The NRC has a machine shop and technicians on the premises, and rear axle mounts were designed for each bike. A crane hoisted the motorcycles to the level of the wind tunnel, which has a removable wall section to permit entry. Each bike was mounted on a balance, set below a turntable, flush with the floor. The balance is an extremely sensitive measuring device linked to computers in the control room. The first step is to zero the force on the balance with the bike and rider in place. Baseline figures are established with the rider in a full racing-style crouch.
Once the fan is operating and the wind speed constant, the test speed, drag, lift and side force are measured by computer, as are the pitching moment (nose-up/nose-down rotation of the bike), rolling moment (tendency to roll to the left or right in a crosswind) and yawing moment (tendency to rotate to the left or right about a central vertical axis). These forces and moments describe the aerodynamic loads on the motorcycle and influence top speed, acceleration, crosswind handling and stability. Generally, a motorcycle with smaller rolling and yawing moments will blow around less in crosswinds, and a motorcycle with less front end lift will tend to respond better to steering inputs at high speed and may be more stable. Motorcycles are more affected by aerodynamic forces than most road vehicles because of their lower densities. Aerodynamic drag, of course, is the focus of our attention here, and it has a large effect on top speed, fuel consumption and top-end acceleration.
Our greatest curiosity, however, regarded one simple question: which bike has the most slippery shape, the Suzuki Hayabusa or Kawasaki ZX-12R? In previous top-speed testing at the Transport Canada speed oval, something seemed to be limiting the ZX-12R's velocity to a "politically correct" 187.5 mph, which failed to match the 190.5 mph we'd recorded earlier for the Hayabusa. The Suzuki had recorded 153.0 horsepower at the rear wheel, while the Kawasaki made a blistering 164.5 horsepower. Yet the ZX-12R was slower. Was it the much-rumored electronic speed control? Or something else?
The wind tunnel provides a simple answer. What limits the ZX-12R's top speed is aerodynamic drag. Despite Kawasaki's unique monocoque frame design, the expertise of the company's aerospace division and various winglets and spoilers, the ZX-12R produces significantly more drag than the Hayabusa. The Suzuki can therefore go faster with less horsepower. It's not the threat of political intervention that has limited the ZX-12R's top speed, but rather the shape and size of the motorcycle. (Please see the editor's note at the conclusion of this article.)
Drag primarily comes from positive pressures pushing back on the front-facing parts of the rider and motorcycle, as well as suction pressures pulling on the backward-facing parts, where the flow has separated. Skin friction, whether from laminar or turbulent flow over the surfaces of the bike's fairing or the rider, are small in comparison, which is why discussions of laminar or turbulent flow on a streetbike are essentially misguided.
Drag is proportional to the square of speed, and to the size of the motorcycle's frontal area. The constant of proportionality is called the drag coefficient, or CD, and is primarily a function of shape. It indicates which shape is superior, but does not define the total aerodynamic drag by itself. The product of the drag coefficient and the frontal area, A, gives the drag. A larger motorcycle with a lower drag coefficient may be faster than a smaller, poorly-shaped motorcycle with a larger drag coefficient. The best measure of aerodynamic drag is the parameter known as the drag area, CDA, which has units of square feet. This can be interpreted as the size of a flat plate that has the same drag as the motorcycle.
A lower figure means less drag, and the Hayabusa recorded a CDA of 3.37 ft2 (0.313 m2), about 8 percent less than the ZX-12R's figure of 3.67 ft2 (0.341 m2).