windshield

 I am considering a windshield for my Hop Sing 250 for bug and weather
protection.  My first thought is that the windshield would improve the
overall aerodynamics, as it seems much more streamlined than my lumpy
body.  Is that right?  Will a windshield result in a slight
performance improvement, or vice versa?  Thanks for any replies.
Conrad "Connie" Eaton

 


It depends on:
1) How big the shield is.
2) Whether or not you mount it correctly.

Even a large shield with the proper "rake" is probably better than the wind
hitting your "lumpy body". ;-)

Even a small shield mounted too straight up will have the aero properties of
a barn door and likely be worse.

 
Well, if you want to use the term "slight", it's true that a curved
handlebar-mounted windshield is aerodynamically better than an
absolutely *flat* plate of equal area.

The aerodynamic term is "coefficient of drag". A human being isn't
flat, a human is curved, and a rider sitting upright has a 0.45
coefficent of drag.

IOW, the human body hitting the air causes 0.45 pounds of drag per
square foot of body area.

The rider cannot change his body shape, but he can reduce his frontal
area by
laying his upper body down on the gas tank.

That's why sportbikes are designed with low handlebars, and the riders
get used to riding all scrunched up like the proverbial monkey and the
football...

A sylishly curved cafe racing quarter fairing might have a coefficient
of 0.30, but only over the area covered by the front side of the
"bikini" fairing. The rest of the rider's body area is still out there
"in the wind".

And there is a huge area of turbulent air *behind* the rider,
resisting his forward progress.

One magazine articles about fully-faired sportbikes claimed that all
the stylish plastic resulted in an amazing coefficient of 0.10.

But, unlike the flat plate windshield being pushed through the air by
brute horsepower, the scientifically-designed fairings slip through
the air without a lot of turbulence on the *rear side*, they smoothly
bend the air around the motorcycle and rider, allowing the slightly
disturbed airstreams on either side to merge back together.

These scientifically designed fairings allow a very powerful sportbike
to go 200 miles per hour, with "only" 200 horsepower.

At that speed, nobody cares about *fuel economy*. The sportbike that
gets
45 miles per gallon while cruising at 60 mph might be getting only 25
miles per gallon running at full throttle.

 . wrote:


I don't think that's right. You don't have speed as a factor there.

A 0.45 coefficient just means 0.45 as much drag as a flat square plate
of the same frontal area.

 

That's true. Aerodynamic drag is a velocity-squared effect, IOW, if
you go twice as fast, you get four times as much drag for any given
frontal area and
shape.

But I didn't want to complicate the discussion with formulae related
to dynamic air pressure, which varies according to altitude, humidity,
and temperature, and the mass of a slug of air...