Cycle Canada
Tunnel Vision
By Bruce Reeve

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
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, howeverm 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 aerodynamics.
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 motorcyles
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 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 aerodynamic lab at the
National Research Council (NRC) in Ottawa, Canada, offered to help. Many
motorcyle clients have used the NRC's wind tunnel for commercial work and
it was ideal for our purposes. Cooper arranged for two days in the tunnel
with the support of the NRC staff. We booked a Hayabusa and a ZX-12R, but
Cooper suggested we bring some other bikes also.

Representing the other extremes 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
neccesary 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 and 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. The balance is an extremelt sensative 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.

Keven Cooper, left, oversees the NRC staff preparing for a run. The bike
is positioned on a balance beam and the weight is zeroed. A turntable
changes the angle of the bike against the wind.

Once the fan is operating and the wind speed constant, the 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 verticle
axis). These forces and moments describe the aerodynamic loads on the motorcycle and
influence top speed, acceleration, crosswind handling and stability. Generally, a
motorcylce 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 for the
Hayabusa. The Suzuki had recorded 153.0 horsepower at the rear wheel, while the Kawasaki
made 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.

The NRC wind tunnel uses a 2000 horsepower DC motor to generate the airflow
that blows past the bike, which is mounted on a balance to measure drag.
Smoke created by burning vegetable-based oil helps trace the air currents
around the bike and rider. We expected the Hayabusa (bottom) and ZX-12(top)
to have similar aerodynamic efficiency, but the Suzuki proved markedly
superior, indicating why it needs less power to go faster than the ZX-12R.

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 the 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.

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 effect, but
essentially, the bikes have similar speed potential, although the Suzuki
has the edge. One thing is certain - the Kawasaki doesn't need an
electronic governor to limit its top speed.

Using the CdA calcualted from the air drag at a single tunnel speed,
we determined drag for a speed range from zero to 225 mph, indicating
the engine power needed to overcome air drag. Rolling resistance also
must be considered. The sum of the air and rolling drag forces multiplied
by the speed gives us the power figure needed to pull that speed. Repeating
the multiplication across our 225 mph speed range demonstrates how the
Hayabusa's superior aerodynamics require less power than the ZX-12R to
achieve a given 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 would also 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 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 the 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.

Frontal area on the Hayabusa and ZX-12R was measured using Photoshop
images with the pixels set to a scaled size, then counted. The Hayabusa
proves to have a smaller frontal area, but the Cd also shows it has the
superior aeridynamic shape.

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 back
end to minimize the wake size (the same reason a fairing that closes
in is superior).

Motorcycles, even racing design systems, 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 "ballon 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 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 sharply cut-off tailsection, which don't "look" aerodynamic. But the
wind tunnel proved otherwise, and the lower drag was recorded by the older
bike, with a CdA of 2.08 ft2 (0.193 m2) compared with 2.20 ft2 (0.204m2)
for the '96 model. The newer bike demonstrated a modest advantage in
reduced side forece, 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 large improvements
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 produces
less drag than the ZX-12R, but still more than the Hayabusa, confirming
there's more to motorcycle aerodynamics than a small frontal area.

Included in our wind tunnel test were a '90 (1) and '96 Honda RS125 (2),
Suzuki Bandit 600 (3) and GSX-R750 (4). The 90' RS125 produced the least
drag, but mostly because of its small frontal area. The GSX-R750 has a
smaller frontal area than the Hayabusa, but creates more drag. The Busa
definitely has a slippery shape.

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.

[Editor's note]: Since this story first ran in the Sept/Oct 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 (ZX-12R), the prototype resulting from
wind tunnel testing and extensive modifications was almost a carbon copy
of the Hayabusa. It was probably the best design they had ever built from
an aerodynamic standpoint. Only Kawasaki's world product planners figured
it was so ugly it would not sell. We all know what's happened since then.

The National Research Council operates two other wind tunnels in Ottawa
besides the 6.5 foot by 10.0 foot facility we used, including this massive
30 foot by 30 foot tunnel near the city's airport (shown). A 9500 horse-
power DC motor turns an enormous fan (you can walk between the blades) at
225 rpm to generate a hurricane of wind. The air circulates in a huge
horizontal loop, which inside resembles some sort of alien cathedral.
Among the clients for the NRC tunnel are NASCAR teams, which consider
the approximately $1530 per hour charge to be a bargain. Wind tunnel
testing is essential to optimize drag, downforce and lift for different
racetracks, and the top teams put every car in the tunnel for aerodynamic
fine tuning, doen on the spot by sheet metal artists.

The NRC's other tunnel is much smaller, and uses compressed air to generate
a 14-second burst that can simulate Mach4.5 speeds for aircraft models.
NRC's wind tunnel facilites and expertise have been used for the development
of a wide range of vehicles, including airplanes, bicycles, cars and trucks,
for commercial clients around the world.

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