Much more stable. Doesn't weave in crosswinds, but only slides sideways a bit. Reacts to bumps in corners better, since the windshield isn't part of the steering system.
Much, much better headlight. More even high beam and better alignment of high and low to each other.
Different wind noise (probably not quieter, though.) See the next section for detaiils, but there seems to be more noise for a given windshield height (compared to the old windshield).
Warmer. With the baffles removed, there is noticeable warmth coming through the fairing in all but the coldest conditions. Much warmer hands, although the hand protectors may go back on if they'll fit. The only wind on my hands is across my fingertips, and this is where the hand protectors have the most effect. Feet still get cold, though. Probably I'll fabricate some air-deflectors for that area. It's better if I keep the balls of my feet on the pegs, but it's less comfortable. To me, this indicates that I don't have to change the airflow very much to get warmer feet, if such a small change in position makes a big difference.
Putting a foot down at lights is harder. The reach to the ground doesn't seem very different, but I have to remove my knee from behind the fairing before I reach downward.
Mirrors are less useful. It's difficult to see around the elbows. Only the outer edge can see directly to the rear, and adjusted that way there's no vision to the side. Some convex add-ons may be called for here.
The bike feels wider, but once I got used to it it's not an issue. I have no hesitation about flicking around someone on the shoulder with this bike.
Greater range, since the RT tank holds a liter more than the Standard tank.
After installing the new RT fairing on the K75, the windshield selection process began. There are many to choose from, and fortunately I had access to a wide variety. I evaluated six windshields:
This was about 1" above my line of sight, and the turbulence from it grazed the top of my head, causing some rumbling. No wind noise. Optical clarity was OK, but not great, and there is quite a reflection from the tankbag in direct sunlight. Changing the winglets did not affect the turbulence. I always wanted to hunch down another inch or two to get rid of the turbulence.
Chin-level for me, better protection than I expected. Wonderful visibility, of course, and it is by far the best-looking. The base is the same width as the mounting area, and it narrows considerably towards the top. The turbulence from the windshield hit me at mid-helmet, so the wind noise was increased and there was some turbulence. But since the edge of the shield was so close to me, and the overall "hole in the air" was smaller, the overall turbulence was low, and the frequency of the buffetting was not as low as the others. By ducking down about two inches, I could get out of it completely, even though my line of sight was still several inches over the screen. This shield has an airfoil on top to help flip the air higher, but the narrow top of it allows crosswinds to change the airflow significantly.
Since it was a bit too low for me, I considered that raising it and adding a vent like the Parabellum had might improve it. I tried several configurations, though, with little benefit; it seems that the turbulence added by increasing the overall height was not removed by adding the vent. In the end, the best way was to raise the back of it by a half inch; this raised the airflow a little at the expense of a little more turbulence. It did not decrease the turbulence. Possibly some more experimentation on this could be productive.
At the base, this one is slightly wider than the mounting area, and it tapers narrower towards the top. At about 2" below my line of sight, I wanted to hunch down with this shield, too, to escape the turbulence. Seems to offer better protection for the height than the stock screen, since it's about 3" shorter but provides a "bubble" that's only about 1.5" lower. But the "bubble" was low enough to make long distances pretty uncomfortable. Lightweight construction was a big problem here, too, as it dimpled and cracked at the mounting points. Optical clarity was good, although it was far enough below eye level that it's not an issue.
The "Gold Wing" of them all, this shield towers a good 4" above my line of sight. It's about as wide as the windshield base, and it rakes back enough to be near my face as I mount the bike. This is the only shield which has a pressure equalization vent - this admits air to the bottom of the shield, which flows upwards and merges with the main airstream at the top. This is supposed to reduce buffetting and I think it's very effective. Although it is by far the tallest of the group, it had less turbulence than the BMW and National Cycle screens. Reaching out towards the screen while riding, I can notice a very strong updraft of air from the bottom vent. Still there was some, and although putting the faceshield down eliminated it, riding this way quickly became unbearably hot. Optical clarity was very good, but there is a noticeable reflection of the tankbag in bright sunlight.
Note: if you ride in the rain at night, you don't want to be looking through your windshield. I got caught this way with the parabellum, and the headlights shining on the road dirt made the windshield impossible to see through. This would have been a problem with the stock screen, too.
In the lowest position, it is several inches below eye level, and rises to about two inches above eye level in the highest position. Airflow is substantially improved over any of the other low shields. In the low position, the airflow hits my helmet about 2/3 of the way up, but there is so much air coming from below the shield that the "boundary layer" is very smooth, rather than turbulent like the unvented screens. This gives some wind noise, but very little turbulence. As the screen is raised to about halfway up, the wind noise disappears completely and is replaced by noticeably more turbulence. Beyond this, there is no appreciable increase in noise, although the air becomes a bit more still as the shield rises completely. Optical clarity is very good, and there are no reflections.
There was no improvement using the tall screen. Wind noise was increased, if anything, and although I had hoped that I could lift the airfow over my head using less windshield angle (and give smoother airflow) this was not the case. The effects were similar to the low screen, but there was more turbulence and a bit less visibility.
The ability to change the airflow is a real plus if you're sensitive to wind noise as I seem to be. Different conditions cause different airflows, and now I can change the position of the windshield to adapt to speed, air temperature (more turbulent when hot), rain, debris ("shields up captain!"), being behind a truck, and whatever. With the shield down I can get plenty of air in my face, and with it up I could open up my future swing-up helmet and eat a Big Mac and fries. No other shield can give that flexibility.
Larger shields punch a bigger hole in the atmosphere and therefore create more low-frequency turbulence (rumbling) behind the fairing, while eliminating the higher-frequency wind noise. No big surprise there. However, there are good and bad ways of punching that hole, and a well-designed shield can be very comfortable.
Another interesting observation is that as windshield height increased, the "bubble" increased by a smaller amount. Venting at the base was the only factor which improved this relationship; the Parabellum and the electric windshield are greatly improved by this feature.