Actually the underside looks lots steep to me... haven't looked at/thought about basjoos' car lately; his is steep too... has he reported any testing, yarn or otherwise, under there? If his works at all it's because of generous radii on the bottom edges so that air rushing under there from the sides has a better go of it. Otherwise I'd wager it's like Hucho says (what did he say exactly?
) but it was a very, very shallow angle.