[PeterS]

Originally Posted by sendler

I offered an answer and further reading to correct the other poster's misconception that the Ecomobile was somehow flying into cross winds because of it's shape. It is not. Motorcycle steering dynamics is fundamentally tied to this discussion. Your question is how to mess them up the least when adding a streamlined body. But you must first know about the effects of trail in sidewinds which most people do not. Which is why I keep bringing it up. I'm not saying that you have to lay on the tank of a streamliner. But I am here to say that if you want the safest performance out of an off the shelf motorcycle in bad weather, you can get it by taking all of the weight off of the bars and by keeping your torso from getting blown to the side with the wind. And I also offered an explanation as to why most long time riders have never noticed it because they are being blown off balance to the wrong side of the bike. An automotive "seat back" style seat in a streamliner will anchor the rider and get the weight off of the bars so the steering can work it's magic.
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Well I didn't really want to sit and type for hours but your hesitance to read and study other good threads has compelled me. I would link some excellent threads Aerohead has written about the rapidly diminishing returns of Kamm tails but you don't like links so let's skip it.
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To summarize what we have already discussed in the other threads:
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Motorcycles automatically lean and corner into a side wind due to the trail in the steering geometry as shown by the dramatic correction of the ecomobile video. As long as the handle bars are not burdened with a terrified rider's rigid arms.
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Keeping the CG on the roll axis high accentuates this.
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Keeping the side CoP low helps push the bottom of the bike out from under the top which accentuates this.
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Streamlining a motorcycle turns it into a vertical airfoil which generates horizontal lift when sidewinds combine with the headwind of forward motion to create an angle of attack. This increases the force beyond what a normal bike would see.
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The greatest lift of an airfoil is located just behind the max thickness. So "spoiling" with a fence along the top and back section of the tail isn't going to help anything. In fact, that is just adding extra area for the side wind to push on above the CG and will act like the winglets on the tips of modern airplanes that will actually increase the undesirable side lift in that area. Pushing the top of the bike over with the wind the wrong way. It was also noted in another thread, (no link) by someone who made a tail with a sharp edge along the top that it tended toward severe oscillations as it knifed through the air. The rounded top of the ecomobile is much better. Modern Moto GP bike have a Kammed off under belly that almost drags the ground under full compression to carry the streamlined air behind the front tire with a ground effect and to lower the side CoP to the minimum (no photo).
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Venting the max pressure differential from one side of the wing to the other across the engine bay or the riders lap is the easiest way to make the increased side lift of a streamliner a complete non issue.
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Bikes corner from leaning and camber thrust. Not from steering and slip angles. The concept of an airplane like rudder on the back of a full aero motorcycle makes an interesting thought experiment for my stance on the futility of an extended fin behind the rear wheel for the purpose of straight line tracking in side winds. A big fin in the back makes the bike into an arrow when it is going straight down the road except for the fact that arrows don't have two tires stuck to the road. The rear tire is fixed and can only roll straight ahead so even with the tail fin, your bike is not really like an arrow. The tail fin can only push the rear of the bike with the wind but it is forcing the rear tire to go sideways in order for anything to happen. Now look at the rudder sticking up high above the cg and far behind it like the low fin is. Which way would you actuate the rudder if you were flying the bike and got hit by a side wind? Would you actuate the rudder to amplify the force of the wind at the rear as you are trying to do with the tail fin? Again, this is not an airplane. The rear wheel is stuck to the ground and the most effective way to generate the lateral force needed to push back against the side wind is to LEAN the bike so it is camber thrusting and steering into the wind. You would actually use the high mounted rudder in the opposite direction that the tail fin provides by trying to rudder the top of the bike, even though it is behind the rear wheel, to ROLL INTO the wind. Think about it. Maybe counter intuitive. The rear of the bike to roll INTO the wind. I also wonder what any yawing action that did make it sideways past the tracking rear tire would do to the trail in the front geometry. If you could have the cg at the steering axis it would tend to stay neutral like the arrow. But the cg will probably be closer to midway between the wheels with a feet forward design. And the yaw force from the tail will have the stuck rear tire patch as the fulcrum. The yaw will pivot on the cg and the contact patch of the rear tire, forcing the steering axis laterally into the wind, causing the trail to counter steer and precess and lean the wrong way, with the wind.
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But I have seen that the full tail is very quiet to turbulence when following big trucks. And it is the most aero. But makes the bike easier to blow off the stand when it is parked.
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Longitudinal CoP, contrary to Foale, provides the best use of the steering trail if centered at a point which is low and directly beneath the steering head. This was demonstrated by some HPV guys (no link) by tying a string at various points along a bicycle and tugging on it while someone was riding. Top speed runs on salt flats are an extreme case where the rear tire is completely at the limit of traction and so the vehicle is essentially flying as it skips along the ground.
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In lieu of venting which still could be done even if the rider is fully enclosed, Spoiling on either side of the nose could also reduce the side lift at a slight compromise of cd if the spoilers are carefully run along the lines of flow. They will have to be much larger than a piece of rope. More like Sheepdog's design at the early part of the thread (no photo) but two of them and moved part way around to the side of the nose.
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That's enough for now. I will edit this post as time goes by when I think of any other things we already knew and forgot.

I take on board what you say and have said often but Ironside DID report and improvement with that little bit of rope .

How come ?