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.
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).
Roughly 90 percent of an engine's power is used to overcome aerodynamic drag at high speeds, while the remaining 10 percent works against rolling resistance. The exact rolling resistance is difficult to determine, and the relative efficiency of each bike's ram-air system is also unknown. But it is possible to calculate a power vs. speed graph using the drag figures measured in the wind tunnel (see above). To achieve 187.5 mph, the Hayabusa needs 147.6 horsepower to overcome drag alone; the 12R needs 161.3 horsepower for the same speed. However, using the wind tunnel data, test weights, our road-test dyno figures for horsepower and a rolling-resistance figure, Cooper calculated that the ZX-12R would have a maximum speed of 187.0 mph and the Hayabusa 187.7 mph. The effect of wind can vary the result, usually decreasing speed unless it's a tailwind. Sidewinds during a test can decrease top speed as a result of the higher drag at yaw. This calculation doesn't include any ram-air effects, but essentially, the bikes have similar speed potential, although the Suzuki has an edge. One thing is certain-the Kawasaki doesn't need an electronic governor to limit its top speed.
After our wind tunnel session with the two bikes, we asked Kevin Cooper for his opinion about why the Hayabusa produces less drag than the ZX-12R. Both bikes have roughly similar shapes, and it would take a great deal of wind tunnel experimentation to determine what makes one better than the other. But Cooper was willing to take some educated guesses. With the bikes parked side by side, one obvious difference between them was that the ZX-12R sits taller, and to Cooper's eye it has a larger frontal area. The ZX-12R's monocoque frame design, which uses a huge aluminum box-section over the engine instead of spars around each side, reduces the width of the bike. But the design seems to have increased height, which exacts a cost in frontal area. The taller shape of the 12R raises the point of action of the drag and side forces, generally leading to more front end lift and a larger rolling moment, respectively. It makes sense that the yawing moment also would be increased, but it was not. This could have been a result of the action of the winglets on the 12R, but Cooper felt it probably was due to a more forward position of the side force on the more slippery Hayabusa.
Kawasaki's explanation for those "canard-like winglets" is they are flow separators which prevent turbulent air from the front wheel from disturbing the laminar flow of the upper fairing. Cooper was skeptical about this claim, however.
Cooper cited several areas where he thought the Hayabusa might have an aerodynamic advantage: the smaller twin mufflers might have lower drag because they are tucked behind the rider's legs, while the 12R's large single muffler is more exposed; the Hayabusa's fairing is wider at the front and closes in, whereas the ZX-12R has a less desirable flat-sided shape; and the lower profile of the Hayabusa better conceals the fork legs, which typically produce considerable drag for their size; a circular cylinder has a drag coefficient of 1.2. The Suzuki also has integrated turn signals and ram-air ducts. But the main reason for its lesser drag, Cooper guessed, was simply the frontal area.
In order to measure frontal area, we photographed the Hayabusa and ZX-12R from the front, using a long lens to minimize parallax distortion, with a measuring stick beside each bike as a reference point. Later we scanned the photographs, enlarged them to an identical scale and close-cropped them. Using Adobe Photoshop, the pixels in the images were adjusted to a scaled half-inch-square size and then counted, which gave us an accurate measurement of the frontal area of each bike, confirming our impressions. The ZX-12R has a frontal area of 6.09 ft2 (0.566 m2), physically larger than the Hayabusa, which is 6.01 ft2 (0.558 m2). But the advantage for the Suzuki is not just in frontal area. With figures for both drag and frontal area, it's possible to calculate the coefficient of drag, which is 0.603 for the 12R and 0.561 for the Hayabusa. The winner of this wind tunnel shootout is the Suzuki.
It's worth remembering, however, that neither of these CD figures indicate a particularly impressive degree of streamlining, since even a typical passenger car has a CD of less than 0.60 and some models are lower than 0.30. A fully streamlined Bonneville speed-record bike might have a CD of 0.10. Such is the nature of streetbikes, where performance derives mostly from extreme power-to-weight ratios.
One of the most striking, and controversial, elements of the Hayabusa is its prominent, drooping snout. Is it possible this unusual shape contributes to the Hayabusa's more slippery aerodynamics? Not according to Cooper, who says one of the enduring misconceptions about aerodynamics is that a sharp, projectile-like nose produces less drag. He offers the example of a brick, which, once the leading edges have been smoothly rounded, can only achieve significant reduction in drag from changing the shape of its back end to minimize the wake size (the same reason a fairing that closes in is superior).
Motorcycles, even racing designs, continue to feature what Cooper calls "styling aerodynamics." A good example on the ZX-12R is the exaggerated torpedo shape of the mirrors, which Cooper says offer little, if any, advantage.
The most immediate way to decrease drag on a motorcycle is for the rider to adopt a crouched position. A smaller rider can produce a 15 percent reduction in drag; tight clothing, which reduces the "balloon effect," can provide another 15 percent reduction. Motorcycles generally have a large separated wake resulting from the unstreamlined shape of the rider, and are called "bluff bodies." Streamlined bodies have a gently closing tail and a very small wake, reducing the pressure drag. Motorcycles designed for speed-record attempts, and to a lesser extent racebikes, typically have enclosed shapes or bodywork that is integrated with the rider's body to produce a smoother wake. For streetbikes, however, styling conventions and comfort demands make this sort of approach impractical.
The next candidates for the wind tunnel test were the 1990 and 1996 Honda RS125 GP bikes. To look at the bikes, you'd expect the more current model to have the more slippery shape. The older RS has a blunter nose and a sharply cut-off tailsection, which don't "look" aerodynamic. But the wind tunnel proved otherwise, and the lowest drag was recorded by the older bike, with a CDA of 2.08 ft2 (0.193 m2) compared with 2.20 ft2 (0.204 m2) for the '96 model. The newer bike demonstrated a modest advantage in reduced side force, yaw and roll, but the '90 model had less drag at every wind angle. Cooper was unimpressed with the sharper and more aggressive-looking nose of the '96 model, but he suggested that the lower drag on the '90 model was probably the result of a better fit between the fairing and the rider
Again using a Photoshop image, we measured the frontal area of the '96 RS125, which proved to be 3.40 ft2 (0.316 m2), leading to a CD of 0.644-which doesn't compare that well to the Hayabusa and ZX-12R. In other words, the RS125 has an aerodynamic advantage because of its smaller frontal area, but not because of a particularly streamlined shape. Generally, the GP bike fairings weren't very good because they left too much of the rider's body exposed. Cooper predicted the racebikes would have a large improvement in performance with good fairings.
Next in the tunnel was a half-faired Suzuki Bandit 600, which produced a CDA of 3.94 ft2 (0.366 m2). The Bandit was followed by a fully faired GSX-R750, which produced a CDA of 3.49 ft2 (0.324 m2). With an obviously smaller frontal area than both the Hayabusa and ZX-12R, the GSX-R produced less drag than the ZX-12R, but still more than the Hayabusa, confirming there's more to motorcycle aerodynamics than a small frontal area.
It seems we may have reached the point with streetbikes where horsepower and aerodynamics have produced the maximum velocity authorities are willing to tolerate, but we have by no means reached the limits of what is possible. For all the hype surrounding Kawasaki's ZX-12R, the wind tunnel shows it fails to surpass the bike it was designed to beat for the most elemental of reasons. The Suzuki Hayabusa deserves to keep its title as the world's fastest motorcycle.
Thanks to Kevin Cooper for assistance with this story.
Editor's Note: Since this story first ran in the Sept./Oct. 2000 issue of Cycle Canada, some interesting info has come to light from our Man In Japan, Don Helle. Eight years ago, when Kawasaki engineers were designing the original replacement for the ZX-11, the prototype resulting from wind tunnel testing was almost a carbon copy of the Hayabusa-only the company figured it was too ugly to sell. And a Kawasaki engineer has confirmed the ZX-12R was in fact "neutered" to a lower top speed in its final stages of development. See Late Braking for the complete story.
|BIKE ||CDA (ft2) |
|1990 Honda RS125 ||2.08 |
|1996 Honda RS125 ||2.20 |
|Suzuki Hayabusa ||3.37 |
|Kawasaki ZX-12R ||3.67 |
|Suzuki GSX-R750 ||3.49 |
|Suzuki 600 Bandit ||3.94 |