What is aerodynamics? It is the study
of moving gases (in our case air) over a
body in motion, and how that airflow will
affect the body's movement through the flow.
In other words it is the study of forces and
the resulting motion of objects through the
In this article we are concerned about car
aerodynamics. In the illustration below, the
moving car is being subjected to several
forces. The driving force is the one that
causes the car to move to the right, while
all the other forces are acting against
that. The weight of the car, the friction
between the tires and the road as well as
the air resistance are forces that
are responsible for slowing down the
movement of the car.
(the forces acted upon a moving object by
the air -- also defined as
has a tremendous effect on the way a car
accelerates, handles and achieves better
fuel consumption. The air resistance should
not be underestimated. As an example, have
you ever put your hand out the window of a
moving car? the higher the speed, the bigger
the force of the air being exerted on your
hand. Motorcyclists understand better what I
mean by air resistance since they can feel
its effect especially at high speeds.
About 60% of the power required to cruise at
highway speeds is used to overcome air drag,
and this increases very quickly at high
speed. Therefore, a vehicle with better
aerodynamics is the one that has the least
effect of air resistance. The less the drag,
the higher the speed that can be attained
and the better the fuel consumption.
Essentially, having a car designed with
airflow in mind means it has less difficulty
accelerating and can achieve better fuel
economy numbers because the
doesn't have to work nearly as hard to push
the car through the wall of air.
To give you an example of how important the
drag or air resistance is: the force acting
against a car in motion by the air is a
function of: Cd x Frontal Area x
Density of Air x Speed Squared.
Air resistance W = Cd. A v2
Where, Cd = drag
A =cross-sectional area (frontal
Q =air density
Speed clearly is an important part of the
equation. At stop-and-go speeds, drag isn't
a big deal, but the faster you go, the more
it matters. At a speed of 70 km/h, you've
got four times the force working against
your car that you have at 35 km/h.
Cd varies for different shapes
and designs of cars and is influenced by how
the air flows around the car. The lower the
Cd the better the fuel
consumption and the higher speeds that can
be attained. The shape of a tear drop has
the lowest coefficient of drag of 0.05.
In 1921, German inventor Edmund Rumpler
created the Rumpler-Tropfenauto, which
translates into "tear-drop car." Based on
the most aerodynamic shape in nature, the
teardrop, this car had a Cd of
just .27, but its unique looks never caught
on with the public. Only about 100 were
The frontal area of the car is an important
factor in determining drag. That is why
trucks and most SUV’s have higher drag than
sports cars. Another factor that designers
take into consideration is the CdA.
the product of frontal area and the drag
coefficient is called
(CdA). Average full-size
passenger cars have a drag area of roughly
(0.790 m2). Reported drag area
ranges from the 1999
at (0.474 m2) to the 2003
at (2.44 m2).
Automakers have been interested in
aerodynamics since the early twenties. But
the need to improve fuel economy in recent
years has pushed aerodynamics toward the top
of automakers' priority lists. Automakers
rely on computer software and wind tunnels
to ensure vehicles meet their aerodynamic
targets. The main concerns of automotive
aerodynamics beside fuel economy are
reducing drag, reducing wind noise, and
preventing undesired lift forces at high
Race cars aerodynamics
Reducing drag is also a factor in
design, where fuel efficiency is less of a
factor, but where low drag helps a car
achieve a high top speed. In racing cars, a
designer's aim is to increase down-force,
increase grip and allow for greater
There's more to
aerodynamics than just drag -- there are
other factors called lift and down-force,
is the force that opposes the weight of
an object and raises it into the air and
keeps it there.
is the opposite of lift -- the force
that presses an object in the direction of
It is important to minimize
to avoid the car becoming airborne.
Increasing the down-force pushes the car
down onto the race track—allowing higher
cornering speed. It is also important to
maximize aerodynamic stability. For best
cornering and racing performance, as
cars, down-force and stability are crucial
and these cars must attempt to maximize
down-force and maintain stability while
attempting to minimize the overall Cd
F1 cars are built to generate as much
down-force as possible. At the speeds
they're traveling, and with their extremely
these cars actually begin to experience lift
at some speeds -- physics forces them to
take off like an airplane. Obviously, cars
aren't intended to fly through the air, and
if a car goes airborne it could mean a
devastating crash. For this reason,
down-force must be maximized to keep the car
on the ground at high speeds, and this leads
to a high Cd.
For F1 cars
the limitations on ground effects, limited
size of the wings (requiring use at high
angles of attack
to create sufficient down-force), and
created by open wheels lead to a high
(0.7 to 1.1)
One cannot discuss this topic without
mentioning the importance of the spoilers on
cars. Most people unfortunately believe that
the spoiler is there for aesthetic reasons.
Of course in some cases it might improve the
looks but the main purpose of it is to help
increase the down force at high speeds and
hence prevent the car from getting subjected
How Automakers Improve Aerodynamics
As I have mentioned earlier, that
aerodynamicists, designers and engineers use
wind tunnels and computer software
simulations in order to achieve best
aerodynamic results. In essence, a wind
tunnel is a massive tube with fans that
produce airflow over an object inside. From
a room behind the tunnel, engineers study
the way the air interacts with the object.
The car inside never moves, but the fans
create wind at different speeds to simulate
real-world conditions. Sometimes a real car
won't even be used -- designers often rely
on exact scale models of their vehicles to
measure wind resistance. As wind moves over
the car in the tunnel, computers are used to
calculate the drag coefficient (Cd).
Computer simulations are starting to replace
wind tunnels as the best way to measure the
aerodynamics of a car. In many cases, wind
tunnels are mostly just called upon to make
sure the computer simulations are accurate.
While some shapes are inherently more
aerodynamic than others, aerodynamicists and
designers subtly shape every vehicle to
reduce drag. They look at all areas of the
car that come in contact with the air. Upper
surface shape, under floor, wheels, bumpers,
fenders and even cooling and engine bay. In
the end they fine tune the way the air
attaches to the vehicle's surface, and the
way it leaves the rear end.
To reduce Cd, designers may make
the following changes:
Round the edges of the front end.
Tune the grill and fascia openings.
Tune the wheel openings.
Place spats (small spoilers) in front of
the tires to reduce turbulence.
Re -design the size and shape of the
outside mirrors and their attachment
Re-shape the water channel on the
Adjust the front fascia and air dam to
reduce drag under the vehicle.
Add side skirts.
Tune the deck height, length and edge
Install a rear spoiler.
Adjust the angle of the rear window.
Tuck up the exhaust system.
Use diffusers to tune air coming off the
underside and the rear.
Install underbody panels that cover
components and smooth airflow.
Today's cars have a Cd around
0.25-O.35 and designers are trying to
achieve lower figures. Radical concept
vehicles have gotten as low as 0.19. A
reduction in Cd from 0.25 to 0.24
will result in a 4% (0.01/0.25) reduction in
fuel needed to overcome drag. Just by
reshaping the car, aerodynamicists can
reduce fuel consumption by up to 25% without
even modifying any parts of the engine or
its management systems.
Refer to the table below for some comparison
of coefficient of drag figures on few cars.
To conclude this article, we will definitely
see in the future more aerodynamically
shaped cars since fuel consumption is a
leading issue in the automotive field. As
for the futuristic sports cars, attaining
higher speeds will always be on top of the
list for auto designers and engineers, who
will strive more to reach lower drag
Written by: Pascal Hayek