Mercedes offers aero wheels optimised in the wind tunnel for the EQS SUV in sizes 20 to 22 inches. They feature claddings, so-called "aeroblades". The tyre geometry is also optimised, the sidewalls and the transition to the tread are advantageously designed. We spoke to aerodynamicists Benjamin Arnold, Alexander Gensch and Alexander Wäschle about the importance of the wheels for efficiency.
Why have the wheels become the focus of aerodynamic development?
Wäschle: Actually, the wheels just get in the way of the wind. On an aircraft, they are therefore simply retracted. Of course, this is not possible with passenger cars; an alternative would be the cover. But as long as we show the wheels as a design element, we integrate them as best we can under the wheel arches so that as little air as possible hits them directly. But that is not enough. In production vehicles, about one third of the air resistance is still due to the wheels. It is therefore worthwhile to consistently optimise wheels aerodynamically.
The rims are only part of a complex system?
Arnold: By "wheel" we mean the complete wheel, because there is also great aerodynamic potential in the tyres. The wheel spoilers, for example, help to optimise the wheel incident flow - these are the downward-pointing lips on the wheel arches in front of the wheels. The wheel spoilers are getting more and more sophisticated 3D geometries. The cooling air from the engine compartment of models with combustion engines that flows into the front wheel well plays just as important a role as the lateral flow over the front bumper and over the doors and side member trim on the rear wheels.
And unfortunately for you, there is not just one wheel/tyre combination per model series, but many individual variants for the customers.
Wäschle: Precisely. Wheels not only have to roll, be aerodynamic and quiet, have low rolling resistance and plenty of grip. They are also a piece of jewellery and an individualisation option for our customers. Each wheel geometry makes a different contribution to the total air resistance. This means that not just one wheel has to work aerodynamically optimally in its vehicle environment, but quite different wheel designs in different inch sizes. What is more: With each wheel size comes a variety of tyres from different manufacturers. And each one has a different aerodynamic effect. This means that every tyre and every wheel is relevant to certification and thus has an influence on consumption and range. As an aerodynamicist, you can really spin your wheels!
Which details in wheels and tyres offer the greatest aerodynamic potential?
Arnold: On the tyre side, width is the biggest aerodynamic lever. With decreasing tyre width, significant Cd reductions can be achieved. In addition, further improvements can be achieved with optimally designed tyre contours. In the aerodynamic evaluation of the wheels, we pay particular attention to compliance with the "aero ring", the spoke design and a small opening area. The aero ring defines the outer area of the rim and should be a closed ring of adequate width. And many other aerodynamic levers remain our secret, of course ...
How does aerodynamic wheel/tyre development work at Mercedes-Benz?
Wäschle: We have bundled all our knowledge in a cross-series "cross-sectional function wheels/tyres" within aerodynamics. This allows us to reliably transfer the latest aero know-how to all model series and to be the central point of contact for all development areas involved. We scientifically develop new aero potentials and optimise and automate the aerodynamic evaluation of wheels and tyres.
How close is the cooperation with other areas such as design?
Wäschle: In close cooperation with design and the wheel and tyre sector, we have achieved a great deal from which the electric vehicles on our new platform benefit. With a lot of creativity, our design colleagues implement the developed aero guidelines without letting the appearance suffer. A feat that succeeds with a lot of dialogue in the design phase. With the courage to come up with new constructive ideas, we were able to resolve the conflict of objectives between good aerodynamic performance and the weight of aero wheels together with our specialist department colleagues.
Gensch: For the experimental investigations, near-series test parts must be created at an early stage. The hardware design envisages the construction and production of so-called carrier wheels made of aluminium, which can accommodate as many different wheel designs as possible in the form of inlays or trim pieces from the 3D printer. Our own design engineers accompany the entire process, from the construction and the design of the operational stability to the creation of the hardware. The trim pieces are manufactured using various 3D printing processes. Updates in wheel designs, which can be both aerodynamic and design-motivated, can thus be quickly and precisely prepared for assessment on the 1:1 aerodynamic model.