Knot a pull cord to the abseil rope

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Neo
Posts: 17
Joined: 26 May 2018, 21:43
Full name: Daniel Mitchell
City: Port Macquarie
State: New South Wales

Knot a pull cord to the abseil rope

Post by Neo » 08 Mar 2021, 17:37

G'day

I have some Imlay Canyon Fire 8.3mm static rope and got some dyneema 3mm from T2 as the pull cord.

I am wondering what is the best way to join two very different diameters?

The best a couple of search parameters comes up with is the overhand knot, same as if joining two similar ropes to later pull.

I haven't played yet but am thinking to use the overhand with a long tail on the 3mm followed by some half hitches of the 3mm around the mainline to prevent the possibility of the smaller dyneema cord slipping through the knot...

Wadayado? Please tell!

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T2
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Full name: Tim Vollmer
City: Blaxland
State: New South Wales

Re: Knot a pull cord to the abseil rope

Post by T2 » 10 Mar 2021, 09:57

Daniel,

I primarily use my dyneema pull cord with a fiddlestick, so don't have a clear solution for you.

A couple points though. Firstly, if you're using a pullcord in a conventional way (i.e. to physically pull your rope down) then you'll need some sort of block at the anchor. My preference in that setting is for a 'biner block. At the point that you've used a 'biner block, why not just attach your pullcord to the 'biner to retrieve it? If you have an eye splice in the end of your Dyneema pull cord, this retains 90-100% rope strength, so is a super secure way to attach the thin pullcord to a 'biner.

The main issue with Dyneema is it is quite slippery (the technical language is that it has a very low coefficient of friction). This is what allows it to slip with conventional knots. So while you're not imparting huge forces during a rope pulldown, there is the risk of it slipping through. Given that, whatever you decide on, you should do some practice pulls (hard ones) in a controlled setting where you can test the system first.

Another other issue with thin Dyneema cords is they can cut into your hands if you are pulling hard. For your system, you'd probably want to clove hitch a 'biner to the cord when you're pulling it, and use that like a handle. It'll make things much better for your hands.

All that said, and to actually answer your question, the two options I'd probably experiment with are a stacked overhand (possibly with the Dyneema cord used to tie an additional scaffold knot on the tail of the abseil rope) or a triple fishermans. I'd make sure I had a decent tail to allow for some creep with the Dyneema.

I'd be curious to hear how you go and what works best with some testing.

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