Double sheet bend; but it’s fine?

Introduction

I was recently undertaking testing to confirm our cave rescue setup (see the post Let’s lighten the load – update).

In cave rescue, as we use the double sheet bend for rigging – being easy to adjust, especially in the middle of a rope or cord – I thought it would be a good idea to test it. 

I was somewhat surprised to find that in 10mm rope, the double sheet bend started slipping at around 10kN and then continued to slip until beaking at around 13kN. In the same rope, a figure-8 rethread bend did not slip and broke at 18kN.

So I went down a side street for a while to see if any other variations of the double sheet bend made a difference to performance compared to a figure-8 rethread and double fisherman’s bend.

As a bonus, I have included a link to 11 videos of the variations tested.

Variations tested

10mm PMI Classic Sport Rope

  • Double sheet bend
  • Double sheet bend – Yosemite backup bight side
  • Double sheet bend – double overhand backup bight side
  • Double sheet bend – double overhand backup both sides
  • Figure-8 rethread bend
  • Figure-8 on a bight rethread bend

8mm PMI Accessory Cord Loop

  • Double sheet bend
  • Double sheet bend – Yosemite backup bight side
  • Double sheet bend – double overhand backup bight side
  • Double sheet bend – double overhand backup both sides
  • Figure-8 rethread bend
  • Figure-8 on a bight rethread bend
  • Double fisherman’s bend

How to tie

Double sheet bend
Double sheet bend
Double sheet bend Yosemite backup
Double sheet bend Yosemite backup
Double sheet bend double overhand backup bight side
Double sheet bend double overhand backup bight side
Double sheet bend double overhand backups both sides
Double sheet bend – double overhand backups – both sides
Figure-8 rethread bend
figure-8 on a bight rethread bend
Figure-8 on a bight rethread bend
Double fisherman's bend
Double fisherman's bend

Test Method

  • A 100kN vertical testbed (60Hz) was used at Aspiring Safety, 1/6 Burdale Street, Riccarton, Christchurch, New Zealand. https://www.aspiring.co.nz.
  • Any cord or rope was used for the testing was brand new.
  • All tests were completed between 12mm pins, rope grabs or 12mm steel carabiners. 
  • 5cm tails were used as the standard when tied off and 15cm tails when not tied off in 10mm rope. All 8mm cord tails were 7cm.
  • All bends and backups had hand tension. All strands were pulled to the get bend tight and compact.
  • Three tests were undertaken on each variation unless otherwise stated.
  • Testing speed was set at 100mm/minute.
  • All the 10mm ropes were tested between rope grabs with the bend in the middle.
  • All the 8mm cords were tested between 12mm pins as a loop.
  • The percentages were worked; for the 10mm rope – the Max Force divided by 27; for the 8mm cord loop – the Max Force divided by 28.6 (cord breaking strength 14.3 x 2)

Materials used

PMI 10mm classic sport rope

  • Diameter: 10mm
  • Weight: 66 g/m
  • Static elongation: 2% (140kgs)
  • Breaking strength: 27kN
  • Materials: Core/Sheath – Nylon/ Nylon
  • Standards: CI 1801 static rope
  • Manufacturer: PMI – www.pmirope.com

PMI 8mm accessory cord

  • Diameter: 8mm
  • Breaking strength: 14.3kN
  • Materials: Core/Sheath – Nylon/Nylon
  • Standards: EN 564 Accessory Cord
  • Manufacturer: PMI – www.pmirope.com

Testing 10mm PMI Classic Sport Rope

Double Sheet Bend

Double sheet bend single

Testing

  • First Slip: 10.47kN
  • Average Max force (3 tests): 13.24 (49%)
  • Comments: Slipped through initially 50-95% of tail length then broke at the bend bight side core (2) and double wrap side (1)
Double sheet bend PMI 10mm

Double Sheet Bend + Yosemite Backup

Double sheet bend yosemite single

Testing

  • First Slip: 3.6kN
  • Average Max force (3 tests): 7.04 (26%)
  • Comments: Bight side slipped through and came apart (1) and stayed together one side (2)
Double sheet bend yosemite 10mm

Double Sheet Bend + Double Overhand Backup

(bight side)

Double sheet bend doh bight side single

Testing

  • First Slip: 7.74kN
  • Average Max force (3 tests): 13.57 (50%)
  • Comments: Broke bend bight side (2) and double wrap side (1)

Double Sheet Bend + Double Overhand Backups

(both sides)

Double sheet bend both sides single

Testing

  • First Slip: 10.58kN
  • Average Max force (3 tests): 13.56 (50%)
  • Comments: Broke bend bight side (1) and double wrap side (2)

Figure-8 rethread bend

fig8 rethread bend single

Testing

  • First Slip: Did not slip
  • Average Max force (3 tests): 18.13 (67%)
  • Comments: Broke at the bend
fig8 rethread bend

Figure-8 on a bight rethread bend

fig8 on a bight rethread bend single

Testing

  • First Slip: Did not slip
  • Average Max force (3 tests): 18.15 (67%)
  • Comments: Broke at the bend tail side
fig8 on a bight rethread bend

Analysis

for 10mm PMI Classic Sport Rope

  • The double sheet bend (DSB) initial failure method is the bight slips (10kN). It does not have enough rope on rope contact (twists and turns) to hold sufficiently. After slipping significantly, it broke at around 13kN (49%).
  • When the DSB is backed up with the Yosemite, it introduces more space around the bight, less rope on rope contact and starts to slip at a lower force (3kN) initially and slips through (7kN).
  • When the DSB is backed up with a double overhand on the bight side or both sides, the bend itself fails. The bends radius is very tight, breaking around 13kN (50%).
  • Both the figure-8 rethread and figure-8 on a bight rethread are very stable bends. The tails do not get pulled in and breaks at the bend at a consistently high force (18kN – 67%)

Testing 8mm PMI Accessory Cord Loops

Double Sheet Bend Loop

Double sheet bend loop

Testing

  • First Slip: 14.11kN
  • Average Max force (3 tests): 17.89 (63%)
  • Comments: Broke at the bight side, the tail from bight side sucked in 60-90% before breaking
double sheet bend loop

Double Sheet Bend + Yosemite Backup Loop

Double sheet bend yosemite loop

Testing

  • First Slip: 9.11kN
  • Average Max force (3 tests): 12.09 (42%)
  • Comments: Bight side slipped through
double sheet bend Yosemite loop

Double Sheet Bend + Double Overhand Backup 

(bight side)

Double sheet bend doh bight side loop

Testing

  • First Slip: 12.04kN
  • Average Max force (3 tests): 16.56 (58%)
  • Comments: Broke at the bend bight side (2) and double wrap side (1)
double sheet bend loop doh bight side

Double Sheet Bend + Double Overhand Backup

(both sides)

Double sheet bend both sides loop

Testing

  • First Slip: 13.22kN
  • Average Max force (3 tests): 16.60 (58%)
  • Comments: Broke at the bend double wrap side

Figure-8 rethread bend

Testing

  • First Slip: Did not slip
  • Average Max force (3 tests): 20.34 (71%)
  • Comments: Broke at the bend (1) and the pin (2)

Figure-8 on a bight rethread bend

fig8 on a bight rethread bend single loop

Testing

  • First Slip: Did not slip
  • Average Max force (3 tests): 19.01 (66%)
  • Comments: Broke at the bend tail side (2) and bight side (1)
fig8 on a bight rethread bend loop

Double fisherman’s bend

double fishermans bend single loop

Testing

  • First Slip: Did not slip
  • Average Max force (3 tests): 23.72 (83%)
  • Comments: Broke at the bend(2) and the pin (1)
double fishermans bend

Analysis

  • The double sheet bend (DSB) initial failure method is the bight slips (14kN). It does not have enough rope on rope contact (twists and turns) to hold sufficiently. After slipping significantly, it broke at around 17.5kN (63%).
  • When the DSB is backed up with the Yosemite, it introduces more space around the bight, less rope on rope contact and starts to slip at a lower force (9kN) and breaks at around 12kN (42%).
  • When the DSB is backed up with a double overhand (on the bight side or both sides), the bend itself fails. The bends radius is very tight, breaking around 16kN (58%).
  • The double fisherman’s, figure-8 rethread and figure-8 on a bight rethread are very stable; tails do not get pulled in and breaks at the bend at a consistently high force (19-23kN – 66-83%)

Testing Summary

Note: all tests are an average of 3.

PMI 10mm classic sport rope

Bend First slip (kN) Max Force (kN) % Comments
Double sheet bend 10.47 13.24 49 Slipped through initially then broke at the bend bight side core (2) and double wrap side (1)
Double sheet bend +Yosemite 3.6 7.04 26 Bight side slipped through and came apart (1) and stayed together one side (2)
Double sheet bend + Double OH bight side 7.74 13.57 50 Broke bend bight side (2) and double wrap side (1)
Double sheet bend + Double OH both sides 10.58 13.56 50 Broke bend bight side (1) and double wrap side (2)
Fig-8 rethread bend None 18.13 67 Broke at bend.
Figure-8 on a bight rethread bend None 18.15 67 Broke at bend.

PMI 8mm accessory cord loop

Bend First slip (kN) Max Force (kN) % Comments
Double sheet bend 10.47 13.24 49 Slipped through initially then broke at the bend bight side core (2) and double wrap side (1)
Double sheet bend +Yosemite 3.6 7.04 26 Bight side slipped through and came apart (1) and stayed together one side (2)
Double sheet bend + Double OH bight side 7.74 13.57 50 Broke bend bight side (2) and double wrap side (1)
Double sheet bend + Double OH both sides 10.58 13.56 50 Broke bend bight side (1) and double wrap side (2)
Fig-8 rethread bend None 18.13 67 Broke at bend.
Figure-8 on a bight rethread bend None 18.15 67 Broke at bend.

Conclusions

The Double Sheet Bend (DSB) and any of its variations are not recommended for any rigging in rescue.

The very thing that makes the DSB adjustable (the bight) is what slips and fails. By the time you go to the trouble of tying backups to attempt to improve performance, you may as well have tied something else that works much better.

In a 10mm rope, the figure-8 rethread bend is recommended, or in the middle of a rope tying a figure-8 on a bight rethread bend is recommended.

In an 8mm accessory cord, the double fisherman’s bend, a figure-8 rethread bend is recommended, or in the middle of a cord, tying a figure-8 on a bight rethread bend is recommended.

Final Thoughts

If someone shows you a new knot, bend or hitch (or variation) and tries to convince you it’s great based on a particular feature, stop, pause and think. Ensure they have (or someone has) tested it in the context you are using it, ideally with the ropes and cords you are using.

Some of you may say ‘its fine’ if you bury it back in the rigging where it’s not taking much force. 

  1. Is it ‘fine’ to use a bend that can’t be used everywhere in the rigging? 
  2. It may be ‘fine’ until someone without the requisite knowledge uses it in a position that takes the full load.

Click the image below and free get access to the testing videos. 

  • There are 6 videos – 1 of each test variation for the 10mm PMI classic sport static.
  • There are 7 videos – 1 of each test variation for the 8mm PMI accessory cord loop.

Note: If you have previously signed up to overtheedge.training use the same login and password.

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