Effective fall factor

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Effective fall factor or as it is sometimes called real world fall factor is the actual Fall Factor that determine the impact force that develops during the breaking of a lead fall. When estimating the fall factor, the ratio between full length of the rope between the falling climber and the belay device on the belayer's harness are taken. Since the rope is dragged over rock and through the binners of the runners, the friction prevents the rope from elongating evenly, thus causing the effective fall factor to be higher, as well as the impact force.

lengths used to calculate the fall factor

The fall factor is the ratio between the length of the fall and the length of the rope used to break it. If [math]h[/math] is the fall length, and [math]L[/math] is the length of the rope that's elongated while breaking it, the fall factor is:

[math]FF=\frac {h}{L}[/math]

We assume, of course, that the whole rope is elongated evenly. But is that assumption justified? In most conceivable lead falls, the rope passes through a few runners, and runs over rock. The friction causes different rope segments to absorb different amounts of the kinetic energy of the fall. The impact force will be larger, and similar to one generated in a higher fall factor fall.

The friction of the rope passing through binners effectively devides the rope into different sections, each with a different tension. If the rope changes direction in the binner not only the leader has more drag, but in the case of a lead fall, the friction separates the segments of length of rope. The more the angle differs from 180°, the more different the tention will be between segments. In this case, the fall factor is no longer a good estimate for the impact force, since the whole length of rope is not equally elongated. A better estimate would be to use an "effective" fall factor, which will allways be higher than the theoretical one. This means the impact force will be higher, and the fall - more severe. This is especially evident by the difference between the force applied to the falling climber, and the force felt by the belayer. Ideally they should be the same, but actually, the difference is not negligible. This difference is actually the sum of friction forces in all the runners. It can be calculated by summing up all the angles of the rope and substituting the total angle in the formula:

angles in the runners



  • [math]T_2[/math] is the force on the falling leader
  • [math]\mu[/math] is the friction coefficient between rope and binner
  • * [math]N[/math] is the number of binners in the system
  • [math]\alpha[/math]is the sum of angles

Without calculating it might seem that the friction is not signifficant, but it is. This is rigorously explained in the article about how do friction devices work? and here we'll give only a short outline:

The friction of a rope on a capstan (strangely enough, the diameter of the capstan has almost no effect) is determined by the angle the rope spans while going around the capstan. A rope hanging from a binner, for axample, will span 180°. For a rope passing through a binner, the angle can simply be calculated as [math]180°-\alpha[/math], [math]\alpha[/math] being the angle between the ropes on both sides of the binner.

further reading

contributions to this page by: : Mica yaniv and others...