Petzl Simple and Stop: attachment mailion modification

[potholer]

On page 45 of ACT, it's a bit vague about what has been observed to happen. There is mention of the sideplates of the descender separating below the lower cam, but without failing or being significantly distorted, which does imply it's something that's been observed in practice - if it was just speculation about jamming, I doubt there'd be as much confidence about a lack of failure or significant distortion.

It seems to be the potential for uncontrolled descent due to loss of friction when a load is removed that is their major concern, and in the rebelay-failure case, that would likely be after the lower caver had got themselves off the rope, which would at least give the upper caver time to realise what had happened with their descender and maybe slap a hand-jammer on for protection.

[Tubo Longo]

This is an interesting thread indeed. Let's see....

About the bobbins and the two cavers during a pick-off, you're right Paul: I made a bit of confusion in my post. They would be both hanging from the same end and not from two different ones, like in the rebelays failure scenario we are talking about.
I also agree this difference, during a rebelay failure, might likely chance the position of the bobbins BUT not necessarily, as you point out (enough rope in between to act as shock absorber). I say might because I'm not totally convinced, since not only the bobbins will have to change completely position, going flat horizontal, but also the brake biner will have to slack free aside enough to allow the bobbins head (or upper part, whatever you like more) to enter into the brake biner.

Now, if I would be in a situation like this, I guess my istinct would be to try to grab the rope even tighter and brake more to regain control (see the burned hand of Gerald), so keeping the brake biner in work (and in place) to some extent. But unless the rope break and/or the lower caver get off rope, I won't be worried to go anywhere because of the huge brake being the weight of the lower caver, as Potholer point out.

A main point if a situation like this (bobbins head stuck into the brake biner) happens is that the outcome will be VERY fast, possibly too fast to do anything when we would realize it: when say fast I'm really talking of seconds, since we would enter in almost total fre fall.
That's why checking our gear is so important.

[paul]

I also agree this difference, during a rebelay failure, might likely chance the position of the bobbins BUT not necessarily, as you point out (enough rope in between to act as shock absorber). I say might because I'm not totally convinced, since not only the bobbins will have to change completely position, going flat horizontal, but also the brake biner will have to slack free aside enough to allow the bobbins head (or upper part, whatever you like more) to enter into the brake biner.

I think you still don't understand my point.

If the weight of a caver were to be applied to the top of a bobbin descender - it wouldn't be horizontal but would end up pointing downwards!

The bobbin is acting as a lever with your weight at one end and the other caver's at the other end but the pivot (or fulcrum) is where the rope is exiting the lower capstan which is much nearer to you. So with the mechanical advantage of a longer portion of the lever, the weight of the other caver will win and pull the bobbin downwards (assuming no large disparities in your weight as compared to the other caver!).

As the rope is bing pulled downwards the braking krab will tend to become slack and hence the top of the bobbin could quite easily foul the braking krab. My mentioning of shock absorbing was in relation to how this fouling of the braking krab would affect the integrity of the bobbin.

I suppose you could test this by clipping a Stop (this won't work with a Simple as it would just slide down the rope!) to a suspended rope, then attach a large weight to the attachment end so that the Stop is in the normal position as when abseling. Now attach a similar large weight to the rope below the Stop and observe its new postiton... Of course in reality this change in position would not be as controlled if a rebelay anchor failed.

A main point if a situation like this (bobbins head stuck into the brake biner) happens is that the outcome will be VERY fast, possibly too fast to do anything when we would realize it: when say fast I'm really talking of seconds, since we would enter in almost total fre fall.
That's why checking our gear is so important.

I agree with you there!

[Tubo Longo]

Paul, I do understand your point. But I realize we are both making a mistake. That's it, the gear we are talking about and how is positioned.

You talk of a Stop with a brake biner clipped on the maillon. Instead I use a Simple with the brake biner clipped on the same biner of the bobbins, so to avoid the danger that the bobbins can get into the brake biner itself (which is what we're talking about).

I agree that the mechanical lever will tend to position the bobbins of the upper caver flat or even downward. My point (but I'm not a physicist, I do talk for direct experience) is that, even with the gear you're talking, that there are factors which weight in, like the difference in weight between upper and lower caver, their distance, how much dynamic is the rope and also how fast and sudden the rebelay fails or, anyway, if and how all these conditions play into the equation.

Once I had a miscommunication problem in a 50m (164') free fall pit. A caver at the bottom got that I delayed my rappel to wait for him to climb while indeed I was already on rope, at the top of the pit, just waiting to have some bags passed to me before to start the rappel. As soon as I started rappelling, the lower one started climbing and we both got the motion each other were giving to the rope as caused by some other factors (he thought the rope was moving 'cause he was moving while clipping his ascenders on, I thought the initial added weight was given by the end of the rope being tangled around some rocks at the bottom).
When the lower caver actually began to climb he did of course pulled my bobbins flat, but not downwards: at the time my brake biner was still clipped on the maillon and the bobbins didn't even got close to get stuck into it. There was some differential in weight, being me some 10 to 20 kg (22.2 to 44.4 lbs) heavier, but still I kept controll of the bobbins all time, even if I was kinda stuck for some minutes, until he bottomed again.

I don't pretend this past experience of mine to be the absolute truth, not at all, but just to means that although you are correct about the lever etc, still there might be a numbers of factors which might give a slight different outcome (in terms of the position of the descender). But that slight it might (again..) means a big difference as to where the upper caver could stand in term of the position of the descender and so to maintain the controll of the rappell, if any. Just think, in the experiment you suggest, at how different the outcome is if the brake biner is positioned your way or my way. In other terms, it might even make the difference between dying on an uncontrolled free fall or surviving with quite a bit of scare.

[fuzzy-hair-man]

I think a Simple or Stop would invert (hang pointing down) with significant weight on the the other end were it not for the brake carabiner. The brake carabiner is acting sort of like a redirect pulling the direction the rope takes more in towards the descender body eliminating as much as it can the tendency to turn upside down.

The Frieno is probably better at achieving this than carabiners.

Putting the brake carabiner in through the attachment maillon or carabiner would help as well I suspect as the brake carabiner cannot be pushed sideways(as it can on the main maillon) so the rope is pulled in closer to the descender. The friction lost because there is less contact of the rope with the descender is at least in part made up for by increased friction on the brake carabiner and further made up for by increased force squeezing the rope into the bollards on the descender generating more friction on the descender as well.

As far as the problem of the head of the Simple or Stop threading through the brake carabiner goes I tried this off rope with my gear and with all the gear on my central maillon it is pretty difficult to do. To me this accident was caused more by the caver taking shortcuts that should not have been taken. In my opinion the practice of using the rope to weight your descender seems ill advised at best (I would call it something worse but shouldn't speak ill of the dead).
The attachment on my maillon actually makes it harder (unintentional) to thread the head of the Simple through the brake carabiner, the descender / maillon joint is prevented from bending right the way back because of the added bar making it slightly more difficult to thread the head through the carabiner.

[fuzzy-hair-man]

I just thought of something you may not have thought of Tubo Longo

A carabiner is only made to be loaded in one direction only right.

A brake carabiner connected in through the carabiner that attaches your bobbin will in normal operation be loaded in the same direction as the bobbin so there is no problem.

But.... when the situation you are discussing happens the brake carabiner will be pulled sideways in a direction at roughly 90 degrees to the direction the bobbin is pulling so you have the attachment carabiner being pulled in more than one direction at once.

I am not sure the force pulling the brake carabiner sideways would be huge but it would be reasonably strong force so it is possibly worth looking at?

[fuzzy-hair-man]

I finally found a compelling reason to have my descender so that it is detachable from my harness whilst on rope.

If for whatever reason as you are descending and the rope below you gets weighted, this could be because a rebelay has failed or someone due to bad communication has started coming up or whatever. The descender will jam and you will be stopped on the rope unable to move, in normal circumstances this wouldn't be a huge deal you could tell them to get off the rope and proceed downwards once they are done. But if communication is difficult or if the person on the failed rebelay below you is unconscious then you would need to leave your descender on the rope and go down to see what's wrong or extract yourself (go up). If you couldn't do this you'd be stuck, not much fun.

So as a result I have added a wing nut to to the back of my maillon attachment which means I will be able to get it undone by hand in an emergency.