This page is dedicated to the thing that slows your scooter down the most. At higher speed, all the tuning effort you do is mostly aimed at overcoming the forces of pushing you and your scooter through air.
Left is the Vespa landspeed record scooter which set the world speed record for a flying kilometre at an average of 171.102 km/h. Scooter Magazine stated this had a 21 Hp engine (at the crank). Right, speed can be gained through a simple Cuppini Screen, this photo shows an aerodynamic shape compared to the less aerodynamic shape of the rider.
Drag Coefficient
http://en.wikipedia.org/wiki/Drag_coefficient
Drag Coefficient is a number that helps compare how easily things of various areas travel through air. This is one of the things that goes into working out how much drag, and therefore how much force required to overcome the drag, a surface has.
For example a man in an upright position has a drag coefficient of 1.0 - 1.3. Where as a Porche has a drag coefficent of 0.3. So if you were the shape of a Porsche, you would require a 3rd the power to overcome drag. It is not hard to see why making yourself more porche like with a simple windscreen on a Vespa (or reducing your frontal area by ducking down), will increase the top speed noticably.
Knowing about drag is important because it's impact is exponential. It is both the speed of the air that hits you and the quantity of air that slow you down. So going twice as fast means twice as much air hitting you at twice the speed. Even on big engine bikes, the drag is the thing that is of most concern, even more than weight as explained in this article: Blown Away Article
Vespa Drag Measurements
Here are some area measurements roughly taken from a front on photo of a Vespa PX (no screen).
Headset area = 0.0711 m^2
Frame area = 0.3173 m^2
Wheel area = 0.0482 m^2
Fork area = 0.0187 m^2
Mirror area (each) = 0.0148 m^2
Engine and Exhaust = 0.0440 m^2
Total Vespa area = 0.5299 m^2
Rider area
Body area (above headset) = 0.3 m^2
Head area (with helmet) = 0.05 m^2
Total Rider area (without screen) = 0.35 m^2
If you look at the above the first thing to note is that the rider makes up over one 3rd of the area presented to the oncoming air. The drag coefficient of a person wearing motorcycle gear, sitting upright, is at a guess = 1 (based on searching google for know things lke bicycle riders, skiers, sky divers etc). So it is easy to see the a curved windshield, which could have a drag coefficient of less than half (based on what is expected for curved surfaces on cars), would have a noticeable effect on top speed. The HP required to overcome drag would instantly drop by one 6th (i.e. instead of requiring 14 HP to do 130Km/hr, you might only require 12 HP).
