Let’s just start this off by saying ziplines are dangerous and I’m far from an expert. Don’t attempt this on your own. However, if you choose to ignore this warning, don’t blame me if you hurt yourself or others. This is dangerous — you have been warned.

When I was little, my father built a zipline for my siblings and me. It was an adventure from the familiar confines of our side yard into the darkest tree tops of the untamed woods next door. We had the quintessential “sloping away” landscape that left the rider high up one of the trees where my dad had fashioned a kick board to speed the rider’s return. It followed the “never get stuck” pattern of having the distant side at a higher elevation than the start so the worst that could happen if the rider didn’t make it to the kickboard and ended up somewhere over unknown brambles is to slowly glide back to safety. My brothers and I pretty much wore that zipline out over the years spending countless hours heaving each other at ever faster speeds into the unknown. When we moved to a new house, the zipline came with us and was set up to an even more perilous height. This thing was critical to my upbringing.

Fast forward many years and not only do we have the house and kid but also the classic “sloping away” yard — something had to give!

Riding the Zipline

Building

Anything worth building is worth overbuilding, especially if little ones are going to be using it. I decided to make a zipline that could easily support two of me so I opted for 3/8" aircraft grade galvanized cable. Particularly, ADVANTAGE 3/8", 7x19 Galvanized Cable from Amazon. This stuff has a working strength of 2,800 lb. and breaks around 14,000 lbs so I figured that would have us covered.

ADVANTAGE 3/8", 7x19 Galvanized Cable
ADVANTAGE 3/8", 7x19 Galvanized Cable
ADVANTAGE 3/8", 7x19 Galvanized Cable

Given the 70 foot span I had to cover, a 6 foot sag and two of me, (very conservatively, 500 lb) this highline tension calculator suggests an anchor force of 1,479 lbs which is around half of the cable’s working strength. I’ll call that easily enough of an overbuild as far as the cable is concerned. The tree on the far side of my line is going to bend before cable strength would ever become an issue.

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I also picked 3/8" cable because that was just over the 9mm minimum the trolly I decided to use required. I have two different devices but this PETZL TRAC Pulley with Vertigo Wire-Lock Zipline Pulley is easily the best.

PETZL TRAC Pulley with Vertigo Wire-Lock Zipline Pulley — $140

The nice thing about this Petzl is how easily you can disconnect it from the line which makes taking the carriage into storage very easy. It also allows quick swapping out things to hang from the zipline such as the “box carriage” (pictured below) which is good for swapping between multiple kids or a harness which really only practically supports one rider.

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The “Box Riding Carriage” — $?

I made the “box carriage” from what I remember we had as kids. Basically it is just some nylon rope with wooden dowels painted with marine varnish. I gave it a few coats so it would last some years in the sun but in retrospect I might get slightly thinner dowels next time so tiny hands can better grip it. I tested this myself first and it holds up brilliantly. You might notice I melted the ends of the nylon rope a little bit with a butane torch just past the knots so it doesn’t unravel.

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Nylon / Wood Detail

Here’s the harness which has become by far the preferred device. (wish I had this when I was a kid!) This is the X XBEN Kids’ Full Body Harness.

X XBEN Kids’ Full Body Harness
X XBEN Kids’ Full Body Harness
X XBEN Kids’ Full Body Harness — $40

It is a bit of a pain to get a kid in and out of this thing but you basically can’t fall off. I’ve needed to use safety pins to keep the straps from eventually tightening up at all the wrong places so it is a little bit of a trial and error thing but overall it is seems to be very comfortable even after hours of use. I’ve found the best policy is to have one harness per kid and swap riders using a carabiner.

Sometimes with the body harness you get twisted up which can be annoying so I found this little beauty which works great. This is the AusKit Swing Swivel. This is really only useful for the body harness setup though because you generally want the “box carriage” to stay facing in the same direction.

AusKit Swing Swivel
AusKit Swing Swivel
AusKit Swing Swivel — $36

As I mentioned from my youth, the best ziplines for kids have the destination anchor higher than the start. This keeps little ones rolling back to safety if they can’t quite make it to the other end. You want some good running room where you can get up some speed and then ideally you want the ground to slope away so you get the feeling of some good height.

We have this fabulous sugar maple in our back yard that looked like a perfect starting anchor. On the other end of my run though is a comparably wimpy weed of a tree that splits off into multiple trunks. I tied two of the trunks together to get better support and anchored fairly high up there. The result is approximately a 70 foot zip across our backyard.

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The Zipline

Anchoring

Anchoring into trees is a bit of a project, especially if you want the tree to continue to live. Having a system held together by friction rather than screwing large bolts into a tree which will eventually subsume them seems like a better solution. The core of a tree is dead and all the life is around the outside near the bark so if you simply wrap some cable around that, you tend to “strangle” most of the live part of the tree. On a smaller tree, especially one that grows quickly, a cable can cut right through a tree and create a weak spot that eventually snaps. To guard against that and give the tree a fighting chance, I use a set of wood chocks to distribute the force a bit. Aside from the obvious, it also allows the trunk to expand over time. Here’s what it looks like on the sugar maple. The cable doesn’t actually touch the tree and everything is held to the tree simply with friction. (you can see the wood chocks are stapled to the wire so they don’t fall out in storms)

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Wooden chocks keep the cable from touching the tree and are stapled in place so they don’t fall.

There are two wire rope loops, one around each tree, and a wire rope between them. Every section is clamped with three wire rope clamps. Why three? Because one should be enough, then a second one for redundancy and a third for overkill. This also helps mitigate a clamp not properly tightened up for some reason. (additionally, I use thread lock compound to make sure the clamp nuts stay where I set them)

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On the lower “start” side of the line, there is a loop around the tree with the chocks pictured above. On the far side of the line at a higher altitude is a similar loop. You can also see the additional loop below mentioned previously which ties two trunks together for more support.

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Tree loop at the far (higher) end.

The actual zipline is a single cable with a tieback and a thimble at each end. This is carefully set to a length that will allow just the right amount of sag in the system. I was tempted to use bottle screws to fine tune tension but opted against it because it seemed like an obvious weak point. This made tensioning a bit more work but once you get it, you generally don’t have to change it.

You’ll also notice the previously mentioned set of wire rope clamps I’m using. I use the same ones everywhere in the system. The main line also uses some stainless steel thimbles so the wire rope doesn’t get pinched.

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Wire Rope Clamps ($20) and Stainless Steel Thimbles ($16)

Here’s what the start side of the zip looks like. This is the end where I did most of my fine tuning because it is near the ground and therefore much easier to reach.

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I connect everything with heavy duty quick links. (Stainless Steel Locking Carabiners — $18 for 2) This might seem like overkill but I wanted to be able to remove the zipline without removing the tree loops or messing with the finely adjusted cable length.

The best tension is one where there is some sag in the system to keep forces against the trees and speeds at reasonable levels but not too much as to make the ride slow. A very tight system with little sag will be fast but dangerous for riders, especially near the endpoints. Aside from the obvious, this is particularly critical with kids because larger kids or even adults may push smaller kids too fast. Having a little more sag will make the system exponentially slow the rider down as they near an end which will help keep things safer. However if you have too much sag, the rider won’t be able to go anywhere so you’ll want to experiment with this a bit.

Tensioning

Getting the cable up there and tight wasn’t as hard as you might think. All you need is a nice set of Ratchet Tie Down Straps and several spare wire rope clamps. I attached the hook on a strap to the wire with the clamps and ratcheted the main line up into place a little tighter than I wanted. Then I looped the main cable through a quick link and got it in place with a thimble and clamps. With the clamps tightened up, I released the straps and saw how the tension looked. It usually takes a few tries to get a good “tight but not too tight” setup that works but it is fairly easy. (no need to muscle it!)

Once I had the line up, I ran a bunch of tests. Beyond the obvious “does it break if I go on it?” type tests, I tried bouncing up and down and swinging side to side. In my case there is a fence on one side of the line so I needed to make sure swinging in the middle of the line didn’t cause a safety issue with kids potentially smacking into the fence. Generally, you want to know where the limits of the system are and have ways to stay clear of them.

There doesn’t seem to be a reason not to have the zipline up year-round. Of course you probably want to bring the trolly / harness setups in so they aren’t exposed to the weather all year. You also probably don’t want the safety risk of others using the zipline while you are away.

As kids grow up, you will likely want to move the endpoints higher up in the trees. This may also require re-tensioning the system as well. You might also want to move the wooden chocks around on the trees from time to time to avoid damaging the tree. Additionally, if you should ever move away, you’ll want to remove your zipline and leave as little trace as possible.

Conclusion

Ziplines are dangerous, but if you design them well, overbuild them and are careful with the way they are used, they can be safe and incredibly fun. Keeping things more temporary and held in place with friction rather than permanently driving bolts through trees seems to me to be better and more flexible overall. Even though I’m grown, I still really enjoy using this zipline so my parting advice is “overbuild, overbuild, overbuild!”.

Written by

Applied CBDC Research @ the Federal Reserve — fmr Circle.com, Bandwidth.com. MIT / Podcaster / Runner / Helicopter Pilot

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