Aerodynamics 101
Aerodynamics 101
What looks fast, isn’t
IT'S A DRAG, DRAG, DRAG PUSHING THE AIR aside. And contrary to what motorcycle companies might like you to think, your bike is probably not as slick as the average minivan.
Compare a modem 600cc sportbike to a pure-bred 500cc Grand Prix racer from four decades ago, and you’d find the current street-based machine to have an advantage in almost every area. Hold something truly exotic, like a 1956 Moto Guzzi V-Eight, up against a 1999 CBR600F4, and there are few contests.
Power? The Guzzi makes just 62 horsepower, the Honda 93. Brakes? Guzzi drums versus Honda discs-not even a contest. Tires? The best racing tires of racing’s Golden Age weren’t nearly as sticky as modem DOT-approved radiais. Top speed? Ah, there’s the interesting one.
The 62-horse Guzzi would annihilate the 93-horse Honda, topping out at more than 160 mph compared to the Honda’s 157.
The reason? Modem motorcycles, as marvelous as they are in many ways, are only slightly more aerodynamic than bam doors. The Guzzi came from a period when few rules restricted GP motorcycles, a period of exuberant experimentation with motorcycle aerodynamics. To take advantage, Moto Guzzi built its own wind tunnel, and fitted its GP machines with “dustbin” fairings that fully enclosed the front wheel. They may look strange to the modem eye, but they were slick.
In fact, modern motorcycles are so non-aerodynamic that their makers “cook the books” when it comes time to brag about aerodynamic drag. Automobile manufacturers routinely refer to a drag coefficient when comparing the ability of their vehicles to efficiently cleave the air. The best cars (Chevrolet Corvette, some Audi, Lexus and Mercedes sedans) boast drag coefficients of around 0.29, while the singularly exceptional General Motors EV1 electric car derives a coefficient near 0.20 from its extreme shape. Reading recent press releases of new sportbikes, you might have the mistaken impression that motorcycles are playing in the automotive ballpark. The Aprilia RSV 1000, after all, is cited as having a “Cxs” of 0.31, which one of this country’s roadracing newspapers translated into a “Cc/” (coefficient of drag) of 0.31. That’s almost as good as the cars, isn’t it?
It’s not even close-but you have to know a little bit about how aerodynamic drag is measured to understand why. A long time ago, technical types noticed that aerodynamic drag was related to three main characteristics: the frontal area of the device traveling through the air, its shape and its speed. For a given shape and frontal area, the actual drag force increased with the square of the speed; that is, when you doubled speed, the drag force quadrupled. But for a given shape, drag force varied linearly with frontal area: Try pushing two bam doors through the air, one with twice the frontal area of the other, and the drag force resisting the movement for the larger door would be twice that of the smaller.
Obviously, some shapes slip through the air better than others, and that’s where the coefficient of drag comes in. It’s a way of relating the drag of any given shape with that of a simple, flat plate. Multiply the actual frontal area of a given vehicle by its coefficient of drag, and you get the “effective frontal area,” the area of a flat plate (think bam door) with the same drag as that particular vehicle. American engineers refer to that effective frontal area with the abbreviation CdA (coefficient Cd times actual frontal area A), and Italians evidently by “Cxs.”
So what Aprilia was referring to was the effective frontal area {CdA) of the RSV, measured in square meters-not the dimensionless drag coefficient Cd normally quoted by automotive specifications. The fact that the 0.31-square-meter figure of the RSV sounds much like a good automotive drag coefficient is meaningless; the two figures are as incomparable as apples and hand grenades. Given that the Aprilia’s actual frontal area with rider is around 5.25 square feet (0.48 square meters), its drag coefficient is around 0.65-worse than that of most pickup trucks. At any given speed, a Corvette has only 60 percent more drag than the Aprilia, despite having 300 percent more frontal area.
But not to pick on Aprilia: Most motorcycle companies resist quoting the drag coefficients of their products because the numbers look so poor compared to cars. That leads to practices such as Honda’s a few years ago, referring to CBR1000 drag properties in unspecified units and numerical values that could not be related to anything. Not to mention the popularity of giving out the CdA in square meters while forgetting to mention the units-because the resulting numbers just happen to look like good automotive drag coefficients.
Fortunately, there’s a history of published tests of motorcycle drag in technical literature, there’s some proprietary information made available to us by manufacturers such as Buell, and drag figures can be calculated independently from top speed and power measurements. And what do all those tell us? Current sportbike drag coefficients are generally in the range of 0.55 to 0.65, with overall effective frontal areas of around 2.9 to 3.5 square feet (0.32 to 0.27 square meters). The interesting thing is when you look at a few examples that diverge from current practice: That Moto Guzzi V-Eight, for instance, had a drag coefficient below 0.45, and an effective frontal area of around 2.0 square feet. That’s a third lower than a slick modern motorcycle like the Suzuki GSX1300R Hayabusa, which has a CdA of around 2.9 square feet and a Cd of about 0.56. Give the Hayabusa the Guzzi’s low drag, and it would exceed 210 mph with the same power that now carries it to 194 mph-a speed increase that, if achieved by horsepower alone, would take an additional 75 horses at the rear wheel! Alternatively, a 95-horsepower 600 with the Moto Guzzi’s drag would go faster (mid-180s) than either the current Honda CBR1100XX or Kawasaki ZX-11.
Another sportbike from the past-the late-’80s Buell RRlOOO-demonstrates one reason why ~ more effort hasn’t been paid to drag reduction.
Erik Buell and company have had most of the relevant Japanese machines in the wind tunnel, but their engineers have yet to measure a street-going machine with a lower drag coefficient than that of the RR: Cd of 0.50, I effective frontal area of 2.7 square feet. But, as even Buell himself is willing to admit, the RR looks big and unusually rounded, and the average sportbike customer is expecting a bike that’s smaller and more sharply aggressive looking. Unfortunately, that conflict between styling and function is an ongoing battle. Lowdrag isn’t achieved by sharp-nosed “penetration,” as many assume, but by leaving the smallest possible hole in the air behind the bike. And that means keeping airflow attached to a motorcycle past the bike’s point of maximum width, allowing the air that has been accelerated out of the way by the bike’s nose to slow back down and fill in the space behind the bike. The difficulty in doing that is the pressure gradient along that airflow path is from low pressure to higher, so cajoling the air to follow is much like pushing on a rope. It can be done, but it must be done carefully and smoothly, or the air tumbles away in turbulence leaving a big low-pressure hole behind the bike and accordingly high drag. And unfortunately, the shapes that work don’t always match up with styling expectations. Not surprisingly, the machines with the lowest drag-the Buell RR1000 and the Suzuki Hayabusa-depart the farthest from those angular shapes that work best at Mach II on supersonic fighters, and cleave closest to those gentle curves and tapers that parted the air for DC-3s some 60 years ago, and still work well on subsonic vehicles today.
So, when it comes to aerodynamics and motorcycles, you may be compelled to answer this question: Do you want speed, or this year’s appearance of speed? They’re not necessarily the same thing. Steve Anderson
