[CoClimber]

I did a bit of research and some fluid dynamic work when I was building my land speed racing moped. It is my belief that you want to do as much as you can to reattach the air behind you to minimize the vacuum. The attached photo isn't the final result but you get the idea. I was not able to get any wind tunnel testing but the bike holds the record for being the fastest 50cc motorcycle to run at the Ohio Mile. It is a 1977 Puch single speed moped so I suspect the aero had to help. The rear section starts out as the width of my butt and tapers as much as possible without the air separating then is chopped flat as per the rules pertaining to how far back the aero may extend.