Bounce Testing Forces

Does anyone know approximately how much force is put on a piece of gear when its bounce tested with a static sling? I realize that there are so many factors that go into it, but I'm looking for a very rough approximation. 2KN? 5KN? 10KN?

I remember a thread a while back where someone wanted to test their old CCH aliens. He clipped a screamer rated to deploy at 2K to the cam, and was unable to deploy the screamer with body weigh bounce testing.

I was surprised.

Disclaimer: I have never aid climbed.

Andy Kirkpatrick has a lot of good info on the subject.

He recommends using loops of different thickness chord to get a feel for the forces involved. 2mm chord should take body weight, but break on a bounce. 3mm chord should be harder to break. And you may not be able to break a loop of 4mm chord.

I don't think it is hard to approach 4 kN. Of course there are many variables. I assumed a 180 lb climber with a bounce that creates a 2 inch drop, using a four foot sling with a UIAA fall impact rating of 20 kN, a reasonable number in view of the fact that nylon slings have been tested and found to have a UIAA fall impact rating of 18 kN.

With these assumptions, the standard fall force equation gives a maximum load of 3.8 kN. This equation tends to overestimate loads for the leader-rope-anchors-rope-belayer situation, because of friction and give in the system, but in this situation it is likely to be much more accurate.

If you can double your bounce, i.e. drop 4 inches on a four-foot sling, you could hit 5 kN.

FWIW RGold, playing around with a load cell I was hitting ~4kN bouncing around on a shoulder length dyneema sling done up as a foot loop (I'm a hair under 200 without gear). Was just curious, so didn't record any logs of the data though.

Aric, I get almost exactly 4 kN for a one-inch bounce by a 200 lb person on a two-foot runner.

I was just guessing at the 20 kN UIAA rating for dyneema slings, but your result suggests it is in the ballpark.

John Wilder wrote: My impression from what little aiding I've done is that bounce testing is, for the most part, not all that useful. Either the piece is bomber and you dont need to test it, or its marginal, and testing it may compromise it anyway- better to just ease onto it and hope it holds. Some of the more experienced aid guru's can probably go into much more detail on whether bounce testing is worthwhile, though.

The aid gurus bounce test.

For example,from Chris MacNamara's online tutorial on big wall climbing (this excerpt from supertopo.com/a/How-to-Big-…):

Bounce Testing

The goal in bounce testing is to generate enough force to mimic a small fall. Proper bounce testing is essential for safety, peace of mind, and speed. If you don’t properly bounce test…

• You might place strings of barely bodyweight gear that will all zipper out in a fall.
• A marginal-looking piece will feel like a time bomb. You will be terrified that the piece might blow at any second.
• You will timidly step up the piece and won’t feel more confident about getting high in your Aiders and/or top-stepping.

Andy Kirkpatrick has a page on bounce-testing at andy-kirkpatrick.com/articl… . Among other things, he says

"Hard bounce testing is by far the most important technique when it comes to dicey aiding, limiting the chance of ever falling – important when falling is unthinkable!"

Fun fact: A full-grown man + aid rack weighs in at roughly one kilonewton, just standing there.

Everyone thank Killis for illustrating that "common sense" can't get you through everything.

Andrew Haag wrote: A KN is a moving force. You can not produce a KN without movement. If you are just standing there it would be a static measurment of weight.

Sorry, but you are wrong.
Newtons are a measurement of force, which is mass * acceleration. Gravity is the acceleration, your body is the mass.
If your logic held true, a truck hanging off a 3kn aid piece wouldn't blow because it was "not moving" and thus not going to exceed 3kN

Andrew Haag wrote: A KN is a moving force. You can not produce a KN without movement. If you are just standing there it would be a static measurment of weight.

If you're going to nitpick other people, you should really try to be correct about it. The force still exists, and can be measured in kN, even when it's only gravity causing it. And "moving force" makes no sense at all.

Yes, to be completely correct Eric should have said "A full-grown man + aid rack being acted upon by gravity exerts a force of roughly one kilonewton", but then he would have sounded pretentious.

rgold's CMac quote is spot-on. Bounce testing is critical for many placements.

John Wilder wrote:fair enough- but since I have little/no interest in anything harder than C1 or C2, my bounce testing needs are little, if any. hard aid climbing is way too freaky for my taste.

That's fair, too. If it's clearly a bomber placement, then bounce-testing is a waste of time.

All this armchair science is great, but there was a good test mentioned - get some cord and break it. Cord always breaks at higher values than actually rated for when I test it, but you'll get a ballpark number by bouncing on loops or stands of cord and snapping them. It'll also teach you how to move on gear, in aiders, to produce lots of force (bounce-testing), or very little force (creeping up on marginal placements).

Andrew Haag wrote: I dont mind being wrong

Well that is a relief. You are wrong. Newtons measure force. A weight hanging in gravity applies a force. Measuring weight in kilonewtons is perfectly valid.

Your driver's license says you weigh some number of lbs because we don't use metric here. If we did it would have your weight in kilograms, which is technically incorrect- grams are mass, but since we all live on the same planet they're used interchangably.

Anyways.

I'd like to encourage people to answer authoritatively when they really understand a topic, and discourage speaking up otherwise. There's enough disinformation out there, this place can be different.

Looks like Andy did some editing while I was writing my reply. Sorry dude.

so a kN is 1000 Newtons right?

Darren Mabe wrote:so a kN is 1000 Newtons right?

Yes, the prefix (kilo) would be applied to the unit (Newton)

...
Kilo = 10^3 (1000)
Hecto = 10 ^2 (100)
Deca = 10^1 (10)
No prefix = 10^0 (1)
Deci = 10^-1 (.1)
Centi = 10^-2 (.01)
Milli = 10^-3 (.001)
...

So, 1 Kilo-newton contains 1000 newtons.

Fig Newtons? I prefer Fig Newman's. But 1,000 of either would be welcome.

Eric Fjellanger wrote: I'd like to encourage people to answer authoritatively when they really understand a topic, and discourage speaking up otherwise. There's enough disinformation out there, this place can be different.

That's much nicer than the "Donny, you're out of your element" comment I was going to make.

Oops.

On a side note, everything becomes clear if you look at the units.... A Netwton is a kilogram * meter / second^2. Acceleration due to gravity is meter/second^2, so clearly the "moving force" thing is out the window.

Aric Datesman wrote:On a side note, everything becomes clear if you look at the units.... A Netwton is a kilogram * meter / second^2. Acceleration due to gravity is meter/second^2, so clearly the "moving force" thing is out the window.

the force is strong with this one

Darren Mabe wrote: the force is strong with this one

Clearly the case, given the beer gut. :-o

The Dread Pirate Killis wrote:According to the posts above, smallest HB/DMMs, BD Micro Stoppers, and Wild Country Zero Cams won't pass a bounce test. They should shear at the cable under body weight bouncing. Anyone tested that?

I'm willing to bet BD has tested that...

But having a strength rating of 2 kN does NOT mean that the nut will break right at 2.00000001 kN. It means 99.87% of the nuts will break above 2 kN. The majority will hold quite a bit more.

However, if your point is that you maybe shouldn't vigorously bounce-test the smallest pro that exists, I would not disagree.