When a climber falls, the energy must be absorbed by the belaying system and in particular by the rope. If the rope is a good energy absorber, it will reduce the impact on the climber. The force sustained by the climber as a fall is arrested is what we call the impact force.
Its value depends on the fall factor, the climber’s weight, and the capacity of the rope to absorb the energy of the fall.

The maximum impact force

All mountaineering ropes are characterised by their maximum impact force, measured in the laboratory under extreme conditions which may not be encountered when climbing: a metal mass, fixed anchor, rope locked. This is the origin of the value given in the technical notice.
In these conditions all the energy of the fall is absorbed by the rope, none of it by friction, the harness or deformation of the human body.
It thus concerns the maximum obtainable impact force in the rope.

Progression of the impact force with use

In climbing, with successive falls, the dynamic capacity of the rope reduces and thus the impact force increases.

Laboratory test
with fall factor 1,77

FOCUS
ON THE LAST RUNNER : THE PULLEY EFFECT

During a fall, the highest runner sustains both the impact force generated by the climber, and the force held by the belayer. These two forces add together. This is what we call the pulley effect. The force held by the belayer is lower than the impact force on the climber because of friction in the karabiner. That’s why, in total, the force exerted on the highest runner is around 1.6 times the force on the climber!