Turning a Stop into a Simple (or bobbin)

[Dep]

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

Correct - the total amount of energy to dissipate over a pitch is given by mgh (mass x little-g x height) little=g = 9.81 m/s ~10m/s
The total rate of energy dissipation is the total energy over the time taken to descend.
[as an aside the wattages dissipated across the cams get surprisngly high for big people at speed]

http://www.wcms.org.uk/pages/scary_srt_physics_feb2005.shtml

But the issue is that there is a critical speed beyond which the Stop will not bite, this has nothing to do with energy dissipation and everything to do with how friction varies at speed.

Also, the stop requires a retarding force on the rope-tail to make the rope wrap tightly around the cams, thus massively increasing the area over which this frictional force is applied.

In theory a stop can be stopped at ANY speed just by grabbing the rope and pulling it tightly agaonst the cams - the only trouble is this requires teflon hands - and again we come back to a maximum critical speed beyond which you cannot grab the rope without flaying your hand - and still being unable to grasp it.

 

[ian.p]

i thoght the whole point of a stop was that it didnt need a force aplied to the rope to stop it once the handle is released given that the whole point of the stop is to stop when the operator is for whatever reason unable to controll the rope ie a huge rock has just fallen on me im unconcois but its all good cos im using a stop which was designed with a dead mans handle for precisly this sort of ocurence if you had to maintain a controll of the rope in order to make the cams bite this would seem to make the stop fairly pointless. 

 

[cap n chris]

Sometimes.

 

[ian.p]

well i gues somtimes is better than no times if it doesnt work why does anyone use it? (i use a simple partly cos i  couldnt afford a stop)

 

[potholer]

But the issue is that there is a critical speed beyond which the Stop will not bite, this has nothing to do with energy dissipation and everything to do with how friction varies at speed.

Well, I was replying to the 'cube of the speed' suggestion.

In terms of the autolock biting, there can be combinations of wear, rope size, rope surface nature, and rope tension where it doesn't bite. To the extent speed is a factor, it is just another factor.
Even if high speed is a factor in autolock failure, it's also an indication that something else has already gone wrong (very quick rope and/or loss of control).

[quote author=DepIn theory a stop can be stopped at ANY speed just by grabbing the rope and pulling it tightly agaonst the cams - the only trouble is this requires teflon hands - and again we come back to a maximum critical speed beyond which you cannot grab the rope without flaying your hand - and still being unable to grasp it.[/quote]
Which makes it like basically any other descender, apart maybe from some more advanced autolock, or a descender backed up with an additional device.

The autolock on a Stop is effectively part dead-man's-handle, and part automatic parking brake, but it isn't perfect at either of those things, nor does it claim to be.
Indeed, the fact that it can't act as a static brake on some sizes/conditions of ropes should make it pretty clear it's unlikely to act as a fantastic dead-man's handle on them either, and it's not surprising that there may be some extra conditions (possibly including speed) which would result in it not stopping a descent even if it might just hold as a parking brake on the same rope.

 

[Dep]

But the issue is that there is a critical speed beyond which the Stop will not bite, this has nothing to do with energy dissipation and everything to do with how friction varies at speed.

Well, I was replying to the 'cube of the speed' suggestion.

Indeed, you actually beat me to it.

In terms of the autolock biting, there can be combinations of wear, rope size, rope surface nature, and rope tension where it doesn't bite. To the extent speed is a factor, it is just another factor.
Even if high speed is a factor in autolock failure, it's also an indication that something else has already gone wrong (very quick rope and/or loss of control).

The autolock on a Stop is effectively part dead-man's-handle, and part automatic parking brake, but it isn't perfect at either of those things, nor does it claim to be.
Indeed, the fact that it can't act as a static brake on some sizes/conditions of ropes should make it pretty clear it's unlikely to act as a fantastic dead-man's handle on them either, and it's not surprising that there may be some extra conditions (possibly including speed) which would result in it not stopping a descent even if it might just hold as a parking brake on the same rope.

Definitely - dead-man's handle it surely isn't.

To some extent I think the problem is in the name. I'm sure this has been said before in this thread.

"Stop" suggests that it should do just that.
Other possible names might be "Creep", "Maybe", etc,

Seriously though it's probably best to just call it a "Petzl Descender"...

 

[martinr]

To some extent I think the problem is in the name. I'm sure this has been said before in this thread.

"Stop" suggests that it should do just that.
Other possible names might be "Creep", "Maybe", etc,

Seriously though it's probably best to just call it a "Petzl Descender"...

Petzl Go! (as in squeeze-the-handle-and-go)

 

[ken]

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.

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.

Well ya, I don't want that to happen 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.

I hate being confronted with my mistakes :-[

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.

Ok, I'll try this

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.