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japhyr
Mar 8, 2004, 9:56 AM
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A while ago I was describing a slackline to a friend, who asked how much force a slackline puts on the anchors. I had figured it out before, but I forgot exactly how much. I went home and figured it out again, and this time I thought I'd put it up for other people to see.
I started by looking at a pretty simple case, a person standing perfectly still at the middle of a static line. If you want to check out the short version: http://www.wanderingphotographer.com/...c_analysis_graph.htm
If you want to check out the full version:
http://www.wanderingphotographer.com/.../static_analysis.htm
Please let me know if you find any mistakes or have any comments, either here or via PM.
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slacklinejoe
Mar 8, 2004, 8:35 PM
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Very nice work Japhyr. Mind if I put a link on my site to your info & site for other tech info geeks like myself?
Under your additional questions you'll have your work cut out for you since impact of bounces and falls rely heavily on the dynamic nature of the webbing to absorb the tension, since all of the other calculations assumed a static line.
A while back quality tester at REI sent me some data on impacts for bouncing, walking and just setting up the line by using a load measuring carabiner.
But somehow his data had to be off as it didn't make much logical sense ie, he said it was right around 100 lbs of tension on a 25' ultra tight line at rest and jumping up and down on that same line never created moe than 500 lbs of tension. Maybe he was really measuring in newtons or another unit or maybe he had it as part of the tightening system thus report reduced numbers instead of on a dead end.
Hard to say how the data was wrong, but I do want to repeat the experiments myself when I find something affordable to measure the load.
At any rate, thank you for the good information.
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coldclimb
Mar 8, 2004, 9:10 PM
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Interesting information. Very cool. :)
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japhyr
Mar 8, 2004, 9:19 PM
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Joe,
You are welcome to link to those pages. I am going to set up some links as well in the near future.
I know the complexities of bouncing and all, and I look forward to analyzing those more complicated situations. It won't be exact, but it will give us a good idea of the forces generated. I think it is relatively straightforward to answer a question like this: "You jump up onto a slackline. At your highest point, you are 1 foot above the slackline. The slackline dips two feet before it brings you to a stop and starts to bounce you back up. What force does this scenario apply to the anchors?" Again, not something everyone needs to know, but fun stuff for us geeks to play around with.
I was also thinking of measuring the spring constant of nylon webbing, if I can get some weights and a place to hang 30 feet of webbing so I can measure the stretch accurately. I'll let you know when I make any progress on this.
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slacklinejoe
Mar 9, 2004, 5:15 AM
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You know, I was thinking and if you want, send me that Excell file and I'll turn it into a web application where people can enter their length and desired drop and it will calculate the tension for them.
I'm thinking of pairing it off with how much tension would be needed to input through a mechanical advantage system to provide the proper pull since the two are pretty related.
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japhyr
Mar 9, 2004, 6:56 AM
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I think that's a more complicated question: "How much do I need to tension a 50-foot line so it will only sag 5 feet?" The answer to this question requires taking into account the dynamic properties of webbing. For high-stretch webbing, you'd need to tension it a lot. With very low-stretch webbing, you'd barely need to tension it at all. For perfectly static webbing, you would not need to pre-tension at all!
That is exactly the kind of question I'd like to answer, though.
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cheezhead
Apr 16, 2004, 7:01 AM
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8^) slacklining looks fun!! i have never heard about it before now. I think i will have to give it a shot. Cool pics on your website too.
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slackydread
May 10, 2005, 3:59 AM
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damn you all are smart. i'm pretty new to this but every question i have gets answered by you braniacs. thanks
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veganboyjosh
May 11, 2005, 5:55 PM
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someone in a post above mentioned a load measuring carabiner. is this about the only way to measure how tight a line is? surely there's others?
the other day i set up my line and piggybacked 3 3:1's, and got it hella tight...but i noticed that simply under tension, without someone on the line, the front biner gate was hitting the pin, and wouldn't open. i got kind a paranoid based on that "biner to the ankle" pic in the other thread, and had to call it a day.
i'd like to know that once i tension the line, i'm still under what i need to be under...
any ideas?
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slacklinejoe
May 11, 2005, 6:21 PM
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In reply to: the other day i set up my line and piggybacked 3 3:1's, and got it hella tight...but i noticed that simply under tension, without someone on the line, the front biner gate was hitting the pin, and wouldn't open.
Carabiners are designed to stretch; the pin locking is part of the way it makes it stronger. The only requirements I recall is that the carabiner pin cannot lock the gate while it is under body weight. That being that case, the pin locking just means you had at least 300 lb + of tension on the line, that's less than 1/10th the webbing strength, so don't use that as a safety indicator. Gate lock under tensioning is common on jump tight lines.
The best estimate I've found for setup is judging sag in the line while standing still, it's about the only thing that can be moderately estimated without some special equipment. Granted, you have to compare it to Eric's chart, but at least that's memorizable for certain points.
I've come up with another way of testing strength that seems to work, but it's a bit overkill to build and only a ball park measurement.. It works by stitch break strength by using the stitch count and thread strength, essentally create a screamer with a calculated load it should handle before breaking.
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chilidogg
May 17, 2005, 11:19 PM
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how many people do you know that walk on a static line?
i've tested dynamic scenarios by placing a dynometer (a pull tester rated to 10,000 lbs) between the slackline knot and anchor... we used an 80 foot threaded line about 35 feet off the ground and found our preferible walking tension (maybe 3 or 4 feet drop in center) to be between 900 and 1100 lbs without anyone on the line.
we had 3 slackliners weighing approx 140, 155, and 165 lbs... with someone on the line walking end to end, the dynometer maxed out around 1400-1600 lbs....
then we took some whippers... most of the falls fell between 1800 and 2500 lbs, although one shock load fall performed by falling directly under the line (155 lb slacker) instead of away from the line created 3400 lbs of force...
if the typical strongest dynamic webbing breaks at 4200 lbs and tying a knot in it reduces its strength to 60 - 70%... uh oh!!! good thing we walk on various tensioned lines, thread them, and build up our knots!
i think the only way to effectively test and record the dynamics of slacklining is to really test every possible scenario you can think of... the basic properties of geometry and algebra will work when we're walking a chain, but that only happens when i'm drunk, walking around the city, and find someone with a chain fence or barricade.
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slacklinejoe
May 17, 2005, 11:26 PM
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In reply to: if the typical strongest dynamic webbing breaks at 4200 lbs
I was under the impression that 4,200 lb test isn't break strength but more of load proof, break strength would be closer to double that. It should come rated with a large safety margin already.
Either way, it's just more of an arguement to avoid knots, especially weaker varieties of knots.
Also, your numbers actually support Eric's analysis numbers fairly well, in his analysis, it only provided numbers for standing still (static load), but using his chart as a guide for the distance and sag the ball park for standing still would be around 1,250. It's the bouncing and walking part that's hard to use formulas for, but I can see how the walking process would add that extra few hundred pounds of tension.
It's just a guess, I think a lot of us would consider 3 to 4 ft sag in a 80 foot line extremely tight. I usually go for around at least 5 on that range, but I guess my preference is a good deal looser.
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greenketch
May 18, 2005, 12:50 AM
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This is a sweet thread, almost breaking into the typical Rc.com rehash of the physics on a recuring basis but not quite. Erics numbers and notes are good. I mess about on a slackline and it's good to find a forum of other slackers.
I work in showbiz and rig tight ropes once in a while. The calculations we use is fairly simple. When you boil it down, the sin of the angle formed times the tension equals the weight on the line. We use a few field calcs to see what the angle would be if our 160lb walker only wants 2 feet at the center of a 100 ft span what is the angle and then the sin of that angle divided into the 160 lbs equals how tight we pull. It seems that this could be pretty easily worked for a slackline also. Just that the numbers won't be as big. We already knew that since we are slackin.
It really is that bouncing and the dynamic nature of the webing that adds a bunch of variables that make it less precise.
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