This is a tricky thing to put a number on precisely. The things that determine friction. Bushing diameter, angle, length of wrench. It is hard to find a good balance because when climbing around the tree, going out horizontally, you dont want the wrench to drag you down, especially us lighter guys. But then for going down the tree, mid air descents, you don't want too little friction at the wrench or it binds. I have heard that hitches bind more on heavier climbers. I am thinking of disigning a sub 170 lb wrench and a 200lb+ wrench that has a little more friction. I have found that the adjustment bushing does not have as much influence on friction as I originally thought. My method of friction measurement is as follows
I have 175lbs of weights in 5 pound, 2.5 lb, 25, lbs and 45 lb plates. I install a wrench on the rope, so far, only Poison Ivy has been tested. Work positioning with static lines in trees is not allowed by ANSI I believe. I personally find them problematic anyway. so I have not tested Static lines such as globe or escalator.
On the wrench I have a rope bag that I can place weights into. If I hang 10 lbs on the tail of the rope, i place plates in the bag until the point that the rope wrench begins to slide. I mark that point and then tie more weight onto the tail.
the results are interesting and more variable than I expcted. I found the wrench will slide sooner when it is warm as opposed to cold . this is true for the rope too. It is not something that is exactly quantified.
this is rounded off version of my results for a wrench in mid position on poison Ivy.
with 5 lb I needed 45 lb to slid
10- 50
15 65
20 80
30 110
35 115
40 125
these results do not follow a clean curve but they folloow more or less a trend. I seem to be getting somewhat lower numbers with the QRP but not much less.
I take the results to mean that as a 170 lb climber, I must hold more or less 40 lbs on my hand/hitch and transfer 130 lbs onto the wrench and I will slowly begin to descend, the more I transfer weight onto the wrench the faster I will descend. Its not so much what percentage of the weight does the wrench take, its more how much weight must be kept on the tail.
I want to conduct these tests with figure eights, atc's and racks as well. Its quite fun and I will keep you all posted if I get the system down enough to really give hard numbers and coefficients of friction etc.
Lots of variables including rope size, even humidity and temperature have effects on the rope wrench. I recommend going with a bigger rope if you are not getting enough friction.
Tests have been done with the wrench in a couple of places and it really comes down to your hitch. IT is very easy to tie a not good hitch that will take you to the ground if your not careful. The danger is more so than in ddrt because there is less rope that goes through the hitch and thus less chance to catch. Well tied hitches perform superbly in the drop tests while poorly tied hitches, especially hitches tied with stiffer cords can definitely take you for a ride. So many variables in play there as well.