Conclusions/observations/philosophy
The key to building a lightweight trailer is not having too much redundant structure. In other words if there are extra parts doing what another part is already doing then the extra part may be doing nothing more than adding weight.
Let’s talk about the frame first, because it is one of the heaviest single items in the tear. What is the function of the frame?
1) The frame provides support for the tear body
2) Provides an attachment location for the suspension
3) Provides for the attachment of the tongue
We can address these one at a time:
1) Here is one of the keys IMO to making a frameless teardrop trailer. As an example, take a shoe box with the lid off and twist it – pretty flimsy. Now put the lid on and while holding the lid on twist it again (it also increases the bending strength, but the torsion is easier to see and more dramatic. It’s orders of magnitude stiffer with the lid on. What we want to do is make a trailer that is strong like the shoebox with the lid on. If we can manage to do that, then we will have a structural box which will react the forces that are applied to it. Even if you now cut a hole in the top of the shoebox it will still be pretty stiff (not unlike the side of the trailer with the door cutout.
There are many teardrops that have used techniques that I believe would lend itself to this type of structural box type construction. You can glue and screw, overlap the sides with the floor, or biscuit – all of these will work. I think the important part is to start with a good foundation or floor and provide load continuity between panels. A composite floor could be as little as 1/8 inch thick plywood bonded to 2 inched of blue/pink foam with the panel closed out by some 2x2s (sandwiched between the plywood facesheets) (alternately, a couple of plies of fiberglass is lighter yet and quite robust when combined with the rigid foam core). We would probably want to bury another 1x2 underneath the galley wall. The 1x2s will facilitate the joining of the sidewalls to the floor as well as the galley wall to the floor. By bonding the plywood to the foam core we turn the plywood into the axial load carrying member while the core reacts the transverse shear loads (don’t worry about that now – I’ll show how to figure that as we go along). The main thing is just to provide load paths so that the entire applied load has someplace to go, and that we don’t load something in a direction that it is weak.
2) Once we get to the point that the box is ‘structural’ – which means able to react loads that are applied to it, we will be able to attach the suspension directly to the body (or box in a structural sense). We would like to avoid applying concentrated localized loads, or at least we need to spread them out a bit. For me, I think that the suspension of choice for a frameless trailer will be the full torsion suspension (to be discussed in more detail later). I think it is a little more elegant and when purchased as a full axle, will probably be easier to align and whatnot. A leaf spring suspension could certainly can and has been used, but as you will see in the upcoming link it requires a fair amount of local reinforcing and I don’t think that the lateral robustness is as good as the full torsion axle. I think that it is a bit more complicated to do; but there all lots of ways of skinning a cat (all of them painful for the cat… unless it’s CRAZY CAT, but then that’s painful for the rabbit… but I digress). Bottom line is that we can locally reinforce the ‘box’ or body to be strong where the local loads are introduced. We sometimes call these types of loads bunch loads as they are similarly concentrated.
What we can do is attach a reinforcing angle to the body that is a bit bigger than the suspension attaching bracket. The concentrated load is then applied to the angle where the load can subsequently spread out over a larger area on the trailer rather than applying the load directly to the trailer in a small spot. By using a full axle, the overturning moments from the outboard wheel offset are reacted by the axle cross tube rather than the trailer body, or frame. The beauty here is that load never leaves the axle, which keeps the concentrated load out of the body. So, through the use of a local reinforcement, and using a full axle, we can reduce the localized punch loads and can attach the suspension directly to the body without a separate frame.
3) We don’t want to leave the teardrop behind so we better attach it to the tow vehicle. There are two basic configurations that I believe will work for this.
a. The first is Andrew’s original light-weight A-frame. It offers a guarantee of lateral stability, reasonable weight, and multiple attach points along the A-frame that will help distribute the tongue reactions over a larger and wider area.
http://tnttt.com/viewtopic.php?t=26891
b. The second is an alternate configuration, presented about the same time, which was a simpler single-beam tongue which offers the advantage of lighter weight, lower cost, and easier construction. If the body is properly reinforced, then I believe this one is a really good choice, though I will lay out methods for either of these to be used.
http://www.mikenchell.com/TheUltralight.pdf -- page 8 and 9 have example layouts.
c. There are a couple of ways that we can use to react the loads from the tongue into the trailer.
i. We can bury a cross member as was mentioned earlier. In the case of the forward attachment at the intersection of the floor and vertical wall, it will be closed out by a 2x2 anyway, so we can use that point. The 2x2 rectangular steel section (in this case 16 gage) can have a couple of angles welded to the sides of the tongue and through bolts could attach through the angles.
ii. We could use inserts or plugs in the composite which we could bury at initial fabrication or we could drill with a hole saw, put in a plug and scarf or taper a patch and glue it from the outside. It’s much better to use a little foresight and do it ahead of time and glue it together initially.
iii. Another method would be to run the tongue all of the way back to the full axle where it could be attached, though I prefer to attach it to the locally reinforced trailer floor.
SO, that’s the general plan. I hope to get some time to do some detailed layouts to show how the various panels and parts will be attached and connected to each other so that there are reasonable load paths and stresses despite not having a full frame.
