Tarptent StratoSpire Li vs Zpacks Duplex
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“Hi Dan, where did this information come from?”
The BPL review of the Notch Li says: “The Tarptent Notch Li applies a bonded-then-sewn approach to adjoining DCF seams”I agree that the sewn seams of the Duplex aren’t going to fail when the tent is used reasonably. They’re fine. My point was just that Zpacks construction methods don’t really provide a reason to choose their product over someone else’s. Their methods are good enough, but not anything special.
If you start by taping, then you’re probably using 1″ double sided tape in sheer (first diagram below. This gives a really strong seam because the force is distributed so widely. It’s much stronger than stitching but tape can slip/creep in really hot or cold conditions. So then you can add sewing to stop the slip (2nd diagram below) and you get a bomber seam but it leaks.
So then you add more tape on the outside like the 3rd diagram. Now it’s bomber and leakproof. But you also want to hide the raw edges of the DCF so they don’t fray, so then you’re left with something like the last diagram. I think this is close to what HMG is doing, who seam to have the nicest DCF seams, but I’m not certain.Whereas if you start by stitching, you get a mediocre seam strength because the stress is concentrated on the stitches and DCF has poor puncture resistance so the stitch holes grow. Adding tape on the outside seals the stitching and also improves the strength a lot if the tape overlaps the panels (like the blue line above) so it’s pretty good but not as strong as having the tape right in the seam (red tape) where there is more contact area with the bonding.
So perhaps it’s semantics since if something is good enough then why worry about it further, but still like things to be as good as they can be unless there is a cost to doing that. And there would be a weight cost to the full method in my diagrams since it’s effectively double taped which probably adds 1/2 – 1oz to the tent.
As always, I prefer durability/ruggedness over extreme light weight. A normal flat seam, rolled together with glue, provides this. With DCF, you would simply use a narrower tape, like 1/4″-3/8″ wide in three layers in the seam itself with a wider 3/4″ to 1″ single sided tape over that, just to make sure there are no leaks.
Like this:
I don’t think the additional “top” layer is really required for waterproofness, because stitching through the glue will pretty much seal the actual sewn seams. Maybe you could use a regular flat seam (no glue) with tape over it? Don’t know…anyway, covering the seam holes may be overkill. But again, durability, reliability, ruggedness are my focal points. Of course, cleaning the needle with a alcohol or acetone rag will be required periodically.
The actual mechanics of glue creep and “Strength Of the Joint” (SOJ) seem to indicate a combination joint. Glue & Stitch is a fairly standard technique. Glue creeps in hot (softens slightly) or cold (gets brittle and cracks.) As it creeps, the first stitch picks up picks up pressure (depending on your viewpoint) transferring it to the second stitch. And, yes, it will also elongate the holes somewhat. This really happens simultaneously from both sides of each panel. Between the combination of glue and the two stitches, the SOJ will actually make up about 90-99% of the fabric strength. Generally, any shredding or tears will NOT happen at the join, but rather in the panel where scratches or other abrasion weakens it. The SOJ will hold up even under a LOT of abrasion, which is what you want. It may leak a bit through the over-tape from seam abrasion, but this is about a year of use down the road. The SOJ is mostly in shear so all the glue lines/stitches act in concert, reinforcing each other. The two lines of stitches through the glue will distribute any load away from the glue, and, pick up more fibers in the DCF than a single stitch. Standard stuff while sewing. Only the glue between them is different, preventing the join from unrolling.
The overall weight of the joins in a two person tent will be about two ounces heavier. But, the overall join durability is increased about 2 to 4 times. Maybe that is overkill, but it sure feels good to know you have a rugged tent in less than sunshine and calm conditions.
Is this clear? My writing is terrible sometimes…
James,
I think there is an issue with that seam design, which is that a lot of the tape is designed in peel.
In the diagram below I’ve drawn your seam, but omitted the stitching and top piece of tape to just focus on the tape in the rolled seam itself (I’ll discuss those other parts after). What happens is that as you pull on the seam it twists (and this would be particularly pronounced here because the seam is to thick) so it would start to peel the outer layers of tape (middle diagram) and if that was all there was to the seam, these would peel apart until you are left with only the middle bit of tape that is in “sheer” so it won’t peel:
So in the diagram above the outer tape will peel apart until you are left with the stronger bit of tape in the center. That would hold, but ugly to have a seam do this.But you’ve got stitching and that top piece of tape. That top piece of tape is going to prevent the peeling on one side (the bottom as you’ve drawn it) because it takes the load, but then the bottom tape isn’t really doing anything. The only thing it could do would be subject to peel forces, so but it’s an inefficient use of tape. But more importantly, the top (or left) side will still start to peel. This will peel back to the stitching, and then all the force will be on that line of stitching and it becomes the weak point of the seam. If that single line of stitching fails then the seam peels to unravel back to the middle piece of tape. So ultimately the seam is very strong because of the tape in sheer in the middle, but this load isn’t transferred to here until several other components have failed. So it’s a lot more complex and heavier that what I’ve drawn, but I don’t think it would function as well.
“The most immaculately constructed DCF shelter I have ever seen was an MLD Grace Duo Tarp and it didn’t have a single stitch in it’s construction. I studied the ridge line seam in particular, and never could quite be certain how it was put together. From what I have read, it is generally considered as bomber as any DCF shelter out there, and MLD has claimed to have not received any reports of failure in the field.
Any one know what seam construction MLD uses, or is it a proprietary secret?”
I just received a .51 DCF Grace Duo from MLD a few weeks ago. Mine does have a stitched hemmed edge finishing the perimeter of the tarp.
The ridgeline is bonded with a simple overlapped seam without any stitching. I have looked closely and cannot discern whether they use tape or a liquid adhesive. The seam looks like this (image stolen from Dan):
The reinforcements at guyout points are bonded without any stitching.
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