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We use acrylic, like most windshield companies, because is better suited to making windshields out of. Acyrlic and polycarbonate have a few differences.
The biggest difference is that acrylic is a brittle material while polycarbonate is ductile. What this means is that acrylic is stiffer and harder, while polycarbonate is soft. This makes polycarbonate harder to crack, however in order to keep it upright without it flopping over you either need to make it very thick, or put a flare in it which generates turbulence. This, along with a lack of understanding of aerodynamics, is why you will see flares or flips in windshields, despite them providing inferior performance. Polycarbonate is accordingly easier to manufacture.
Polycarbonate itself will turn foggy when exposed to oxygen or UV light and is very prone to scratching, so in order to protect it it is coated in a thin layer of another material. Although this optical coating is somewhat scratch resistant, if you do scratch it the shield is destroyed. Any attempt to buff out the scratch will only peel away more optical coating. Acrylic is very scratch resistant, and easy to repair if scratched.
Acylic is optically superior; it is crystal clear and will have a transparent glossy edge. Polycarbonate is mostly clear and will have a darkened edge. Can Am Spyder Accessories guarantees that your windshield will provide a calmer and more quiet ride than any other, and that you will be satisfied with your purchase. Take your shield for a ride for a couple of miles; if for any reason you are not completely satisfied with it, you are welcome to return it in like-new condition for a full refund with no restocking fee.
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.
All 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.