Tight curtains or slack curtains?
I would suggest that convex sided trailer would deflect wind. I dont anyone would argue with that?? So the question is whether a flat or concave sided trailer is going to be more/less susceptible to side winds? Assuming both sides are equally tight/slack and that the trailer is empty, (if fully loaded then the trailer will act as a solid wont it?), then the upwind side will be concave, and may be more resistant to the force of the wind, BUT, the down wind side will become convex, and so there will be less of a partial vacuum on the downwind side*. Will those two forces balance each other out?
Maybe itd be good to have a tight sheet on the windy side and a slack sheet on the downwind side? Or maybe the shape of the side is irrelevant if its formed as an effect of the wind and is not rigid?
Do more tautliners fall over than boxes?
Maybe its because theres more of them on the road? Maybe it`s because they have a greater unladen weight?
*Consider that streamlined vehicles have a long tail as well as a convex/pointy front.