A Japanese automotive OEM requested LMS Engineering Services to investigate the possibilities of optimizing chassis design for impact harshness – the seat accelerations caused by a vehicle crossing obstacles such as expansion joints on the road surface. Simultaneously, the customer wanted to understand how the chassis optimization for impact harshness would affect other performance attributes. As the design of the suspension influences a wide range of vehicle performance attributes, including vehicle dynamics, ride comfort and NVH, this presented quite a challenge to LMS consultants. To achieve a balanced functional performance that meets both handling targets and NVH demands, the consequences of optimizing chassis attributes on the NVH behavior of the vehicle needed to be understood. LMS engineers deployed powerful CAE modeling and analysis methods to succeed at predicting and optimizing chassis performance in the early development phase without compromising NVH requirements.
Multibody modeling as a first step towards harshness simulations
The first step towards impact harshness simulation consists of building a full-vehicle multibody model that includes the following subsystems: front and rear suspension, wheels and tire models, steering system, driveline, powertrain mounting system and car body. The front suspension unit is a McPherson suspension with spring damper system, lower arm, subframe, anti-roll bar, knuckles, drive-shafts and tie-rods. The multilink rear suspension includes all the arms, lower and upper, the spring-damper system, the subframe and the rear anti-roll bar. The steering system model includes the rack and the rack casing, the steering column as well as the steering wheel. The driveline of the vehicle is modeled conceptually using functions that describe the torque that is applied to the drive shafts in order to establish the chosen car speed. The engine mounting system is modeled as a rigid body that is connected to the vehicle chassis using non-linear bushings. The front and rear subframes and the car body can be represented both by means of rigid parts or flexible bodies. Tires were represented by CDTire models – LMS Comfort Durability Tire.
Although most multibody models are based on the assumption that systems are composed of rigid bodies, it is sometimes essential that some carefully chosen components are modeled as
flexible bodies representing the operational elastic deformation of certain components. Accordingly, LMS engineers modeled most subsystems as rigid bodies, including suspension components, based on their inertia properties and center of gravity location. However, the subframe and trimmed body were modeled as flexible bodies by performing a Craig-Bampton modal reduction of the Finite Element (FE) models. The modal base consists of the normal modes in the frequency range of interest. The static modes at the connection points between the body and the suspension are included to compensate for truncation effects. A structural damping ratio was assigned to the flexible bodies.
Picture 1 & 2: Longitudinal and vertical acceleration at seat rail during the front tires obstacle passage at 40 km/h.
Accurately modeling tire-obstacle interaction using CD-Tire
When performing impact harshness simulation, it is essential that the interaction between tires and obstacle is represented accurately. To achieve the required precision, LMS engineers selected the tire/wheel models of the CD-Tire model family. Altogether, CD-Tire features 3 tire models, i.e. CD-Tire 20, 30 and 40, based on a macroscopic physical description of tires. As the specification number of the CD-Tire model goes up, the complexity of the tire model as well as the related computational effort increases. Since CD-Tire models are adaptive – the most appropriate model complexity is automatically selected depending on road surface quality and simulation objectives – additional computational effort required by the more complex models is activated only if necessary. CDTire models can handle digital road surface information in three dimensions: height profile as function of both x and y. To obtain Longitudinal and vertical acceleration at seat rail during the front tires obstacle passage at 40 km/h. the profile of a specific road or track, a scan of the surface is required. Two different types of CD-Tire were used to run the impact harshness simulations. A CD-Tire 20 (rigid ring) model is used in the transient phase of the analysis, all the way up to the obstacle. Then, the more complex CD-Tire 30 (flexible ring) model was selected for the obstacle-crossing phase. The switch between both tire types occurs automatically.
Virtual harshness tests reveal the significant effect of body and subframe flexibility
The road surface model used for impact harshness simulations is relatively straightforward. It consists of a smooth road profile with a cleat of 10 mm high and 50 mm wide. The tires perpendicularly impact the cleat and both front and rear tires respectively cross the cleat simultaneously. The simulation was run at various car speeds ranging from 20 to 70 km/h, accrued stepwise by 10 km/h. To make a distinction between the contribution of the rear and front suspension, engineers carried out separate simulations for the front and rear tire crossing the obstacle.
The effect of the flexibility of the vehicle body depends on both the characteristics of the suspension and the body and must therefore not be overlooked. To understand the effect of the body flexibility, LMS consultants calculated the front and rear subframes and the car body both all rigid and all flexible. Ultimately, front input results show that taking body flexibility into account leads to a significant reduction of the longitudinal acceleration of the driver’s seat rail, whereas the effect on the vertical acceleration is minimal. Rear input results show that body flexibility impacts the vertical acceleration while not affecting the longitudinal acceleration.
From the simulation data, the vertical and longitudinal accelerations on the
driver’s seat rail were selected for monitoring. The peak-peak amplitude of the time history of the responses was plotted at different speeds. The correlation between the calculated and measured front wheel center accelerations was high when the front tires traversed the obstacle. Thus, the validity of the tire model dominating the wheel center response was established. Similarly, the correlation between the calculated and measured driver’s seat rail accelerations was high when the front tires crossed the obstacle. Last but not least, the peak-peak amplitude of the longitudinal and vertical acceleration of the driver’s seat, as predicted by the multibody model, occurs within the dispersion of the test data.
Picture 3 & 4: Longitudinal (top) and vertical (buttom) peak-peak acceleration at seat rail during the front tires obstacle passage at differnt speeds.
Early impact harshness optimization contributes to rationalized development processLMS consultants successfully implemented a CAE based approach for harshness modeling and analysis to impact harshness from early on in the vehicle development process, before prototypes are built. The approach is based on time domain simulations using full vehicle multibody models. Flexible model components like the trimmed body and subframe were modeled as flexible bodies to enhance the accuracy of the multibody model. Comparison of the simulation results and measurement data shows excellent correlation, validating the proposed approach to optimize harshness performance before physical prototypes exist.
Being able to optimize chassis design and understand the effects on other attributes in the development process offers huge advantages to car manufacturers. Not only does it ameliorate the performance of the vehicle, but it also leads to a reduction of the development time and cost. In today’s exceedingly competitive automotive market, a rationalization of the development process is much sought after. LMS Engineering Services helped the customer to optimize chassis development while transferring the know-how and technologies to ensure that the customer benefits from the results in subsequent development projects.
