UKC

I saw this Instagram recommended by an American climbing website: https://www.instagram.com/p/CMgB34BjBT-/

The author is a guide and claims to be putting to bed various myths about the physics involved in a fall. I find his calculations dubious, based on false assumptions and drawing spurious conclusions. But, perhaps I'm missing something? What do you think?

It would be great in particular if someone with an engineering brain can explain whether a FF1.9 really does involve much higher forces than a FF2, and address the issue of whether or not clipping a "Jesus piece" actually invites a high fall force than not doing so. Also, comments on the specific set ups. There is also a post by this guy on using a similar system for trad climbing.

Thanks!

The post is correct, but only for a direct lead belay off the anchor. With a single piece taking a FF1.9 fall, the force on the piece is double the tension in each stand of the rope. With a FF2 fall onto a munter hitch direct on the belay, only one strand is in full tension and so the force on the anchor will be much lower, added to which the load will be shared between two pieces.

Edit: of course, clipping a Jesus piece when you're just belaying direct off the harness is still a very good idea, since otherwise such a fall is going to be very hard to hold.

Where did this 'Jesus' name originate? America I suspect...

Mmmm.  I'm not at all sure I agree with the instagram post.  For a start, his table of forces seems quite high: he has FF1 putting 6kN on the climber and 11kN on the top anchor.  Petzl reckon those forces are 4kN and 6kN respectively, tested with a grigri (which is a reasonably harsh catch) https://www.petzl.com/US/en/Sport/Forces-at-work-in-a-real-fall

In an ideal system, then yes, his fundamental point - that there's a greater force on the jesus piece than you would put onto the anchor if you didn't place that piece - is correct.  

In the real world, friction and slippage do so much that you won't get any where near those forces.  The Petzl experiment shows that even in a FF1, the force to the belayer never really goes above 2kN.

But there's another point here - the jesus piece directs the force on the belayer upwards, which is what they expect.  Without it they will be twisted through 180 degrees with a high chance of losing control.  This chap seems to be belaying direct off his anchor with an italian hitch, which kind of gets around this problem, but it's not a very common way to belay.

Lastly - so what if the jesus piece fails?  If it is not placed than you definitely need to deal with FF2 hitting your anchor.  If it is placed, and it fails, then it has just dissipated most of the force before it hits the anchor, and put you in exactly the position that you would have been without it but with a falling climber who is now moving much more slowly.

Playing devil's advocate - what if the failure of the jesus piece compromised the anchors or the belayer?  I guess that if the jesus piece is close by the anchor, then it could affect the ice or rock that the anchor is placed in?  Or hit the belayer..?

I suspect it's what you scream as you pass it?

I believe it was Palestine...

I'm a bit surprised he quotes Fall Factor 1.9. That means the leader climbs quite a long way before placing the first ice-screw runner (having already clipped the Jesus Piece which is part of the anchor). If the leader was concerned about the forces on the Jesus Piece an earlier first ice-screw runner would solve that. Higher up the pitch the runners can be more spaced of course without increasing the forces.

I've rarely climbed steep ice and haven't used ice screws as runners for quite a few years, but I regularly clip the highest anchor point as a "Jesus Piece" on rock. I normally only do this where the highest anchor is pretty high (at least 1.5m above the belay point), it's a solid anchor (otherwise I wouldn't normally have gone that high to place it) and would hope to place the next runner pretty soon afterwards so the fall factor onto the Jesus Piece would not normally exceed about 1.0. These days I normally try to have three anchor points for the belayer as well, the "Jesus Piece" being just one of the three. All that assumes ideal conditions of course - often compromises are necessary.

Interested in whether others would consider what I do as safe, or whether it would be better always to find an early "Jesus Piece" runner independent of the belay anchors. The problem with this is that I won't normally find as good a placement, otherwise I would have already chosen it as one of the anchor points.....

Martin

This is a common misconception which we discussed at length on MP recently.

Firstly you can't dissapate force. Secondly the energy required to break for example a Rock 1 is the energy gained by a climber falling 5cm, in other words negligeable.

Dissipate is the wrong word to use with force in this case, I agree.  Better would be if I'd said that the deceleration of the falling climber by the first piece would reduce the maximum force exerted upon the anchor.

Your value of 5cm of fall is suggesting that you would only do 50J of work to break a Rock #1, which at 7kN suggests that it only extends about 1.4cm before it fails - is that about right?  

But is it not the case that much more work than this will be done in the extension of the rope whilst the nut is holding, though?

That would be highly counterintuitive. Are you saying that, in theory, if you were to able to place a succession of Rock 1s at 5cm intervals, and clip them all with a non-dynamic belayed cable leading to an 80kg weight, then dropping the weight from only 2.5cm above the top piece would snap the whole lot in turn?

Just realised as well, that it isn't anything to do with the energy required to break the piece, but rather the energy dissipated via the dynamic rope prior to the piece breaking. This could be anything up to marginally short of 100% of the fall, given enough of a dynamic component, which would leave an effective fall distance of the height of the piece above the belay.

"There are a lot of myths and misguided notions about ice climbing systems and strengths. All of this can be debunked with a general understanding of climbing physics."

Perhaps I missed it, but I didn't see in the post which particular myth or misguided notion he was trying to debunk.

I haven’t read it all properly but his third statement:

is jibberish.

1. What does “dissipate a maximum impact force” mean?

2. The test is that in a Fall Factor 1.77 fall of an 80kg mass, the peak force should not exceed 12kN.

It is a set test to give a measurable and comparable result. It’s not applicable to every and all circumstances.

I shall read it properly later.

That would be an ecumenical question…

I've chucked the bit of paper away but the nut actually held 10.2kN, the extension and thus energy is suprisingly low.

Other parts of the system MAY play an important role but the piece itself failing removes virtually no energy from the system. There is a paper from Beverly about sequential failure and ropes which covers this.

Probably!

If you do loads of testing with a hand-powered rig you soon get a good feel for what takes loads of energy, a stretchy climbing rope you are pumping for ever, a karabiner just lean on the handle and ping!

It's an interesting setup but not sure where I'd ever use it in practice - bolted multipitch climbs maybe? 

As far as the physics goes, it has a bit of the "we model the climber as a frictionless point mass" about it. Like others I suspect the real world forced would be far lower than those quoted.

A fall onto a runner exerts twice the force onto said runner because the belayer opposes the initial force with an equal and opposite force (minus some friction between the rope and the carabiner).

If you decide to clip the belay anchor to facilitate catching a fall it should be bomber. If your belay anchor is sketchy and there is a risk of a factor 2 fall you're much better off belaying directly to the anchor with a Munter Hitch until the leader has placed a bomber runner. Munter Hitches are a great way to progressively brake a falling climber

Why would it not be better to have the belayer in the system too? Surely they will absorb some of the energy? Providing they make the catch! 

The term "Jesus nut" was coined by John Long, who referred to the use of the term among helicopter pilots and mechanics as some hardware---a nut in fact---that had better not fail.  Long's promotion of the concept in the context of climbing involved placing something really good soon after the anchor as a way of protecting the anchor.  I don't think Long ever intended the term to be extended to one component of the belay anchor itself and indeed the concept is nonsensical if protecting the anchor is a goal.

Making the catch on a factor 2 fall can be extremely traumatic, much more so than most people imagine. The belayer will most likely be pulled straight onto the anchor, and will be unable to control the braking action as they'll be struggling just to hold the rope.

If the climber falls directly onto a Munther Hitch on the anchor, it's much easier to brake progressively. In fact it's not possible to brake suddenly with a Munter Hitch. Obviously thin leather gloves are big help.

If you have a small person belaying a big person it becomes a no-brainer.

More likely Rome or somewhere Roman since Jesus is the latinisation  (if that's not a real word then it is now) of Joshua.

(And Joshua is of course merely the anglicisation of a name that would originally have been in Hebrew or Aramaic - but it's much closer in sound to the "original" than Jesus)

Thanks for that. I thought it must be an American source. So un-British!

Well, it seems I was missing something. Thanks for the thoughtful replies.