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Originally Posted by greenjoe
Above 65mph the rear of the bike vibrates, as a lack of a better description, seems to "shutter".
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The aerodynamic term is flutter.
Happens to the best of 'm.
(Just about all aircraft companies have had trouble with flutter. Uncured, it can shear of tails, wings, ... so you might want to take it seriously.)
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At around 75mph it seems to smooth out some what, I'm thinking because the vibration is so fast.
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Likely.
And likely to kill the structure - both tail and bike if the tail is mounted sturdily - when not sorted.
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I'm planning on narrowing & reducing the height of the tail section, any other suggestions are welcome.
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Narrowing it won't help much unless it's really far too wide - I don't expect you made it much wider than the rider ?
Reducing the height will help a lot as it'll also reduce the bending moment - less weight situated closer to the fixing point on the bike will lead to reduced vibrations.
Reducing weight will lessen the effects of the vibration, but it's two-fold as weight can also dampen vibrations.
Make it smoother in the aerodynamical sense.
Don't overdo the angles, the top deck is too steep anyways.
Cut the boattail off at the rear, rather than make the angles too steep in order to make a pointy end - you don't really need the pointy end.
The theoretical optimum only works best at 0° wind direction, but you're almost never going to go straight into the wind.
Round off top, bottom and front edges.
The bottom edge seems like a hard straight ridge.
Curving it to the inside will likely help - if not with the flutter, then with side-wind issues.
There's a big gap between the windscreen and front fairing, towards the rear fairing.
That literally sucks, and creates a relative vacuum where you sit.
This alone could introduce vibrations through the rider, onto the steering bars. Happened to me with a flyscreen on a BMW R1200R roadster.
You've probably seen windscreens and fairings with holes - they relieve pressure by letting some air into the "vacuum" - or spoilers atop windscreens.