Fall factor and impact force

Fall factor and impact force are two important concepts in the physics of rock climbing falls. To understand rock climbing falls, it is important to remember a fundamental law of physics: when an object falls, it stores energy.

force of impact or thrust

During the fall stopping process, the impact energy will be absorbed by the elongation of the rope, the displacement of the protector (the protector who puts the rope below), and the body of the climber (the climber above), and the energy will be transferred to the protection chain in the form of a force, which is the impact force.

We usually have to focus on the impact forces transmitted to the climber, the protector and the anchor point.

The impact force is related to all the important factors of energy absorption, such as rope extension, protector displacement, climber's body, and rope sliding in the device.

Tests have shown that the impact force acting on the human body cannot be greater than 12kN, otherwise it will cause damage to the human body.

Theoretical fall factor (impulse fall factor)

The fall factor is often used to indicate the severity of a washout fall.

The fall factor is the ratio of the fall height to the effective rope length.

The effective rope length is the length of the rope that is actually involved in the process of absorbing the impact force, not the entire length of the rope.

Theoretically, during climbing, the value of the fall coefficient can be between 0 and 2, with larger values indicating more severe falls. In power rope certification, a fall factor of 1.77 is recognized as the most severe fall.

During climbing, the severity of a fall does not depend solely on the height of the fall, but also on the length of the effective rope, because the longer the rope, the more energy it can absorb.

In the two example cases below, the fall heights are the same and the energy absorbed is the same, but the severity of the fall increases with the excessively large impulse fall coefficient of the Example 2 system.

Example 1 has a fall factor of 0.4 and Example 2 has a fall factor of 2. Therefore, in Example 1, although the height appears to be very high, the actual fall factor is very small, so the whole system is safe and reliable.

Impact factor in practice

The theoretical fall coefficient does not take into account the friction between the rope and the rock as well as the friction between the rope and fittings such as the main locking quickhook. This friction prevents the rope from extending over its entire length. As a result, only a portion of the rope (the solid part of the rope in the diagram below) absorbs the energy of the fall: this is known as the effective rope length. It is therefore more important to focus on the fall factor in practice. Obviously, if the climber does not take the necessary steps to reduce rope drag, the actual fall factor may increase rapidly. In this case, the fall will be worse for the climber rushing down.

Ability of different ropes to absorb a fall

Both power and static ropes need to have the ability to absorb a fall, but the requirements are different. In the certification standards, static ropes are required to withstand more than five falls with a fall factor of 1, and power ropes are required to withstand more than five falls with a fall factor of 1.77.

The impact force increases exponentially with an increase in the drop factor. Therefore, the ability of a power rope that can withstand a fall factor of 1.77 to absorb an impact force is much stronger than the ability of a static rope that can withstand a fall factor of 1 to absorb an impact force, and is much higher than two times.