Slippery shapes

There have been plenty of very slippery Audi designs over the years – from the Type C Streamliner to the NSU Ro 80, Audi 100 (C3) and more recently the Audi e-tron GT. The wind tunnel has often been used to produce body styles that slip as easily through the air as possible, but with the new A6 Sportback e-tron, revealed to the world just weeks ago – Audi’s design and aerodynamics teams have achieved a new benchmark.

With its drag coefficient (Cd) of just 0.21, the A6 Sportback e-tron becomes the most aerodynamic design in the company’s history. The Avant version of the all-new model series is also a slippery customer with its Cd of 0.24, but it’s the Sportback that takes the top honours when it comes to aerodynamic superiority.

“From the very start of the project, we placed a high value on efficiency and range and pursued very ambitious goals, regarding the A6 e-tron,” says aerodynamics developer, Andreas Lauterbach.

“From the beginning of the project, the designers shared their drafts with us so that we could provide initial aerodynamic evaluations. In an iterative process – firstly using virtual simulations and later the physical model in the wind tunnel – we went on to optimise the basic vehicle body. In particular, the basic proportions with the slim greenhouse and the sloping roofline contribute to good aerodynamics.”

Fellow aerodynamics developer, Matteo Ghelfi, says that the process was exhaustive, with extensive work conducted both on computer simulations as well as in the wind tunnel to achieve the extraordinary result.

“In all, we ran over 1,300 simulations on the car and spent countless hours in the wind tunnel and in meetings collaborating with surface experts and designers,” Ghelfi says.

“Looking at the car overall, neither side had to make any major compromises. When all was said and done, and I called my colleague in Design to tell him that together we had achieved the Cd value of 0.21 for the A6 Sportback e-tron, he could hardly believe it,” recounts Lauterbach.

Every conceivable area that could be refined was scrutinised and adjusted to achieve the result. The switchblade cool-air intake under the Singleframe grille alone, which helps the air to flow around this area with few losses, realised a Cd advantage of 0.012, which is equivalent to about 12 kilometres of range in real terms.

“The underfloor also plays a significant role in a car’s aerodynamic performance,” says Lauterbach. “On the A6 Sportback e-tron, we added radii, optimised stiffening ribs and breakaway edges at critical points. The rear diffuser is another element crucial to aerodynamics –  due to the smooth underfloor, this part is exposed to direct airflow and pressure recovery has a positive effect on the Cd value.”

The differing body shapes of the two cars obviously required different treatments for best results and different solutions to overcome their unique respective challenges. 

“The Avant has an additional diffuser spoiler that compensates for the fundamental difference in aerodynamics between the Sportback and Avant silhouettes,” says Matteo Ghelfi.

“This means that the underfloor airflow differs for the two A6 e-tron derivatives … this is another reason the Avant uses wider 3D bumpers to improve airflow around the front wheels.”

Even the different wheel options available within the range came in for attention to see that they too contributed to the slippery shape.

“Formerly, wheels basically only had to meet stability requirements,” says Rim designer Andreas Valencia Pollex.

“Today, we develop and design intelligent aero wheels that are as efficient as possible because the wheels, and even the tyres, have a major impact on the range of an electric car.”

“To achieve perfect aerodynamics, the rims had to be somewhat flat so that the air that hits the front of the car is directed sideways around the body without much turbulence. We want the wind to flow along a wall rather than a collection of geometric shapes. So, we also developed a 21-inch wheel with aero blades made from special plastic for the A6 e-tron.”

It was a painstaking process, but the results speak for themselves with sleek shapes that are at once visually appealing and in keeping with the Audi design language, but also serve the practical application of contributing to the impressive range of these new Audi models.

“To be honest, in the beginning, we weren’t sure if we would be able to achieve the values we were aiming for,” says Lauterbach. “Getting to that last thousandth of a Cd value is the hardest part, but in the end, we exceeded our goals.”