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I want to rappel off of 10mm dynamic rope. The rope will be fed through both sides of the ATC device (parallel). Is it safe to use a 4mm cord for my French Prusik auto-block knot?

There is a similar question [1] but it only refers to ascending, while I am asking about descending. Note that my question covers using a Prusik around 2 parallel ropes, while with ascending the Prusik would be around a single rope (based on what I've heard from others on this thread).

[1] What's the minimum diameter cord to use when climbing a rope using a prusik

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    Whether you are going up or down makes no difference as to the diameter of the cord needed – Charlie Brumbaugh Nov 15 '18 at 7:07
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    Either the answers 'over there' should make clear that climbing or rappelling makes no difference (then it is a duplicate), or there should be an answer here along the lines of Charlies comment (and it's not a duplicate). That may be obvious to many, but not to the OP, or he wouldn't have asked. – Jan Doggen Nov 15 '18 at 8:56
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    Why do you want to use 4mm cord when thicker cord is more accepted? In rock climbing, it is best to stick with established accepted procedures. – Qudit Nov 15 '18 at 10:31
  • @CharlieBrumbaugh up is single strand taking full body weight while down is double strand and only partial weight. – StrongBad Nov 15 '18 at 16:14
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There is a significant difference between using an autoblock for rappelling/lowering and a Prusik for ascending. In the ascending case, the Prusik typically goes around a single strand of the rope while when rappelling, the Prusik goes around both strands. This means you could theoretically, get away with a thicker cord when rappelling. Another difference is that if the autoblock is below the rappel device, then the loads are substantially different since there is friction from the rappel device and the chance of shock loading the system is very different. When the Prusik is below the rappel device, there are additional issues with unloading the Prusik since there is a lot less space to work.

I have no doubt that a 4mm cord in pristine condition would provide sufficient holding strength on two strands of 10mm rope to support your weight and any shock loading. I would worry about wear and heat build up with such a thin cord. I also think it would be a nightmare to manage.

Overall, I think the answer is yes you can do it and it will provide additional safety over no autoblock, but if you can use a more common diameter cord (6mm or 7mm), I would go with that.

  • Thanks for the response. It might be worth focusing more on the fact that the Prusik is beneath the ATC, which greatly reduces shock loading and the non-shock-load force exerted, as it isn't really "holding my full body weight" in the traditional sense (due to the massive amount of friction from the ATC). – Chris Morris Nov 16 '18 at 17:11
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I would say no. 4mm is just too small. That's really accessory cord at that point.

Considering it's only about the ability of the cord to constrict on the rope and block, the diameter is always a tradeoff between having lots of grabbing power and being easier to loosen after having been loaded.

There are two general rules where I'm from:

70% diameter of your climbing rope (so for a ~10mm rope that'd be 7mm prusik)

half diameter + 1mm (so 6mm this time)

It comes down to general preference then but keep in mind with cold or numb hands, the smaller the cord, the harder it is to loosen from the rope. 4mm is asking for trouble on that front for no good reason. On a smaller rope, sure, the tightness is a least of two evils, but on a bigger rope, it's unneeded. We haven't touched on the resistance of smaller diameters either or other factors which the other question covers.

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