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Abstract
The racing situation requires a high performance tuned vehicle incorporating a high performance engine, aerodynamic body, and tyres. Among these three important factors the tyre is the only component which can transmit the vehicle force to the road, and is the most difficult part to tune, since it involves complex phenomena and a wide range of operating conditions.
Development of a theoretical tyre model is necessitated to predict the tyre forces and moments according to various road conditions. Many empirical tyre models have been studied in the past as it is efficient to conduct experimental curve-fittings. However, these carry inherent problems from the unrealistic tyre testing equipment and conditions. On the other hand, the theoretical tyre model can purely simulate and make predictions taking into account the causes and effects of the tyre operating phenomena. Therefore, it can enhance the accuracy of the tuning and selection of the tyre.
The aim of this research project was to provide an understanding of tread friction interacting with the road surface in various operating conditions in order to enhance the knowledge about the application to theoretical tyre model development. The approaches employed to achieve the research aim mainly involve experimental investigations and numerical simulations including the development and application of Gaussian/non-Gaussian surfaces at the contact interface in order to predict frictional behaviours of tread in a realistic way.
The review and analyses of the literature involving the empirical and theoretical research field of tyres revealed that existing information regarding the effects on the complex interactions between tyre operating variables was found to be sparse and only qualitative. Recently, a great deal of improvement of theoretical tyre models has been made with innovative representation of road surface asperities. However, this surface model employs only a few simple shapes to represent the road asperity shapes. This should be further improved with inclusion of a greater variety of shapes.
Currently the simulation software packages used for the tyre forces and moments prediction are mostly multi-body physics software packages such as MSC.ADAMS for the computing efficiency. In order to maximise the accuracy and evaluate the interaction phenomena in detail, FEA can be utilized.
In order to evaluate the effects of important operating variables on frictional behaviours of tread, the main parts of the model: tread material properties and the surface roughness representations, were developed. Each part of the model was investigated separately and then applied to the model. Equivalent Gaussian/non-Gaussian surfaces were generated using the same roughness parameters, in order to assess the effects of these parameters on the friction corresponding to a given roughness.
The friction tests were conducted with a unique testing apparatus and the experimental rig was designed based upon piezo-electric load cells such that the road surface was sliding over the tread rubber sheet. The experimental schedule was conceived with the aim of making consistent measurements in order to examine the data with respect to the causes and effects of the four individual experimental variables: tread compound, surface roughness, sliding velocity, and normal load. The experimental data were expressed in terms of the average friction coefficients over the sliding motion.
The important model parameters which control the frictional behaviours were determined by a trial and error method to give a match between the friction test results and their simulation results. These parameters were the local friction coefficient corresponding to the effect of the microscopic texture of the contact bodies and the friction stress limit, which represents the limiting shear stress during friction. Comparisons were conducted to verify the validity of the FE models for the predictions with different conditions.
Lastly, the prediction of friction for the load controlled (LC) mode, which is a more realistic tyre contact condition, was conducted, with the important model parameters obtained by comparison between the experimental results and their simulations. The effects of each variable on the friction of the tread was discussed in conjunction with the interaction of the operating variables. The conclusions derived from the research conducted to achieve each of these specific aims are summarised separately.
These analyses led to the confirmation of the general and specific trends of the friction in contact between tyre tread and road surface. Specifically, the successful applications of the Gaussian/non-Gaussian surfaces enabled a discussion of the effects of each roughness parameters on tread friction. It has been revealed that not only the effects of the relative roughness could cause the variations in the apparent friction coefficient, but also more significantly the skewness and kurtosis could have the effects on the frictional behaviours of tread rubber, and should thus always be included in simulated surfaces. It has been verified that the FE model of the tread-road interaction as a tool to investigate the causes and effects of the operating variables can be used to predict the detailed interaction of the tread rubber and road asperities.