BikeTech

The wind screen
As a newbie I do not comprehend the excitement of race bikers when there doing well over 200 km/h (120 mph). There is not a windscreen that will protect you at these speeds, so the best way to drive at these speeds is comfortably in your car (excuse me: I mean "cage"). My BMW 650GS came with the tall BMW windscreen. Above about 80 km/h, no positive effect is to be gained from this screen. I'm 1m80 (6 ft) and when I ride the thumper, the wind hits me just under my chin. Riding 100 km/h is already uncomfortable and 140 km/h (90 mph) is quite unpleasant and only useful for some quick passing.
As a newbie I had not realized this BMW screen was that poorly constructed, so I started browsing the internet again to figure out a solution. I found that there are three screens available (in Europe) that fit the (new) F650GS: The tall BMW screen, the BMW Dakar screen, and an Ermax tall screen. Multiple stories and theories existed on the web about these screens, with lots of impressions but no hard data. In addition to the commercially availabe screens there are also some non-standard fixes. I'll describe most of them below.

How to improve your screen and what about "Jet Compression"?
Based on info from the FAQ pages of the Chain Gang, I modified my existing tall F650 GS BMW screen by adding 12 mm (1/2") plastic spacers under the top screws. I had to make the two side holes in the screen a bit oval to allow for the new position of the screws. Easily done with a low rev drill. The more upright position helped a bit, but not a significant amount.
Wind impact point moved from my chin up to my nose (I'm 1m80 - 6 ft). Big deal.

What to do next? Adding one of those little plastic winglets on top of my screen (made by Saeng) did not make too much sense to me. I played around with the idea to build the "Jet Compression" windscreen that was mentioned on the Chain Gang site. This was a story that seemed to have appeared in a German motorcycle magazine. When and where is not known and some charts that were supposed to show why the system worked were no longer present.

The document can be found here. It basically means that the author built a funnel on top of his screen. The funnel increased the density of the air and caused it to "blow away" the air that hit the screen from the front. Sounds logical, but how to build one? I have seen a somewhat similar gadget on an older BMW, but that strip was only 5 cm (2"). A funnel should be much longer to have any positive effect.

Out of curiousity I posted the basics of this story at an aerodynamics engineering site, just to see if one could design such system based on aerodynamics theory. The feedback from the experts was that this theory about a "23 degree air compression and expansion angle" was basically bullocks and that there were too many variables existing for any decent planning of such screen. Trial and effort seemed the only way forward, but if you realize that you will have to buy LEXAN (as used by BMW) or some other poly-type material at $ 150 per square meter, you'll need to make a mold for the form, and be proficient with a heatgun or use a temperature controlled oven to bend these 5 mm thick screens, you realize that this is not going to be an easy task. Expect to mess up at least two screens and a few days of work before you may have a useable prototype.

Earplug intermezzo
I admitted defeat and started to wear ear plugs. That did ten times more for the noise than any screen will ever do. However, I still would like to be able to ride about 70 mph without them. By the way, I have been using Wilson pilot earplugs for many years on board the plane (as passenger) to silence out the engine noise that causes most of the tiredness after intercontinental flights. They cost virtually nothing. Last batch I bought was at http://aircraft-spruce.com/main.html. Can't get any better and they're only about $2.

Ermax versus BMW screens
Anyway, having read multiple articles and comparisons between the BMW and Ermax screen I decided to give the Ermax a try. I ordered the "tall" transparent screen ($92 in Holland and had to be shipped from France) and received it two weeks later. Note: Ermax did not ship the long M5 screws that you need with the screen so I had to get these from my local hardware store.

Key difference: The Ermax screen is far more upright than the BMW screen and is a bit curved away from the screen at the top, which helps messing up the wind speed. Note that the Dakar screen has a much more curved screen top. Overall result is that the wind now hits me at the forehead: Altogether a 15 cm (6") improvement ($16 per inch). Still not enough to ride with an open helmet, but a little better when you're doing 70
mph. I did notice that the form of the Ermax causes buffeting. The BMW tall screen does not have this problem.

My conclusion is that the Ermax moves the "quiet" zone a bit up, but not enough if you're 1m80. I guess that Ermax caters for the French market where people are still a bit less tall than in Norther Europe. You will still need ear plugs. At speeds above 90 mph there is quite some buffeting. Everything basically starts to get blurred at that speed. Side wind from the screen hits the mirrors, which are sometimes blown out of position. If you're up to about 1m75 (5ft10) I expect the Ermax to be more effective than the tall BMW screen. If you're taller and already have the tall BMW screen or the Dakar screen, the Ermax is not worth the money. I noticed that the BMW tall screen is made from LEXAN (polycarbonate) while the Ermax looks made of thick Acrylic glass. It took only a day to get some scratches on the Ermax screen (and therefore couldn't return it to the store).

No screen better than any screen?
I installed the Ermax over the small stock screen that originally comes with the F650GS. While trying to get the right type of screws for the Ermax I drove around for a day with only the small screen, and concluded that up to speeds of about 50 mph it does'n matter at all if you have a screen or not.
Altogether I'm still looking for a more optimal solution. Now that I have two screens I will use the BMW screen to apply the "Trevor adjustable screen" solution (see below).

Some screen theory
To get comparison numbers that can be used for future reference I measured the height of the screen by taking the distance from the top of the (fake) tank, a pretty stable element on my bike, to the top of the screen (vertical measurement only). Results: The tall BMW screen, unaltered, measures 32 cm (about 13") tank-to-screen top. My altered version (adding some spacers on top screws) has 38 cm (little over 15") tank-to-top.
The Ermax buffeting screen measures 45 cm (18") tank-to-top.
Using the shape of the BMW tall screen I measured the maximum height that Trevor's adjustable screen will be able to go: 44 cm tank-to-top, so a bit less than Ermax. Trevor's height is 1m76 (5ft10.5), so that's why his screen is perfect for him. Since I'm 1m80 (6 ft) I guess I need about 50 cm (20") to be able to ride with less wind problems.

My general conclusion from these measurements is that a very basic principle applies: As long as the top of the screen is at about eye level you will have significant less wind impact. It apparently does not matter very much how the screen is shaped and whether it has the extra top spoiler or not. The easiest way to see if your screen modification is successful (besides trying out) is to tape some loom to the top of your screen and see what happens with these tiny strings at different speeds. Raising the screen above your eye level is also an option, but in that case you need to raise it substantially higher (at least 10 cm / 4") above eye level to avoid visibility problems. This last solution will probably look a bit silly on an all-road bike.

Go to next page for screen alternatives: Rando Mega screen and Clever Trevor screen.