Ultralight Floor

[levsmith]

Hey all,
I posted earlier in the constuction tips and techniques section on my idea for a miniature fiberglass td and and it was recommended doing a foamie instead. I'll paste my intro here as well to save you time.

I am a motorcycle guy and just got me a big black lab. He loves going for rides in the pickup but I like going for rides on the motorcycle. So I have been thinking about and working on a design for a miniature teardrop that I can pull behind the motorcycle to take him along and also for a place to crash at night should I go on any longer trips. My only problem is that my motorcycle is only a 650cc so it doesn't have the power to pull a whole lot comfortably. I am guessing 300lbs would be about as much as i would want to pull and being the dog is 100lbs, that leaves me with the challenge of building something that weighs no more than 200lbs. Basically all i am wanting is an enclosed shell, no kitchen or anything like that.

One part I am stuck on is whether to go frameless or not. I have read a few threads on here about torsions boxes but havent seen a whole lot. Just going by shipping weight on northern tools website, the coupler, rubber torsion suspension, hubs, wheels and tires are right at 100lbs. So I have about 100 lbs to work with on the body/frame. The size I am thinking about is 36"wide, 30"tall and the flat part of the floor at 60"long.

Does anybody know of any builds like this? Any opinions? Am I absolutely nuts thinking I can build something less than 200lbs?

So here are a few questions for you.
Like it says, I am undecided on whether to go framess or not. I'm not sure I can build a torsion box strong enough and still be lighter than something like the harbor freight bolt together trailer with a plywood floor. Does anyone know of any resources to help design torsion boxes for certain weights? Normally I overbuild everything but I dont want to add anymore weight than necessary on this one.

Does anyone know how much the typical foam weighs per cubic inch or cubic ft?

Next question should be an easy one. If I understand correctly, most of you guys cover your foam with canvas. I think I would prefer the look of fiberglass. would one layer of say 6oz fiberglass be enough or would I need more than that? And what do you use to atach your foam panels together before canvasing?

I'll attach my plans below to give you guys an idea of what I am thinking. These were designed with fiberglass in mind, so of course the walls are thinner than they would be for a foamie.

Thanks for any help,
Levi

[KCStudly]

Another builder expressed an interest in foamies today. Here is a starting point and some references (links back here to foamies) that I posted in capnTelescope's build.

[atahoekid]

I built my Road Foamie with a single 6 oz layer of glass and It is plenty sturdy, but remember it will take multiple coats of epoxy to get a nice finish so you may want to consider that as you add up the weight. I looked for data concerning weight of foam panels but could not find any. I used epoxy to "glue" sheets of foam together. Some people use Gorilla Glue, others have used TiteBond II, still others have used Gripper primer from Glidden. The most important thing holding your foam together in it"s box shape is the "sock" of fabric surrounding the foam. The sketches look great!!! I want to see this get built!

[GPW]

When fiberglassing , one trick the plane guys use to save weight is to apply the fiberglass , and then squeegee all the excess resin off, leaving the wetted glass cloth with just enough resin to secure it , and no more (the resin is Heavy) ... Finish is just a matter of priming (and sanding) and painting after that ... In planes weight is EVERYTHING ...

[angib]

If you are thinking of going frameless, the design you've modelled is not ideal. You still need an A-frame (a single tongue is not suitable for a frameless design) and so you have to design the body to connect to the A-frame. Any point along the side walls is fine for the rear end of the A-frame - though you can usually extend that point back less than 1 foot to meet the front axle mount (like the Ultralight chassis below).



But then you need a second pair of hard points where the A-frame passes under the front of the body. Your design with its roll-under front means that there isn't a hard point where the loads from the A-frame can be fed upwards into the bodyshell. The two alternatives are:

1) Bring the front/roof down to floor level somewhere near vertical (let's say no more than 45 degrees from vertical);

2) Keep the shape you've got but bring just the middle front/roof down to the floor vertically - like a bit of a tongue box. This middle bit should be at least the width of the A-frame at that point.

The extreme version of (2) is to extend the middle section forwards until you don't need an A-frame at all, like in this frameless design I did:

[GPW]

Am I wrong or does that trailer have the profile of a Duck head ...??? Very Cool actually ... Foamie Duck .. I like that !!!

[levsmith]

Thanks for the awesome replies so far everyone. I am working on changing the design a little bit and will upload some updated pictures soon. I never thought of needing a "load bearing wall" on the front, but you are probably right. That's what I'm changing now.

now back to my original question, does anyone have any ideas for a floor if I go with an a-frame? I looked at some hollow core doors tonight but the skin wasnt thick enough. I could press it in with just my hand while having it vertical. I would have hated to see what a knee would have done while horizontal! I am thinking about building an insulated torsion box floor but not sure how thick everything needs to be.

[levsmith]

Does this look better angib? Anything else I missed?

[matthewh]

Not that I want to discourage your project, but just so you know, Kompact Kamp trailers in Myerstown PA makes a nice pet trailer for motorcycles.

http://www.kompactkamptrailers.com/#!pet-trailer/c1dt3

I've seen them... They're pretty cool.

[levsmith]

Not that I want to discourage your project, but just so you know, Kompact Kamp trailers in Myerstown PA makes a nice pet trailer for motorcycles.

http://www.kompactkamptrailers.com/#!pet-trailer/c1dt3

I've seen them... They're pretty cool.

Yeah I saw those when I first started looking, but didnt like the price and I wanted something just big enough for me to sleep in as well for some longer motorcycle trips. I'm not sure why but all motorcycle trailers seem very expensive. Maybe I'll find out why if I start building my own

[angib]

Does this look better angib? Anything else I missed?

Yeah, that looks plenty 'vertical' enough to be a strong support where the A-frame crosses it.

For the floor, I think you have to build it so it's 'tough' enough for what you want. A sandwich floor will probably be the lightest but if you have to thicken the top skin a lot to meet your toughness requirement, you might be able to go down to a 3/8" single skin with regular cross beams. But if you want light, you will have to accept that you can't kneel on it without the mattress being present, in which case two 1/8" skins stuck to a 1" foam core will support the sleepers' weight.

A foam core isn't essential for light weight and strength - closely spaced beams that are reasonably small will also do the same job. Something like 1"x1" on 12" centers is plenty strong enough (the beams actually don't need to be as wide as they are deep). But there are builders who seem to think it is unmanly to ever use timer smaller than 2x4 - and that would drive the weight up!

[levsmith]

Does this look better angib? Anything else I missed?

Yeah, that looks plenty 'vertical' enough to be a strong support where the A-frame crosses it.

For the floor, I think you have to build it so it's 'tough' enough for what you want. A sandwich floor will probably be the lightest but if you have to thicken the top skin a lot to meet your toughness requirement, you might be able to go down to a 3/8" single skin with regular cross beams. But if you want light, you will have to accept that you can't kneel on it without the mattress being present, in which case two 1/8" skins stuck to a 1" foam core will support the sleepers' weight.

Are you saying a 1" foam panel with 1/8 ply glued to both sides would be strong enough to support me and the dog (~250lbs) while only being connected to the a-frame at the 4 points (one above each suspension and 2 points up front)?

A foam core isn't essential for light weight and strength - closely spaced beams that are reasonably small will also do the same job. Something like 1"x1" on 12" centers is plenty strong enough (the beams actually don't need to be as wide as they are deep). But there are builders who seem to think it is unmanly to ever use timer smaller than 2x4 - and that would drive the weight up!

Would you mind explaining a little more what you are talking about here? Is this another option besides the sandwiched foam panel? I'm not sure if you are describing a torsion box here or a wooden frame. Sorry for my stupidity!

[angib]

As a (former) structural engineer, it's hard to remember that most people 'know' what holds up what, and for the most part they are completely wrong.

The strongest part of a teardrop is its side walls, not its frame. Even light plywood when made into a beam 4 feet deep has enormous strength. So all structural questions in a teardrop revolve around how is this part connected to the side walls? Another part of huge strength is the galley bulkhead - though this may be only 2 feet deep, so not as strong as the side walls. So the floor is just a structure that spans between the two side walls (and the galley bulkhead) and stops people from falling out the bottom. It is not holding anything else up, just itself. Of course all this assumes the body is built properly with decent joints between the parts.

And even in a frameless design, it is likely that the A-frame will run diagonally under the floor and it does not harm to rest the floor on the two A-frame members. So now the floor is really just spanning a fairly small triangular space between the A-frame members and the galley bulkhead - what, four feet along each side?

The foam in a sandwich structure can be thought of as a continuous beam connecting the top and bottom skins and it can just as well be replaced by a series of small beams that provide the same connection. This does make the sandwich rather one-directional whereas a foam-cored sandwich is strong in all directions. The amount of connection needed between the two skins isn't huge - I suggest 1"x1" because that gives a 1" wide gluing surface top and bottom, but something like 1/4"x1" beams more closely spaced would probably be stronger but an absolute nightmare to build.

Whenever the phrase 'torsion box' appears you should assume what follows is rubbish, as there is no such thing as a torsion box in most applications. The sills of unibody vehicles are one of the rare exceptions, but then cars/trucks are subjected to serious torsional loads and trailers are not. (Rant over!)

[levsmith]

Sorry if I came off as a know-it-all with my comment as that was not my intent. I guess I was just surprised and making sure I understood you correctly. So you are saying I could either use the sandwiched foam method or make a grid of 1x1's (or something thinner) and sandwich that, correct?

also you say the galley bulkhead is a huge part of the structure. I dont plan on having a galley, is this something to be concerned about?

thanks for your help!

[KCStudly]

Since you do not have a rear hatch, you don't have to worry about a bulkhead.

My floor has both a 2x2 perimeter frame with 2x2 and 1x2 xmbrs (all light weight select cedar) that are infilled with 1-1/2 thk blue foam, then sandwiched between 5 mm (3/16 inch) thick 5 ply Okoume marine ply.

Plenty strong enough. I wouldn't want to dance on it, but I can crawl and lay on it, and once the mattress is in, spreading the load, it will be fine. During construction, I have a piece of 1/2 inch ply laid down on top so I can walk on it without having to think about it.

It is not the lightest, but due to the size, I had seams in the skins that I wanted to back up with xmbrs, and I have attached my walls to the floor using screws, so I wanted the beef of the 2x's.

I'm a believer in the foam core for its insulation value, both thermal and acoustic.

For your much smaller footprint, and reinforced glass construction, I would think that you could use very little wood, maybe just some anti-crush blocks where you will bolt thru the trailer frame mounting features. If you are glassing the bottom I don't know why you would need a full bottom skin. Maybe a thin 1/8 inch ply on top before glass, just to minimize how much epoxy and glass is needed to get the penetration resistance up (plywood being less expensive and lighter than epoxy and glass... I'm guessing).