#alpinhack: Rappelling with aid lines (Rad Line & Co)

Alpine sport climbing routes are often abseiled. Such routes are often climbed with single ropes. An aid line can be used to retrieve a small haul bag and enables abseiling over the full rope length. Here we explain the problems and questions that frequently arise when abseiling with aid lines.

Can you use a single rope and a 5 or 6 mm cord for abseiling?

Yes, you can also use ropes with different diameters for abseiling. A single rope and a 5 to 6 mm cord can also be used.

Which connection knot is suitable?

It is much easier for two bag stitches to become entangled during abseiling than one. — Fig. 1 Well-intentioned, but badly hit.Two sack stitches are much easier to pull off than one.

A simple sack stitch is sufficient as a connecting knot, even with ropes of different thicknesses. Even when connecting a single rope with a 6 mm Kevlar or Dyneema rope, the strength of the sack stitch is 4-5 kN before the knot unravels. This strength on the single strand is far higher than the forces that occur during abseiling.

Firstly, when abseiling, you are hanging from two ropes, which means that only half of the load acts on the connecting knot. Secondly, when abseiling at the belay, loads of only 70-150 % of the body weight occur. 70 % if the end of the abseil is already further below the belay and the rope absorbs part of the load through friction when resting on the rock. 150 % if the person abseiling is abseiling very abruptly close to the belay.

Even if you go for it and rush through a free-hanging section and then brake very abruptly, forces of only 2.5 kN maximum could be measured at the abseiling point (see Hellberg, bergundsteigen #123). In the worst case, around 1-2.5 kN will act on the stand. This means that the connecting node is only loaded with 0.5-1.25 kN, which corresponds to 50-125 kg.

These are loads where every knot is ‘still bored’. Under no circumstances should you tie two sack stitches one after the other on the rope ends. This greatly increases the risk of the rope getting caught somewhere on the rock when it is pulled off the knot (Fig. 1).

The breaking strength of a 5 to 6 mm cord with a single rope cannot be determined, as the sack stitch knot begins to ‘roll’ until the ends slip through before the breaking load is reached. This is the reason why, as a back-up, the ends of the sack stitch should protrude 30 cm when abseiling. Even an initial roll then poses no danger.

More details on breaking strength and material properties can be found in the article ‘Hilfsleinen’ from bergundsteigen #67.

Is it absolutely necessary to use a high-strength Dyneema or Kevlar cord?

No. Even a 5 mm polyamide cord in combination with a single rope has a strength of 4 kN. However, a conventional cord offers little comfort when hauling the haul bag, as it has too much stretch and also has a significantly lower edge strength than Kevlar or Dyneema cords.

Which abseiling device is suitable?

Fig. 2 Two carabiners increase the friction.

The tube works as an abseiling device. If you combine a thin single rope with a cord, the braking effect can be very low, which is why you should use two carabiners under the tube (Fig. 2).

What can be done to prevent the rope from shifting when abseiling from the belay station?

Anyone who has often abseiled with ropes of different thicknesses will have noticed that the rope shifts in the abseil ring at the start of the abseil. This is because the thinner rope in the tube is subject to less braking force. As a result, it slides upwards towards the abseil station, while the thick rope is slowly pulled off during the abseil.

This happens until either the connecting knot touches the abseil ring or – if the connecting knot is on the side of the thicker strand – until the rope rests on the rock. As a rule, this phenomenon stops after a few metres of abseiling. However, if you abseil completely overhanging and the knot is on the side of the thicker rope strand, this can be a problem.

Slipping is avoided if the knot is placed on the side of the thin rope and rests against the abseil eye. If you have already threaded and unthreaded the rope and do not want to undo the connecting knot and knot it again on the other side of the abseil ring, the tube of the second abseil end can also be inserted into the rope.

Abseil down the blue single rope. The rope is pulled off via the red auxiliary line — Fig. 3 The blue single line is used for abseiling. The rope is pulled off via the red auxiliary line. However, techniques with carabiner suspension are often problematic, as the carabiner can easily get caught when pulling off.

The friction in the unloaded tube already prevents the rope strands from moving. When the second person begins the abseil, the thin rope is held by the lower climber by hand, just enough so that it cannot move, but still allows the upper climber to abseil.

After the first few metres, friction on the rock will prevent slipping anyway. Techniques with a carabiner attachment to fix the single strand to the belay so that you can then abseil on the single strand are rather problematic. Although the rope cannot slip, it is often difficult to pull off due to the increased friction. In addition, a rope with a fixed carabiner can easily become entangled when being pulled off. This is only a good option for steep abseils (e.g. canyoning), where the rope cannot easily get caught on the rock (Fig. 3).

What should I bear in mind when pulling the rope?

Fig. 4 Always pull off using the aid line.

It is always best to pull off at the aid line (Fig. 4). This assumes that the knot is placed on the side of the thin rope strand – which also avoids the problem of wandering described above. This is because the thicker, stiffer rope falls down. If you were to pull off on the thicker strand, the thin auxiliary rope would fall down along the rock, which would lead to more frequent snagging and an increased tendency for rope entanglement. When abseiling over several rope lengths, it is therefore worthwhile to always tie the connecting knot on the side of the thin rope strand when pulling off (Fig. 5).