here is another interesting link with some crude testing
Stability Or Control
summary:
for aerodynamically stability, you want the weight up front and the side area in back (like a dart or an arrow). You don't want too much stability though or you can't turn
For crosswind stability in a car/trike you want the center of pressure to coincide with the center of gravity.
On a bike, they concluded experimentally that a side force (center of pressure) in front and perhaps some steering geometry changes made side forces manageable or even self correcting. (though aerodynamic instability introduces new complications at higher speeds, which there may be ways to address, like active aerodynamic stability)
Quote:
"This experiment is so easy to do that I hope you repeat it. I am tempted to leave out the results but, for the curious, here is what we found.
1.
With the string tied at the head tube, Max pulled sideways (gently at first!) and I found that it was very easy to make a slight steering correction to return the bike to roll-and-yaw equilibrium and to keep the path essentially straight. With a little practice, I was steering and rolling the bike slightly and could resist as much side force as he could pull. Sharply varying side forces (gusty winds) were tried next with the same ease of control.
2.
Next, we moved the string back to the seat post simulating a CP aft of the CG. We kept the height above ground the same. Here the control required was much more difficult. With practice, I could steer and roll the bike to counter this side force but there always were several big swerves and the heading always changed. A varying "gusty" side force was very difficult to
control -- most of the effort went into roll stability (keeping balanced) and the heading went all over the road!
3.
Finally, we moved the string back to the head tube and reversed the front forks to increase the trail. Now the side force also produced a large steering torque. This torque steered the bike "down-wind" which resulted very quickly in a roll angle "up-wind", just what is required to "lean into the wind". With a loose grip on the handlebars, the bars wiggled around as the string was jerked but the bike kept going nearly straight.
The interesting conclusion is that the "aerodynamically unstable" location of the CP forward of the CG is the easiest to control and appears preferable over an "aerodynamically stable" configuration! Control appears more important than stability for this situation. The experiment we tried did not go to very high speeds so I am not suggesting that this result is valid at higher speeds. My experience with large, frame-mounted front fairings has generally been good at speed (on long hills) in moderately gusty winds.
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