Line type

[dexter]

Hello, After experiencing a hard opening skydiving the weekend before last, I got to wondering (I'm not blaming my whack on linetype, it just got me wondering)...

What effect does the stretch in dacron have in openings? Does it make us open faster or slower or not matter at all? Would using a material with little to no stretch result in slightly faster openings? I know it will make my 3.5 sec slider off jumps hurt a ton.

Should we just leave the vectran and HMA at the DZ? Any thoughts out there?


Stay safe out there! ---Dex
dexterbase@hotmail.com

[bandito]

Here is my 2 cents...

Stick with dacron.

It stretches, thus increasing the moment of deceleration, which in turn exponentially increases your comfort zone.

Let us also think of the dramatic forces you normally exert on your BASE gear, do you really want to tear one or two critical attachment point on opening?

Again, I vote for dacron

cya

Rob :-)

[imported_Tom Aiello]

I've also heard speculation that by giving a bit more, dacron can be more forgiving of body position or packing in terms of maintaining opening heading.

I jumped a BASE canopy with spectra lines a while back. It did have bad heading, but it was a very sheisty old canopy to begin with. It also had marvelous flare (it had five control lines, though, so I'm not sure the line material was the primary contributor).

I'll stick with Dacron until someone can make a good argument for more static line types. I've given a lot of thought to the faster opening argument, and I just don't buy it.

If you are really interested, I know a rigger who jumps a Dagger with spectra lines. I could see if he'd talk to you about it.

--Tom Aiello
tbaiello@mac.com

[Mike]

"I've also heard speculation that by giving a bit more, spectra can be more forgiving..."

Unless I'm misreading something, I think you got that backwards. Spectra line gives (stretches) much much LESS than dacron.

For a dedicated slider up canopy, maybe. Openings might be faster and more consistent (in terms of opening speed, not heading). Spectra lines also tend not to go out of trim as easily as dacron. But slider off, it won't make your canopy open faster...it will only feel like it. Your risers, conn. links, and brake settings will have to absorb that impact alone. If something is going to break on opening, I'd rather have it be above the links than below.

Also, free-stowing spectra lines in a tail pocket can be very scary. Tension-knot city. No thank you.

[imported_Tom Aiello]

Sorry. Definitely a typo. I did mean, as you surmised, that Dacron was more forgiving as to heading. I'll edit the original.

Thanks.

--Tom Aiello
tbaiello@mac.com

[jon593]

I watched with interest one of the participants of a Cave trips openings because he had a microlined canopy. The openings were dramaticly more sharp and even appeared to be more erratic after bottom skin stretch. The canopy seemed to smack hard and secondary opening shock was also sharp and could be uneven causing direction change.

leave the cheese cutter lines for the DZ

jon

[bps]

> Spectra lines also tend not to go out of trim as
> easily as dacron.

Hi Mike --

You may have a typo here, but just so people don't get confused, Spectra lines go out of trim way before Dacron does. Spectra lines shrink dramatically from the heat that's generated when the slider grommets slide down your lines. Easily demonstrated by grabbing any Spectra-lines canopy that has at least 300 to 400 jums on it. Take a look at the outside A lines (when compared to the other A lines). You'll notice the shrinkage immediately.

Yikes, gotta go catch a load. (I'm a closet skydiver!)

C-ya,

Bryan

[Mike]

I'll give you the benefit of the doubt on this one. I haven't played with skydiving parachutes much in the last 3 years. I was basing my assumption on theoretical information and what may now be some antiquated rules of thumb. Practical experience obviously trumps both.

But I just can't let it go. Now that I think about it, I have seen a lot of spectra line canopies go out of trim. My old PD-170 (in my closet collecting dust-I'm a "closet skydiver" too) is a great example. But your explanation doesn't seem to add up. For one thing spectra has a very low coefficient of friction compared to other line materials, so there shouldn't be much heat generated in the first place. And if anything, the heat generated by the slider would tend to expand the material, not shrink it (just guessing here). What I am wondering is, how do you know the shrinkage of the outside lines is not actually creep (elongation over time) in the relatively highly-loaded inner lines?

[3ringheathen]

I think Dacron was chosen over Spectra for good reason and has become the default standard in BASE.

I did a bit of reading on helmet design and it turns out that truly effective helmets essentially work simply by spreading the force of impact over time. The research indicates that milliseconds matter. To extrapolate from this I'd guess that the benefit of a faster opening, even if true, would be more than offset by the increased shock load on your gear and body. I'm guessing we're talking about milliseconds here, which won't make much difference in your opening altitude, but may very well make a significant difference in opening shock distribution.

Aside from the issue of opening shock, HMA and Vectran are newer materials, but IMO, are not suitable for BASE jumping (I don't even think they're a very good idea for skydiving).
HMA and Vectran don't necessarily produce visible signs of wear, and are much less abrasion resistant than Dacron or Spectra. Combine this with the abuse a BASE canopy is certain to take: tree landings, bushes, concrete, mud, dirt, and so on, and you're just begging for line failure really fast.
My .02,
Josh

[crwper]

Accidentally posted twice. Sorry!

[crwper]

Sounds like you're thinking of the way a block of metal, like most materials, shrinks in the cold and expands in heat. Polymers aren't like most materials, though.

Think of heat-shrink tubing. You apply heat, it shrinks. My understanding is that the heat causes some bonds in the polymer to break, and when they rebuild, they do so in a lower-energy, shorter, configuration.

The polymer, like most materials, will probably expand initially when it's heated, but the heat generated by friction is enough to rearrange the molecule, and then you're talking about something completely different from heating and cooling a block of metal.

The amount a given polymer shrinks when heat is applied, unless I'm mistaken, is mainly a factor of how easy it is to rearrange the bonds. The bonds of Dacron, for example, are slightly more resistant to heat than the bonds of Spectra. Kevlar and Vectran are even more resistant to heat, but suffer the problem that because they are relatively "stiff" materials, they tend to fail all at once.

On a high-performance skydiving canopy, it can be worthwhile to trade longevity for a material which will keep the trim of the canopy over its entire lifespan -- provided we are very careful to re-line the canopy often, even if it looks "fine".

On BASE canopies, which are typically very large, I don't think the trim issue is such a big deal. The properties of Dacron are fairly well understood, and their reliability is proven. I don't think a stiffer line will make the canopy open faster, it will just make it so the canopy opens at the same speed, but your body feels more of the opening shock.

Michael

[Tree]

With regard to spectra line, it is common knowledge (not theory) that spectra line shrinks tremendously from the heat generated by slider grommets. Spectra line is reallly not suitable for base, IMHO, due to the lack of strectch. Spectra will be more likely to break at the moment of force on a hard opening whereas dacron will stretch through the moment. Vectran and HMA will never be an acceptable line for base canopies because of poor abrasion wear characteristics. I doubt that HMA will be around on skydiving canopies for long as they are already snapping without warning (pretty much the same thing that happened when Kevlar was tried out as suspension line 7 or 8 years ago). Vectran is great for swooping machines but does require additional attention and care to remain safe. Stick with what is proven. Base is a lousy place to try "new and improved" materials.

Tree :-)

[Tree]

With regard to spectra line, it is common knowledge (not theory) that spectra line shrinks tremendously from the heat generated by slider grommets. Spectra line is reallly not suitable for base, IMHO, due to the lack of strectch. Spectra will be more likely to break at the moment of force on a hard opening whereas dacron will stretch through the moment. Vectran and HMA will never be an acceptable line for base canopies because of poor abrasion wear characteristics. I doubt that HMA will be around on skydiving canopies for long as they are already snapping without warning (pretty much the same thing that happened when Kevlar was tried out as suspension line 7 or 8 years ago). Vectran is great for swooping machines but does require additional attention and care to remain safe. Stick with what is proven. Base is a lousy place to try "new and improved" materials.

Tree :-)

[dexter]

Thanks for the input guys, you confirmed my thinking. Dacron is the only acceptable BASE suspension line material currently available.


Stay safe out there! ---Dex
dexterbase@hotmail.com