How effective is a friction device in front of Tandem Prusiks?

How effective is a friction device in front of Tandem Prusiks?

Back in 2002, yes that’s over 15 years ago, we started putting a friction device in front of Tandem Prusiks (pretension) in the backup belay line as part of a two rope system for Mountain Rescue. We attached a 1m sling in front of the Prusiks (to prevent any Prusik/device interaction) and then connected a climber belay/rappel device (aka ATC).

Tandem Prusik with Pretension

The friction device was to overcome a specific problem of managing the Tandem Prusiks on a heavy belay rope for longer slope lowers we often encountered. It had the added benefit of pre-tensioning the backup belay rope with a set amount of friction. The mainline did most of the work and speed control of the load.

We didn’t consider at the time if this would work for failures with a small amount of rope in play. So more recently in 2016, we tested a climbers belay/rappel device as a pretension in the backup belay line. What we found is very little force went onto the Prusiks as the friction device takes the load. On all occasions, we were able to bump (i.e., disengage) the Prusiks by hand after taking a drop. However, during this testing we had the friction of an edge involved so decided to undertake drop testing to simulate a worst-case scenario.

We had undertaken some previous drop tests using 10mm static and semi-static rope with 7mm Tandem Prusiks (Let’s lighten the load). Our baseline number for a 1m drop on 3m of rope with a 200kg load was an average of 8.22kN and 171mm of slip.

As we were testing for a backcountry audience, we did drop tests with a variety of friction devices including a brake bar rack (caving), figure-8 descender (canyoning) and belay/rappel device (climbing).

Testing Setup

  • Undertook a 1m drop on 3m of rope (with the 3m measured from the Prusiks)
  • 200kg mass
  • 10mm semi-static rope
  • Friction devices including brake bar rack (Aspiring Safety Abseil Rack), figure-8 descender (Petzl Huit) & climbers belay/rappel device (Black Diamond ATC)
  • 120cm 15mm Spectra sewn sling extender
  • Rock Exotica Enforcer load cell.

Tandem Prusiks on Load Cell Friction Device on 1.2m sling

Testing Notes

  • We used a rigid tower on Takaka Hill, Nelson, New Zealand
  • New rope and cord used for testing.
  • Friction devices had had some previous use.
  • The 10mm rope (Kordas Iris 26.5kN) tests used a bowline knot at the load end.

Testing results

7mm Tandem Prusik with a Brakebar rack (4 bars used) on a 1.2m sling.

  • Force: 2.44kN
  • Slip: 120mm
  • Prusiks: Both Prusiks welded onto the rope.
  • Rope/sheath: Slight glazing, minor hardening, and furring of rope.
  • Device: the rope bent the Rack

7mm Tandem Prusik with a climber belay/rappel device on a 1.2m sling.

  • Force: 1.40kN
  • Slip: 40mm
  • Prusiks: Short Prusik welded on. Long Prusik free and undamaged.
  • Rope/sheath: Slight furring in 3 spot locations as it passed through the ATC.
  • Device: the ATC not was damaged.

7mm Tandem Prusik with a Fig-8 descender (small hole) on a 1.2m sling

  • Force: 2.14kN
  • Slip: 20mm
  • Prusiks: Both Prusiks were tight, however, free and undamaged.
  • Rope/sheath: Rope furring over 70mm at the small hole on the 8. Rope hardening on the initial impact area.
  • Device: the Figure-8 descender was not damaged


  • All the friction devices performed well. The Prusiks on average saw 2kN and slipped 60mm. This result is on average over a 6kN and 375mm reduction from Tandem Prusiks alone (Let’s lighten the load).
  • The rope wanted to straighten the brake bar rack. Therefore, it bent like a banana. In the unlikely event of failure, the rack would be unusable for any further abseiling. (Note: recommend your rack is tested to a minimum of 15kN)
  • Despite the low amount of force (1.4-2.4kN) 2 out of the three tests had the one or both of the Prusiks welded on to the rope. I would recommend continuing to use Tandem Prusiks to dissipate the energy and not drop back to one Prusik.

Final thoughts

The added benefits of using a pretension – friction device in front of the Tandem Prusiks –  in your backup belay line for lowering as part of a two rope system include:

  • Works well with a drop test 1m drop on 3m of rope as only a small amount of force ends up on the Tandem Prusiks.
  • Gives set amount of friction on the rope. You don’t have to think about balancing the tension between the two lines. You just get what you get.
  • Allows you to manage the Prusiks efficiently especially when you have lots of rope out. The pretension does the work so you can control the Prusiks.

Grant at Over The Edge Rescue.

Thanks to:

Testers: Ollie Polson, Chris Whitehouse, and Maarten Haenen
Use of the Tower: Nelson Speleo Group
Weights: Oz Patterson

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