In reply to Paul Evans:
I was emailed by Paul Evans because he was looking for technical data on how larks-footing slings directly to wire affected their strength.
I have always regarded this as a big taboo, but was not aware of any technical reports. I spoke to Fred Hall, our Technical Director, and he also cited the practice as being dangerous because of the small diameters and tight angles involved. However, he was also not aware of any formal technical reports on this topic.
Thus we then proceeded to tensile test a variety of different sized nuts that had been larks-footed to a variety of different webbings.
I should stress that the results here are not conducted in a scientific or controlled environment; they are just snapshots to give one a general idea of how this practice may reduce the strength of the system.
The results were all obtained using new product, thus the results are likely to be at the top end of those than could be expected. This is because nicks, cuts and abrasion damage on the nut/tape would be likely to reduce the strength of the system.
The drop tower was in use and therefore we had to use the main tensile tester. This means the force will have been applied relatively slowly to the tape/nut system; thus again the results shown are probably higher than would be achieved in a real life scenario.
The worst scenario occurred when using skinny slings on small nuts with thin wire. Thus a small size nut (wire of diameter 2.3mm(BD Stopper 5) to 2.5mm(Wallnut 1)) attached to an 8mm Dyneema sling via a larks-foot broke at 5.5 to 6.2Kn.
When attached to large nut with thicker wire (3.2mm on a Wallnuts 5 -11) the 8mm Dyneema tapes broke at 7.5 to 8.0Kn.
The results for 10mm Dyneema webbing were from 6.8 to 8.6KN
The results for 11mm and 12mm Dyneema webbing were from 9.3 to 10.9kn.
Severe tear and rupture damage occurred on all slings prior to failure, Thus it is likely that, even should the sling hold the fall, the sling would have to be replaced.
As a reference it is fairly easy to generate 7kn in a fall and hard falls can generate 8kn-9kn. Again I would suggest that these figures are the top end of what you could hope to see the system cope with because all the kit was new.
Thus I would seriously discourage any climber from larks-footing any Dyneema sling directly to a wire nut.
It is worth pointing out that the DMM 3 and 4CU units use 3.5mm wire that is then covered with a stiff 2,5mm PVC tube. These units are slung on 11mm or 12mm Dyneema. These achieve a safe 3 Sigma 12KN (extended) and 14kn (doubled) strength. We could put this wire/plastic combo on the nuts , but the weight penalty would be huge.
Regards
Simon Marsh
DMM