Something scary to think about!

[iain]

I get 3:1 and 11:1 using the "T" method (since both are complex pulley systems)

[iain]

how 'bout the loads on the anchors?

[RobBob]

(only on edge of thread) ...anybody see the show on Natl Geo channel on the SAR in NZ? Pulleying that guy out of that tiny crevasse? Ugggg!

[CascadeClimber]

But what about the friction in the system, and the force of gravity on the anchor points and the rope, and hydrogen bonds between the sheaves and the rope, and what about the moon and its gravitational effect and...ouch. I think I just hurt myself.

[iain]

(way off-topic, sorry) here's a funky one, what's the mech. adv. on this?

 

[CascadeClimber]

If L1<>L2, then 2-1, I think.

 

If L1=L2, then 1-1??

 

 

[iain]

yeah sorry, both loads being equal. Guess this works out to mechanical disadvantage:

 

[forrest_m]

iain -ok, maybe I'm dense, but I don't see any mechanical advantage at all. If the loads are the same, the middle pulley is just a dummy, right? I mean, the ropes might as well be tied at that point, and the two upper pulleys just change the direction of loading.

 

If the loads are different, the lighter one will just ride up until it hits the upper pulley and becomes "fixed", giving your pull a 2:1 advantage on the remaining weight.

 

assuming friction is 0, of course.

 

[crap, you guys just type too fast for me]

 

Edited December 5, 2002 by forrest_m

[iain]

the middle pulley just works as a differential to even out your pulling force on each load, and since your pull is split in half over the pulley, you actually have to pull twice as hard.

[forrest_m]

but where's the "disadvantage"; in your second diagram, each weight is .5T, and the total pull is T, i.e. pull is equal to the total weight being lifted; so no advantage but no disadvantage either, right?

[CascadeClimber]

Overall it is 1-1, but the effect on either load alone is 0.5-1, I think.

 

Did this thread have something to do with climbing??

 

[forrest_m]

yes, I think it was "never go climbing with physics geeks, they take forever to set up an anchor"

[iain]

This is sortof pertinant to climbing. A good understanding of mechanical advantage couldn't hurt when improvising a rescue.

[allthumbs]

I've found that to be true about engineers, scientists, school teachers and sailors. They all drive me fucking up the wall with their analytical horseshit. I had a blowboater ask me about british thermal units as it applies to sailboats once...WTF?

[Greg_W]

I must admit that I am guilty of this. I'd spend half a day setting up a perfect anchor if I had the time. I'm better at slamming in something bomber and calling it good now, but when I was starting out I was tedious.

[CascadeClimber]

So getting back to the original issue, if the force introduced into the system by the falling climber is constant, who can tell us where the additional compensating force comes from?

 

Bueller? Anyone?

 

[Peter_Puget]

Just got around to reading this thread. here is a fun link to play around with:

Click on the fall simulator

 

One this whole discussion got me thinking about is the usefulness of lots of crappy pro even if its function is simply to mimick a "screamer."

Last week I climbed Desert Song in J-Tree. The second pitch has difficult to protect climbing right off the belay. Every thing was bomber at the belay but who wants to hold a 30' factor 2 fall? Our solution was to dump some more pieces effectively creating an additional anchor for a directional and then place a few pieces we expected to blow out in case of a fall.

 

PP

 

 

[Smoker]

CC writes:

So in Alex's example, the spiral fracture occurred because the belayer was looking up at the leader and was spun around by the fall. But if he was oriented differently, with no slack, the force would not have spun him and he would have felt much less force overall.

 

I'm no engineer so dont get too technical with me or my head will spin.

2 points to make here,

 

1) Most of us are standing at ice belays, with some ability to move about to avoid being bombed so a fall past the anchor onto the belay device of any distance will most certainly be felt in a most unpleasant way, bringing the strongest of us to our knees or possibly to the breaking point as the leader plummits past. (Remember that a 10 leader fall with no gear in equates to 20+ before the belayer is wrenched down in line with the fall)

 

2) The need for a multi directional anchor is paramount if you plan on belaying a leader that one minute is 30 ft out with no gear and above you (hence the need to position yourself in the line of anticipated pull if said ballsy leader ever does place gear) but the belayer must also be able to move should ballsy leader launch from said 30' lead and change the anticipated direction of pull. When the slack comes out of the system the belayer had best be oriented towards the fall (downward) and the anchor truly bomber and multi directional.

 

 

[CascadeClimber]

Why can my small brain not discover how to "reply with quote"???

 

Don't situate the belay directly below the line of climbing for the next pitch.

 

Regardless, a leader fall onto the belay is going to be highly unpleasant for everyone involved, regardless of position, rope system, how much Smoker-custom-roast-espresso you had that morning, or how many hail-Marys you said the night before. Perhaps the best position is with your head tucked between your knees so you can kiss your ass goodbye. So leaders, get a damn screw in to protect the belay on multipitch ice routes.

 

Can it please get cold now so we can stop talking about ice and start climbing it?!?!?!?!?!

[wdietsch]

Why can my small brain not discover how to "reply with quote"???

 

ditto, help us dumb brothers on this one ... how do you get the "In reply to box" thing ?????

 

 

So leaders, get a damn screw in to protect the belay on multipitch ice routes.

 

or anything ... pins, chocks, cam ... I have alway been a beliver in protecting the belay .... just about lost my ass on the NR of Stuart once, never again ... given three anchor points for a belay I would gladly give up one of them to protect the leader and belay off two

 

oh, shit, hey ... what does this "quote" button in the instant UBB Code commands do? ... if that is what I have been looking for I really feel stupid!!!

 

 

 

 

 

 

[wdietsch]

Can it please get cold now so we can stop talking about ice and start climbing it?!?!?!?!?!

 

testing 1, 2, 3 .... I think my dumb ass figured it out

[icegirl]

Can it please get cold now so we can stop talking about ice and start climbing it?!?!?!?!?!

 

amen brother!

 

ice ice ice ice ice......

 

[Pencil_Pusher]

My brain hurts from all of this. Thanks to Alex and CascadeClimber for at first confusing the hell out of me. I think I understand it better now.

Why I had to look at a Mountaineers theory of mechanical advantage pic and quiz is beyond me. Iain.

[chriss]

Finally able to log on this thing.

 

Cascade. The compensating forces, and I'm assuming you mean the compensating anchor forces not the force due to the belayer, come from mass x accelration of the anchor. If the anchor stays attached to the ice (rock, tree, etc.) and its stays attached to the earth, the mass is big enough as it can hold the force with very, very little accelration (as good as 0).

I'm glad you brought up the fact that the elastic stretch and force absorbtion of of ropes is NOT linear, meaning no K factor. It's not that simple with polymer materials.

And I agree with you about not using a doubtful belay anchor as leader pro. If possible the belayer should help hold any fall and not put so much force on the anchor.

 

And, did anyone note that the spin that probably caused the broken leg was due to the anchor rope sliding around the harness. One of the posts said the rope to the anchor slide to one side of the swami and the force of the fall spun him around to the other side. If he would have clipped the belay device to the loop of rope at his tie-in, ..... very little movement.

 

 

chris

 

[Alex]

chriss, yes Maurice and I noted that had he clipped into his belay loop or boith swami and leg loops with the pearabiner, a differnt outcome might ahve resulted, since he would not have had the entire belay move around to the back of the swami,

 

Alex