Post by Brian Roebuck on Mar 5, 2007 19:30:08 GMT -5
The French wrap self belay is an interesting safety device that looks like it would actually work even in a panic situation (which is where it needs to work the most!). It is more "stuff" to deal with when rappelling but when you consider the following things it is clear you can get in trouble very fast. While rappelling you only have one point of attachment, your momentum downward is not in your favor, and the possibility of rockfall rendering you unable to control things all add up to a far more dangerous situation than during climbing. Having something to slow or stop your rappel in an emergency situation could easily save your life. I have never used a french wrap but I suspect it takes practice in controlled conditions to use properly. In fact all vertical gear should be used in training before it ever goes underground with you. Your life depends on it.
I always climb with three points of contact. If I get hurt etc I won't fall unless the rope itself breaks. When I rappel only the rack and my caribiner keep me attached to the rope. The rack can be configured (incorrectly) or misused to allow me to lose control and fall almost as fast as in a freefall. Once the speed of rope moving through the rack gets fast enough it melts slightly on the hot metal surface of the rack bars (glazing) which in turn acts like a lubricant. I suspect the nylon surfaces in contact with the bars "out-gases" during this time as well providing a sort of gas bearing effect to further reduce friction. It takes only minute quantities of solid nylon to create large amounts of gases when heated (solids are much more dense than gases) so looking at a glazed rope gives little clue about such a physical event. All of this gives you very little friction to slow yourself back to a controllable speed. When you reach that point you likely cannot recover and that can happen so fast you don't have much time to react. Only superhuman strength in pushing the rack bars together can create some usable friction by squishing the rope between bars that might be enough to slow you.
There are cases where a french wrap would be impractical or even more dangerous such as through a tight spot at the top of a drop etc. In this case it is up to the caver to decide if he or she is willing to risk the rappel. Most do and do just fine. But some don't. It's always the other guy that gets hurt - until it is you.
As an engineer I often have to think about things that can get out of control and hurt people or machinery. Doing a safety analysis on rappelling would be interesting and would point to many things that could go wrong. Perhaps we all should consider french wraps or other rappel safety devices. Maybe the real answer to rappelling safety has not yet been invented.
It's certainly something to think about.
Good caving to all.
Brian Roebuck NSS 34626 RL (FE) ----- Caving is far too serious to be taken seriously.