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PeterS · 09-19-2011, 06:19 AM

Good info here .
Aerodynamics | FF Web

It is widely believed that bodied PTW's are susceptible to sidewind disturbance. This is sometimes cited as the reason for their ban in motorcycle racing. Sidewinds may disturb an FF if it has the 'wrong' characteristics but it is actually quite easy to achieve excellent cross-wind stability.

Sidewinds act on the whole vehicle, tending to roll it, on it's contact patches, out of the wind. This effect is modified by the shape of the vehicle and normal PTW steering geometry.

The shape of the vehicle has two effects. If the shape has more side area behind the CG than in front of it, it will "weathercock", turn into wind, and this is a desirable effect. The other effect is that if the side area is high, especially at the front, it will increase the tendancy of the vehicle to roll out of the wind. This is an undesirable effect.

This is the basis of the 'notched nose' shaped used in the Voyager and Cmax designs.
(Old and new | FF Web).
In the course of development it also became clear that flow separation devices damp and reduce these 'shape' related effects.

PTW steering geometry modifes the effect because the steering rotates away from a sidewind, the trail element in the geometry providing the lever. This is the same effect as normal counter-steer. The vehicle will then roll into the wind with gyro and tyre-coning effects resisting the wind-induced counter steer to provide self-balance at some resultant angle to the wind.

This effect can be modified by bodyshape - countered by a high frontal side area, or assisted by a suitable tail area.

It can also be modified by PTW geometry. Dynamic trail provides directional stability, so large trail figures will generate a higher resistance to deflection of the steering by a given sidewind - and reduce the steering angle of a given deflection. High trail figures therefor reduce the automatic counter-steer effect.

Conversly, low trail figures generate a relatively large steering rotation for a given sidewind, and the dynamic trail effect is less. Therefor short trail will produce steering that is more reactive to sidewinds, and this allows adjustment of the speed and energy of the self-balancing effect.

HCS systems readily tolerate rapid self-balancing, which minimises the detectable disturbance and is desirable.

09-19-2011, 06:19 AM	#47 (permalink)
PeterS Master EcoModder Join Date: Aug 2011 Location: East coast of Australia Posts: 393 Yella Peril - '80 Mercedes 240D sedan Thanks: 15 Thanked 41 Times in 17 Posts	Good info here . Aerodynamics \| FF Web It is widely believed that bodied PTW's are susceptible to sidewind disturbance. This is sometimes cited as the reason for their ban in motorcycle racing. Sidewinds may disturb an FF if it has the 'wrong' characteristics but it is actually quite easy to achieve excellent cross-wind stability. Sidewinds act on the whole vehicle, tending to roll it, on it's contact patches, out of the wind. This effect is modified by the shape of the vehicle and normal PTW steering geometry. The shape of the vehicle has two effects. If the shape has more side area behind the CG than in front of it, it will "weathercock", turn into wind, and this is a desirable effect. The other effect is that if the side area is high, especially at the front, it will increase the tendancy of the vehicle to roll out of the wind. This is an undesirable effect. This is the basis of the 'notched nose' shaped used in the Voyager and Cmax designs. (Old and new \| FF Web). In the course of development it also became clear that flow separation devices damp and reduce these 'shape' related effects. PTW steering geometry modifes the effect because the steering rotates away from a sidewind, the trail element in the geometry providing the lever. This is the same effect as normal counter-steer. The vehicle will then roll into the wind with gyro and tyre-coning effects resisting the wind-induced counter steer to provide self-balance at some resultant angle to the wind. This effect can be modified by bodyshape - countered by a high frontal side area, or assisted by a suitable tail area. It can also be modified by PTW geometry. Dynamic trail provides directional stability, so large trail figures will generate a higher resistance to deflection of the steering by a given sidewind - and reduce the steering angle of a given deflection. High trail figures therefor reduce the automatic counter-steer effect. Conversly, low trail figures generate a relatively large steering rotation for a given sidewind, and the dynamic trail effect is less. Therefor short trail will produce steering that is more reactive to sidewinds, and this allows adjustment of the speed and energy of the self-balancing effect. HCS systems readily tolerate rapid self-balancing, which minimises the detectable disturbance and is desirable.