Jul 17, 2013 at 7:43 pm #1305521
Background: I'm always interested in improving Virga's tarps and I've been giving some thought to the corner tieouts. Mainly asking the question "How can I distribute stress across a wider area of the tarp?"
Awhile ago I toyed with the idea of attaching the tieouts flush with each edge of the tarp. I figured this would put stress along the edges which, due to their reinforced hems, would be stronger than the actual body of the tarp.
I never got around to experimenting with it and then recently I noticed that the Spinntwin's corner tieouts are done exactly this way (based on photos I've found using google).
Anyone try this method of attaching tieouts? How does it compare to the standard orientation? Can you still get a taut pitch? Does it introduce wrinkles in the body of the tarp?
Any feedback is welcome!Jul 18, 2013 at 8:40 pm #2007507
If I'm understanding the Spinntwinn tie-out design, it seems similar to tieouts that were used (and maybe still are) by the hammock maker Warbonnet, for their tarps. I started a thread a couple of years ago about a tarp project, and the discussion eventually wandered onto the topic of this kind of tie-out:
I think it's a good way to make tie-outs, but all of the edges need to be relatively deep catenary curves to avoid wrinkling.Jul 26, 2013 at 7:46 pm #2009886
Thanks for the post Colin – I read the thread you referenced.
Being that I'm a climber first, I'm well aware of how force is multiplied when the angle between "anchor points" increases (see the middle column of the chart on this Wikipedia page for a simple explanation: http://en.wikipedia.org/wiki/American_death_triangle). For this reason, while I think Warbonnet's method (and my theory posted above) for attaching tie-outs, although creative, is ultimately a bad idea: Let's say the corner of your tarp has 100 lbs of force on it. At 90 degrees, Warbonnet's method will cause each leg of the tie-out to experience 71 lbs of force, for a total of 142 lbs – quite the increase.
For this reason I created a new tie-out design where I staggered the attachment points by 45 or so degrees. This results in only a slight increase in force (using the above numbers the load of 100 lbs is increased to 108 lbs), but I'm able to spread that force over way more area on the corner of the tarp than previously (the traditional tie-out design that you see in 99% of tarps out there).
It's a little more time-consuming to build, but from now on I anticipate all Virga tarps to have the staggered tie-out design. I will be putting it to the test in a few days when I take my latest Spectre tarp to the Sierras for two weeks.
Additionally, I'll post up some photos of the new design (it should all make sense once you see it) on Virga's facebook page ASAP.
Cheers!Jul 26, 2013 at 10:44 pm #2009908
Yes, I'm familiar with that general problem. It's the reason that ropes suspending hammocks need to withstand forces much greater than a person's body weight.
I guess I don't quite see how it is a problem in this application, though. In your example, 100 lbs of pulling force on the tie-out (which is more force than most tie-outs will experience and enough to pull out most stakes) produces 71 lbs of force on the hypothetical cordage within the perimeter hem in a tarp of the Warbonnet design. Selecting and using cordage strong enough for that is trivial. Any cordage that anyone would consider using for the perimeter of a tarp would likely be much stronger than that. The ordinary braided fishing line I use for guylines has a breaking strength of 350 lbs.
In any case, I'm interested in your design. I'd love to see photos. I'm not on Facebook, though.Jul 27, 2013 at 11:40 am #2009992
It's not really a "problem" – tarps have been working just fine for years, and the numbers I used in my example, while mathematically correct, are probably a lot higher than any force one would experience in the real world.
Also, my concern is not a guy line breaking; it's a tie-out ripping off the tarp (or ripping the tarp itself). So minimizing forces (actually spreading that force over greater surface area) makes sense.
What I found with my new design is that since it is stronger, I can use less material and actually made the whole tarp lighter.
Only one advantage is spreading the force out. The other is that the tie-out attaches over a greater surface area which minimizes chances of stitching or adhesive failure.
I took photos today and will try to upload soon.Jul 27, 2013 at 11:54 am #2009994
Dan DurstonBPL Member
I'm looking forward to pictures :)Jul 27, 2013 at 3:27 pm #2010051
"Also, my concern is not a guy line breaking; it's a tie-out ripping off the tarp (or ripping the tarp itself)."
My post was a little muddled. I was talking about a line that goes within the perimeter hem, not guylines:
The corner on the left seems similar to the Spinntwinn and Warbonnet corners, and the one on the right is the kind I meant to refer to in my last post. I think they function basically the same way. The line running within the perimeter hem is the line that would be under 71 lbs of load in your example, I think. I think the load on that line is actually an advantage of that design, because it keeps the edges of the tarp taught and only pulls on the fabric panel in the warp and weft directions, not in the diagonal direction. It seems to me that it completely avoids any concentration of forces at the corners. When the tarp is pitched, the perimeter lines form a rigid frame and the fabric panels are subject to tensile loads evenly across the entire length of their edges. It makes corner reinforcements unnecessary.
This thread is about your corner reinforcement innovation, I realize. Sorry for veering off topic a bit. I'm looking forward to the photos and details.Jul 27, 2013 at 3:48 pm #2010061
Jerry AdamsBPL Member
@retiredjerryLocale: Oregon and Washington
I think there'de be a wrinkle in the fabric right in line with corner.
Why would you remove that little triangle piece of fabric?
If you have two triangular pieces of fabric that you sew together with a seam, and that seam goes into the corner, and the tie-out is sewn through that seam, then you don't need corner reinforcement.
Even without that seam, do you really need a corner reinforcement? Has anyone tried this and the fabric ripped?
Just being skeptical. If your method works then it's a good idea. I see what you're saying about having a frame around the perimeter. Hmmm…Jul 27, 2013 at 9:45 pm #2010141
More photos to be uploaded soon…
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