Zip line connection to a backyard play set: need advice

[Roger Feeley]

I’m putting in a very short (40’) zip line for my grandsons aged 3 and 5. One end is a large maple tree and the other end is a wood backyard play set that I put in and is anchored in concrete 24” deep.

I will be approaching the play set at about a 45 degree angle. My non-engineer gut tells me that it would be better to run the cable through the nearest leg and attach to the farthest leg.

Some parameters:
— the cable will be low for several reasons. I want to make it nearly impossible for an adult to ride. I want the seat low enough that a fall won’t matter. It will be just high enough that I don’t garrote myself when I mow with the lawn tractor. Making it low also means that the boys can operate it themselves (still with supervision).

— the play set is pretty old, probably 20 years. It was given to us by a neighbor. The bottoms of the legs were rotted so I made ‘sister’ posts anchored in concrete and bolted the old legs to the new.

So, should I attach to the nearest leg or the farthest one?

[Malcolm McLeod]

Short answer :: neither.

Run the cable over a (honkin' big) bracket with snatch block on the near leg (a steel reinforced 10x10PT oak), then embed a screw anchor eleventeen feet deep* in the yard and attach the cable. Repeat at the tree.

* - this assumes a solid granite yard. For dirt, go deeper. Much deeper!

Joking aside, most people grossly underestimate the tension in a cable due to a side load (aka - a zip line). Here is a link to engineers discussing methods to calculate such; enjoy. TMI? Basically, you can't calculate it without knowing the loads and stretch in the cable. It is an iterative process.

A 40lb grandson is a piddling little vertical load, but toss them on a seat in the middle of a 40' cable span and they will generate a surprisingly huge tension in the cable, and the playset has to resist that tension. I'd run the cable over the near leg and attach it as close to ground level on the 2nd as you can get. Then test it. Carefully. ....Make sure no one has a video camera or you may very well be starring in an episode of Epic Fails on WeTube.

Then be prepared to toss it all in the burn pile. ...and buy 'em LOTS of popsicles!!

[Roger Feeley]

Thanks Malcom,
I found a calculator that seems to do the math for me.
https://www.ropelab.com.au/highline-tension-calculator/
The amount of sag seems to be a big deal. If I have 12” of sag, the force on each end isn’t that different from the weight of the boy. I put in 1” and it went up to ten times the weight.

[Adam Herman]

Thanks Malcom,
I found a calculator that seems to do the math for me.
https://www.ropelab.com.au/highline-tension-calculator/
The amount of sag seems to be a big deal. If I have 12” of sag, the force on each end isn’t that different from the weight of the boy. I put in 1” and it went up to ten times the weight.

Pythagoras is a B. lol.

i would use everything rated for overhead lifting, not that crap you get at ace/borg/etc, all forged hardware. static climbing rope and climbing hardware would also be great. tie a figure8 in each end, locking carabiner to your forged eye and you are set. no screwing with shitty cable clamps, turn buckles, etc. also easy to roll up when not is use and hang on a hook on the playset or tree.

a cable running through a vertical member and on to the the next at the same level will gain no strength at all assuming each vertical member is the same. a cable going through the post, and then angled down to the ground will redirect the side lode to the cable anchor at ground level, and keep the force on the post mostly vertical. the post holds the cable up but the tension is held more by the anchor at ground level. depending on angle/etc.



I was the head rigger in an 1100 seat theater, and got to have robin hood fly on my rigging, and previously held rigging and crane operator certifications working on and offshore in the oilfields. also, a MechE.

if you draw up some quick diagrams, that would be helpful.

[Roger Feeley]

Thanks to both Adam and Malcom,
Im constantly amazed by the depth and breadth of knowledge and experience here.
I think I’m going with a combination of The anchor down low and as much slack as I can tolerate. I’m using a kit my daughter bought. All the parts look pretty beefy.

1100 seat theater, huh? If you get a chance, see “Come From Away” about a town in Newfoundland handling the grounded jet passengers during 9/11. We all know that “Peter Pan” is the technical directors worst nightmare. This musical is as harrowing but for the property master. And it’s really entertaining.

[Adam Herman]

Thanks to both Adam and Malcom,
Im constantly amazed by the depth and breadth of knowledge and experience here.
I think I’m going with a combination of The anchor down low and as much slack as I can tolerate. I’m using a kit my daughter bought. All the parts look pretty beefy.

1100 seat theater, huh? If you get a chance, see “Come From Away” about a town in Newfoundland handling the grounded jet passengers during 9/11. We all know that “Peter Pan” is the technical directors worst nightmare. This musical is as harrowing but for the property master. And it’s really entertaining.

I loved the big complicated shows, we were the tech site for miss sigon's run, a couple steps off of broadway. The equipment all shows up from the rental companies, lights, sound, set, actors, road crew and they figure out all the technical issues, how to fit it into the least amount of semi trucks and rehearse before going "on the road" that was very fun. Our small engineering college team was always beating loadout records vs. the big union houses.

let us know how the rigging goes.

[Jim Koepke]

I will be approaching the play set at about a 45 degree angle.

That is a bit of a steep drop for a zip line intended for an adult let alone a 3 or 5 year old child.

jtk

[Roger Feeley]

That is a bit of a steep drop for a zip line intended for an adult let alone a 3 or 5 year old child.

jtk

whoops. Not 45 degrees vertically. That’s 45 degrees horizontally. If we label the 4 posts like a compass, the cable could pass through the north post and tie to the south post. My bad writing.

[Malcolm McLeod]

That is a bit of a steep drop for a zip line intended for an adult let alone a 3 or 5 year old child.

jtk

I looked twice at that too, but reconciled that this must be 45deg in plan view - - not elevation - - and thus it relates to how the cable passes over/thru the legs. ...but MAYBE for the adults-only zip line??

Anything descending on a 45deg or greater slope (cable) is essentially accelerated like it is in free-fall. (Don't believe me? Stumble on a 12:12 pitch roof.)

[Jim Koepke]

(Don't believe me? Stumble on a 12:12 pitch roof.)

You couldn't get me up on one of those!

jtk

[Ronald Blue]

Are you manufacturing it from scratch? Fabricating all of it? My son had something very similar to this for a few years until the kids moved on to other forms of fun. It worked fine.
amazon.com/CTSC-Stainless-Backyards-Enjoyment-Accessories/dp/B07TZMY5D1/ref=sr_1_3?crid=18T61N3MAFJXB&dchild=1&keywords=zi p+line+40+ft&qid=1608672526&sprefix=zip+line%2Caps %2C204&sr=8-3

[Roger Feeley]

Are you manufacturing it from scratch? Fabricating all of it? My son had something very similar to this for a few years until the kids moved on to other forms of fun. It worked fine.
amazon.com/CTSC-Stainless-Backyards-Enjoyment-Accessories/dp/B07TZMY5D1/ref=sr_1_3?crid=18T61N3MAFJXB&dchild=1&keywords=zi p+line+40+ft&qid=1608672526&sprefix=zip+line%2Caps %2C204&sr=8-3

i made one once but not this time. I got a kit off of Amazon that’s similar to what you have. Mine is from Slackers.

my brother is building one that will be 200’ and pass over a pond. Yikes!

[Mark Hennebury]

Newfoundland, Roof's and cables.

I lived in Newfoundland for twenty years, 1970 -1990; my father was born there. Great place, made a lot of great friends there.
I have just shored up a sagging roof ( roof spread) on our 125 year old brick farmhouse....with cables.
My first time using cables, quite interesting actually.

[Bill Dufour]

My niece had one for her young kids. poorly designed play structure came with the house. The play structure was actually well made but the zip line was not. If you rode it all the way down you ran your face into the support post. Boys could spread their legs and land on the family jewels to save there faces from being split open instead.
The physics of a horizontal rope make it is impossible to pull hard enough to make it level. It will always sag down in the middle unless it is fully supported.
They make sag tables for correcting tape measures used in surveying. these are tensioned with spring scales to exact pull tensions.
Bill D

[Roger Feeley]

This one has a 5’ compression spring around th cable at the end to stop the rider. No bonking the noggin or the jewels. I set it up yesterday and I think I got what I want. It’s a very short ride that won’t appeal to older kids I...think... the cable is high enough not to clothesline me when I mow which means that the 3 and 5 year old won’t have far to fall.