Stitch Newbie

[Matt]

I’m making a pack out of Dyneema Composite Hybrid.  It seems that I need to stitch the long vertical seams of the pack body (among others) multiple times for strength.  Does the second pass go right over the first pass, or does the second pass go next to the first pass?  I am concerned that the needle on the second pass will tear the thread from the first pass if they are on top of each other.  But I am also concerned about creating swiss cheese fabric if they are next to each other.  Am I making a mountain out of a molehill? I just can’t find an answer to this seemingly basic question.

[Greg Pehrson]

I haven’t worked with Hybrid DCF, but I think the best solution for the long vertical seams may be a flat- felled seam, which is extremely strong and has enough spacing between the two lines of stitches that perforation shouldn’t be a problem.

I would think that if you tried to sew the second pass right over the first pass, it would be nearly impossible to line up the stitches exactly (going into the same holes) so you’d have many more stitch holes in the same line much closer together, leading to increased chance of perforation. Even if you _could_ line up the stitches I would think that a second pass would enlarge the original stitch holes creating a similar problem.

I’m sure someone with direct experience with DCF will weigh in which will be more helpful; this is just my experience with other relatively delicate fabrics.

[Jerry Adams]

I try to offset the second (and third) rows of stitches.  More so it doesn’t weaken the fabric.  It’s like you’re perforating it and it might rip there.  Or you’re spreading out the area on the fabric that takes the load.

But I often sew directly on top of the first row.  Sort of accidentally.

The only times I’ve seen rows of stitching failing is when I only did one row.  Like when the nylon fabric stretched, straight row of polyester thread which doesn’t stretch as much so it broke.  The Dyneema probably doesn’t stretch as much though.

For the seam running into the corner of a silnylon tent, the fabric stretches more, so I do zigzag stitch with the polyester thread so the seam will stretch even if the thread doesn’t.

 

[R]

Zpacks Arc Blast, DCF Hybrid fabric. It doesn’t look like a felled seam on the outside, but it does look like there are two seems at the upper left of the interior photo.  Not sure what’s going on there.  It’s entirely possible the two photos are not of the same actual pack.

The problem w/ felled seams is at lest one seam is a top stitch which exposes the threads to external abrasion.  None of my commercial packs use felled seams.  That said, none are Cuben/DCF.

I suppose if you did two seams directly on top of each other with twice the stitch length you normally would, you’d be at the expected frequency of thread holes.  The advantage would be that if a thread gets severed, the other seam would prevent the entire seam from coming apart.  The disadvantage, besides twice as much sewing, and the need to carefully align the second seam, is that the thread hole distribution won’t be perfect; some segments will have a higher hole frequency than others, resulting in potential weak spots where the fabric may be more prone to failure.

[Hoosier T]

The biggest issue with felled seams is that you really can’t do them on the vertical seams of a pack. You might be able to do it on the first one or two but as soon as you have a enclosed pack, there’s no way to sew the felled seam all the way to the bottom of the pack.

In the above photos, the second row of stitching is just an extra row of stitching sewn in the seam allowance outside of the structural row. This does add some additional strength. After the second row is sewn, they just taped the seam allowance down as part of the seam taping.

[Lester Moore]

there’s no way to sew the felled seam all the way to the bottom of the pack

Could you use flat felled stitching on the vertical seams of the pack if the bottom of the pack is finished last, as in this pack:

http://www.mountainultralight.com/2011/09/make-your-own-cuben-fiber-backpack.html

 

[Hoosier T]

I suppose it depends on how tall of a pack you’re making. You would likely have to do the top half and bottom half in separate passes which would leave you with lock stitching somewhere near the middle of the pack. The problem isn’t entirely that the bottom is usually closed up, it’s that you have all the pack material opposite your seam to try to keep out of the machine. Take a piece of paper, fold it over, staple the long edges together. Now lay the resulting tube down and try to imagine how difficult it would be to fell that with the other side of the tube in the way.

 

[Nathan Meyerson]

It is certainly possible to sew flat felled seams on vertical main-body panels of packs, top to bottom so long as there is no frame or other stiffener keeping the pack fabric from compressing, and the circumference of the pack allows enough space to compress. It is certainly a pain in the butt, as it requires a lot of fabric smooshing, but by no means impossible.

Ive done dozens of packs of varying sizes this way never had one that was Impossible. Makes one yearn for a post-mount sewing machine, but isnt in the realm of unfeasible for tenacious folks. Most production sewing outfits will ignore this method as it is tedious and adds negligible strength. It mostly stays in the realm of artesian or cottage manufacturers.

Regarding stitches on DCF. Long stitch lengths of 5mm or more will work best long term. Don’t ever sew a second row of stitches directly over the first if you can help it. a 1/8″ or 1/4″ spacing between parallel stitches is recommended. Even more so, adding a layer of 1″ cuben fiber seam tape over a flat felled will help  spread out tensile forces and keep the likelihood of seams tearing (which is an issue with non-woven materials) to a minimum.

[Nick Smolinske]

I would definitely not add a second stitch line anywhere near the first when working with DCF. In fact when working with DCF I prefer to avoid straight stitching when at all possible, instead opting for lap seams with a triple-step zig-zag. But I haven’t worked with the hybrid before and it has the advantage of the polyester face fabric to help hold stitches.

I think the best would be to sew a flat felled seam, because it will be easier to tape on the inside for reinforcement and waterproofing. Or, just sew a regular seam, fold it over one direction and sew again – if you’re taping you don’t have to worry about folding under the raw edge.

 

[Geoff Caplan]

I suspect that a felled seam may be overkill for this application.

I have an alpine pack by Lowe which I bought in the early ’70s. It did a few alpine seasons, and I’ve used it for many hundreds of day walks.

I recently took a look at the construction, and the long seams are simply a couple of lines of straight stitching in a heavy thread. No signs of wear or failure – and it’s carried some heavy gear.

OK – it’s made of a heavy Cordura, and you wouldn’t build it that way for a thru-hike – but if the seam lasts for over 40 years of regular use I would say that’s good enough. No experience of Hybrid DCF, but this assumes that it’s comparable to Cordura in the way it takes a stitch.

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