Quote:
Originally Posted by MetroMPG
For what it's worth, I've never seen a low-drag vehicle with a "finned" undertray - they're just smooth. The fins are something I've only ever seen on racing vehicles (or wannabe racing vehicles). That may be significant. Something to think about.
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This is a bit elementary looking but it is nevertheless very instructive. According to them diffusers, particularly rear diffusers, and major players in the aero game:
Superhachi.com - Downforce Basics
To quote that page:
"The most effective diffusers are closed on the sides by walls that extend close to the ground, in effect forming a sealed tunnel (sometimes referred to as a "ground effect tunnel") only open at the ends. Because air cannot come in from the sides, it is forced to accelerate through the entrance of the diffuser to fill it. This acceleration decreases the pressure of the air at the inlet of the diffuser creating downforce. "