RAD LINE joining knots
Tests of joining knots useful for rappelling and glacier travel with RAD LINE cord.
Warnings
- Carefully read the Instructions for Use used in this technical advice before consulting the advice itself. You must have already read and understood the information in the Instructions for Use to be able to understand this supplementary information.
- Mastering these techniques requires specific training. Work with a professional to confirm your ability to perform these techniques safely and independently before attempting them unsupervised.
- We provide examples of techniques related to your activity. There may be others that we do not describe here.
1. Rappelling on RAD LINE: recommended knot for joining two strands
The tests show that knots which are carefully tied and tightened before use provide enough strength for normal rappelling forces. The primary question isn't about the strength of the knot, but of its resistance to capsizing. Capsizing "consumes" the tails, so the best precaution - regardless of the knot used - is to leave long enough tails (30 cm minimum).
The flat overhand bend, with long enough tails, offers sufficient strength, unties easily enough after use, and seems to get stuck less than others when retrieving the rope. So it is recommended for joining two strands of RAD LINE for rappelling.
For more information, see the test results
Notes:
- The knot was not tested when jammed against a rappel ring
- The knot was not tested for joining two ropes of different diameters
- Testing was done only on RAD LINE cord
WARNING: figure 8 knot behavior can vary significantly depending on whether it is correctly or poorly tied, correctly or poorly tightened. It is also often difficult to untie after use. This is why the flat overhand bend seems more reassuring for this application.
The double fisherman's knot, strong and reliable but also difficult to untie, is known to get stuck easily when retrieving the rope.
2. Roped progression with RAD LINE: recommended knot for joining two strands
The flat overhand bend, or flat overhand bend with stopper knot, are recommended for joining two strands of RAD LINE for roped progression.
For more information, see the test results
The tests show that the force of a fall can be enough to capsize a poorly-tightened figure 8 knot or cause a flat overhand bend to slip. So it is essential to carefully tie and tighten the knot and to leave long enough tails (30 cm minimum).
But in a glacier travel situation, we also know that a bulky knot is an advantage in helping stop a fall (the knot can jam against the crevasse lip).
Tying a stopper knot in the tails thus seems very appropriate:
- The recommended 30 cm minimum tails are long enough to tie a stopper knot
- The knot's bulk is an inherent advantage in a fall
3. Note on tightening the knot
In this study, a knot is considered "well tightened" if the 4 strands have been individually tightened. Simply pulling on the two strands together on each side of the knot is insufficient.
4. Test results
WARNING: the data presented are valid only for RAD LINE. They could be very different with a cord of different diameter or material, especially in the case of a cord with a slippery Dyneema sheath, or any joining of ropes or cords of different diameters.
4.1. Pull test, two strands of RAD LINE tied together
Figure 8 knot correctly tied and tightened:
First capsize between 5 and 7 kN.
Further capsizing between 5 and 7 kN.
Rope released after successive capsizing consumed all of the tails. (Each capsize consumes 2-3 cm of tail.)
Figure 8 correctly tied but poorly tightened :
First capsize between 1.2 and 2.6 kN.
Further capsizing between 5 and 7 kN.
Rope released after successive capsizing consumed all of the tails.
Flat overhand bend correctly tied and tightened :
First capsize between 4 and 5 kN.
Further capsizing between 2 and 3 kN.
Rope released after successive capsizing consumed all of the tails. (The first capsize consumed 2-3 cm of tail, further capsizing consumed about 1 cm of tail but at lower forces).
Flat overhand bend correctly tied but poorly tightened :
First capsize between 3.5 and 5 kN.
Further capsizing between 2 and 3 kN.
Rope released after successive capsizing consumed all of the tails.
Double fisherman's knot:
Cord breaks at 9 kN.
No capsizing before failure.
4.2. Fall arrest test: two strands of RAD LINE joined together
The pull tests were supplemented by fall arrest tests.
For repeatability, tests were done in a lab: arresting an 80 kg rigid mass falling 35 cm. A real fall into a crevasse would no doubt be longer, but many variables can reduce the impact force: rope friction on the crevasse lip, belayer displacement, harness deformation, knot tightening, energy absorption by the body... In comparison with forces measured in the field, we see that the laboratory impact force values, with a rigid mass, are slightly greater than in reality.
Figure 8 knot correctly tied and tightened:
Force measured at the anchor: greater than 5 kN.
Very little rope slippage in the knot.
Figure 8 correctly tied but poorly tightened :
First capsize at less than 2 kN, then the knot tightens and stops the fall with a force close to 5 kN: danger if the tails are too short!
Flat overhand bend correctly tied and tightened :
Force measured at the anchor: between 3 and 4 kN.
More than 10 cm of tail slippage in the knot: danger if the tails are too short!
Flat overhand bend correctly tied but poorly tightened :
Behavior very similar to a correctly tightened knot: danger if the tails are too short!
Flat overhand bend with stopper knot:
Force measured at the anchor: between 4 and 4.5 kN.
Slippage in the first knot is stopped by the second knot, which does not slip at all; this is the most reassuring configuration in our tests.
