Impact (Kn) absorbing knots?

I was wondering if anyone has ever experimented, or know of knots that would potentially absorb impact forces in a fall scenario, much like a screamer?

some possibilities I was thinking of were a 6-8 inch single fishermans knot of accessory cord untightened with an overhand on a bight on each end. This would be hypothetically clipped to a piece of gear and the rope like a draw, tightening and slipping through itself in a fall.

also, possibly, if you can envision a fishermans knot on a bight? another possibility would be a series of "daisy" knots in accessory cord.

Ill try to post some pictures after i do some testing.

any thoughts?

i think by definition of a 'knot', i am afraid not.

all a screamer does is lower the peak load by extending itself. distributing the load over time (impulse), and hence not as high of an impact. i suppose if you design a knot to slip, you might achieve this idea. but knots are not really designed to slip.

interesting thread (so to speak)

The figure-8 already does this. In one test situation, it absorbed energy equivalent to having an extra five feet of rope in the system, which could be significant when the leader isn't very far out.

If taking multiple falls, you'd have to untie and retie the tightened knot to continue to get the effect.

if poor dan-o was still alive, he had personal test experience with how to get the most absorption out of knots...

check out the bulky ass knot he uses in the old Masters of Stone rope jumping...

rgold wrote:The figure-8 already does this. In one test situation, it absorbed energy equivalent to having an extra five feet of rope in the system, which could be significant when the leader isn't very far out. If taking multiple falls, you'd have to untie and retie the tightened knot to continue to get the effect.

If thats true, I would think a butterfly knot might do even better.

Any common knot will absorb some energy as it tightens under load, with the figure 8 certainly exhibiting this characteristic as noted above (think of how much extra rope is needed to tie-in--some amount of this will slip out over time when loaded).

The only knot I know of that has been explicitly stated as being an energy absorbing knot is the false butterfy (from Alpine Caving Techniques, a French caving book). Instead of being a normal mid-lin knot as the regular butterfly, the false one slips under load and basically becomes a slip knot unless there's a biner in the loop. This slipping of more rope material over time would absorb some energy, though I'm not sure how much it's used.

Here's a thread about butterfly knots ('alpine' and false)

have there been studies about how much a clove hitch moves under stress? I'm sure that might absorb some energy too

A couple years back I set up a toprope solo by tying one end of my rope to the chains. I had overhands on a bight every 5 ft that I would clip into as I climbed (clipping the higher with one runner and then unclipping the lower with the other runner). Before I really got too far off the ground I tested it, and it was extremely dynamic! (at least for that first time) Although there were at least 15-20 knots...

I'm wondering how much you can really "absorb" with a knot method like this. Let's think about it this way. The energy of a fall has to go somewhere. In the case of a dynamic rope a lot of it is stored in the rope as it stretches. If perfectly elastic the rope will release "all" of this energy when the weight is taken off. However, the energy also goes into friction. Friction at the ATC, Caribiners, inside the rope itself, etc... And if the belayer gives a soft catch and allows himself to take a ride, the energy goes into the potential energy of the belayer who is now a few feet off the ground. In your scenario of a knot absorbing energy the only way I can see energy being absorbed is through the friction.

You have, however, lowered the impact force by increasing the time. My question is how much can you really do that with some of the basic knots we are talking about. What is the ratio of the total rope stretch vs the small amount of rope the knot pays out? And that relationship to time? Seems fairly insignificant. Why not get some Johnny Jumper springs and attach that to your harness? Or better yet, oil filled shock absorbers on your draws? Ok seriously now, aren't there cords that stretch that you could use that would be far better than a knot? I could swear "Freedom of the Hills" mentions some shock absorbing things like sewn slings that are meant to break at the stitches absorbing energy until they are fully open. Now something like this that you can attach to your harness that is renewable, that is what we need to be talking about..!

As said above "the figure 8 already does this [absorb]. Some people tighten this animal down tight before setting off, and I always wonder why they want to sacrifice the cushion. As far as screamers go, I say avoid it by 'place early place often' ethics.
Steve

not to mention:
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Alpine Butterfly
« Reply #15 on: March 23, 2006, 05:15:26 pm »

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According to a nice report proced by the Combined Services
cs-caving-association.com/R…
the false butterfly has strengths of

transverse 53%
Longitudinal 67%
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ukcaving.com/board/index.ph…
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And the false butterfly sounds pretty complicated to get just right... Although very interesting information, and an interesting idea. now I'm thinking...

There are some very simple devices that create energy absorbing knots and are resettable. One is the Kong Slyde. It's a small plate with rounded edges and takes 9mm cord.

storrick.cnchost.com/Vertic…

I've used them and they will absorb quite an impressive amount of impact. Very simple, but effective.

These devices are good for via ferrata routes, as a fall onto one of the cable mounts will create very high impact forces, much higher than most roped lead falls.

strong agreement regarding the above statement of figure 8 absorbtion. Whenever setting out on a scary s or vs lead I loosely tie the 8 follow though looping the tail back through the base of the 8. This absorbs quite a bit od shock upon loading. I know many climbers with heaps of experience on sketchy/severe gear routes who do the same. I believe this can help distribute the load over both time and space having significant effect.

I am very surprised to hear of a common or shared understanding of common knots (i.e. retraced 8) enhancing the shock absorbing potential of the system. I believe it does but never gaged safety margins on the notion.

I'd suggest that few responses, so far, really engaged the question in the original post. It's a very interesting question and one not commonly discussed.

What about knots included in systems that might be intentionally employed to reduce shock loading? Really intriguing idea. Could be on the anchor, climber or belay device?? How about used intentionally in vector loads?

Most interesting. Thanks!

Although an interesting idea, isn't rope on rope friction a bad thing? Could it put unecessary wear on the rope or even burn through? Any knot designed to lessen impact forces will extend the fall, right? How about having a length of accessory cord attached to your harness tied in a prusik, klemheist, or bachman knot around the rope and pulling a length of the climbing rope through it toward yourself to create a 'reserve' that would slip through the prusik, klemheist, or bachman knot in a fall? This may cause too much friction and risk burning the rope. Maybe not a knot at all, but a device attached to the rope that would replace the prusik/klemheist/bachman. For example a figure 8 rappel device. What about a 'chain of eights,' (the knot, not the rap device) tied in series in front of your harness?

rgold wrote:The figure-8 already does this. In one test situation, it absorbed energy equivalent to having an extra five feet of rope in the system, which could be significant when the leader isn't very far out.

Do you have a PDF or link for this study? The testing that the CAI did on their tower in Padova showed that the energy absorbed by a tie-in knot was inconsequential. I'd be interested in seeing any more recent reports.

For the OP, the best option will always be something tested and calibrated, such as Screamers or any of the Via Ferrata gizmos. Relying on knots will always be a guess at best.

Interesting question. This idea is discussed in a book I have, "The Ontario Rock Climbing Association - Safety Manual".

In this book they describe an adjustable sling that has energy absorbing qualities. Basically, it is a variation of 2 prussik type knots tied into a sling that will absorb energy as the 2 knots slide toward each other. The book includes a table showing amount of force absorbed vs. prussik wraps.

You can adjust the amount of force absorb by varying the number of wraps of the prussik knots (fewer wraps/less force absorb - more wraps/more force absorbed).

If one is concerned with this "sling" failing, you could back it up with a full strength sewn sling that would come into play if and when the adjustable sling's knots butted up against each other or if it failed.

Kn-kiloNewton=224.8 lbs.of force

Length of Fall
Fall factor = ---------------
Length of rope out
Fall Factor 2 is the max. you should encounter (18Kn)

CalmAdrenaline wrote:I was wondering if anyone has ever experimented, or know of knots that would potentially absorb impact forces in a fall scenario, much like a screamer? some possibilities I was thinking of were a 6-8 inch single fishermans knot of accessory cord untightened with an overhand on a bight on each end. This would be hypothetically clipped to a piece of gear and the rope like a draw, tightening and slipping through itself in a fall. also, possibly, if you can envision a fishermans knot on a bight? another possibility would be a series of "daisy" knots in accessory cord. Ill try to post some pictures after i do some testing. any thoughts?

7mil. double fishermans.

AJS wrote:have there been studies about how much a clove hitch moves under stress? I'm sure that might absorb some energy too

yes. i tested this and found that the clove slips about 5-6 inches as it is tightened to 1100 pounds. after this, it did not slip further, the rope broke at the hitch at 2800 pounds.

the amga did a much more detailed study and found that if the clove hitch is tied with the load strand next to the spine of the carabiner, the rope broke as above. however, if the load strand was on the gate side of the carabiner, the clove hitch slipped.

in either case, the knot is too tight to be untied after being loaded to 1000 pounds or so - it can only be removed by cutting it off the carabiner.

Geir Hundal wrote: yes. i tested this and found that the clove slips about 5-6 inches as it is tightened to 1100 pounds. after this, it did not slip further, the rope broke at the hitch at 2800 pounds. the amga did a much more detailed study and found that if the clove hitch is tied with the load strand next to the spine of the carabiner, the rope broke as above. however, if the load strand was on the gate side of the carabiner, the clove hitch slipped. in either case, the knot is too tight to be untied after being loaded to 1000 pounds or so - it can only be removed by cutting it off the carabiner.

I've wondered about this problem. Is this a legitimate reason to use a different knot for an anchor attachment? Shouldn't there always be some dynamic characteristics in the anchor build?

Clyde, the reference for my comment is

From Measurement of Dynamic Rope System Stiffness in a Sequential Failure for Lead Climbing Falls, J. Marc Beverly and Stephan W. Attaway,

mra.org/services/grants/doc…,

Recommendations section, page 22:

"We were able to make accurate measurements of the system stiffness and show that knots play an important role in system stiffness. The figure-8 follow through knot absorbs an equivalent of nearly 1.5 m (5 feet) or rope for the first impact force. After that the knot is 'hardened' and has less absorptive ability....Although not of practical use on bolted sport routes, this could have major consequences when falling on questionable anchor points."

The testing used a belay without slippage. Much, perhaps not all, of the CAI testing used real belayers. Perhaps once the rope starts slipping through the belay device, as it invariably does in those tests, the further effect of knot-tightening is inconsequential.

Lots of other interesting results in the Attaway-Beverly paper, the main one being an affirmative conclusion about the oft-debated issue of whether climbing ropes recover in the fractions of seconds between sequential anchor failures. For those whose memory goes back this far, Ken Cline's shock wave analysis on rec.climbing seems to have been confirmed.

Edit 2/27/2012: The link above to the Attaway paper is dead. Here are two current links: google.com/url?sa=t&rct=j&q…

google.com/url?sa=t&rct=j&q…