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[JBrochu]

About crossloading the belay loop.. about the time that the EKD was becomming the rage for rappeling someone got the chop useing an overhand fig eight as a rap knott. the story went they were freaked out about the EKD so why not use  a fig 8 instead. Turns out the fig eight rolls real easy in that configuration and one of them got chopped. I took that info and deducted that clipping directly into your tie in loop could load the fig 8 in the same or simeler direction that caused the aformentioned accident. My own decision based on stories I read on the internet.. YMMV

There is no doubt the flat figure eight is dangerous for joining rappel ropes as it rolls at pretty low loads. However, the figure eight follow through and flat figure eight are not the same thing, so it doesn't make much sense to me to make deductions on one based on the other. My own decision based on logic.



 

[tradmanclimbz]

Regardless of the circumstances if you look at the pure math/odds it is quite telling.
 Every accident has a story and a list of excuses. Those are varriabls that can not be controled. What can be controled is to simply look at all accidents that result from a failure of the tie in knot. Does not matter what the story behind the failure is just look at the numbers. Despite the fact that the vast majoriety of climbers use the  fig eight the majoriety of tie in knott failures happen with the bowline..  Vegas odds and the house always wins

[tradmanclimbz]

JB if you cross load the tie in loop in two directions at the same time you have replicated the exact same direction of load on the fig eight as would be experienced in an overhand 8 rappel situation in effect turning it into the same knott for that dirrection of pull?

[tradmanclimbz]

Just thought about this a bit more and the only difference i see is that you have one less tail. You stil have one tail that could concievably pull through if the knott rolled?

[JBrochu]

After that rappel accident somebody did some testing and the load it rolls at was fairly low. Don't take this as fact because I'm going off of memory but it was something like 170 kg. Again, going off memory only the inside loop of a figure eight follow through (without the tuck) doesn't roll until something like 750 kg, and that's without a backup knot. With a backup knot it probably can't roll at all, practically speaking.

So I think they have to be different.

And I swear I'm not really into all this numbers stuff. I just want to know enough to know the methods I'm using are safe--I don't particularly care about exact numbers and mechanism of failure and whatnot. When Mark posted that picture with the skull and crossbones for loading the figure eight follow through with a tuck I just wanted to make sure I hadn't misunderstood what I learned about using that loop for an eight with a backup knot. Then it kind of snowballed into geeksville.

[DLottmann]

However, the figure eight follow through and flat figure eight are not the same thing, so it doesn't make much sense to me to make deductions on one based on the other.

Some confusion here as I feel the term "flat figure eight" is misused, or at-least mis-understood. Lets break the variations down:

1) Figure Eight Follow Though (or retraced - standard tie in knot, this is the knot we are concerned about loading from within the loop formed

2) For lack of a better name, the Figure 8 EDK- this is the knot that proved fatal when used in place of a EDK, and is tied just like the EDK but by adding one more twist...

3) In-Line Figure 8- This is a very strong but hard to untie knot that looks just like the other 8's, but is vastly different. To join two ends you would first make the "8" in one end, then re-trace the 8 from the opposite end. The big difference is you don't just grab the rope ends and wrap around EDK style. This knot would actually block any possible rolling, but is not feasible as a tie in as it REQUIRES both ends of the rope to tie... It is this knot that I have also heard called the "Flat Figure 8" and should not be in this discussion as it is impossible to employ it as a tie-in...

So, JBrochu, it is absolutely prudent to make deductions from what Tradmanclimbz reported, the two knots in question where #1 and #2 above. While created differently, they are structurally the exact same when loaded within the loop, but in the case of the rappel error the "loop" doesn't exist, the knot structure however is identical, and not intended for a strong load (even failed under body weight rappelling).

That being said, if it rolls in your described scenario a backup knot would prevent failure... You know it's this weakness loading an 8 in this manner that is why we use butterfly's on a bight or direction 8's in glacier travel... they don't have that weakness...

[JBrochu]

So, JBrochu, it is absolutely prudent to make deductions from what Tradmanclimbz reported, the two knots in question where #1 and #2 above. While created differently, they are structurally the exact same when loaded within the loop, but in the case of the rappel error the "loop" doesn't exist, the knot structure however is identical, and not intended for a strong load (even failed under body weight rappelling).

Ok, but then do you know why they roll at vastly different loads? Not counting the backup knot--from what I've read they roll at vastly different loads even without the backup. They said the backup essentially makes it bombproof, but even without the backup it's very strong.

 

[DLottmann]

Ok, but then do you know why they roll at vastly different loads?...

Who says? Source?

[JBrochu]

Ok, but then do you know why they roll at vastly different loads?...

Who says? Source?

I've got to go digging again for where I found the 750 kg figure. But I found Goldstone's post explaining why the inner loop is very strong and why the EDK method is not...

Clipping into the rope loop does not load it perpendicular to the anchor strand when belaying the second off the harness or when catching a factor-2 leader fall.

When belaying the leader, it is possible to get such loading, instantaneously. (As far as I know, the figure-8 has never been tested for rolling under off-axis instantaneous impacts.)

In leader falls where the anchor actually comes into play, the loading of the anchor strand means the situation is completely different from the EDK loading tests, in which neither of the ends is under any tension. Tension in the anchor strand will inhibit rolling for the same reason an extra stopper knot does for the EDK; it prevents rope from feeding into the knot, a prerequisite for rolling behavior.

That leaves only low instantaneous loads that are not enough to lift the belayer onto the anchor. The BMC article does suggest a stopper knot to back up the figure-8. Many climbers do this anyway, and such a knot would eliminate even the remote possibility of the figure-8 knot rolling.

With my half ropes, I use bowlines with "Yosemite finish" and double overhand backup after that, then I clip both loops, so no worries for me.

Probably the best tie-in knot of all is the rethreaded bowline on a bight, which puts two loops in the harness tie-in points. Using such a knot and clipping those rope loops would be beyond bomber.

But getting back to the figure-8, tie a backup knot as you probably should anyway and clip the rope loop with no conceivable worries.

Here is a link to the entire thread. I cannot figure out how to link just to the post the quote came from and there is no assigned post number for me to reference.

http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?post=2574312


edit: from the same thread...

Moyer's tests suggest that a well-tied figure-8 can roll at 750 lbf, which would mean the rope loop would need a rather hefty 1500 lbf for rolling to occur. At that level, there is no doubt that the belayer is going to be yanked hard against the anchor, in which case tension on the anchor strand will, I think, make rolling impossible. This is, of course, doubly hypothetical in the presence of a back-up knot on the figure-8.

[DLottmann]

Ok, my head is spinning a bit, but answer this... If you skip the butterfly in the setup you posted a pic of why not just use the belay loop for what u are describing??? The doesn't seem to be any reason to use the tie in loop, unless you climb often on harnesses with out belay loop....

[JBrochu]

Actually just switched a few weeks ago to my first harness ever with a belay loop--so I hadn't considered that.

[DLottmann]

Actually just switched a few weeks ago to my first harness ever with a belay loop--so I hadn't considered that.

LOL all this theory for naught... just use the belay loop