Building a 150 foot zipline

August 23, 2009 | categories: engineering, projects | View Comments

My brother and his wife bought a house in Michigan a few months ago, and Sharon and I went out to visit them last weekend. The house is on a wooded lot a few miles outside of town. There are other houses that you can make out through the woods, but nobody really nearby. They probably own an acre or two of woods, and they abut a 15 acre park (which previously housed a privately-owned 1/3 scale steam railroad-- I'll save that for a later post).

At some point, we were standing around in the yard when Ben pointed out an overgrown field about 30 yards through the woods. Ben had previously discovered steel posts that suggest the field was in fact a grass tennis court that hadn't been maintained for at least a decade. As soon as I saw the outline of the tennis court, I thought, "These weeds will be subdued by this man. I will burn fossil fuels in an age-old ritual: the subjugation of the wild to man's will."

A few days later, I dragged Ben's 6.5 horsepower beast of a lawnmower out of the shed and fired it up. 30 minutes later, nothing taller than three inches lived within the perimeter of the court.

My brother's newly discovered grass tennis court, neglected for decades, after I mowed it The tennis court, recovered

Once the tennis court was recovered, the landscape was changed. Instead of a house in the woods, we had in our possession a house overlooking a small athletic field. There was talk of a croquet pitch or maybe a small soccer or frisbee game. I mowed the rest of the field, and we thought about the possibilities.

Later in the day, we talked about maybe putting a zipline in the yard on the other side of the house, but it didn't really seem like the right spot. The lawn was something of a valley; I didn't see an easy way to position the zipline so the rider wouldn't crash into the tree at the bottom. As it was getting dark, I walked around the yard and considered the possibilities. Coming around the corner to the tennis court, I noticed that the court was a good ten feet lower than the house, and there was a nice tree right at the edge of the yard. Ben pointed out that there was a massive tree on the far side of the tennis court. I paced the distance off at around 150 feet.

As I was lying in bed that night, I did some calculations in my head. The potential energy of a person at the top of the hill would be converted into kinetic energy at the bottom of the hill. If we ignore wind resistance and friction in the pulleys, we can estimate an upper limit of the speed.

<img src="http://chart.apis.google.com/chart?cht=tx&chl=mgh = frac{1}{2}mv^2">

The m cancels out, so either we all die, or nobody does. When you reach the bottom, the maximum possible speed is

<img src="http://chart.apis.google.com/chart?cht=tx&chl=v = sqrt{2gh}">

h is around 10 feet, and g is 32 ft/s^2. This means that roughly

<img src="http://chart.apis.google.com/chart?cht=tx&chl=v = sqrt{640}approxsqrt{625} = 25 frac{ft}{s}">

I went to sleep thinking that a zipline where you landed at a 25 ft/s sprint was about as dangerous as I wanted.

The zipline site, looking back up the hill The zipline site, looking up the hill

The next day, we went to Home Depot and picked up 200 feet of 7x19 1/4" steel cable, a 24" piece of schedule 40, 1" PVC pipe, and some cable clamps. A little internet research suggested that REI might have some cheap pulleys that could bear the load. They didn't have exactly the pulleys we were hoping for, but we found some that were good enough. We also got two carabiners. The advertised breaking strength of everything involved was at least 2000 lbs, more than 10x the weight of the heaviest rider. (I was expecting the cable to be slightly slack, so I wasn't worried about the 1/sin force amplifying effect if the cable were taut.)

All the supplies needed for a badass zipline The supplies

In the afternoon, we laid out the cable. We didn't have any good way of tightening the cable other than Ben pulling on it as hard as he could while I clamped down the fittings real quick-like, but the terrain was such that a slightly slack line worked great. You skim about 2 feet off the ground most of the way, before running to a stop as you intersect the flat tennis court. (The videos below suggest an alternative "crash-and-burn" method of landing, but we were going for distance then. It's relatively easy to land on your feet if you do it when the cable is at shoulder height. Cable at thigh height-- not so much.)

Zipline trolley closeup The trolley

The pipe-trolley forcing alignment with the cable worked well; I think a trapeze mounted at a single point would make it harder to launch and land safely. After a few runs, I noticed that the axles in the pulleys were too hot to touch, due to the friction from the nylon sheaves. That was mitigated with a few squirts of Boeshield T-9, everyone's favorite waxy lubricant. In the future, if the zipline proves popular, we might raise the start a foot or two and switch to a Petzl "Tandem Speed" pulley, which has ball bearings instead of a nylon sheave riding on a fixed axle. There's also talk of adding a slip-n-slide in the landing zone.

I knew that the zipline was a success when my brother's wife went out and rode it three times in a row by herself the next morning.

Videos and more photos below.

Sarah rides the zipline

Ben, method air on the zipline

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