lightweight or underbuilt? your opinions, please

[jmtk]

I want to build a lightweight teardrop since we'll be towing with a smaller car (Subaru Impreza). I'm convinced sandwich construction is the way to go for lightweight strength. My question is how thin is too thin for the skins? Here's what I'm currently thinking of doing. All sandwich construction will be filled with rigid foam insulation of the right thickness to match the framing and all surfaces glued with polyurethane construction adhesive and screwed together.
Sidewalls: 1/8" birch inside and outside skins with 3/4" framing and insulation
Roof: 1/8" birch inside and outside skins with 1.5" spars and insulation
Floor: 3/8" birch top layer and 1/8" bottom layer with 3/4" framing and insulation
Tear dimensions will be 56"w x 96" x 48" h.

As an experiment (and to see how hard this was before committing to it ), I picked up some 1/4" luan, insulation, 1x2s and polyurethane construction adhesive to put together a half-scale sidewall (2'x4'). Here's what it looked like before the top layer was glued on:

After it was done and the glue dried, I put a couple 2x4s on the driveway, laid my prototype sidewall ends on top of them, then stood in the middle of the 4' span. It barely flexed and encouraged me to shoot for the thicknesses listed above. I wasn't planning on fiberglassing unless someone convinces me I'm making a big mistake. I intend to have a Uniflex exterior.

Our tear will mainly see use on paved roads, but living in the west, we'll be hitting some gravel washboards, too. Nothing terribly abusive, but hopefully it won't rattle apart. I'm planning on buying the plywood at the end of the week, so I need to make up my mind about this. I look forward to your comments.

Jeanette

[madjack]

Jeanette, I think you are on the right track...I plan on a futer build doing something similar. with 3/16 bead board panelling on both sides of 3/4 framing...dont forget to include support framing for doors/galley/cabinets when you make up your panel...
madjack

[asianflava]

Everything on my trailer is made the same, 1/8" plywood (birch or luan) with 3/4" framing (1x lumber), with rigid insulation (R-Tech). The roof, floor, walls, cabinetry are made from glued up panels. I used epoxy on the floor and walls Titebond on the cabinetry, and polyurethane glue on the roof.

[mikeschn]

Very impressive...

Have you considered using Extruded Polystyrene Insulation, (the pink stuff you get at home depot?)

Many builders use lightweight stick construction... check out the Lil Diner Build journal, starting on this page...
http://www.steve-frederick.com/diner3.html

Mike...

[angib]

Jeanette,

I'd call those specs perfect for a lightweight trailer - anyone wanting to build light would do well to copy them. I like your choice of decent plywood too - the thinner plywood is, the more its quality matters.

The one point I would raise is the puncture resistance of 1/8" ply. You're obviuosly not the sort of person who wants to be able to kick their trailer without causing damage, but if you're going to be driving on gravel/dirt roads, there's some risk that stones could puncture the bottom end of the 'roof'. If you wanted to prevent that, you could glue a second layer of 1/8" ply over, say, the bottom foot or two of the 'roof' as a stone shield.

I can't see the need for fiberglassing if you're using a good paint system.

Andrew

[Ira]

Jeanette, that looks like a plan to me. I would just suggest MAYBE reconsidering the floor:

Do you really need sandwich construction here? Like, would you be better served by just using a thicker piece of ply, and no framing sticks? (Weight-wise.)

But I guess if you do it as your planning, you're not going to use a wood frame, correct? That you'll just lay the sandwiched floor into the chassis?

Also, how come 56" wide?

[GregB]

Hey Jeanette,

Have you found a good source locally for birch ply? I can find lauan but I'm having a tough time locating Baltic Birch. I am planning on the same type of wall style, though I was going to do 1/8" exterior with 1/4" interior. BTW, I rcommend the pink or blue board as well, better strength and insulative value.

GB

[jmtk]

Thanks for all the reassuring comments. I'll be going with it as is. Greg - MacBeath's has everything you could possibly want with Baltic Birch up the ying yang. Ira - I definitely want insulation in the floor. We camp year round and in below freezing temps. A lot of people just glue the insulation to the bottom of the ply, but it's exposed to road abuse. I like the idea of closing it up with some plywood. 56" is because the futon mattress we'll use is 53" wide, and subtract off two inches for the two sidewalls and you have a little left over. Another reason is because we might want to stick a Yakima rack on the top with a luggage carrier (there goes the lightweight ) and we already have some unused Yakima bars that are 58" wide.

Now I just have to beat Greg to MacBeath's to buy out their 1/8" Baltic Birch

Jeanette

[Artificer]

Jeanette, that looks like a plan to me. I would just suggest MAYBE reconsidering the floor:

Do you really need sandwich construction here? Like, would you be better served by just using a thicker piece of ply, and no framing sticks? (Weight-wise.)

But I guess if you do it as your planning, you're not going to use a wood frame, correct? That you'll just lay the sandwiched floor into the chassis?

Also, how come 56" wide?

I did a couple calculations on this. Unless I'm wrong (a definite possibility) for the addition of 20 lbs of foam plus glue, the floor will be several times as stiff. This added stiffness relies on strong foam and a good glue bond. You also end up with an insulated floor.

I noticed a big difference in my floor between just 1/2" ply on 24" center to center support, and after I added 2" of foam to the bottom and fiberglassed it. (1 layer of 17oz. +mat)

If you're not going to have floor supports, other than the 3/4", I would probably up the foam thickness to 1 1/2" Go with extruded polystyrene (pink or blue) rather than expanded (white bead board). Its much stronger.

Take a look at the thread: Mike's SIP thread

[rampage]

Jeanette,
What weight range are you shooting for? The Subaru should be able to tow more than other cars in its class due to the all wheel drive. Its sounds like you have a good plan.

[critter]

hey jmtk,
sounds like your on the right track to me and heres another idea for you.
if i understand you your building a 1/2 scale kinda model more or less to see how stuff is going to work and how tuff its going to be to build.why not finish the model and make a dog house out of it,put wheels on it and put it in the back yard!that is iffan you got a dog?

[jmtk]

Thanks for the tip on using the pink foam, I didn't know it was stronger. I'll definitely switch to that. Art - I'll consider the thicker floor. That's the one place I wanted some feedback from you all. I knew of a number of folks using 1/8" for the walls (most notably AF and Rik), but hadn't come across anyone with those specs for the floor. Ira - yes, the sandwich floor becomes the wooden frame that rests on the steel frame. Rampage - the Subaru specs say less than 1000 lbs for a trailer without brakes, 2000 lbs for one with brakes. My current estimate is it'll come in about 650 lbs with all removables taken out (battery, mattress). In Utah, unladen trailers under 750 lbs don't have to be registered. Loaded with our usual assortment of gear, my estimate has it coming it right around 900 lbs. Critter - no dog, but my 5 year old would fit Good feedback, everyone. Thanks.

Jeanette

[mwolford]

Hi,
I don't know anything about how strong what combinations of materials will be, but I was amazed at how strong the walls of our Aliner are after learning what they are made from. Below is a picture of the piece that I cut out while installing a Fantastic fan. It is only 1/8" vinyl covered ply, 3/4" foam, covered with an outer aluminum skin. Very strong!
Mike

[Mitheral]

A steel exterior door is often just a couple pieces of embossed light weight sheet metal sandwiching a sprayed in foam core. The only wood is around the edges and around the lockset.

[Arne]

regarding foam, does pink come in 3/4" thick? I went with white foam with foil on both sides (tuff-R or high-R?) to get the 3/4" I wanted....

h/d only has pink in 2" thick, at least that I could find.

I never checked around and don't know which foam is the most rigid and the best to use to strengthen sandwich walls.