Wire for a Zip Line or Flying Fox?

A Zip Line makes you measurably cooler… First of all, when well-built (and thought through) they are marvellous, thrilling, and at times – incredibly useful. Secondly, when shoddily constructed or poorly thought out, they can be a liability that can border on lethal. So let’s assume that you want to build something safe for both the users of the Zip Line, the environment, and attached structural points. Beyond budget and safety concerns – the installation environment, overall purpose, line length, vertical drop, anticipated load, the planned lifetime of the installation are primary factors in choosing the components of a successful Zip Line.

Overall Materials, Components and Construction:

Unfortunately, there is no ‘right’ set up. As the primary factors noted earlier vary with respect to your project – the recommended materials change.

How do I estimate the bigness of the project?

While all Zip Lines need to be ‘safe’, that judgement actually translates into the concept of ‘fit for purpose’.  Describing a small “Residential” installation and comparing it to a large “Commercial” installation helps to identify how and why primary factors can vary enormously. In a similar way, a compact car and a double decker buss are both vehicles that can carry children to school – but they are NOT the same. Firstly, lets call a “Residential” installation a Zip Line that is a supervised temporary structure for small children with a minimal vertical drop and a length of under 10M in the backyard of a house or cottage. Next, lets call a “Commercial” installation a Zip Line that exists as a stand alone, permanent structure to ferry 4WD transports across a 1 kilometre gorge with a 200M vertical drop installed in a public location. Clearly these examples are somewhat related in purpose, but would be implemented with vastly different resources and form greatly distant points on a line linking all Zip Line Projects. And for the purposes of this article, we will focus on the “Residential” installation end of the spectrum.

Can the Zip Line be taken down?

If the Zip Line is designed as a temporary or removable feature, then provision should be made so that the components can be readily assembled and used – then disassembled, cleaned, maintained and stored. Clearly, something that is never taken down will be installed with a focus on low maintenance and durability whereas a temporary Zip Line will need to highlight ease of deployment and adjustment. A Zip Line between two trees is a temporary structure and should consider how to avoid killing the trees while maintaining an adjustable yet stable deployment scenario.  Generally, the permanence of the structure affects the ancillary components that attach, adjust, and enable the main zip line to function. And for the purposes of this article, we will focus on the temporary installation end of the spectrum of permanence.

How does the Installation Environment change things?

If you are installing a zip line in a dry inland location – you have a benign environment that will inherently limit corrosion. Low grade (G1570) Galvanised Wire Rope (aircraft cable) and Electro-Plated Zinc (Galvanised) Fittings will work and have minimal cost. Upgrading to Marine Grade (G2070) Galvanised Wire Rope and Hot Dip Zinc (Galvanised) Fittings will cost a bit more but result in a much more durable installation. Conversely, if you are installing a flying fox in a Rain Forest location near the ocean – you have a highly aggressive environment. Grade (G304 / A2) Stainless Steel Wire Rope and  grade fittings (G304 / A2) will work as the lower cost alternative. Upgrading to Marine Grade (G316 / A4) Stainless Steel Wire Rope and marine grade (G316 / A4) fittings will cost a bit more but result in a much superior installation – both in function and appearance. And for the purposes of this article, we will focus on the benign end of the spectrum of the environment.

How Strong a Zip Line do I need?

Clearly, anticipated load (both from the weight of the wire itself and the passenger/sled) affects the required strength of the Zip Line. This observation incorporates both the load and the length of the Zip Line. But rest assured that unless your zip line is > 100M, then the weight of the wire itself can be ignored. As well, characteristics such as a high speed zip line with a large vertical drop has additional requirements but that is beyond this discussion and could not truly be considered “Residential”. With wire rope, thickness and strength vary directly (as cable gauge increases so does cable strength and cost). So Zip Line strength comes from  both thicker (greater gauge or overall diameter) cable/wire rope. Strength from thickness also applies to the size/thickness of the individual fibres resulting from the structure of a wire rope (e.g. 1×19 if stronger than 7×7 which is stronger than 7×19) but generally, the wire structure used by “Residential” zip lines is 7×19. And as  “Residential” projects <100M with only a passenger/sled load to consider the recommendation is 7×19 wire rope in either a 6mm or 8mm or 10mm diameter.  With the most conservative safety factor (10X) applied to the certified MBS of each wire (24.7kN for 6mm, 43.0kN for 8mm and 68.7kN for 10mm) with the load applied orthogonal to the wire (which cuts its effective strength by 50%). So by way of example, using marine grade 7×19 galvanised aircraft cable (wire rope), the recommended Safe Working Loads in your application are:

Practical Advice

So in practical terms – if my 100kg Fat Aunt Martha plops on my zipline after two chardies, then my attachment points had better be able to stand up to an instantaneous static load of ~200kg (note the “doubling” occurs because of the orthogonal application of force)… Lets hope she doesn’t bounce because then a dynamic load makes things MUCH worse. So 6.0mm may seem adequate, but I would rather not have to prove that in court or at an inquest. So – strength wise, we recommend the 8mm (5/16″) Wire Rope as a minimum as it will accommodate even for the odd very fat drunk adult (although I would NOT recommend the latter for other reasons…). You can’t go wrong by going thicker as it will be safer – but the implementation is both harder (physically) and more expensive in a non-linear fashion. Thus thicker wire is always stronger and if the rest of your structure is adequate, then it is arguably safer.  Just please remember that in EVERY structure there will be ‘the weakest link’. Zip lines are no exception. Generally, that link and the potential failure mechanism of the structure should be understood and planned for in advance of deployment.

An Example “Residential” installation.

Zip Line Example
Zip Line Example

Application:

Lets suppose you have an ~44M distance, then a recommended wire set up is:

Other bits

that you may want to acquire or consider include:

  • Crushable Landing Zone: a way to negatively accelerate the passenger before they hit the bottom anchor point – springs or a collection of old car tires.
  • a ROBUST trolley pulley/sled -with handlebars and IDEALLY a dead man’s brake so that if someone freaks out and lets go – the trolley is effectively braked
  • a full body harness or climbing belt for people who get freaked out and let go, and
  • a good insurance policy.
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