thoughts on rigging a single line rappel.

Here ya go...same rope, different day...

rappel from knot against ring with backup biner

Whew...safe!

Brian,

I think you may be overlooking the potential for the 8-on-a-bight to roll completely off the bight. Impossible with the biner.

-Zach

ZachR wrote:Brian, I think you may be overlooking the potential for the 8-on-a-bight to roll completely off the bight. Impossible with the biner. -Zach

I understand the concern, which is why I use the EDK to tie rope ends together for rappelling off both strands...but...I'm loading one side of the knot, not both. Also, the knot is jammed against a ring, which, might change the load path a bit (or spread it out).

I'd be curious to know what type of force would be required to roll a a figure eight on a bite when loading only one strand, though. My bet is its a tad more than my body weight (factor of ten at least?).

So, my point is, I thinks there's zero potential for that knot to roll. If I suddenly gained about 4000lbs...maybe I should stay away from ice cream? Ha ha.

Might be some data out there from testing...anyone?

Here is some data for you: user.xmission.com/~tmoyer/t…

Well-dressed pre-tensioned Fig8 rolled at 590-750 lbs. Having an un-pretty knot reduced those numbers.

http://www.bwrs.org.au/sites/default/inline-files/abstract.pdf

"The Abnormal Figure 8 Knot is dangerous due to roll back slippage. It is possible that this knot when poorly packed and with short tails could completely undo with loads as low as 50kgs. Even a well-packed knot could roll back at 200kgs."

Brad W wrote:Here is some data for you: user.xmission.com/~tmoyer/t… Well-dressed pre-tensioned Fig8 rolled at 590-750 lbs. Having an un-pretty knot reduced those numbers. bwrs.org.au/sites/default/i… "The Abnormal Figure 8 Knot is dangerous due to roll back slippage. It is possible that this knot when poorly packed and with short tails could completely undo with loads as low as 50kgs. Even a well-packed knot could roll back at 200kgs."

Yeah yeah, old news.

Not talking rappelling from two ropes tied together with a flat figure eight.

Loading one strand with the knot against a ring. Very different.

Tried to simulate it with a 3mm piece of cord through a small keeper hole on an old Chouinard "air traffic controller". Very loosely tied, tries to pull through, but, it self cinches up and, the pull cord side of the rope adds an element of fun too. Asymmetric, and, only one strand pulling against the looser strands from the pull side.

I'd still like to pull one at high loads, just for fun.

Sorry, I know the data isn't an exact match to the situation. I was trying to get out the closest data I could find. Please post any other results you find/create yourself.

IMO, my personal order of likelihood of things going wrong on a single-rope rap are as follows
1. Rap the wrong line
2. Knot pulls through
3. Knot rolls/comes undone (if doing full rap with pullcord)

#1 just requires weighting the rap before heading down to check.
#3 is easy to manage if you tie a nice, clean, safe-for-rap knot

#2 is the crux IMO. If you always climb in the same area and KNOW your knot is big enough to always jam the rings, even after it's tightened from you rapping on it, that's one thing. But like the Yosemite accident shows: a slightly bigger ring or maybe a new smaller rope you just bought, and that equation can change for the worse.

Just like everything else, what setup you use is a personal decision (plus partner). It just didn't seem like a good idea to let a topic asking about the subject be closed without pointing out potential flaws. Now whoever reads this has a fuller story and can decide for themselves...