fiberglass teardrop trailers?

[R Keller]

And if all this conversation is about getting a lighter trailer, why not just make it in plywood of the right thickness? 1/2" and 3/4" ply is for making houses, not trailers.

Andrew (ducking and creeping for cover)

Andrew: Hey! No one got on your case for that statement! Since you brought it up though, what, in your opinion, would be the right thickness for plywood?

Just as an aside, when I was researching materials/techniques for building my trailer, I came across a lot of people who are using thick Styrofoam as a base for homebuilt hovercrafts. I think most were laminating thin plywood onto the Styrofoam but some were using fiberglass (or glass fibre, if you prefer).

Styrofoam doesn't have a huge shear strength rating, but in some calcs I did for my construction, it was more than enough. Even given the relatively low shear strength of the foam, the maximum load for my sandwich panels based on a given deflection is governed far more by the bending force than the shear force. In other words, the structure will deflect more than the allowable maximum if overly loaded based on the lack of bending resistance of the skins before the shear strength of the foam comes into play. The bond between the core and the skin does have to be good to prevent localized buckling of the skin, though.

Since this was for a sandwich with 5/32" plywood on each side, it would be interesting to run some calculations using much thinner fiberglass skin.

And even if the low shear strength of the Styrofoam doesn't allow for the maximum (bending) strength for a given panel with fiberglass skin, it might be plenty strong for trailer purposes?

Rik

[campadk]

Hi Mark!

Good to see more interest here in Ottawa!

I hear one of the Dodge dealers here actually had a tear for sale... not sure where. Have you seen it?

[angib]

The ducking and covering was just a recognition that I'm not any sort of expert, just another guy with an opinion!

My logic on ply thickness is: if 1/8" ply (with some framing) is OK on curved surfaces like the roof and front, why is much thicker ply needed elsewhere? And if a 1/4" ply side with some framing held up a 'Trailer for Two', why would thicker ply be needed now? Of course a 3/4" ply side will be easier to build, since it doesn't need framing, but the payoff comes in weight. Thinner ply will need framing to keep it flat, but if you're adding insulation, you're putting in some framing already.

For boatbuilding materials, I like the ball-peen test - how hard do you want to be able to hit a structure with the ball of a ball-peen hammer before you damage it? Anything should withstand a light tap, but I can't see the need for a leisure vehicle to withstand an "arm-swinging" blow. This test is fairly relevant to teardrops since a 1/8" roof skin gets stiffness from being curved but doesn't score any higher on the ball peen test.

I stand corrected on ply-foam sandwiches - all my experience is on fiberglass-foam sandwiches (and lightweight ones at that). It's worth noting that on a racing dinghy deck, the foam core will be around 1/4" thick with skins around 1/32" while a small yacht cockpit floor will have a 1/2-5/8" core with skins well under 1/16". These thicknesses seem scary, but they do withstand the gorillas jumping on them.

When looking at ply-foam sandwiches, it's worth looking at just-ply sandwiches - that is, put a few continuous frames between the ply skins (which you probably have to do, for door openings, etc) and you've got enough shear strength without the (insulation) foam being structural. Glueing the foam in place won't do any harm, but a 1x2 every 18 inches is enough.

Andrew

[R Keller]

For boatbuilding materials, I like the ball-peen test - how hard do you want to be able to hit a structure with the ball of a ball-peen hammer before you damage it? Anything should withstand a light tap, but I can't see the need for a leisure vehicle to withstand an "arm-swinging" blow. This test is fairly relevant to teardrops since a 1/8" roof skin gets stiffness from being curved but doesn't score any higher on the ball peen test

Andrew,

Funnily enough I did this test on some test panels that I made prior to my trailer construction. Just wanted some tangible feel of how tough the panels were. I was pleasantly suprised that they took quite a blow. And the opposite side (interior) didn't get damaged at all, with the outer plywood and foam taking all of the abuse.

I stand corrected on ply-foam sandwiches - all my experience is on fiberglass-foam sandwiches (and lightweight ones at that). It's worth noting that on a racing dinghy deck, the foam core will be around 1/4" thick with skins around 1/32" while a small yacht cockpit floor will have a 1/2-5/8" core with skins well under 1/16". These thicknesses seem scary, but they do withstand the gorillas jumping on them.

Didn't mean to imply that you were mistaken. You're talking about thin marine construction where shear strength is going to be very important. Probably not as important for thicker Styrofoam panels for use in trailers. Also, while the maximum deflection was still the determining factor for my plywood-foam sandwich panels, it is true that the shear strength was less than the bending strength. So, you'd want to make sure that the design deflection load wasn't exceeded or you could end up with shear failure in the foam.

When looking at ply-foam sandwiches, it's worth looking at just-ply sandwiches - that is, put a few continuous frames between the ply skins (which you probably have to do, for door openings, etc) and you've got enough shear strength without the (insulation) foam being structural. Glueing the foam in place won't do any harm, but a 1x2 every 18 inches is enough.

Question: wouldn't this leave the structure more open to localized buckling of the skins in between the framing members? I would think that glueing the foam on the full surface would be a lot stronger? However, I am also just an interested amateur when it comes to this stuff, and probably the limited knowledge I have is of the "dangerous" variety.

Rik

[mikeschn]

Funnily enough I did this test on some test panels that I made prior to my trailer construction. Just wanted some tangible feel of how tough the panels were. I was pleasantly suprised that they took quite a blow. And the opposite side (interior) didn't get damaged at all, with the outer plywood and foam taking all of the abuse.
Rik

Hey Rik,

What did you glue the foam panels together with? Titebond II or something similar? And you covered the entire surface with glue?

Mike...

[R Keller]

System Three marine epoxy mixed with plastic minifibers. Not cheap by any means. But gap filling, strong, and flexible. There are also other adhesives (including some construction adhesives) around that won't "melt" polystyrene. I did a test panel or two with Titebond II just to see how it worked, but I used epoxy since it is gap-filling (when thickened) and can handle some flexing.

I built the panels by gluing down (Titebond II) the poplar framing to one side of the plywood and temporarily screwing it down to my work table to clamp. Then I cut the Styrofoam to fit. Next, I "wet out" the plywood surfaces, framing and Styrofoam with unthickened epoxy. Then, I applied the thickened epoxy mixture to the bottom plywood panel (the one with the framing) with a notched spreader and stuck the foam pieces in. Then I applied thickned epoxy to the foam and framing with a notched spreader and put the other piece of plywood on top. The whole sandwich was clamped using screws (where they would not be visible in the final interior), bar clamps and weights (water filled containers and cinder blocks).

It was a very labor-intensive, and expensive way to go. On the other hand, the panels are relatively lightweight, insulated, extremely strong, and should last a good long time. Also, the method met my requirements of no off-gassing toxic substances.

Rik

[angib]

Yeah, once you've used epoxy/microfibers, you're spoilt for anything else! And when you try using silica and microballon fillers as well, you can just paint on joints stronger than any cabinetmaker's with a tongue depressor! It really is the wood-worker's dream.

But I feel it's about eight levels of technology more advanced than is appropriate for a home-build trailer...

Question: wouldn't this leave the structure more open to localized buckling of the skins in between the framing members?

Yes it might, but does it matter? There is almost nowhere in a trailer that you are looking for much compressive strength, so a bit of local 'load-shedding' (nice aircraft stressers' term) will do no harm. For a floor, as long as (a) the skin will carry the load to where the frames are and (b) the 'stiffener' formed by the two skins and frame will carry the load on it, that'll do.

Andrew

[R Keller]

But I feel it's about eight levels of technology more advanced than is appropriate for a home-build trailer...

Now you tell me! And to think...when I started the project I was worried that this method woudn't be strong enough! Well, at least I can jump up and down on my roof!

Rik

[mikeschn]

I would like to be able to jump up and down on my roof. Would you make me a roof panel? 52" long please!!! Oh btw, please leave a recess for the solar panel...

Mike...