Why is drop tower testing harsher than real world climbing?
Short version: Petzl testing shows that dropping a real climber rather than a “rigid mass” decreases impact forces on the climber and the top piece of gear by almost 50%.
Many of us have seen those drop tower gear-buster videos, where a heavy weight is released and snaps a Dyneema sling rated to 20+ kilonewtons like it was a shoelace. Impressive, no?
But, we also know that that hardly ever happens in the real world.
Why are real world forces lower than the drop tower, and by how much?
Fortunately the clever engineers at Petzl had the same question, and here are some answers. Disclaimer: there are many variables involved in testing forces like this. This is not a comprehensive study with definitive results, but more of a way to get people thinking about general technique and assumptions.
In the diagram below, there is no belayer. The Grigri is tied off at the bottom, with the rope essentially fixed. (If there was an actual belayer involved, and if they had a tube style device such as a Reverso, that would further lower the forces. But that also introduces an extra variable, so perhaps Petzl decided not to include it.)
Notes . . .
If your French is a bit rusty: climber = “grimpeur”.
The gray dot is called “anchor” in the caption. That’s the top gear placement, not the lower anchor where the Grigri is attached.
That’s impressive! When a real climber is involved in the system instead of a concrete block, there’s a huge reduction in forces. That's good news for your top gear placement, your kidneys, and also why those gearbuster test videos may not apply too well to real life.
The drop tower has only the dynamic rope to absorb energy from the fall. In the real world, we have additional variables such as the squishy and force-absorbing human body, the displacement of the belayer, and rope slipping through the belay device. Even with a pretty harsh fall factor of 0.7, force on the top piece of gear is only about 5 kN with a real climber, compared to 9 kN with a rigid mass.
So, another way to think of it, is that having real bodies involved decreases force in the system by about 50%. That is a very good thing!
Now, to be clear, I'm not saying you should be taking factor 1 falls on a Dyneema tether. In that case, there is only your squishy body, and no belayer displacement or rope slippage through a belay device, so forces are going to be a bit higher. But still less than the infamous concrete block drop test.
Some of you are wondering, what about falls greater than factor 0.7? Petzl did test some factor 1 falls, with a real climber and belayer. See that at the link below. You can't really test much more than factor 1 with real people, because somebody's probably going to get hurt.
From the Petzl website:
“These tests revealed very significant differences between the two protocols: the force on the climber increases by up to 70% with a rigid mass.
These differences are explained by the many factors, other than the rope, that contribute to fall energy dissipation: the absorption of the two bodies, belayer displacement, rope slippage in the device...
Conclusion
These tests help us evaluate falls involving real people.
Such measurements necessarily carry a high degree of uncertainty, but help provide important information:
The forces at work in a real fall differ greatly from the results of standard testing.
In practice, factors other than the rope contribute to dissipating the energy of a fall.
To understand a fall, one must take into account all of these factors and not focus only on the rope.
It is difficult to control all of the factors that dissipate fall energy. However, it is easy to influence the potential for belayer displacement. Belayer displacement helps dissipate a significant part of the energy and thus limits the forces at work. On the ground, it is essential to allow displacement to occur for a dynamic belay. At the belay station, it is wise to use a long tether, when the situation allows it, to allow displacement to occur.”
