I just added the comments below to a thread in the "Foamie" forum. I am adding a copy here because it relates to the overall history behind the Tip Top Tier Drop...
As this will essential be two open “halves”, only attached by a piano hinge at the back and lock down clamps at the front (when traveling); will foam give enough rigidity to keep it from flexing, or do you think I’d need a stick frame (say 2x2) with XPS?
For what it’s worth, here are my two cents:
I don’t worry about the closed state, nor do I consider the key question to be rigidity of the erected configuration (top raised and latched)....
Yes, it is true that a opened pop up does not have the rigidity of the classic tear drop or even that of a simple fixed box trailer, even though the two halves are latched together. This is especially true with a telescoping top because (unlike the base), the top can not have cross corner reinforcement (it has to nest over the bottom). However, my chief concern in designing the Tip Top Tier Drop was always the stability of the top
during erection before it is latched to the bottom. Diagonal flexing was my major worry because the top had to be manually raised (by an elderly weakling with no assistance) and that meant holding it in the raised position with one hand while latching with the other. Finding the right balance between stability (rigidity), cost and weight was my aim and that drove me to experimenting with very marginal construction to see what I could achieve.
Manual lifting also put a limit on open height since I had to be able to reach a height clearing the latches. I knew this would be problem with camp sites that “rolled off” in the back, but what I under estimated was how often that would be the case. In fact, many camp site pads have a log back stop (which often ends up under the rear of the trailer) and a steep drop-off beyond it, creating quite a step down off the back of the trailer. We keep a stool handy for such situations, but sometimes that
stool is needed just to raise the top.Your diagrams show the reverse, the top is hinged at the rear, which has an obvious advantage in such a case. Also, your flip-up end and side walls should reduce lift weight, but there are set up exposure advantages with telescoping designs. In any case, I find clam shell designs well matched to manual erection constraints.