This is a good article but there are a couple places where he miss-spoke.
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Tony Foale Designs, article on motorcycle aerodynamics.
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From the article:
Him..."The component of the wind side force, acting at the front of the machine is passed to the road surface through the tyre via the steering axis, but the steering axis is in front of the tyre contact patch by the amount of the trail, and hence the sideways force on the bike will tend to steer the wheel with the wind. Exactly the situation that we might wish to avoid"
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Me...Bikes don't make lateral acceleration without first having to lean. He is correct in saying that the trail will take any side force that acts on the bike and translate it through the steering geometry to turn the front wheel WITH the wind. But this exactly what we DO want. The wheels are way below the CG of the roll axis and much lighter than the core of the bike/ rider, so the nose of the bike moves with the wind very little while the wheels counter steer freely out from under the bike by a great amount. Which makes the bike lean and corner into the wind.
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His 3 summations at the end are all not quite right.
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Him..."1. Smaller wheel;----- The precessional forces will be reduced in line with the reduction in weight close to the wheel/tyre circumference. This will reduce the coupling between yaw and roll movements."
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Me...He is saying a lighter wheel is more stable but I disagree. It is true that a lighter/ smaller tire and wheel will make less gyroscopic force but if you want more stability, you want more wheel. Yaw forces from a side wind acting on a vehicle with front and rear tires planted to the ground will be miniscule compared to the roll forces and the lateral forces. Unless there is a huge overhang to shift the Center of Pressure, the yaw force will be applied somewhere between the two contact points and have very little leverage. The gyroscopic interaction of yaw, roll and steering in the front wheel with trail is ideal. Side winds will act well above the contact patches and try to roll the top of the bike with the wind. The precession of the movement of the top of the wheel to the right is to steer the front of the wheel to the right. Good. Exactly the counter steering input we want in order to lean and corner to the left, into the side wind. There will now be a slight yaw of the front of the bike to the right, not so much from the side wind, but as the rear wheel tracks to get back in line. Any slight yaw of the front of the bike to the right (whether from steering or the force of the side wind acting at the aero point of max differential, which will always be forward of the CG of a fast moving bike, even with a tail), will precess to roll the top of the wheel into the wind. Very slight but in the right direction again. And the counter steering input that the trail imparts away from the wind to the right, precesses the same way as the yaw, by forcing the top of the front wheel to lean into the wind. Good again.
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A bigger wheel/ tire is more stable. We already knew this.
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He then states that less trail is better in side winds:
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Him..."2. Less trail;----- The component of the wind side force, acting at the front of the machine is passed to the road surface through the tyre via the steering axis, but the steering axis is in front of the tyre contact patch by the amount of the trail, and hence the sideways force on the bike will tend to steer the wheel with the wind. Exactly the situation that we might wish to avoid."
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Me...Automatic counter steering from increased trail is exactly the situation we might wish to implement. More trail equals more stability. At the expense of requiring heavier steering efforts.
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Him..."3. Zero offset;----- With normal steering geometry with about 1-2" of offset, the bulk of the wheel side area is forward of the steering axis, this combined with the effect of today's large tyres and discs means that a considerable steering force can be generated by the action of a side wind on the wheel. But with the zero offset geometry used on the hub-centre arrangement, this wind force is balanced about the steering axis and no turning effect is produced."
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Me...Not quite right again although much less of a factor than having too little trail. Looking at just the wheel assembly, a slight bias of the CoP toward the front will tend to generate a beneficial counter steering force.
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It is a good article and he clearly addresses the aero issues of side winds and at what point they act on a fast moving vehicle which is nowhere near the static side CoP, but (like many of my own hurried posts) some of his statements could benefit from some proof reading and constructive discussion.