As motorcyclists, we share a relationship with the wind that is intimate. Our exposure to it is one of the defining elements of motorcycling. At lower speeds, the movement of air around us seems soothing and beneficent. With increasing velocity, however, the air becomes noisy, harsh and fiercely resistant to motion.
Motorcycle aerodynamics sometimes have been disparaged as crude or inefficient, but this is an incomplete assessment. We choose to be in the wind; perfect aerodynamic enclosures eliminate what makes a motorcycle a motorcycle, which is why racing regulations strictly prohibit fairings that detract from the two-wheeled ideal.
Anyone who has ridden into a strong headwind, however, knows the importance of aerodynamics for a motorcyclist. For sportbikes, a slippery aerodynamic design assists both rider protection and efficiency, permitting a higher speed for a given amount of horsepower. Aerodynamics becomes more important the faster you go because the power required increases as the cube of speed-in other words, to double your speed, you need eight times as much power to overcome aerodynamic drag. You can quickly reach a point where speed is achieved more easily by improving aerodynamics rather than adding horsepower.
Earlier this year we began to wonder about the relative aerodynamic qualities of the two fastest motorcycles on the market, the Suzuki Hayabusa and Kawasaki ZX-12R. The 12R in particular seemed something of a mystery, having been announced with much fanfare, yet proving to be slower in top-speed tests than the Suzuki. Some explained the result as a measure of political correctness: Kawasaki had capped the 12R's speed potential voluntarily to avoid antagonizing European authorities.
One way to help determine why the more powerful Kawasaki ZX-12R was slower than the Hayabusa would be to measure aerodynamic drag in a wind tunnel. The drag measurement then could be used to calculate theoretical top speed, working with horsepower at the rear wheel and an estimate of rolling resistance.
Wind tunnels, however, are not found on every street corner. Fortunately, Kevin Cooper, a principal research officer of the aerodynamics lab at the National Research Council (NRC) in Ottawa, Canada, offered to help. Many motorcycle clients have used the NRC's wind tunnel for commercial work and it was ideal for our purposes. Cooper arranged two days in the tunnel with the support of NRC staff. We booked a Hayabusa and ZX-12R, but Cooper suggested we bring some other bikes also. Representing the other extreme in frontal area would be a pair of Honda RS125 Grand Prix bikes, a 1990 and 1996 model provided by Phil Unhola of Moto Canada (613/596-2552, www.motocanada.com). We also brought a Bandit 600 test bike and were able to obtain a new Suzuki GSX-R750 from the Wheelsport dealership (613/749-2020, www.wheelsport.com) in Ottawa.
The tunnel we used was built in 1940 and has a 10 foot wide by 6.5 foot high test section, which provides comfortable room for a motorcycle. Wind is generated with a four-blade fan driven by a 2000 horsepower DC motor, located two and one-half stories below the chamber, which circulates the air continuously in a vertical loop. Although the fan can generate a wind speed of 310 mph, a high velocity is not necessary to measure drag accurately, and for our tests the wind speed was set at a constant 62 mph. The effect of side winds is provided by rotating the motorcycle on a turntable to produce an approaching wind flow from one side or the other. The wheels are stationary, since wheel rotation effects have been found to be small.