The original mission was to construct a shock-absorber ( screamer) out of components found on an average rack.
Furthermore, the rig had to be "reusable". This has an advantage over mission-specific, shop bought shock-absorbers, which are "use once".
Although some manufacturers offer a "return for a re-stitch" service, this is not really viable mid-route or if you are based in the middle of nowhere.
After several false starts, I eventually came up with a satisfactory rigging, which is described here.
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Fig. 1
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Components
You will need:
- A couple of carrier bags. I have only used plain carrier bags but I suppose
the ones from your favourite supermarket will do just as well.
- Two karabiners.
- One 60cm sling as the back up.
Note: Click on the pictures for an enlargement ( 1-2mb)
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Fig. 2
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Construction
1. Roll up the polythene bag as tightly as you can. Using the "bottom of the bag" end, tie a slip knot with a 1 inch tail.
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Fig. 3
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2. Clip a karabiner into the loop and tighten gently.
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Fig. 4
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3. Add the other karabiner and loosely secure it with an overhand knot.
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Fig. 5
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4. Clip the handles of the polythene bag back into the first karabiner. Feather the
overhand knot until all strands of the "dog-bone" ( the tape bit of a quickdraw) are equalised.
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Fig. 6
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5. Add the back up sling.
Note: All rigs must be backed up by a sling.
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The Theory
1.
The polythene quickdraw is used to connect suspect protection to the rope.
In the event of a fall, the polythene will dissipate additional energy by
deforming. This should ( in theory) reduce the impact force on the suspect
protection, making it less likely to fail. The additional 60cm sling provides a
backup in the event of the polythene "dog-bone" failing.
This is NOT a multi-use device but can easily be rebuilt using another carrier bag.
Fig. 7
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Testing
2.
The Lemming and I have tested this rig at a super secret location. Once we stopped giggling, the drop test clipping the Death Quickdraw directly into a harness, was conducted as follows:
- In all tests, the Death Quickdraw was clipped slip knot end up, at a bolt.
- Body weight only loading on a single polythene bag "dog-bone":
As you would expect, the polythene started stretching as soon as your weight loaded the quickdraw.
I estimate, each polythene bag will be able to take a static 15-20kg load.
Note: This has NOT been actually measured.
- Fall factor 2 loading on a single polythene bag "dog-bone": The "dog-bone" failed!
This is to be expected as a fall factor 2 will generate a massive load on it.
The Victi, err! Tester did notice a small jerk but nothing so severe as to put him off eyeing up the cute rock-chick in the strappy vest.
So in effect, the "dog-bone" failure did absorb significant energy from the fall factor 2 drop.
- On post fall inspection, we noticed:
- The polythene did not stretch to the full length of the 120cm backup sling. It had deformed to about 30cm, then failed ( Hooke's Law).
- For this test, we deliberately used a 120cm sling as we were planning to test to destruction.
Note: A 30 - 60cm sling is recommended for the live rig.
- The point of failure was actually at the slip knot.
- The force of a fall factor 2 fall, caused the slip knot to tighten and shear through the polythene.
- You can also see that the bottom overhand knot has been tightened by the fall but did not suffer any visible damage.
- At the time of writing, we have not conducted further tests with a multi-polythene bag set up, nor have we tried different knots. I'm afraid, the jury is still out on this one.
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As with any Chris Tan Death Product™, usual disclaimers apply. Use at your own risk! < Usual laugh follows>
Note: Usual copyright & disclaimers apply. If you would like to use the information or any of the pictures contained in these pages,
contact Chris Tan by first removing the Walnut.
Last Updated: 22 May 2006
