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Willanhanyard
Jul 31, 2012, 4:28 AM
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Registered: Jul 31, 2012
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So I have really been wanting to build a rope swing, and I thought maybe you guys could help me.
Heres my plan:
Get a ~500' rope and stretch it between two trees on either sides of a small ravine/valley. Get another ~150' rope and attach it to the middle of the 500' rope in the middle of the ravine/valley. The other side of the 150' rope thats hanging down is where I will harness myself in and then start swinging.
So I have a few questions:
1.) Is this idea remotely possible?
2.) What kind of rope should I use? Polypropylene is pretty cheap and lightweight, but I've heard it falls apart in the sun. Polyester sounds to be a good option, but it's pretty expensive if you want a 200m section. I don't know about climbing rope either. It is also super expensive, and I don't know if there are any advantages to using it in a rope swing.
3.) Where is a good place to buy rope this size? I have a good place to buy climbing rope for when I am climbing, but I don't know where to get huge 200m lengths of rope.
4.) How do you figure the amount of force exerted on the anchors on either side of the valley where the 500' rope is anchored?
If you want me to draw up a sketch of what I am talking about, feel free to ask.
Thanks! 
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sungam
Jul 31, 2012, 1:47 PM
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Registered: Jun 24, 2004
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I really hope you are trolling. Please don't do this, it's pretty clear you aren't equipped with the necessary knowledge and experience to pull it off safely. No offesene, I just don't want you to get hurt. The force multiplication from that kind of set up is obtusely high.
Pun intended.
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Willanhanyard
Jul 31, 2012, 6:18 PM
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Well ya I would hope you realize I am a complete noob. I am wondering what the forces are though.
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crasic
Aug 2, 2012, 6:45 AM
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Registered: Jul 26, 2012
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Willanhanyard wrote: So I have really been wanting to build a rope swing, and I thought maybe you guys could help me. Heres my plan: Get a ~500' rope and stretch it between two trees on either sides of a small ravine/valley. Get another ~150' rope and attach it to the middle of the 500' rope in the middle of the ravine/valley. The other side of the 150' rope thats hanging down is where I will harness myself in and then start swinging. So I have a few questions: 1.) Is this idea remotely possible? 2.) What kind of rope should I use? Polypropylene is pretty cheap and lightweight, but I've heard it falls apart in the sun. Polyester sounds to be a good option, but it's pretty expensive if you want a 200m section. I don't know about climbing rope either. It is also super expensive, and I don't know if there are any advantages to using it in a rope swing. 3.) Where is a good place to buy rope this size? I have a good place to buy climbing rope for when I am climbing, but I don't know where to get huge 200m lengths of rope. 4.) How do you figure the amount of force exerted on the anchors on either side of the valley where the 500' rope is anchored? If you want me to draw up a sketch of what I am talking about, feel free to ask. Thanks!
This only for educational purposes and you should not do this because you have no idea what you are doing, I'm only going to answer 1 and 4 in the name of science.
1. Using a tyrolean traverse (your horizontal line) as a rope swing anchor is feasible given good anchors and other engineering considerations. Keep in mind though that even the world expert in rope jumping/swinging (Dan Osman) fell to his death due to a misrigging issue, if an expert with years of experience and infamous rigging meticulousness can misrig and die, so can you.
4. The forces on the anchor are related to the angle that the rope between them forms with your tie in in the center, the tighter the rope, the higher the forces. They go as 1/2sin(x). Meaning if your rope is so tight that when you hang on it the dip only forms a 1degree angle, the force on the anchor on either side is 30 times whatever force is put on the swinging rope, if you are just sitting, its 30 times your body weight, when swinging you will exert 2-3 times your bodyweight on the swinging rope and thus the force on the anchors is 90-110 times your bodyweight. If we are talking a more reasonable 15 degree angle the anchor multiplier is 2. Keep in mind that this does not include the tension required to keep the tyrolean rope taught on its own (only the additional tension to keep your ass swingin), which can be rather significant, its a simple calculus problem to figure out the tension required given the rope/cable weight per unit length (incorporating any stretch or dynamic properties requires some cleverness, but still a freshman physics problem).
edit to add: The length of the tyrolean, or width of the ravine does nothing to change the calculation, except that with a shorter length tyrolean the tension required to keep the rope self-taught is reduced, since this can be actually pretty significant (hundreds or thousands of pounds for a 500ft traverse) it can reduce the ultimate load on the anchor, but the swinging load doesn't change.
(This post was edited by crasic on Aug 2, 2012, 6:47 AM)
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