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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.
Last edited by IanIsInTheGarage; 01-24-2019 at 07:39 AM.
Reason: F! stupid webpage removed all my text.