Descending

[DrewB]

i am going to OWN you mr glen!!

phht, a mattress indeed

[joe into trees]

Yay, grudge match!

 

Can I propose that the loser has to do a ballet dance at the next arborcamp talent night?

[DrewB]

im thinking more, shots in town:)

[joe into trees]

The only right way to run this comp would be:

 

15 metre descent, to land on a thin plank placed across a mudpit. That way you have to bring your descent under control, and land lightly... and if anyone burns through their hitch (cough, couldn't happen with a spiderjack, cough) they just fall in the mudpit. No harm done.

 

There could also be wrestling in the mudpit later. I hadn't realised before, but this is something that is definitely lacking in tree climbing competitions.

[DrewB]

thin plank across stinging nettles......

[kevin bingham]

Aww. It's SO cute when the hitch guys try and do fast descents.

 

Like watching a sumo wrestler enter a ballet contest... you know he's going to lose hopelessly, but your heart just goes out to the big guy.

 

This is what i would say for doubled line descents of any sort spider jack or not. A doubled line descent has to burn through twice as much rope as the SRT set up. I got my money on Drew B.

[atree]

bit clearer what i mean here, Ropeguide all the way. i challenge anyone to a descent race on my hitch:) hehe

 

Tried it today,Works a treat .

[joe into trees]

A doubled line descent has to burn through twice as much rope as the SRT set up.

 

Hi Kev, guys,

 

That got me thinking for most of the day: Kev's dead right of course but I think it’s only half of the story. I haven’t seen anyone do a fast, long descent with a RW or unicender (I know you can go quickly with a figure-8) so I’m speaking from ignorance a bit, but I do know that with the SJ you can get something so close to free-fall as to make almost no difference. The limiting factor is only your body’s instinctive fear of rapid falling and the potential for a sudden (bone-shaking) stop.

 

Nevertheless, this imaginary race-to-the-ground brings up all sorts of interesting points about DdRT in relation to SRT climbing.

 

Straight fall

 

First off, if you imagine that there’s no risk of a sudden stop, and the climbers are just racing straight into the ground as fast as possible, it just comes down to who can get the necessary amount of cordage through their system quick enough, which is in turn decided by who can get the least amount of friction in the system. The maths of coefficients of friction are really complicated when it comes to climbing systems, as most are multiple relationships which are affected by how much weight the climber has in each part of the system. In a (theoretical) situation where a climber had his SJ fully opened with no use of the block, there would be next-to-no force on the running (prussic) side of the line. Consequently, the friction at the pulley would be very low. To test how this works, try pulling down on a line running over a high-quality pulley – almost no resistance. Now try hoisting a lighter climber into the air when he’s hooked up to one end of that line, and you’re pulling down on the other – almost impossible, usually takes a couple of people.

 

Net resistance for the SJ climber: very small amount of friction over pulley. For the two hitch climbers (one on SRT and one on DdRT) everything comes down to how much they are able to get their hitches to lose all contact with the line. Assuming that there is even a small amount of contact, everything changes a lot. As Kev points out, the DdRT climber has to get 2 metres of rope through his system for every metre that the SRT climber does, but if there’s no contact at all this doesn’t matter, and it just comes down to a question of whether there is more friction in a pulley (with no weight on one side of the rope) or a rope wrench... with the climber’s full weight on it. In this case, I would guess that the RW would slow the guy down more, and the DdRT climber would hit the ground first.

 

The truth is however that you can’t get a hitch completely free of the rope. This friction, however slight, reduces the friction of the RW but increases the friction at the DdRT pulley... and the two metres of rope running through the DdRT climber’s hitch for every metre of descent starts to become a problem. In both cases, I think the relationship follows half of a proportional sine curve, however in the case of the RW the relationship of friction-at-hitch to friction-at-RW would be inversely proportional, within limits.

 

Result of all this, is that I think that with properly configured systems, set up as if you were going to keep climbing, but then racing (from say 15 metres) straight into the ground, the SJ climber ought to deck out first.

 

The easy way to run this race, if we give the hitches the benefit of the doubt and say that it's possible to open them up so well that there's no contact with the line at all, would be to take two 80kg weights up a tree, tie one to a rope running through a pulley, and one to a RW with no prussik attached. If you drop them both, which would hit first?

 

Controlled fall

 

The really interesting bit is that you just can’t make yourself fall that fast. I’m curious to try, maybe with a dynamic belay line and using dynamic rope or a series of energy-absorbers at the high point. The important race is really one in which each climber has to bring themselves smoothly down to the ground. This is where I reckon that a lot of the current discussion on the forces/speeds of SRT climbing really fall short. In DdRT, at rest, half of your weight is on the spliced/termination side of the line, and only half is on your friction adjustor. As you ‘open up’ the adjustor (and descend) the net weight supported by the system is decreased... up to the point where you are falling, and there is almost no weight on your high point. (I would be very interested to be able to work out whether it is ever possible to get more than half of your weight onto the spliced side of the line whilst descending... does anyone know?) The force applied here is proportional to the friction between the adjustor and the line, with the upper limit of 50% of your weight (multiplied by any shock force) and the lower limit defined by how little friction you can achieve between the adjustor and the line.

 

In SRT climbing, the friction adjustor (whether that is a figure-8, unicender, RW-and-hitch etc) has to deal with 100% of your weight. The force applied is still proportional to the friction between the adjustor and the line, with the upper limit of 100% of your weight (again, x any shock loading) and the lower limit defined by how little friction you can get between the adjustor and the line.

 

(OK, I’m cheating by using weight as a force, but it’s easier and this is complicated enough)

 

A lot of the internet discussion of SRT vs DdRT seems to miss the fact that, although twice as much rope is passing through the DdRT adjustor, the force being applied to it (which in turn affects friction) is halved. To check this out, try ‘loosening’ the klemheist on a footlock prussic, then dropping it down a line. Once in a while, it falls all the way to the bottom. Try it again with a weight on it (no, not a climber... they do all sorts of things like scream and grab at the line which make for bad science) and it will bite much more quickly.

 

In a controlled fall, all of the climbers are trying to apply a steady amount of friction, enough to keep them at a speed where if they are suddenly brought up short (twist in the rope, fingers slip off the adjustor) the resulting stop won’t be too brutal... and they are also trying to avoid the heat-destruction of their hands, cordage or devices! The SRT climber has to let 15 metres of rope through his device, but that device is experiencing the full load of the climber. In the case of a figure-8, this works fine, and you can go zipping down then come to a neat stop, and (so long as you get the thing off your rope before it burns through!) there’s no harm done. So, in this race, my money would probably be on the guy using the figure-8.

 

Between the other ones, I really don’t know. I would guess that the friction taken by the metal of the RW would probably mean that the RW climber could save his hitch much better than the DdRT hitch climber, and that he could therefore get down quicker, all other things being equal. Rope passing over a pulley, under tension, from the prussic to the spliced side of the line as the climber descended would benefit from the coefficient of friction at the pulley, but the faster the climber went the less this would be.

 

My guess would be that the finishing order in this race would be determined more by how reckless the climbers in question were than anything else, but if you assume that all of the climbers were able to really ‘open up’ their gear, then the limiting factor would be collateral heat-damage to the equipment rather than anything else.

 

I look forward to the videos of this competition, but do be careful!

Edited January 31, 2012 by joe into trees
added some extra spiderjack propoganda

[DrewB]

" determined more by how reckless the climbers in question were than anything else"

 

Oh you've started something now Harris, I hope you feel bad when the bones start snapping!!

[joe into trees]

Oh you've started something now Harris, I hope you feel bad when the bones start snapping!!

 

Me? All I did was suggest that a real man isn't afraid to go hurtling into the ground at a horrific speed. Surely that is a founding principle of modern arboriculture? In fact I can prove, scientificallally, that impact velocity is directly proportional to testicular volume.

 

Chicks dig blisters.