Dr. Pete Donzelli did a research project about this same topic a number of years ago. His work was with rigging and slam dunk loads.
He titled his article...All in all, its the length of the fall...maybe a bit of a generational mystery!
If anyone would like to read the articles that I have from Pete, drop me an email:
canopytree@earthlink.net
Keep Pete's memory alive.
Here's something that he wrote:
Saving Friction
Dr. Peter Donzelli
With the advent of false crotches for arborists' climbing lines has come much discussion about friction in the climbing system. While the Friction Saver (tm), one particular false crotch for climbing, has many benefits to both the tree and climber, the name may be a bit misleading. In fact, this device is not really "saving" friction, but is more correctly "displacing" friction. This can have some important effects on the ropes and knots used in tree care, so an understanding of the science behind this can help the climber work more safely.
It is best to start with some definitions.
First recall that a force is the action of one object on another, such as a person pushing a box across the floor. Friction is a specific type of force that resists relative motion between two objects. So as you push the box to the right, friction between the box and the floor pushes back to the left. You need to exert a force greater than this friction to cause the box to move. Another important fact about friction is that it is a dissipative force.This means that the work you do against friction is lost (to either heat or sound) and cannot be recovered to do something else useful.
Now, we need to understand the physics of climbing a tree with ropes, knots and a saddle. The rope starts at the saddle (the working end of the climbing line), goes up over a limb or through a false crotch (the standing part of the climbing line), passes through the friction hitch (tautline, Blake, etc.), and has the running end on the ground. When the climber is stationary, so the climbing line is effectively a large, fixed loop, half of her weight is supported by each leg of the climbing line. This means that the friction hitch is also supporting half of her weight. This is true whether the climbing line is over a twelve-inch white oak limb, or installed through a false crotch.
This changes a bit once she grabs the friction hitch and starts to descend (some people faster than others!). Now there is relative motion between the climbing line and the tie-in point (branch or false crotch), and between the climbing line and the climbing knot. If there is relative motion, there must be friction at each of these places. What is important to realize is that for any given climber, the total friction from both of these places must be a constant, every day and all the time. Any less than this and the climbing system no longer functions to keep you aloft. So what is different between the twelve-inch oak limb and the false crotch? Well, the climbing line will have more friction over the limb, and less with the false crotch. This means that the friction hitch is making up the difference between the two systems.
Recognizing that the amount of friction in your climbing system must be constant helps explain a lot. If you recently switched from natural crotch climbing to a false crotch, you may find that your knot bights tighter, and that the tail of your rope (or split tail) may burn out more frequently. For this reason many people using the false crotch have switched to Blake's hitch as a climbing knot, since it does not tend to get as tight. Of course, one limitation of the Blake is the tuck under two coils, which is more susceptible to burning on quick descents. On the other hand, using a false crotch will significantly reduce wear on your climbing line, and, coupled with a split tail climbing system, can save money in the long run.
Just a few ideas to help you climb safely.
