Ideas for an Ultralight "Monocoque"

[Shaggy]

Hi all, I am playing with ideas for very light Teardrop. Target weight is under 400 pounds, I would like to use the lightest of Timbren suspension systems "axle-less" units. http://timbren.com/products-page/400lbs/asr400s02/

Basic idea is a very minimalist trailer, that on it's own will be pretty bendy. The shell of the camper itself will be built around a plywood bed platform, with bulkheads and box outs front and rear. Foam is shaped around that, and then fibreglass wrap. The base is formed around the frame, and then bonded, and possibly bolted or riveted as well. the intention is the fibreglass and minimal frame together are sufficiently rigid/strong for towing. Typical rear galley under a hatch out back. Doors and hatch made out of light frame with fibreglass over foam. Part of my design involves temporary formers between the bulkheads to permit shaping of the foam roof. Floor plan is around an inflatable mattress about 77"x 52" with a head height of 44-48" Overall dimensions of 100" x 68" height 60"

Projected tow vehicle is a ford focus, so wanting to keep weight down, and not much wider/taller than the car. So far, estimated weight for the frame, such as it is, is coming in at 150# including suspension units, wheels, including spare. I am coming up with similar numbers or slightly less for the plywood.

I have no idea what the glass will weigh, other concerns are around the finishing of the fibreglass, which I expect to be the biggest chore in the building, and if it might be too light. This will be a very aero design, and a teardrop is close enough to an aerofoil shape that it might generate enough lift make the trailer become unmanageable.

Please hack away at my ideas, and chime in if you have relevant experience with any of this.

[GPW]

Shaggy , one day we’ll have to discuss “ the Ballast" !!! It was required on the earliest Foamie trailers for fear they would float away ....but they didn’t ...

[KennethW]

The axleless units require a ridged frame to hold them straight.
A leaf spring spreads the load.
fiberglass is a lot heaver then glue, canvas and paint.

for a super light TD I would do a 1 1/2 inch foam walls and roof and 2 inch foam floor with canvas wrapped around the outside of it all. Inside canvas with 1/4 ply on the floor.
The trailer you might be able to use thin angle iron glued along the side to mount the springs on with thin wall square tubes running from the back spring mount for the hitch.
Just some thoughts.

[KCStudly]

Two plies of 6oz FG and epoxy weigh exactly the same as one ply of 10oz duck canvas, TB2 and latex paint; I tested it measuring samples of about 1 sq ft using a gram scale accurate to .05 gram.

The ideal ratio of epoxy to glass cloth is 1:1 based on the sq yd weight of the cloth, but you do have to account for whatever soaks into the foam and any wood framing. I am seeing a consistent use of about 1.5:1 on my build, so two plies of 6oz adds about 30oz per sq yd (12oz cloth + 18oz epoxy).

On my 9ft 8in x 64 inch wide Benroy-esque profile, glassing all but under the floor I estimate it will add about 32 lbs.

For the glass this did not include high build primer or paint.

[GPW]

Nor does count what will be sanded off to get everything flat . So it comes out a little lighter ...

When we hear Monocoque , we think aircraft ... http://aeroegg.com/different-types-of-fuselage/

[Shaggy]

Two plies of 6oz FG and epoxy weigh exactly the same as one ply of 10oz duck canvas, TB2 and latex paint; I tested it measuring samples of about 1 sq ft using a gram scale accurate to .05 gram.

The ideal ratio of epoxy to glass cloth is 1:1 based on the sq yd weight of the cloth, but you do have to account for whatever soaks into the foam and any wood framing. I am seeing a consistent use of about 1.5:1 on my build, so two plies of 6oz adds about 30oz per sq yd (12oz cloth + 18oz epoxy).

On my 9ft 8in x 64 inch wide Benroy-esque profile, glassing all but under the floor I estimate it will add about 32 lbs.

For the glass this did not include high build primer or paint.

That's excellent information, thank you. From your numbers, it looks like i could well keep it under my target weight.

[Shaggy]

The axleless units require a ridged frame to hold them straight.
A leaf spring spreads the load.
fiberglass is a lot heaver then glue, canvas and paint.

for a super light TD I would do a 1 1/2 inch foam walls and roof and 2 inch foam floor with canvas wrapped around the outside of it all. Inside canvas with 1/4 ply on the floor.
The trailer you might be able to use thin angle iron glued along the side to mount the springs on with thin wall square tubes running from the back spring mount for the hitch.
Just some thoughts.

The Timbren units have an alternative mounting method. If you don't have the option of a box section on the edge, you can use a 2" square steel tube as brace between the units. I am planning on a fairly thick aluminum angle that will be bolted to the suspension units, which will then be bolted to 2 aluminum channels running the width of the trailer. Both of those are to be bonded and screwed (or bolted) to the body. Adding the 2" steel tube is I think, possibly not necessary, but more of a belt and braces attitude.

My feeling is that yes, I could go traditional style on a solid deck/trailer, but the design I am working on should be somewhat lighter, and more durable overall.

[Shaggy]

Shaggy , one day we’ll have to discuss “ the Ballast" !!! It was required on the earliest Foamie trailers for fear they would float away ....but they didn’t ...

That's reassuring, but from what I've read so far, mine will, if I'm successful, be quite a bit lighter. Also, quite aerodynamic. The general profile is closer to a wing foil as well. I don't think it will be a problem, just trying to forecast problems ahead. If it does, I can put a functional spoiler on the roof!

[GPW]

At the speeds we drive ( or are supposed to ) it shouldn’t be a problem being light ...

[Shaggy]

At the speeds we drive ( or are supposed to ) it shouldn’t be a problem being light ...

Oh yeah, like I said, probably not a problem, and I really don't think it will take off, but maybe lighten things up enough to make it a little unstable? I kinda like the idea of a spoiler, just to see people reactions.

[aggie79]

Here's a link to a composite wall, composite frame build that weighed 340# for the shell, frame, torsion axle and wheels & tires: http://www.tnttt.com/viewtopic.php?f=50&t=44293.

Less than 400# can be done but it usually involves exotic materials and/or techniques.

[GPW]

Light does not mean unstable when towing ... ( ever tow an empty trailer ? ) Seriously it’s the weight distribution and how well the axle is aligned as to how it will tow ... A foamie cabin adds a bit of drag , and weight, but usually nowhere near what the trailer is designed to handle ... And unless you make your trailer unusually tall and with a very narrow wheel base , the chances of wind blowing it over in a hurricane are still pretty good ... Normal proportioned trailers do not seem affected as much ( none have been reported other than Catherine's) .... Even the 8’ tall FoamStream seems unaffected by large trucks flying by ... remember most of the weight is down low (trailer and flooring) really helping the trailer to stay upright...
And speaking of Monocoque , did you see this ?

[GPW]

Here is the one that blew over from a very strong crosswind ... ... viewtopic.php?f=55&t=44431 Read it for yourself and see what you think caused it to blow over...

And the equation by NMM ... posting.php?mode=quote&f=55&p=1134586

[Shaggy]

Here's a link to a composite wall, composite frame build that weighed 340# for the shell, frame, torsion axle and wheels & tires: http://www.tnttt.com/viewtopic.php?f=50&t=44293.

Less than 400# can be done but it usually involves exotic materials and/or techniques.

Cool, thanks for that link, I would have expected that to come out quite a bit lighter than 340# will have to do some comparison calculations. It is a bit bigger than I'm planning though.

[Shaggy]

That's an interesting design DPW, do you have a link to the thread?

I'm really not expecting it to fly away, but it has been mentioned as a concern, so I'm keeping it in mind with some options to deal with it. At this point, I’m still blue sky dreaming, I still need to get my hands on some sample ply for the bed base and internal bracing. Then I can see if the weights I'm projecting are reasonable. I know most of what I'm thinking of has been done before, but not whether my dedication and skill levels are up to it.

At this stage, my design is forming more on what I feel, is appropriate for strength. I am usually pretty good at that, but this is a lot further out from stuff I've done in the past.