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Evaluating the Effect of Light Weighting Through Roll Stiffness Change on Vehicle Maneuverability and Stability
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on November 21, 2019 by SAE International in United States
Event: NuGen Summit
Objective To achieve better fuel economy and reduced carbon footprint, OEMs are reducing the sprung and unsprung mass. This translates into a reduction in stiffness which profoundly deteriorates the handling/road holding characteristics of the vehicle. To model these changes in stiffness, modifications are made to suspension roll stiffness at the front and rear. This study compares different configurations of roll stiffness and evaluates vehicle behavior using frequency response characteristics and phase change of Yaw Gain recorded. The present work associates acquired data with subjective feedback to outline the shift in vehicle balance emerging from a variation of sprung and unsprung mass ratio. Methodology To study the frequency response characteristics of the vehicle, the pulse input is chosen for this. An ideal pulse input’s Fourier transform represents constant amplitude over all the frequency ranges. By giving a single input, the system is subjected to a range of frequencies. The changes in roll stiffness at the front and rear axles are achieved through addition of Stabilizer Bars of different diameters and stiffness. The test is conducted using a Front Engined Front Wheel Drive vehicle with similar track conditions, ambient parameters as well as loading conditions (Test Engineer + Measuring Instruments) to maintain conditions as uniform as possible between different iterations. The data is logged using a six-axis inertial and GPS measurement system which uses a Gyro based sensor to log angular rates. The output variables chosen to estimate the handling response of the vehicle were Maximum Yaw Gain, Frequency for Maximum Yaw Gain, Yaw Gain at 1 Hz and phase difference. Result The frequency response characteristics are plotted along with the phase lag of different iterations. The iterations were compared on the basis of the lateral shifting of the peaks observed on the frequency response curve as well as the subjective feedback from the test engineer. The following are the findings: • Increase in Front Roll Stiffness increases the understeer level in the vehicle but makes the vehicle more responsive. • Increase in Rear Roll Stiffness increases (more gain observed than front) the oversteer level as well as makes the system more non-linear. The system becomes less responsive. • Reduction in Roll Stiffness in the Front Axle generates more delay in the vehicle system from the input to output. Limitations • Findings should have been confirmed with more transient state tests and inputs to further quantify the subjective evaluation and relate the results mathematically. • The study only considers changes in stiffness through Stabilizer Bar, however, the impact of other Front and Rear suspension components impact needs to be understood. • Effects of the change in stiffness observed on other parameters elsewhere on the vehicle are not taken in consideration. Distinctiveness Light weighting results in reduced stiffness if geometry and material are considered as constraints. This study is conducted to study the impact of change in stiffness and subsequently, the reduction of weight. Existing works conventionally estimate/investigate the impact of a stabilizer bar for reducing the roll moment and angle for the purpose of improving ride and comfort. This work investigates the effect of the stiffness of a stabilizer bar on yaw characteristics and its effects on handling and road/holding. Conclusion The results aid in providing a path for designers and test engineers to predict changes in the vehicle behavior upon changing the weight and subsequently stiffness or vice-versa. We can predict that the front stiffness plays a more significant role in deciding the vehicle balance (understeer or oversteer). The rear stiffness has an impact on the linearity of the frequency response, that is, it talks about predictability. These findings can be used to justify a change in the sprung and un-sprung mass ratio for light weighting and its impact on vehicle maneuverability and stability.