Picking & Installing a Backyard Zip-line Cable
Zip Line Cable & Wire Rope Clips
Cable for zip lines can vary greatly according to the site conditions, the expected load, as well as the length, pitch and sag of the zip line when fully installed. Choosing the proper cable is more important than it may seem at first because there are great tension forces created when a load is placed in the center of the zip line. That is why you are wise to buy cable that is many times stronger than the load you expect on the zip line. We recommend 3/8″ and larger sized aircraft cable for zip lines, although 1/4″ cable is enough for some backyard zip line applications.
Zip Line Cable Considerations
- Location – if the zip line is near the ocean, we suggest stainless steel cable which resists corrosion from salt in the air better than regular galvanized aircraft cable (GAC). Elsewhere, we prefer the GAC because the cable is cheaper and about 10% stronger.
- Cable Sag – are you installing the zip line tight or in more of a “U” shape? The tighter your cable is installed, the larger diameter you should buy.
- Zip Line Ride – Larger cable like 1/2″ wire rope rides smoother.
- Cable & Trolley Wear – Most serious zip line trolleys are made for 3/8″ – 1/2″ cable – they will run on smaller cable, but the sheaves and the cable may wear out faster.
- Zip Line Length – The longer the zip line, the bigger the cable you will need. This is because long zip lines have significant weight of their own, which is a constant tension stress on the cable (really-they can exert 1000’s of pound of pull in each direction under their own weight).
- Take note of Cable or Wire Rope with a Fiber Core! – Sometimes you will find cable where the center strand is fiber instead of a 7th strand. This is never used by professionals in the zip line industry because it is not as strong as if it was 100% steel. For instance, the link above to 3/8 is a fiber core cable (6×19), which means that a true 7×19 GAC would have a higher safe working load.
How to Connect a Zip Line Cable
There are many acceptable methods for terminating a zip line cable with various brackets, bolts, and wire rope fittings. Make sure that you always follow manufacturer’s instructions for all hardware applications. If you are connecting to a building or other structure, then get an engineer or builder to advise you. If you are connecting the zip line to a tree or pole, then please read on.
Wrapping VS Through-Bolting
Whether to through-bolt or wrap the cable around the tree is a matter debated by experienced zip line builders, so don’t expect a simple answer for all situations. If you are using a pole, then through bolting is almost always the preferred method because it is a cleaner look and the pole isn’t alive. If you are connecting the zip line to a tree, this is where the debate comes in. Some advocate bolting through the tree while others insist on wrapping cable around the tree. There are pros & cons to each cable termination method:
Why to bolt zip lines to trees:
- You don’t need to wrap cables around trees, which can girdle them.
- The blocks used to spread the cables out from the trees cause a lot of growth interference.
- Redundant Back-up Loops are easily added that don’t girdle the trees.
Why to wrap zip line cables around trees:
- You can see every part of the system to inspect
- You don’t need to drill into the tree, causing harm
- Making adjustments does not require drilling a new hole.
Bolting to Poles
Bolts should be kept at least a foot from the top of the pole and connected with appropriate hardware. The following zip line bolts are commonly used on challenge courses for cable terminations on Class 2 poles. Always check the ratings to ensure that these parts will work for your zip line.
- 3/4″ Forged Nut Eye Bolt
- 5/8″ Forged Thimble Eye Bolt
Wire Rope Fittings
The actual fittings on the cable fall into three categories:
- U-Bolts – these are no longer commonly used because they reduce cable strength by 20%.
- Fist Grip Clamps – these are better because they don’t deform the cable as easily, but they still reduce the cable strength by 20%
- Ferrules – these are preferred by professionals and only reduce the cable strength by 5%.
Last note on fittings – make sure to follow the manufacturer’s recommendations. Download the crosby specs for clamps for the specific size cable you are using. For most backyard zip-lines, you will need 2 swaged ferrules or 3 clamps, but don’t assume, check the data for current recommendations and keep zip-line safety ranked #1.