Half a Catenary Curve?

‘Half’ a cantenary curve will have no measurable difference than a ‘whole’ cat curve with less curvature. The catenary curves used in tarps and tents are just a tiny piece of the bottom of a very large catenary curve and are nearly identical to a circular arc (blue in picture below). As such, it doesn’t matter which piece of the circle you use, they are all the same. When I developed my catenary curve spreadsheet, I plotted a number of cat curves vs circular arcs and the differences were tiny( in the range of 1/64″). Keep in mind that the deflection in ‘half’ a curve is not half of the deflection of the ‘whole’ curve. In the picture below the first drawing shows 3″ deflection over 96″ with a cord drawn to the midpoint to accentuate what half a curve would look like. The bottom drawing shows 1.5″ deflection over 48″. Hope this helps.

Derrick, I used 1.5"-2" of deflection on mine (I use a 14' pole that I bend and tie guy line to both tips to hold it's shape, then trace along the pole to mark my lines) and I've been really happy with out it turned out.

Jordo, Thank you. I think I prefer that technique to marking with a ruler, but wonder what type of pole to use, How long is the edge? And height of your tent, out of curiosity? It looks really nice. That is exactly the curve I am after. Thank you for the information. Derrick

Lance, Thanks. That is very helpful. Of course, not bringing the deflection back to zero would shorten the edge length. As you point out, any curve has deflection at the center. Thank you Derrick

After reading about spread sheets here for many years, am so glad to have read Jordo's post and his photo that illustrates the success of his method. Catenary cuts have always been a nightmare for me, and while there were some successes, the wasted time spent on the failures was frustrating. An early attempt that mostly worked was the zip seam joining the front vestibule covers of this tent: But the curve on the back side of the inner net on this tent was difficult to tailor, and a short 6" sewn in carbon spar was needed at the floor to keep it taut: The result of Jordo's method should be no different than the result of flexing a hoop or dome tent pole into an arc; namely, a parabolic curve, or one that arcs slightly more at its center. Beyond me is the math and physics that produces the parabola, and determines which parabola is best for a tent ridge where two sloping tent walls are joined. Having seen commercial tents with a more pronounced arcs in the parabola near the top, or highest point, some have occasionally wondered here if a symmetrical parabola will work when tipped up on a steep angle. The arcs on Aarn tents appear symmetrical (scroll down): http://www.aarnpacks.com/#!pacer-tents/c204h With others, not so sure, as with this Black Diamond vestibule with an arc that appears more pronounced near its bottom: http://blackdiamondequipment.com/en/tents-and-bivys/ahwahnee-tent-vestibule-BD8101370000ALL1.html#cgid=shelter&start=22 All the wonderers here seem to have come away from their threads reassured, but their reasons expressed for being so never seemed entirely convincing. But think I'll go with Jordo's m/o for now. Thanks for a simple alternative to spreadsheets.

I agree with Jordan that 1.5-2" deflection is pretty good for a mid. I just made a 9×9 foot mid with 1.5" deflection on the ridges and 2" deflection along the bottom. I haven't been able to properly pitch it (just one pitch inside out in my garage to seal the seams, without the best anchors), but it seems good so far:

I think you must avoid a big cat curve along the base when you make a mid. Even if the base is flat, there will be a curve. (elasticity of the ridge) Less steep slopes, give more stretch You can see the result In the first brown example, the cat curve everywhere is 2 inches In the second green one, only the sides and not the base has a 2 inch curve. The last (brown) one has a 1-inch curve on its ridges and non at the base.

Ivo, I am working with cuben. Would the same philosophy apply regarding the bottom edge with no cat curve? Thanks Derrick

Derrick, I never worked with cuben but for what I know about it: I think that for the ridges, 1 inch is more than enouch to avoid crinkles There is almost no stretch on the ridges The base is always under tension so there is no problem You can make a cat curve but if the gap at the base is to big, rain, snow and wind will find their way in bad weather. If you want a lower gap for ventilation, you can extend the pole I suppose.

I have a 14' pole sitting around that I bought for a tent project (similar to a TarpTent Scarp 2) that I have unofficially abandoned. You can easily use any fiberglass pole as well if you have a large dome tent sitting around. Here's what I've found to work well but it's much easier since my pole has "grommet tips". 1) Tie as knot (I use a slipknot since it's easy) to one end 2) loop the line around other end a few times and pull the line towards the middle 3) Lay the pole onto my fabric/template 4) Pull the line to increase the cat cat or let some line out to reduce it 5) Once I'm happy with it, I just make a basic hitch knot to hot the line in place and then mark all the lines at once. (or really, you can just leave the line tied in place but take it off the pole until you're ready for the next one) The PANELS on my tipi are roughly 96" tall and 64" wide (so the hypotenuse that gets the cat cat would be using 96 and 32 = 100" long) Overall size of the tipi is 72" tall and averages a little under 10' in diameter (110" and 128 ")

These instructions are great, which I will follow. I have 3 different length ridgesedges to cut (6 sided assymetrical tent- pole forward) and ideally I would not put any cat curve on the bottom edges, unless necessary. Does anyone have experience with cuben pyramids and the need for catenary curves on the bottom edges. My Ultamid 2 is cat cut there, but as Ivo indicates, a cat cut causes the edge to rise. Ideally, I want it to lay straight as I expect to use this shelter in snow. Thanks again, Derrick

Here's a recent cuben mid: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=107186 He appears to use, well, no bottom edge, actually, as he has a snow skirt. Half way down the thread is a pitched cuben mid that appears to have cat cuts on the bottom edge. Maybe you can get details from its creator.