A Jaguar is known to handle like a dream, and at fast highway speeds, you hear only the quiet purr of the engine – not a glimmer of annoying road noise. Jaguar is one of the few vehicle brands associated with nothing but superlatives -- in this case top-of-the-line comfort and superb drivability. To reach this level of performance, the expert engineering teams from Jaguar and LMS combined a multi-attribute chassis-balancing process with a cutting-edge test-based tire model from LMS International. The multi-attribute methodology helped Jaguar optimize their engineering process and balance road noise requirements on the much-anticipated new XJ without compromising its renowned best- in-class vehicle handling. LMS Engineering consultants provided technology transfer throughout the project so that Jaguar’s experts can continue to deploy the multi-attribute process.
Maintaining Jag’s brand image
Throughout the decades, Jaguar has maintained a clear brand image. Jaguars are known for power, style, quality and – most importantly – driving dynamics and comfort – areas which are the focus for the company’s expert engineers and their leading-edge automotive technology. Case in point is the chassis and suspension design of the XJ series, Jaguar’s flagship full-size sedan. A third-generation XJ was introduced in 2003 with a ground-breaking aluminum body and chassis that minimizes the use of heavier steel parts.
The aluminum design is 60% stiffer and 40% lighter than comparable all- steel vehicles, translating into improved fuel economy and handling. Furthermore, a Computer Active Technology Suspension (CATS) system constantly monitors and tunes the car’s four-wheel independent double- wishbone suspension to match driving conditions, providing a comfortable cruising ride and automatically adjustments for greater maneuvering control and responsiveness.
A redesigned XJ is underway. A key goal of the project is to optimize the design of the chassis to maintain current ride and handling behavior while lowering interior NVH road noise – quite a trick considering the XJ is already extremely quiet. Compounding the engineering challenge, ride and handling requirements often conflict with those related to road noise. Historically, traditional prototype testing performed late in development was used to lower road noise, but noise levels typically were not reduced significantly because engineers had only enough time to evaluate a few alternatives with this time consuming trial-and-error method and had to remain within the boundaries set by the ride and handling department.
Building full vehicle concept models
To ensure an efficient process and successful project outcome, Jaguar opted to work with LMS Engineering Services to perform a systematic multi-attribute optimization of the vehicle chassis. The multi- attribute optimization process required engineers to create full- vehicle concept models to predict different functional performances for road noise and ride and handling. To reach the targeted road noise performance and maintain the handling, engineers optimized and balanced the functional performance quickly and efficiently to reflect the required Jaguar brand values.
The road noise model was a hybrid representation consisting of test- based and simulation-based building-block sub-models assembled in LMS Virtual.Lab Noise and Vibration using a frequency response function
(FRF) based sub-structuring method. The chassis sub-frame (including all the different suspension connections) and the various suspension links were represented as flexible-body finite-element analysis (FEA) models. For computational efficiency, these FEA models were reduced to modal representations. The car body (frame, doors, side panels, etc.) was a test-based representation created from FRF measurements taken by LMS Test.Lab. A revolutionary test-based NVH tire model unique to LMS was derived from FRF experimental data, along with three-degree-of- freedom road displacement measurement data.
New test-based tire model
In order to respond to the automotive industry’s need for accurate representations of road-induced input forces on the vehicle structure – an input that remains the same regardless of suspension or body design configuration – LMS engineers developed a NVH tire model. A physically correct tire representation can achieve accurate predictions of structure-borne road noise in the early vehicle development stages when prototypes are not available for measuring the forces or resulting noise.
Traditional automotive industry methods for handling road noise phenomena involve applying experimental spindle forces or vertical displacements to the tire patch. Each method has limitations with respect to absolute accuracy or input dependency on suspension characteristics. For an accurate design evaluation, an invariant input that can reproduce measured vehicle cabin response is required. This is why LMS decided to develop a new NVH tire model derived from experimental data along with three-degree-of-freedom tire patch input displacements. When coupled with a vehicle model, road noise can be predicted accurately up to 300Hz, enabling virtual testing of the suspension and body design changes.
To achieve the 300Hz upper limit of the target frequency range, the LMS NVH tire model accounts for interior cavity acoustic effects as well as the flexibility effects of the wheel rim. Tire manufacturers typically make modal models available to the vehicle OEMs, but effects such as wheel rim flexibility and cavity acoustic effects are not included. Fully meshed FE models have been created to include these effects, but a high level of accuracy is difficult to achieve especially for an automotive OEM, who typically does not have accurate construction geometry or material properties.
"For these reasons, an experimentally derived NVH tire model is an attractive approach to accurately study structure-borne road noise in the early stages of vehicle development," explained Joris Van Herbruggen, Vehicle Development Manager at LMS Engineering Services.
"When the NVH tire model is coupled with representations for road displacements, sub-frame, body and chassis in a full vehicle model, road noise can be accurately predicted for frequencies up to 300 Hz at the driver seat as well as front and rear passenger locations in the vehicle interior."
Multi-attribute optimization yields first-class results
In a similar fashion, LMS and Jaguar engineers worked together to create a full-vehicle ride and handling model for predicting handling and steering feel during steady-state cornering, step steer inputs, double lane changes and other vehicle maneuvers. LMS Virtual.Lab Optimization correctly adjusted these models using a Design of Experiments (DOE) process to determine various "best of" chassis parameters for both road noise and ride & handling for bushing stiffness, modal frequencies and local stiffness in suspension attachment points on front and rear sub-frames.
"Thousands of different chassis parameter combinations were explored in this multi-attribute optimization process to find the "best- possible" optimized road noise using a target curve while keeping handling performance within specified boundaries. The final result was a set of modified chassis design parameters. The predicted road noise levels were lower by as much as 2dB," stated Joris Van Herbruggen.
"Thanks to the know-how and pragmatic attitude of LMS Engineering Services experts, the technology transfer from this study will enable Jaguar engineers to leverage this leading-edge work in future development projects. We will continue to use LMS solutions to maintain our brand image for the next-generation luxury vehicles that drivers have come to expect from Jaguar." Faruk Turgay, Manager Vehicle NVH at Jaguar Land Rover.
