ATC devices and rope slippage?

This is not a "how to fix this" question, just curious.

Belaying a heavy guy in the gym recently, as he hung and rested, the rope very slowly oozed up through the ATC, fully braked, both hands on the rope, taking my locked hands with it, if I allowed it. This, on a top rope, with very little friction.

Here's the question. Is there any formula or approximation for the weight that is needed to cause this to happen? How much weight the belayer needs to hold that the brake doesn't hold? Awkwardly phrased, sorry, hope someone gets what I'm asking.

Again, I don't need to be told to add friction, use a grigri, learn to hip belay, any of that. Just curious if this has been measured and quantified.

Thanks! Helen

I seriously doubt it has been "measured and quantified." Depends on the weight difference, the slickness of the rope, how much rope drag there is and your braking power (grip strength). Next question.

There are to many factors, the wear on your device, the carabiners you are using, the wear on those biners.  Also, an ATC has incredibly large slots and I hate this about it whereas a reverso has smaller slots that lock better so you may want to try those.

There was a mini-study published that measured force required to hold climber with different belay devices.
You can increase friction by attaching ATC with two carabiners. When TR belaying someone considerably heavier, add extra twist around rope at the anchors, it will reduce force on the belayer's end.
Brake assisted belay devices are really nice to use when your climber is hanging a lot.

use a grigri

If they're going to hang a while I'll take a bit below my butt cheeks with my non-primary brake hand and "sit" on it with it wrapping up to my other side. The friction from it wrapping behind me usually means I don't have to grip very hard. Primary brake hand never lets go for obvious reasons.

I haven't seen anyone else do it. It works well for me and the biggest issue I can see is failing to keep your primary hand engaged at all times in case the rope slips to a worse position. 

Old lady H wrote: This is not a "how to fix this" question, just curious.

Belaying a heavy guy in the gym recently, as he hung and rested, the rope very slowly oozed up through the ATC, fully braked, both hands on the rope, taking my locked hands with it, if I allowed it. This, on a top rope, with very little friction.

Here's the question. Is there any formula or approximation for the weight that is needed to cause this to happen? How much weight the belayer needs to hold that the brake doesn't hold? Awkwardly phrased, sorry, hope someone gets what I'm asking.

Again, I don't need to be told to add friction, use a grigri, learn to hip belay, any of that. Just curious if this has been measured and quantified.

Thanks! Helen

Well there´s nothing I haven´t measured on the various ATC´s but your question is confusing me, what do you mean by "taking my locked hands with it, if I allowed it"? Just holding the rope in the braked position does virtually nothing, you still have to hold it from moving through. And is it an ATC, an ATC XP or an ATC XP Guide? They are all different.

I find that ATCs with teeth (ATC-XP, Guide, Reverso, etc.) have very little if any slip when holding for me, even with heavy climbers.  I suppose I've never quantified it, but I do think the reverso bites the hardest.

Does your gym put a loop in the pre-set topropes?  I find that makes it effortless to hold climbers on any tube style device.

Danny Poceta wrote: use a grigri

Sorry H, but sometimes the simplest answer is the best answer.

Jim Titt wrote:

Well there´s nothing I haven´t measured on the various ATC´s but your question is confusing me, what do you mean by "taking my locked hands with it, if I allowed it"? Just holding the rope in the braked position does virtually nothing, you still have to hold it from moving through. And is it an ATC, an ATC XP or an ATC XP Guide? They are all different.

Howdy, sir! I'm one of those who have read through some of your very accessible papers. 

Let's say ATC XP guide, since that's what I have. With both hands firmly grasping the rope, in front of the belayer, holding the rope in full braking position, with enough weight, it will also require some muscle from the belayer to keep rope from creeping through the device. I'm not talking about slipping through hands, just watching rope pull up (very slowly) and taking the belayers hands up also.

I realize there are lots of variables, but if the only variable is the weight on the climbers end, do you have an idea at what point the brake of the device alone will not be enough?

Thanks, Jim!

Most everyone else, enjoy yourselves answering questions that aren't being asked, lol!

I have plenty of options to choose from for more friction, or to hold a climber working a project. But hey, sitting there, holding the dude, the mind wanders, eh?

Best, Helen

I think you need to provide the weight of the climber and the various angles of the rope at every point it changes directions to.give you a number.

Old lady H wrote:

Howdy, sir! I'm one of those who have through some of your very accessible papers. 

Let's say ATC XP guide, since that's what I have. With both hands firmly grasping the rope, in front of the belayer, holding the rope in full braking position, with enough weight, it will also require some muscle from the belayer to keep rope from creeping through the device. I'm not talking about slipping through hands, just watching rope pull up (very slowly) and taking the belayers hands up also.

I realize there are lots of variables, but if the only variable is the weight on the climbers end, do you have an idea at what point the brake of the device alone will not be enough?

Thanks, Jim!

Most everyone else, enjoy yourselves answering questions that aren't being asked, lol!

I have plenty of options to choose from for more friction, or to hold a climber working a project. But hey, sitting there, holding the dude, the mind wanders, eh?

Best, Helen

Another variable you need to consider is grip strength. In essence, the ATC multiplies your grip strength so your equation looks something like this:

In order for the rope to not slip, you need to exert this amount of grip strength: (w - f) / m   where w is the weight of the climber, f is the friction that is created by all the rope bends and biners in the system, and m is the factor at which your belay device and belay biner combo multiply braking strength. If you fail to exert that force on the brake strand, the rope will slip through the device.

So the answer to your original question can only be calculated if you can figure out the value of each of those variables. The reason why nobody seems to give you the answer you're looking for is because measuring each of those requisite variables is either impossible or highly impractical. 

Rope diameter could play a role

eli poss wrote:
So the answer to your original question can only be calculated if you can figure out the value of each of those variables. The reason why nobody seems to give you the answer you're looking for is because measuring each of those requisite variables is either impossible or highly impractical. 

How is it that complicated? 

The belay device has a certain (high) coefficient of friction. If the climber is hanging still, the frictional force plus the tension in the brake line is enough to counteract the gravitational force of the climber. If the line is slipping just slightly, there isn't enough tension given the frictional force to counteract the climber.

For a given device+rope combo, the tension required in the brake line to hold the load is proportional to the weight of the load. It will require twice as much tension in the brake line to hold up a 200 pound climber as a 100 pound climber. You can include as many variables as you want, but the basic relationship is really simple. With a heavier climber, you're pushing the limits of what that system will hold without creeping.

I like how Stephen Attaway lays this out - see his The Mechanics of Friction in Rope Rescue.

"taking my locked hands with it" means the relevant belayer parameter is their triceps strength (rather than weight or grip strength).

BTW, Helen, while I respect your desire to not be told things you already know, the question makes me wonder if you've considered using your thumb.

You mean how much do you need to "hold" the rope just to keep the climber hanging there?
Naturally I measure this, it defines the characteristics of a device when you are rapping because we know that applying a hand force of more than ca. 6kg(f) is uncomfortable for longer periods. I´ve only a data set for the ATC XP with a 9.5mm rope so depending on your rope and the friction over the top belay things will vary. A hand drawn graph!

Just holding the rope in the braking position but not restraining it I got 18kg braking force so top-roping about enough for a child.

Old lady H wrote:Belaying a heavy guy in the gym recently, as he hung and rested, the rope very slowly oozed up through the ATC, fully braked, both hands on the rope, taking my locked hands with it, if I allowed it. This, on a top rope, with very little friction.

You either have exceptionally weak triceps or your "fully braked" position is not fully braked.  While ropes and devices vary I can generally hold a heavy climber with a single finger and a thumb and just minor tricep pressure.

-Learn how to orientate your brake hand(s) with respect to the device to maximise brake force.
-Learn how you cock your wrist to reduce grip strength required.
-Learn to lock your arm or wrap the rope to reduce the stress on you tricep when somebody is hanging for extended periods

Best device for the situation you describe is the Gri Gri, but you could  use TWO carabiners instead of one, or one carabiner with a more rectangular rather than round cross-section.   Both those methods will add friction to the system.  I use the two carabiner method when rappelling with skinny ropes (less than about 8.0) when a overhung route is involved.

This can happen when you belay with a rappel device

Darren Mabe wrote: This can happen when you belay with a rappel device

Who ever does that?

Old lady H wrote: This is not a "how to fix this" question, just curious..........

Again, I don't need to be told to add friction, use a grigri, learn to hip belay, any of that. Just curious if this has been measured and quantified.

Thanks! Helen

Guess you´re used to telling men stuff and being ignored  

When I belay fat slobs who can´t get up the route and I´m bored holding them I just stand on the rope