Thanks Sendler.... that's a well worded explanation .One that I reached myself a while ago !
The wind off the nose is called "apparent wind" IIRC ,as sailors and sail designers refer to it . It seems that the opening behind the screen is effective in either spoiling the lift by essentially having a big gap in the foil or allowing pressure to equalise between the two sides ....and spoiling the lift!
There doesn't seem to be too many options to this though as a good equally balanced aero shape will be very unstable at highway speeds. One minute we will have an apparent wind off our nose and we will be compensating for it with lean, then a truck will block it and our compensation will be to much (?). It seems likely that a shell that generates lift both sides has the potential to be very unstable.
Here's one I designed earlier
It's too suit a Royal Enfield fitted with a diesel engine that I could run on biodiesel .The engine would only produce 9hp so a decent drag coefficient is essential. I'd like to achieve .5 but I'm not sure it's possible on a road going bike.
and Craig's for comparison.
BTW, I wouldn't bother with Craig's extended tail.The Kamm tail is quite effective .