Does it make sense to use fall arresters in rock climbing to reduce the forces involved?

Question

I understand that the fall factor depends on the actual length of the rope that will absorb the energy during a fall. The longer the rope, the smaller the rigidity of the system, and thus, the smaller the forces involved.

The rigidity of the rope is, simply put, proportional to the rope Cross section, the elastic Modulus, and inversely proportional to the length of the rope. So the longer the rope, the smaller the rigidity.

I was wondering if decreasing the system rigidity by means of an additional 'spring' connected in series at the climber side would make sense. In this case, the system rigidity would be calculated as the product of rigidities divided by the sum of those (like a paralell connection of Resistors in an electrical circuit). This Spring would reduce the decceleration and therefore reduce the involved forces (special focus here on the climber and the anchorage point). Of course, this is a simplistic view.

Does it make sense? i have googled it, and found that fall arresters are used by people working at heights, but not in rock climbing. It is mentioned that they are useful when working with static ropes (ropes with very high rigidity), which makes a lot of sense.

However, I could find nothing when it comes to rock climbing. I understand that sometimes the anchorage point could be not fully reliable, and reducing the forces on it would be welcome for the sake of security.

Answer 1

When rock climbing, it is true that sometimes a single anchor point is not fully reliable. In that case, it's MUCH better to improve safety by building a redundant anchor than to reduce the force of a fall by 50% using a fancy device like a fall arrester. Typically, 2-4 trusted anchor points (bolts, trees, or gear) are connected using a static rope (or something similar). Using some knots and carabiners, a "master point" can be created so that as long as one of the anchor points doesn't fail, the climber will be fine.

Dynamic climbing rope is then passed through this master point, and connects the climber to their belayer. Depending upon the forces involved, this rope can stretch anywhere from 10-30% the total length of the rope when a climber falls. An additional spring (or "fall arrester") is totally unnecessary in this case. All climbing equipment is designed to handle forces much greater than would be expected during a typical fall. In other words, if the fall arrester saves you by reducing forces by only 2x, then something in your setup is horribly wrong and will likely fail soon afterwards anyways.

Using a device such as a fall arrester could actually make things more dangerous by reducing the amount of control a belayer has over the distance their climber falls. For example, on an overhung route where the climber falls into open space, it is common for belayers to jump slightly (ending in the air) so that their climber falls farther, decelerating more slowly and comfortably. If there is a dangerous rock shelf or potential for the climber to hit the ground in the event of a fall, the belayer can do the opposite: sitting back or even backing up from the wall quickly to reduce the distance the climber falls. This results in an abrupt jerky catch for the climber, but is preferable to the climber breaking both of their legs by hitting the ground or rock shelf. If there were a fall arrester in this system, such a save would not be possible.

Answer 2

A rope brakes falls elastically (except for the hardest falls), various devices you likely call "fall arrester" (screamers, rope brakes used for via ferrata) do it inelastically. The elasticity of ropes is designed to lead to the maximum forces considered safe when applied to a harness. It would be easy to design more elastic ropes, but a softer rope means a longer fall distance, and a longer fall distance can mean a grounder.

The maximum forces on a rope do not actually depend on how far you fall but on the fall factor: fall distance divided by rope out from the belay (biner friction causes some adjustments here). The maximum fall factor is usually taken as 2 for falling from above a belay (the climber falls twice the distance of the rope out). It can be even higher with an active belayer who takes in rope in such a belay station fall.

For fall factors larger than 1 (given reasonably standard climbers' weight), the rope tends to stretch inelastically as well as elastically, so climbing ropes have specified a maximum number of such falls after which the rope is to be retired.

Because rope stretch scales with fall height naturally, other braking systems have become mostly irrelevant. Exceptions are screamers (for limiting the force on protection points of questionable strength) and rope brakes (usually used for via ferrata where the fall distance can be a multiple of the rope available for braking). Inelastic brakes have the advantage of more or less constant braking force, thus requiring minimum braking distance for a given maximum force. They have the disadvantage of being one-shot. Screamers have to be replaced after triggering, rope brakes have to be rethreaded after triggering (it depends on their design whether that's even possible or advisable).

So anything you put in series with the rope in regular climbing messes with the tradeoff of fall distance and utility that the rope as a primarily elastic fall catcher has been designed for. In the case of using a screamer on a weak protection point, modifying the tradeoff for a targeted situation is intentional. As a general strategy, it is ill-advised.

There may be a point in using an additional rope brake on the harness of particularly light climbers (like children) since the rope will not stretch as much in a fall as it has been designed for. It is comparatively unusual to have hard lead falls with children however.

Answer 3

As always, it depends.

There are devices, usually called "screamers", designed to be used instead of a regular runner to decrease the peak load in the event of a fall.

However, they are bulkier and heavier than regular slings and usually not neccessary since the energy absorbsion of the rope and the strength of a bolt or a well placed nut or cam is enough by a large margin.

But they do have their place. Sometimes you need to rely on sub-optimal or downright sketchy protection that never the less must not fail. This might be hard trad climbing, aid or ice climbing. In those cases, carrying a screamer or two might be what you need to stay safe. Or at least safe-ish. Or at the very least to be able to convince yourself to push on knowing you did what you could.