Vibration analysis of flexible aluminium frame for a lightweight electric vehicle

A solar powered three-wheeled electric vehicle has been developed using aluminium alloy round tube in order to benefit from its lightweight property. However, the low material modulus of flexibility reduces stiffness and consequently decreases the bending frequencies of the structure. As a result, flexural vibrations especially in the vertical direction of the structure tend to increase accordingly and could affect the ride performance and comfort of the driver. A rigid sprung mass model of a vehicle body which has been used to investigate the effectiveness of a suspension system in ride comfort analysis is no longer suitable to predict the vibration responses accurately. Therefore, this study intends to develop a suitable full-vehicle model to represent the lightweight electric vehicle by incorporating the bending effects of the aluminium frame and use the model to analyse vibration responses of the vehicle due to road surface roughness. Integration of the vertical bending vibration of the vehicle flexible structure with the sprung mass was done by applying the Euler-Bernoulli beam theory. The electric vehicle is represented as 6 degrees of freedom full-vehicle model coupled with 4 degrees of freedom seated human model. A modified sprung mass equation of motion has been developed to include the effect of frame flexural vibration comprises of three vertical bending modes for both longitudinal and lateral bending of the vehicle frame. The numerical analysis is performed using MATLAB Simulink to determine the vertical vibration acceleration which affects the whole-body vibration of the driver inside the vehicle. The simulated accelerations of floor and seat are compared with the measured root-mean-square acceleration when moving on three different road surface conditions for validation. Furthermore, safety and health risks due to the vibration exposure are evaluated based on the guidelines provided by ISO 2631-1. The results show that the full-vehicle model with the frame flexibility effect produced better prediction on the vertical vibration acceleration values compared to the rigid frame full-vehicle model. The model with flexural vibration effect produces almost similar vibration accelerations with the root-mean-square acceleration values of the floor and seat having low vibration input from road surface condition. Optimisation the damping constant of the passive type primary suspension system could effectively improve the effects of the driver whole-body vibration exposure especially for the coarse road surface condition. The developed full-vehicle mathematical model of the lightweight electric vehicle that considers the flexural vibration effect of the vehicle frame is able to represent the actual vibration behaviour of the vehicle with vibration acceleration difference of less than 10%.