Newbie design questions

[TD Beej]

Hello Everyone,

I am still working on refining my design and have a laundry list of question. I am also open to comments and suggestions.


Plywood:
How thin can I go with the plywood? Home Depot has 3/16th inch luan (5.2mm) and when I gave it a little curve it seemed to be real solid. Since every surface except the belly has a curve I am thinking that it is plenty thick enough, would it be possible to go to 1/8th inch? What about the spar (?) spacing, how close do I need them.
I am still waffling about insulating and making a sandwich of 1/8th outer shell and 1 inch of urethane foam insulation, could I go even thinner with the ply in a sandwich? Where do I even find suitable plywood, other then the 3/16th luan I found at HD.

Cooler:
I am planning on building a custom cooler, use 2 or 3 inches of urethane foam and glass it together, have two sides, one for cold and the other for colder. Is and exterior dimension of 2’x2’x4’ enough or should I try to push up the size a bit (2 adults and 4 kids).

Wheel Location:
I ran the spreadsheet with guestimated numbers, do these seem reasonable (see pictures below). If my build is heavier build is heavier my tongue weight goes up fast, or if my galley is off by a lot I start having problems there too. I am thinking that moving the wheels forward 3 inches is probably safe move since it would be easier to add tongue weight then to remove it.

Battery:
I see on a lot of builds that the lighting is provided by a car battery. I am thinking of just doing LED lights run by a couple of small batteries and then a small quiet generator to run the espresso maker and coffee grinder. A solar powered vent would help but am I missing something?

Hinges:
If you look at the drawing the pivot points for the rear are right at the very edge, does anyone know of a source for such hinges strong enough or will I need to fab. them. The outside wall curves so the hinges will need to reach out a bit to provide the correct positioning.

Thanks,
Beej

[Prem]

ANSWERS in BOLD type:

Plywood:
How thin can I go with the plywood? Home Depot has 3/16th inch luan (5.2mm) and when I gave it a little curve it seemed to be real solid. Since every surface except the belly has a curve I am thinking that it is plenty thick enough, would it be possible to go to 1/8th inch? What about the spar (?) spacing, how close do I need them.
I am still waffling about insulating and making a sandwich of 1/8th outer shell and 1 inch of urethane foam insulation, could I go even thinner with the ply in a sandwich? Where do I even find suitable plywood, other then the 3/16th luan I found at HD.

Hardwood store, not HD. Most use two layers of 1/8" plywood (often luan door skins) glued/layered together for the roof and hatches. If you're familiar with fiberglass and epoxy techniques, I'd put a layer of 6+ oz. cloth or chopped glass fibers in the resin between the plywood layers to really strengthen that length of roof you've got in your plans.

Cooler:
I am planning on building a custom cooler, use 2 or 3 inches of urethane foam and glass it together, have two sides, one for cold and the other for colder. Is and exterior dimension of 2’x2’x4’ enough or should I try to push up the size a bit (2 adults and 4 kids).

Couple of blocks of ice take up a lot of space. 30" x 30" x 48" (exterior) is what I'm going to build next summer with 3/8" MDO ply, 2"foam sheet, Bondo, cheapo polyester resin and 6 oz. cloth.

Wheel Location:
I ran the spreadsheet with guestimated numbers, do these seem reasonable (see pictures below). If my build is heavier build is heavier my tongue weight goes up fast, or if my galley is off by a lot I start having problems there too. I am thinking that moving the wheels forward 3 inches is probably safe move since it would be easier to add tongue weight then to remove it.

300 pounds of tongue weight. Maybe a load equalizing hitch set-up too. But why are you going with tandem axles in the first place? One 3,500 or 5,000 pound axle and 8 bolt, 16 inch tires would suffice with a 4" channel steel chassis.

Battery:
I see on a lot of builds that the lighting is provided by a car battery. I am thinking of just doing LED lights run by a couple of small batteries and then a small quiet generator to run the espresso maker and coffee grinder. A solar powered vent would help but am I missing something?

Deep cycle RV batteries look like car starting batteries. You can charge the battery from your tow vehicle via the trailer light plug (7-wire set-up). I use a deep cycle RV battery and run all 12VDC LED lights and a boom-box via a plug-in 12VDC to 120VAC converter.

Hinges:
If you look at the drawing the pivot points for the rear are right at the very edge, does anyone know of a source for such hinges strong enough or will I need to fab. them. The outside wall curves so the hinges will need to reach out a bit to provide the correct positioning.

www.teardrops.net Ask Grant. He's got it all available.

Post photos as you go so all of us get our necessary mental stimulation.

Best wishes,

Prem

[WesGrimes]

Very interesting design. I second the question on the tandom axles. You are adding a lot of weight there and it really is not needed. Looks cool though, so maybe that is part of the equation.

I have played with the thoughts of a fiberglass cooler as well. I found studies on styrene leaching from polyester resin into water from the fiberglass. Pretty nasty stuff, so it scarred me away from the idea.

Coolers are made from polyethylene in a rotomold typically. But then again, polyethylene can leach bispenol, so I don't know what is worse.

After researching the toxicity of everything going into my build I have learned that ignorance really is bliss

[TD Beej]

ANSWERS in BOLD type:
Follow-ups in Italics:

Plywood:
How thin can I go with the plywood? Home Depot has 3/16th inch luan (5.2mm) and when I gave it a little curve it seemed to be real solid. Since every surface except the belly has a curve I am thinking that it is plenty thick enough, would it be possible to go to 1/8th inch? What about the spar (?) spacing, how close do I need them.
I am still waffling about insulating and making a sandwich of 1/8th outer shell and 1 inch of urethane foam insulation, could I go even thinner with the ply in a sandwich? Where do I even find suitable plywood, other then the 3/16th luan I found at HD.

Hardwood store, not HD. Most use two layers of 1/8" plywood (often luan door skins) glued/layered together for the roof and hatches. If you're familiar with fiberglass and epoxy techniques, I'd put a layer of 6+ oz. cloth or chopped glass fibers in the resin between the plywood layers to really strengthen that length of
roof you've got in your plans.

Good idea, and it only gets me to about ¼” which isn’t to bad. But why does the glass in the sandwich make it stronger, it seems to me that just bonding would give you all that strength.


Cooler:
I am planning on building a custom cooler, use 2 or 3 inches of urethane foam and glass it together, have two sides, one for cold and the other for colder. Is and exterior dimension of 2’x2’x4’ enough or should I try to push up the size a bit (2 adults and 4 kids).

Couple of blocks of ice take up a lot of space. 30" x 30" x 48" (exterior) is what I'm going to build next summer with 3/8" MDO ply, 2"foam sheet, Bondo, cheapo polyester resin and 6 oz. cloth.

Yes the more square shape is more efficient shape for storage, however the urethane is almost twice as efficient as an insulator, and the sheets come with radiant barrier (foil) which will also help. I am actually thinking of going wider so that the ice blocks could tuck into the ends and stay out of the way and the hatches would remain the same.


Wheel Location:
I ran the spreadsheet with guestimated numbers, do these seem reasonable (see pictures below). If my build is heavier build is heavier my tongue weight goes up fast, or if my galley is off by a lot I start having problems there too. I am thinking that moving the wheels forward 3 inches is probably safe move since it would be easier to add tongue weight then to remove it.

300 pounds of tongue weight. Maybe a load equalizing hitch set-up too. But why are you going with tandem axles in the first place? One 3,500 or 5,000 pound axle and 8 bolt, 16 inch tires would suffice with a 4" channel steel chassis.

Why the tandem axle: Better ride, skinnier tires will provide the better aerodynamic but provide a larger contact patch. I know this is one area it would be easy to go back and simplify and may end up doing it but I also have a few crazy ideas I would like to be able to go back and do so I am holding onto these dreams for now. B^)



Prem, thanks for the info on the batteries and hindges too.

Wes, I not planning on drinking ice melt but maybe it is worth checking into in terms of off-gassing as is a reasonably sealed space.

Thanks for the info!
Beej

[Prem]

Wes,

REALLY interesting build you've got there! I especially marveled at the skeleton built OFF the chassis.

I always build ON the chassis and without predetermined exactness. I'm a sculptor. So it's wonderful to see someone do the opposite!

My last build was a half build. I only had to do the interior and it was a great project for a winter even when the trailer was parked outside:

http://www.tnttt.com/viewtopic.php?t=25377&start=0

Thanks for the album photos. NICE work!

Prem

[Prem]

Beej,

"Good idea, and it only gets me to about ¼” which isn’t to bad. But why does the glass in the sandwich make it stronger, it seems to me that just bonding would give you all that strength."

The glass holds the epoxy like a mini-sponge so that you get resin penetration uniformly. (Make it extra wet with resin by pre-coating both sheet surfaces BEFORE laying on the glass and rolling the glass cloth with even more resin.) PLUS (and this is amazing), that 1/16' layer of additional laminate--the fiberglass--makes for much more strength. You could use fiberglass mat and get even more of both factors or, like I previously mentioned, put a load of chopped glass fiber in the resin. With epoxy, the more resin one uses, the stronger the joint. Epoxy is the cure for dry rotted oak frames in a wooden yacht. You just chip out the rot and build a "dam" of heavy tape or wood walls and pour in the resin with a lot of chopped glass fibers or strips of mat. It's stronger than the white oak.

Don't you want a roof that can support your weight if need be? That's how to do it in 1/4 inch thickness (with cross members in the ceiling every foot of course).

Are you planning on covering the roof with aluminum or just a painted surface? If painted, I'd glass the entire roof too. (It's cheaper and future maintenance-free to use aluminum sheet.)

As far as the "quality" of the ride...well, that means that your springs actually move. A stiff ride is not a quality ride. Your trailer gets banged around going down the road if there's no cushy spring. Hence, the tandem axle idea is going to be a rough ride given how little weight is on the suspension unless each axle is rated at 1,000 - 1,200 pounds.

What size and shape steel are you using for the chassis? What's going to be the loaded weight finished on top of the suspension?

Ask MadJack what he thinks. He's really good with this subject.



Prem

[TD Beej]

Aluminum would be nice but the seems would be a problem. figuring I will end up putting a glass layer on the outside.

For the axles I am figuring progressive rated springs so that they will be initially soft but be firm enough to take the to 100% of the load when needed.

I am still trying to figure out the frame. Originally I was thinking space frame tub to get the maximum strength and stiffness for weight. I am continuing to ponder, but it seems the best way to keep the weight off. It also depends on what I finally settle on for a suspension. I've played with the idea of making a simple ring and then rely on the structure to provide the regidity. Looking at some of the vintage plans it doesn't look unreasonable to make wood do this work.

[Prem]

Beej,

Single axle:





Prem

[oldtamiyaphile]

I am still trying to figure out the frame. Originally I was thinking space frame tub to get the maximum strength and stiffness for weight.

I've been thinking about an alloy space frame myself, it's a lot of work though. Sure could be light and strong though.

I am thinking a truss rail frame like the above posted glider trailer might be a good compromise:

http://acemobilewelding.co.uk/html/glid ... ect_2.html

They weigh around 1000lbs sheeted in steel (I think). Seems to me that this is the way to go for a light but bullet proof chassis. Look at those tiny cross members. The floors are steel sheet and a sailplane weighs around 900lbs.

[Prem]

Beej,

Ouu...good photo find! Trusses work great.

One could get by nicely with a much lighter truss structure on that sailplane trailer and not as tall either (for your app). Thicker wall square tubing on the bottom to carry the spring hangers/torsion axle and thinner wall square tube 12' above supported by same as uprights spaced every foot or two.

I built a 22' boat trailer to haul a heavy 26' wooden boat that I built once. The 4" channel steel flopped and flexed. I welded 1/2" x 1/2" x 6" square rod uprights on the top of channel steel every 2' and then a run of same on top of those the entire length. That totally killed the flex and flop. I angled the top rod down at the ends so they made about a 15 degree intersection with the bottom channel steel.

Prem

[oldtamiyaphile]

I say thick wall tubing the entire length is a waste, my plan is to keep the thin walls for the bottom rails, and just box the section where the axle will go with 10mm (alloy in my case) plate.

I'm thinking a 4" tall truss would be plenty.

[TD Beej]

The difference between a space frame is a truss frame, but is explicitly using a third dimension for nodal points. For example the sailplane trailer is a pair of trusses, however if the floor were also truss that would provide the third dimension and represent a space frame. For the sailplane trailer however the hoop roof and skin provide a stressed skin and making a third truss unneeded to maintain rigidity.

The flatbed trailers most of the teardrops are built are heavy for the strength relative to other methods but they do support the floor well are easy to find and build with so will likely remain a popular choice.

B^)

[Prem]

oldtamiyaphile,

Sure, as long as one keeps it very plumb so that the walls bolted or screwed to it aren't leaning out or in. That's the only reason I suggested 12" high truss structure. It's a bit easier to keep plumb from the welding/heat warp.

For that matter, why not weld the entire wall as a truss:

http://www.flickr.com/photos/cargoconversion/3860271544/

My RV build #5 had a 1" thin-wall square tubing framework. It was light and very strong. Only the floor joists were heavy 1" x 2" rectangular tubing.

Prem

[TD Beej]

Prem,

Actually the frame structure I've been playing with so far is coincidentally 12" high canted trusses on the sides. The floor, front, and suspension support representing the third dimension.

I wanted to ask you another question, for the curved roof/walls. I like the idea of using the 1/8th sheets laminated together w/ epoxy and fiber because I could put the curve in before installing. Do you have any idea how much extra curve I would need to put in so it would maintain the correct curve after removed from the rig?

Thanks,
Beej

[Prem]

beej,

Sounds like by using the term "space frame" you mean "exoskeleton." The problem is though, that a trailer chassis has to support those gravity and flex points where the spring hangers (or torsion bar plates) are attached.

If you look under an old AWARD camping trailer from Canada or some of the "light" and long toy-haulers, you'll see that they conserved weight and materials in the frame rails by varying the height and punching out rolled-edge holes (essentially a truss) in the frame rails from tall near the torsion bar plates to short near the tail and the tongue.

The skin and roof on an AWARD are a sandwich of vinyl-coated sheet metal making an exoskeleton. AWARDs have no structural uprights in the walls.

In the alternative, there's my favorite in low-tech from the 1930s in L.A.:



There used to be a photo on a website of one of these with all the skin removed to repair the 1" round tubing truss in the floor. There is almost no structure above the floor except for three or four hoops strategically placed where the skin changes curve. The ultralight skin and the gazillion aluminum rivets carry some of the weight/flop. Exoskeleton.



Prem