Teardrops n Tiny Travel Trailers

Is there any formula or rule of thumb to decide the spacing of spars along the curve of a profile?
I have an 800mm radius to bend 4mm marine ply over....

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I admit I did not use any type of formula to determine the actual spacing of my spars. With that said, "Conch Fritter"'s profile is actually a continuous curve, although the top does look kind of flat. I wanted my spacing to be about 8" (200mm), so I took the total length of the profile and divided it by whatever number to get that spacing, which told me how many spars I needed. I got the 8" number from others that have built similar type systems. It worked out great. I also added in four 1/4" all-thread rods that tie the two sides together, so the walls/roof are in a constant compression/tension condition. This made it massively strong.

dave

Bending of the wood will increase strength along the curve, so I would say you can use larger spaces, in general, on a large curve than on a flat. Just make sure to have enough spars for fastening things (skins, roof vent, solar panels, rack or canopy attachment points) later on.

You can do a "mock-up" of the curve with small blocks, to see how it works on the bench prior to the actual build. This should let you know how it will set up when it's time for the skin.

I spaced my spars close enough to support the curve, but far enough to not make it like a tank. My spars ended up being somewhere between 8 and 12" apart. But don't forget, you need 14 1/4 opening for the roof vent.

Thanks for the pointers

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Snicker, Snicker Mike - you only need the 14+" space if you actually use a roof vent. Some of us do weirdo things and put in a floor feed duct system. And if you are going to do ducting, then make it two chambers, one for intake, one for exhaust. We use 200mm computer fans (2 in each plenum) for fresh air exchange. Plus, no problems with rain and wind.

Yeah, I know, I've always gotta be the smarta##.



dave

Mine are around 12 " except for the fan area

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In my experience, 12-inch spacing seems to be the magic mean. I've had trouble using 16-inch spacing going around curves any smaller than 24-inch radius. I've used 8-inch spacing around curves smaller than a 12-inch radius. With your radius, you could probably get away with 12 inches, but 8 inches works, too.

What's the best wood for spars?

gcardogm wrote:What's the best wood for spars?

Most people use 1x2's or 1x3's for spars. These are commonly available in whitewood (SPF), hardwoods (oak, poplar, maple, etc.) and a few select other species, like radiata pine and cedar. Some people use 2x2's or ripped 1x2's from dimensional lumber, which might be SPF, Hem/Fir, or Southern Yellow Pine.

Anyways, I'm getting ahead of myself. Most people install an interior ceiling that hides the roof spars, so looks don't matter.

- The #1 choice seams to be poplar: It's a strong, good-enough hardwood that doesn't easily split because it's soft.
- I'd say the #2 choice would be radiata pine. It's very affordable, strong, and easily machines.
- Some people use cedar. I haven't. It's extremely rot resistant, but it splits easily when kiln-dried.
- If looks DO matter, then you get into the realm of red oak, maple, hickory, alder and how to finish them, and that's beyond the scope of this thread. Suffice it to say, most any premium hardwood would suffice as a roof spar.
- Lastly, some people use dimensional lumber. I've tried this, and I generally don't like it. It's hard to find or machine a piece of dimensional lumber, regardless of the species, without a honkin' big knot that just ruins the strength of the whole spar. I have found cheap dimensional lumber, like 1x2 or 2x2 furring strips, to be a pile of headaches.

Well I went with 30cm (12in).... Now I need to get some material to bend over them! Oh and I did include a 35.5cm square provision for the vent...

Lookibg good edge what state are you in?

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Queensland.

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Cool Vic here

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Just for future reference, and for the sake of any others watching to learn, it would have been much better to have increased the second vent spar spacing so that it remained a single uninterrupted piece full span. By keeping the same spacing and cutting that spar into segments, the pieces more or less become fillers/blocks, not spanning members. Yeah, it will still stiffen the roof panel some when you attach it, but I would be very mindful to watch for sagging at this location during the glue up.

Is it too late to add another spar bracketing the fan frame? If not, I would.