Windshields for motorcycles are made from either polycarbonate (Lexan) or acrylic (Plexiglas). Each type of plastic has advantages and disadvantages.
Polycarbonate is an extremely strong plastic. Polycarbonate is about as transparent as glass. Polycarbonate cuts and forms easily at both room temperature and at higher temperatures. For machining purposes, you can work with polycarbonate pretty much the same as you would aluminum. Polycarbonate has a major drawback for windshield use: polycarbonate picks up water from the air. The water eventually makes the polycarbonate cloudy. This water will form bubbles if you heat the polycarbonate to forming temperatures. So, before you can form polycarbonate, first you have to place it in a drying oven at about 200° for about 12 hours. Because of this, only companies that manufacture polycarbonate make windshields. Polycarbonate is sensitive to ammonia, so glass cleaners like Windex should not be used on polycarbonate. Polycarbonate windshields need a coating to protect them from chemicals and prevent them from absorbing water from the air. This optical coating is difficult to apply uniformly, resulting in optical distortion. It also scratches and cannot be repaired with plastic polish. By far the most popular polycarbonate for motorcycle windshields is GE Lexan Margard MR10, aka "quantum coated." GE polymers was recently bought by a Saudi Arabian firm, Sabic - see GEPlastics.com. We don't buy products from countries that fund terrorism.
Acrylic is only about 3% as impact resistant as polycarbonate. Normal acrylic shatters upon impact, and therefore is considered an unsafe material for windshields. Acrylic is very chemically resistant, and is more transparent than glass - glass absorbs about half again as much light as acrylic does. Acrylic forms easily at high temperatures, about 300°. However, machining acrylic at room temperature is difficult. It's not very easy to cut acrylic with a saw or drill holes in acrylic without shattering or weakening the material.
Polycarbonate is a DOT approved material for making windshields; normal acrylic is not. Some states require DOT approved windshields, and therefore in these states a normal acrylic windshield is actually illegal, however these laws are rarely enforced. Normal acrylic can be shattered by an impact from a rock moving at speeds as low as 15mph.
We use a special high cost acrylic called Impact Modified Acrylic. This form of acrylic is DOT approved for windshields. We use only DOT certified impact resistant plastics to make Calsci windshields. Our windshields will not shatter if hit by a rock. We test our windshields by shooting them with a .22 caliber rifle and verifying that the windshield maintains its basic integrity without shedding small pieces that could impact your face or eyes. No windshield can protect you against everything, but we do our best to make certain that our windshields protect you against the small rocks frequently thrown up by other vehicle's tires.
Even though Calsci windshields are designed so that you look over them, not through them, we use only optically correct shapes that will not distort your vision if you do look through the shield. If you look through one of our shields at a dividing line on the highway, you'll see essentially no bending of the straight line. You'll never get a headache from looking through one of our shields.
There's a very understandable desire for a very small attractive shield that will throw the air completely over your head. Can't be done. Laws of physics. Some people put little adjustable wings on their shields promising this; the wings can make a shield act 3cm-5cm taller than it is, but that's about it, and then the top of the shield has three parallel edges instead of just one in your visual field.
Stock shields are designed to look sexy on the showroom floor and sell bikes. Really, in almost all cases, the manufacturers are completely uninterested in the aerodynamic performance, they're interested in the marketing / sales performance. And their experience in wind tunnels is mostly on things like the CBR, so they're thinking punch a small hole in the air at 280kph, they're not thinking produce a calm quiet ride at 120kph.
I'm all about long distance touring comfort, riding 6 to 10 hours per day then being able to do it again tomorrow. I understand this means many think my shields look like barn doors, and I have essentially no customers under the age of about 34. On the other hand, guys over about 45 are completely uninterested in the small sexy shields: we mostly feel like we've already taken our life quota of abuse, and we certainly don't need to take more from our chosen hobby. If you're under 30, I'll talk to you in about 10-15 years. You'll feel very differently then.
Why don't we use wind tunnels? Wind tunnels are made to measure lift and drag, not noise and turbulence. You put a model on a pedestal attached to strain gauges and start up the wind. Lift is the pull upwards on the pedestal; drag is the push backwards. This is what wind tunnels have measured since they were invented by the Wright brothers. CBRs go into wind tunnels because at 180mph aerodynamic drag is everything. Those fancy looking smoke trails you see in many car ads? The wind tunnel is operating at about 1-2 mph. Any faster and the smoke pulls apart and you can't see a thing.
Nearly all of our windshields have vents. These vents are part of the aerodynamic design of the shield, to reduce turbulence and noise. They are not there to make a flow of air on the rider. When you're riding on the highway, any windshield is pushing air away from the rider. This leaves a low-pressure pocket between the windshield and the rider. Some riders feel this low-pressure area as a push on their shoulders, "back pressure." The air flowing past the windshield wants to drop into this low pressure area. If the outside air is allowed to spill into the area between the windshield and the rider, the result is turbulence, noise, and drafts. When outside air spills into the rider area, it almost always falls in a curved path, causing spinning vortices of air. These vortices are noisy and can cause the battering and hammering on your helmet reported by some riders. Our windshields and vents are designed to funnel air into the rider region to relieve this low pressure area and greatly reduce the tendency of outside air to spill in. The vents are designed so that the air coming through them is quickly dispersed, leaving almost no detectable air flow at the rider. Our goal is to produce almost completely still air on the rider with no back pressure.
Why don't we put louvers on our vents? Air sticks to any surface; immediately at the surface the air is not moving. As you move away from the surface the air speed picks up with distance. The curve of airspeed vs. distance from the surface is called a Poisson curve. As you go to higher and higher speeds the Poisson curves from adjacent surfaces on the louvers move outwards until they touch. When they touch, that's the maximum air flow speed for that gap. Typical 1/2" louvers will choke off air flow to a maximum speed around 40 mph or so; above that speed you need more and more air flow to compensate for the growing vacuum behind the windshield, but the louvers have maxed out. So the louvered vent becomes less and less effective as your speed increases to 80 mph or beyond, and the windshield becomes more noisy and has more turbulence as you pick up speed.
I get a lot of emails, "Can you make me a windshield with a reverse flip to kick the air up over my head?" Yes, I can, but I won't. Air is a spring - there are shock absorbers made with only air as the spring. When you kick a spring, it kicks back. Putting energy into the air like this is exactly the opposite of what we're all about. Windshields with reverse flips and non-fair shapes generate semi-periodic chaotic swirls of turbulent air, called Von Karman vortices, after Theodore Von Karman. These vortices, or pockets of turbulence, grow as they move away from your windshield. If you feel your head being rocked or even slammed side to side or front to back as you ride, this is Von Karman vortices at work. Some manufacturers, to my own astonishment, actually claim to produce these vortices on purpose, apparently with the idea that some turbulence is "good" and will somehow perhaps cancel out the "bad turbulence." We work very hard with the design of the shape of our windshields and the location and size of the vents to eliminate all Von Karman vortices.
Theodore Von Karman emigrated from his native Hungary to the US in 1930 to become the director of the aerodynamics laboratory at Caltech. Mark learned his aerodynamics in Von Karman labs at Caltech. Calsci windshields are designed using aerodynamic engineering principles that guarantee our shields do not generate turbulence. These are the same shapes that minimize drag and maximize fuel mileage.
The shapes of our shields are all solutions to Laplace's equation, ∇²φ = 0, which guarantees a fair shape, that is a continuous second derivative. Laplace's equation governs much of the world around us; solutions include aerodynamics, space-time near a black hole; and the electron orbitals of atoms. Notice the hydrogen orbitals below - these are some of the solutions to Laplace's equation Most of the orbitals look like spheres, doughnuts, or rain drops - the basic aerodynamic shapes.
All our shields are laid out on a computer and cut with an industrial cutting laser. Our shields are symmetric to within a thousandth of an inch (.025mm). All mounting holes are also drilled with the laser, guarantying an excellent fit to your bike. This precision is necessary to be certain your riding experience will be precisely the same as all our other customers, and precisely what we engineered for your bike.
Our windshields are designed by Mark Lawrence and Carl Porter. Carl has a Bachelor's degree and a Master's degree in engineering from Ohio State University. Mark has a Bachelor's degree in engineering from the California Institute of Technology, and is currently working on a PhD in physics at the University of Southern California. Carl and Mark don't agree very well about college football teams. Mark has a bit more than 550,000 miles of motorcycle experience. It takes about 6 weeks, eight to twelve prototypes, and typically several thousand miles to finalize a windshield design. Our windshields are not just a stock windshield made a bit wider and taller. We build and modify our prototypes until the resulting windshield is quiet, comfortable, and attractive.
For those of you who are interested in history, here's where the word "Fairing" comes from.
Early in the development of aviation, it was realized that the important thing for an airplane was to have a lot of lift and very little drag. An enormous amount of drag happens if you lose laminar flow - that is, if instead of smoothly following the surfaces on the airplane, when the air breaks away from the surface it will form spinning vortices which tumble around and wreck the airflow all over the place. This is called turbulence. The exact same problem was known from laying out the keels of ships, for water flow around a ship hull is a lot like air flow around the skin of an airplane. This problem was analyzed by mathematicians. They learned something: they could predict the points at which the air flow (or water flow) would break away from the surface and start to become turbulent.
A curve which has no breaks in it is called "Continuous" by mathematicians. A curve which has no sharp corners in it is called "smooth" by mathematicians. Smooth means the first derivative of the curve is continuous. At any given point, a curve has a radius of curvature. If there are no sudden jumps in the radius of curvature, the curve is called "Fair." A Fair curve has a continuous second derivative. It was learned that turbulent flow always starts at a point on the skin where the curve has an abrupt change in the radius of curvature, that is a point where the curve is not fair, or a point where the second derivative is discontinuous. So, you can't just stick a wing onto an airplane fuselage - the sharp corner where they meet is not even smooth, much less fair. The designers found they had to locate places like this on the aircraft skin and cover them with some smoothly curved sheet metal. These pieces of sheet metal are called "Fairings."
In the '70's, when gas mileage became important, automotive companies quickly hired some aircraft designers to help them make their cars have less drag. Shortly after that, the automotive companies started putting pressure on the computer programmers to make certain that all the curves on an automotive body were fair. Some companies became quite obsessed with this: Honda at one point announced that they had determined that surfaces which had a continuous fifth derivative were most pleasing to the eye, so they wanted their cad/cam systems to only design curves which were smooth, fair, and also had three more levels of derivative continuity. I don't think they got very far, as very few programmers can handle the mathematics of c5 continuous surfaces.
Of course, until about 1970, there basically was no such thing as computer aided design. To lay out the curves for the hulls of ships and large bombers, Boeing many years ago built a building with an unbroken wooden floor which was bigger than a football field. They would clear this building, and draw a coordinate graph on the floor. Then, the designers would tell them exact points where they wanted the hull skin or aircraft skin to be. The engineers would hammer nails into the floor at these points. They would then take very long, very thin strips of oak, soak them in water, and tie the oak strips to the nails. The oak will naturally form a shape of least energy, which happily enough is a shape which is both smooth and fair. The engineers would wait for the oak to dry, then trace the lines on the floor of the building. This then became the master drawing for the bulkheads. The thin strips of wet oak were called "splines," which is why today curves in mcdraw and autocad are called splines, although essentially none of the programmers know this either. Most of our bombers and battleships in WW II were laid out in this building, because this was what we had.
In General Relativity, Einstein assumed that the universe itself was curved, but in a smooth and fair fashion. His reasoning: anything else would have been mathematically ugly, and he didn't believe God did ugly things. Since then, several people have made alternative theories of gravity where the universe does not have to be smooth and fair. None of them have worked worth beans, however. It seems God does in fact have a sense of aesthetics. Later, it was pointed out to Einstein that his theory included the possibility of points where the universe was neither smooth nor fair. These points are called singularities, or more popularly black holes. Not all scientists believe in black holes, and Einstein was skeptical.