This thesis examined the difficulties of generating traction from 4x4 (light truck) tyres in pure sand conditions. Investigations conducted in the Cranfield University Soil Dynamics Laboratory measured the tractive performance of a range of production and prototype 4x4 tyre tread patterns to quantify the effect of tread features upon tractive performance. The investigation also quantified the amount of sand displacement instantaneously occurring beneath the tyre, by a novel application of radio frequency identification (RFID) technology, which determined sand displacements to an accuracy of ±5.5 mm. A limited number of normal contact stress measurements were recorded using a TekScan normal pressure mapping system. This technology was employed in a new manner that allowed pressure distributions to be dynamically recorded on a deformable soil surface. Models were developed or adapted to predict rolling resistance, gross thrust of a tyre and the gross thrust effect due to its tread. Net thrust was predicted from refined versions of equations developed by Bekker to predict gross thrust and rolling resistance. These were modified to account for dynamic tractive conditions. A new tread model proposed by the author produced a numerical representation of the gross thrust capability of a tread based on factors hypothesised to influence traction on loose sand. This allowed the development of a relationship between the features of the tread and its measured gross thrust improvement (relative to a plain tread tyre), from which a total relationship was developed. The tread features were also, in combination with the wheel slip, related to the sand displacements and net thrusts simultaneously achieved. The sand displacement results indicated that the majority of the variation in displacement between the different treads occurred in the longitudinal (rearward) direction. This effect was influenced by the wheel slip, as increased slip caused greater displacements, so the differences between the treads were greater at higher slips. The treads that generated the highest relative displacements also derived the higher gross thrusts (up to +5% extra gross thrust compared to a plain tread), although at the higher slips this also caused increased sinkage. As sinkage increased, the rolling resistance increased at a fester rate then the gross thrust, and thus the net thrust reduced. To prevent this effect the wheel slip should be limited to a maximum of 20% at low forward speeds (approximately 5 km/h). Current market forces dictate that the biggest benefit that tyre manufacturers could offer in desert market regions would be to optimise road-biased tyres to suit loose sand conditions. The modelling developed indicated that this could be achieved by maximising the number of lateral grooves (and thus lateral edges) featured on a tread, however care would have to be exercised so as not to compromise the necessaiy on-road capability. The models could also be used to quantifiably determine from a choice of possible tyre treads, the tread that would offer most traction on pure loose sand.

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This thesis presents an initial Finite Element (FE) based modelling investigation aimed at supporting the development of 'smart' tyre or intelligent tyre technologies. Physical tests carried out with a stationary (non-rolling) and rolling experimental tyre are used to enhance understanding of tyre behaviour in the contact patch and validate the modelling methodology. Simulation results with the explicit FE package LS-DYNA are then used to characterise the internal stresses and strains at several positions in the tyre tread. Two separate FE models are developed to simulate the stationary and rolling tyre behaviour at the macroscopic level. The models differ only with respect to the mesh density in the circumferential direction, the mesh through the cross section is identical. The complex tyre structure is represented as a rubber and reinforced rubber composite, and the mesh specification and the material descriptions used in the models are discussed. The structural behaviour of the stationary experimental tyre under normal load is simulated. The inflation of the tyre, the wheel fit and the normal loading against the horizontal surface are represented. Simulation results are also presented when a subsequent longitudinal or lateral load is applied to the stationary tyre. These analyses were conducted to determine the longitudinal and lateral tyre stifffiesses, respectively. The predicted normal load-deflection characteristics and contact patch dimensions (length and width) are compared with a reasonable degree of success to those obtained in the full-scale physical tests. The longitudinal and lateral simulations also appear to give realistic tyre stiffnesses. The contact patch dimensions give a good trend-wise agreement, but the length and width are greater than the experimental measurements. A parametric study is carried out and this disparity is related to a deficiency in the performance of the contact algorithms. It is concluded that it not straightforward to accurately predict contact patch behaviour, and therefore the internal transient stresses and strains in a rolling tyre in absolute terms. However, the good trend-wise agreement suggests that the modelling methodology should be capable of predicting internal transient responses which are related to the 'actual' deformations in the contact region. To simulate the rolling tyre behaviour on flat bed and drum surfaces, consideration is given to the inflation of the tyre, the wheel fit, the normal loading and the rotation of the tyre. Numerical instabilities are found to occur and these are related to imperfections inherent in version 950d of the code. This version was, at the time, the most up to date release. The current release is version 960 and it does not contain many of the imperfections in the earlier version. Thus, the flat bed simulation is repeated using the current version. The predicted contact patch stresses are presented and a reasonable correlation is achieved with the experimental data. The internal stresses and strains are then characterised at a number of selected positions in the tread region. These stresses and strains are discussed in context with the development of smart tyre technologies and are useful as a guide to the most appropriate location for an in-tyre sensor (or sensors).

The work presented in this thesis is dedicated to the study of transient tyre dynamics and how these influence the dynamic behaviour of the vehicle and its driveline, with the main focus being on low-to-zero speed manoeuvres such as pull-away events. The bulk of the work focuses on the amalgamation of the hitherto disparate fields of driveline modelling and detailed tyre modelling. Several tyre models are employed and their relative advantages and disadvantages analysed. The observed dynamic behaviour is correlated to the inherent structure of each tyre model in order for the most appropriate for driveline studies to be identified. The main simulation studies are split into two parts: the first comprises a study into isolated driveline dynamics; where the yaw, pitch and roll behaviours of the vehicle body are neglected. A relatively detailed driveline model with an open differential is used with tyre models of increasing complexity with the aim of determining when increased model detail fails to increase the accuracy of the results. The second part is concerned with the study of how the dynamics of the vehicle body and suspension affect tyre model performance and associated effects on the driveline behaviour. For this, the driveline and tyre models are incorporated into a full six degree-of-freedom vehicle model with full suspension effects. Frequency migration on low-μ surfaces is successfully explained via the decoupling of the vehicle and driveline inertias. Frequencies observed in FFT analyses of the simulation results correspond to those obtained through eigen-analysis of appropriately modified state-space models with varying degrees of coupling that reflect the vehicle travelling on uniform low- or split-μ surfaces. The main finding of the thesis is that this decoupling theory can also be applied to high-speed take-off manoeuvres, as it is the position along the tyre slip-force curve that dictates decoupling; i.e. if the curve has saturated. This leads to the effective traction stiffness being zero, which modifies the equations of motion and subsequently the system eigenvalues. A series of measurements are taken in order to verify the findings from the simulation work. Manoeuvres analogous to those simulated are carried out. It is found that only the simulation of split-μ conditions is necessary, as the results from the low-μ test show a similar pattern to those seen on the split-μ surface.

The work presented in this thesis is devoted to the study of mechanism of tyre force generation and its influence on handling dynamics of ground vehicles. The main part of the work involves the development of tyre models for use under steady-state and transient operating conditions. The general capability of these models is assessedin a full vehicle simulation environment. The interaction between tyre and vehicle dynamics is critically evaluated and the observed vehicle behaviour is related to the inherent characteristics of different tyre models. In the field of steady-state tyre modelling, two versions of a numerical tyre model are developed. The modelling procedure is carried out in accordance with the viscoelastic properties of rubber, which influence the mechanical properties of the tyre structure and play a significant role in the determination of friction in the tyre contact patch. Whilst the initial simple version of the tyre model assumes a parabolic pressure distribution along the contact, a later more elaborate model employs a numerical method for the calculation of the actual normal pressure distribution. The changes in the pressure distribution as a result of variations in the rolling velocity and normal load influence mainly the levels of self-aligning moment, whilst the force characteristics remain practically unaffected. The adoption of a velocity dependent friction law explains the force generating behaviour of tyres at high sliding velocities. The analysis is extended to the area of transient tyre behaviour with the development of a tyre model appropriate for the study of transient friction force generation within the contact patch. The model incorporates viscoelasticity and inertial contributions, and incorporates a numerical stick-slip law. These characteristics are combined together for the successful simulation of transient friction force generation. The methodologies developed for the modelling of transient friction and steady-state tyre force generation are combined and further extended in order to create a generic transient tyre model. This final model incorporates a discretised flexible viscoelastic belt with inertia and a separate fully-dynamic discretised tread, also with inertia and damping, for the simulation of actual prevailing conditions in the contact patch. The generic tyre model appears to be capable of performing under a variety of operating conditions, including periodic excitations and transient inputs which extend to the non-linear range of tyre behaviour. For the evaluation of the influence of the aforementioned tyre models on the handling responses of a vehicle, a comprehensive vehicle model is developed, appropriate for use in handling simulations. The two versions of the steady-state models and the generic transient model are interfaced with the vehicle model, and the response of the vehicle to a step-steer manoeuvre is compared with that obtained using the Magic Formula tyre model. The comparison between the responses is facilitated by the definition of a new measure, defined as the non-dimensional yaw impulse. It is found that the transience involved in tyre behaviour may largely affect the response of a vehicle to a prescribed input.