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| Motorcycle Fairings and Windshields
This page Copyright © 2003-2007, by Mark Lawrence.
Email me, mark@calsci.com, with suggestions, additions, broken links.
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Windshields
Plastics
Windshields for motorcycles are made from either polycarbonate
(Lexan) or acrylic (Plexiglas). Each type
of plastic has advantages and disadvantages.
Polycarbonate is an extremely strong plastic. Polycarbonate is about
as transparent as glass. Polycarbonate
cuts and forms easily at both room temperature and at higher
temperatures. For machining purposes, you can work
with polycarbonate pretty much the same as you would aluminum.
Polycarbonate has a major drawback for windshield
use: polycarbonate picks up water from the air. The water eventually
makes the polycarbonate cloudy. This water
will form bubbles if you heat the polycarbonate to forming
temperatures. So, before you can form polycarbonate,
first you have to place it in a drying oven at about 200° for about
12 hours. Because of this, only companies
that manufacture polycarbonate make windshields. Polycarbonate is
sensitive to ammonia, so glass cleaners like
Windex should not be used on polycarbonate. Polycarbonate windshields
need a coating to protect them from
chemicals and prevent them from absorbing water from the air. This
optical coating is difficult to apply
uniformly, resulting in optical distortion. It also scratches and
cannot be repaired with plastic polish. By far
the most popular polycarbonate for motorcycle windshields is GE
Lexan Margard
MR10, aka "quantum coated." GE polymers was recently bought by a
Saudi Arabian firm, Sabic - see
GEPlastics.com. We don't buy products from countries that fund
terrorism.
Acrylic is only about 3% as impact resistant as polycarbonate.
Normal acrylic shatters upon impact, and
therefore is considered an unsafe material for windshields. Acrylic is
very chemically resistant, and is more
transparent than glass - glass absorbs about half again as much light
as acrylic does. Acrylic forms easily at
high temperatures, about 300°. However, machining acrylic at room
temperature is difficult. It's not very
easy to cut acrylic with a saw or drill holes in acrylic without
shattering or weakening the material.
Polycarbonate is a DOT approved material for making windshields;
normal acrylic is not. Some states
require DOT approved windshields, and therefore in these states a
normal acrylic windshield is actually
illegal, however these laws are rarely enforced. Normal acrylic can be
shattered by an impact from a rock
moving at speeds as low as 15mph.
A special high cost acrylic called Impact Modified Acrylic is
available. This form of acrylic is DOT
approved for windshields. We use only DOT certified impact resistant
plastics to make Calsci windshields. Our
windshields will not shatter if hit by a rock. We test our windshields
by shooting them with a .22 caliber
rifle and verifying that the windshield maintains its basic integrity
without shedding small pieces that could
impact your face or eyes. No windshield can protect you against
everything, but we do our best to make certain
that our windshields protect you against the small rocks frequently
thrown up by other vehicle's tires.
Optics
Even though Calsci windshields are designed so that you look over
them, not through them, we use only
optically correct shapes that will not distort your vision if you do
look through the shield. If you look
through one of our shields at a dividing line on the highway, you'll
see essentially no bending of the straight
line. You'll never get a headache from looking through one of our
shields.
Aerodynamics
Nearly all of our windshields
have vents. These vents are part of the
aerodynamic design of the shield, to reduce turbulence and noise. They
are not there to make a flow of air on the
rider. When you're riding on the highway, any windshield is pushing air
away from the rider. This leaves a low-
pressure pocket between the windshield and the rider. The air flowing
past the motorcycle wants to drop into this
low pressure area. If the outside air is allowed to spill into the area
between the windshield and the rider, the
result is turbulence, noise, and drafts. When outside air spills into
the rider area, it almost always falls in a
curved path, causing spinning vortices of air. These vortices are noisy
and can cause the battering and hammering
on your helmet reported by some riders. Our windshields and vents are
designed to funnel air into the rider
region to relieve this low pressure area and greatly reduce the
tendency of outside air to spill in. The vents
are designed so that the air coming through them is quickly dispersed,
leaving almost no detectable air flow at
the rider. Our goal is to produce almost completely still air on the
rider with no back pressure.
I get a lot of emails, "Can you make me a windshield with a reverse
flip to kick the air up over my head?" Yes,
I can, but I won't. Air is a spring - there are shock absorbers made
with only air as the spring. When you kick a
spring, it kicks back. Putting energy into the air like this is exactly
the opposite of what we're all about.
Windshields with reverse flips and non-fair shapes generate
semi-periodic chaotic swirls of turbulent air, called
Von Karman vortices, after Theodore Von Karman. These vortices, or
pockets of turbulence, grow as they move away
from your windshield. If you feel your head being rocked or even
slammed side to side or front to back as you
ride, this is Von Karman vortices at work. Some manufacturers, to my
own astonishment, actually claim to produce
these vortices on purpose, apparently with the idea that some
turbulence is "good" and will somehow perhaps cancel
out the "bad turbulence." We work very hard with the design of the
shape of our windshields and the location and
size of the vents to eliminate all Von Karman vortices.
Von Karman Vortices - the source of countless headaches.
Theodore Von Karman emigrated from his native Hungary to the US in
1930 to become the director of the
aerodynamics laboratory at Caltech. Mark learned his aerodynamics in
Von Karman labs at Caltech. Calsci
windshields are designed using aerodynamic engineering principles that
guarantee our shields do not generate
turbulence. These are the same shapes that minimize drag and maximize
fuel mileage. If you love math, give
us a call and we'll tell you all the gory details. Hint: our shapes are
all solutions to Laplace's
equation, ∇²φ = 0, which guarantees a fair shape, that is a
continuous second derivative.
Design
All our shields are laid out on a computer and cut with an
industrial cutting laser. Our shields are
symmetric to within a thousandth of an inch (.025mm). All mounting
holes are also drilled with the laser,
guarantying an excellent fit to your bike. This precision is necessary
to be certain your riding experience
will be precisely the same as all our other customers, and precisely
what we engineered for your bike.
Our windshields are designed by
Mark Lawrence and Carl Porter. Carl has a
Bachelor's degree and a Master's degree in engineering from Ohio State
University. Mark has a Bachelor's degree
in engineering from the California Institute of Technology, and is
currently working on a PhD in physics at the
University of Southern California. Carl and Mark don't agree very well
about college football teams. Mark has a
bit more than 500,000 miles of motorcycle experience. It takes about 6
weeks, eight to twelve prototypes, and
typically several thousand miles to finalize a windshield design. Our
windshields are not just a stock windshield
made a bit wider and taller. We build and modify our prototypes until
the resulting windshield is quiet,
comfortable, and attractive.
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How to choose the best
windshield height for you.
The yardstick method below gives the correct answer, including
adjustments for your riding posture, handlebar
height, seat height, etc. The table below is only approximate. To use
the table, get a size estimate from the table
using your height and pants leg length. If you're between sizes, use
the larger size if you live in a colder
climate, or the smaller size if you live in a warmer climate. People in
Florida or Houston need somewhat shorter
windshields, people in Seattle or Juneau need taller windshields. Both
the yardstick method and the table are for
people who want to look over the windshield. If you want to look
through the windshield, add about 3" to your
windshield height. Of course the quietest possible ride is when you're
looking through the windshield, but only
about 1% of our customers are interested in looking through a
windshield.
Younger guys (under 35) typically want about an inch or two shorter
windshield to get a sportier feel. Older guys
(over 45) typically want a more quiet and comfortable ride and prefer
about an inch taller shield.
If you want coverage for your body up to your shoulders, but your
head in undisturbed full-speed air, then
your windshield's top edge should visually hit the ground about 10' -
12' (3 - 4 meters) in front of the front
tire contact patch.
Yardstick Method of determining
windshield height
- If your stock shield is mechanically adjustable, put it in
the lowest position. If your windshield is
electrically adjustable, put it about 1/4 to 1/3 of the way up.
- Get a yard (meter) stick - free at Home Depot or most
hardware stores.
- Park your bike on level ground. Measure 30' (9 meters) from
your front wheel contact patch. Drop something
on the pavement, keys, a rock, whatever.
- Measure 30' (nine meters) more, you're now 60' (18 meters)
in front of your bike. Drop something else,
wallet, ex-girlfriend, whatever.
- Tape the yardstick along the center of the windshield with
masking tape or something, with the 20" mark
aligned with the top of the windshield and the stick pointing up.
- Sit on your bike and look at the two things you dropped on
the pavement. Try to sit with your normal riding
posture. Don't cheat - if you slouch a bit when riding, slouch a bit
now.
- You can sight along the yard stick and see how many inches
up from the top of the shield you see the 30'
and 60' marks. This tells you how many inches taller you would like
your windshield.
- Your optimum windshield height is somewhere between these
two heights. Lower for warmer climates, sportier feel
and more air flow. Higher for colder climates / quieter riding / more
wind protection. Our shields are typically made
in 1.5" (4cm) increments to help you get the best height for you.
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Table: Height v. Pants Inseam (Leg Length)
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28" = 71cm |
29" = 74cm |
30" = 76cm |
31" = 79cm |
32" = 81cm |
33" = 84cm |
34" = 86cm |
35" = 89cm |
36" = 91cm
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| 5'7" = 170cm |
M |
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| 5'8" = 173cm |
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M |
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| 5'9" = 175cm |
L |
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M |
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| 5'10" = 178cm |
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L |
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M |
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| 5'11" = 180cm |
XL |
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L |
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M |
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| 6' = 183cm |
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XL |
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L |
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M |
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| 6'1" = 185cm |
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L |
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M |
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| 6'2" = 188cm |
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XL |
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L |
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M |
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| 6'3" = 191cm |
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L |
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| 6'4" = 193cm |
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L |
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| 6'5" = 196cm |
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If you buy a windshield from us, you can wrap it in plastic wrap a
couple layers thick, then cut the plastic wrap
away from the vent, then mount it. The plastic wrap will protect it
from bugs, scratches when mounting, etc. If you
might send the shield back for an exchange, please don't scratch it
first. And, trust me, as impressive as your
state's bugs are, I already have as many bugs as I need. Ride the
windshield for a half hour or so. If you bought the
wrong size, we'll trade you. Or, put some masking tape on your shield
where you would like it cut, and we'll cut it
to your spec. We want happy customers.
Cleaning your Windshield
We make our own cleaning fluid. When you buy a windshield from us,
we give you a small spray bottle of
cleaning fluid and a micro-fiber cleaning towel. Here's our "secret"
washing fluid recipe, the result of
testing about 25 different formulas:
Makes 1 quart = 1 liter windshield washing fluid
- 1/2 cup = 100ml ammonia (double this if you have a lot of
bugs)
- 2 cups = 400ml isopropyl (rubbing) alcohol
- 1.5 tsp = 8ml car wash detergent (don't substitute dish
soap or laundry detergent)
- 2 cups = 500ml water
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We recommend using only micro-fiber cleaning towels on your
windshield.
Repairing your Windshield
Scratches in our windshields can be easily repaired with any good
plastic polish. Plexus and Novus are
two good brands. You can easily buy a 3 bottle kit of Novus #1, #2, and
#3 on Ebay for about $10. We use #1
in house to clean and polish our windshields, and #3 to repair any
small scratches that happen during the
manufacturing. #2 is used to remove any tiny scratches left by the #3
process.
To repair a scratch, use a good cloth - a used diaper, or the
microfiber cloth we supply is good. Rub
across the scratch with #3 until the scratch is visually gone. Then rub
against the #3 direction with #2
to clean up the area. Finally, use #1 to polish the windshield.
Modifying your Windshield
You may paint your Calsci shield with pretty much anything. Paint
from the rear of the shield, and preferentially
use Acrylic Enamel paint. From the front of the shield it will look
fantastic.
To drill a hole in your windshield, it's best to use a slower speed
drill, like a battery powered drill. Also it's
best to use acrylic bits, available from Craftics.com. You can do this
with normal tools if you're very careful.
Drill a small pilot hole in the center, 1/8" or 3/16" (3-4mm). With the
full size bit, drill from the front of the
shield just enough to make the full diameter hole on the front, but not
all the way through. Now turn the shield
over, support it on a piece of wood, and drill from the rear all the
way through. Drilling from both sides like this
minimizes the chances of chipping the shield as your bit completes the
cut.
To cut away part of your shield, first cover most of the front of
the shield with masking tape. You don't want to
scratch your shield. Mark on the tape with a black felt tip pen where
you want to cut.
Cut the shield with a band saw, saber saw, or dremel tool using a
blade with 12 to 14 teeth per inch. Cut triangular
pieces off the corners in preparation for rounding them. Or you can use
the top piece you cut off to mark corner curves on
your new top edge.
You can remove smaller sections of plastic using a sanding drum
attachment on a dremel tool. Mark the shield first
with a black felt tip pen and cut to your mark. You can draw directly
on the plastic, then later wash off the ink with
alcohol.
Sand new rounded corners and straighten your
cut edge using a floor mounted belt sander with 80 to 120 grit
sandpaper.
If you don't have a floor mounted belt sander, you can get similar
results by securing a hand held sander upside down in a
vise and holding the windshield over that. Use a sharp piece of metal,
like an open scissors, to scrape the sanded edge.
This will give you the same edge as sanding with about 600 grit. Scrape
with the scissors to break the sharp corners on
the edge.
When making custom cuts, we use a band saw with a 12 tooth per inch
non-ferrous metal blade; a floor mounted
belt sander with 80 grit sandpaper; and a plastic scraping tool from
Craftics, available on Ebay, Amazon,
DelviesPlastics.com, ProfessionalPlastics.com or Craftics.com.
Fairings
For those of you who are interested in history, here's where the
word "Fairing" comes from.
Early in the development of aviation, it was realized that the
important thing for an airplane was to have a
lot of lift and very little drag. An enormous amount of drag happens if
you lose laminar flow - that is, if instead
of smoothly following the surfaces on the airplane, when the air breaks
away from the surface it will form spinning
vortices which tumble around and wreck the airflow all over the place.
This is called turbulence. The exact same
problem was known from laying out the keels of ships, for water flow
around a ship hull is a lot like air flow
around the skin of an airplane. This problem was analyzed by
mathematicians. They learned something: they could
predict the points at which the air flow (or water flow) would break
away from the surface and start to become
turbulent.
A curve which has no breaks in it is called "Continuous" by
mathematicians. A curve which has no sharp corners in
it is called "smooth" by mathematicians. Smooth means the first
derivative of the curve is continuous. At any given
point, a curve has a radius of curvature. If there are no sudden jumps
in the radius of curvature, the curve is
called "Fair." A Fair curve has a continuous second derivative. It was
learned that turbulent flow always starts at
a point on the skin where the curve has an abrupt change in the radius
of curvature, that is a point where the curve
is not fair, or a point where the second derivative is discontinuous.
So, you can't just stick a wing onto an
airplane fuselage - the sharp corner where they meet is not even
smooth, much less fair. The designers found they
had to locate places like this on the aircraft skin and cover them with
some smoothly curved sheet metal. These
pieces of sheet metal are called "Fairings."
Notice the fairings on the wing-fuselage joint of this DC-3.
I jumped out of one of these once, and it was working just fine at the
time.
In the '70's, when gas mileage became important, automotive
companies quickly hired some aircraft designers to
help them make their cars have less drag. Shortly after that, the
automotive companies started putting pressure on
the computer programmers to make certain that all the curves on an
automotive body were fair. Some companies became
quite obsessed with this: Honda at one point announced that they had
determined that surfaces which had a continuous
fifth derivative were most pleasing to the eye, so they wanted their
CAD/CAM systems to only design curves which
were smooth, fair, and also had three more levels of derivative
continuity. I don't think they got very far, as very
few programmers can handle the mathematics of C5 continuous surfaces.
Of course, until about 1970, there basically was no such thing as
computer aided design. To lay out the curves
for the hulls of ships and large bombers, Boeing many years ago built a
building with an unbroken wooden floor which
was bigger than a football field. They would clear this building, and
draw a coordinate graph on the floor. Then,
the designers would tell them exact points where they wanted the hull
skin or aircraft skin to be. The engineers
would hammer nails into the floor at these points. They would then take
very long, very thin strips of oak, soak
them in water, and tie the oak strips to the nails. The oak will
naturally form a shape of least energy, which
happily enough is a shape which is both smooth and fair. The engineers
would wait for the oak to dry, then trace the
lines on the floor of the building. This then became the master drawing
for the bulkheads. The thin strips of wet
oak were called "Splines," which is why today curves in McDraw and
Autocad are called splines, although essentially
none of the programmers know this either. Most of our bombers and
battleships in WW II were laid out in this
building, because this was what we had.
In General Relativity, Einstein assumed that the universe itself was
curved, but in a smooth and fair fashion.
His reasoning: anything else would have been mathematically ugly, and
he didn't believe God did ugly things. Since
then, several people have made alternative theories of gravity where
the universe does not have to be smooth and
fair. None of them have worked worth beans, however. It seems God does
in fact have a sense of aesthetics. Later,
it was pointed out to Einstein that his theory included the possibility
of points where the universe was neither
smooth nor fair. These points are called singularities, or more
popularly Black Holes. Not all scientists believe in
black holes, and Einstein was skeptical.
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California Scientific
4005 Seaport
West Sacramento
CA 95691
Since Jan 2, 1985
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Sales@CalSci.com
800-284-8112
916-372-6800
Revised Monday, 08-Dec-2008 01:37:47 PST
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