Speaking of which, does anybody know the Young's modulus (E) of 1" tubular webbing. I tried looking it up a while ago, but was unsuccessful. If you know Young's modulus/modulus of elasticity, you should be able to find the forces on a weighted line a completely different way than using "basic geometry" and "high school math." To do so, you basically treat the webbing like a spring, and figure out the stress due to extension.

Also, does anyone know if a slackline set up can be treated as elastic (allowing you to use simple statics equations)? For the most part, the webbing returns to it's original, unloaded state, but there is some permanent elongation. The webbing also seems to stretch a greater percentage than most other materials would within their elastic region.

Tried that, and it won't work cleanly. Webbing's elasticity varies too damn much. From batch to batch of the same color of web from the same company varies, much less once you consider different manufacturers, the effects of humidity and how it becomes less elastic with wear.

I love how nerdy climbers can be. Such a range of people from pot smoking college dropouts (no offense to anyone) to guys with PhD's in physics or mechE all out there enjoying themselves to the fullest!!! Have fun and never stop learning.

TRADitionally yours,

Cali Dirtbag (soon to be Master's of Physical Therapy)

Do those calculators find the force using purely trigonometric means

Things that blow that calc all to hell:

Standing on 1 side means more force is on that side and less on the other

Any movement on the slackers part, what I believe WILL fix that limitation is take the additional weight the slacker is imparting on the line and feed that back as the input for slacker weight, i.e. if jumping and landing back down creates an impact equal to double your weight, then input that back as the slacker weight and you'll be in the ball park. Part of the reason I can't trust that completely is under that the system isn't static and time and stretch/recoil enters the equation.

For this one couldn't you just recaluate the angle created at each point? What you should end up with is at each anchor is a different value of force in the y direction, however because the line is still in equilibrium the force in the x direction should stay the same.

Anyways if we did have all this then we could calculate the impulse of the person when he lands on the line, and thus the downward force. Then you could go back to using statics and trig to find the horizontal force on the anchors.

My head hurts. :?

Don't worry about it man. I'd avoid the math side of things too if I could get away with it. I built that calculator doo-hikey just to avoid having to answer questions about this stuff since people kept asking.

I guess what you could do is though is use statics to find the worst case scenario, ie find the instantanious force that the person would generate if they were instantly stoped the second the landed back on the slackline after jumping. Then you could go back to the wonderfull world of statics!

fat lotta good that didja, huh?

I e-mailed Blue Water to try to get some data about the elastic properties of their webbing, but i didn't get anything back from them. They said
"Do you own a load cell or dynamometer? You must determine the actual peak
load to calculate." that was all :(

Can't say i know exactly what they means, but maybe some of you guys do.

I e-mailed Blue Water to try to get some data about the elastic properties of their webbing, but i didn't get anything back from them. They said
"Do you own a load cell or dynamometer? You must determine the actual peak
load to calculate." that was all :(

Can't say i know exactly what they means, but maybe some of you guys do.

Best guess:
It means that elongation % is a curve and not just a normal % of stretch. It also means shy of having a load measuring device we're pretty much left to statics and trig (or at least as far as I know).

The elongation would be a curve, since nylon is a thermoplastic material. However, it should operate in a fairly straight line until it approaches failure. The slope of that line is the elastic modulus.

I should have qualified that with the statement that the curve is highly variable depending on factors such as environment, age (even unused web will get more static with age), usage and treatments.

Blue Water man says:
"I can supply you with elongation figures for specific loads. The "tension"
is the variable and the unknown in a slackline w/o a load cell. I figure
zero to X lbf in 250 lb. increments? Will that work? This will only apply to
BW climb spec webbing. Other webbings may vary [they]. If it out of
curiosity the figures will be of value. If it is out of need for end use I
strongly suggest using a Yates screamer as a load limiter in the system.
There are different models with different activation forces. They work"

i e-mailed him back and asked him to send all the data he could. We'll see what fun numbers we get back.

Hey slacklinejoe... want to fund a research project in exchange for the results?

Man you engineering students can take the fun out of slackline in a hurry!!!
I'm with Larry, "you guys are hurting my head!"
I'm sure this is not my kind of thread but, dynomometers have been put on slaklines before.
When Darrin did the highline between two buildings in Long Beach for Ripleys, one of the stage hands had a dynomometer and they rigged it in the set up!
Darrin told me that when he got the line walkable it read 800lbs. He also said that when he was on the line, it only increased the load to 850, I don't know about the load of a fall because Darrin never ever fell! as a matter of fact he did that walk leashless!!!

and the Blue Water man says:
"Age won't be a critical factor. You'll likely wear web out before age
becomes an issue. Wear is the biggest problem as I see it. Webbing loses A LOT of strength with small nicks [or] at the sides. The worn/ cut spots throw the web out of balance and it can lose a great deal of strength. DON'T use worn webbing!! UV is the biggest long term problem especially if the web is left set up on a regular basis. Water reduces nylon strength by 25% or so but nylon regains the strength upon drying. Wet web will elongate more. %age increase of elongation I'm not sure off the top of my head. Dampness- same as wet but not as big of a problem. The nylon produces say nylon will not mildew. But the accumulation of dirt, oil from your hands, sunscreen etc. can get attached to the nylon and those things will mildew.
I'll get my lab guy running testing this a.m. if he's caught up on regular
materials testing."

I think i love this guy.

Actually, I could have told you most of that. I believe however, that if the nylon is stretched a lot while wet that won't necessarily regain all of it's strength. It would depend if the wet nylon is strained enough.

With it being wet, it looses 25% of it's strength so it makes sense that it also takes somewhere around 75% as much of a load before it starts going static when compared to normal.

Interesting!!!! It seems that the force of him being on the line is absorbed by the stretching of the material so that it doesn't increase as much as you would expect. The elasticity of the webbing acts as a shock absorber so that the entire load isn't transferred to the anchors.

The amount of load before it permantly makes the web less dynamic than new. I.e. when wet it reaches that load faster than when dry because it's actually weaker in that state.

At first glance, this seems correct. At second glance, it seems impossible. The third (or maybe not until the 100th) time you look at it, you figure it out. If the person is standing still on the webbing, there is no way around the statics equations. The load HAS to be transferred to the anchors in the exact amount calculated via trig. It doesn't matter whether you are using a bungee cord or a chain.

The slackline tension will always be the same, given a line length, weight on the line, and angle from horizontal (or equivalently, change in height from nonweighted to weighted line). It just happens that Darrin was able to get his non-weighted line to a tension close to the tension of his weighted line. The fact that he was able to get the numbers so close is due to the large stretchiness of webbing (I think).

*Sweet: I'm a top roper.

and the Blue Water man says:
"Age won't be a critical factor. You'll likely wear web out before age
becomes an issue. Wear is the biggest problem as I see it. Webbing loses A LOT of strength with small nicks [or] at the sides. The worn/ cut spots throw the web out of balance and it can lose a great deal of strength. DON'T use worn webbing!! UV is the biggest long term problem especially if the web is left set up on a regular basis. Water reduces nylon strength by 25% or so but nylon regains the strength upon drying. Wet web will elongate more. %age increase of elongation I'm not sure off the top of my head. Dampness- same as wet but not as big of a problem. The nylon produces say nylon will not mildew. But the accumulation of dirt, oil from your hands, sunscreen etc. can get attached to the nylon and those things will mildew.
I'll get my lab guy running testing this a.m. if he's caught up on regular
materials testing."



so, sorry to change the subject a bit, with uv being a huge killer to webbing just how safe are those trad anchors. you know the ones where people have left webbing lashed around a rock years ago and expect people like me to rap off of.

i also got this, although i'm not quite sure what it means. anyone??

Basic trigometry is 100% accurate in finding the tension in a slack line.
.......
Of course dynamic moves like jumping would greatly increase the forces.

If I get a reasonable number for the modulus, and the line does indeed follow Hooke's law(or some other known function), I can do a dynamic solution of the line force and displacement, i.e. take into account bouncing, jumping, walking etc.
It's basically just Newton's second law, it's all these empirical constants that are difficult to find...

I was surprised to see so much head scratching..

and wondering if some of these super long lines you see with some freak crossing a gapping chasm, were close to their breaking strength.

The problem has been coming up with a reliable mathmatical formula for what happens when you've got impact on the line line bouncing (or momentum making less impact) and still get a very close number to what is seen with dynometers - that's what we're shooting for that is the complicating factor in this thread.

The problem has been what the hell is going on if they take a leashed fall, bounce or jump on the line.

patto - no offense man, but it looks like your trying to make concrete conclusions from total guesses at the forces involved. I think we're looking for something a bit more concrete. In the case of jumps, the bigger the air, the longer your mass has to accelerate - and don't underestimate the air achieved by some of those big air gurus.

As for the analysis page, yes, I've studied it a lot, that addresses the static loads on the system, as a matter of fact I used that as a basis of my slackline force calculator on my site that lets you enter the appropriate values and it'll spit out the static load. Force Calc

I've thought about doing the same thing. The problem with this approach is that the modulus of the webbing will change over time.

How 'bout modeling the line as a spring-damper system?

Ok, so I'll throw this out using where Patto was going. We've all pretty much agreed that a leashed fall would be the worst thing going on for a single person on the line.

What about taking their mass, accelerating it the distance of the fall and inserting their accellerated mass as the new static weight? I know that ignores the disipation over time, but it might serve to give us a worst case scenario.

Who cares about the oscillation of the [damped] system?

We're screwed because Young's modulus is so incredibly variable, and webbing is far from linearly elastic.

Ok, so I'll throw this out using where Patto was going. We've all pretty much agreed that a leashed fall would be the worst thing going on for a single person on the line.

What about taking their mass, accelerating it the distance of the fall and inserting their accellerated mass as the new static weight? I know that ignores the disipation over time, but it might serve to give us a worst case scenario.

Also remember that a two foot jump is probably an almost impossible standing jump as it is measured as movement of the centre of mass not the height of the feet. My typical standing jump on firm ground is probably only a foot.

Are you absolutely certain that nylon webbing isn't linearly elastic?

WOW :)

I guess it wasn't a standing jump then :)

Yes. It's definitely not linearly elastic under slackline tension. A material is linearly elastic when the ratio of Stress:Strain is constant. The easiest way to see that webbing doesn't hold to this is by tensioning your line, then coming back to it an hour later (or after bouncing on it). Although there is still the exact same amount of webbing stretched between your two anchors (strain is still the same), there is less tension (stress has decreased).

Simple Example:

Time 1:
Tension = 900 lbf
Strain = 10%

Time 2:
Tension = 700 lbf
Strain = 10%

I'd love to take a bunch of different brands, colors, ages, etc of webbing to the lab and play with them.

But that's because it changes over time. What I'm wondering about is if it's linear over a short period of time. A jump only lasts a couple of seconds, so maybe it behaves linearly over that short period.
The data supplied earlier in the thread by BlueWater seems to agree fairly well with this(I plotted the data in Matlab together with a linear best fit curve):
http://home.online.no/...igen/linetension.png

Iltrip, thanks for posting that. That's the problem with data analysis, you often find what you're looking for... :oops:

What does A,B,C,D and E stand for in the polyester graph?

Even if it's not linear, though, as long as the curve is known, you can find a dynamic solution. The problem is that a result from one batch of webbing probably wouldn't be comparable to another...