How do knots in slackline webbing influence the breaking strength?

Tear it up

Introduction

Since a few years the low-tension lines have become more of a standard. This new style brings new styles of rigging, one of these is tying knots in the back up webbing. But what does a knot in webbing actually hold? How does the knot influence the breaking strength of the webbing? As SLACKTIVITY we also had the goal to test out multiple knots to connect our back-up webbing to the mainline in our Type B webbings (redTube & pinkTube). This is done by making a knot in your back up webbing, and connect it to a T-Loop with a quicklink.

WARNING: Knots can be complicated and hard to check properly. Don’t use them if you’re not 100% sure that the knot is correct, or if there is nobody that has the knowledge to double check it. Even after tying a knot 20 times, mistakes can still happen. Always double check each other rigs.

Method

Test setup

For this testing we used a static break test machine. On both ends we used a seaHorse for connection, the seaHorse was fixed to a sling with the back pin, that was attached to the breaking test machine. The knot was attached to the front pin in the loop, and the “walking end” was locked in the seaHorse in a single wrap. A pretension between 0.1kN-0.2kN was put on the knot with the machine. Then the test started to pull with a speed of 100mm/min until the knot broke completely, or so much strands in the webbing broke that the force dropped down below 40% of the highest tension reached. The tension was measured with 100Hz. Then the maximum force was read off the graph on the computer. The sample size on most knots was only one (n = 1). Therefore you should only see this as an indication and not as an exact number about how much a certain knot holds in a specific webbing. Read more about this in the discussion.

For this test we used all the different types of webbing Slacktivity has. In the table below you can read all the specifications. With the wide variety of webbing that we tested, it makes it easier to compare your (not SLACKTIVITY) webbing to the webbing that is tested here. Be aware that this is still not a 1:1 comparison because the weave, the material, the stretch etcetera might be different. All knots were tied as accurate as possible. This means all strands were as nicely as possible on top of each other and nicely aligned. For the sleeving of the halfMarathon, pinkTube was used. Sleeving of 1 inch webbing was not possible due to lack of sleeving for 1 inch. It was also tested if the way the knot is tied influences the breaking strength, a messy fig. 8, one with half a twist and a regular one were tested 3 times on halfMarathon.

Table 1 - webbing specifications
Table 1. Different webbing types used to break test the knots. *Minimum Breaking Strength in the seaHorse weblock in a single wrap (n = 5) calculated with 3-Sigma

Knots

We broke a wide variety of knots that are or can be useful in slacklining. A few knots might not be familiar to you, therefore here’s a list of all the knots we test with a short description. See attachment 1 for pictures of all the knots.  

  • Frost knot / Threaded
    An overhand tied with 4 layers of webbing. The knot can be threaded through a piece of tubular webbing to protect your webbing and potentially make it stronger.  
  • Figure 8 / Half Twist / Double / Threaded.
    Well known end loop knot that is used a lot in climbing. Can be tied with 2 (normal) or 4 (double) layers of webbing. The normal version can also be tied with half a twist so the webbing is sits better on top of each other. The webbing can be threaded through a piece of tubular to make the knot potentially stronger. 
  • Figure 9 / Unfinished
    A knot quite similar to the figure 8, but with an extra pass underneath the crossing, and only then through itself. Tend to be easier to untie in webbing after seeing a load. The Unfinished 9 is a knot created by Charles (Chillington) Chater. Instead of passing the whole bite through the knot, you make one more bite and put this through the loop. This means the knot ends on a bite. This bite can be easily pulled out to undo the knot. WARNING: ALWAYS CONNECT BOTH LOOPS IN THE UNFINISHED 9 OR ELSE THE KNOTS COMES UNDONE DIRECTLY 
  • Double Fisherman or Barrell knot / Connect
    Most often used to connect two ropes together. In this case the double fisherman was tied and connected to the seaHorse pin. Instead of passing the whole tail through the knot, it was finished on a bite. This is in the same way as how you would tie it in your webbing when the knot is made in the middle of your line and not on the end.   
  • Bowline / Mid – Bowline
    Quite known knot. Holds similar breaking strength in rope as a fig. 8. The knot is easy to untie after the knot has seen tension. In this test a single bowline was made. The mid – bowline means that the tail was not passed through the knot, but a bite was passed through. This makes it possible tie this in the middle of your line (for example in a case where you want to use it as a back up to a bolt or other anchor). 
  • Overhand
    Simplest knot out there. Creates a single loop to connect something to. Harder to untie than a fig. 8 due to the small knot.   
  • Clove Hitch
    Normally tied around an object. In this case useful to connect your backup webbing to a T-Loop with a quicklink. Basically two hitches, with the last hitch underneath the first one. 
  • Girth hitch
    Easy to untie, quick connection. In a slackline application it might be useful as a way to connect your backup webbing to a T-Loop with a quicklink. 
  • Alpine Butterfly
    Famous alpine knot usefull in a lot of different situations when made in rope. Can be tied in the middle of the line. In a slackline application it might be useful as a way to connect your backup webbing to a T-Loop with a quicklink. or to back up your webbing to another anchor. The useful thing about this knot is that it can be loaded in all directions: Loop – strand 1, loop – strand 2, strand 1 – strand 2.

Results

After breaking a lot of knots the following results came out. If you want to play with the sorting or filter, follow this download link to see play with the sorting in all the tables: https://goo.gl/1zobZ4 
In attachment 2, you can find a graph with the average, minimum and maximum strength reduction calculated with the results of the different webbings combined. The table is meant to show reduction of strength in knots. Be sure to also check the value in actual kN. An 50% breaking retention in Black & white is still 21.2 kN, in 20Tube that’s only 9.0 kN. The colour doesn’t say anything about safety.

Overview of all the knots tested
SLACKTIVITY Overview of knots tested
Table 2. Breaking value in different webbings with different knots (n=1)


Table 3 & 4 other knots tested

In the table first the 100% value is stated. For this we used the breaking strength of the particular webbing in the seaHorse weblock in single wrap. This number was calculated by using a Sigma 3 calculation. The breaking strength retention percentage is calculated by dividing the breaking strength by the 100% value. This leaves us with the percentage of how much the knot holds compared to the breaking strength in the weblock. In table 3 you’ll find some extra tested knots. For the different figure 8’s the 3-Sigma was calculated (table 4.). This calculation shows the probable lowest and highest breaking strength according to the tests (n=3) that were done. The threaded frost knot & figure 8. could only be tested on the halfMarathon due to lack of sleeving for 1” webbing. Also some messy / bad knots were tested to check how much tying your knot nicely influences the breaking strength. This is only done in the halfMarathon and pinkTube due to the amount of webbing we broke so far. We also tested different way of tying a figure 8. & frost knot to see if this influences the breaking strength. In image 1, 2 & 3 you can see the different fig. 8 tied. In image 4 & 5 you can see the different threaded frost knots tested.

Different Figure 8's tested

Image 1, 2 & 3. Messy fig. 8, fig 8. half twist & regular fig. 8.

Different Frost Knots tested
Image 4 & 5. Threaded frost knot and messy threaded Frost knot.

Conclusion

After these tests it’s not possible to write down a conclusion due to the lack of retests (sample size, n =1). The tests are meant to create a data pool of breaking strength of knots. It’s up to the user to decide what knot he or she uses depending on his or her own safety margins. However the following observations can been made:

  1. The threaded frost knot (nicely tied) holds 98% of the breaking strength in NEW halfMarathon. No threading leads to a breaking strength of 51% in this test. It’s unsure if this Frost Knot was tied in the same way as the threaded one (see discussion). In the threaded figure 8 (75%), the strength of the knot showed an increase of 15% compared to the average of 60% (lowest 58%).  However it seems that threading your knot increases the breaking strength of the knot / webbing. This needs further testing with a higher sample size to determine statistical significance of the results
  2. A messy fig. 8 in halfMarathon leads to an MBS of 10.44 kN / 40% (n = 3) compared to a regular fig. 8 (n=3) 13.75 kN / 53% or one with half a twist (n=3) 17.06 kN / 65%, this is an average reduction of 12% and 25% in three tests. See table 4. for 3-Sigma calculations. There is a significant difference between the breaking strength of the half twists and the messy Figure 8. This means a Figure 8 with a half twist, is stronger than a regular one. A messy threaded frost knot showed a breaking strength of 65%. Compared to the threaded nice frost knot (98%) this is a reduction of 33%. Presumably the way a Frost Knot is tied influences the breaking strength of the knot. This needs further testing with a higher sample size to determine statistical significance of the results.
  3. It seems that the type of webbing influences the breaking strength significantly. Where the 20m tubular (20mm, tubular weave, nylon) held 78% in the frost knot, halfMarathon (20mm, flat weave, 3 layered construction, polyester, 4,5% @ 10kN) only held 49% in this case. It’s hard to say which webbing is best for knots, it’s easier to read out of the table and see what knot might be the strongest in your webbing according to this one time test.

These results can’t give an ultimate conclusion or recommendation on what to use and what not to use due to the lack of retest. Therefore it’s up to the user to judge what knot fits there webbing the best due to the difference in breaking strength reduction in different webbings. These observation leads to the following conclusions about tests that still needs to be done. Read the discussion first, after writing the discussion some tests were added to the recommendation list. The following tests needs to be done in the future to further discover the influence of knots in webbing:

  1. For the frost knot, does different tying method influence the breaking strength? Walking end on the outside, or sandwiched in the middle? See discussion point three below.
  2. It’s unclear how knots in webbing perform under a shock load or cyclic loading. Tests with a small sample size can be done to explore new hypothesis to then create further research ideas.
  3. The Data Pool of broken knots is far from full, a lot more samples are needed to see how reliable these numbers are. It might not be necessary to tests all the knots again. Only for some knots that are really useful for slacklining a sample size of at least > 10 would be recommended.

Discussion

This article is meant to create an idea what knots do to your webbing. In that it succeeded. But what can you say about the results? The tests on most knots are only done once (n=1). Therefore it’s not possible to say, a fig 8. reduces the breaking strength by 40%. Also you can’t compare this webbing to the webbing you own due to it being a different weave, different material, even a different batch. UV radiation and usage over the years lowers the strength of the webbing1,2. A test done by Samuel Volery on a 2 year rigged piece of redTube (see reference for more details) the webbing retained 69% (25kn) of its original breaking strength in the seaHorse (36kN). A test done by Emil Hübner on Polyester webbing (White Magic and Core 1 by Landcruising) showed that the webbing held respectively 65% and ~80% of the original breaking strength in a Zilla Weblock from Landcruising. Both lines were used in different circumstances, and therefore it’s hard to say how much a line really loses its strength. Therefore it’s important to look at the test results with a critical eye, because saying this slackline with a threaded frost knot will break at 95% is far from possible due to the many different factors involved (slight difference in tying, webbing, UV, abrasion that it has seen, shock load etc).

Another important thing to realize is that the static pull test is not comparable to a real life scenario where the knot is probably used in. In a slackline application the knot always sees a cyclic load (load – unload) when under tension. When not under tension (back up knot behind the weblock or backup webbing end knot) it’s impossible to judge if the webbing behaves the same as in a static pull test. The friction moment in the knot due to it settling might cause an earlier failure. The double fisherman that was tested for example had to be reset due to the walking end becoming to long for the machine due to tightening and sliding of the knot. In a rapid event where this knot is used as a backup it’s hard to say how it will perform. Dynamic drop test with different types of knot is necessary for in the future.  

The Frost Knot is a knot that’s easy to tie wrong. Ryan Jenks3 responded on a question about knots in back up webbing on SlackChat. He mentioned that he broke two threaded frost knots with Parsec webbing from Raed. The knots had a strength retention of 50% and 72%. This is a big difference for the same knot. Same results were seen in the halfMarathon were one (Frost Knot b.) showed a retention of 63% the first Frost Knot we tested had a strength retention of 49%. After analyzing the knots it was clear that the knots were tied differently. It’s unclear if the knots Ryan tied were also different from each other. See image 6 & 7 for the difference.

Different way of tying Frost Knot
Image 6 & 7: Walking end in the middle of the knot (left) and walking end on the outside of the knot (outer layer) (right).

After breaking both knots another time the Frost Knot with the walking end in the middle held 13.3kN and the other one held 16.8kN. See results in table 5. For Frost Knot a. it’s unsure which tying method was used. It seems that the way you tie it the breaking strength is influenced. Presumably this is due to the bigger bending radius in the Frost Knot with the walking end on the outside. With this low sample size it’s not possible to concluded this for sure. Further testing and breaking of knots is needed.

Table 5 - Frost knots
Table 5. Results of different tied frost knots in halfMarathon

All in all these discussion points show that these tests are far from proving anything about knots in webbing. However it fulfilled in the goal of making the community aware of the influence of knots in their webbing.


References 

1 Breaking test by Samuel Volery - https://www.facebook.com/SLACKTIVITY/videos/1213192232141811   

2 Break tests done by Emil Hübner - https://drive.google.com/file/d/15yIMC-fmbyjai_cQUz0SqQnjmNAlbjF-/view?fbclid=IwAR2NrgGr28ZjeM3vywZI4e3MU2xxJWx8Ipf04NuNwciFxsIrUZEMgy986U0 

3 Comment by Ryan Jenks - https://www.facebook.com/groups/slackchat/permalink/2344000402340490/  


Attachment 1.

Unfinished 9Figure 9

Double Figure 8

Figure 8

Frost Knot outer layerFrost Knot inner layer

Double FishermanDouble Fisherman Connect

BowlineBowline Mid

OverhandAlpine Butterfly

Clove HitchGirth Hitch

Attachment 2. 

Graph 1

Graph 1. Average, minimum and maximum strength reduction calculated with the results of the different webbings combined. Graph by Florian Beyer / Lepyruvate