Stringing the Cable

Cable Starting Point
Cable Starting Point

The first step was to string a cable between the two trees.  The height of the cable would be determined by the amount of slope.  I didn’t know how much slope the cable would need when I started.  But I wanted the rider to be near the ground when he / she finished the run.  I took a best guess at how high the starting point would be to have a cable slope that let the rider go fast enough, but not too fast, and still end up near the ground.  I could always adjust it later if it was too high or too low.

Pressure Treated 2x4 Blocking
Pressure Treated 2×4 Blocking

I attached the cable to the starting point about twenty feet up the tree.  I didn’t want the cable to bite into the tree bark and eventually damage the tree, so I cut some pressure-treated 2x4s to hold the cable of the tree.

Engineering Questions

There were a number of questions regarding the engineering for this project that needed to be addressed before starting.  Here are a few that immediately came to mind:

  • If I string a cable between two trees, how much tension should be on the cable?  Too much and it might pull the trees together, especially when a weight is hanging on the cable.  Too little and the cable will sag – maybe to the point that a rider would stop in the middle.
  • How steep should the cable slope be?  Too steep and the rider may gain too much speed to land safely.  Too shallow and the rider may not make it to the end.
  • How much weight will the cable hold?
  • What is involved in stopping the rider safely?  If the rider is going too fast and there’s nothing to safely stop him / her, then he / she will smack into the tree and sustain injury.  And any system that safely slows a forty pound rider may not work with a hundred sixty pound adult, since the adult will have quadruple the momentum that needs to be stopped.
  • Based on the answers to some of these questions, how much tension will be on the cable, and what size cable should be needed to withstand the tension?

My general approach was to use experimentation and trial-and-error to figure out the answers to these questions.  Then, based on the results of these experiments, I would be able to come up with a design that should work and provide a reasonable margin of safety.

 

 

 

Zipline Kit

The easiest approach was to purchase a kit that has everything I need to build a zipline.  I found this kit on Amazon and ordered it.

Zipline Kit
Zipline Kit

The kit included a trolley, cable, harness, and helmet. It was less expensive to buy these items as part of a kit than it would have been to purchase each separately.

When the kit arrived at my doorstep two days later (it was ordered via Amazon Prime) I was anxious to see how it worked.  I spent maybe an hour stringing the cable between the two trees and getting the trolley installed on the wire.

I noticed a couple of things right away that I wanted to eventually change.  The cable was made of quarter-inch steel, but I wanted something bigger and stronger.  My research showed that a three-eights inch cable had a tensile strength of 14,400 lbs (quarter-inch cable is 5,480 lbs), so I would want to use this stronger cable on my final configuration.  Also, the trolley seemed to be well designed and made and included bearings in the wheels.  But once the trolley was installed on the cable it could not be removed without disassembly.  The cable threads through the trolley housing; removing the trolley from the cable required a disassembly of the housing.  Other trolleys are designed so that they can easily be installed and removed from the cable without having to disassemble the housing.  The significance of this is that I’ll have to either leave the trolley on the cable in all weather, or go through a hassle to remove it for storage.  I’ll use it for now, but may want to upgrade later.

The quarter-inch cable included some hardware for fastening the cable.  While the tensile strength is 5480 lbs, the strength of some of the hardware components is much less.  Since the cable is only as strong as its weakest component, that was a concern.  I’ll have to consider the strength of any cable components when I eventually replace the cable with the larger diameter one.