In 1957, Lotus was launching their Elite model, a beautiful light little sports car for which the coefficient of drag (CD) was 0.29, following in-depth wind tunnel testing and aerodynamic concerns and developments. Let’s mention that no CFD (Computational Fluid Dynamics) program or computer were available then as it is in use today. This recent development tool is the source of major detail clean-up, allowing for usual looking cars to be more aerodynamic (less drag).
A CD of 0.29 was a real feast for any production type vehicle, as cars were more in the high 0.40 range at that time. In fact, it took probably more than 3 decades for any high production car to reach this level of low drag.
I remember study cars in the 70ies. Most manufacturers had their aero development programs, reaching 0.19 of CD or less, and not only Europeans, as Ford was very active in aero researches showcasing with their “Probe” prototypes. The model number 5 was reaching 0.13 of CD, which was among the very lowest of any concept vehicles.
It was common to read a CD number from a car manufacturer as an argument of performance in the 90ies. Although, no national testing norms were established, opening the door to abuses from some manufacturers, publishing unrealistic figures generally from scaled wind tunnel models generating less drag with no internal flow or cooling, etc…
The publication of the CD figure of new models almost completely disappeared at the same time car constructors were switching from aerodynamic efficiency concern to heavy marketing concerns, creating “things”, consumer objects, targeting (or sometime creating) markets to sell to defined and redefined population segments.
Today, we see in the streets cars and SUV designed 4 to 5 years ago looking like army vehicles, designed to take advantage of the war/defense concern among a segment of the population. Aerodynamic attention got blown in the wing, if we can say so, as various marketing schemes of triggering appeals through tricks of “crossing over”, various type vehicles into adult toys, induced people to buy, even, and most of the time, above their financial means.
For some good reasons, EPA has changed their highway consumption rating since 2008 (this is about the official and famous MPG). The tests are now made at 80 mph instead of 60, which suddenly push car manufacturers to reconsider again aerodynamics as a more important subject, although timidly for some of them.
To understand this, let’s keep in mind that the aerodynamic drag generated by a vehicle increase at the mathematical square of the speed. As an example, if a vehicle generates 100 lbs of drag at 60 mph, it will become 178 lbs at 80 mph. 150 lbs of drag at 60 mph will become 266 at 80 mph and a lower starting figure such as 75 lbs at 60 mph will become 133 at 80 mph.
Let’s also remember that less drag a vehicle has, less contribution to the fuel consumption it will have. The CD (drag coefficient or coefficient of drag) is great to know and I am looking forward to see the CD being published again, providing it will be regulated by the EPA (Environmental Protection Agency) or norms established by SAE (Society of Automotive Engineers).
Nevertheless, we have to be aware that the CD is a dimensionless number that need to be scaled. The next important factor to keep in mind and ultimately to keep track of, is the frontal area or the surface of the vehicle seen at the front as a flat panel penetrating the air. The frontal area is measured from the ground (not from the bottom of the chassis), up to the roof and including every details that can be seen if one is looking at the front of a vehicle from let say 20 yards distance.
The Coefficient of Drag Area as it is called is the CD multiplied by the Frontal Area and represents the factor defining the quantity of drag that is expressed in Force at a defined speed and air density. This is the real number called the aerodynamic drag, making a vehicle consuming more gas faster it goes, or bigger the vehicle is, such as an SUV compare to a car, or less aerodynamically efficient, such as a truck generating major turbulences at the back.
One of the very best SUV today is reaching a CD of 0.35, in comparison to the best CD for a car at the present time, which would be 0.25 for the Mercedes E-Class 2009, the Toyota Prius 2010 and let’s note older productions such as the Audi A2 (unfortunately, only European market) from 2002, as well as the little coupe Honda Insight from 1999.
Nevertheless, let’s not forget that we have to multiply the CD by the frontal area (A) of these vehicles to find a realistic drag factor. An SUV can be from a quarter to twice the size of a car. A CD of 0.35, best for an SUV, is low average for a car, meaning that an SUV is, hands down, a “drag queen”, burning more fuel for no other reason than marketing push.
Let’s look at an example with published numbers. The 2008 Mercedes E-Class had a CD of 0.27 and a frontal area (A) of 2.11 square meters. The effective drag factor (CD x A) was 0.57
The new 2009 Mercedes E-Class has a lower CD of 0.25, which is very good, although, for some reason, the frontal area is now 2.32 square meters, which makes the CD x A climbing to 0.58.
I know, it is not a lot of increase but why creating such a good aerodynamic improvement if it is to erase this advantage by re-designing a bigger car that has a higher effective drag factor?
Mercedes admit to be able to reduce easily the CD to 0.20 and today, with more sophisticated wind tunnels and CFD programs, one could be reaching levels that the study vehicles of the 70ies were attaining, but this time for real. Citroen launched their DS19 in 1955. It was a futuristic odd looking car at that time, although the aerodynamic efficiency was way up (CD of 0.31) and the car became a commercial success above prediction. Many other examples are available to prove that a well designed car is commercially viable. It does not take to mold some population taste or trend into what a vehicle need to look like to sell. Efficiency, through the ages and various industries, has always paid dividends.
The new Ford Taurus has a CD of 0.32, which is not that great these days, nevertheless represent an improvement on the previous model produced until 07. Its CD was 0.34.
No frontal area numbers have been found for either models but it is obvious that, as the new production is based on the defunct 500 car, even with lowered roof line, the surface area is bigger, as the 500 was designed for “high chairs”, kind of looking like a mixture of car and some of the appeal of SUV with which drivers and passengers can still look down to other drivers from a few inches above. The effective drag factor must be higher on the new car as well, which, in this case, is not creating the most efficient product.
Beside the use of big engines, high frontal area vehicles and poor coefficient of drag has distinguished this Country from the rest of the world and created the most gas guzzling nation. We burn more fuel per day than the rest of the world put together (read again, it is true!), meaning more than the other 6.4 billions people from which some are driving vehicles.
Many subjects can contribute to reducing oil consumption, as we will review them in time. Nevertheless, aerodynamics is of great importance and need to be treated again seriously, as vehicles need to become highly efficient to regain their status of automobile and shed off what commercial hypes have made of them: objects that happen to be vehicles and for which we pay dearly the consequences in many ways.