With more than 100 years of success in designing clutches and shock absorbers for a variety of OEMs, the ZF Sachs team is highly adept at handling development challenges facing suppliers. Component and full-vehicle development issues at ZF Sachs are examined and solved with careful attention to NVH and with the support of LMS products and services.
Increasing demands for higher levels of comfort mean manufacturers must demonstrate the continuous improvement of their vehicles’ acoustic features in order to remain competitive. Because engine and power train noise have been greatly reduced in recent years, the dominant source of noise in vehicles is now suspension-related road noise. The trend toward lightweight construction means that this problem will only become more pressing in the future, so good damper designs are of primary importance.
According to Dr. Alexander Kruse of the ZF Sachs NVH department, noise and vibration are critical to a customer’s overall positive or negative perception of passenger cars. In light of the importance of interior noise to the customer, and of the importance of shock absorbers to low interior noise, his department contributes expertise throughout the product development cycle. The team tests, models and refines the noise and vibration quality of their components in the context of the complete vehicle from the time the initial calculations and simulations of shock absorber valve parts are made to the point when the OEM test-drives the vehicle.
Subjective evaluation of interior noise
Shock absorbers, the part of the vehicle subject to the greatest stress, form a transfer path for various chassis noises (primarily tire noise). The shock absorbers also convert low-frequency road excitation into high-frequency excitation in the vehicle body, which is perceived by passengers as a noise disturbance. In the damper, additional noise and vibration are created from the dynamic force and collisions of the valve components, the hydraulic flows, cavitation and foaming effects as well as friction. The sound and vibration created by all of these forces, and transferred to the passenger compartment, can be characterized by the subjective terms “chuckle noise” and “swish noise”.
An OEM does not always set quantitative targets for the subjective perception of interior noise. The effect of the damper on interior noise is evaluated subjectively by the customer, so the NVH department’s goal is to achieve a minimum of unpleasant or unwanted noise, coming from the shock absorber.
Measuring noise and vibrationThe better the understanding of a shock absorber’s dynamics and interaction with the vehicle, the more effective the optimization and the lower the noise. By using NVH analysis to gain insight into the dynamics of the structures, and to guide design modifications, ZF Sachs can optimize the performance of the damper, lower the level of noise in the passenger compartment and improve the overall performance of the vehicle. Dr. Kruse works with measurements of the damper’s displacement on the lower mount and of the behavior of its valve, piston and the upper mount. He measures the forces acting on the damper, its structure, and the points of noise and vibration transfer from the damper to the vehicle.
Testing is done in a strict order
Evaluation of the car is always conducted on the road to determine the exact driving and road conditions that cause a problem and the frequency range of that problem. A binaural head with microphones picks up the sound levels in the compartment as the driver and passenger would perceive them. Road conditions are then reproduced on the 4-poster rig where more detailed measurements can be done to find how this problem is perceived inside the car. The noise is finally related to a shock absorber parameter, for example, the position of the shock absorber.
Optimization tacticsThe actual optimization of the shock absorber is done on the hydropulse rig. ZF Sachs employs three separate and tightly targeted design tactics: optimize the damping force to reduce the force excitations; optimize its dynamic structural properties to reduce the damper’s vibrational response; and optimize the damper mount to reduce the noise transmission to the passenger compartment.
Follow-up measurements of the interior noise levels confirm that these and other tactics reduce noise significantly. In one instance, the structural analysis of the strut module showed that the frame was too rigid, so ZF Sachs relaxed the stiffness to reduce vibration. The comparison of measurements taken before and after the optimization of the mount showed a noise reduction of 3dBA.
In the event that an optimized damper still causes problems, road noise transfer path analysis is done – tracking the path of road noise and vibration through the vehicle: from the wheel to the shock absorber module, through the car body and into the passenger compartment. The entire test-optimization process has to be repeated several times, until the OEM is satisfied with the shock absorbers. Shock absorbers also influence the ride and handling of the car, so optimization must balance the reduction of interior noise with optimal ride and handling.
LMS products are used for a wide variety of applications at ZF Sachs, in the test cells and for mobile testing. LMS CADA-X acquisition software in combination with the LMS SCADAS III front-end measures the airborne and structure-borne noise during the simulations on the 4-post test rig. LMS Pimento is used for mobile measurements while LMS Roadrunner is used for the hydropulse rig.
The CADA-X Sound Quality module is used to listen to and filter the sound, and then calculates psycho-acoustic metrics like loudness, sharpness and roughness to make an objective evaluation of sound quality. Annoying noise resulting from the impacts of valves is analyzed using time frequency analysis in CADA-X TMON.
Using simulation to prevent noise and vibration problems
CAE products and methods also contribute to NVH improvements. A ‘physical’ model of the shock absorber prototype is built, to predict the force and damping characteristics of the absorber. The structural model and ANSYS CFD (computational fluid dynamics) software are combined to tune valve performance. Dr. Kruse looks forward to using more CAE in his development process, “The team only has access to the prototype for about one week, so optimizing that time is critical. We are achieving the transition from trouble-shooting to designing low-noise systems. It is made possible by carefully combining test-derived data with our simulations of design variations. CAE will enable us to shorten the refinement cycle for noise and vibration considerably. We plan to work very closely with CAE.”
LMS products are used for a wide variety of applications at ZF Sachs, both in the test cells and for mobile testing.
