Audi examines link between body flexibility and handling

  • 11-Sep-2012 05:02 EDT

The stiffness of the bushings on the chassis is always a conflict between NVH and handling. The overall goal of the project at Audi was a new target parameter for chassis stiffness. The LMS body flexibility method is a global analysis, but it lets engineers hone into key areas to determine where a change is really important for the overall vehicle performance and where it is not.

Ask any NVH engineering expert what their toughest technical challenge is today and you will likely hear about finding the balance between realistic target settings and having the right type of validated data to know where early-stage parameter changes will affect the final design or mission-critical brand attributes and crucial performance targets such as weight, emission levels, and fuel economy.

Like other automotive OEMs, the NVH engineers at Audi use benchmark analysis to set design targets and dig through the data to find potential areas of improvement, which can be further investigated using simulation techniques. In regard to body flexibility, its influence on comfort and vibrations has been part of the development process for quite some time, but body flexibility’s influence on handling was apparent but had never been clearly analyzed precisely.

“We wanted to take one more parameter into consideration in the development of the entire car to improve our knowledge regarding the exact influence of the body on handling, so that we can improve our functional targets catalogue,” said Antoine Guellec, the project leader and an acoustic expert at Audi.

“This approach really fit with our idea of the right way of developing vehicle proper ties. We found that the LMS body flexibility solution and especially the way they determined and analyzed the best possible body load measurements is very advanced and matched how Audi thinks,” said Guellec.

“Our job as engineers at Audi is to create that premium sporty car with superior ride and handling that Audi is known for,” said Dr. Eckhard Plank, who served as NVH mentor to Guellec. “We hope that this project will open the door to other departments regarding the advantages of a hybrid development approach. Ideally, we would like one whole process where you could see the influences of changing a single parameter on the overall vehicle performance in real-life situations.”

The LMS body flexibility technology focuses on strain gauge instrumentation to identify the chassis to body interface loads—a testing technique coming from the world of durability and transfer path analysis. This project is the first time that the chassis was integrated into a study in such detail. The specific setup used 90 strain gauges, three LMS SCADAS Mobile systems, and a sensitized steering wheel to gather the first sets of data on the Audi proving ground close to Ingolstadt and the famous Nürburgring.

“Besides the fact that this was the first time we integrated the chassis on this scale, the toughest part of the job is knowing exactly where to place the strain gauges to get the right type of data. We worked in LMS Virtual.Lab to make sure that we selected the right instrumentation locations, using the trimmed body FE model. This allowed us to precheck our test setup way before we hooked up all those strain gauges to the Audi and hit the tracks in Germany,” said Theo Geluk, project leader from LMS Engineering Services.

After collecting the operational load data on the test tracks in Germany, the Audi was disassembled and prepared for further study. Theo Geluk and his colleagues spent several weeks analyzing the data.

“Basically we determined what were the forces and what were the deformations. With our process, the key advantage is that you can split up the resulting deformations into each individual mode’s contribution. Not only into global mode contributions like lateral bending or torsion, but also the local flexibility contribution is identified. Using our body deformation visualization tool, you can evaluate the forces at each interface node together with the actual deformations of both the interface node and the full body,” said Geluk.

“Calculating body deformation in the time domain—selecting the right data segment of a specific maneuver and comparing a base vehicle to a modified vehicle—is a unique LMS technique, which permits customers like Audi see the real impact of modifications in terms of a better balance between vibrations and handling.”

“This project really showed Audi that it is possible to implement handling and body flexibility requirements early in the CAE process. In theory, we’ll be able to look at data from all of the critical departments from handling, ride and comfort, and acoustics and determine the optimal local and global stiffness requirements – early in the vehicle concept phase – while we are still using the FE model. With this process, we would engineer a more robust body structure practically in the earliest project stage, so we can match the best balance of comfort and handling in a later testing phase while optimizing the chassis settings,” said Guellec.

“Integrating this type of solution in our CAE process is incredibly important in this day and age where we all need to reduce cost.”

Jennifer Schlegel, Senior Editor, LMS International, Leuven, Belgium, wrote this article for AEI.

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