Wind and Automotive Design – 4

24 opel calibra L

Author’s sketch of Open Calibra (published on a Korean magazine, Auto)

It is essential for automobile designers, especially exterior designers to have the basic knowledge of aerodynamics including the effects of air on automobiles and the aerodynamic design characteristics of the car body. Tt should be taught in “Exterior Design 101” course. In the 1980s and 1990s, the interest in aerodynamics was particularly high, and many cars with aerodynamic characteristics were designed.I wrote in 1988 (when I wrote the original article), “.. many designers are focusing on the design of aerodynamic cars…” I still remember spending quite some time working on the final clay model in order to reduce rear lift in the wind tunnel at MIRA in England, the largest wind tunnel in the world. It is a little disappointing that the attention to aerodynamic characteristics has been lost and designers are focusing heavily on brand and eye-catching designs these days.

In this last issue of Wind and Automotive Design, we will look at some of the cars that have been recorded as remarkable milestones in aerodynamic automotive design.

Examples of aerodynamic vehicle design

There are a lot of examples of aerodynamic automobiles since many designers have been involved in the design of aerodynamic vehicles for quite some time. Even a slightly strange car, the Rumpler Tropfenwagen of 1921, surprised many with astonishing aerodynamic characteristics of a Cd 0.28 as found at a wind tunnel tests. In fact, the German name Tropfen means droplet, which looks like a droplet from the top view, and it was designed with aerodynamics in mind. Even the driver’s seat was in the middle in order to make it in a droplet shape.

1923_Rumpler Tropfenwagen

Rumpler Tropfenwagen (1921)

This time we will look into the case of Audi 100 with a Cd 0.30 for the first time as a mass produced car, Opel Calibra achieved Cd 0.26 for the first time and Ford Probe IV Concept with 0.15 for aerodynamic experimental car.

1.Audi 100: Cd 0.30 for the first time in the production car. 1982.

When Audi 100 was announced to have an excellent aerodynamics with a Cd of 0.30 in 1982, not many believed it. It was because it wasn’t looks as a very aerodynamic car. There was neither air spoiler nor extreme air dam, and any necessary feature as an ordinary passenger car was compromised for aerodynamics. In a nutshell, the car was not showing off its aerodynamic excellence. However, the fact that a mass-produced passenger car achieving a Cd of a 0.30 was similar to when Jim Hines broke a record of 100m splint with a historic run at 9.95 seconds in 1968.

The aerodynamic excellence of the plain-looking Audi 100 can be easily identified by analyzing the design in detail. For easy analysis, the following points are found when compared with the older 1976 version of Audi 100.

Audi 100 old and new.jpg

Older (1976. left) and newer (1982. right)versions of Audi 100

– The entire body has more curved surfaces. The 1976 Audi 100 more angular.
– The engine hood is more slanted, resulting in a lower nose height.
– The angles of the windshield and the rear window was further increased to suppress the occurrence of vortex.
– The radiator grille opening is smaller and larger bumper air intake with easier air flow.
– From the side, the 1976 model looks almost horizontal, while 1982 Audi 100 has slightly higher trunk.
– Lower surface of the body is much smoother.
– Surface of wheel is smoother and flush without extrusions
– The windshield and rear window are completely flush with the bodywork.
– Side windows are completely flush with pillars and window frames.
– The gap between the bumper and the body was minimized as the steel bumper was replaced with a plastic bumper.
– Wheel arch lips are eliminated and smoothened.
– The opening of the rear wheel arch is minimized and thus the amount of air flow into it is significantly reduced
– Concealed wiper

audi 100 in wind tunnel

Audi 100 in wind tunnel

The Cd of the previous model was reduced from 0.45 to 0.30 as a result of the above design improvements. The design of the Audi 100 is to create a great aerodynamic vehicle without having to become less ordinary. In addition, the aerodynamic knowledge gained from the design of the Audi 100 actually influenced the design of other cars.

25-Audi 100.jpg

Author’s sketch of Audi 100 (published on a Korean magazine Auto)

2. Opel Calibra Cd 0.25. 1989

The development of aerodynamic vehicles has been particularly successful in Germany, which seems to be related to the fact that each country focuses on automobiles. Germany is especially well known for making rational and optimized performance cars, so the development of an aerodynamically optimized car would have seemed like a mission to German designers and engineers. Just like many faster splinters emerged once Jim Hines broke the record with 9.95-second run of a 100-meter splint, more aerodynamically improved cars have been developed since Audi 100 set the new record of a Cd 0.30, which was considered impossible for production cars. Opel Calibra (1989-1997), which set a new world record of a Cd 0.25, is the most noticeable one among them.

Opel Calibra wind tunnel

Compared to the Audi 100, which has a generally plain look, the Calibra is a two-door coupe with a sleeker front nose and an angular rear deck, all of which make Calibra more aerodynamic design. But it does not compromise what you need from a daily car.

The main components of the aerodynamic design features of the Calibra are:

– Basic body configuration composed of slant nose, fastback, high deck, angled trunk, etc
– A cover filling the space between the lower body fuel tank and the rear bumper
– Bumpers actively including front and rear air dam
– Front bumper with integrated radiator grille
– Grill and bumper (some models) with flaps to block unnecessary airflow

The technological advancement that allowed computer simulation also contributed to the optimization of the aerodynamics. It must save quite a bit of time and effort that would have been spent on modifying clay models in the wind tunnel.

Opel Calibra air velocity and pressure.png

Computer simulations of air velocity and pressure on exterior surface

3. Ford Probe IV Concept Cd 0.152. 1983

Since 1979, Ford has made considerable progress in aerodynamic automotive design through the development of the Probe Concept series. As the name suggests, the Probe is intended to serve as a “probe” to discover the extreme aerodynamic characteristics among other innovative initiatives and it actually produced many remarkable achievements.

In 1979, the Probe I Concept recorded a Cd of 0.22, and in 1985, merely 6 years after the first generation, the Probe V Concept recorded a Cd of 0.137, equivalent to that of an F-16 fighter. The design of the Probe Concept series also partially influences the design of cars produced under the name Ford Probe (Ford Sierra 1982 was based on Probe III and Ford Probe was partly based on Probe IV and Probe V).

The Ford Probe VI Concept, which was released for the fourth time in the Probe series in 1983, recorded a Cd of 0.152, attracting a lot of attention to the many innovative design and technical elements introduced into the design. Most of them are difficult to apply to realistic production cars of the time, but there are many things that are later introduced into actual production as production technology advances.

22. Ford Probe IV.jpg

Ford Probe IV Concept

The Probe IV has a low and sharp nose, a slanted engine hood, flush windows, front and rear wheel arch covers, and there is no radiator grille at all. The parts that reduce air resistance and lift are as follows.

– Front wheel arch cover: It is used to eliminate the vortex resistance around the tire and in the wheel well. There is a tire housing that pushes the resilient body ‘skins’ when steering.
– Strakes are installed keep air flow away from rolling tires to reduce the resistance around the tires.
– Valley fan: This is the cover of the body developed under the Probe III, which adjusts the flow of air with the function controlled up and down according to the speed.
– Engine cooling system: The air stream sucked from the side of the body turns around the rear of the tail lamp, cool down the radiator, and then exits behind the body. This treatment has an effect of suppressing the occurrence of vortices on the rear surface of the vehicle body.
– Vehicle attitude control system adjusts the ground clearance according to the speed change by using the speed sensitive device. The ground clearance is adjusted to 10cm at 16km/h, 8.5cm at 64km/h, and 5cm at 64km/h to simultaneously reduce the lift and resistance.

Ford Probe I and V 2

Ford Probe I Concept (1979. Left) and Ford Probe V (1985)

The cars introduced in this article were developed more than 30 years ago, and their effort and investment in aerodynamic automobiles have been applied directly and indirectly to today’s automotive design. As Charles Jordan, former vice president of GM design, introduced in Part 1, says, the challenge for designers is to design cars that are both aerodynamically optimistic and beautiful and suitable for the use, using knowledge and data developed through the endeavor thus far.

[end]

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.

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