Introduction

Soft shackles are a very cool new item in the slackline industry. They have a ton of potential to lighten the load on our backs when we are transporting our gear from one place to another.

There are a number of ways to build a soft shackle, all of which are not created equally. Knowing this, I wanted to test the difference in breaking strength between a few of the more popular ways of building soft shackles in order to determine which way is best for our application.

The strength of soft shackles relies heavily on the knot and the diameter of the body where the noose wraps around. Each of these 4 varieties of soft shackles provides different style knots and different diameter bodies. It is our goal to find which of the varieties offers the best knot strength and largest diameter body without sacrificing too much material.

Methods

There are a number of ways to make a soft shackle, each with their own pros and cons. I will show you 4 different methods and describe what the benefits of each are.

First, you will need to acquire a length of rope to use for your soft shackles. I would highly recommend using Dyneema 12-strand rope, such as Amsteel Blue (check out the specs for this line here:Amsteel Blue).

The recommended size to use for any of the methods below is a minimum of 1/4" (6mm) and a maximum of 5/16" (8mm). This will yield you shackles that are not too big and extremely strong. For master point use, or any use that sees the full load of the slackline, I would recommend going with 5/16" (8mm), no matter what method you use to tie the shackle. For any connector that is not seeing the full force of the line, the 1/4" (6mm) line will work perfectly.

The strength of a soft shackle is HIGHLY dependent on how well the knot is tied and set. Any of the below soft shackles can break at tensions well below the average if not perfectly tied. The "higher-strength" styles are especially susceptible to this issue.

It is not recommended to make your own soft shackles due to the fact that setting the knot is extremely critical. Without this important step, your soft shackle can fail well below the rated strength (sometimes as low as 1,500 lbf)!!

Method 1 - Diamond Knot

Average Strength: 136% of single line strength

The first method is known as the Diamond Knot method. This variety of soft shackle is the easiest to make and hardest to do wrong.

Here is a great guide on creating the entire soft shackle using this method: http://www.animatedknots.com/softshackle/

Here is a nice video on how to create a beautiful soft shackle using this method: https://www.youtube.com/watch?v=jH66tGsWv_Y

Here is a calculator for determining how much rope is needed: http://www.animatedknots.com/softshackle/ (scroll down)

PROS

• Requires the least amount of rope - This variety of soft shackle needs far less rope compared to the other varieties below. To find how much rope you need to create a soft shackle of a given size, check out the calculator here: http://www.animatedknots.com/softshackle/
• Extremely easy to tie - Even if you mess up the leg lengths, you can still expect to see at least full line strength with this variety. For example, if you are using 1/4" Amsteel Blue as your rope, you can expect the strength of the soft shackle to be at least 8,600 lbf, which is the strength of a single strand of the Amsteel.
• Leaves one strand in the body of the shackle - Since one tail is buried inside the other, the body of the soft shackle appears to be 1 strand. This makes managing the connector extremely easy, especially when dealing with connecting other ropes.
• Least variance in strength - The diamond knot method, since it's such an easy knot to tie well, has been shown to have the least variance in strength. This is likely due to the fact that having an absolutely perfectly tied knot is not as critical with this method. The other methods that can achieve higher strengths rely on perfect knots, which means the strength will vary far more.
• Consistently breaks at the same place - Over and over again in strength tests, this method breaks right at the weakest point of the shackle: the knot. This is great information to know as it means it's a reliable connector. As soon as we know the strength of the shackle, we can be fairly certain that it will break at that value at the same location every time.

CONS

• Least strong of the methods - Compared to some of the other methods, the Diamond Knot has been shown to be the weakest of these 4. However, our professionally made soft shackles are achieving 125% of single line strength reliably and consistently.
• Leaves rope tails on top - This method will leave the tails of the rope on the top of the knot. This can be a hassle if installing the soft shackles on a regular basis. There are methods to make this less of an issue (cover the knot and tails with electric tape).
• Difficult to open eye after loaded - Without the installation of a small pull-cord in the noose, it can be very hard to open the soft shackle after is has been loaded for a length of time.

Method 2 - The Better Diamond Knot

Average Strength: 144% of single line strength

This method uses the same knot as method 1 but has a different noose style to allow for easier opening and closing.

Here is a guide on how to create this type of soft shackle: http://www.animatedknots.com/softshackleedwards/

Here is another guide that has a calculator for line lengths: http://l-36.com/soft_shackle_9.php

Here is a video on how to make them: https://www.youtube.com/watch?v=s8QJEdYqe_E

PROS

• Leaves one strand in the body of the shackle - With the exception of the small un-buried section
• Easier to open noose - With the addition of the small un-buried section near the noose, you can easily open it up to slip the knot in or out.
• Requires the least amount of rope - This variety of soft shackle needs far less rope compared to the other varieties below. To find how much rope you need to create a soft shackle of a given size, check out the calculator here: http://l-36.com/soft_shackle_9.php

CONS

• Least strong of the methods - With the same weak-point as the regular Diamond knot shackles, these are also on the low-end of the strength spectrum for soft shackles.
• Leaves rope tails on top - This method will leave the tails of the rope on the top of the knot. This can be a hassle if installing the soft shackles on a regular basis. There are methods to make this less of an issue (cover the knot and tails with electric tape).

Method 3 - The Button Knot

Invented by Brion Toss.

Average Strength: 219% of single line strength

The button knot is a variation on the diamond knot that increases the strength of the knot as well as adds girth to the body of the shackle where the noose wraps around. These 2 things combined allow this method of soft shackle to retain a much higher percentage of strength compared to the other methods above. This is the method we use for our professionally made Soft Shackles in the shop.

It is especially important to tuck the tails after setting the knot with this variety as it is needed for a high strength retention. It's also extremely important to have the leg lengths be the same. There can be a large variation in strength if these 2 rules are not followed.

Here is more information on this type of soft shackle: http://l-36.com/high_strength_soft_shackle.php

Here is a guide on how to tie the button knot: http://l-36.com/button_knot_top.php

Here is another guide on how to tie the button knot: http://www.bethandevans.com/pdf/button.pdf

Here is a video on how to make these types of shackles: https://www.youtube.com/watch?v=SLhQlJqfjEM

PROS

• Easy to tie - Once you get the tying of the knot down, this method is quite easy to build, with the exception of tapering the tails.
• Easier to open noose - Since the body of the shackle is two legs, the noose is very easy to open, even after a large load has been put on the shackle.
• Very strong - With an average strength of 220% of single line strength, these shackles are exceptionally strong. The bigger diameter body combined with a better knot make these things ideal for high tension applications.
• No tails on top - The button knot allows the tails to leave the bottom of the knot, which makes for a smooth knot all around. This eliminates the wear and tear on the noose when using the shackle many times.
• More surface area in contact with anchors - This means that there is less potential for critical abrasion damage, which is the main failure mode for anything made with Dyneema.

CONS

• Uses more rope - This type of shackle uses more rope than the Diamond Knot methods above.
• Has 2 strands in body of shackle - This can be an issue sometimes if you are not careful with how you install the shackle, especially in combination with other ropes. It's not a huge deal if you are mindful of installing the shackle.
• Takes a long time to make - This method takes considerably longer than the Diamond knot methods. This is mostly due to the buried tails, which can be very time consuming. If you instead just cut the ends at an angle instead of tapering the tails, you can save a lot of time. This should not affect the strength of the shackle, in theory.

Method 4 - The Big Overhand

Average Strength: 223% of single line strength

This is another method that is much stronger than the Diamond Knot methods above. It uses a very large overhand knot at the end of the shackle with 2 spliced loops on each tail. This makes a very large stopper knot and increases the diameter of the body of the shackle where the noose goes around. In tests, this is showing a similar strength to the button knot, but has a few downsides that make it not the best solution.

It's vital to make sure the tucked tails extend out beyond the overhand knot into the body of the shackle. This is to ensure the noose wraps around a second of the line that has tucked tails. This will increase the diameter of this section of the shackle, thus increasing the strength retention.

You can see a document on how to build this type of soft shackle here: http://www.bethandevans.com/pdf/improvedsoftshackle.pdf

This method requires you create 2 eye splices: one on either end. Here are instructions for doing this: http://www.animatedknots.com/longbury/

PROS

• Easier to open noose - Since the body of the shackle is two legs, the noose is very easy to open, even after a large load has been put on the shackle.
• Very strong - With an average strength of 223% of single line strength, these shackles are exceptionally strong. The bigger diameter body combined with a better knot make these things ideal for high tension applications.
• No tails on top - The big overhand knot allows the tails to leave the bottom of the knot, which makes for a smooth knot all around. This eliminates the wear and tear on the noose when using the shackle many times.
• More surface area in contact with anchors - This means that there is less potential for critical abrasion damage, which is the main failure mode for anything made with Dyneema.

CONS

• Uses more rope - This type of shackle uses more rope than the Diamond Knot methods above.
• Has 2 strands in body of shackle - This can be an issue sometimes if you are not careful with how you install the shackle, especially in combination with other ropes. It's not a huge deal if you are mindful of installing the shackle.
• Takes forever to make - Since you have to create 2 spliced eyes with proper buries, this method takes the longest to tie. When making a bunch of them, I was averaging around 20 minutes per shackle with this method.

We will start with a length of 3/16" Amsteel Blue rated at a breaking strength of 5,400 lbf by Samson Ropes (seen here: Amsteel Blue). We will be making 5 shackles of each variety.

This size of Amsteel is exceptionally small for this type of application. We had to use a small size due to the capacity of the break test machine. If you are thinking of making your own soft shackles, USE 5/16" AMSTEEL! This will ensure you have a strong enough connector for any type of line you rig.

Using the exact same length of rope for each soft shackle (48 inches), we will use the Diamond Knot technique (shown here: http://www.animatedknots.com/softshackle/) to make these soft shackles.

Each soft shackle will have the knot set to 2,000 lbf using the break test machine, all during the same evening.

We will report the breaking strength of the soft shackles in "% of line strength", using the rated value of 5,400 lbf as our "line strength". We will also be testing this line strength value by doing a simple spliced loop test between two 5/8" shackles.

Each shackle variety will be pull-tested between two 5/8" shackles on the same day to ensure the same testing environment.

The results of this test will be observed and described below. No statistical analysis will be run on these results.

Results

Here is a table with the results of our tests:

As you can see, the variance within each group is big! We had one strange occurrence with the big overhand knot where the knot pulled through itself. This particular method is very delicate and requires a perfect tie.

We had the most variance with the Button Knot method, mainly due to the one low value. I suspect this is due to a non-uniform loading during the test. This could have easily been fixed by setting the shackle up better before the test.

The method with the most consistent results is the Diamond Knot. It also shows the least strength of the varieties.

Pictures of Results

Control Samples

Diamond Knot Samples

Better Diamond Knot Samples

Button Knot Samples

Conclusion

It's great to see how well each method holds up in the strength testing. With as little information there is online about the various methods, it was nice to have some concrete numbers behind some of the tying methods. It's hard to say which method is best from the data above alone. With some practical use testing, which I have also done, it's quite clear which method is the best (the button knot). However, there is more to it than just strength and usability, especially if you are making your own. The relative difficulty in tying each variety plays a huge role in what style you should use.

If strength is a huge issue, the option to use a larger size rope will always be a good way to ensure this. No matter what variety you use, 5/16" Amsteel Blue will be plenty strong for any slackline application.