Backup Extender Strength on Girth Hitches

1. Introduction

Segmented highlines becoming more popular nowadays. Having a loose backup with loops between the taping points is quite standard since some years, allowing additional dampening of mainline dynamics. The logical progression points towards standardized length, mainly the 50 m length interval for splitted highline segments.

Loose backup results in terms of standardized length, that the backup segment needs to be longer than the mainline itself. While some prefer dedicated backup segment length, it is obvious that a more adjustable solution has more benefits. The additional backup length depends on some factors like prefered backup loop sizing, mainline tension and stretch, body weight and sportive usage of the highline itself.

Backup extenders offer this adjustable solution, choosing just the desired additional length and connect it to the standardized backup webbing piece. The connection method is just as simple as it is - girth hitching backup sewn end loop and a sewn end loop at the extender together.

The aim of this little break test study is to examine the strength of this loop to loop girth hitch connection method and to spot potential weakness and dependencies.

The study is structured in 3 parts. First i tested an extender of relative light webbing material (Aki Sonic Light), secondly i used super sturdy Sonic II webbing for having a strong extender and lastly i break tested 2 extenders of moderate to low stretch webbing (Aki Polar and White Magic).

The test setup was girth hitched connection between 2 Zilla 3 weblocks (double wrap method). Tested on my AkiSnap test rig (test speed at around 600 mm / min). My high precision load cell (S-shape with both side ball heads) delivers 1600 values per second to the display.

2. Sonic Light Extender

Sonic Light Extender connected to a Polar ALB sewn end loop. As expected the Sonic Light extender broke at loop sewing inlet with 26,07 kN.

Sonic Light Extender connected to a Wizard ALB sewn end loop. The extender webbing broke in the Zilla 3 weblock at 27,39 kN.

Sonic Light Extender connected to a White Magic sewn end loop. The Sonic Light extender broke at loop sewing inlet with 26,63 kN.

The extender broke never at the girth hitch itself. The weak spot is usual the single strand webbing inlet at the extender loop sewing.

3. Sonic II Extender

The aim of a stronger extender is to use the full strength potential of a stronger backup webbing, e.g. having a > 30 kN backup system.

Sonic II extender connected to Wizard ALB sewn end loop. The extender webbing loop sewing broke at 34,91 kN, the extender loop itself broke subsequently in the girth hitch. The test time length was longer than the other test due to the high stretch and high load. Wizard ALB sewn end loop shows its full potential ; ).

Sonic II extender connected to White Magic sewn end loop. The White Magic webbing broke at the inlet of the loop sewing with 31,53 kN, as expected.

Sonic II extender connected to Soma sewn end loop. The sewing of the Soma end loop ripped at 29,18 kN (other single tests with this Soma end loop broke the webbing at the sewing at similar loads). Subsequent break of Soma webbing in the girth hitch.

Again the results of the break tests showed that the girth hitched connection is not the weak spot. Its usual the webbings sewn loop.

3. Low Stretch Extender

After examining extenders with high stretch polyamide webbing i was just curious about how an extender made of lower stretch webbing behaves.

Polar extender (no ALB sewing) connected to Polar ALB sewn end loop. The extender webbing broke at the extender loop sewing inlet with 26,8 kN, as expected (the relative high strength value for the non-ALB sewing is due to higher strength of new batches of Polar (close to 30 kN in Zilla 3 Double Wrap).

White Magic extender connected to White Magic sewn end loop. The extender broke in the girth hitch at 30,54 kN.

Extenders made of very low stretch webbing may break in the girth hitch.

4. Conclusions

Based on the test results i come to following conclusions.

Its logical that girth hitched sewn end loops offering very high strength compared to just single strand knots, because of the double strand distributed load inside the sewn loop.

Extenders made of high stretch webbings are prefered over low stretch webbing extenders:

• high stretch webbing adapts better to the deformations in the girth hitch, allowing better webbing strength retention
• extenders with high stretch webbing break usually not in the girth hitch connection, the single strand inlet into the loop sewings is the weak spot
• high stretch extenders may have better shock dampening capacities in case that the backup has to do its job (mainline failure)
• high stretch webbing sewn loops have better strength retention while anchored with soft shackles (please find my other study in my blog)

The girth hitch connection should be carefully tied and tightened. But compared to other knot techniques its quite simple.

The extender loop at the split connection location acts as Soft Thimble when placed inside the mainline loop while using soft shackles as connectors.

Aki Slacklines is offering a light and a strong backup extender in various length starting at 2 m length up to 12 m length. Both backup extender versions are made of stretchy polyamide. They have an "ecological touch" too, since we use short rest pieces of the webbings, which would be otherwisely have no meaningful usage. Accordingly the pricing is quite fair, since the webbing and sewing work is not full priced.