The frame separation that sometimes occurs demonstrates the strength and
stiffness of the body shell of the Airstream. If the "house" was entirely
supported by the frame, if the frame sagged the body would also sag.
Instead the body holds itself up despite the loss of support by the frame
in the rear.
You can be sure that it is not good for the shell to be left without
support from the frame. It is not intended to hold itself up - it is only
intended to contribute to the stiffness of the trailer assembly.
The trailer shell, being of aluminum, expands and contracts at a
different rate from the steel underframe. The expansion and contraction
of aluminum and alloys containing aluminum has contributed to the
disrepute of aluminum sheet roofing. It tears the nail holes in the
roofing, generating leaks.
The top of the shell is heated by the sun, as is the side exposed to the
sun, while the shaded side remains at the temperature of the ambient air.
This generates stresses on the skin which must be dispersed. The generous
curves of the Airstream help dissipate the stresses as the curves bow in
and out slightly, unlike aluminum skin trailers that have flat sides and
roofs. On those, creases in the sheets provide some relief, and the
corners and edges are usually curved, which also helps to relieve stress.
My old Spartan appears at a glance to have flat sides and roof, but when
you look closely you can see that the sides have a slight radius. The
roof has a slight crown and is curved from front to rear, which is its
prominent design feature. My Silver Streak similarly has a curve to the
sides from top to bottom. The roof has a crown and ridge. Plus the side
sheets are creased. This allows thermal stresses to be relieved.
Al