it could be aero certainly... but I'm pretty sure you want to be fully shielding the *outside* of the setup.

Berner started the covers for Lance/Fabian back in the day... then Vision had it on their Metron set. Raltech as mentioned and a couple of others as well.

If you have a SRAM Red derailleur you can download a file on Thingiverse and print one off yourself. This guy's looks like what you would actually "want" out of a cover.

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