Posted on Wednesday, September 17, 2014 - 01:10 am:

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quote:

"Unfortunately, that conflict between styling and function is
an ongoing battle. Lowdrag isn't achieved by sharped-nosed
"penetration," as many assume, but by leaving the smallest
possible hole in the air behind the bike. And that means
keeping the 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 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 hehind 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
gental curves and tapers that parted the air for DC-3s some 60 years
ago, and still work well on subsonic vehicles today."
[end quote]

They also list the Hayabusa with a CdA of around 2.9 square feet
[coeffficient Cd times actual frontal area A] and
a Cd of about 0.56

The Buell RR1000 is listed with a Cd of 0.50, effective
frontal area of 2.7 square feet.
Erik Buell and company say their engineers have yet to measure
a street going machine with a lower drag coefficient than the RR1000.

There's also a lot of info about the calculations and numbers
used by bike manufacturers too. Article says that 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.

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