The new Audi A8, BMW 7 Series, and Porsche Panamera have an underbody with an extreme focus on acoustics and aerodynamics. Röchling Automotive, their engineering partner and supplier, helped develop their lightweight chassis construction with aerodynamic underbodies—brand builders especially for premium cars.
The advantages of underbody covers of the Röchling type are broadly acknowledged in the engineering community for aerodynamics, acoustic absorption, underbody protection, thermal management, and recycling potential superior to PVC underbody coatings. The trend toward covering the underbody is literally covering the segment full scale, yet there is still a huge diversity regarding materials and processes. A weight comparison reveals the differentiation most significantly.
The complete Seeberlite underbody of a current 7 Series has a mass of 5 kg (11 lb), some 39% less than that of the outgoing model or 21-37% less than its closest competitors. “Which is even more astonishing, if you take into account the competitive or even considerably better acoustics,” emphasized Dr. Klaus Pfaffelhuber, Director of Predevelopment and Acoustics at Röchling Automotive. “Regarding idle-exterior, drive-by, or interior noise, we are always ranking among the top or are even number one.”
Acoustic comfort is a key feature of the material. The process parameters of Softlofting allow for a remarkably good fine-tuning of the material’s acoustic properties. The main control options within the process are temperature and pressure. Crucial are migrating clipping levels and smooth material-protecting transformations, hence the process’ Softlofting name.
“Further tuning options are offered by the parameters of the raw material: fiber length, mix ratio, additives. The press itself must be capable of soft motion control. Even if it looks pretty simple: Softlofting truly is not trivial,” said Ludwig Huber, Chief Technology Officer of Röchling Automotive. “It comprises much fine-tuning. The real creation of value happens totally invisibly from the outside within the closed tool on a chemical level.”
Basically, Seeberlite is a sandwich structure with core and coating differentiated by distinguished densities but without a tight outer skin. Thus, the components represent an acoustic, active-absorbing surface on the road side, as well as on the chassis side, but without additional absorbers as usually used up to now for densified substrates.
In comparison with such a part split into two layers, the new engine cover boasts twice the equivalent absorbing surface—at a significantly lower overall package height and with a single-resin sandwich.
Progress on the acoustics front is even more evident when benchmarking against former parts without the additional absorber, for example, on each side of the exhaust. In that area, the equivalent absorbing surface of Seeberlite based on test rig measurements is four to five times greater.
“Thanks to this multifunctional and efficient approach, acoustic comfort can contribute to lightweight construction,“ said Pfaffelhuber. “Overall this leads to a considerable reduction of outer and inner noise, particularly of the high-frequency noise of diesel and gasoline direct-injection engines, further of the very disturbing tire and road noise.”
But what about mechanical and thermal resilience considering the low weight? Stone chip testing is performed according to specification with a shooting machine under a 30-45° angle. There is no piercing, even by steel pellets. The abrasion test treats the underbody to a granite rock scrubbing, 10,000 times to and fro. Again, the material was only scratched on the outer skin.
Among the most demanding kinds of mechanical stresses is a flush-water impact when crossing a big puddle or spray water on wet roads. Carwash compatibility was routinely confirmed.
“With Seeberlite, we even stand the steam cleaner test without problems,” said Dr. Egon Moos, Product Manager of Underbody Systems at Röchling Automotive. On the other hand, the material looks quite open-pored. Moos explained: “An essential feature of Seeberlite is to dry very quickly. This was proven by comprehensive tests under real-life conditions.”
Currently, the underbody accounts for about 30% of the aerodynamic optimization potential. Wind forces occurring at high speeds stress the cover’s mounts with several hundred Newtons. In these areas, the material is partially densified so it can withstand pull-out forces better.
The densifying is done only in spots, with dimensions limited to the crucial areas. This is one of the main advantages of the material and process. As in tailored blanks for steel chassis construction, the enforced segments of the Röchling underbody are only where necessary. On special shaker test rigs, the vibrations are scanned by laser and visualized by computer animation so stiffness can be optimized further or verified.
“High stiffness also facilitates directly integrating functional parts like air nozzles,” said Huber. “We realize the inlet ramps for the cooling airstream by, respectively, transformed and densified underbody sections—fully integrated, without additional components. This, once again, eliminates a multipiece module and an assembly process.” This focused cooling approach improves the “thermal operation reliability or durability” of the whole car.
Despite being open-pored and the large dimensions, even strict emissions specifications are complied with. The hydrocarbon figure, which is relevant in California, for example, is 6 mg for the total underbody. This is still true at a temperature stress of 120ºC (248ºF), for which the material is designed. So, even the exhaust pipe near the engine can be covered widely if a heat-shield layer is applied.
Samuel Kiefer, Röchling Automotive, wrote this article for Automotive Engineering.