What in the Heck is a Fall Factor?
This article originally appeared on Climbing
Falling is the opposite of climbing, which can explain why we apply every membrane between our ears toward getting up the rock, but dedicate precious little to hurtling down it. Falling, it seems, isn't just to be avoided, it's to be struck from our minds.
But fall we do. Sometimes a little, sometimes a lot. Then, we take it on blind faith that everything will work properly. The gear will hold. The rope won't break. The belayer isn't asleep. That edge isn't that sharp. For the most part our systems are sound and work when we need them. Yet things do go awry. A partner of mine once fell just 20 feet and broke a half-dozen wired nuts in a row, zippering all the way to and past the hanging belay, which (phew) held.
Our accidents, large and small, reinforce the need for caution. Part and parcel is education--you can't become a safer climber without knowing the fundamentals. Here, we'll debunk (or rebunk) the elements of falling.
Impact Force
Like a sack of potatoes dropped earthward, when you fall, you hit hard. When you pitch off a route and come to rope-stretching halt, you, the rope, your gear and your belayer feel this as "impact force." When you buy a rope, notice the "maximum impact force" rating on the hangtag. For a cord to be UIAA/CE approved as a single rope, it must absorb enough energy to limit the impact force from a test fall to 2,697 pounds force. (1/2 and twin ropes are tested with lighter weights.) Today's ropes have impact forces ranging from 1,600 to 2,500 pounds force. It is worth noting that the force limit is for the first drop only, and on a brand-new rope. Subsequent drops will reach higher forces, sometimes much higher, depending on how the rope is engineered. Rope stretch, or "elongation" is also a factor to weave in: more stretch usually means lower impact forces, and an increase in the distance you'll fall.
The hangtag number for maximum impact force and elongation are rough guidelines to keep in mind, but know that a rope's maximum force is measured on the "leader's" end of the rope only. The forces placed on your protection and belayer are quite different. In fact, due to a little devil called the "pulley effect" (rope goes up to the pro, then down, creating an inverted "v" which multiplies the force on the pro), the load on your top piece of gear is roughly 1.66 times the force you feel (figure one, next page). This load multiplication is crucial because it can exceed your rope's listed maximum impact force.
This doesn't mean that your rope will break--won't happen--but it should yellow flag micro-gear placements, old quarter-inch bolts, fixed tat and other bits of pro with relatively low breaking or pull-out strengths.
Impact forces, however, are theoretical until you get on the rock and fall, which is what our official “crash test dummies” did. For our tests, we fell off bolted, overhanging routes and recorded the impact forces registered at the top bolt. In all we lobbed off 30 times and recorded forces from 1,100 to a jolting 2,200 pounds force, and we never fell farther than 12 feet. Our falls pulled up well short of the gamut of situations you can encounter, but do give you an idea of the forces involved in typical falling situations.
Lessons to Remember
You can impact gear well beyond your rope's maximum rating--although this is unlikely in common climbing situations.
A fall that generates as little as 1,000 pounds force on the top anchor can still pull hard on the belayer. Always position your belayer (or anchor him) so he can cope with a severe yank and not lose control of the rope.
After a fall, let your rope "rest" for 10 minutes to recover its elasticity. Falling, then getting back on the rock and immediately falling again can result in escalating impact forces.
Think of a rope as water in a five-gallon bucket. Every time you fall, you dip out a cup in terms of energy absorption. Eventually, the bucket, and the rope's ability to absorb energy, is drained. Track your rope's falls, know that over time it will lose its elasticity, resulting in higher and higher impact forces, and retire it accordingly.
Fall Factor
We instinctively feel safer falling five feet 20 feet off the ground than we do lobbing five feet, 60 feet off the deck. Paradoxically, how we feel and how serious the fall really is can be diametrically opposed. Due to the "fall factor," it's not so much how far you fall, but how much rope you have out to absorb energy.
The fall factor is the Richter Scale used to rate the severity of a fall. A higher number equals a harder fall. To determine the fall factor, divide the distance of your fall by the amount of rope that catches it. In the examples above, a five-foot fall on 20 feet of rope is a factor .25 fall (five divided by 20), while the five-foot fall on 60 feet of rope is a factor .08 fall. Since .25 is greater than .08, the fall closer to the ground generates a higher impact force than the fall higher up. You can take comfort in knowing that UIAA/CE approved ropes must survive five, factor 1.78 falls, which are difficult, though not impossible, to achieve in real life. (The maximum fall factor is 2; an example would be a 20-foot leader fall on 10 feet of rope.)
In practice, it is easy to achieve a factor .5 fall, which is a far cry from factor 2, but still severe enough to get your attention. Say you climb 24 feet up a sport route and fall below your next clip, dropping 12 feet. We replicated this and the .5 factor fall pegged the needle up to 1,700 pounds force--harsh enough to feel in your guts, and requiring a knife to cut apart the impossibly tight figure-eight tie-in knot. Our original plan called for increasing the fall factor up to 1, but due to the severity of the .5 falls, we cancelled the harder falls and decreased the factor to .25, more in line with what you'd experience in a typical fall at a crag where you are placing pro every body length. The result? Lower impact forces.
Lessons to Remember
Higher fall factors generate higher impact forces.
Avoid falling low on a climb, where there's not much rope in the system and the fall factor will be high.
Place frequent protection early on a pitch. This will lower the distance you fall, reducing the fall factor.
Ditch your ego, and place protection immediately after each belay. Failure to do so can result in a factor 2 fall onto the belay--a nightmare scenario.
Rope drag can increase the fall factor because it causes uneven rope loading: The rope on the leader side of the rope-drag point can take most of the full load, while the rope on the belayer's side can receive almost no load. Use slings to minimize drag and keep the fall factor as low as possible.
If possible, after a jolting fall, untie and switch ends of the rope.
Want to lear how to get yourself out of tight spots? Check out our online course “essential self rescue.”
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