History of the Drag Coefficients in Cars

The first cars were made before the Wright brother’s era and before the invention of wind tunnel tests. Wind tunnels were extensively used in aircraft design but car designers were slow to notice that aerodynamics effect car’s performance. At the start of 20th century, a number of technical universities were set up in Germany, which put Germany way ahead in aerodynamic research. Zeppelin Airship works was the first company to study automobile in wind tunnels, and that was in 1921.

i: Drag

Drag is the type of friction and it’s a force acting to oppose the relative motion of body in a fluid. It was observed that the drag is the main reason for the inefficiency of car bodies. Drag depends on two things;

• Cd
• Frontal area

and it can be subdivided into two sections:

1. External flow drag(due to car’s body, mirrors and wheels etc)
2. Internal flow drag(due to engine cooling , heating and ventilation)

There are three main causes of external flow drag:

• Pressure drag is caused by pressure between the back and front of the car
• Skin friction drag is due to tendency of fluid to stick to body’s surface
• And Interference drag is due to interference between different components of body.

The drag coefficient is assumed to be constant in specific range of Reynold’s number.

ii. 1890-1920: The Start of Drag Reduction Era

“La Jamais Contente” made by racer Camille Jenatzy, was the first car ever to cross 100 kmph barrier in 1899. It was shaped like a torpedo so the body of car was streamlined but the driver and wheels lost much of the advantage of car’s body shape, nevertheless, this was the first attempt at designing car to be aerodynamically advantageous.

Next came “Alpha Romeo Ricotti”, which was an aerodynamic prototype designed in 1914, it was shaped like tear drop and had impressive high speed of 140 kmph.

As the principals of aerodynamics were becoming available to engineers, they began to design and experiment with cars. The falling drop of water was considered to be aerodynamically perfect, and after First World War many aeronautical engineers from Germany started building cars, primarily because Germany was banned from aircraft manufacturing.

Edmund Rumpler was one such engineer who built “Rumpler Tropfenwagen” in 1922. He was the first person to conduct wind tunnel experiments on cars. this car had drag three times lower than other cars of that day. The most influential designer of this era was Paul Jaray, who was originally a zeppelin airship designer. He designed cars with smooth body surfaces, integrated fenders and headlights, cambered windshields and other innovations. This design was eventually copied by BMW, Audi, VW and others.

“Jaray cars” decreased Cd from prevailing 0.64 to just 0.29. Wind tunnel experiments by Klemperer showed that minimum Cd for half body with wheels close to earth was just 0.15 and so subsequent streamlined cars were designed, keeping half body design in mind. Although the real cars had Cd of 0.4 but this was a good improvement over Cd of 0.7 of earlier cars; this increase in Cd was due to bumpers, windows wheel fairings etc.

Streamline cars were not very popular as they were uncomfortable and not very practical. They had only experimental usage and limited success. The most popular expectation to this rule are “Porche 911 series” along with “VW Beetle”.

Another innovation borrowed from aerodynamic principals was Kammback cars, in which truncated trailing edge design has slightly higher Cd, as compared to the full airfoil design, but this provides practical design and easier production techniques for example Kamm K5 car designed in 1939 had Cd of just 0.37.

iii. 1940-1970: Drag Reduction Era

After the Second World War the trend however was reversed, design of cars moved away from aerodynamic principals as cars in United States concentrated on big spacious bodies as they were more comfortable, resembling bath tubs and cars in Europe although small and boxy because of small roads, both types were equally not aerodynamic and had high Coefficient of drag. The US cars although had bumpers which were streamlined as well as streamlined headlights and wheels which did decrease drag to a certain extent, this was the only development in this era.

iv. 1970-2010: The Modern Drag Reduction Era

This trend however changed after the petrol crisis of 1970 that made oil not as affordable as it used to be hence more efficiency was demanded form the cars. So aerodynamic design started to improve and Cd started to decrease. The drag improvement of 1970’s were based on “detail optimizations” in which they made minor local modifications in order to obtain significant total drag reduction. These details included curving of the edges, camber of panels, and location of spoilers and side mirrors.

The result is that detailed optimized car like “1974 VW Scirocco” had Cd of 0.41 compare to Cd 0.42 for streamlined design of “1969 Opel GT”. From late 1970’s onward the work on aerodynamic shapes was picked up from where it was left in 1930’s so “Shape optimization” was introduced. In this method a low drag conceptual design was taken and in small steps a practical car was developed out of it.

The first car designed employing these methods was “Audi C3 100” which was made in 1983 and had just 0.3 Cd. This car also introduced flush windows which quickly became norm. Recently detail and shape optimizations have been combined and with state of the art CFD methods trial and error methods could be employed in designing, as wind tunnels are no longer necessity. Modern cars has Cd in range of 0.25-0.30 and the future target it to bring it lower than 0.25.