Fall Factors

[Joe]

My daughter had to do a Grade 9 science project of her own design, so she decided to a little study of Fall Factors (with just a little help from her Dad). The tests were to record the maximum force applied to an anchor with varying lengths of rope, and from different fall lengths, as measured by a simple dial scale. I won’t go into detail, but the most surprising consistency is it doesn’t take much of a fall to generate 15 to 20 times the original weight. For example, we dropped a 5 kg weight 50 cm on a 1 meter rope tied directly to the scale, and it measured 76 kg. Then we dropped 5 kg 1 meter on a 1 meter rope, and it measured 97kg. I ain’t no engineer and this is a Grade 9 project, so I avoided nerdy crap like kilonewtons, mass times acceleration, elongation rates, etc. But my simple take home message is this: Don’t bounce while rappelling when using suspect anchors. (PS. Sport climbers who never venture far from bolts can ignore this post.)

I had always assumed that it would be difficult to generate twice your bodyweight of force on anchors when rappelling. Now I fully realize how wrong that assumption was. And when it comes to leading with ice screws in manky ice, well that’s just a little too scary to dwell on. For general climbing application, use solid rappel anchors. But if none are available, rappel very smoothly and slowly!

[jdhohmann]

Wow, Great description, who would have thought such a force was applied by simply rapping down.. I'll be cautious next time

[Andrew]

I am also not an engineer, however, while reading your post, it occurred to me that 1 meter of rope will not stretch nearly as much as a climbing rope of full length such as 50, 60 or 70m.

It may be possible to figure that a 5 kg weight dropped 1 meter on a 50m rope would not become quite so heavy because it has more rope to stretch and slow down the weight, thus distributing that force over a longer time.

From the mind of ...

[Ammex]

I am also not an engineer, however, while reading your post, it occurred to me that 1 meter of rope will not stretch nearly as much as a climbing rope of full length such as 50, 60 or 70m.

It may be possible to figure that a 5 kg weight dropped 1 meter on a 50m rope would not become quite so heavy because it has more rope to stretch and slow down the weight, thus distributing that force over a longer time.

From the mind of ...

But a 5kg weight dropped 50 or 100% of the total rope's length, as in the experiment, would produce similar results, until it's falling long enough to hit terminal velocity, then adding more rope would add more relative stretch... or something like that.

[Stan]

I remember at least one documented fatality (in Russia, mid 90's) with exactly the same scenario: a climber clipped into the main anchor lost his balance and fell ~1 m from the belay ledge (fall factor 1). The pear-shaped biner on his anchor attachment (no daisy chain, just a piece of 10 mm static rope) got caught loaded along its short axis and ruptured. The climber continued his downward trip for another 600 m.