Rope break strength question... want to build a tire swing :)

[ajf3]

Hey all -

Been years since I took the physics classes where you figure out the weight break for ropes, chains, etc... figured some of you guys were probably more current on it!

I seem to remember that a rope breaks easier with a perpendicular load than with a horizontal load & I kind of need to know that for sure since I don't have any good branches to hang the tire from. I'm thinking I'll string a rope between two trees - about 20' apart, put a swivel or something in the middle & then hang the main tire rope down from that.

I've been looking at some 3/4" hemp rope that rated for around 5k# - is that gonna be enough to keep the tire (say 30#) a couple of kids (say 250# to be safe) from breaking the support ropes?

Thanks!

 

[MikeyIs4Dcats]

rope and chain will have a higher breaking limit if pulled in a linear fashion. If you load it at an angle, it reduces the limit. it does not matter if the rope is pulled vertical or horizontal so long that the load is linear

 

[TuxDave]

(250/2)/sin(theta) where theta is the angle between a line parallel to the ground and the rope from the swivel and the point on the tree.

I think that's how the math works in the physics world.

90 = the rope is parallel with the load --> 125#
0 = the rope is perfectly perpendicular to the load --> infinite tension needed.

 

[ajf3]

haha, I get that - gravity doesn't matter

I'm talking about the tire hanging vertically down from the middle of a horizontally strung rope. It won't be perfectly level, I'm sure there will be a couple of feet of sag in the middle, so how much would that angle reduce the load capacity? If it's less than like 90% I think I'm good...

 

[TuxDave]

haha, I get that - gravity doesn't matter

I'm talking about the tire hanging vertically down from the middle of a horizontally strung rope. It won't be perfectly level, I'm sure there will be a couple of feet of sag in the middle, so how much would that angle reduce the load capacity? If it's less than like 90% I think I'm good...

1 feet of sag for 10 feet of distance (halfway between the trees) has theta = 5.7 degrees which makes the tension required = 1258lbs?

Edit: oh wait, The load is 280lbs making the tension 1410lbs.

 

[Matthiasa]

Eh if you are worried just overdue it, or test it out yourself.

 

[vshah]

you might get a lot of oscillations on the horizontal rope when the kids are swinging - might be unpleasant/scary

 

[ajf3]

Yeah - I was going back & forth between a horiz rope of chain, but the chain is so much more $$$

 

[keird]

pftt! Use steel cable and make a zip line.

In a serious response; make a peace symbol with your fingers. If your angle is less than your two spread fingers, you're fine. If it's greater than that angle, you're putting an undue load on your anchor points.

I'll try to find a reference with the astronomical loads at various angles, but your anchors (trees) will likely move quite a bit if you fashion something like you've described.

 

[keird]

Edit: fixed wiki link

Reference equalization

 

[keird]

Why don't you make a sturdy yard arm? You can hang the rope from that.

 

[lxskllr]

Don't forget your knots. Knots can cut the working load by as much as 50%. I don't like your whole idea. I'd try to find a different way of hanging a swing.

 

[DrPizza]

I've thought about it & thought about it. Not liking the idea. Get a couple of 4x6's & a little bit of hardware & build a simple swingset.

 

[ajf3]

Just gave the swingset away on CL - they were getting too big for it Now I have to come up with this uberkewl 20' tire swing thing we've been talking about...

 

[ShawnD1]

Eh if you are worried just overdue it, or test it out yourself.

This. It should be strong enough that a full size obese man can swing on it without it showing signs of breaking.

Don't forget your knots. Knots can cut the working load by as much as 50%. I don't like your whole idea. I'd try to find a different way of hanging a swing.

I don't like rope or knots either. I think the OP should use chains.

 

[ajf3]

Hmm... maybe it's not so bad? I ran the numbers with that formula on the wiki - their chart goes to 120 degress which is 71% of the load. When I ran it with 170 degrees it worked out to 101% of the load.

...so, if my load is 300#, each of the almost horizontal ropes would be loaded at just about 300#. With a rope rated for 5400# that seems way sufficient, even if you cut it in half due to some knotting.

Seems like the question now is what will a 300# sideways load do to the trees at 20 foot high, right? Is my thinking sound here?

 

[lxskllr]

Don't forget shock loading...

 

[Kntx]

I think any good rope would be fine. When I was a kid we had a tire swing supported by one of those crappy plasticy yellow ropes. We wailed on that thing for years and it was fine.

 

[vshah]

no one else is concerned about the horizontal rope causing bouncing / oscillations?

what i mean is, when the tire is at its max height on a swing, the horizontal rope will be stretched out in 1 direction. at the max height there momentarily won't be any tension coming from the rope the tire is attached to. the horizontal rope will start to pull back, causing a slight bounce every time the tire changes direction...

or am i just totally wrong and confused?

 

[lxskllr]

no one else is concerned about the horizontal rope causing bouncing / oscillations?

what i mean is, when the tire is at its max height on a swing, the horizontal rope will be stretched out in 1 direction. at the max height there momentarily won't be any tension coming from the rope the tire is attached to. the horizontal rope will start to pull back, causing a slight bounce every time the tire changes direction...

or am i just totally wrong and confused?

I think that could happen, but it wouldn't be a primary concern of mine. Kids will figure out what the scare limits are, so unpredictable tire movement won't be so bad. My biggest concern would be breakage, and damage to the trees. You could effectively girdle the trees, and kill them off above the ropes.

 

[Matthiasa]

Atot...
The only place where making a rope swing is seen as a monumental engineering challenge.

 

[0roo0roo]

not just girdling, but just abrasion damage could eventually weaken the tree/branch.

 

[DrPizza]

Why not configure it like a giant capital "Y", rather than a "T"?

p.s. it would be hilarious to see a video of the two trees collapsing inward as someone started swinging higher & higher.

 

[Anubis]

Atot...
The only place where making a rope swing is seen as a monumental engineering challenge.

 

[vshah]

Why not configure it like a giant capital "Y", rather than a "T"?

but then there will be rotation at the point of connection to the trees...couldn't that cause abrasion/wear?