My parents have some acreage in California on the coast with trees. One of my brothers (Ian) has taken on the task to create a totally epic tree zip-line, between 190 ft and 250 ft long (depending on the direction he decides to go).

It is going to be pretty amazing when it is done, but making a long zip line isn’t exactly easy! I did some of the calculations for Ian on maximum velocity and cable tension under load while estimating the friction from the trolley. We’re going to need a cable to handle 4500 lbs to 6000 lbs.

Here’s some of the resources for zip line design that I’ve come across for those who are interested in the physics, or who are crazy enough to try this themselves:

• http://www.ziplinerider.com/
• http://zipline.wvu.edu/
• https://www.physicsforums.com/threads/calculating-tension-on-chain-fence-with-someone-sitting-on-it.207321/

The WVU site is a good starting place even though it seems most of their javascript calculators are broken and the navigation is a bit odd (look for the top right-hand column for sub-section links). I recommend keeping your info in a spreadsheet anyway if you intend to do this yourself.

If you intend to put in a zip line yourself, plus do the cable tension calculations under load, here’s another resource to double-check yourself: https://web.archive.org/web/20211225091438/http://photoshipone.com/calculating-cable-tension-for-cable-cam-rigging/

Ian and I are still in discussion, but hopefully we’ll have lots of time to do load testing and sanity checking low to the ground before Ian builds the platforms (12 ft – 18 ft off the ground) for take-off and landing. First, the cable tensioning/tightening mechanism is going to be important to figure out before you’re high-up in a tree. I think we need to attach notched 2×4 strips around the top and bottom trees to protect it from the cable wrapping around it. The cable clamps will need to be rated for 6000 lbs for sure. A tie-back cable for the top tree back to ground anchors (other trees?) may also be needed to handle the 3000 lbs of tension under load.

The trolley friction (across a wide-range of weights from 50lbs to 250lbs) will play into how much slope will be needed (between 6% to 9.5%) and cable tension required to safely arrive at the destination platform. Tuning the slow-down mechanism at the arrival point is going to be especially important if we ever hope to let our kids ride this thing (vs adults smashing into a mattress on a tree 15 ft up on a platform).

The hope is to have this all figured out with testing by Christmas! If we make that goal, I’ll post some pictures.