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flat/convex
I have no good science to contribute here.
I do recall reading at least one specific treatise on the difference but it didn't stick,I didn't flag the article.
One practical element of the MOON type disc,which borrows heavily from aeronautical technology,is that at the time it was introduced in hot-rodding,it would clear the dust caps on the spindles,something a flat cover would not.
With modern cars and different wheel offsets it may not be an issue for some vehicles.
A flat cover will not work with my trailer without hole-cutting the center.
Another aspect is strength.The convex shape adds tremendous rigidity to the cover which allows thinner material for same strength,and consequently a lower polar moment of inertia,something extremely important in racing.
Ryan Aeronautical calculated that the fabric wheel covers added 7-miles to the range of the Spirit of St. Louis and helped keep Charles Lindbergh out of the English Channel.
I believe convex is better aerodynamically,the proof is out there.I think modern aerospace technology speaks to the issue.
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