Wind and Automotive Design – 3

This article was originally published on the September 1986 issue of Monthly Design, a major Korean design magazine.
It was a little over 30 years, so I revised and supplemented the article partially to fit today’s context. I hope this is helpful for those who design automobiles.

2. Aero parts

Aerodynamic parts play important role whether the shape automobile is aerodynamic or not. There are numerous possibilities to improve aerodynamic factors with specially designed part in addition to the main body.

• Air dam
The air dam is typically installed in the lower part of the front and rear bumper of the car body to suppress the vortex which is likely to occur in these areas, thereby reducing resistance and lift. Therefore, the difference between the presence and absence of air dam is significant. However, this air dam does not always effective. In the case of an automobile with an excellent aerodynamic characteristics, it is effective to install a small-sized air dam relatively rearward, and if not, install an inclined large air dam in the front portion. The rear air dam attached to the rear bumper suppresses the sudden rise of the airflow passing through the lower part of the vehicle body and the generation of vortex thereby reducing the lift and resistance and preventing the dirt from adhering to the rear window or tail lamp. is. However, installing it in a vehicle with poor aerodynamic characteristics may result in increased resistance. Also, air dams should be installed at where the vortex not significant. The air dam is also designed to be integrated with the bumper.

concord-air dam large

Author’s sketch of an air dam for Kia Concord front bumper (1988)

• Air spoilers
The air spoiler functions similar to an air dam, but its installation location is different – typically rear end of the roof or trunk. If the connection between roof and backlite is not aerodynamic enough, installation of an air spoiler can be beneficial as it functions similar to a roof kick-up. Air spoiler may be installed on the rear tip of the roof, mainly in the case of one-box or two-box vehicles. This suppresses the lift acting on the vehicle body and suppresses dust adhering on the backlite.

vehicross air spoiler

Air spoiler on a two-box SUV – Isuzu VehiCROSS

In the case of fast-back or notch-back cars, it is common to install the rear spoiler at the edge of the trunk. When the rear spoiler is installed, the speed of the airflow over the vehicle body is reduced thereby increasing the pressure acting on the rear portion of the vehicle body. This increased pressure acts as negative (-) resistance and negative (-) lift, reducing resistance and lift simultaneously. However, if the spoiler is too large, it can increase the air and ground resistance, so it is important to select the right shape and size or air spoiler to get the right amount of effect to reduce resistance and lift.

 

15. effect of rear spoiler.jpg

Effect of air spoiler on fast back sedan

• Strakes

Tires on a moving vehicle are a problem in aerodynamic aspects because they generate significant vortex. The strakes function to prevent airflow from flowing into the tires as much as possible. That is, the convex shape is mounted on the front and rear of the tire to adjust the air flow. This can reduce the air resistance by about 5%. However, in real design, it is solved to the extent that it forms a rocker panel (a panel beneath the door) that is not too big.

17. strake E.jpg

Strakes on Ford Probe IV concept

ford probe strake

Strakes on Ford Probe production

• Wheel arch cover

As described in the previous article, covering the wheel arch helps controlling the air flow around the tire. This can reduce the vortex resistance between the wheel arch and the tire. However, it is not widely used because of the complexity of design compared to its effect.

icon_8.jpg

Author’s design of taxi with covered rear wheel arch (1993)

• Wind deflector

The large freight cars experience significant amount of vortex due to their box-shaped bodies and the space between tractor and trailer, and thus the air resistance becomes serious and causes bad gas mileage. Wind deflectors are designed to improve this by allow the airflow over the body to flow smoothly into the container or trailer. This deflector has become common after many years of experimentation and has been made in various designs and structures.

truck aerodynamics.jpg

A truck installed with a wind deflector designed to match the cabin

3. Design of underbody

The drag of air flowing through the underbody of an automobile is greatly increased as it passes through a complex chassis parts such as an engine, an axle, and a suspension. Therefore, simplification and smoothing of the underbody has been examined. Making the underbody smoother can greatly reduce the occurrence of vortex, thus reducing resistance and lift. However, this is not only difficult to construct, but also because it is cost-prohibitive. Therefore, in reality, only a part of design or a degree of protruding portion as much as possible is applied to a high-end sports car.

19. flattened under body PROBE

Ford Probe III concept with flattened underbody design

Hip-up design, slowly raising the rear part of the underbody, is also useful to lower the lift force of the rear part of the body. The inclination angle for a hip-up is preferably about 3 degrees. However, this method also has different effects depending on each automobile design.

4. Control of airflow within vehicles

The airflow that influences the aerodynamic characteristics of a car can be divided into two – an air stream flowing outside the vehicle body and an air stream passing through the inside the vehicle body. Aerodynamics often refers to the case related to external air flow, but in the latter case it also accounts for 12% of the total resistance and should not be ignored.

There are two kinds of air flowing inside the vehicle body – an air flow through engine room for cooling and combustion and an air flow for indoor ventilation. The resistance of the airflow flowing through the engine room is about 6% of the total resistance, which is significant. Sometimes excessive amount of air stream enters into the engine compartment, which should be and can be controlled to prevent unwanted air from entering, which can significantly reduce the drag. Designing the structure of the radiator grille rationally can be helpful. Some uniquely designed grilles let air flows in at the low speed such as when climbing the hill and minimize air when at high speed. There is also a grille that can automatically adjust the amount of closure according to the temperature of engine.

The shape and location of the radiator grille is also related to the generation of resistance. While generally simple forms are aerodynamically advantageous. Ventilation performance can be improved when the air intake air flow and the air outlets are well positioned. The air inlet is should be located where the air pressure is positive (+) and the outlet should be at where the negative (-) air pressure is.

5. Surface of the body

Of course vehicles with smooth body surface have better aerodynamic performance than those with rough surface. The resistance caused by friction between the surface and the air is about 9% of the total resistance, which is larger than what we would. No designer would intentionally design body with rough surfaces. Designers need to focus on the details such as the steps between the window and the window frame, the gaps between exterior components for example. A window frame with no protrusion is called a flushed window as found on faxed glasses on highway buses. Flushed windows can significantly reduce the resistance and wind noises. The disadvantage of the flushed window, however, is that its structural design is quite difficult if the window needs to be regulated.

Along with this, it has become common to lower the resistance by adhering the windshield glass and backlite directly to the body without using rubber molding for many years. The spacing between the bumper and the fender has also significantly reduced due to advances in production technology, which has greatly contributed to aerodynamics. For example, the distance between the bumper and the fender was 10mm in the late 1980s (Kia Pride), 4, 5mm (Kia Sephia) in the early 1990s, and 0 and 1mm in the late 1990s and became a norm for most vehicles today.

6. Other
In addition to the above, there are mirrors, lamps, wipers, and antennas that are related to the aerodynamic performance of the car. The mirror is essential for driving, but it affects various aerodynamic characteristics depending on its shape and location. Mirrors mounted on both sides of the door cause a wind noise. Also, the shape of the mirror is gradually streamlined considering the resistance of the air, and the streamlined mirror can reduce the resistance by about 50% compared with the flat one. One of the disadvantages of the door mirror (as opposed to fender mirrors, which is popular in Japan especially on taxies) is that it tends to generate a vortex which makes dust adhere to the window. To prevent this, a design of passing the wind between the base of the mirror and the window is often used.

20. door mirror

The surface of the tail lamp may collect a lot of dirt due to the generation of vortex, so sometimes it is not visible even when the lamp is turned on. This phenomenon is prominent where it gets foggy wet often. The tail lamps older Mercedes models had a recessed groove on the surface to avoid dirt in the recessed part of the lamp. The design of these lamps has been changed to a sleek design, which is very disappointing because of its appearance and its abandonment of functions.

old and new benz tail lamps.jpg

Old Mercedes Benz tail lamp with grooves (left) compared to recent one with smooth surface

Since the windshield wipers are located where the air pressure is negative (-), they sometimes floats up when traveling at high speed, and it may cause a wind noise. In order to prevent this, the wiper may be designed as a concealed wiper that hides into the body. Most cars designed these days have either concealed or semi-concealed wipers.

Note: Some of the illustrations are from the Aerodynamics special issue of Car Styling

[continued on part 4]

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