Vertical caving terminology and methods > General hardware
A measure of the shock load that a rope might experience, based on how far the fall was, onto what length of rope, calculated as fall_distance divided by rope_length, where fall_distance is the distance before the rope starts to stretch as it absorbs the shock. The actual distances are almost irrelevant to the shock load, only the ratio between them matters, since the rope's elasticity scales up at the same time as the fall distance. A person falling 2 metres onto a 2 metre length of rope (meaning that they fall from the same height as the rope's anchor) creates a fall factor 1 shock load. A person who climbs 2 metres above the same anchor, and then falls 4 metres onto a 2 metre length of rope creates a fall factor 2 shock load. European static ropes are not, in general, designed to cope with anything more than a fall factor 1 fall, and neither is the person experiencing them. A fall factor 1 fall with a static European rope can produce a shock load of 12 kN, which is near the limit of human endurance - a human could well be injured at 6 kN or less in a sit harness. This is why ropes are so heavily over-engineered, and why cavers (unlike climbers) try to avoid shock loading ropes. Dynamic ropes can absorb much more energy, producing a much lower shock load at the same fall factor. American static ropes typically produce higher shock loads at the same fall factors, as they can be far more static than European ones, so a fall on an American static rope is even more serious than with European static rope.
With rigging set up for SRT, the maximum fall factor that should ever be expected is 0.3, but the fall factors can be much higher for bottom roping. Several factors influence the fall factor in real cases, such as how many carabiners the rope passes through, and how easily it passes through them. These can make the rope act like it is shorter than it really is, increasing the fall factor. The rope length is only counted between the object that is falling, and the belay device that catches the fall. So for example, if a belayer at the pitch base is top roping a climber, using an Italian hitch located at the pitch head, then the fall factor relates to the length of rope between the Italian hitch and the climber, not the part between the belayer and the Italian hitch. Of course, it still relates to how far the climber falls, and with top roping, that is largely governed by how much slack is in the lifeline. The calculation gets very confused when the belayer gets pulled up into the air as they catch the climber, which often happens with belaying, or they intentonally jump into the air, which a good climbing belayer will. It also does not account for knots tightening and harnesses taking some of the impact, which softens smaller falls such as those below 1.5 metres.
Fall factors cannot represent falls which result in a severe swing, where the energy from the fall is converted into the swing without the shock load being applied to the rope. They can only help describe the severity of a fall where the rope has to absorb the shock directly. Fall factors relate only to systems with stretchy ropes. They do not apply to cables, chains or slings (especially Dyneema), which effectively have no stretch, and so can be treated as a rope with no length. In systems where there is no rope to stretch, the stretchiness of the object that is falling (such as the human or their harness) is the only part that can absorb the energy, so the distance fallen becomes the major consideration instead of the fall factor. This is because the actual force experienced is a product of the fall distance divided by the slowing down distance, and the fall factor is just a nice and easy way to work with that, when you have a stretchy rope. (This is also a simplification. It actually comes down to that popular physics expression f=ma, where a=dv/dt.) It does not realistically model what can actually happen with SRT, such as slipping off a ledge with a traverse line onto cows tails. It cannot cope with the mix of dynamic and static rope, the sit harness and the caver's own body movements, carabiners and hangers adding fall distance without shock absorbancy, the knots changing the shock absorbing nature of the rope, and most of all the traverse line heading in two directions, not directly away from the fall. Fall factors are only really useful when dealing with falls on a lifeline.
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