However this force acts on the side area and if the centre of pressure is behind the centre of gravity then the vehicle starts to behave like a weather vane and turns towards the high pressure side.
Very common misconception among many people that should know better. Trying to yaw the bike around a pivot point of the rear wheel with a long tail like a dart causes the front steering geometry to react with gyro precession and translation of the force through the trail to counter steer the tire patch toward the wind which leans and corners the bike with the wind any time the front tire still has traction. The only time a long tail really helps the bike stay straight would be in a high speed/ low traction situation like when running on salt. As Terry has stated above that the truncated tail was a little easier to handle in the wind than the full tail. He has tried it off and on both ways so should know.
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The problem with creating an airfoil shape with full attachment of flow is that you have created an efficient vertical wing that creates lift (side) when side winds shift the apparent vector of the oncoming air stream to give it an angle of attack. This will occur with or without a tail but will be more pronounced the more perfect the airfoil with a tail.
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Craig found out that venting the the pressure differential across his lap was a big help when he tried closing in one side side of his bike and immediately took the door back off because cross wind performance was worse. I also see that modern faired world bikes such as the CBR250R and R3 have obviously drawn on their companies design experience by incorporating a scoop like shape in the sides windshield/ dash area to vent pressure across the bike. And a second technique which is the implimentation of spoilers via the sharp protrusions at either side of the headlight to spoil the attachment and kill the lift (side) whenever there is a cross wind.
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So here are some recommendations that have come from 5 years of heated debates with some really good engineer types on Ecomodder.com for an aero motorcycle that doesn't get too unruly in cross winds.
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Obviously we would want the most efficient pure airfoil shape draped over the top view of the bike and rider with a concession to an acceptable rear overhang via Kamm truncation at the upper to fit a license plate, tapering down to a more efficient J form behind the wheel at the bottom. There is no stability advantage to a full length tail and it makes the bike difficult to park although a complete pure form all the way to the back is more efficient.
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The side view should also fit the template as posted on Ecomodder with minimal front overhang, as low to the ground as possible while retaining reasonable ground clearance with the exit sweeping up at the template angle to clear driveways.
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Height is the real problem that gives side winds leverage to lean the bike over the wrong way so a low seat height and low over all height is a target. At which point you see most designs must choose head forward or feet forward. Sit up and beg riding positions are the worst for cross winds since the wind can push on a higher area of the rider and their upper body is not anchored to the bike so they get blown over by the wind and the bike follows their lean.
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Design an effective vent across the rider's hands and in front of the battery.
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Add a curved "Y" spoiler rib starting at the stagnation point of the nose and curving up to either side of the wind screen. Effective tests have been done on other dust bin set ups with some 2 inch rope.
