I have a Memphis Fats 21" on my R. I havent ridden in many years so I dont remember exactally what to expect from the wind. I do feel I get too much head buffeting though. It almost feels more like a vibration and I cant tell exactally where the wind might be coming from but it is very tiring. Would a windshield too tall cause it or is it most likely coming from below? I had a 19" but I seemed to be looking directly at the edge of the shield and it bothered me. I figured a 21" would be good for 2 up riding better than a 17. I guess its the wind that feels like someone is slapping rapidly on my helmet. Any ideas? How quiet and smooth should I expect?
Using lowers has been well discussed here, but if I may, I have a few notes with regards to the turbulence that comes off of the top of the windscreen. I had major head buffeting issues at one point as well. As has been well pointed out here, buffetting usually comes from one of two places. Below the shield or above it.
The trick is figuring out which one you are dealing with. The air coming off the top of your windscreen is usually very turbulent compared to the air that would be hitting you if you had no windscreen at all. Some windscreens suffer from this more than others. I suppose a perfectly designed windscreen would not do this, but I don't believe there is any way to at least prevent that the air coming off of the windscreen to be faster moving
than air that has not (any aerodynamic experts, please chime in here). So if at all possible, you do not want your face or the neck opening, or vents, of a full face helmet in that "dirty" air.
Some helmets are more effected by this than others for a number of reasons. A good full face helmet ususally is a little taller on your head than most open helmets. Full helmets also often have vents protruding from the top that may be just high enought to catch the turbulent airflow coming off the top of your windscreen. Another issue with full face helmets is that the air can get up under your chin and with no easy way to flow right through the helmet it starts "rocking your world"; often in this case, opening your visor a crack will stop or drastically reduce the head shaking. Sometimes just using a different helmet can drastically effect buffeting. Of course with the prices of a lot of helmets, this can be a much more expensive solution than changing windshields
I found that a good way to tell if your buffetting is coming from the turbulence at the top of the windshield is to simply lower your head a bit by slouching and see if it goes away. As well, if you are wearing an open face windshield and you are experiencing buffetting, but do not feel it on your face, just try to sit a little taller in the saddle and then see if you feel the turbulence directly on your face after only sitting a few inches taller. If you do, then it is quite likely that some buffetting was being caused by that same turbulence hitting the top of your helmet while you were in your normal riding position.
If you are able to determine that the buffetting was being caused by turbulence off the top of the windshield, you have only 3 choices. A taller shield, a shorter shield so that the turbulence isn't hitting your head, or a different model of shield all together that might do a better job of controlling the airflow and turbulence that is occuring at your particular head height with your particular helmet.
Hope this is of some use and that I have made sense.