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Gri gri slippage (Read 5775 times)

mrjonathanr

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#25 Re: Gri gri slippage
November 11, 2022, 05:37:49 pm
We bought a Cinch which was great (and is a really good backup when ascending a fixed line as it self feeds) but did need a totally different technique and you couldn't just pass it to someone and expect they'd be familiar with it.

This. I bought a new grigri after going sport climbing again and wondering how likely it was a partner unfamiliar with my Trango was going to drop me. Too likely, was my conclusion. Still love the Cinch tho

Paul B

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#26 Re: Gri gri slippage
November 11, 2022, 06:04:45 pm
You bought it off me I think?

mrjonathanr

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#27 Re: Gri gri slippage
November 11, 2022, 07:54:10 pm
Yes, I did Paul. It’s great, it’s just that I don’t want to die onsighting a sport route because my partner happens to be unfamiliar with the thing.  Pretty much everyone knows how a grigri works so… I bought one.

Wood FT

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#28 Re: Gri gri slippage
November 12, 2022, 09:37:15 am
Tl:dr: Grigris don't kill people, belayers do
Guns don't kill people, GrisGris do, I'm a fucking GriGri and I might kill you!

andy_e

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#29 Re: Gri gri slippage
November 12, 2022, 11:03:35 am
Did you hear that on a documentary on BBC2 Wood FT?

mrjonathanr

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#30 Re: Gri gri slippage
November 12, 2022, 11:16:38 am
The device requires tension in the rope to engage the cam, so if this tension isn't present or is low enough, you won't engage the braking assistance.


I got dropped exactly this way, due to falling slowly because of friction, where I was out of sight and close to a bolt when I fell, by Peter Croft, of all people. He stopped me when I appeared round the roof. He thought I’d wanted slack.

Tl;dr Chris says hi.

Moral: treat it as a normal belay plate, with benefits. Hand on the brake rope at all times

andy moles

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#31 Re: Gri gri slippage
November 12, 2022, 01:25:05 pm
The part that I don't quite get regarding the friction aspect is: it makes sense that the cam does not engage initially if there's quite a bit of friction in the system, but there's never going to be so much friction that you don't still accelerate pretty fast in falling (otherwise you would have barely been able to move beforehand, and you'd be dropping so slowly it would be more like being lowered than falling.)

So as soon as your velocity is great enough to cause injury, isn't the rope going to be pulling through the device at a speed that should engage the cam regardless of friction?

Edit: thinking about this some more, I guess it's the more gradual acceleration isn't it...if friction is the factor causing slippage, rather than rope diameter or sheath condition (and having a hand on the rope of course), the cam will engage eventually, but perhaps not before you've hit the floor.
« Last Edit: November 12, 2022, 01:34:23 pm by andy moles »

danm

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#32 Re: Gri gri slippage
November 12, 2022, 04:16:08 pm
The way I try and think about it Andy is that if all the rope is moving at the same speed, it doesn't really matter how fast it's going, the cam isn't going to engage.

petejh

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#33 Re: Gri gri slippage
November 12, 2022, 04:32:49 pm
I think it’s the rate of change. Like Dan says you could accelerate to a million mph but if the rate of change was below the ROC required to activate the cam, it wouldn’t activate.

andy moles

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#34 Re: Gri gri slippage
November 12, 2022, 05:15:06 pm
Interesting, thanks.

Johnny Brown

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#35 Re: Gri gri slippage
November 12, 2022, 05:42:41 pm
I don’t agree with that interpretation. The device works on friction between rope and cam. The faster the rope is moving the more potential it has to engage the cam, if it touches it. The problem is holding the rope and device, as in the video, where the rope isn’t touching the cam sufficiently. Acceleration does have a role, in that it is more likely to disturb the path of the rope out of a poor position, but it’s not acceleration that engages the cam, it’s friction. Hence why in the vid the cam does engage eventually.

Unlike drag in air or water, solid-on-solid friction does not simply increase with speed, so the critical thing is the quality of the contact between the rope and the cam. Hence why a slow pull will engage the cam if things are in contact, but a fast one may not. But we tend to get in the habit of trying to minimise the contact for clipping.

When I tested Shunts as used for fall arrest in shunting or rope access back up, we found the behaviour at low speed was not representative of that at falling speeds, where the device required a higher load to start slipping than it did to continue slipping. But grigris don’t slip once engaged in my experience.

Moo

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#36 Re: Gri gri slippage
November 12, 2022, 07:38:20 pm
That follows because standing friction is greater than moving ( kinetic ) friction between two objects, eg its easier to keep something moving than it is to get it it moving in the first place.

danm

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#37 Re: Gri gri slippage
November 12, 2022, 08:14:52 pm
I don’t agree with that interpretation. The device works on friction between rope and cam. The faster the rope is moving the more potential it has to engage the cam, if it touches it. The problem is holding the rope and device, as in the video, where the rope isn’t touching the cam sufficiently. Acceleration does have a role, in that it is more likely to disturb the path of the rope out of a poor position, but it’s not acceleration that engages the cam, it’s friction. Hence why in the vid the cam does engage eventually.

Unlike drag in air or water, solid-on-solid friction does not simply increase with speed, so the critical thing is the quality of the contact between the rope and the cam. Hence why a slow pull will engage the cam if things are in contact, but a fast one may not. But we tend to get in the habit of trying to minimise the contact for clipping.

When I tested Shunts as used for fall arrest in shunting or rope access back up, we found the behaviour at low speed was not representative of that at falling speeds, where the device required a higher load to start slipping than it did to continue slipping. But grigris don’t slip once engaged in my experience.
Can you quantify a quality of friction? No, me neither! The tension in the rope is required as it causes the rope to try and straighten its path, the frictional qualities of the rope and cam material then come into play to create a force which moves the cam. The reason a big fall tends to activate the cam more easily is not because the rope is moving at a higher velocity and therefore there is more friction, it because of inertia. One end of the rope is accelerating rapidly, one end is not, and tension will result. Greater accelerating equals more tension as F=ma. We could of course solve this argument by arranging an away day at Lyon to do some testing!

crzylgs

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#38 Re: Gri gri slippage
November 12, 2022, 09:06:19 pm
Just hold onto the rope that is attached to your friend/loved one/relatively unknown climbing partner.

andy moles

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#39 Re: Gri gri slippage
November 13, 2022, 07:41:18 am
Just hold onto the rope

Yes, that is the bottom line here.

All the rest is academic if you've got a hand on the brake rope.

Though it's interesting to understand exactly how the Grigri works (or doesn't in particular circumstances). Especially because belaying a leader is not the only way the device is routinely used.

I realise my original question may be confusing because in all the references to 'friction' I'm talking about friction on the rope between the device and the climber, not friction between the rope and the device itself, which is evidently important.

SamT

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#40 Re: Gri gri slippage
November 13, 2022, 12:06:02 pm
"Keep a hand on the brake rope"

Yes, obv.. but isn't that the issue? (I certainly find it a problem), that its hard to quickly pay out slack without using the thumb on cam method, which entails not having your hand properly on the brake rope.

Now I have to admit, I didn't know about the fore finger under the lip method till now. I'm sure I'm pretty bad at short roping partners, as I don't like taking my hand off the brake rope (long trad upbringing).  If they needed a lot of slack quickly, I'd usually get one short rope snag, then take my hand off the brake rope, hold the whole grigri body, with thumb on cam, deliver slack, then quickly find brake rope again.   :(  I genuinely really didn't feel comfortable at all when resorting to this.  Especially 2nd or 3rd bolt for instance. They'd be on the deck before you knew what had happened if the cam didn't catch instantly, plus there is no opportunity to reflex pull in that bit of slack.
I don't really do a mass of sport or indoor roped climbing, so perhaps this is why I've never really got on with my grigri. 

However, from now on, I'll be using the finger under the lip method and 3 fingers on brake rope. , but only for that moment they need quick slack.  I'd like to think that in 30 years of belaying people, I don't think I've ever dropped anyone and I have the learnt reflex action of holding a fall using the brake rope hand.


mrjonathanr

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#41 Re: Gri gri slippage
November 13, 2022, 12:45:07 pm
"Keep a hand on the brake rope"

However, from now on, I'll be using the finger under the lip method and 3 fingers on brake rope. , but only for that moment they need quick slack.

This method is easy when you get used to it and you never stop having 3 fingers safely holding the brake end. In the event of a fall you’d naturally put your forefinger round the rope too, given its stating position. Just needs a bit of practice.

danm

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#42 Re: Gri gri slippage
November 13, 2022, 12:47:59 pm
If I'm belaying with a GriGri (which I'll admit, these days isn't very often) I make sure I have clear floor space front and back, and I can usually manage to give enough slack without having to resort to disabling the cam by paying out slack normally and taking a big forward stride if necessary. You need to be an active belayer for this to work i.e full attention on the climber, moving around and getting the hands in the optimal position, anticipating the climber clipping etc, but that's the sort of behaviour that should be encouraged anyway. Unfortunately I think many accidents/near misses are essentially a result of people who treat belaying as a break between their own activity. I've been guilty myself of this in the past and it was the one time I nearly dropped my partner, which taught me a very valuable lesson.

mrjonathanr

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#43 Re: Gri gri slippage
November 13, 2022, 01:37:45 pm
I think many accidents/near misses are essentially a result of people who treat belaying as a break between their own activity.
This.

andy moles

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#44 Re: Gri gri slippage
November 14, 2022, 07:06:06 am
"Keep a hand on the brake rope"

Yes, obv.. but isn't that the issue? (I certainly find it a problem), that its hard to quickly pay out slack without using the thumb on cam method, which entails not having your hand properly on the brake rope.

My technique for paying out slack quickly (with a GG2) is to pinch the device between thumb and forefinger, thumbing the tip of the cam nearest me, with the brake rope running so that I still have hold of it with my back three fingers. I think this is what Petzl recommends, and it feels reasonably robust to me.

What is meant by forefinger under the lip method?

Another method I've seen used is to have a firm grip further down the brake rope, so you've got a loop of slack to pay out from when thumbing the cam, which seems fine too if well practised.

mrjonathanr

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#45 Re: Gri gri slippage
November 14, 2022, 07:13:09 am
This is what Petzl recommend.

andy moles

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#46 Re: Gri gri slippage
November 14, 2022, 07:31:07 am
This is what Petzl recommend.


Gotcha. It may be I've slightly misinterpreted the Petzl diagram previously. Though forefinger under whole device feels like it does much the same job, maybe depends on the size of your hand a bit though.

mrjonathanr

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#47 Re: Gri gri slippage
November 14, 2022, 05:08:13 pm
I think the danger with the forefinger under device method is that under stress, we grab, and it’s possible to clamp the cam open in this position as thumb and finger are opposed.

A finger under the flange is offset from the thumb on an insecure feature to hold. It’s unlikely you’d exert enough force to keep the cam open in a fall this way.

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#48 Re: Gri gri slippage
November 14, 2022, 06:16:30 pm
Enjoyed the original video. Clear and entertaining like most of his work. Familiar issues to most here I would guess but it’s good to point out it is usually a combination of several factors that lead to problems. It is intuitive that a new, slick, thin rope is going to reduce the likelihood of locking but I hadn’t thought of the effect of rope drag.

I think the danger with the forefinger under device method is that under stress, we grab, and it’s possible to clamp the cam open in this position as thumb and finger are opposed.

A finger under the flange is offset from the thumb on an insecure feature to hold. It’s unlikely you’d exert enough force to keep the cam open in a fall this way.

I was going to post something similar but got distracted by the need to fix the fridge!

An instinctive panic grip whilst holding the whole grigri - fingers underneath, thumb on the cam - is a known mode of failure.

I had a quick play with a well-used grigri 3 and nearly new 9.2mm dry treated rope, a fairly slick combination. Holding the grigri with just one finger underneath and thumb on the cam, it still seemed possible a panic grip might defeat the cam. With the index finger hooked under the lip - the current Petzl recommendation - there is no way I can stop the cam locking against a sharp-ish tug.

I started with a sticht plate then tubes. Perhaps because of this I have been over-cautious belaying with a grigri and tended to short-rope people. It took me a few sessions to learn the finger-under-the-lip technique with current model grigri but I’m now much slicker with it and it seems, on balance, the best of the devices I've used.



andy moles

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#49 Re: Gri gri slippage
November 16, 2022, 08:00:44 am
I was belaying with a Grigri yesterday and adapted to the finger under lip position - not only lower risk of 'panic clamping', but actually more comfortable than finger under device as well. Can't believe I'd missed this being pointed out before.   :oops:

 

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