The Highs And Lows Of Zipline Tension


It's your turn to ride down the flying fox. Standing. Waiting. Helmet, check ... harness, check.

You stare along the cable as it fades off into the distance. It's a long way down if something goes wrong. Your mouth begins to go dry. What's going on? Is it fear? Excitement? Either way you can feel the tension mounting.

Don't worry. For many first-time zipline riders that kind of tension is normal. It quickly gives way to exhilaration.

When I talk about "zipline tension" what I really mean is tension in the cable itself. Like, how tight it is.

Because, just as Goldilocks discovered, having the one that's just right makes all the difference. We want one where the tension is not too high. But a cable with low tension is no good either.

Why is that? And what's the secret for hitting that sweet spot?

Here's what we discovered ...

Low tension ruins the ride

When a zipline is slack the speed along it will be less. Where's the fun in that? The whole point is to fly fast and scare your zits off.

Clearly a slightly loose cable may not make much of a difference. But as tension gets lower so does the cable. A participant might not even make it through the last part. Sag in the cable makes gravity pull it back as the trolley tries to travel uphill. For low hanging ziplines a heavy rider might even scrape their feet along the ground.

So ... slower, shorter and possibly dangerous. Low zipline tension does not have much going for it.

High tension damages the system

Increased zipline cable tension adds stress to every component in the system. Wire, shackles, anchors, posts and turnbuckles. All are affected though most things cope. Loads should still be within the safe working limit of hardware if designed correctly. However, the result will be higher wear-and-tear. Translation: maintenance costs.

The real danger is tangential.

For example, as cable gets tighter its diameter gets smaller. If the ends are terminated with wire grips what's going to happen? Suddenly they have less to hold on to. You guessed it. They could let go and WHAM. The system goes from being a zipline to a slipline.

Another example from an actual case study. High tension caused an anchor bolt to tear 35mm through the timber of a hardwood post. It had a washer the size of a match-book but even this was not enough to resist the pull. [See photo: Common Causes Of Zipline Failure]

With the bolt no longer seated firmly against the post additional forces were now at work upon it. Eventually side loading and yanking of the bolt would have made it snap. Sad for the rider and embarrassing to the owner.

What it takes to get the tension "just right"

There's a magic formula for setting the right level of tension.

Bottle screw open
With the ideal cable length turnbuckles should never be more than 80% open or closed. This makes the system adjustable for temperature swings in different seasons.

I don't just mean data you can get from specs. That's easy. Like a 12mm cable has a minimum breaking load of 12.45 tonnes and is safe to 1800kg (SWL).

While screaming down a zipline, the wind in your hair and getting a rush of adrenaline ... who cares for any of that?

The formula involves,

  • Surveying the site accurately for cutting the ideal cable length
  • Judging seasonal variations and providing adjustments for it
  • Making better hardware choices to eliminate potential risks
  • Load testing and recalibrating to lock in sag at the proper height
  • And, of course, tightening to the correct number of kilo-newtons.

You see, it's all about creating the perfect ride. One that's not too slow, or short. And one that you can walk away from and say,

"Dang, that was fun!"