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Author

Knots

lance

27-Nov-2007
4:15:50 PM

Would someone be able to tell me which is the stronger knot out of a figure 8 on the bite or a double figure 8 on the bite. It will just be used for abseiling off a single anchor point.


gordoste

27-Nov-2007
5:32:33 PM

Not exactly sure what you mean. A figure of 8 on the bight is commonly used for tying in to the rope when climbing (although it is tied by rethreading a single figure 8). A double figure 8 would be a figure 8 tied in two ropes. What is a double figure 8 on the bight?
(preparing to feel stupid when someone points out the obvious)


penguinator

27-Nov-2007
5:56:47 PM

I suppose you would get a certain amount of redundancy using a double figure 8 with a single anchor point, but they are mainly used when there are two anchors.

IMO a figure 8 on a bight is fine for a single anchor.


IdratherbeclimbingM9

28-Nov-2007
11:23:27 AM

Although it is possible to tie a double figure of 8 on a bight of rope (by forming and passing a second bight of the doubled over end loop, through the original end loop; - to form what some climbers call 'bunny ears' similar to a double bowline knot), what you end up with is a certain amount of redundancy and the ability to adjust tension evenly to two anchor pieces.
You will not have a stonger knot as the strength largely comes from the tightening point radius (within the knot), and both are in essence the same ie a 'fig 8'.

Re strength of knot; some sources quote the overall strength of a fig 8 knot as being 62 % of the breaking strength of the rope (ie a 38% reduction due to the knot).
Many people prefer a bowline (or doubled bowline), for ease of untying the knot after loading however its strength is almost identical (?).

Most modern 11 mm ropes are good for 3000+ kg so the remaining 60% strength after a knot is tied in it (say conservative 1500 kg) is more than sufficient for abseilers weight plus additional rescue equipment / patient etc.

It all comes back to your anchor (how good is it? ... heh, heh), and possible human error in tying the knot, along with other factors like sharp edges rubbing the rope due to poor rigging ...


AlanD

28-Nov-2007
1:06:59 PM

When Andrew Pavey (MD of Spelean) did the tests back in the 70's at UNSW, the bowline came in at 50% reduction and what you guys call a figure 8 at 35% reduction in strength.

What you guys need to be very careful of when quoting stats is that what climbers call a figure 8 knot not actually a figure 8, but may be referred to as a figure 8 loop or a double figure 8, I know Andrew's work referred to the term double figure 8, but is what you refer to a figure 8. When you see stats, make sure of the actual knot which has been tested, preferably visually. The origin of almost all knots is sailing, but the names have been corrupted by specific types of users and this can lead to confusion.

Just to emphasise this, the UNE web site (linked through the Bowline knot on the Chockstone’s Tech tips, refers to the single fisherman’s knot, but shows actually just one side of the double fisherman’s knot and the Chockstone’s double fisherman’s knot shows it both correctly ties (lower pink rope picture) and incorrectly tied (being tied) and there is a difference in strength.

** I'm knot having a go at anyone or any groups, just trying to get the point across that care needs to be taken i.e. that the correct knot is actually tied.


sliamese

28-Nov-2007
6:54:43 PM

unless you dipped your bowline/figure 8 etc in battery acid you wont break it! i think the main reason to choose a know is how easy it is to equalise anchors. IMO its usually better to have a sling/cordalette making an equalised anchor with the rope attached so that if something did happen you can perform rescues easier, get ropes in and out of the system easier etc etc.

but in terms of bean counting id say the breaking strains would be very similar.


AlanD

28-Nov-2007
7:27:12 PM

The safety margin is much smaller than people think

Take your 3000 kg rope,
reduce that by 40% because of the the knot = 1800 kg
give the rope some wear (minor damage to the sheath and a bit of git in the core) 25%= 1350 kg
You have an 80 kg climber, with a 2.3 factor fall (UIAA standard) = 184kg

Safety margin = 7.3

7.3 is fine, but if you're doing a rescue or the rope is over a sharp rock when you take the fall, that safety factor is much lower.


sliamese

28-Nov-2007
9:23:47 PM

please explain how a fall factor can be 2.3????? that shows how much can be learnt in forums generally. basically its never gonna be the knot that breaks. gear pops, ropes cut and your spine breaks before the rope snaps. if anyone can tell me about a rope breaking im all ears, ive never heard of it. and i mean breaking, not cutting! the only examples i can think of are due to chemical interference.

Disclaimer: im not an idiot that thinks my rope is invinceable! i think people get too caught up in numbers. learning things about keeping ropes away from sharp edges, re-belaying, rope care etc etc is more important!


AlanD

29-Nov-2007
12:38:50 AM

Ok, the webpage I referred to get the fall factor of 2.3 was incorrect. 2 is the max that can be achieved. But my point still remains, after correcting for the 2.3 back to say 1.8, you end up with a safety margin of 9. I weigh about 100kg, not 80kg, so that will cut my safety margin and if I was doing a recue, that margin would be further cut by 50%. Potentially you could have an extremely low safety margin.

I fully admit that the likely hood of a rope braking is low during a fall, but the potential does exist, particularly if the rope is not well maintained and regularly inspected.


muki

29-Nov-2007
8:28:51 AM

On 28/11/2007 AlanD wrote:
>The safety margin is much smaller than people think
>
>Take your 3000 kg rope,
>reduce that by 40% because of the the knot = 1800 kg
>give the rope some wear (minor damage to the sheath and a bit of git in
>the core) 25%= 1350 kg

Given that the bulk of the strength of a rope is its core, I think that a 25% reduction in strength is too
much, maybe if a quarter of the core was shot then you could apply those figures, but not otherwise.

>You have an 80 kg climber, with a 2.3 factor fall (UIAA standard) = 184kg
>Safety margin = 7.3

Also have to say that a figure of 2,3 is pretty out there, maybe even impossible to generate this
but it does make your theory sound more knowledgeable to quote facts and figures,
though only they were the correct figures!

>7.3 is fine, but if you're doing a rescue or the rope is over a sharp
>rock when you take the fall, that safety factor is much lower.

And trying to use this situation, to then say that if you were doing a rescue would further reduce the
safety margin is just plain crazy! who would climb up 60m from the anchors with a quarter of the rope
shot, and then connect another climber to themselves and jump off?
If I ever get in trouble and need a rescue...... please don't come to my aid, I'll take my chances with
somebody else. BP


skink

29-Nov-2007
9:25:46 AM

On 28/11/2007 AlanD wrote:
>The safety margin is much smaller than people think

>Safety margin = 7.3

Wow, with absolute worst case numbers, my rope is still 7.3 times strong enough if I factor 2.3 onto it...

I don't know about the rest of you guys, but that's actually a bigger safety margin than I was expecting


skink

29-Nov-2007
9:30:59 AM

On 29/11/2007 AlanD wrote:
>I fully admit that the likely hood of a rope braking is low during a fall,
>but the potential does exist, particularly if the rope is not well maintained
>and regularly inspected.

Never has a rope broken in a climbing fall except where severely compromised by chemicals or sharp edges (as far as I know - anyone heard/seen different?).

Therefore, I strongly disagree that the likelihood of a rope breaking during a fall is low, I think the likelihood is closer to non-existent.

ademmert

29-Nov-2007
9:35:18 AM

i've seen the sheath break a couple of times


skink

29-Nov-2007
10:16:24 AM

On 29/11/2007 ademmert wrote:
>i've seen the sheath break a couple of times

break, or be cut?


AlanD

29-Nov-2007
11:01:03 AM

On 29/11/2007 bomber pro wrote:
>Given that the bulk of the strength of a rope is its core, I think that
>a 25% reduction in strength is too
>much, maybe if a quarter of the core was shot then you could apply those
>figures, but not otherwise.

You're assuming that the core is only damaged by being cut in one location and filaments that make up the core do not lose strength with age, unfortunately this is not the case. If you rope gets grit into the core, that grit can start cutting the individual filaments or groups of filaments, without any damage to the sheath. It's also the nature of polymers to lose strength overtime as the between the polymer chains break, heat being a major influence.

>Also have to say that a figure of 2,3 is pretty out there, maybe even
>impossible to generate this
>but it does make your theory sound more knowledgeable to quote facts and
>figures,
>though only they were the correct figures!

Yawn. See the correction above to the 2.3 figure. Yes, by definition 2.3 is impossible to generate. Figures do put things more into perspective, even they are guessimates, to many people see that there rope or sling, beaner are rated to take a certain load, divide that by how much they weigh and thing that they have a safety factor of 40. Knots, damage to the core from cutting, age, the type of fall, loads etc reduce that significantly.

>And trying to use this situation, to then say that if you were doing a
>rescue would further reduce the
>safety margin is just plain crazy! who would climb up 60m from the anchors
>with a quarter of the rope
>shot, and then connect another climber to themselves and jump off?>

From my understanding, a 4 metre fall generates the same fall factor a 120 metre fall. Surprising anchor points fail, including those above you, while you might not delibrately jump off while doing a rescue, the situation can develop through the failure of other items.


muki

29-Nov-2007
11:10:02 AM

I am involved in a rescue group, and we have so much redundancy that the rope, and a complete anchor
system could fail, and we still would be totally OK, I know that the factor 2 could happen at 4m as well
as 60m, but if the anchor fails during a rescue that has no redundancy, then it really won't matter if the
knot has reduced the strength of the rope, will it?

Ronny

29-Nov-2007
11:27:36 AM

Ok, much as I like a bit of chockstone bitching, I'm going to break up the monotony with this short anecdote about how strong ropes are.

A few years back I had a 20y.o. 244 volvo, and on the way back from Araps one day it broke down (surprise surprise). So the next day my brother is towing me from my house to the mechanics and we've used an old piece of climbing rope doubled over. We get to a round-about, stop and then start to pull out into the intersection. Then from out of nowhere a car full of school age girls comes screaming through the intersection from the right. They obviously don't see the rope (or me frantically yelling and waving out the window) and decided to drive between my car and my brother's.
They realise really late, hit the skids and hit the rope at some (not insignificant - say 20kph maybe?) speed. I'm most worried about them running into one of us, so both me and my brother have the brakes on. They end up getting pulled up by the rope, with the front wheels of their car sliding/skidding slightly to the right as their car slid along the rope. But the rope didn't break.

Since then I've figured that if my old retired rope (admittedly doubled over) can take a car hitting it sideways while already under tension it was probably ok for me and some fatass belayer...

Ok, now back to the made up numbers and the nit-picking...

mikl law

29-Nov-2007
12:18:25 PM

opinion of a scared fat bloke?
ropes don't break - they cut or untie or weren't tied to begin with (or are so "strong" that the loading on gear is impossible and it fails)
I still don't believe a Volvo 244 broke down, did you fail to pick up a Swedish hitchhiker or some other karmic misdemenour?


muki

29-Nov-2007
12:25:43 PM

Hi Ronny, I too have used ropes outside of their designated chore, my use of chords was to redirect 3
& 2 ton RSJ beams inside the St Pauls cathedral in Melbourne, the main crane (that I installed using a
six to one pulley system) was doing the bulk of the lifting, then I had climbing rope to do the
redirection work.
The load that the 3 ton beam generated on the redirect, would have easily been 2/3 of the primary
load,
that was a static loading with redundancy's, and did the job perfectly.
I have no problems with falling onto my rope during a lead, that's what they are designed for! even with
a reduction in strength from tying knots in it, and will add that being a climber, I am well aware of the
reduction issues of knots and am quite happy with the amount of overkill these ropes are built with to
deal with that reduction, as has been said before on this thread a rope has never "broken" without
some reason like chemical exposure (Yosemite climber) or extreme weathering (Dan Osman) to my
knowledge. BP


AlanD

29-Nov-2007
12:26:33 PM

Similar, I used to be part of NSW Cave Rescue. I agree, building in redunancy is critical, but sometimes s*** happens. Even if you reduncancy is lost during a complete failure of your top anchor set ups, if you have rebelays placed further down, then the strength of the rope becomes an issue again.

There are 22 messages in this topic.