In cave rescue, we had been using the Double Sheet Bend for rigging – being easy to adjust, especially in the middle of a rope or cord and easier to untie after significant loading.
I was recently (2021) undertaking testing to confirm our cave rescue setup (see the post Let’s lighten the load – update), so I thought testing the Double Sheet Bend (as part of this) would be a good idea.
I was somewhat surprised to find that in 10mm static rope, the double sheet bend started slipping at around 10kN and then continued to slip until breaking 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 in performance compared to a figure-8 rethread and double fisherman’s bend.
Note: This testing has been done in the context of joining static ropes and accessory cord for rescue work. For information on joining static ropes for pull-through retrieval (see the post Which bend for joining ropes?)
As a bonus, I have included a link to 11 videos of the variations tested AND (as of May 2023) added all the testing into a 100-page+ report.
Author’s Note: This post was originally published in March 2021 and has been updated for accuracy and comprehensiveness.
Table of Contents
Variations tested
10mm PMI Classic Sport Static 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
Test Method
- A 50kN 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 used for the testing was brand new.
- All tests were completed between 12mm pins, rope grabs or 12mm steel carabiners.
- In 10mm rope, 5cm tails were used as the standard when tied off, and 15cm tails when not tied off. In 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.
- The 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 out; for the 10mm rope – the Max Force was divided by 27; for the 8mm cord loop – the Max Force was divided by 28.6 (cord breaking strength 14.3 x 2)
Materials used
PMI 10mm classic sport static rope (EZ bend)
- 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 Static Rope
Double Sheet Bend
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 + Yosemite Backup
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 + Double Overhand Backup
(bight side)
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)
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
Testing
- First Slip: Did not slip
- Average Max force (3 tests): 18.13 (67%)
- Comments: Broke at the bend
Figure-8 on a bight rethread bend
Testing
- First Slip: Did not slip
- Average Max force (3 tests): 18.15 (67%)
- Comments: Broke at the bend tail side
Analysis: for 10mm PMI Classic Sport Static 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, 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 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 break at the bend at a consistently high force (18kN – 67%)
Testing 8mm PMI Accessory Cord Loops
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 the bight side sucked in 60-90% before breaking
Double Sheet Bend + Yosemite Backup Loop
Testing
- First Slip: 9.11kN
- Average Max force (3 tests): 12.09 (42%)
- Comments: Bight side slipped through
Double Sheet Bend + Double Overhand Backup
(bight side)
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 + Double Overhand Backup
(both sides)
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
Testing
- First Slip: Did not slip
- Average Max force (3 tests): 19.01 (66%)
- Comments: Broke at the bend tail side (2) and the bight side (1)
Double fisherman’s bend
Testing
- First Slip: Did not slip
- Average Max force (3 tests): 23.72 (83%)
- Comments: Broke at the bend(2) and the pin (1)
Analysis: for 8mm PMI Accessory Cord Loops
- 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 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 break at the bend at a consistently high force (19-23kN – 66-83%)
Testing Summary
PMI 10mm classic sport static rope
| Bend | First slip (kN) | Max Force (kN) | % |
|---|---|---|---|
| Double sheet bend | 10.47 | 13.24 | 49 |
| Double sheet bend + Yosemite | 3.60 | 7.04 | 26 |
| Double sheet bend + Double Overhand bight side | 7.74 | 13.57 | 50 |
| Double sheet bend + Double Overhand both sides | 10.58 | 13.56 | 50 |
| Fig-8 rethread bend | None | 18.13 | 67 |
| Figure-8 on a bight rethread bend | None | 18.15 | 67 |
PMI 8mm accessory cord loop
| Bend | First slip (kN) | Max Force (kN) | % |
|---|---|---|---|
| Double sheet bend | 14.11 | 17.89 | 63 |
| Double sheet bend +Yosemite | 9.11 | 12.09 | 42 |
| Double sheet bend + Double OH top side | 12.04 | 16.56 | 58 |
| Double sheet bend + Double OH both sides | 13.22 | 16.60 | 58 |
| Fig-8 rethread bend | None | 20.34 | 71 |
| Figure-8 on a bight rethread bend | None | 19.01 | 66 |
| Double Fisherman's bend | None | 23.72 | 83 |
Conclusions
The Double Sheet Bend (DSB) and any of its variations are not recommended for any rigging in rescue.
What makes the DSB adjustable and untie after loading (the bight) is what slips and fails. By the time you go to the trouble of tying backups 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 excellent 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 use.
Some of you may say ‘its fine’ if you bury the bend back in the rigging where it’s not taking much force.
However, consider the following:
- Is it ‘fine’ to use a bend that can’t be used everywhere in the rigging? AND
- It may be ‘fine’ until someone without the requisite knowledge uses it in a position that takes the entire load.
Testing Videos and Report
Testing Videos
Click the button (or image) below and get free 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 for overtheedge.training, then use the same login and password.
