Forums: Skydiving: Gear and Rigging:
Stretching lineset back to trim

 

First page Previous page 1 2 3 Next page Last page  View All

popsjumper  (D 999999999)

Dec 29, 2012, 1:12 PM
Post #26 of 73 (991 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Quote:
If your car needed an oil change would you try and 'fix' the oil, or would you just do what needs to be done and change it.

If your guitar was out of tune would you change the strings? More than likely you would tune the guitar by adjusting the strings many times before deciding to replace them.

In any event canopy suspension lines are a unique case, awkward analogies don't mean much.
Speaking of awkward analogies.
LaughLaughLaugh

Hmmmmm. I can't remember the last time my life and well-being depended on guitar strings. But then, I'm an old fart and conservative so take it from there.

Do the right thing and buy or make a new line set.


(This post was edited by popsjumper on Dec 29, 2012, 1:12 PM)


pnuwin  (D 112233)

Dec 29, 2012, 2:53 PM
Post #27 of 73 (976 views)
Shortcut
Re: [phoenixlpr] Stretching lineset back to trim [In reply to] Can't Post

Quote:
I'd rather say Spectra is going out of fashion, HMA and Vectran are used on more and more canopies.

HMA and Vectran have their own disadvantages.

HMA is very sensitive to UV light. It loses a lot of strength when exposed to the sun for relatively short periods of time. According to PD, it also loses its strength when experiencing peak loads during a hard opening for example. Both HMA and Vectran are less resistant to wear.

Spectra is low bulk, very resistant to wear and nice to pack with its naturally slippery feel. If we could solve the trim/shrinkage issue, it would be my preferred line type.


Martini  (D 23756)

Dec 29, 2012, 3:53 PM
Post #28 of 73 (961 views)
Shortcut
Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

Quote:
Do the right thing and buy or make a new line set.

The right thing according to who? Linesets don't simply go from "good" to "bad" as you are obviously aware. They begin to go out of trim gradually and start the process from jump #1. If better trim quality can be safely maintained by a simple procedure then that is the right thing to do.

Have you ever tried this procedure? Do you have evidence that it has no merit or that it is dangerous? If not then your opinion isn't particularly useful and contributes nothing to this discussion. I'm not a rigger, I claim no special scientific knowledge. But I have used this technique on three of my own canopies with varying degrees of success and no adverse results. I was shown this process by a man who has many years of canopy design and construction experience, this experience gives him a great deal of credibility. So continue to be a conservative old fart naysayer or instead maybe take a more open-minded approach. Either way, blue skies Pops.


popsjumper  (D 999999999)

Dec 29, 2012, 6:13 PM
Post #29 of 73 (940 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Quote:
Do the right thing and buy or make a new line set.

In reply to:
The right thing according to who? Linesets don't simply go from "good" to "bad" as you are obviously aware.
You are right. I am aware. Did you have a particular reason for re-stating the obvious?

In reply to:
If better trim quality can be safely maintained by a simple procedure then that is the right thing to do.

Keyword "IF". You seem to think this has been tested and recommended by manufacturers of any type. Run it through the mill and see for sure. As it stands, you have no basis in fact.

OK you just want to argue.

In reply to:
Have you ever tried this procedure? Do you have evidence that it has no merit or that it is dangerous?
Nope on both counts. And, like you, I have no evidence that it does have merit.

The fact that I don't have, nor do you have, evidence one way or the other is why I recommend "buy the damned lineset". What makes you think I should do this on YOUR recommendation? There's already info out there that says it weakens lines. You choose to disregard that?
And, until this idea gets formally tested, I will NOT try this "procedure".

My contribution, because you will not see it, is your method has not been tested for safety. If you want to be one of those who will blindly try anything anybody tells you to do, have at it. I would ask that you don't lead our young jumpers down that path, thank you.

There are no Gods in this sport. Everybody is subject to error, regardless of past history. Anybody that tells you to do something they dreamed up just may be wrong...regardless of who they are but you wouldn't know it.

Again...Buy the damned lineset and be done with it. The activity has not been tested for safety validity that we know of as yet.

Good day, sir.


(This post was edited by popsjumper on Dec 29, 2012, 6:15 PM)


Martini  (D 23756)

Dec 29, 2012, 7:09 PM
Post #30 of 73 (921 views)
Shortcut
Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

Your position then is that my canopy should be jumped until it needs a new lineset and I should just "Buy the damned lineset and be done with it". That's a really fascinating statement since you have no idea of what canopy we're discussing, it's condition, the number of jumps on the lines or their condition and trim or the performance degradation of this canopy if any. You couldn't possibly know these things because the canopy in question is purely hypothetical. I'm in no way insisting that you use this process, that's your decision. I believe that this is a useful topic and that the OP asked a reasonable question. If your contribution here is that this technique is untested then we allready have agreed that no formal testing has been done. Since I have not had nor heard of any issues I'll continue to use this technique. You do your thing, I'll do mine. Continuing to insist that I or someone else buy a new lineset is dogma.

By the way I do have evidence that this idea has merit, that's why I use it. The evidence is simple: I end up with lines that are measurably more in trim than before. One canopy in particular demonstrated markedly improved openings after the lines were brought closer to spec trim. Is this scientific proof? Obviously not. Am I concerned about safety? Of course. If I see evidence that stretching lines weakens them to a degree where they are unsafe then I'll reconsider, so far that's been anecdotal. Doing casual internet research on that subject has brought no result. Since line strength degrades inevitably and as a factor of many environmental conditions it would be interesting to see the effect of line shrinkage and stretching.


(This post was edited by Martini on Dec 29, 2012, 7:27 PM)


kuai43  (C License)

Dec 29, 2012, 11:03 PM
Post #31 of 73 (882 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Your position then is that my canopy should be jumped until it needs a new lineset and I should just "Buy the damned lineset and be done with it". That's a really fascinating statement since you have no idea of what canopy we're discussing, it's condition, the number of jumps on the lines or their condition and trim or the performance degradation of this canopy if any. You couldn't possibly know these things because the canopy in question is purely hypothetical. I'm in no way insisting that you use this process, that's your decision. I believe that this is a useful topic and that the OP asked a reasonable question. If your contribution here is that this technique is untested then we allready have agreed that no formal testing has been done. Since I have not had nor heard of any issues I'll continue to use this technique. You do your thing, I'll do mine. Continuing to insist that I or someone else buy a new lineset is dogma.

By the way I do have evidence that this idea has merit, that's why I use it. The evidence is simple: I end up with lines that are measurably more in trim than before. One canopy in particular demonstrated markedly improved openings after the lines were brought closer to spec trim. Is this scientific proof? Obviously not. Am I concerned about safety? Of course. If I see evidence that stretching lines weakens them to a degree where they are unsafe then I'll reconsider, so far that's been anecdotal. Doing casual internet research on that subject has brought no result. Since line strength degrades inevitably and as a factor of many environmental conditions it would be interesting to see the effect of line shrinkage and stretching.

+1
After all, you can't trust those infernal horseless carriages anyway, muh horse's always got me where I'm-a goin', dag-nabbit! Wink


Premier ianmdrennan  (D 25821)
Moderator
Dec 30, 2012, 1:00 AM
Post #32 of 73 (869 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

If this was considered a good idea, manufacturers would offer a "retrim stretch" option...after all, there'd be money to be made.

Just saying Wink

I don't believe anyone is arguing against the improved opening (BECAUSE the lineset is more in trim), but the bigger questions is: At what cost?

Ian


(This post was edited by ianmdrennan on Dec 30, 2012, 1:01 AM)


popsjumper  (D 999999999)

Dec 30, 2012, 5:41 AM
Post #33 of 73 (829 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

OK you are missing or denying the point so have at it.

-Canopy type has nothing to do with it so why interject that?
-Reasonable question? Yes, of course.
-I have evidence that skydiving won't kill me. I haven't been killed yet. Hey! It's the same reasoning you are using.
- I certainly hope that the evidence you see about the strength issue is not breakage on a HP landing.
-Pretty much all canopies will demonstrate better openings with lines in trim...nothing new there.
-I have no vested interest in your well-being...the sport does.

I am not trying to make you change your ways. Simply pointing out the fallacy of your reasoning. You are welcome to continue in your vein. Again, please do not propagate that type of reasoning to our youngsters.

Go ahead and reply. I'll not reply to you again. Let the thread proceed as it will.

And again, Good day, sir.


popsjumper  (D 999999999)

Dec 30, 2012, 6:00 AM
Post #34 of 73 (822 views)
Shortcut
Re: [kuai43] Stretching lineset back to trim [In reply to] Can't Post

Understanding the history of the sport and the problems encountered and overcome will help you understand the reasons why the old-timers are trying to teach you what they know.

Trying to help the youngsters avoid making the same mistakes others have made in the past is getting to be a thankless job. Kids apparently want history to repeat itself. I sincerely hope you do not fall into that fallacy.

Those who stick around for a while always eventually come to find that they were off-base about much of their "youngster" ideas. Knowing and understanding the history will help speed up that precess.
Talk to your guys at Orange....listen to what they have to say.


timgaines  (C 107228)

Dec 30, 2012, 6:24 AM
Post #35 of 73 (817 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Quote:
Will stretching this new material will make the line substantially weaker than the original line? Yes.

Good to have info on this. What is your source? How much is Spectra weakened by heat shrinkage? How much further weakening is caused by stretching the shrunken line?

Physics/Material Sciences/Engineering 101

The exact tensile strengths, I am struggling to find, mostly because a lot of this information isn't freely distributed.
There is a paper here but you cant view it unless you are a subscriber or you buy it but some of the figures are visible.
http://www.sciencedirect.com/...ii/S0142941802001277

Look down to figure 12 which is the closest scenario to this. Increasing temperature reduces it's Youngs Modulus and Tensile Stregnth. No material is 100% elastic in it's properties (it won't go back to exactly how it was when you started) so constantly heating the lines 100's of times will weaken this. No weak analogies required.

Once you then start stretching your now weakened fibres, that remaining tensile strength has to be distributed over it's new length ie If it was 10 metres long and had a tensile strength of 100MPa but you now make it 20 metres long it only has a tensile strength of 50MPa.


Premier ianmdrennan  (D 25821)
Moderator
Dec 30, 2012, 7:42 AM
Post #36 of 73 (802 views)
Shortcut
Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

Quote:
I have evidence that skydiving won't kill me. I haven't been killed yet. Hey! It's the same reasoning you are using.

Even outside of the context of this thread, this is an incredibly important statement for skydivers to realize. I've been reading a spectacular book "Deep Survival" (that I recommend ALL skydivers read btw) and while it has nothing to do with skydiving there are a lot of things that help make sense of why we learn, and do, certain things. One topic covered is "Negative Reinforcement"....essentially: Doing the same thing 1000 times with success is not indication of a good behavior but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption (Paraphrased).

Ian


(This post was edited by ianmdrennan on Dec 30, 2012, 7:43 AM)


AggieDave  (D License)

Dec 30, 2012, 8:35 AM
Post #37 of 73 (778 views)
Shortcut
Re: [ianmdrennan] Stretching lineset back to trim [In reply to] Can't Post

Quote:
Doing the same thing 1000 times with success is not indication of a good behavior but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption (Paraphrased).

To be more direct, I would phrase it "doing the same thing 1000 times with out consequence is not an indication of good behavior, but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption."

Our sport's history is littered with examples that highlight this in many different ways. From gear design, gear modification, to aircraft operations and maintenance, skydiver decisions and judgement.


Martini  (D 23756)

Dec 30, 2012, 8:55 AM
Post #38 of 73 (772 views)
Shortcut
Re: [timgaines] Stretching lineset back to trim [In reply to] Can't Post

I can't see the data but thanks for the research. Is the reduction in strength described while the material is still heated or after it has returned to ambient? BTW I would be surprised if heating/shrinking/stretching didn't degrade tensile strength, the question is by how much especially in the skydiving environment.

Also note that the lines will never be stretched beyond their original length. What's more is that the shrinkage/stretching is relatively small by percentage. For example if an "A" line on my Sabre-120, which is 278 cm long, shrinks by 10 cm (which is nearly 4 inches) that's less than 4%.

I'm not disputing the likelihood of tensile loss, only interested in whether the loss is of significance in lines that are incredibly strong and not typically highly loaded.


Martini  (D 23756)

Dec 30, 2012, 9:06 AM
Post #39 of 73 (767 views)
Shortcut
Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

We can certainly agree to disagree to some extent. Here's the original post:

"Has anyone heard of taking a Spectra lineset and stretching it back into trim? this is something ive heard about and have a friend who is jumping a canopy with the lineset stretched and he said it was like a new canopy.

Does anyone have experiences with this good or bad?"

My response to his question is yes I have experience with this and it has been good.

So the breakdown becomes a matter of functionality which I have proven to myself and safety which is not proven except by default. My perception is that this is a low-risk behavior.

And I agree that you and I have no need for further argument.


sundevil777  (D License)

Dec 30, 2012, 9:36 AM
Post #40 of 73 (757 views)
Shortcut
Re: [timgaines] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Once you then start stretching your now weakened fibres, that remaining tensile strength has to be distributed over it's new length ie If it was 10 metres long and had a tensile strength of 100MPa but you now make it 20 metres long it only has a tensile strength of 50MPa.

From what do you make this conclusion? The fibers are not like a ductile metal.


Martini  (D 23756)

Dec 30, 2012, 9:45 AM
Post #41 of 73 (763 views)
Shortcut
Re: [ianmdrennan] Stretching lineset back to trim [In reply to] Can't Post

Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

"Deep Survival"........what a terrific book. "Doing the same thing 1000 times with success is not indication of a good behavior but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption (Paraphrased)." is a pretty well established philosophy though, we are always surprised when bitten by our family dog.

Like anything that involves risk, swimming in uncharted waters should be done thoughtfully. That is not to say that we should never attempt to experiment or change at all. In this case I believe the risk to be low enough to justify the benefit. I perceive the risk of causing a line to break to be small while receiving a perceptible benefit in improved line trim. I feel fortunate that this option was shown to me and I choose to use it. It's interesting that skydivers readily accept Spectra shrinkage without wondering if there is a remedy, it's reasonable that they would be wary of an unfamiliar one.


AggieDave  (D License)

Dec 30, 2012, 12:11 PM
Post #42 of 73 (740 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

So let me get this straight, you believe that no one will tell you what you want to hear because it is a vast conspiracy to pad the profits of the canopy manufactures?

Right...


sundevil777  (D License)

Dec 30, 2012, 12:48 PM
Post #43 of 73 (730 views)
Shortcut
Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

So let me get this straight, you believe that no one will tell you what you want to hear because it is a vast conspiracy to pad the profits of the canopy manufactures?

Right...

A former canopy mfg/designer did already tell him - not what he wants to hear, but that isn't what you want to hear.

The mechanism of the lines shrinking may indeed lend itself to stretching with no ill effect. I definitely do not think this is so unlikely.


(This post was edited by sundevil777 on Dec 30, 2012, 12:50 PM)


JerryBaumchen  (D 1543)

Dec 30, 2012, 1:06 PM
Post #44 of 73 (721 views)
Shortcut
Re: [timgaines] Stretching lineset back to trim [In reply to] Can't Post

Hi tim,

Quote:
tensile strength has to be distributed over it's new length

I might not be the brightest Mech. Engr. to get a sheepskin, but it has always been my understanding that tensile strength is a function of the area of the material and not the length.

Just what I have learned, I 'think,'

JerryBaumchen


AggieDave  (D License)

Dec 30, 2012, 1:22 PM
Post #45 of 73 (715 views)
Shortcut
Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

Edit:

I realized that I really don't care. Do what you want with your canopies, get your linsets to "last" 1000 jumps. Good luck.


(This post was edited by AggieDave on Dec 30, 2012, 1:28 PM)


julio_gyn  (A 705)

Dec 30, 2012, 2:20 PM
Post #46 of 73 (701 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

I know nothing about line strength, but the lines will build up heat from the top down on the lines, making me think the lines will be shrinking more on the bottom. and whenever you stretch it again you can't control where it will happen, so it will be weakening more some part of the lines (maybe bottom?) while the stronger (top?) will stretch less....

and again, I'm just following the thread and trying to see the scenario on real life....


5.samadhi

Dec 30, 2012, 2:32 PM
Post #47 of 73 (695 views)
Shortcut
Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

So let me get this straight, you believe that no one will tell you what you want to hear because it is a vast conspiracy to pad the profits of the canopy manufactures?

Right...
Thats a little dramatic Smile
I think he is trying to say that you won't get support from manufacturers on this topic (although waiting for BG to chime in?) because they have a financial bias that strongly predisposes them toward being conservative on this issue. Not saying they are being malicious or purposefully deceptive, just that the money bias they have toward the issue is muddling their subjectivity.

Does that make sense to you? That is how I took his reasoning (you were not being charitable to him and were attempting to label his line of reasoning as illogical because you disagree with his idea? you are bored? WHatever).


sundevil777  (D License)

Dec 30, 2012, 2:37 PM
Post #48 of 73 (692 views)
Shortcut
Re: [julio_gyn] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
the lines will build up heat from the top down on the lines,

How do you make this conclusion?

In reply to:
whenever you stretch it again you can't control where it will happen,

"Control" may not be possible in the strict sense, but it might be that the areas that shrunk are where the stretching will occur. It depends on the mechanism of how the fibers shrink, and what happens during stretching.

The lack of interest by some to want to know is, interesting.


Martini  (D 23756)

Dec 30, 2012, 3:26 PM
Post #49 of 73 (673 views)
Shortcut
Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

Well Dave, if that's what you heard then it's only because that's what you wanted to hear. Right...

Do you have something useful to say or just want to stir the pot?


AggieDave  (D License)

Dec 30, 2012, 3:28 PM
Post #50 of 73 (671 views)
Shortcut
Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Well Dave, if that's what you heard then it's only because that's what you wanted to hear. Right...

Do you have something useful to say or just want to stir the pot?

I did and had a long post written out about it, then realized that I really don't care and people will continue to do what they want regardless. Hence the previously shown edited comment.

Do what you will and best of luck for it.


First page Previous page 1 2 3 Next page Last page  View All

Forums : Skydiving : Gear and Rigging

 


Search for (options)