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jeffrdu
Mar 23, 2013, 2:30 PM
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I have some brand new tech cord that I was using to sling a big bro. Triple fisherman's knot. Problem is...I don't like the length of what I ended up with. Now that all my knots are set, would it be a problem to untie it and use it in a hexentric I have that needs to be redone? I think the length would be better on it. I just don't know how much knots weaken the cord, and if untying the knots and retying other knots in potentially other places on the rope is risky.
From what I've read, I think actually falling on the cord is the real issue.
Thanks for any input.
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dagibbs
Mar 23, 2013, 4:50 PM
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When people talk about knots weakening rope/cord, what they mean is that when the rope is stressed, the knots are where the rope generally fails, and this is due to the uneven pressures on the rope because of the tight turns the rope undergoes -- more of the stress is taken up by less of the diameter.
So, untying knots, then retying them elsewhere, or even in about the same place, will be fine.
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marc801
Mar 23, 2013, 5:30 PM
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dagibbs wrote: So, untying knots, then retying them elsewhere, or even in about the same place, will be fine.
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wivanoff
Mar 24, 2013, 2:01 PM
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jeffrdu wrote: I just don't know how much knots weaken the cord, and if untying the knots and retying other knots in potentially other places on the rope is risky.
What dagibbs and marc said. You're fine. There's probably more flexing from it dangling on your rack when climbing or from rope movement when you place it than retying those knots a few times.
EDIT: Some folks use Tech Cord to make their cordelettes. Those get tied and untied regularly. Although regular nylon cord might last longer, we don't hear about Tech Cord cordelettes regularly failing from to much flexing.
I thought Kevlar was bad but Spectra, Technora, Dyneema were OK for reasonable amounts of flexing.
I'll post what I found while researching high tech cords. This was mostly gleaned from manufacturer's websites. I am not a chemist, so if anyone has more current information please let me know and I'll edit this post:
Kevlar: Apparently, Kevlar's surface is very rough and fibers under load "sand" each other to dust as the pass by each other when bending over an edge. So, Kevlar is not so good for our climbing applications.
Technora is an aramid, as is Kevlar. But, while they are both aramids, Technora is called a para-aramid and seems to have better fatigue resistance when flexing.
Pelican Rope says this about Technora: "Technora® shows little loss of strength during repeated abrasion, flexing and stretching which makes this rope the perfect choice for pulling, winch, sheave and turning block applications. Exceptional molecular stability ensures a high modulus of elasticity, low creep and stress relaxation, as well as low thermal shrinkage. Highly resistant to heat, acids, alkalis and organic solvents"
Technora is used in timing and vee belts - both of which take a lot of flexing.
Dyneema and Spectra are both very similar. Both are High Modulus Polyethylene (HMPE) although the manufacturing process may be slightly different. They both seem to do pretty well with strength, abrasion resistance and fatigue resistance when flexing.
Vectran is a liquid crystal polymer (LCP). The manufacturer lists it as having these qualities: "High strength and modulus, High abrasion resistance, Excellent flex/fold characteristics, Minimal moisture absorption, Outstanding cut resistance." I think Vectran is $$$ compared to the others and may not be as available..
Beal 5.5 Dyneema = Dyneema core
BD Gemini = Kevlar core (no longer sold)
Bluewater 5mm Titan = Dyneema core
Edelweiss 5.5mm Aramid Cord = Technora(?) core
NE Rope 5mm Maxim Tech Cord = Technora core
Sterling Powercord = Technora core
Sterling Vectran = Vectran core (not sure if this is still available)
Spectra® is a registered trademark of Honeywell.
Dyneema® is a registered trademark of DSM Dyneema.
Vectran® is a registered trademark of Kuraray.
Kevlar® is a registered trademark of DuPont.
Technora® is a registered trademarks of Teijin.
I currently use Maxim Tech Cord for Gunks tie offs on rigid stem Friends and old hexes. It's tied with a TRIPLE fisherman's and replaced every 3-4 years.
Again, if anyone has more current/more accurate information please let me know and I'll happily edit this post. But, please post a verifiable link, not just your opinion or something that you heard from a friend.
(This post was edited by wivanoff on Apr 9, 2013, 11:19 AM)
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jeffrdu
Apr 6, 2013, 11:57 AM
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Ok, thanks everyone for your input. That was helpful.
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