Hi all i got a 87 k 100 it has a tall screen with a deflector on the top edge i get no wind noise up to 40 mph then it gets realy noisey if i move my head close to the screen it gets quieter i have noticed there are some holes under the undersid of the front fairing also there are no gaters fitted to the top of the fork tubes any ideas please how to keep it quiet

I will think you find it is just turbulence from the screen.
My bike has the standard screen with the deflector and with my height (193cm) it is hopeless. I managed to crudely but dramatically improve the situation by installing a piece of perspex to the top of the screen.
However, it put too much pressure on the screen which started to flex at speed.

The holes on the underside of the fairing originally had rubber boots installed but may have been removed to aid cooling of the tank same as mine.

In short you probably need a taller screen. As for the lip or deflector at the top, I am in two minds if it improves the situation or not but it would have been tested in a wind tunnel (we hope) before production but for what height rider??
Our friends in the US seem to be able to purchase Lexan cheaply from their local hardware store and make their own modifications accordingly.


Regards,

Phil

My 86 K100RT had the standard screen (the raked back in your face one) with the deflector. At up to 50 MPH it was fairly quiet, but above that it got progressively louder. Although noisy there was very little turbulence. I found that putting my hand at the back of the screen and pushing it forward at the top by a couple of centimeters made a huge difference. I replaced the deflector with a home made one a bit taller and it was much better. That screen got broken two years ago when I let the bike fall against a petrol pump ( brain fade, I thought it was on the main stand). The screen I have on it now is from a K75RT (95 I think) and is lower and "straighter up", and the noise and turbulence from it are terrible. I have not got around to solving this problem yet, but will probably have to get a taller screen when funds allow. I am 6ft tall, (183cms) with a "long body" so I am well over the current screen. I always wear earplugs, after a 120 mile journey withouth them left me a splitting headache and ringing in my ears for several days.

Well, this is easy to explain. In stead of big formulas, here is a small schematic representing the wind drag on the back of your windshield. You can see that if you are very close to the windshield, you are a bit in an air bubble.
The same principle apply to the wind drag over your head.
BMW was selling the K100rt with a tall windshield. The windshield had to be cut by the dealer to be adjusted to your size.
This is one of the reason why I installed an electric one. I can set it up to get the wind on my face or to be totally (or almost) protected.

Here is the wind drag against a flat surface


Here is the wind drag against a curved surface


You can notice that the trajectory of the wind drag is more elongated when the wind hits a curved surface.

By installing a deflector, you can modify the shape and direction of the wind drag.
If you have a deflector well adjusted, the turbulence may pass over your head but hit your passenger. It may as well cool down your neck.
The angle of a deflector is more important than its size. You may want to try adding some spacers under your deflector to modify the angle.

BMW stopped installing the gaiters and replaced them with small plastic air scoop. This extra air flow helps keeping the under tank a bit cooler.
The 1983 models had problems with excessive heat under the tank. The gas was boiling. The first modification was to ad a reflective heat shield under the tank.