I’ve always wanted to build a treehouse and a zipline. I don't have a perfect spot for a treehouse, so for Christmas this year I decided to build a zip line for the kids (and me!)
The first thing I wanted to do is figure out what
size cable I need.
There is all kinds of information out there (mostly
commercial links to purchase equipment) but one thing that is absent is clear
data on what type (size) of cable can be used for a specific length of zip line.
Everybody wants to sell you trollies, ropes and hardware, but no one wants to
take ownership of actual cable specifications. Moreover, many steel cables for
sale don’t specify breaking strength or full ratings. You have to operate on a
few assumptions and do your own math.
I found two suitable trees about 170 feet apart and
cleared some brush for clearance.
Here are a couple of formulas I found online after
quite a bit of searching. One takes into account the length of the cable. I’m
guestimating a sag of 15 degrees, or 10 foot sag, and the rider plus cable is
250 lbs.
Here are the 2 formulas I found. They both result in
similar tension, so I believe I’m in the ballpark.
Tension=(Weight of cable, trolley and passenger x
Length of cable in feet) / (8 x Sag of cable in feet)
=(250*170)/(8X10)
=42500/80
=531
lbs
And another formula
T=(Mg)/(2⋅sin(θ))
where Mg is the total weight of rider + cable.
=250/(2sin15 degrees)
=250/.5176
=482 lbs
1/4" Cable
Pully
Cable Grip (for pulling the cable, you don't HAVE to have this but it makes it a LOT easier)
Cable Winch
Here is River giving it a go.
Using the winch and cable grabber.
I had to build a little platform at the top to get enough drop.
Some friends trying it out.
