The air hasn’t changed in the 100-plus years since the beginning of the automotive era. Well, okay, it now smells generally less like a typhoid ward or horse manure. But air acts as it always has when you jam an object through it. Ever notice that the vehicles intended to skirt the air’s resistance share some fundamental traits? A low, rounded front end. A plunging roofline that leads to a taffy-pulled, tapered rear end that ends abruptly. And, of course, ugly wheels, either on display or covered by spats. That’s the work of generations of aerodynamicists toiling over equations and wind-tunnel experimentation. It’s work that was gleefully interrupted beginning in the second half of the 20th century. This era, the tail end of which we are in now, is defined by powerful internal-combustion engines fed by cheap fuel available at nearly every street corner. So put away your weird prewar Tatras and streamliners; we’re going to drive this damn barn (’58 Imperial, Hummer H2, etc.) defiantly through the air.
This story originally appeared in Volume 14 of Road & Track.
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Mercedes-Benz, which has produced its own stately but wasteful machines over the years, has a different view of the future. Called the Vision EQXX, it’s an electric four-door-sedan concept that employs a radically efficient aerodynamic form. In recognition that the energy of current electric batteries quickly depletes and takes a long time to replenish, there’s no room to sacrifice slickness to indulge style. With a highly efficient drivetrain and a light curb weight (for an EV), the EQXX managed a trip from Stuttgart, Germany, to Silverstone, England, fully 747 miles, on a single charge. Its absurdly low 0.17 drag coefficient was critical in making that happen.
We asked longtime Mercedes aerodynamicist Alexander Wäschle to walk us through the EQXX’s more notable aero achievements.
“The basic shape of the car is very important. The smooth, round front and greenhouse are particularly important on the Vision EQXX, as is the long, smooth boatlike taper at the rear. To reach such a low drag coefficient, you have to optimize each part and each contour around the complete car.” Rear-seat passengers will have to be short to clear the plunging roofline.
“At Mercedes-Benz, designers and aerodynamicists have been working closely together for decades. We know one another personally and have a deep understanding of the goals of our counterparts. While designers think in terms of proportions and aesthetics, we as aerodynamicists work with technology and functionality. We are constantly looking for compromises or completely new solutions. One good example of that is the active rear diffuser, which extends rearward by [7.8 inches] at speed. It delivered significant progress for aerodynamics—an improvement of 10 aerodynamic points [or Cd 0.001]—while giving the designers considerably more design freedom at
the rear end.”
“If you look at cars with a drag coefficient of 0.17 (like the W125 Rekordwagen from 1937 or the C 111-IV), they have one thing in common: They cover the rear wheels to create more efficient aerodynamics. To be honest, it doesn’t look elegant, and that’s why the designers were not happy with it. Another important aspect is the track width. Designers love to create a good stance, which means the tail should be broader than the front. As aerodynamicists, we prefer it the other way. The compromise: Design accepted a [two-inch] narrower rear track, and we sacrificed the spats. To be honest, when looking at the Vision EQXX: It was the right decision, even if the drag coefficient could have been better with spats.”
“The sharp-edged rear end is similar in concept to a Kamm tail, but it’s CFD [computational fluid dynamics] optimized for the complex three-dimensional wake behind the car and defined by the length of the car as well as by boundary conditions of the styling. Its interaction with the active diffuser also has a big impact. All of this minimizes the ring vortex formed in the car’s wake.”
“Specially designed tires from Bridgestone and smooth-faced wheels are mounted completely flush with the body to reduce drag.”
“When looking at every detail, we obviously talked about using cameras instead of side mirrors. In the end, it was a trade-off between drag and energy efficiency. As mirror cameras require dedicated screens that consume energy, the aero-optimized mirrors won. To reduce the frontal area, we shaped a double mirror base.” Typically, side mirrors add between two and eight percent to the overall drag of a design. The EQXX’s are below two percent.
“We are learning a great deal from the technology program of which the Vision EQXX is part. This applies not just to specific aerodynamic measures but also to the digital development processes we used. Aspects of this are already flowing into our development processes for future production models.”
“Wheels play a big role in generating aerodynamic drag. Therefore, a lot of different measures are leading the air smoothly along the wheels to reduce aerodynamic drag. It starts with the air curtain in front of the front wheel, which leads the air coming from the front bumper directly to the aero-shaped rim covers. Behind the front wheel, the air breather collects the air from the wheel well again and carries
it without separation along the front door.”
“A cooling plate installed in the vehicle floor keeps the electric drivetrain cool under normal driving conditions without any aerodynamic disadvantages. When more cooling is needed, ducts at the leading edge of the nose open and allow air to pass through a radiator and out through flow-optimized ventilation openings in the hood. This arrangement significantly reduces cooling drag.”
Daniel Pund took the Executive Editor role at Road & Track in 2020 to help re-invent the venerable magazine brand. For nearly 30 years, Pund has toiled away as a feature writer, car reviewer, editor, and columnist for every car magazine that matters (including Car and Driver and Autoweek) and a few that didn’t. He’s also contributed to Esquire and GQ and other general-interest publications.
