Wind Stress on Cuben

[Ryan Jordan]

We are in the midst of a comprehensive research project on wind testing of ultralight shelters.

One of the things we are looking at is the attachment of guyline tie-out points to high-stress areas on shelters.

Here's an example of stitching on a Cuben Fiber shelter in response to a load of about 15 pounds.

I'm wondering if any of you have any field experience that has resulted in stitching failure on tie-outs of Cuben tarps/shelters resulting from the stitching cutting the Cuben and ripping out in response to wind stress?

[Travis Leanna]

Very cool project!

Thank you for becoming more involved in BPL, Ryan. These types of projects and research are what we love to see!

Hello Ryan,

I have been doing these type of testings with Cuben Fiber for the last three hiking seasons. Testing everything from 1.43 down to 0.34 (presently involved in a 2 season endurance testing of a 0.34 cf tarp) and in nearly ever case where the threads where not taped pre-sewing and post sewing there were thread pulling issues. The vast majority of them have been around the tie-out squares/triangles and the apex of the shelters.

My testings have never been of the scientific type (ie: sewing tie-outs and putting weights on them, much like what Steve Evans did and documented in his whitepaper a few years back) but rather putting the shelters up, using them, sometimes letting them stay up and get abused by the weather, and most recently I have started intentionally over tightening the guy lines to see how tight different methods of sewing/taping perform (suppose that is somewhat along the lines of what Steve Evans did.)

My overall approach to the matter after all of these tests is that it is not so much the weight of the material that is important as it is the importance of using taping. Those shelters that have applied tape before sewing and than again after sewing have been extremely difficult to have threads ripping it.

This also leads to the necessary discussion/research of different widths of tape. I have seen a few manufacturers using tape that is so thin it probably is not doing any good at all, while others are using cf tape that is much wider than I would have expected to see on a CF shelter (such as the SMD Skyscape X, which uses the widest cf tape I have seen in any shelter.) How much greater resistance to thread pulling can be gained by using different widths of cf tape is crucial in the development of CF tarps/shelters that are expected to be put under serious wind/snow conditions, I feel.

I cannot give great details on the direct issue of wind to cf thread pulling as there is not a whole lot of wind here in the middle of the Redwood forest, but I have been able to test for cf thread pulling in other ways and it has been a fun adventure blowing through a lot of cf material in the quest to find what endures the best.

John B. Abela
HikeLighter.Com

[Dan @ Durston Gear]

Here's a link to Steve's tests that he did a year or so ago….it's well done research:

http://www.suluk46.com/RandD%20-%20RD20%20Cuben%20Tieout%20Tests.html

[Daryl and Daryl]

Ryan,

No field testing to add here.

From recent discussions and postings on the subject, however, I'd guess that the 9 observations listed in the post below would contribute to a strong tie-out on cuben.

here

Looking forward to your work on the subject. Could be very useful.

Thanks.

Daryl

[a b]

I just performed a test on my MLD .74 oz/sq yard Cuben Patrol Shelter.
I used a corner tie out.
Hanging the corner of the tarp over the back of a closet door, I loaded the corner tie out with 2 gallon milk jugs full of water. (I know the picture is a bit unclear. You can just see the other jug behind the first.)
Thats about 16 lbs.
Actually one of the jugs is frozen cause i use it for ballast in my freezer for when the power goes out but obviously it weighs the same.
As you can see from the photos, the construction of the tie out on my Patrol is reinforced with nylon fabric and the gross grain is double bartacked.
All the tie outs on the tarp are reinforced like this.
Here are the pictures.
.
.
.

Before you call me crazy for trying this, let me explain: I never had a doubt in my mind that this would hold.
I used this Patrol on the AT and had some incredible windstorms and microburst thunderstorm events. Never once did I doubt the cuben my shelter was made from.. the stakes stuck in the ground will give way long before the material or stitchwork would.
The construction of this tarp and reinforcement of the tieouts is far superior to what is shown in Ryan's picture above.
I am a little surprised at the crude nature of the stitch work in that picture. I think any fabric would have shown that kind of stress and needle hole stretch given that crude stitchwork.
The construction technique used is far more important under load than the material itself.
Note: No cuben shelter was harmed in the making of this post :)

[Ryan Jordan]

16 lbs should be easily accommodated by any shelter!

Let's assume you tension every tie out point to a minimum of 10 pounds. For Cuben, you should be able to tension to a minimum of 15 lbs for tarps and tarp tents where you have high surface area and little structure, so you have adequate tension in the panels.

Then, add wind loading on that, which on tarps and tarp tents, we see 5-15 pounds in response to 25-30 mph winds.

So if a tie out point can't at least handle a 30 pound load (and a dynamic one at that, which generates more stress than the static milk jug hang), then _________ (you fill in the blank). All of us well know that 30 mph winds are not an unreasonable thing to encounter in virtually any backcountry scenario. Heck, I've seen 40+ mph gusts in the trees on the AT. Summer T-storms out West, or hurricane residuals back East, can generate a lot worse. (And don't even get the Brits started on this, they'll laugh at our 40 mph winds and call them "a tad breezy").

The more insidious types of failure are those that occur when you aren't noticing, i.e., that repetitive stress that s-l-o-w-l-y works those thread cuts bigger…and bigger…and bigger.

[a b]

Ryan, I was responding to your photo of a 15 lb "static" load on a tie out on a piece of cuben fiber.
The point I was trying to make was the importance and bearing of construction of the tie out itself.
In the case of my shelter a hybrid approach was used. There is the cuben fiber itself, a layer of nylon fabric, and the gross grain of the pullout.
The nylon triangle spreads the force over a wide area of the cuben. The gross grain is sewn through both the cuben and nylon patch using double bartack stitching.
As for dynamic loading I suppose anything done in the wild is going to have too much variability even if the shelters are side by side, one might experience more of a gust than the other; witness the "trailer park effect" in tornado alley.
So are you suggesting a laboratory type test where tie outs sewn to cuben fiber are tested to destruction?
For what it's worth.. i ain't volunteering my Patrol for that!
This will be an interesting series of articles!
Right on man.. or as well say in Santa Cruz.. Dude thats rad!

[Terry Trimble]

I have seen the Granite Gear Cuben stuff sacks they look like they are ultra sonic welded.

I was was thinking if I was in the cottage industry maker of cuben tarps. I would sonic weld thicker cuben at guy out points with thicker cuben guy loops also sonic welded to the tarp. Basically it would be a stitch less tarp with all the seams and edge ultra sonic welded also. It would not leak at the peak and would be stronger.

http://www.emersonindustrial.com/xx-XX/branson/Industries/textile-machinery/Pages/allindustr

Terry

[James Marco]

Yeah, thread stitching on cuben can be a real problem. Especially with 30mph winds, and the potential for snapping, or flapping fabric (impact) adding stress beyond what is normal. 40mph it becomes down right damaging.

One of the big factors IS the impact. Wind stress can load and unload rapidly, imparting an impact "moment" to the fabric. Any fabric is subject to this. Alleviating it is simply a matter of using elastic strapping or elastic guy lines or sumply an elastic hair tie on the loop.

The low stretch nature of kevlar, carbon, Dyneema, or spectra fibers used in cuben materials is at odds with the fairly elastic nature of the plastic films between them. I anticipate that any threading over the fibers will cause a stretching of the material between them. Hence, as Ryan's photo shows so clearly, elongated holes where the stitching has damaged the "body" material as it stretched, locking around the fiber (wich doesn't to any large degree.) This will cause some stretching around the fiber, but, since it was not damaged, it remains intact, but thinned. Since it is still bonded to the cross grain fiber, it doesn't spread. The result, if allowed to continue to failure, would be a ragged edge starting at the origonal score line. The period of the failure would be the strength of the fibers, and the skin. Indeed, I expect at some degree of stress, you could get a "creeping" tear, it will fail in slow motion as the body picks up stress, stretches and finally tears, then the next one does the same. Generally, sails are directed fibers, ie, directed stresses to the points of attachment. Fibers are laid to pick up these stresses directly. We only adapt cuben cloth to tarps and packs because it is light. Sewing is not the same.

Stitching cuben would not be my first choice for construction methodology, therefore. The damage to the body skin will always dictate the weakest point at the seam because it is a combination of the skin and fiber that is the overall strength. I think this was pretty much the result of the "bucket" test. Once you hit the stress exceeding the strength of the film, it may hold together, but you get some elongation of any stitch holes, till the stress can be distributed among surronding material to compensate. As is seen in Ryan's photo. This can be looked at as a fail at that point. You have exceeded the strenght of the film, it is stretching. I would call it a mid-failure though, because you still have a compensation reaction, among the surrounding material. For a sail, this is bad, eventually fatal. For a shelter, not necessarily fatal.

Stitch length becomes important in distributing loads. One in every other "box"? One in every 3-4 "boxes?" Anyway, it will lead to a weakness, never a way to totally compensate, again, going back to the bucket testing.

Really, bonding, with appropriate glues, is much prefered. I would suggest simply bonding heavier material at the tieouts, pole pockets, etc. Say, 1.1ounce materal to .53 material for stitching. Cutting and adding lapped triangles should work, OK along the edges for guy lines and pockets. Gluing on patches should work for other tieouts. Generally, stitching should be avoided for maximal strength. I think of cuben as reinforced plastic. Not as a single material like silnylon, though ripstop has similar characteristics. Multiple layes of fibers going different directions would help a lot. Fibers in the same direction probably not much. Totally eliminate the problem? Probably not.

Anyway, I don't have any real experience with cuben, soo, I won't say more. Just a few thoughts…

[Dan @ Durston Gear]

"I have seen the Granite Gear Cuben stuff sacks they look like they are ultra sonic welded."

Granite Gear's Uberlight CT3 stuff sacks are just bonded with adhesive…nothing fancy. The adhesive slips over time a bit and my wife's actually came apart just from her stuff her sleeping bag into it. The cuben didn't fail….the adhesive just slipped until the two panels came apart.

Mine (shown) has slipped a bit (middle of photo), but nothing like hers which slowly came wide open after much use.

[Jason Elsworth]

If I remember rightly this is exactly why UK ultralight, Colin Ibbotson, stopped using Cuben tarps. However, the cuben tarp he used didn't seem to me constructed to best practice standards.

I have my first Cuben shelter on order, a Trailstar, but I will be hanging onto my Silnylon Trailstar for a bit, as here in NZ 40 mph winds, especially where I live, are considered moderate. So I want to see how things go for a bit.

[Terry Trimble]

Dan,
I did not know the seams were glued on the granite gear bag. I can see how the glue has changed color on the bag in the picture from aging. I was thinking a sail makers forum would be the best place to look at how sail makers glue and sew cuben fiber because sails have more stress on them than tarps .
So I found the sail anarchy forum reading a few post on sailing anarchy cuben is being used less in sailing in less you have a world cup sailing racing budget or deep pockets. I did find a letter from cuben fiber corp. asking to be part of the sail comparison shoot out they were doing at sailing anarchy.

http://www.sailinganarchy.com/general/2004/cuben%20fiber.htm

From what I read they are using a Dimension Polyant heated ultra bond gun and sewing machine to assemble the sails. I was also thinking if anyone on BPL fourm knows a cuben fiber sail maker maybe they could pick his mind on simple sewing and bonding techniques.
Photos of a Dimension Polyant ultra bond gun:
http://schurrsails.com/ultrabond.htm

I used to make fighter kites and we would make a sew less fighter kites with Cyanoacrylates glues aka: {superglue} we glued patches of folded nylon over kite cloth on the kite for carbon fiber spars.

I was thinking a hour glass shaped one piece of fabric guy out point made out of 70 d ripstop nylon folded over and glued to the cuben fabric would make a stronger sew less guy out point. You could also use a large/wide Zigzag stitching like I have seen used in sailboard sails would put less stress on cuben fabric. For example of the guy out hour glass picture I sketched below.

I have to admit I have not worked with cuben but I have worked a lot different nylon fabrics making packs including Xpac and .75 oz. nylon kite cloth. We would use a large zig zag stitch when sewing fighter kites. I have been looking around the internet and looking at the sewing techniques sailmakers use to learn more about cuben.
Hope my information helps you guys.
Terry

[Luke Schmidt]

THIS is the kind of forum (and article hopefully) that makes BPL great!

[Harald Hope]

Figure Four backpack/cuben fiber analysis

suluk46.com cuben tear/failure tests on tie-outs (pdf)

These are both good information.

The first one note the hole tearing, sadly they don't share what stitch lengths they came up with for optimal strength, but did note there is one.

To me, after ordering some .74 oz cuben just to see what it's like, it's basically self evident that threads are going to tear that flimsy sheet of plastic/mylar, that's obvious.

What is interesting in the pdf is how little difference there was in pounds between bonded and sewn patches on tieouts., but that's talking about catastrophic rip apart failure, not more fine grained where you look at when the holes in sewn start to expand. However, those were differences of between about 150 and 200 pounds to break it, in other words, either method is fine for the raw strength real world applications will apply. But that's only talking about full failure.

I couldn't get much information from the first pic / posting Ryan made, what weight cuben is that, where is the stitch, how many stitches per inch, what is the zig zag width?

As the Figure Four article notes, these are important considerations.

I'm going to resist using that cuben material until i have a better sense of this. Personally I have no interest in playing with anything less than the .74, that's already too thin in terms of the plastic sheeting.

I'll definitely be interested in seeing the results of that testing. It seems intuitively obvious that a full bond between sheets, by glue, heat, or tape, is going to avoid those types of stretching holes, since the force is evenly distributed and not focused on one thread per x millimeters. Maybe a nylon patch/reinforcing really is the way to go.

[David Ure]

Can't wait to see the results of testing.

I also concur that it is great to have Ryan back active on the forums!

[Benjamin Moryson]

Is there any information in advance which shelters will be tested??

[Jeremy Platt]

I definately had some stitch induced stretch/rip on my zpacks hexamid after a couple of taught pitches, however the stitches haven't moved at all in the last couple of years after I seam sealed them all. Has anyone else found this too?

[Ryan Jordan]

I'll test anything you guys want to see (and manufacturers want to provide). Just let me know what your "votes" are.

[Terry Trimble]

I was looking at article on the shore sails website it looks like they use 2.5oz cuben for racing sails. I know I am comparing apples and oranges because we use lighter weight cuben. Here's a quote about the strength of the cuben sails from the article.

"Where's the proof of success for the RaceSail in Cuben Fiber?
Bill's Cuben Fiber RaceSail main for Pamir, a Swan 55, was used at the Heineken Cup. In 30+ knots of breeze, this new main survived in race-ready condition while molded sails were self-destructing. This boat's mainsail weighs only 49 lbs. That's right! A main for a Swan 55 which weighs 49 lbs. and survived 30 + knot winds successfully."

Link to the article:
http://www.shoresails.com/shore%20international%20web%20site/Sail%20Consulting/Cuben%20Fiber.htm

I also have been thinking that maybe stitching is the problem has anyone made tarp panels
with the guys outs being part or integrated into the main tarp panel when cutting out the panel. Then just folding the guy out over and glue to keep the integrity of the fabric. The guy outs would be part of the tarp panel and you would not have the extra weight of nylon patches,crossgrain ribbon and thread so you could shave a couple grams off your trap weight.

Then do some weight testing of different weight of cuben fabric weights and fabric durability we use to see if they tear faster or less compared to sewn on guy out.

I did quick sketch of one piece panel of tarp with the guys outs folded that you would fold over and glue.

It kind of fun thinking out of the box about tarp design hope this helps you guys.
Terry

[Benjamin Moryson]

MLD DuoMid
LocuGear Khufu
HMG Echo One
MLD Trailstar

[Chris Peichel]

Has anyone tested a sewn seam where the stitching has been covered with a thin layer of seamgrip? I know with other fabrics seamgrip has helped reinforce seams and minimized the tearing of the fabric.

I have not yet worked with cuben fiber but hope to someday when funds allow.

[Rob Daly]

to sewing Cuben, at least on high stress areas. I would stick to glue or tape.

[nick beaudoin]

This is an interesting topic but there are some broad strokes being painted about sewing vs bonding.

I am a sailmaker at North Sails and one of the only sail lofts in the world that builds Cuben spinnakers. They are 400 to 900 square meters in size and cost up to $180,000 each depending on size and cloth (cn 5 up to cn18) used.

We sew all Cuben spinnakers. The seems contain both 3m (9485) double sided tape and sewing. We have never had any failures yet, with our sails competing in the worlds most difficult offshore races. We have in the past seen failures when sails were constructed using Q-bond only.

My point is that if you use a small needle (size 90) and light thread (v32) and triple step stitch it shouldn't tear the cloth, and if it did the nature of Cuben would stop it from yielding. Ryan's picture at the top of the thread look like a larger needle was used and zig zag stitch creating nice holes to yield under load

So depending on application, I think there is merit for both methods of construction.
nick

[robert v]

Lightheartgear sews and bonds their cuben tents, they feel it is the most secure way to construct a tent of this material. Since I have a Solong 6 tent made of cuben, I'd be curious to see how their tents compare to other manufacturers, some who sew, some who bond, and some who do both.

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