Originally Posted by BadIdea
Is the diagonal allowed to go right to left?
For SCCA current rules, yes.
From a engineering perspecitive, I understand that this diagonal is only there to keep the main hoop from "folding" sideways, and this will work equally well when the bar is slanted in either direction. You end up with a pair of triangles either way, and they pretty much work equally well resisting collapse in both orientations.
In an open top car, it is common to see guy's using a "1/2 hoop" design that reduces drag by having less bars exposed above the bodywork.
When that is done, since there is only a bar over the driver's head, the diagonal will slant from top left to lower right when viewed from behind, but with a full main hoop it usually can go either way.
FWIW, in my opinion, keeping as many bars away from the driver as possible means less chance of blunt trauma in an accident. For this reason it is my personal preference to keep the upper portion of this diagonal bar away from my head.