| The level of vehicle comfort and quality perceived by the driver and passengers can be severely diminished due to unwanted noise, vibrations, and harshness arising external sources such as road surface irregularities and wind pressure distribution around the vehicle due to aerodynamics, or internally propagated from the engine/powertrain, the suspension, and the tire/wheel assembly. The periodic force variations produced by a nonuniform tire/wheel assembly are known root causes of excessive torsional steering wheel vibrations known as "steering nibble". Though several studies have sought to investigate this issue through modeling or experimentation involving the entire vehicle or specific subsystems involved, there has yet to be established a direct link between objectively measured tire force variations and the severity of the vibration levels perceived by the occupants. Thus, the primary objectives of this research are to develop, as part of a toolkit for the characterization of the first-order transmission of steering nibble vibrations, an analytical model consisting of the integrated subsystems involved, validate the model results through experimentation, identify the key parameters influencing vibration transmissibility, and establish a correlation between the subsystem properties and the subjective steering nibble rating.; Following a literature survey, analytical models of a nonuniform tire, double-wishbone suspension system, and rack and pinion steering system are reviewed and refined such that an integrated subsystem-level model can be formulated and implemented. Each subsystem is validated against experimental results sourced from a database formed as part of a collaborative effort with this project to insure proper input-output relationships of the subsystem modules and complete integrated model. It is observed, consistent with expectations, that the nonuniform tire model provides an oscillatory force input to the wheel spindle, the magnitude of which is affected by the relative motions of the suspension assembly, tire tread band, and road interface. The tire force input, namely the tangential force variations, serves to excite a significant rotational response of the knuckle about the kingpin axis, providing an oscillatory force input to the steering system via the tie rods. From the experimental testing, it is revealed that the steering system response is characterized by a nonlinear compliant friction which causes the resonant frequencies of the system to shift with tie rod force amplitude.; In the particular case of the vehicle chosen for analysis, the 2004 Ford F-150 4x4 SuperCrew, a new production pickup truck known for customer complaints regarding nibble vibrations, it is determined that at 13.8 Hz, a frequency corresponding to the tire/wheel rotational rate at a vehicle speed of 75 mph, the tie rod forces generated for a range of tire nonuniformities serve to excite an equivalent resonant mode of the steering system at approximately the same frequency. A parameter sensitivity study is performed and it is shown that through proper tuning of the inertial and frictional properties of the steering system and suspension parameters, the vibration transmission could be substantially decreased. Further analysis may establish a relationship between tire design parameters and force/vibration transmissibility. The subjectively-correlated threshold for the angular vibration of the steering wheel is established at about 400 deg/s2, a response which, around the critical speed of 75 mph, is achievable even with modest tire nonuniformity due to the nonlinear resonance present in the steering system. |
