Is a twisted line weaker?

[riddler 0]

Post continued from Incidents

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When a line, such as Spectra, is twisted enough, it is no longer as strong as it used to be. When it's twisted, the load on the fibers is no longer evenly distributed, and its strength is reduced.

This is what doesn't make sense to me. I guess I think in terms of a rope. Old school ropes were nothing more than fibers twisted together (I actually made a rope once by twisting twine together). The fact that the fibers are twisted doesn't make the rope less strong than if all the fibers were straight.

My understanding of why steering lines break so often is because they take more of the opening shock than any of the other lines. This is because they are on the back of the canopy, which is taking the majority of pressure during the initial opening stage as the canopy starts to inflate. At least that's the way it was explained to me by a rigger
Can anyone clarify? Would a twisted steering line really be significantly weaker than an untwisted one?

Trapped on the surface of a sphere. XKCD

[rainman 0]

I heard this too, and although I was also surprised to hear that a twisted line is weaker, it is really easy to demonstrate.

If you take about a 4-inch part of one of your lines and hold it between your thumbs and indexfingers, then twist it a couple of times, you will see that when you pull it taut about half the fibers are stretched, and half the fibers are loose. Obviously the load on the line is now only supported by the fibers that are taut.

Disclaimer: I'm not a rigger or an instructor, but I was shown this and it makes a lot of sense to me.

Rainman

[RTB 0]

I've never thought about a twisted line being weaker but one factor effecting the wear on steering lines is that they pass through a guide ring. When you steer and flare the canopy they get worn more in that part of the line.
What also happens is increased risk of tension knots or lines getting caught together when a line is twisted.
Edited to add: Of course the slider also wears more on the steering lines, and the other outer lines, since they press out against the slider grommets the most.

[Gadget 0]

According to my rigger (who is also a militaryrigger in the belgium army and did testing on this subject)Twist's and knots in lines can reduse the strenght up to 50 %.So please ntwist those lines.It isn't so much work

If people from Poland are called Poles, why aren't people from Holland called Holes???
My logbook

[ZigZag 0]

But we really have not much to go on. A good picture of the line in question, especially of the area were it broke, would provide more clues of its type and condition. Suspension lines overall often brake in the area of the fingertrap were the running end inside the line terminates, so usually a few inches above the connector links, at the cascades or below
the attachment points of the canopy.
It would be a good idea to also inspect the slider grommets and for the presence of slider bumpers.
The slider grommets may have small nicks that abraid the lines. These nicks are most often the result of the grommets impacting onto metal connector links.
We know from experience that the type of suspension lines used on canopies are quite reliable, but in the absence of good maintenance, the chance of failure increases dramatically.

[AndyMan 7]

To me, the opening characteristcis of a canopy with twisted lines are a much greater concern then an decrease in strength.

Twisted lines in effect become shorter, and if one side is twisted more then the other (which is usually the case), twisted brake lines will contribute to wildly off heading openings, and can induce a spinning malfunction.

Personally, I consider a spinning malfunction to be a greater risk then broken steering lines... I can land with broken lines, I can't land with a spinning mal. Also, broken steering lines will most likely result in a slow speed malfunction, giving you ample time to execute emergency procedures if you choose.

_Am

__

You put the fun in "funnel" - craichead.

[base283 0]

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According to my rigger (who is also a militaryrigger in the belgium army and did testing on this subject)Twist's and knots in lines can reduse the strenght up to 50 %.So please ntwist those lines.It isn't so much work

knots YES, but twists NO on the "50% reduse" A Bowline knot reduces the strength by a whopping 50%...51% for an outside Bowline. Twists actually make the line stronger by giving a dynamic compress to the fibers under load and thus absorbing some of the kinetic energy applied to the line by allowing some yarns to take the load before others (That is how ropes are made), Line wear is a different story. It will make the line wear more pronounced on the edges of the line. Edges that are produced by the torsion of the twists.
And as someone stated, tension knots come with the twists territory.
The simple 3strand twist loses very little tensile strength with surface abrasion when compared to the woven strand. I think I read that 3% compared to 25% respectively.
take care, and inspect those lines..
space.

[FrogNog 1]

> Twists actually make the line stronger by giving a dynamic compress to the fibers under load and thus absorbing some of the kinetic energy applied to the line by allowing some yarns to take the load before others ...

I can see where this would make sense, but I believe if the "earlier-loaded" yarns reach the end of their stretchability and begin to tear before all the other yarns begin to load, the result is called "stacking" and the rope can tear completely if the load remains higher than the sum of remaining loaded, not-yet-torn yarns can sustain.

The "dynamic compression" dealie is why I thought lines and webbing were made of weaves - allowing the line/webbing to stretch more than some/all the individual fibers could as the weave lengthens and narrows under tension. I do have to admit, IANAE, though.

I'd still want my woven line/webbing to be as untwisted as possible so it could function up to its original design parameters.

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Pull.

[rendezvous 0]

I'm not a materials engineer so I might be wrong but here is why I think twisted lines break faster than straight lines: when you pull a fiber it's loded perpendicularly to it's crossection. That's the orientation under which it takes the maximum load. When you twist the line you tend to orient the cross section at the point of twist parallel to the line of force i.e you introduce a surface with shear. The fiber of which the line or rope is made is not as resistant to shear as it is too longitudnal loading. Coming to when you make a rope out of a twine, well the twist in the twine is not extensive enough to make the individual fibers experience a larger shearing load. If you twist the lines enough the strands that are subject to large shearing forces will fail. In case of parachute lines, the weaking of the lines due to twisting is compunded by the wear that the slider movement intorduces on the outer fibers making them even weaker when they are experiencing shearing and longitudnal loads.

[rickfri59 0]

My question is.... If one line breaks and you simply release the opposite toggle, will the canopy fly straight? Or is there still enough tension on the "good" line, from the toggle reaching the stop ring on the riser, to keep the canopy in a turn?

[FrogNog 1]

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My question is.... If one line breaks and you simply release the opposite toggle, will the canopy fly straight? Or is there still enough tension on the "good" line, from the toggle reaching the stop ring on the riser, to keep the canopy in a turn?

I don't know the answer to this question, but because I might find out in the air one jump and not like the answer, I (will) have a hook knife.

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Pull.

[Hooknswoop 19]

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My question is.... If one line breaks and you simply release the opposite toggle, will the canopy fly straight? Or is there still enough tension on the "good" line, from the toggle reaching the stop ring on the riser, to keep the canopy in a turn?

Depends on how the toggles have been set up and if the steering lines have shrunk. So, the answer to your question is 'maybe'.

Derek

[webracer 0]

Here is my opinion, backed up by those I trust with significantly more experience than me.
Yes, a twisted line is weaker. When you load a twisted line, you are loading 50% of the threads more than the other 50%. If you twist one up, you can see how some fibres take more load.
Untwist your brake lines often. I never let go of my toggles until I put them back on the risers. My lines stay pretty much twist free.
The other side of twisted lines is that they are shorter. This may or not be significant enough to cause a problem, but it can't do any good.
Twisted lines also get uneven wear from the slider, and some fibres may shrink more than others due to heat.

Troy

I am now free to exercise my downward mobility.