DIY Caternary curves ?

[joseph daluz]

Has anyone made a tarp with Cat curves?

If you allow a string to sag between two tacks on the wall, will that get you a cat curve? Would be a PIA to transfer that to the fabric though

[Dwight Shackelford]

I was just thinking about how to do that just a while ago. The idea that came to mind was to set the string up, with the curve, then right next to it, somehow get the nylon very taut behind it.

My first idea was to take some spray paint and spray over the string and tarp so the string would leave a line, but I guess that wouldn’t be a good idea.

The other idea is to take a magic marker and put dots along the tarp where the string is.

I think getting the tarp tight is going to be the problem.

I guess the other thing you could do is get some carpenters roll paper, set the up behind the string, and spray paint that. The just cut along the line and use it as a pattern.

If spray paint is too messy, do the magic marker thing. Then you’d have a pattern to reuse.

Thing is though, is the slope going to be the same for tarps that are angled out, rather than just straight up and down. By angled out, I mean when erected. By straight up and down I mean just hanging vertically when trying to measure the slope.

Dwight

[Vick Hines]

Remember, the cat curve on fabric is not the same as the classic catenary curve formed by a line, cable or chain. The fabric curve is complicated by the fact that some of the pull is on the bias of the fabric – the angle at which the pull is not parallel to the weave of the fabric. Therefore, the curve for a ridge line does not follow the catenary formula(don’t ask). Fabric distorts more than a line and needs a different catenary shape.

There are two good ways to establish a catenary curve. The easiest is to get a long baton – either a piece of wooden molding or, better yet, a bar of aluminum (something like the 3/16″ X 1″ stock available in most hardware stores). The aluminum bar is a good straight edge when laid flat and, turned up on its side, will bend uniformly when used to establish a curve. Lay out the cloth flat and straight and tape it down. Then use weights such as bricks at the ends of the ridge line to hold the bar in place. Offset the center however much your design requires using another brick to keep the baton in place. I use at least 1/2 inch offset per foot of ridge line. That is an arbitrary number that puts a 5 inch curve on a 10 foot ridge line. More will work, but reduces the tarps coverage area and headroom. Less may not pull it taut. If all you want to do is make your sewing mistakes less visible, reduce the offset to 1/4″ per foot. That works for me. It does not give a firm catenary. I use it on “square” tarps – those without catenary.
The other method is trickier, but gives an exact catenary for the fabric you are using. Lay out one panel of the fabric. Anchor each corner firmly especially one ends of what will become the ridge line. Then stretch the ridge firmly and uniformly from the each end by moving the anchors. This will pull the fabric out of shape and will produce the very wrinkles the catenary cut is designed to remove. Anchor the stretched fabric and use chalk or Magic Marker to dot along the part of the panel that is mostly not wrinkled. This is usually the between the last large wrinkle and a smaller, half-hearted wrinkle before the fabric levels out. These dots are unlikely to form a neat line, but they will serve as a general guide for laying out the baton or bar as discussed earlier. This curve will not look exactly like the other curve, and will need two center anchors. No matter, it will still pull the tarp taut. Mark and cut one panel, then use it as a guide to mark and cut the other one.

Personally, I prefer to lay out a simple curve as in the first description. It works, it is easy, and I have messed up with the second method. The only virtue of the second method is when using unfamiliar fabric that has more stretch than normal, such as PU coated ripstop nylon from an unknown source. That stuff can really sag and needs a lot of catenary. The stretch method will show you how much.

[Lance M]

You can also use an Excel spreadsheet to calculate the deflection between the straight ridgeline and the catenary curve. Easy and accurate. Plot it onto your fabric, connect the dots and cut. I have a simple spreadsheet if you would like to try it.
Here is a sample (credit to Dwight for tweaking the graph):

[Rich DeLuca]

Vick,

Your method sounds very interesting, but I am having a hard time envisioning the setup. Could you describe the first method again…maybe with pictures?

[Dwight Shackelford]

Lance,

I’d be interested in that spreadsheet for calculating the points.

zydeholic at yahoo dot com

Thanks.

Dwight

[Vick Hines]

No pics, sorry. It’s just a bent stick. Aluminum bar works best. Use bricks as anchors. Nothing complicated – a brick at each end and one in the middle.

[Mark]

Ron at Six Moon Designs offers DIY instructions for the Nightwing Tarp, which has a catenary ridgeline. See page 7 of the PDF instructions. You could probably extrapolate from those measurements curve if you needed a different length.
-Mark

[Daniel Roberts]

Lance, I’m interested in that excel file too. whiteagle al gmail.com

thanks,
daniel

[Rich DeLuca]

Lance,

Can you send me your spreadsheet? delucr attt sbcglobal dottt net

[Dwight Shackelford]

Is there a place on this site that we could archive some of these files, if the creators of the files are willing?

Dwight

[Sam Haraldson]

The NightWing instructions that Mark points out are in a .pdf format that would be perfect for taking to a local printer and having them plot off at full scale. It might be a few dollars but there’s a perfect catenary curve that could be traced. It’s been on my list of things to do for sometime.

[Roger Caffin]

> Then use weights such as bricks at the ends of the ridge line to hold the bar in place. Offset the center however much your design requires using another brick to keep the baton in place. I use at least 1/2 inch offset per foot of ridge line. That is an arbitrary number that puts a 5 inch curve on a 10 foot ridge line.

This makes a nice curve, but it is more like a hyperbola than a catenary curve. The catenary curve is based on a COSH() function.

Really it doesn’t matter whether you use a catenary or a hyperbola, as long as it is a nice smooth curve.

Excel, a straight edge, and a long ruler are the simple solution.

[Author]

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