... center of pressure offset from center of gravity? ...
Correctamundo! Give that man a kewpie doll!
There are two approaches described in Hucho.
- add rear side force
- design in high drag in crosswinds
The first is the rear fin approach, exemplified by the retractable spoiler on recent Porsche 911 variants. A sidewind hitting the front causes a yawing (spinning) force rotating the car away from the wind. That same wind, hitting the rear fin, causes an opposing force, rotating the car into the wind. Net result is a little side slip, but no change in the car's direction of travel.
The second is neat. It calls for a low drag shape in the absence of crosswinds, but one with increasing drag as the crosswind angle increases. As it works out, you don't change drag much until the crossing angle hits a threshold, above which angle the drag increases quickly, and rotating torque drops off. After looking at car designs for the last year or so, I think the crosswind stability feature in the C-Max is the windshield in relation to the A-pillar. Notice that the windshield is recessed a little lower than the A-pillar, which has fairly sharp edges. This clearly collects water off the hood/glass and onto the roof, rather than around the A-pillar and onto the side windows in the absence of crosswind, and in straight ahead wind, does little else.
As the wind direction turns increasingly off-axis, this recess starts to disturb air flow off the glass onto the side of the car, eventually causing to to separate from the surface rather than wrap around it. This separation is a transition from laminar flow to turbulent flow, the latter causing the desired drag increase as the wind's angle of attack becomes more severe.
I suspect all of us with grill blocks, air dams and other aero improvements owe a great deal to Ford's engineers, for giving us a design that we can safely alter, if we choose. At the same time, I can't see the rear window add-ons of the '14 and later models lowering drag, but I can see them reducing crossswind sensitivity!
PS Paul, you're thinking verticalheight... the instability is due to rotation about a vertical axis, in the plane of the road.
Edited by fbov, 08 July 2014 - 04:52 PM.