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This seems particularly relevant to what I'm trying to do .... I'm not after a record breaker but a bike that can be ridden in traffic and that means turbulent air .
Indifference
Extreme efficiency can lead to extreme problems. In general the cleaner a shape and the closer the airflow over it, the more sensitive it will be to disturbances in that airflow.
For FFs these problems chiefly amount to sensitivity to turbulence, where the vehicle is noticeably buffeted by turbulence, typically on motorways, at speed, in traffic. Ideally an FF should be “Indifferent” to these disturbances.
Poor indifference is caused when attached (lamminar) airflow over a surface changes direction or pressure. Sideways lift generated across a smooth nose may be balanced by sufficient tail area but the sudden cessation, or reversal of the airflow will instantly cancel the side load and this will be felt in the steering or attitude of the vehicle.
This is avoided, on cars as well as FFs, by enforcing separation of the airflow from the surface at chosen points, rather than allowing the separation line to wander about the surface generating unpredictable lift effects.
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