What is the best road useful cd?
 

Using the calculator this forum kindly provides and working through various fairing shapes and real world testing is interesting .
For instance this Royal Enfield Dreamliner from 1953 that I've posted before works out at a cd of .66 from the published speed and fuel consumption figures .

http://pic40.picturetrail.com/VOL282.../397431799.jpg

Not a bad figure for something built primarily as a weather protection shield with some consideration of drag . When we come to something like Craig Vetter's fairing though things have really improved !

http://pic40.picturetrail.com/VOL282.../401920866.jpg

At the moment it seems Mr Vetter's fairing is about as good as it is possible to get bearing in mind the problems with a perfect NACA foil ....ie side lift in traffic and disturbed air . The openings at the sides seem sufficient to spoil the side lift but still provide a good airflow .

Does anyone know the cd of the Vetter fairing ?

If the frontal area is 7 sq ft I would guess that the Cd is around 0.35 ???

.35 ?! That would be remarkable especially with the side openings, I hope you are right . Even .5 would be spectacular on a bike .

With some engineering and modelling work 0.2 may be possible.

Do you mean with side openings?

Probably not. I am working on both styles. It is a hot topic what with side wind loads and egress issues. It depends on preventing flow separation at maximum width along the shoulder to hip line.