I always use a yosemite backup...

During a climbing course I took the instructor said a study had been done with figure eight knots in which the last part of the knot was undone and it was still shown to hold nearly all of it's strength without coming undone. Never have been able to find that test though... Someone should do that test.

There are three properties of a knot: strength, security, and releasability. You described "security" but used the word "strength." You say "last part of the [figure 8]," but it's a total guess as to what you mean. Do you mean some kind of stopper knot or finish after the final retrace? Do you mean the final part of the retrace?

Assuming you mean completing the retrace, but skipping a back-up/stopper/finish, well then consider the previous decades of climbers using the fig 8 retrace in this way as your "test."

I like to keep 4 fingers between my loops and my 8, 2 fingers between my 8 and my fisherman, and 2 fingers of tail... give or take a couple mm.

Yah, I'm anal like that.

Thus turning your "safety" knot into a finishing knot. Don't worry, you're just as safe either way.

If your finishing knot is not snug against the main knot, then it has absolutely no impact on the load at which the knot begins to creep. That is, you may as well have not tied the finishing knot. Which is why I've taught it as a finishing knot, not a "backup" or "safety" knot. This also allows for yosemite finishes without really damaging their sense of how the finishing knot affects the main knot.

He got a comment about the tail being too short, but he responded rather authoratatively, "It's 5 times the diameter of the rope. It's enough."

It is not really a backup.

Semantics. How about Yosemite finish?

d.

A guy in the gym today tied in the usual way, with a retraced figure 8 knot. The length of the tail he had left over after retracing the knot was about 5 cm, no more than 6cm. He got a comment about the tail being too short, but he responded rather authoratatively, "It's 5 times the diameter of the rope. It's enough."

I've never heard this 5 times the diameter of the rope guideline for the length of tails leftover from tying knots before. The gym uses Sterling 10.1 Slim Gyms.

I keep thinking about how a lot of climbers are particular about having tails of sufficient length for double fisherman's knots, double overhand knots, water knots, etc., especially when rappelling.

Can anyone shed any light on this?

A guy in the gym today tied in the usual way, with a retraced figure 8 knot. The length of the tail he had left over after retracing the knot was about 5 cm, no more than 6cm. He got a comment about the tail being too short, but he responded rather authoratatively, "It's 5 times the diameter of the rope. It's enough."

I've never heard this 5 times the diameter of the rope guideline for the length of tails leftover from tying knots before. The gym uses Sterling 10.1 Slim Gyms.

I keep thinking about how a lot of climbers are particular about having tails of sufficient length for double fisherman's knots, double overhand knots, water knots, etc., especially when rappelling.

Can anyone shed any light on this?

If I have enough to do A Yosemite finish when my knot is tight I'm happy. Actually to be honest I'd rather tightly set my figure eight with a short tail, then have a long tail and a loose knot.
My point, a well seated figure eight is a good one. However I've seen loosely tied figure 8s snug up in a fall with the tail still being loose and unaffected. The knot works great, Ive never heard of a figure eight failing b/c of a short tail. Lets stop knit picking!

Not really pointless semantics though. A backup knots exist, and finishing knots exist. They serve different purposes. When people start using the terms interchangeably, ambiguity creeps in.

And then we all die?