so often that it's clear more people should be asking this question, and finding the answer.
First off, let's take a look at how a belay device should be set up:
The belay carabiner is connected to the belay loop of the harness, and the rope runs down from the climber, through the belay device, and exits down to the brake hand. The gate of the carabiner is on the opposite side of the brake hand. It's textbook.
And let's look at the correct way to tie in to a climbing rope:
The rope runs through two loops of the harness. The upper loop, called the isolated tie in point, keeps the knot in front of the harness in the case of a sideways fall, and the lower loop, called a rope locator loop, lifts the leg loops when you fall, putting the climber into a seated position, which is a safer position to fall in.
So the question is why do we belay from only one loop and tie in to two, and why don't we set up our belay like this:
The answer is pretty simple, really. If you've taken an Outdoor Adventure Club Gear & Anchor Class, you know that carabiners are weak when they are pulled in three different directions, which we call tri-axial loading. Setting up your belay device in this manner will load the carabiner in three directions when the climber falls, creating a tri-axial load.
Additionally, the belay loop of the harness is typically the strongest part of the harness. The belay loop is typically rated to 22kN (about 5000 pounds of force), while the rest of the harness is typically not rated above 16-18 kN.
So the belay loop is stronger than the rest of the harness and eliminates tri-axial loading. In both cases you are a safer climber if you set up your belay system through the belay loop.
If you don't trust me, how about Chip Miller, the Global Sales Manager for Metolius Climbing. Here's how he answered the question:
Carabiners are designed to be the strongest when weighted along their Major Axis. When you use your belay/rappel loop, your biner is loaded correctly, along its Major Axis. When you run your biner through both points on your harness and then connect it to a device, you are tri-axially loading your carabiner, therefore reducing its ultimate values. Additionally the belay/rappel loop is the strongest part of a harness, typically able to withstand forces in excess of 22 kN. Harnesses fail around 16-18 kN. This however, does not mean it’s ok to tie in to your belay loop. The friction of the rope on the loop has the potential to make it fail and at much lower than it’s ultimate values, but a carabiner eliminates this friction.
Any other questions?
See you on the rock,
Outdoor Adventure Club
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