Wind and Automotive Design – 2

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.

7. MG EX 181.jpg

MG EX181

Following the basic knowledge of aerodynamics, I will explain how to design an aerodynamic vehicle by categorizing it into several categories here. First, it is about the design of the automotive body.

1. Aerodynamic Body Styling
Aerodynamics is one of the important considerations in designing the exterior of a car, along with brand image, style, usability, manufacturability and crash safety. The most important role in improving the aerodynamic characteristics of a car is the basic style of the body. Even if equipped with aerodynamic parts such as a spoiler, it is not effective if the basic body style is not aerodynamic enough.

8. front and side wind.jpg

The lowest drag coefficient recorded so far as an automobile-like object is 0.04, but this is not feasible as an automobile because it did not consider the driving device and the driver, so thus it cannot be an automobile. The lowest resistance recorded as a real automobile is known as the 0.12 recorded by the EX 181, a test car produced by British MG in 1957. This is a single-passenger speed record vehicle in the form of a droplet, The actual production and operation of such a specially designed car is subject to many problems such as road conditions, preparation, serviceability, and cost.

ARVW.png

Volkswagen ARVW equipped with turbo charged 6-cylinder diesel engine. 362.07km/hr.

As a result of more substantial research since 1980, aerodynamic test vehicles such as Aerodynamic Research of Volkswagen (ARVW) in Germany and Ford’s Probe IV have recorded a resistance of 0.15. In order to design an exceptionally aerodynamic automobile, it cannot be solved simply by making it streamlined, so it is necessary to study the effects of various kinds of air according to actual driving situation. Let’s look at aerodynamic design for each part of the body.

– Nose
The front part of automobiles is called a nose. The angular nose shape changes the flow of wind rapidly and generates a vortex above the engine compartment and thus a lower, soft, and inclined slant nose is preferred. This is evident in the changes in the design of the BMW passenger car noses. Even with the same slant nose, the effect on air resistance and lift is different depending on the angle.

11. shapes of nose and cd.jpgThis is evident in the changes in the design of the BMW passenger car nose. Even with the same slant nose, the effect on air resistance and lift is different depending on the angle.

Old and new BMW noses.jpg

Changes in nose angles and shapes of old (left) and new BMW 3 series

The shape of the bumper also plays an important role. It is advantageous to have a shape close to a triangle as seen from the side rather than a flat bumper. However, since the function of the bumper is absorbing and spreading the impact as widely as possible, aerodynamics cannot be the main driver for the shape of bumpers.. In addition, the smaller the gap between the bumper and the vehicle body is more advantageous. In recent years, a method of integrating a bumper and a balance panel (a body panel under the bumper, apron apron) is widely used. (At the time of original writing in 1986, it was common to see body and bumper to be made as separate parts).

– Windshield
Windshield literally means a shield against wind, and it is common sense that the angle of inclination of this windshield is closely related to air resistance. Experiments on windshields with various inclination angles have shown that the angle of the windshield is most preferably around 30°.

The curvature of the windshield glass as well as the angle and the shape of the A filler (the pillars on both sides of the windshield) are also important. Particularly, the shape of the cross section of the A-pillar is very closely related to the problem of not only the resistance but also the occurrence of the wind noise (sound that winds when driving). This is because the gaps and steps around the A-pillar and the rain gutter that caused the rainwater to flow, as the groove speeds up, increases the wind noise and generates vortices, which causes dust and rainwater to gather on the windows. to be. Therefore, in recent years, it has become increasingly common to remove the rainwater grooves of the A pillar and to embed the rainwater grooves between the filler and the door so that the rainwater flows down. (At the time of original article, rain gutter, similar to something like a trough at the end of a house roof, was common on automobiles). In some cases like in Toyota Prius, roofs are designed to prevent rainwater from flowing towards the side of vehicle.

prius roof

A roof of Toyota Prius. It is designed to prevent rain water from flowing towards the doors. The doors are higher than the roof, too.

– Backlite (sometimes called a rear window)
Like the importance of the windshield, the angle and curvature of the backlite is also very important. This is because the airflow that has flowed along the surface of the vehicle body is separated from the rear window as described in the previous section of the vortex. The vortex thus created pulls the body back. Therefore, the angle of the rear window affects the air resistance and lift.

12. windshield and rear window angles

Angles of windshield and backlite and Cd.

The visual angle of the rear window is the angle between the horizontal line and the window. The actual angle, which is important for aerodynamics, is the angle formed by the rear window and the modified reference, which is the final angle of the roof. If the rear end of the loop is curved or slanted, the actual angle of inclination is smaller than what is seen with the eye, and is much larger if there is a kick-up (a raised roof to reduce lift) at the rear end of the roof.

For notchback cars, not only the rear window but also the height of the trunk are important and it is generally known that the notchback style is better on Cd and Cl than on the fastback. It’s interesting because fastback looks more aerodynamic than notchback. In the past, some notchbacks were designed with a high deck style to make them more aerodynamic.

Sephia 1 - red rear 16-9.jpg

Kia Sephia 1993 with high deck style

The angle of the rear window affects the adhesion of dust to the rear window due to the generation of vortex as well as air resistance and lift. If the inclination angle is less than 35°, less dust is adhered. If the inclination angle is more than 35°, more dust is adheres, so it is necessary to install the wiper in such case. The curvature of rear window is also important. Rounding the rear window can significantly reduce the resistance because less vortex would be generated.

– Roof
Generally, the surface of the roof itself is not an issue, but its edge is. As mentioned in the rear window section above, the roof edge must be properly designed to make the angle of the rear window reasonable. Adding a rear kick-up would be helpful, too. Kick-up, which is not higher than the height of the roof, can significantly reduce the drag coefficient of a passenger car with a rear window angle of about 30°, and forms a small static vortex to prevent dust from attaching to the rear window.

14. kick up and vortex.jpg

Static vortex can be generated by rear roof kick up

– Body side
Important parts of the side of the body is the window area, the wheel arch, the tumblehome, and the turn under. Tumblehome and turn under are important issues in automotive design because they relate to resistance to side wind, interior space, and window mechanism. In other words, if the curvature is emphasized to increase the stability against the lateral wind, the head room of the occupant is infringed and it becomes difficult to develop such a window mechanism.

Making window area flush is to remove the protrusion of the glass surface and the pillar portion of the window. The flushed window has the advantage of low friction resistance and wind noise, while also smoothing the overall style. The flushed window can be made with rather complicated mechanism that allows the pillar to be placed inside the glass and, in rare cases, a small secondary window is made within a fixed glass.

Although the design of the wheel arch is seem less important, the air flow between the tires rotating at high speed and the inside of the wheel arch is very complex and therefore has a significant impact on drag. Thus, some designs have partially or fully covered wheel arch, and quite often, the blister design is applied to fender to reduce the resistance.

honda insight.jpg

Honda Insight with fully covered rear wheel arch

blister fender.png

Audi Quattro with fender blister (left) and wheel lip (right)

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

[continued on part 3]

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