Another Cuben Tent Question?

[Sam Farrington]

Bought some Cuben from Cubic Tech, and have been exploring different ways to incorporate it into a tent; but something has been bugging me, and could not put my finger on it.

So, decided to print out and re-read Lawson Kline's winter 2009/2010 thread, "Cuben Tent Question." It reminded me of Prince Myshkin's conversations in a Dostoyevsky novel I'm reading, but I'll spare you all about that. And the automatic updates on the Avatars that made some of the posts incomprehensible made be wonder about revisionist history in a totalitarian cybertech paperless world; but I'll spare you all about that too.

One item that caught my eye was Franco Darioli's New Year's Eve post, and his obervation that,
"Cuben is a manufacturing process not a particular fabric or laminate. So many variations are possible, however they all have some "creep", that is, with pressure they go out of shape. Like say Gladwrap, once expanded it does not go back to the original size."

So I looked at a sample of the CT Cuben I've been using for various tests, and pulled and stretched it, diagonally to the fabric threads. It became quite mishapen, and unlike, say, silnylon, did not resume its original shape. So I pulled and stretched the mishapen area on the other diagonal, and this made a large pockmark in the fabric that would not resume its original shape.

A major concern is that when subjected to high winds, a Cuben tent wall may permanently distort into a concave shape, and thus become incapable of remaining taut and resistant to flapping, billowing, snapping, internal sagging and dripping, and all the other plagues associated with poorly designed and/or constructed tents. A slightly lesser concern is whether a hailstorm could make a Cuben tent wall look like a cobblestone street.

So then I tried the diagonal stretching on a lighter (.5 oz) 1K Cuben sample, from Zpacks, and found the stretching and pockmarking was much less.

So then I looked at a torn label on the material from Cubic Tech sold as "CT2K.08" and found that it read, "CT2E.08."

The Cubic tech info states that the "K" signifies a polyester film, but nothing about "E". I have emailed them about this and am awaiting a response.

So many of you have posted about experience with shaped Cuben tarps, and lately even tents to some degree, that I am hoping you might share any experiences, good or bad, along these lines. If you are reluctant to post publicly, I am at [email protected], and will keep any eMail responses strictly confidential.

I would very much appreciate any guidance anyone might provide on this issue, as I have invested some considerable $; but mostly, really do not want to make a much more valuable investment of time and effort if it is going to be wasted. Being MYOG folk, I trust you know what I mean.

Thanks.
Sam F.

[ziff house]

You are lucky to have a big chunk of cuben. i have not noticed any of the problems you mention, i can pull the material out of shape but i can also pull it back.

[Lawson Kline]

The E Film is actually better then the K film since it has lower air permeability and is stronger. They usually charge $1.00 more per meter for it but were giving it as a free upgrade for a while.

The permanent deflection you notice is called "creep". This has to do with the polyester film's and Dyneema fiber's low modulus of elasticity (aka no stretch) The higher the concentration of Dyneema fibers to film the less this happens. I personally don't think wind on a 2k/e wall material would ever cause creep. Maybe bursting but defiantly not creep. Then again they only use cuben fiber in race spinnakers due to its creep so I really don't know if its possible or not. Essentially all materials have some sort of creep and over time don't stretch back to their original size. Just some are more noticeable then others. I personally have never had a problem.

[Dan @ Durston Gear]

Playing with cuben, I've noticed that you can distort it a bit if you intentionally try, but there is a limit to how much it distorts and with the CT2K.08 that I've mostly played with, the amount of stretch possible was pretty minor.

As it relates to a tent, I don't think this is an issue. If anything, the areas of your shelter that are under the most tension are going to be what stretches a bit, and this stretching would spread the tension over a greater swath of the material, thus giving you a better pitch overall.

I can't imagine hail ever causing significant 'creep'.

I made a tent fly with CT2K.08 and while I could create some 'creep' when intentionally pulling on the material, I never noticed this to be occurring with the finished tent fly.

[Franco Darioli]

My initial comment was to point out that there is the possibility when discussing Cuben (as well as pretty much any brand) that members are discussing apple and oranges.
In other words the lightest type of Cuben is not going to behave the same as the heaviest and so on.
Now if creep can become a factor in tent design or not it remains to be seen but maybe it should be taken into account.
And no, I do not think that creep from stress is a good thing…
Since some comparisons with sails have been made and Samuel has alluded to certain designs that facilitate an initially flat surface to turn concave under wind pressure, that is something that sail makers use to their advantage.
The "fastest" part of a sail (where the wind is concentrated) is called "reach" .
Think of how sunlight from warm becomes burning hot when concentrated (with a magnifying glass for example).
When a sail is under wind that "reach" area has a lot more pressure than any other part.
The point here is that "Cuben" may work in one model and fail in another that looks to some as having a similar construction.

To understand how a small flat surfaces can make a big difference a good example is the VW Beetle.
The "regime" wanted a car that could sustain 100 KM PH fully loaded and with a relatively small (economical) engine.
To do that the Beetle was designed with the help of a wind tunnel resulting in a very low drag (CX) for that time.
Later when a square bumper bar and flat headlights were added, suddenly the drag increased significantly.
Later on it dropped again when a curved widscreen took over from the flat type.
BTW the panels are designed so that the airflow from the top and sides is forced ito the rear of the car where the air cooled engine is.
So although with tents it has to do more with aeroelasticity than aerodynamics some of the principles do apply.
However since I have no technical expertise whatsoever, I could be totally wrong..
Franco
i can pull the material out of shape but i can also pull it back.
sure you can….

[James Marco]

Yes, this is a concern. While not directly by winds on the fabric, we know that the effect can be concentrated a lot at poles and guy lines easily exceeding a fabric's or plastic film's ability to recover. Reinforment patches, tapes, none or few needle holes, double and triple reinforced pole "cups" have been around for a long time.

Example:
10×12 flat tarp: I use a hiking staff to push the center up. Over a period of time, this has created a permanent "dimple" in the tarp. Pulled tight at each of the 4 corners, it will create ripples extending in both directions from center.

The creep or stretching is minor, but it opens the fabric slighly leading to leaks. With the silnylon, it is easily resealed with a thin coating(20 or 30 to 1, thinner and silicone calk.)

With cuben film, I sort'a doubt it will lead to leaks, but, because the film is different from the reinforcing dynema (carbon, kevlar or whatever,) it could result in weakening of the fabric and eventual blowouts between the reinforcing fibers. I suspect this would be contributed from separation of the film from the supporting reinforcement. With no way to repair this, other than a patch, replacement would be the only option. Recycling the undamaged material where it is possible, of course, since it would be localized to pole and tie out areas. This is really no different than a silnylon or canvas tent. Only the actual mechanism for the failure is changed. The type of failure remains the same.

In either case, fabric or film, increased weight is expected. This is also part of the durability of tents and tarps. UL stuff will eventually go bad. Part of the price we pay for participating in the sport.

[ziff house]

answer is that in the test case you mention and in the case above using single poles, a lot of force is being concentrated in a single spot. In practice it would be difficult to exert that same force over a large area, it is possible to design a tent so that forces are directed that way. In the case above i would glue 'spears' to strengthen the stress points, should work.

[Sam Farrington]

To all:
Thank you much for your helpful responses.
Especially for the info on the E (Lawson) and experience with the Cuben fly (Dan), although I recall Dan, that you parted with the fly not too long after completion.
Guess I needed some reassurance – it is much appreciated.

Franco, the concave (or convex from the other side) distortion in Cuben sails was also posted by someone on another thread I could not find. Perhaps it was you, don't know. Am encouraged by Dan's and Lawson's doubts and reports of this not happening in tent or fly walls.

Am determined to avoid either sagging (nylon) or permanent concavity (Cuben?) in the dome panels. The lightest coated polyester I could find was taken from a large Canadian Chinook tarp, and is about 2.1 oz/sq yd. Using this instead of the Cuben for 1/2 the canopy would add over a quarter pound to the tent, defeating the goal of light weight.

The only tie-outs planned for the dome will run from small loops at both sides of each pole sleeve – Dynema cords from the loops will join a few inches past the top of the pole sleeve, after which one cord will lead to the stake. Could use catenary cut triangles instead of loops to spread out the pulling force and cause less distortion in the canopy?

May also add tie-out loops at the center of the base of each of the two door panels at the tent front (think Akto or Moment with 2 doors). This is done on Mr. Wong's Luxe Firefly, and is shown at the end of the YouTube video on the Equipoutdoors site. This would be just to limit flapping in high wind, not to support or anchor the tent, so should not be the source of major stresses on the fabric.

So I will proceed with Cuben for the front half of the canopy, and may use the 1K instead of the 2E; not because the 1K is a quarter ounce lighter, but because it was more resistant to distortion when stretched diagonally. Don't ask me why the lighter fabric film would be more resistant, have no idea. If I use the 1K, will post the 2E for sale cheap, then someone else can get lucky, Ziff.

If the Cuben does permanently distort, the seams will be designed so it can be readily replaced. Who knows what may become available in polyester by then. Will post the outcome either way.

James, assume you were referring to a silnylon fly. If you ran seams to the center of the fly where the pole is pushed, you would probably lose most of the protection afforded by the expansion of the fabric. Suppose you could bond on a large disk-shaped reinforcement piece of silnylon so that the pole pushes at the center of the disk, to limit the "dimple," and increase the life of the fly. (See BPL's recently republished article on bonding silnylon) With Cuben, I think you would have a much larger and more permanent dimple.

Thanks, again,
Sam f.

[Dan @ Durston Gear]

FWIW, I used the cuben fly for about 10-15 nights before I sold it. I wasn't intending to sell it, but I received a generous offer for it and I always love to try new gear so I said yes. The sale had nothing to do with the qualities of cuben. I love this material for shelters.

During my time with the Cuben fly, I did experience the highest winds I've ever encountered while hiking (enough to make me wonder if a pole would snap) and heavy snow fall (about 4" of pure slush) that was causing the tent to collapse until I knocked it off. Obviously I don't have any long term experience, but the more I use cuben the more confident I get with it.

I have on hand quite a few cuben variations so I just spent a few minutes playing with them to compare their 'creep'. I played with CT0.3K.08, CT1K.08, CT2K.08, CT3.5K.18 and CT5K.18. I found the 1K & 2K variants to be fairly similar in terms of stretch. I couldn't confidently say I found one more stretchy than the other. The 0.3K material had about 50% more stretch than the 1K & 2K variants, while the 3.5K.18 and 5K.18 had about half the stretch of the 1K & 2K variants.

With all of these materials, the stretch is far more noticeable if you are pulling right on the diagonal (ie. 45 degrees) since the spectra runs at 0 and 90 degrees. With the 1.2oz and 1.5oz spectra you have to be pretty much right on the diagonal to notice anything. With this in mind, if you orient the cuben in your shelter so that the lines of stress aren't on a diagonal to the material then this seemingly would reduce the 'creep'. Another possible technique would be to orient your reinforcement patches so that the spectra in these runs on the diagonal, so it's not susceptible to creep in the same direction as the underlying material.

Another approach to this whole issue would be to use high bias cuben. High bias cuben has spectra running on the diagonal (ie. +/- 45 degrees) in addition to the 0 & 90 degree orientations, so you would seemingly get radically less stretch/creep. CT1HBK.08 would be a really neat product to use. This stuff weighs 0.86oz/yd. Very little has been done with high bias cuben so far, because it costs even more than regular cuben. I think it's another $5/yd or so.

[James Marco]

"Could use catenary cut triangles instead of loops to spread out the pulling force and cause less distortion in the canopy?"

I think that would work. Distributing the live load from wind pressure will help a lot in comparison to letting it build up at support points.

Cat cuts will help a bit by distributing flapping tensions (live loads,) too.

My example was from an old silnylon tarp…no biggie. I'll make another when it gets bad. I use a smaller tarp these days, mostly.

[Sam Farrington]

Thank you for the additonal feedback and clarifications.

Dan,
Thank you for the more detailed follow-up on your experience with the Cuben fly.
Got an email response from CT saying essentially the same thing as Lawson posted on the E grade material.

Starting from scratch, would probably purchase the 'high bias' material, but at this point, the Cuben I purchased is way too expensive to toss out and replace. There was a fairly negative post on one of the threads about the high bias stuff, and that may have dissuaded me from looking into it. In any event, your experience with the 2K is very encouraging.

If there is going to be any permanent distortion of the Cuben, in the form of concavity (as with he sails), don't think it matters how it is oriented. Can just imagine the sun beating down on the plastic on one of those sails, stretched to the limit by the wind over a long period of sailing. Such distortion could also be occasioned by driving rain. You may have seen the You-Tube video of the tent shower testing. One panel of a nylon tent became so distorted under the water assault that the coating failed. Your experience suggests wind and water will not be a problem with distorting the Cuben in something as small as a tent panel. And you may recall from other posts that the only part of my canopy that even approaches horizontal will be protected by a small cap-fly that will probably be the best grade silnylon I can find.

But just in case, it happens that I designed the tent so that with the exception of the panel edge seams, which will also be bonded, there is to be no sewing anywhere on the surface of the 1/2 part of the canopy that will be Cuben. The reason for this was all the negative reporting about sewing Cuben; but it will also make it much easier to remove and replace the two Cuben panels if they do distort.

The other half of the canopy is going to be Epic Malibu, which a number of folks have advised against (some vehemently); so the whole project is pretty risky. Talk about prototyping! Nevertheless, am still trying to minimize the risks so that the labor will be rewarded by reliable use, not just experimental results from a prototype.

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