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Jul 6, 2011 at 4:07 pm #1276374
I have to first say that I have found the resources offered within this forum to be very motivating, as well as giving me the confidence that the mythical world of tent making is not quite so beyond the basic capabilities of those who choose to try, as well as apply themselves to learn. With that being said, there are some aspects of design and construction that I need to defer to the minds of more experienced dabblers.
My design for the Fly portion of my tent includes some side panels which are triangular in shape. I have calculated the finished design dimensions of these side panels at 77" x 56" x 67". Now I have to calculate the actual size of the raw fabric piece that would be used to construct the given finished dimensions, allowing for a 1" additional dimension for the purpose of generating a proper felled seam along each side. I realize that I cannot just add an inch to each linear measurement of a side of a triangle, as this will not provide for a full 1" from the center point of the panel for said seam. I have not found an appropriate online calculator to tell me the correct linear additions to a triangle which I wish to expand by 1" from the center point in all three directions.
Does anyone have any directions in which to point me as to how I may arrive at such dimensions?
Nicholas
PS .. I have other questions but I will post them in a different Thread.
Jul 6, 2011 at 6:36 pm #1756614Nicholas,
I think most would draw the triangle on paper or plastic, full sized or to scale and then draw parallel lines 1” out.
If you are familiar with Pythagorean’s theorem and basic trigonometry, and had a right angle to base it on, you could figure the length of the new parallel lines mathematically.
Because you don’t have a right angle to start with, you can use the law of cosines to figure the angles you do have, and then fall back on Pythagoreans theorem and basic trig to figure what you're after.
Here’s how to calculate the first angle.
Rearrange the numbers to calculate the second angle. The third will be 180 minus the first two. It will then take a lot of steps using trigonometry to arrive at the three new corners.
A computer aided design program like Sketchup should allow you to draw parallel lines and measure between the intersections of the new lines.
Probably easiest to just draw a pattern and add parallel lines 1" out.
Hope this helps.
Jul 8, 2011 at 6:03 am #1757150As Lance says, I'd just draw the panel you want, and then measure a 1" seam allowance parallel to the outline, and draw the larger outline.
If you enter the triangle in SketchUp, you can use the Offset tool to add a 1" offset to your panel. It will also tell you the length of each side, and the area of the triangle… (right click: 'entity info')
Or you can do it analytically, starting with the Cosine rule:
a^2 = b^2 + c^2 – 2b.c. cos(A)
Lance didn't call it that, or express it formally, but that's what he used… As he says, there's a lot of tedious mucking about to construct the corners
Jul 9, 2011 at 8:08 pm #1757638Nicholas,
If you want to pursue the analytical route, I can send you a spreadsheet which does the calculations. PM me with your email address if you'd like a copy.
Here's what the spreadsheet looks like:
Jul 9, 2011 at 9:03 pm #1757655Wondering why you started with the sides of the triangle, creating a trig problem to find the base and height; rather than with the base and height, which would create a geometry problem to find the lengths of the sides.
Jul 10, 2011 at 5:57 am #1757693Mine was more of a cerebral journey beginning with the problem of reconciling the high cost of lightweight tents with my own limited usage potential. I thought that surely it would be a mind expanding exercise to see what shape I could come up with applying three general needs:
If I could not find a double walled tent system that was less than 3 lbs, less than $200, and gave me over 25 ft2 of interior space without requiring shock cord poles or tying it to trees ( not permitted at Philmont), surely I could achieve those parameters designing and constructing it myself.
I knew i wanted to go with a dual trek pole core, so I started there. I knew i wanted to use paracord for the structural reinforcements as well as support structure for the panels, so I started with those elements, which then define the sides of each panel. Thats how I arrived where I am now. I am sure that there are more efficient ways of doing this, but with my limited math and design aptitude, I am relying upon my artistic design skills to guide me :)
Nicholas
Jul 11, 2011 at 10:28 am #1758067I see Lance has done it again… (nice).
Which got me looking at the sketch I abandoned on Friday. And, looking again, the trig isn't that tedious; drawing the seam allowance around the original triangle, and drawing perpendicular lines from the lines forming each corner to each of the seam allowance outlines, it all becomes clear…
It turns out that, at each corner, the two lines forming the corner are extended by the same amount, which is SA*(1/tan(theta)+1/sin(theta)), where SA is the required seam allowance, and theta is the internal angle of the corner.

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