Do 5mm (vs 8mm) rapides cause undue force for rope pulls?

One often hears that small quicklinks require greater force to pull a rope, leading to stuck ropes in canyoneering adventures; one source uses this claim to recommend 7mm over 6mm quicklinks. Perhaps the migration to single-rope techniques makes this academic.

Let’s compare forces required to pull 8.3mm polyester and 11mm nylon (dynamic) ropes through 5mm and 8mm rapides (thus bracketing the 6mm and 7mm debate).  We'll measure the force required to pull  the ropes up and  through the quicklinks, against the weight of a 5 lb dumbell.

8mm_vs_5mm

You can see the thickness of the rope made the greatest difference; the size of the rapide was a relatively minor effect. Possibly some of the extra force required for the 11mm dynamic rope, is taken up by the energy of stretching. The inefficiency of the pull over the 5mm link is a little greater – but comparable – to the inefficiency of pulling a rope over a carabiner. A 13mm rope (13mm is the inner diameter of curvature of a 5mm rapide) certainly will get stuck; but I don’t know any canyoneers who use ropes over 10mm.

 

How was the test done? I used a 44x2 lb AWS (CE-certified)  hanging scale, calibrated on small dumbbell weights. To record the forces, I took videos of the digital screen with my smartphone attached to the scale through a flexible tripod, and used only the "plateau" forces. A fuller description of the test protocol is here.

 

Each point on the plot represents 4 pull tests (hence the error bars). To get a value for each of the four tests at each point on the plot, ~20 times were used to define the “plateau" on a single pull test, and I averaged the 3 highest values from each plateau. I had the rapides hung from the top of my stairwell, and had ~20’ of rope up and through a rapide “pulley,” down the other side. The far side had a 5lb dumbbell, and I walked down the stairs on the other side, trying to maintain an even rate as I raised the dumbell. The force went up as the weight was lifted, plateaued, then went down at the end of the test.