caver86 wrote:Feeding the rope is really just taking the rope's weight off of the bars so it can move freely while keeping tension on the bottom bar so it does not move.
That's close ;-). It is actually possible to have more friction than is accountable due to rope weight. Imagine a rack with 10 bars on it. If all 10 bars were threaded and there was not much spacing between the bars, you would probably not move down rope even if you had 2 ft. of rope below you.
The friction provided by a rack has a lot more to due with the material the bars are made from (for example, aluminum bars tend to have greater friction than stainless steel), the spacing of the bars (five bars on a shorter rack offer greater friction than five bars spaced out on a longer rack), the type of rope (slick, new 9mm static rope is much faster that an old, stiff, fuzzy 11 mm), and the weight of the person on rope (a heavier person tends to be faster while a lighter person slower - all other things equal), as well as whether the rope is wet or dry, clean or muddy, or freezing.
When approaching a rappel I recommend the person take a look at the surroundings to determine the "speed" of the rope under the prevailing conditions - something you will only really appreciate with experience. The advantage of a rack over some other descending devices is you can fairly easily change the amount of friction to control your descent. For a novice vertical person I recommend to always begin with all the bars engaged - if that setup proves to provide too much friction to move you can always carefully take a bar off until you can move.
I find that when I have the right amount of friction I can actually provide fine tuning adjustments for my speed with just my hand placement - perhaps a slightly tighter grip with my control hand or just moving my control hand to below my thigh so the rope rubs against my leg as I rappel.
It is possible to "feed" the rope through your rappel device. However, there are a couple of issues which come to mind regarding this practice. First of all, feeding the rope through the rappel device can be very tiring. If you have to feed the rope for more than a few meters it can tire you out fast. Descending a rope should require only minimal effort from the rappeller, and you should conserve your energy for any maneuvers you might need to perform such as crossing a knot, deviation, or rebelay.
caver86 wrote:If you mess up, you really could pop a bar off. :panic: Also, I don't think I can get as smooth of a rappel using that technique. I do think a smooth rappel is key. :caver:
Also, when feeding a rope through your descender, particularly on a rack, there is a real possibility that if you are not carefull and pay attention to your rack, a bar may pop off the rack, dramatically reducing the amount of friction. One way to lesson the risk of a bar popping off is to grasp the lower bars with your left hand while feeding the rope with your right (control hand).
Furthurmore, I find it difficult to descend smoothly when feeding the rope. When I have to feed the rope through my descender I notice my descent tends to become bouncy.
Whenever you are on rope you should really strive to be as smooth and fluid in your motions as possible. A jerky motion - either descending or ascending - transmits small shockloads up the rope to the anchors. I see no reason to test the anchors in this way. If you find yourself having to feed rope through your descender, decrease your friction if possible until you begin to descend - slowly - at a controllable pace. You should be constantly monitoring your descent anyway - looking to see where the rope goes, looking to see if there are any obstacle you will encounter such as a knot, redirect, or rebelay, or unexpected not. In this way if you feel your descend is faster than you feel comfortable with you may always add friction to the system.
The original question was whether six bars was safe for a rack. The short answer is: it depends - too many bars may result in too much friction and having to feed the rope which can be very tiring and can lead to a jerky motion while descending. If this is the end result, there is too much friction and it would probably be more efficient to drop a bar.
Steven