[Richard]

Harry it is a high strength skin bonded to an overall cage structure that makes the monocoque. On a raised rail chassis the walls are only attached at the floor line and where they join the other walls and roof.

Think of it this way. Build a one foot by one foot square out of 1 inch wood framing. Can you flex it? Sure you can, it's only attached at the corners. Now screw a thin piece of plywood or aluminum to the entire perimeter of the frame. Now you can't flex it if you tried