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Fall Factors Explained

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falling_apple_2For millennia people like you and I have looked at the world and tried to understand what it is made of and understand the forces that act upon it, and its these observations that have developed into the branch of science that we have come to recognise as physics. I don’t think I’d be wrong in suggesting that a great many people will acknowledge the outstanding contributions of those who have dedicated their lives and careers to the pursuit of such comprehension, and it is something which I’m able to admire, but largely from afar. Thankfully, most climbers are more concerned with overcoming physical laws rather than observing them, but you’d be a fool not to seek to know your enemy.

As far as climbing goes, the downside of gravity, if you can look past the witticism, is the strain that it exerts on a climber and their safety system during a fall. Most of the components that a climber might use to make up that system would be described as static, meaning that they are not designed to absorb energy through movement. There is, however, an obvious exception, and that is the climber’s rope. Climbing ropes are purposely designed to stretch under load, and it is this key feature which helps to diminish the influence of one of natures greatest forces. As you might expect, the rope’s capacity to absorb energy is cumulative: the more rope there is in the system, the more energy the rope is able to absorb. If we could look at this ability to absorb energy in relation to the amount of energy generated, we could start to quantify the fall’s impact on the climber and the rest of the safety chain. This is where fall factors come in to consideration. A fall factor is simply a number used to express the severity of the forces encountered during a climbers fall. It is calculated by dividing the length of the fall by the length of rope available to hold that fall, that is rope that has has been paid into the safety system by the belayer.

Fall factors illustrated
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Consider a climber leaving a belay stance part way up a multi-pitch route. If that climber were to ascent 10 metres above the stance, as shown in example 1, and fall without first placing a runner, he would fall the 10 metres back down to the stance, plus a further 10 metres below the stance before the rope became tight and his partner could arrest his fall. In this extreme example, the climber has fallen a total of 20 metres with only 10 metres of rope in the system. To express this as a fall factor we would divide the distance fallen, in this case 20 metres, by the amount of rope paid out, which is 10 metres. 20/10=2. So, this would be expressed as a factor two fall, which is the highest factor that can be realised in the event of a lead climber falling.

Looking at example 2, the climber has wisely place a runner before leaving the stance. In the event of a leader fall, that single runner will prevent the whole force of the fall being borne by the anchors at the belay, which could potentially be catastrophic, were they to fail. The climber then continues his ascent and falls at the same point, 10 metres above the stance. This time, because of the runner placed at the stance, the climber falls a total of 18 metres with only 10 metres of rope in the system. Whilst this isn’t quite as bad as the scenario outlined in example 1, the climber still suffers a violent fall: an 18 metre fall divided by 10 metres of rope equals a fall factor of 1.8, which is very high.

Example 3 shows that the climber has made the same 10 metre ascent, but his runner is now 5 metres above the stance. This time, when he falls, there is still 10 metres of rope in system, but the climber drops 5 metres to the runner and a further 5 metres before the rope becomes tight. Things are looking up for the climber: a 10 metre fall divided by the 10 metres of rope in the system gives a fall factor of 1, which is starting to look a little more comfortable.

In the final example, example 4, the climber has placed two runners before falling: one at 5 metres above the stance and the second at 8 metres. This is a far safer prospect than the three previous scenarios. In total the climber drops 4 metres, and there is still 10 metres of rope between him and the belayer. A 4 metre fall divided by 10 metres of rope gives a rather more appealing fall factor of 0.4.

What about leading on an indoor wall? Let’s say that the lowest quickdraw on a climbing wall is at 3 metres above the ground, and all subsequent quickdraws are spaced 1.2 metres apart. A climber who has made the first clip, ascended to the second, but failed to make the second clip before falling, would fall a distance of 2.4 metres. At this early stage of the ascent, there is approximately 3.2 metres of rope in the system. The clip is 4.2 metres above the ground, but we need to deduct a metre as the belayer’s harness is likely to be around 1 metre from the ground. In this example, the fall factor would be 0.75: 2.4/3.2=0.75. This is likely to be the highest factor you would experience on a climbing wall. if the climber were to fall at the third clip, the fall factor would have reduced to 0.55, 0.43 at the fourth clip, 0.35 by the fifth and so on. By the time the climber had reached a height of 30 metres, which is plausible, the fall factor will have reduced to 0.08.

Things aren’t always this simple though. there are other considerations that can influence fall factors which aren’t so easy to quantify. Friction, for example, can have a dramatic affect because it is able to reduce the amount of rope that is available to absorb the energy of a fall. If a rope runs over an edge, which is one way drag might be encountered, the rope below that point may be unable to reach it’s full dynamic potential. This might mean that only the rope above the edge works at full capacity, so to speak, to safeguard the climber. Although not usually an issue on indoors walls, bends in the rope caused by poorly extended runners can also create excessive friction.

In summary, fall factors are a useful and simple medium to quantify the severity of forces that may result from leader falls. Understanding the circumstances in which high fall factors present themselves is a principal strategy in reducing the likelihood of their occurrence. If you find yourself in a position where you cannot avoid the risk of a high fall factor, that very awareness might cause you to tread lightly and take more care.  Either way, just being aware of a risk will give you the opportunity to manage it that little bit better. Knowledge is, after all, power.