Hello,
i earned this exprience by build velomobiles.
the quickest VMs had all a flat belly.
Other geometries gain the airflow under the bottom -> more drag
Diffusor-> gains the airflowspeed -> windresistence gains ³
But a few VMs are constructed with rounded belly
http://www.bhpc.org.uk/Data/Sites/1/...Miles/q/q1.jpg
Greetings,
Patrick