Quote:
Originally Posted by California98Civic
Figure 4 in this same article shows the fastback with slightly higher Cd than the notchback, too. But if fastbacks are inferior for drag too, then why are so many low drag vehicles kinda in the fastback shape? The Tesla Model S, the Prius, and the Gen 1 Insight are the most obvious examples. Why aren't those cars notchbacks? One can optimize a fastback to bring down lift (e.g. a well-tuned undertray & diffuser), but why start with a higher lift and higher drag shape in the first place if ALL of them are supposedly so inferior? What's the advantage of a fastback? Cargo area?!
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The big issue with fastback shapes is the generation of strong trailing vortices off the inclined pillars. (As far as I am aware, CFD is still not very good at showing the effect of these.)
So for example, Rob Palin (Tesla aerodynamicist on the Model S) told me he spent a lot of wind tunnel time in changing the shape of the C pillar (A pillar, too) to reduce vortex development.
Because of the small wake that can be developed by a fastback, drag can be low -
if the trailing vortex development of the fastback shape is heavily optimized.
(I think we will see a return to fins on fastback shapes - I am convinced that, based on my measurements on my Insight, their presence can reduce trailing vortex strength.)
In addition, because there is good attached flow down fastback shapes, a rear spoiler can increase pressures on the rear hatch / boot (so decreasing drag, because there is a forward component to the force developed) - and the spoiler can be designed
so that the wake is not increased in size.
In summary, I think - as shown by the cars you mention - fastback shapes can be very low in drag, but it's not all that simple to get them right.