Panel orientation for cuben tarp

[Steve B]

Should the grain of the cuben fabric run parallel to the ridge line, or parallel to the long edges of the tarp?

Thanks for your thoughts, cheers
Steve

[Colin Krusor]

Steve, you haven't said anything about the shape of your tarp. Is it a rectangle? The conventional way to make a cuben rectangular tarp is to orient the material so the fibers are both perpendicular and parallel to the ridgeline, which is the only seam. What other options were you considering?

[michael levi]

Either 90 degrees or parallel in my opinion, it's probably not going to make a difference in failure rate. Tie out points, bonding, etc are more prone to failures.

[Colin Krusor]

If the fibers in a cuben panel are perpendicular to something, they are also parallel to it (and vice versa). I don't see how they can be "either 90 degrees or parallel" to anything. Standard cuben has fibers running in two directions, at 0 and 90 degrees (perpendicular to eachother).

If you look at cuben under a microscope, the fibers appear as a grid. The material might appear to have a one-directional "grain", but if you hold it up to a light and look more closely you'll see a grid of fibers running at 90 degree angles to eachother. Maybe Steve doesn't realize this and that was my reason for not understanding his question.

[Steve B]

Sorry, I should have stated that the tarp is not rectangular, but along the lines of a patrol shelter. The edges are not parallel to the ridge line.

Thanks for your help
Steve

[Steve B]

My thinking is that if the grain of the fabric runs parallel to the ridgeline, the ridge line would be stronger, resisting stretching. If the grain of the fabric runs parallel to the edges, the edges would resist stretching.

Which is more important?

Thanks for your input
Steve

[Sam Farrington]

Steve,
Looking at Google's images for a Patrol Shelter, it appears that you will need a catenary cut for the roof seam. So it's the top edges that I think you will have to cut slightly across the grain to get the arc of the catenary cut. You can orient the material so the bottom edges are with the grain.

In woven fabrics, there are a warp line and a weft or fill line, perpendicular to each other, in the grain. One runs in the long direction of the role, the other across the roll, and frankly, I can never remember which is which, except that on some fabrics, the one that runs in the long direction is stronger. And I've no idea if that is the case with Cuben (except with their bias layups, that run in more than two directions). I doubt that this will be an issue, as the material was originally designed for sails that need to be strong in both directions.

As for stretch, Cuben doesn't stretch very much in any direction, unlike woven nylon that stretches in all directions, and woven polyester that stretches on the bias.
It has been said that with heat and prolonged use, Cuben will permanently stretch, as when a sail develops a permanent ballooned curvature. Just by manipulating the material, I have seen that the mylar outers (not the dyneema fibers) will stretch a little when warm, but it does not return, right away anyway, to its original shape; that is, it is not elastic. But, this doesn't seem to have been an issue in small tarps reported on here on BPL.

Hope you have read a lot of the stuff here on catenary cuts and tie-outs, as it is essential to know about for a durable product, and Cuben is expensive.

[Steve B]

Good point; with the catenary curve the threads will be cut either way. In other words, the same threads of the fabric will not extend from pole to pole in either situation, as they will be cut into. Originally orienting the grain parallel to the line from pole to pole might give a bit more threads parallel to the final curved cut (right in the middle of the curve), but not much.

I got the bonding down good, been doing some testing. Same for the tie outs. I will post lots of pictures and patterns (google sketchup) when done. It has been a long time in the making, at least in planning in my head and reading discussions here.

Cheers
Steve

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