Abstract
RESEARCH AND/OR ENGINEERING QUESTIONS/OBJECTIVE
In search for improved handling performance, road vehicle manufacturers and racing teams favour the solution of a stiffer suspension setup for improved handling dynamics and minimising lap times. This is often at the expense of ride and tyre performance when vehicle body control is prioritised. However, what is the influence of the damper coefficient? Does it vary widely over a variety of spring stiffness? How does the suspension setup affect the mean friction coefficient (i.e. “grip”) available from the tyre? This paper will investigate such questions and consider the difference of the mean vertical loading compared to the static loading (i.e. “grip loss”).
METHODOLOGY
A two degree of freedom (2DOF) ride model in Excel has been constructed to simulate the bounce motion of the sprung and unsprung mass of a road vehicle including the tyre contact with the road. The road profiles analysed include a step-up and a step-down input functions. A sensitivity study has been performed concentrating on varying the natural frequency and critical damping of the system. On the basis of this sensitivity analysis, the response of a non-linear damping coefficient has been studied in order to obtain the optimal solution to maximise the tyre’s grip potential.
RESULTS
In this paper the root mean square load variation of the tyres that relates to the available “grip” and body motion will be presented as a function of the suspension’s natural frequencies and compression/extension damping coefficient. The main finding to be highlighted is that the grip is improved at lower natural frequency and the critical damping has a non-linear relationship to the suspension’s natural frequencies. The body response and positioning is also investigated to review the balance between driver comfort and handling response.
LIMITATIONS OF THIS STUDY
This numerical study does not take into account wheel rate variation due to suspension geometry and assumes the body and joints have no compliance.
WHAT DOES THE PAPER OFFER THAT IS NEW IN THE FIELD INCLUDING IN COMPARISON TO OTHER WORK BY THE AUTHORS?
This sensitivity study focuses on the suspension and body natural frequencies, critical damping and their relationship to tyre mean load variation. Previous research has been focused on optimising damper characteristics at a narrow spectrum or fixed suspension natural frequencies for improved ride or handling by accessing body control and acceleration.
CONCLUSIONS
The study shows having a lower natural frequency suspension setup reduces the tyre mean load variation and grip loss. However the relation to the corresponding critical damping ratio is also shown to be non-linear highlighting the impact that changes in overall vehicle mass can have in tyre grip performance. Minimising the vehicle’s grip loss indicated also a reduction in mean body vertical acceleration, providing a more comfortable ride.
KEYWORDS – Tyre Load, Grip, Handling, Spring and Damper