Industrial Strength Backyard Zip Lines

How to Build A Zip Line

Posted on 05 July 2016

Zip Line Cable Elevation and Slope
The recommended height difference between the beginning and ending of a zip line (the anchors) is a 6% decline of the zip line’s total length. 
An example of this is: A 100 foot zip line will have a total of 6 feet in height difference and a 200 foot zip line will have a an approximate 12 foot height difference.
Starting with the first anchor, look through a sight level, holding it steady and focusing it on the ending anchor. When the sight bubble aligns with the center line in the sight, have helper mark the level point on the end anchor.
Use this formula: Subtract (A) the height of the sight level off the ground from (B) the height of the end anchor’s point off the ground to find (C) the elevation change of the ground. B-A=C
You can now factor in the elevation change to calculate beginning and end anchor heights.
Zip Line Cable Sag
When a zip line is properly hung it will never be guitar string tight, there will be a natural sag that develops which positively contributes to the acceleration of the rider down the zip line.

When testing the cable with a weight it should sag below the end anchor, and approximately 2% of the zip line’s total length. (don't worry it doesn't have to be perfect.)

The zip line cable should also have a 7 foot ground clearance as well.

Example: For a 100 foot zip line, the end of the cable would anchor 2 foot higher than the lowest point of the cable when a test weight is on the line. The minimum height on the ending anchor would be 9 foot to accommodate 7 foot ground clearance. 

Anchors

All tree anchors must be at least 12” thick in diameter. Only healthy, sturdy trees are suitable as anchors. Never attach cable to trees with excessive decay, cracks, exposed roots, diseases, excessive lean, lightning damage or poor tree architecture. Free-standing poles (without guy-wires) must be 12” diameter, minimum. Poles must be sunk into ground at least 4’ or 2’ plus 10% of the pole’s height, whichever is greater.

Eyebolts

Eyebolts that are used to terminate a zip line to a pole or tree must penetrate the anchor entirely in order to be secured with a washer and nut.

Bolted Trolleys (uncommon)

If the trolley does not mount freely on the cable, be sure to thread the cable through the trolley before terminating the zip line to the anchor. 

Otherwise, the trolley wheels can be un-bolted to allow the trolley to assemble onto the cable. 

Cable Tension

When dealing with cable lengths up to 100’, you can create a loop at the end with a cable clamp. Wrap around the anchor and loosely clamp the intersecting cable. Pull loop to tension line, then secure cable with three clamps.

If the cable is too difficult to tension a cable by hand, hitch the loop to a vehicle or winch. A winch and cable grab combination is most practical, requiring no cable loop.

Cable Clamps
U-bolt portion of clamps should press against dead end of cable. Space clamps 1-2 inches apart.
Tighten clamps  with a torque wrench to “moderately past snug” Approximately 30 foot per pound.
Turnbuckles

Turnbuckles are used to adjust cable tension. While major adjustments require a winch and cable grab, the turnbuckles are useful for fine tuning.Turn the buckle to screw the shafts in or out. This often requires a bar or wrench, slid into the buckle, to leverage the rotation.

It is recommended in the zip line industry to back up every turnbuckle with a cable and clamps.

Note: Turnbuckles extend the start of the zip line ride even further from the tree, so if your platform is too small, you may not have the space to safely mount on the cable.

Riding Gear

Do not under any circumstances use chain link carabiners, only link attachments directly to trolley. Multiple carabiners can link to trolley. Do not put more than one attachment in a carabiner.

For any life support connections, (where the link is bearing the rider's weight) use a locking carabiner.

 

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1 comment

  • Dan: February 20, 2017

    Okay, I finally get it! Thank you!

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