UKC

Over the weekend I was climbing a multipitch trad route with bolted belays. There were four of us climbing in two teams. After 7 pitches it was getting late so we decided to back off, with the knowledge that there was an escape route at the top of pitch 5. The other pair went first and didn't find the walk off route, so they continued abseiling. They had two 50m ropes of different diameters. My climbing partner and I abbed 2 pitches then found the escape route and walked down. We got back to the base of the crag in the dark, with no sign of the other team. We grabbed some food and headtorches for them and walked back up to the top of pitch 5.

Turns out they'd ended up having a few issues with their ropes. Firstly, they had threaded the thinner, stretchier rope through the anchor. As they abseiled the thin rope stretched and the knot moved further down on the thick rope side, meaning that thin end of the rope ended up being considerably shorter. I think they had a bit of faff reaching the next anchor as a result. We met an instructor who helped them out and told them that when abseiling on two ropes of different diameter it is best to thread the thicker diameter rope through the anchor so that if there is any movement due to differences in rope stretch, the knot will come up hard against the anchor (potential warning here for cases where the knot can pass through the anchor). This was new information to me and is lesson 1.

Secondly, while abseiling they hadn't been keeping track of ends. When finishing an abseil the two strands will be nicely separated by your belay plate (provided you haven't been spinning around on the way down). The first person can keep the strands separate while taking ab gear off the rope and has some time to get rid of any twists all the way to the ends of the ropes. This can happen while the second person is abbing down- it doesn't take long and can save a lot of faff later on. As soon as the ropes are released, the twists are free to go back up the rope. This is the second issue my pals had.

With both of them at the lower anchor, they tried to pull the ropes and discovered they were stuck. It was dark at this point and they didn't have headtorches, so it was a little unclear whether this was because the thin rope got pinched in a groove near the top, the twists introduced to much friction or the knot got stuck (they were using two overhand knots). At any rate, they then found themselves having to climb back up the ropes.

For this, they free climbed one at a time with guide mode self belay. This was working nicely until they came up against the badly twisted ropes and the knot that had moved down. For the twisted ropes they untwisted them by spinning around, and with the knot they stood on a ledge, released the guide mode plate and asked the instructor who passed them to feed the rope through the anchor until the knot was back in the right place. Eventually everyone made it to the ledge and we all walked off from there.

In short, my friends faced a few problems as a result of stuff that is a little subtle and I haven't thought about before. Abseiling with different diameter ropes means that you need to consider rope stretch as well as knot choice carefully. Twisted ropes can be a pain when abseiling, but if you have to reascend the ropes they can make it much more awkward and it is worth untwisting them when the opportunity arises. Headtorches are awesome (less subtle).

I am curious to hear the thoughts of the hive mind. Have you encountered any issues with abseiling using different diameter ropes? Is there anything you would have done differently if you had been in my friends' shoes?

I would highly recommend the book Down by Andy Kirkpatrick.

It covers all these issues and many others in detail.

Hi.

It is hard to ensure the thick rope is through the ring on each rap as we tend to alternate ropes as part of process on multipitch abs.

When abseiling on somewhat different diameters, like 9 and 8mm, both members put either their plates or prussiks on at the same time. The rope then can't creep for the first person down. So the first person will get down to the next station.

The creep happens because the thinner rope slides more easily through the plate. The second person just needs to be aware of this and relax their hand slightly on the thicker rope.

With a major diameter difference, say 9 and 5.5mm, different techniques are used. See multipitchclimbing.com

What was the diameter of the ropes? I have done multiple abseils with different diameter ropes (a 9.1 matched with a 8.5) and did not have these problems. I had only one a time a slight move of the ropes towards the side of the thicker rope in an abseil over a large overhang where you do not touch the rock. But the move was minimal.

Thanks, I'll check it out. I've only read his autobiographical stuff.

Thanks, that's a great tip about the belay plates. If I'm down first I usually start threading the pull line through the next anchor while the second person is descending, so as you say it means the pull rope alternates.

For tag lines I have used a few of the common canyoning rigging techniques but I'd be very reluctant to try some of those in a mountain setting. I think the snag probability would be pretty high. I'll have a look at that link.

Seems odd that the ropes became SO twisted.  I know this can happen due to the way that some twin bolt belays are arranged but it can also happen big-style if you join the ends of the rope together.  Did your friends do that, rather than knotting each end separately?

The thin one was an 8.1. I'm not sure about the thicker one, but I would have guessed 8.5 to 9.

I'm not sure, but it seems likely. I had spotted them knotting the ends together on an earlier abseil.

I guess if you grab the end of one rope and pull (or thread through the next ring) things could get very twisted. Rather than sort the twists out that have arisen below the plate, then pull/thread.

I frequently thread through the next ring as part of pulling down.  If you've kept the strands well-managed per comments above, it keeps them separate, and is a bit quicker.  You do have to remember that the "pull" rope will alternate (I tie a different knot in the bottom of the pull rope to manage this)

I just got some more information. They threaded the anchor so the knot was on the side away from the rock. When they pulled the knot side it pinched the other rope strand. This could have been avoided by having the knot on the rock side.

I would have set the ropes up with one as a tagline if there was any great difference in the diameter of the ropes and abseiled on the thicker one. Bit more likely to get jammed in a crack but you are only likely to get an inch or two of movement.

Also take headtorches on a route which I may not finish in daylight (from experience at uni, stumbling about on top of a crash in the dark is a pretty frightening experience!). 

I used to abseil with an 11mm and 9 mm pair of ropes regularly. I used a double fishermans knot, never had any problems.

I regularly still abb with a joined 8mm rope and a 9mm rope, no problems.

The infamous overhand knot was banned in our rescue team, for abbing, after one came apart after a few people abbed down the same ropes. Having witnessed that, I will never use it for joining ropes in an abseil. 

What do you mean by 'came apart'? Do you know how this happened? I didn't think this was supposed to be possible. Small risk of the knot rolling or creeping, yes (hence dressing it well and ensuring a decent tail, or possible a second overhand snugged up against the first to be very sure), but I hadn't heard that the overhand could come undone if properly tied and dressed.

This is a great idea 

It was tied together by an outward bound instructor, also team member, so presumably done properly, but I didnt see that. Several people abbed on it, including me, then it was pulled down, at that point it separated. Its use was then banned in our team. I refuse to abb on it now. I have heard American climbers call it the Euro death knot, but don't know why that is.

the term euro death knot is used in two ways.

1. To refer to the normal flat overhand. It just looks like it will kill you. But won't if dressed and with 45cm tails

2. To refer to the flat figure of eight. Which might well kill you

I have never heard of a dressed overhand falling apart. I would say it is hard to think how especially if several people had abseiled on it. It would cinch down. I would say most abs in the world are done on it, so very well tested 

http://publications.americanalpineclub.org/articles/13199808000/Fall-on-Roc...

That report says they'd left only 7 to 8 inch tails. They need to be much longer, at least 30 cm, a double overhand is safer

One wonders if they could have accidentally tied a fig 8 rather than an overhand.  Adding the extra turn is very easy to do - it's almost too easy as our muscle memory kicks in, and we do tie a lot of fig 8s.  Whenever I tie an overhand for abseiling I always ask myself to check if I have actually tied an 8.

Sounds like it was a loose knot and didn't fall apart, but pulled through - as you might expect.

With that retying it could have also been re-tied as a flat fig-8. I tied a poorly dressed flat fig 8 for my book and abbed on it. It failed and the length tail it pulled through itself was impressive.

One thing I noted in those experiments was that if an overhand rolls once it really locks down tight and will not roll again with the forces in abseiling. This does not seem to be true of the fig 8, which on rolling tightens less and hence could roll multiple times. I think there is some pull test data somewhere showing this difference.