Forces for raps on thin cord (3.2 to 5.5 mm)

(Takeaways: A supermunter can be S-L-O-O-W-W-W;  force excursions during "dynamic" actions may matter; Amsteel pretty good. Click on images for larger versions.)

This report gives the forces seen at the anchor by hanging raps on thin cords. The cords were: 5.5 mm Titan (nylon sheath on Dyneema core), Beal 5mm nylon, and 3.2 mm Amsteel Dyneema. The motivation was a HOWNOT2 video about a special rappable 5mm cord, and the problem of finding rappel devices.  Personally, I prefer to rap thin lines on variants of the munter/supermunter, which I use in this report; The Munter with extra twist is ilustrated here. Munter-type hitches, on a small carabiner, are not that "smooth;" there is some jerkiness during the descent, which certainly is felt at the anchor.

The tests involves a 10’ rap in my stairwell, which has a massively-reinforced ceiling anchor:

A Linescale 3 loadcell hangs below the anchor, sampling at 40 Hz. I weigh just 142-150 lbs in gear, adjust accordingly for your own weight.

My “bail” rig for go-light travel is 1/8” (3.2 mm) Amsteel, with a light harness and an Attache carabiner. Sometimes the “harness” is just 12’ of  5/8" (16mm) climbspec tubular webbing tied as a hasty harness. The Attache carabiner has an annoying tendency to stick, so in sandy terrains I may use a double-wire gate biner. Please, this is a bail rig, not for fulltime use in rappels. I’ve found that abrasion, NOT HEAT, is the main concern with Dyneema raps. From personal experience, there is a 10% chance one of 12 yarns in Amsteel will be cut DURING THE PULL when the line gets stuck on sharp projections. While this is not a catastrophic abrasion, it can be expensive, and it may reflect the rough rocks for my raps.

Individual tests

We start by examining  two raps on Bluewater 5.5mm Titan cord (nylon sheath over a Dyneema core), with a single strand tensile strength reported as 13.7 kN (3080 lbs). This cord is extremely stiff as purchased, and needs to be run (full length and weighted) several times over a carabiner to develop sufficient flexibility for knotting. Below are the forces measured in trials with 1) single- and 2) two-strand supermunters. The single strand rap is from a southwestern CEM knot (3 half-hitches), which decreases the strand strength by roughly 40-50%. Note the slowness of the supermunter on two strands—up to 45 seconds to descend 10’ in a hanging rap. At the end of the single-strand supermunter rap, I pulled the CEM knot. At the end of the two-strand supermunter rap, I took three hops, from 12-18” above the nadir, to see how much force small drops would create at the anchor. The max of 560 lbs is probably about 1/3 of the breaking strength of the CEM knot that allows this to be a single-strand rap.

Here are links to the corresponding videos (note videos average 30MB): Titan_1_strand , Titan_2_strand

The next tests involve:

 A) two strands of Titan in a single Munter with an extra wrap, and B) two strands in a supermunter on 5mm Beal nylon. The Beal nylon has a reported strength of 6.5 kN (1461 lbs) per strand, so the nylon double-strand is likely much stronger than the Titan single-strand (on a CEM knot) in the discussion above. Note the nylon double-strand supermunter time is close to the Titan single-strand supermunter time.  Again I take hops at the end to see the force felt at the anchor. The difference in “hop” force between the two ropes is not that great, as a lot of energy is absorbed by my body.

Here are LINKS to videos of the raps (note videos average 30MB):  Titan_xtra_Wrap, Beal_Nylon

Lastly, here is the force felt at the anchor for 3.2 mm Amsteel, double strand supermunter, with three hops at the end to show the force at the anchor on a mild dynamic move. At the end of the last hop, the Amsteel was quite stiff. I bag Amsteel after every rap, and it tends to regain softness and some “construction stretch” before the next use, due to the bends while bagging. The supermunter rap is fairly “fast,” and can be made faster by using a single Munter with extra wrap instead. Video is here.