How to safely use micro wires

Would a micro wire really hold your fall? Before you trust your life to that tiny bit of metal, we got Steve Long to run you through the essentials of using micro gear.

What are micro wires?

Micro wires are the smallest pieces of protection equipment available, designed to slip into tiny cracks less than half-a-centimetre wide. They come in various shapes and sizes, but can be characterised as either a tiny alloy wedge, threaded onto a swaged wire, or a brass wedge, with two holes soldered to each end of a length of wire. Brass wires were originally handmade in Australia by the enterprising Roland Pauligk (and therefore called RPs), and were a game-changer. Nowadays, Pauligk is retired, but several companies have followed his lead.

Will micro wires hold my fall?

Before worrying about holding power, let’s look at the forces generated by a leader fall. Without getting bogged down in maths, the key factors involved are: your weight, rope stretch (the more rope that’s been paid out, the lower the impact force on the top anchor) and the distance fallen. However, the bad news is that the top runner takes far more of the loading than either the climber or the belayer – about equal to the sum of the two. What does this mean in real terms?

Imagine a climber has run out 40m of rope and is sprinting for the belay, three metres above the last runner – and takes a fall. There’s a lot of rope out, so the fall factor is reasonably small (around 0.15), however, a falling body (of say 80kg) generates a lot of energy and so we can anticipate a potential impact force on the top runner of around 5kN. Their flight will be considerably more than six metres (double the height above the runner) due to rope stretch and other factors. If they hit a ledge before the rope comes tight, that will reduce the load on the anchor, but most likely damage an ankle. Suddenly, that hairline crack at eye level seems more appealing as a potential placement. But will it hold?

How strong are micro wires?

Micro wires are impressively strong for their size. The limiting factor is the wire diameter, which is why soldered nuts are often stronger than swaged loops. The smallest wires have a rated strength of 2kN – pretty close to the force generated by just hanging from it on the rope. Hence the tiniest wires are only recommended for bodyweight, but that might be enough to allow you to lower – carefully! – back to safety. You could fall further if the wire rips, so careful down climbing on a snug rope is preferable. Once we move up to the wires rated for leader falls (e.g. the original RP2 and DMM’s IMP 2), then micro wires would stand a fighting chance of holding that 5kN load.

Holding power

A micro wire won’t get a chance to display its impressive holding power if the crack has brittle edges (granite has a reputation for shedding micros if a crystal breaks), and keyhole slots may be weak for an outward pull. For all nut placements, the ideal is to have both working planes of the metal in full contact with solid rock, and this is even more crucial for micros. It’s worth carrying an assortment of shapes (both standard and offset) in order to find the optimum contact surface.

Improving your odds

We can reduce the impact loading on the top anchor by avoiding unnecessary friction lower down; so double-rope technique brings twin benefits of ‘stretchier’ ropes and less rope drag, particularly if you take care to avoid rub points by extending runners. Wires are weakened by tight bends, and it’s been shown that the performance of thin wires can be improved by up to 10% by using larger-diameter karabiners.

Providing a dynamic belay is also important: a bit of give in the system, including bodies absorbing some of the force and the belay plate allowing a bit of rope to pull through, helps to significantly reduce the impact force. However, my best tip for maximising the potential for marginal gear to hold a fall is to share the load between two or more.

In the UK we generally steer away from self-equalised slings, but for running belays these are actually the best solution: the loading will be equally shared between the anchors. Sure, if one placement rips it will shock-load the other(s), but it’s far less likely to rip if it’s only taking part of the load. And, in good placements, equalised bodyweight anchors will become strong enough to hold a whipper.

Steve Long is the development officer for the Association of Mountaineering Instructors (AMI). He works as a climbing and mountaineering instructor and guide, and is chair of the UIAA Training Standards Panel.

Expert Q&A

Calum Muskett is a North Wales trad warrior. He’s never happier than when sketching about above a nest of marginal RPs. He’s also a BMC ambassador. Find out more about Calum at www.muskettmountaineering.co.uk.