Turning a Stop into a Simple (or bobbin)

[cap n chris]

Stops exhibit interesting behaviour on pitches in excess of 30m, as Ken has alluded; there are several differences with long(er) pitches.

I accept that there may also be additional factors as those mentioned above such as glazing, choice of braking method etc..

Rope elongation/bounce is the main difference on a long pitch - something which you won't notice affecting your descent progress until you've tried it - the elongation of EN1891 semi-static is sufficient over a long pitch to result in a bounce effect which creates peaks of load on the rope which will overcome the friction supplied by the Stop surfaces - this can result in an alarmingly jerky descent, often sufficient to even pull the rope through your right hand fast enough to create rope burn EVEN if the Stop handle is being gingerly depressed: this disturbing characteristic will often result in people who are doing long pitches for the first time having a traumatic time and almost certainly moments of uncontrolled plummeting, especially if they are of the "squeeze to descend" handle-operation type: it can be overcome by knowing it is likely to occur and altering your use of the Stop proactively by ceasing your descent at around 30m and waiting for the rope to settle; then hold the rope in your right hand nice and low by your side and tightly - subsequently squeeze the handle quite boldly and FEED the rope with your right hand rather than allowing it to slide through your palm; release the handle so that friction is restored and then SLIDE your right hand down the rope to hold the next metre or so for feeding; repeat the process, stopping any bouncing each time so that there are no peaks of load which will upset the friction on the Stop surfaces.

The problem, as Ken has described well, is the uncontrolled descent when the rope is fast enough to burn your right hand!

Another characteristic of long pitches is that you won't know about any marginal friction issues with your (worn) Stop until the weight of the rope from below starts to become less assistance to your braking style as you descend and the rope weight becomes negligible; to begin with you'll have a nicely controlled descent which will tend to become faster and faster and bouncier and bouncier with jerkiness and slippage kicking in to catch you out significantly once past 30m, on 10mm and probably kicking in well before that (i.e. 20m onwards) on 9mm. If the rope has patchy changes in friction resulting from mud or moisture alternating along its length that will compound the unpredictability of your descent.

Abseiling? - piece of piss!

Not necessarily.

 

[potholer]

it very well could be something else. 
The thread was about stop versus simple and if anyone has had problems, which was better or worse in which situation.

The thread was initially about turning a Stop into a Simple.

Concerning Simple vs. Stop, it may well be that a Simple is a bit smoother for descent than a Stop.

However, if something odd and external to the descender caused a sudden rushed descent on one occasion, that's not necessarily an argument against a Stop unless it's known what happened, and that it wouldn't have happened with a Simple.

 

[Walrus]

So on a long pitch would it be better to turn the Stop into a Simple (temporarily) so you could handle (no pun intended) it better, or would the Stop's handle be of more use?

 

[damian]

... by ceasing your descent at around 30m and waiting for the rope to settle; then hold the rope in your right hand nice and low by your side and tightly - subsequently squeeze the handle quite boldly and FEED the rope with your right hand rather than allowing it to slide through your palm; release the handle so that friction is restored and then SLIDE your right hand down the rope to hold the next metre or so for feeding; repeat the process, stopping any bouncing each time so that there are no peaks of load which will upset the friction on the Stop surfaces.

I would argue that this will actually cause bouncing and make the descent more awkward precisely because you are stopping and starting. Far better, surely, to descend slowly and at a consistent speed to minimise any bounce. To this end, the skill of feeding the rope below into the descender at a constant and steady pace needs considerable practice ... but it is very effective.

 

[darkplaces]

... by ceasing your descent at around 30m and waiting for the rope to settle; then hold the rope in your right hand nice and low by your side and tightly - subsequently squeeze the handle quite boldly and FEED the rope with your right hand rather than allowing it to slide through your palm; release the handle so that friction is restored and then SLIDE your right hand down the rope to hold the next metre or so for feeding; repeat the process, stopping any bouncing each time so that there are no peaks of load which will upset the friction on the Stop surfaces.

I would argue that this will actually cause bouncing and make the descent more awkward precisely because you are stopping and starting. Far better, surely, to descend slowly and at a consistent speed to minimise any bounce. To this end, the skill of feeding the rope below into the descender at a constant and steady pace needs considerable practice ... but it is very effective.

I found the way Cap 'n Chris describes but excluding handle bit gives good measured control. Yeah if you stop and start suddenly some bounce will occur but slowly and with practice it really makes you feel in control of the whole procedure. Letting the rope run though my hand makes me fill I have lost grip. I'v been teaching some DP'ers  SRT recently and they commented that the feed, stop regain some rope and feed in again meathod to be better then letting it run though the hand. What ever works for you I guess. But its all worth trying, and doing what works for you.

 

[Dep]

I like upsetting people and I tend to be a little blunt sometimes so here goes nothing new.
Ken, sounds suspiciously like control was not being maintained by the right hand, the handle is not the break your right hand is (I'm sure you know this). Are you saying the friction in the device with breaking krab was not surfactant to control or halt the decent?

...
there wasn't ANY breaking with a krab or the stop at all. the krab was on my leg harness and the stop in my central. by the timeI realized what was happening I had burns on my hand and then everything started giong downhill very fast.
...
Couldn't pull the rope over the stop (couldn't pull it up because it kept slipping and there was a weight on it somehow) and i free falled.
...
The speed picked up and I couldn't pick up thre rope to lock it over, trying this I burned my hands. Let go of the handle and it went faster......then I screamed :spank:

Sounds like operator error to me from your description Ken.

... I couldn't pick up the rope ...

NEVER take your right hand off the rope - that is what controls your speed.

If you're going fast enough to become uncomfortably warm in the palm of your hand then you are going too fast.

This isn't really anythng to do with clutch and plummet - this is basically loss of control whilst abseiling. Had you done this with a simple, Fig-8 or Italian Hitch you would never have made the mistake in the first place.
But not the fault of the stop, just lack of training combined with misuse. Mind you Petzl cold do a hell of a lot better with their documentation, forget the wordless diagrams and just use pain old fashioned words to explain the 'pitfalls' (no pun intended) of this device.

LesW also made the point about teaching people to control speed wityh the right hand and/or braking krab and not relying on the stop's red-handle 'brake', Darkplaces says much the same thing.

Two weeks ago I taught my 12 year old son to abseil for the first time - I deliberately chose a fig-8 for his first go as it teaches the idea that you NEVER let go of the tail-rope. (I did also take the rope tail up into a seprate belay just in case so in the worst case he could only ever fall a few feet into the bight)


I've said this before and I'll say it again: The analogy is like the clutch/throttle balance on a car; red handle is not a brake but a clutch, the throttle is the variation in friction through your braking krab if you have one or your right hand feeding the tail-rope into the Stop.

The friction across the stop and braking krabs reduces at higher speeds - not sure of the physics here but it is or should be empirically and hopefully instinctively obvious that there is a critical speed beyond which you will loose control over the rope and be unable to stop it. Not having your right hand on the rope and doing the main work is one way to get yourself into this situation.

Once moving the 'brake' isn't that good, especially on a moving rope and even worse if the stop is worn or the rope very new. Releasing the handle has little effect unless you also pull back on the tail-rope with your right hand which causes the rope to bite harder across the cams and increases the frictional force.

 

[docfunk]

Regarding the "bounce" on long decents, when I was working on the second severn crossing I had a single working line of 175m9the piles are 150ish)with no re-belys,and was carrying a36v hilti drill with spare battery various bolts and spanners and a flask of coffee whilst abbing down to say halfway there was quite a bit of bounce, but not too much to worry about ,yes my stop did get a bit warm but again nothing too alarming,what I am trying to say is if you know what you are doing,know about rope stretch and how your stop reacts to wet/dry/well worn rope there should be no problems at the end of the day 99.99(recurring)% of the time it is lack of knowledge and experence that leads to the afore mentioned mishaps,I am not pointing fingers but perhaps people should get to know there kit inside out before using it in places where,if even a tiny mistake happens it is game over.

 

[Dep]

...
but perhaps people should get to know there kit inside out before using it in places where,if even a tiny mistake happens it is game over.

Well said.

 

[francis]

I find that for getting a smooth descent it helps to "wank" the controlling rope instead of gripping it constantly. This is particularly easy with a handy as it has so much friction that you don't have to grip the controlling rope very hard.

Francis

 

[footleg]

I find that for getting a smooth descent it helps to "wank" the controlling rope instead of gripping it constantly. This is particularly easy with a handy as it has so much friction that you don't have to grip the controlling rope very hard.

Always wondered why it was called a 'handy'  :ang:

 

[potholer]

Another characteristic of long pitches is that you won't know about any marginal friction issues with your (worn) Stop until the weight of the rope from below starts to become less assistance to your braking style as you descend and the rope weight becomes negligible; to begin with you'll have a nicely controlled descent which will tend to become faster and faster and bouncier and bouncier with jerkiness and slippage kicking in to catch you out significantly once past 30m, on 10mm and probably kicking in well before that (i.e. 20m onwards) on 9mm.

On a long pitch, the change in friction from rope weight below should be gradual, and you should be aware of how it's changing from the way you adjust your grip to deal with it.
Personally, I prefer my descents to be roughly constant-speed, and to have an upper speed limit determined by me, not by the weight of the rope below me.

Francis is right - the vibrating-right-hand technique does usually work pretty well on descents where bounce could be an issue. It's much easier than stopping and manually lowering oneself in stop-start steps, which I'd reserve only for descents that couldn't be controlled any other way (like extremely quick rope, or abseiling-for-two after self-rescue).

I'm still puzzled about the description of the problem descent. I can understand a heavy caver on a Stop finding a given bit of rope a bit faster than an average caver might find it. However, going from having to actually feed rope in to a descender at the top of a pitch to an unstoppable fall over a distance of only 45m or so, where the change in weight of rope below should only be 3-4kg seems hard to understand, given how much friction a gloved hand and braking krab can provide. Even if bounce were involved, bounces go down as well as up, and stopping descent and putting a lock on in the low-tension phases of a bounce shouldn't be hard.

 

[potholer]

[quote author=c**tplaces]I'v been teaching some DP'ers  SRT recently and they commented that the feed, stop regain some rope and feed in again meathod to be better then letting it run though the hand.[/quote]
I dare say it feels more controlled initially, but it's not something I'd encourage people to carry on doing when not actually necessary, especially if I was ever going to be waiting for them to descend a decent-length pitch, let alone a whole long series of them.

 

[Dep]

...
given how much friction a gloved hand and braking krab can provide.
...

If it they were being used as does not appear to be the case here...

 

[Dep]

...
given how much friction a gloved hand and braking krab can provide.
...

If they were being used as does not appear to be the case here...

Sorry - no modify button again.

 

[ken]

If they were being used as does not appear to be the case here...

Sorry - no modify button again.

[/quote]

if you're refering to my post,  :spank: first read, then write.
If not, ignore.
The braking krab was attached to my leg harness.....and why the stop sliped, I still don't know. I'm D**ned sure not gonna try it again just to prove a point.
After three years of SRT, (I've never been on a ladder untill last week) I can't explain it myself. It was the longest pitch up till now that I've done (and it could have been the rope because it wasn't mine and I didn't look at it ahead of time).
And personal rescue on a 10 meter pitch I've also done with 55 kg (rescue partner) and it never slipped like that, even then.

 

[potholer]

... (and it could have been the rope because it wasn't mine and I didn't look at it ahead of time).

Did anyone else notice the rope being particularly variable in friction?

 

[ken]

... (and it could have been the rope because it wasn't mine and I didn't look at it ahead of time).

Did anyone else notice the rope being particularly variable in friction?

yes, but only after I was down and could holler up that I had problems. They weren't as bad as my accident, but noticeable. Never even thought about that 
ok, I should have thought before I wrote  :spank:

 

[Dep]

If they were being used as does not appear to be the case here...
Sorry - no modify button again.

if you're refering to my post,  :spank: first read, then write.
If not, ignore.
The braking krab was attached to my leg harness.....and why the stop sliped, I still don't know. I'm D**ned sure not gonna try it again just to prove a point.
After three years of SRT, (I've never been on a ladder untill last week) I can't explain it myself. It was the longest pitch up till now that I've done (and it could have been the rope because it wasn't mine and I didn't look at it ahead of time).
And personal rescue on a 10 meter pitch I've also done with 55 kg (rescue partner) and it never slipped like that, even then.
[/quote]

Hi Ken, yes I was referring to your post.

As I understand it you started sliding too fast for the brake on the Stop to be effective, in effect your Stop became a Simple. The rope was moving too fast for you to hold it or stop it with your right hand.

My contention is that a correctly used braking krab and right hand technique do not allow what happened to you to happen - ergo you did something wrong. Presumably your post in this thread is an attempt to find out just what it was that was wrong so you (and others) can avoid doing it again.

The speed of descent should always be such that you can stop the rope with your right hand by pulling the rope-tail upwards against the braking krab. This method works equally well for a Stop, a Simple or even the good old fashioned Fig-8, it does not rely upon the red-handled 'brake' - and therefore works even if the 'brake' fails.

In your post you said that you could not 'pick up the rope'. This suggests that either you weren't holding it at all (silly) or that it got away from you as you accelerated (careless), in which case you exceeded the critical speed at which you could no longer control things. Both options come under the heading of user error.

The problem with critical points is that all is well until you cross what may be an unseen boundary - and then it all goes pear-shaped, suddenly and with no chance to recover.

I have tried (and seen others) placing the braking krab on my leg-loop but I didn't like it as it did not feel solid enough.
With the braking krab through the D-ring, when it is pulled tight the Stop-Dring-brakingkrab assembly all pulls tight and effectively becomes a single fairly rigid unit, against which you can pull the rope-tail with your right hand.
Once you pull out the slack in the metalwork there is no further movement allowed in the various bits and so all the force you apply goes into pinching the rope against the braking krab.

There is no question but that this makes a very good braking set-up.

On the other hand, having the krab attached to your harness leg-loop is not rigid.
Pulling on this rope is lifting your leg up, or pulling your harness loop into the flesh of your thigh, that's where the force you are applying is going - instead of being directed against the rope where it rubs the braking-krab.
Similarly lifting your leg/hip up reduces the tension in the rope and thus the friction against the braking krab.

IMO this method is less efficient as not all of your applied right-hand force goes where you want it to.
But with the braking-krab on the D-ring this is not an issue.

So on the basis of what you wrote (which I read very carefully) this is what I think happened. You discovered (the hard way) a niggly non-obvious critical aspect of braking.

You were descending in your perfectly OK way when an unexpected variation in rope friction suddenly put you on the wrong side of this critical point. And once beyond this point a normally insignificant shortcoming of your braking method meant that you lost control and could not regain it.

The great advantage of this forum is that unusual occurences can be discussed and people forewarned of something that they might not otherwise have anticipated. What I take away from this is the knowledge that abbing at a safe speed is great until something unexpectedly changes to put you over the speed limit - so I will always make sure I have something in reserve, abseiling as fast as possible (allowing of course for rope-glazing, heating etc) is not a good idea and I will slow down a tad.

... and why the stop sliped, I still don't know ...

This comes back to what I said about a critical point or speed.

the coefficient of sliding friction is always less than that of static friction. Basically once something is already moving the friction reduces. In this case once the rope starts traveliing over the Stop cams at a certain speed nothing is going to stop it, especially if the braking-krab is ineffective (because you can't hold the rope) and the rope allowed to run freely into the Stop; there is nothing to force the rope to bite against the cams.

 

[Geoff R]

Following Deps thoughts, I ponder that the Stops own brake system must have only a certain capacity to absorb energy for any given rope / body weight / rope tension on its tail. There may indeed be other variables specific to the Stop  (I am excluding outside influence adding to this brake capability such as using a brake krab or grabbing the tail rope tighter with your gloved hand). 

If my ageing memory is correct, as velocity increases, the amount of brake energy needed to be removed to maintain a certain speed and not accelerate more, increases in close proximity to a cube law.

Get an object running down a hill and as its speed picks up, you quickly find in a very very short time scale that you have no chance in hell of stopping it. 

The action of the Stop also suggests its brake capability would vary with rope velocity passing through it 

If my ponders should be correct, this could suggest that with no outside influence (such as your right hand adding to the Stop's own braking force) that there could be a certain descent velocity for a given set of conditions where the Stops brake on its own just can not cope.  A speed of no return so to speak (until you hit a knot or the ground or do something like grab the tail rope )

This speed would obviously vary with differing circumstances. 

However just using the Stops own brake there is one other possible influence 

The Stop of course has a handle where you can exert more braking pressure manually; so I ponder that in the case of no other outside influence (eg you have dropped the tail rope so cant use this to brake) that there may be a point where excess pressure on the Stops handle effectively becomes useless, as the maximum brake force you are able to physically apply by the handle at a particular high rope velocity is just not sufficient to slow you. This assumes of course you do think to pull the handle correctly (rather than apply the plummet option)

Lots of ponders here  ....  which may equal peoples experience, or may not ...  :doubt:





 

 

[potholer]

Energy is proportional to the square of the speed.
However, energy is also proportional to change in height.
My first stab at analysis would be:

If someone of mass M is travelling at a given speed X m/s, to maintain that speed, the descending system (descender and rope) has to absorb an energy of MgX over 1 second (since that's the energy gained in 1 second by travelling at X m/s.
If the person is descending at 2X m/s, the descending system has to absorb energy of 2MgX over 1 second.
For maintaining speed, energy absorption should be linearly related to speed