Accidents on backyard zip lines happen way too often. This one in Minnesota could have been avoided.
DO NOT: build your own zip line without educating yourself first. Below are the key components of a safe zip line.
A typical backyard zip line can easily put 800-2000 lbs of tension on its anchor points during use.
Trees – Healthy
–> 12” or larger diameter is a great anchor and will support that tension.
Tip: Protect your tree’s health by putting tree protectors around the trunk.
Slings (tow cables) minimize tree damage. Destroying the bark and tree tissue invites disease and insects.
Dying/dead trees are catastrophic
DO NOT: Imbed a lag bolt into your tree and expect it to hold. It won’t.
The bolt must penetrate the trunk entirely and be reinforced with washer and nut on the back.
Note: A single bore hole into a tree is less damaging and invasive than a cable wrapped around it…restricting its growth and weakening it over time.
• Free-standing poles – minimum of 12” diameter, sunk 4’ into the ground.
• Supported poles – minimum of 8” diameter, sunk 2′ into the ground. Anchored via guy wires (i.e. cables) to strong ground.
Tip: For wooden poles, eyebolts or slings should be anchored 12” or lower from the top for proper leverage.
DO NOT: Most buildings and backyard play structures are not designed to withstand the horizontal loading of a zip line cable.
NEVER ANCHOR TO YOUR HOME’S DECK! It was not built for lateral tension such as a zip line would exert.
Thickness of Cable
–> Use 1/4″ cable for zip lines 200′ or less in length; 7000 lb minimum breaking strength.
–> Use 5/16″ cable for zip lines 500′ or less in length; 9800 lb minimum breaking strength.
–> Use 3/8″ cable for zip lines 1000′ or less in length; 14,400 lb minimum breaking strength.
–> Runs over 1000′, you should really consult with and have a professional install this kind of system for you. Major weight loading, anchoring and safety calculations are required.
Good info to know:
– Minimum Breaking Strength: it’s the minimum amount of weight you would have to have on the product to make it fail completely.
– Safe Working Load: This refers to the max amount of weight you should put on a piece of equipment day-in, day-out without damaging it.
Slope and Tension of Zipline
–> Your zip line cable needs to drop in height from beginning to end about 6 – 8’ per 100’ feet of distance to operate at a thrilling…yet safe speed.
– Your riders weight will make the cable sag an additional 2 feet (per 100’ feet of distance) below your bottom anchor point.
–> Too tight is dangerous.
– Tension affects the speed and braking distance required
– More tension = more strain on the anchor points when loaded.
Tip: Allow 2 ft of ‘sag’ in your line per 100′. Any tighter, and the cable could fail when rider weight is applied.
–> Is generated by a combination of the slope and tension.
– A zip line with a high cable tension will start slowly but accelerate throughout the ride to the end.
– A zip line with low cable tension will start faster, but can run out of speed towards the end.
Tip: If your zip line is running a little bit slow, tighten your turnbuckle.
If it’s running too ‘hot’ , loosen your turnbuckle, providing more slack.
–> A rider’s weight is multiplied 3-4x when placed on a horizontal cable between to anchors.
–> Be sure your anchors are sufficient to withstand these sheer loads.
Good Rule of Thumb: While zip line is in use, make sure the cable hangs below the end anchor by 2% of the zip line’s length.
So…a trolley on a 100’ zip line at its lowest point should be 2’ lower than where the cable is anchored on the far end.
Challenge: Achieving Proper Tension
–> For zip lines over 100′, achieving proper cable tension by hand is very difficult
Winch pull – cranks slack out of the line
Cable grip – pulls the cable directly up to the tree for anchoring
Tip: Minor tension adjustments can be made using a turnbuckle
Active Braking Systems (rider involvement)
–> Leather Glove on Hand – grab the line with your hand to slow
-> BrakeHawk Braking System
– Pull down on top grip
– Activates brake pads on cable
– Slows rider based on pressure applied
Passive Braking Systems (no rider involvement)
–> Bungee Brake
– Block placed near end of run
– Bungee anchored to block and tree
– Slows rider gradually as trolley meets block and stretches bungee
Tip: This is one of the better braking systems
–> Spring Brake
– Spring placed near end of run
– Stops rider abruptly as trolley meets spring
– Rider must hang on tight
–> Tire Brake
– Tires placed near end of run
– Stops rider abruptly as trolley meets tires
– Rider can be propelled off zipline
–> Capture Block
– Operates like a bungee brake
– Braking applied by another person by friction to rope tied to block
– Often used in commercial operations
–> Can put more riders through quickly
–> Results in less in-cable rescues due to recoil
–> Rope friction applied matches rider speed
–> *Requires a second person* at all times
No Braking System
–> Gravity Brake
– Braking applied by extra slack in cable
– Problem: different sized people will travel further and faster
–> End Anchor Padding
– Easily most dangerous
– Need I really say more?!?
1: Always have your kids wear their bike helmets or buy them a zip line helmet.
2: There are multiple ways to ride a zip line.
Trolley Handle only
– For older teens and adults
Trolley Handle + Seat
– Easy on and off
– Not for tall ziplines
Trolley + Harness
– Safest riding method
– Always used on commercial ziplines
– Requires multiple harnesses for multiple riders
Test before Trying
Weight Test: Should be done at the center of the zip line with at least 350lbs of load (hang a rope from the trolley and have two adults bounce their weight on it).
Then check/retighten all cable clamps, bungee block bolts, and turnbuckles.
Never exceed designed weight limit in the actual operation of the zip line.
Speed Test: Have a test rider sit on the seat, and lower them down the zip line by walking or running alongside them holding a rope attached to the trolley.
Increase the speed of each run until you are confident that the zip line will not be too fast or over extend the bungee brake if the riders are allowed to zip freely.
Once accomplished, your zip line is ready!