Not a major aero issue really but it used to be (in my mind at least) that open wing uprights were considered very draggy...

BUT I notice high end race cars don't seem to care any more........

Anyone know if they are more draggy than closed or made with Ali sheet for example?

and if so why not wrap them, make them out of sheet or whatever?

 

I have just had a click through my photos of last year's B12Hr, reasoning that the outright players there - BMW, Merc, McLaren, Porsche, Bentley, Lambo, etc, would provide a guide. Nope. There was no consistent pattern, so I conclude if they can't agree, there can't be much in it. FWIW, the quickest car there down Conrod, the Daytona, had a truss mount.

I am not sure how valid that quickie analysis might be, my starting point was the same as yours, but now maybe I would limit myself to just considering strength, stiffness and weight ...

 

Most racing that uses car chassis, like Time Attack is a blend of racing and selling parts. You can't judge that they all have open and assume it has no aero penalty. You can assume that it's not a huge penalty or they would have a blend of open and closed resulting in all closed. You need to judge aero on a racing division that dedicates itself to 100% parts zero compromise. Only sport I know like that is F1. See below. I think all the open uprights you see are a result of two main drivers; 1. It's cheaper to produce in material. 2. It looks better.