Thanks to all for trying, but I'm sorry to say I don't believe any of you! (That sounds very horrid of me!)
Abdeckung translates as Cover, but it doesn't cover anything?
As for air ducting; I don't think so, it's not big enough to do anything of any use.
Possibly it might direct air onto the rectifier, if indeed it is on the front of the frame on models fitted with the "Abdeflap". The rec would be right in the direct airflow anyway.
Maybe it could smooth out the air before it swirled under the tank into the air box?
Splash guard? Good if there wasn't a gap all round under the bottom bearing!
I've got a theory that it might be something to do with guarding a pinch zone in the event of a frontal impact, but that's a bit off the wall, although I suspect it's something legislative rather than practical.
It seems a bit too robust for an air duct in steel when plastic would do that cheaper and lighter.
As for blued steel, that's just nasty. It's new function is to take up room in my scrap bin.
Happy to hear more theories..
