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The unique difficulties associated with low frequency and large amplitude ride vibrations of off-road tractor are summarized. Concept of a cab suspension system for improving the ride quality of off-road tractors in the bounce, longitudinal, lateral, pitch and roll modes is explored. Influence of suspension parameters on the ride performance is presented followed by selection of optimal suspension parameters. It is shown that a cab suspension would provide improved performance in the longitudinal and pitch modes alone. Ride analysis of the cab suspension with a sprung seat reveals satisfactory bounce ride. Roll and lateral ride of the off-road tractor can be improved significantly through alterations in the cab geometry. The ride performance of the optimal suspensions is assessed with reference to ISO (International Standards Organization) specified “fatigue decreased proficiency” (FDP) boundaries.

Low frequency terrain induced vibrations transmitted to the off-road vehicle operators are quite severe and exceed I.S.O specified “fatigue decreased proficiency” limits. In this paper, the ride improvement of an agricultural tractor is sought through effective designs of passive seat suspensions. The dynamic analysis of existing bounce suspension seats is carried out to establish its ride performance behaviour. Optimal bounce seat suspension parameters are selected with an objective to maintain the ride vibration levels within 4 hours exposure “fatigue decreased proficiency” limits. The roll and pitch ride vibrations, perceived by the operators, can be attenuated through a gimbal arrangement mounted to the bounce suspension seat. The optimal parameters of the combined seat isolator are selected using parametric optimization techniques. Also a horizontal isolator, attachable to the bounce or the combined seat isolator, is configured.

Transient fluid slosh within a partly-filled tank could impose high stresses on the tank structure and affect the directional performance in an adverse manner. A three-dimensional nonlinear model of a partly filled circular cylindrical tank with and without baffles is formulated and analyzed to derive the pressure distribution over the wetted tank surface. The baffles and end caps are modeled with curved shapes in accordance with the current standard. The analyses are performed for 40% and 60% fill volumes and different types of baffles, including single-nozzle and multiple-orifice baffles, using the FLUENT software under time varying acceleration fields representing simultaneous braking and turning maneuvers. The pressure data are further analyzed to evaluate steady-state and transient slosh forces, load shifts along the longitudinal and lateral axes, and the roll, pitch and yaw moments imposed on the tank structure.

Ride quality of articulated vehicles is investigated via computer simulation in view of secondary suspension parameters. A tractor-semitrailer vehicle is modelled incorporating primary as well as secondary suspension. The ride vibration levels at the cab floor and at the driver-seat interface are evaluated using power spectral density approach. The effect of various vehicle parameters, such as secondary suspensions, primary suspensions, axle loads and tires on the vehicle ride quality is presented, and the significance of secondary vehicle suspension is specifically emphasized. A software package is developed to evaluate and assess the ride performance of articulated vehicles with suspended seat and cab. A limited validation of the computer ride model is achieved via field measurements.

A six degrees-of-freedom ride dynamic model of a cab-over-engine supported on elastomer mounts is developed using lumped parameters. The lumped parameter model is analyzed for its free vibration response, while assuming the cab structure to be rigid. A finite element model of the suspended cab is developed and analyzed to establish the influence of flexibility of the cab structure on the ride dynamics of the tilt cab. The vibration modes of analytical lumped parameter and finite element models are compared to the dominant ride frequencies of the vehicle measured in the field. The lumped parameter model is then modified to achieve a comprehensive ride dynamic model for its further use in ride performance analyses.

In this study, the enhancement of road friendliness of Heavy Goods Vehicle is investigated using two methods to control the resonant forces: (i) Determination of optimal asymmetric force velocity characteristics of the suspension dampers to control the wheel forces corresponding to the resonant modes; (ii) Optimal design of an axle vibration absorber to control the wheel forces corresponding to the unsprung mass resonance mode. An analogy between the dynamic wheel loads and ride quality performance characteristics of heavy vehicles is established through analysis of an in-plane vehicle model. A weighted optimization function comprising the dynamic load coefficient (DLC) and the overall rms vertical acceleration at the driver's location is formulated to determine the design parameters of the damper and absorber for a range of vehicle speeds. The results show that implementation of tuned axle absorbers can lead to reduction in the DLC ranging from 11.5 to 21%.

Technological developments in road vehicles over the last two decades have received considerable attention towards pushing the safe performance limits to their ultimate levels. Towards this goal, Active Front Steering (AFS) and Direct Yaw-moment Control (DYC) systems have been widely investigated. AFS systems introduce corrective steering angles to conventional system in order to realize target handling response for a given speed and steering input. It is thus expected that such an action under severe maneuvers may cause one tire to reach saturation while the other tire may be capable of developing more force. This study, therefore, proposes an Active Independent Front Steering (AIFS) system capable of controlling a wheel independently. At low speeds, the proposed AIFS system will modify the steer angle with speeds while maintaining pro-ackerman geometry similar to an AFS system. In doing so, it will realize a target response defined as one provided by a neutral steer system.

An Active Independent Front Steering (AIFS) offers attractive potential for realizing improved directional control performance compared to the conventional Active Front Steering (AFS) system, particularly under more severe steering maneuvers. The AIFS control strategy adjusts the wheel steer angles in an independent manner so as to utilize the maximum available adhesion at each wheel/road contact and thereby compensate for cornering loss caused by the lateral load transfer. In this study, the performance potentials of AIFS are explored for vehicles experiencing greater lateral load transfers during steering maneuvers such as partly-filled tank trucks. A nonlinear yaw plane model of a two-axle truck with limited roll degree-of-freedom is developed to study the performance potentials of AIFS under different cargo fill conditions.

A generic tank cross-section is formulated to describe the geometry of currently used tanks in transportation of fuel oils and bulk liquids, and to explore optimal tank geometry for enhancement of roll stability limit of tank vehicle combinations. The tank periphery, composed of 8 circular arcs symmetric about the vertical axis, allows more design flexibility in view of the roll stability limits than the conventional tank shapes. A shape optimization problem is formulated to minimize the overturning moment imposed on the vehicle due to c.g. height of the liquid load, and the lateral and vertical movement of the liquid bulk within the partly filled tank. Different optimal tank cross-sections are proposed corresponding to varying fill conditions, while the total cross-sectional area, overall height and overall width are constrained to specified values.

Dynamic interactions of urban buses with urban roads are investigated in view of the vibration environment for the driver and dynamic tire forces transmitted to the roads. The static and dynamic properties of suspension component and tires are characterized in the laboratory over a wide range of operating conditions. The measured data is used to derive nonlinear models of the suspension component, and a tire model as a function of the normal load and inflation pressure. The component models are integrated to study the vertical and roll dynamics of front and rear axles of the conventional and modern low floor designs of urban buses. The resulting nonlinear vehicle models are thoroughly validated using the fieldmeasured data on the ride vibration and tire force response of the buses.

It has been widely reported that the overall vibration comfort performance of static and dynamics seats is strongly influenced by the biodynamic behaviour of the seated human body. The contributions of the seated occupant to the overall vibration attenuation of the coupled seat-occupant system are experimentally investigated as functions of the nature of excitation, static and dynamic properties of the seat, and the sitting posture. The study involved two different seats with natural frequencies in the vicinity of 1.5 Hz and 4 Hz, which would characterize the low natural frequency suspension as well as high natural frequency seats employed in automobiles and some industrial vehicles. The vibration isolation properties of the seats are evaluated with a rigid mass and two human subjects under different vibration excitations, including swept sine, broad-band random and standardized vibration spectra of selected vehicles.

General purpose tank vehicles often carry partial loads in view of variations in the weight density of the liquid cargo and are thus subject to slosh loads during highway manoeuvres. The magnitude of destabilizing forces and moments due to liquid slosh is strongly related to a number of vehicle and tank design factors, such as tires, suspension, articulation mechanism, weights and dimensions, tank geometry and fill level. The rollover threshold of the tank vehicle is compared to that of an equivalent rigid cargo vehicle to demonstrate the destabilizing effects of liquid slosh. The rollover threshold of the tank vehicle is evaluated for a number of tank design factors. Influence of tank size and cross-section on the rollover threshold of the tank vehicles is investigated. The study concludes that the lateral load shift and thus the rollover threshold is strongly related to the tank cross-section geometry.

The longitudinal load transfer encountered in a partly-filled ellipsoidal tank truck, subject to a straight-line braking maneuver, is investigated as a function of the location of partition walls, deceleration and the fill level. The response characteristics of the truck equipped with a compartmented tank are evaluated in terms of dynamic load transfer, stopping distance, braking time and time lag between the front and rear axle wheel lock-up. The braking response characteristics are derived as a function of the load shift, and number and location of partition walls. Road tests were performed on an airport fuel truck, equipped with a 3 m long tank with two movable partition walls. The simulation results derived from the test vehicle model are compared to the road test data to demonstrate the validity of the analytical model. The results show good correlation with the measured data acquired under straight-line braking maneuvers performed under different fill levels and initial speeds.

This paper summarizes the methodology and findings of an investigation to determine friction coefficients between typical loads and trailer deck materials in static, sinusoidal, and field measured random vibration environments. To conduct the tests in a controlled laboratory environment, a special sled-deck fixture was designed. Provisions were made to allow a change in sled-deck contact materials and to vary the load on the sled. For dynamic testing, the deck was subjected to sinusoidal and field based random signals obtained from tractor-trailer traversing paved and gravel roads. The results reveal that under vertical vibration, these friction coefficients can be as low as 25% of their corresponding static values. The random vibration tests revealed that friction coefficients can fall below 75% of the mean value for up to 35% of the total test duration. Thus, loads that rely on static friction for security may be highly prone to load shifts in a dynamic environment.

The directional dynamics of partially filled articulated tank vehicles is investigated via computer simulation assuming constant forward velocity. The directional response characteristics of an articulated tank vehicle is investigated for various steering manoeuvres and compared to that of an equivalent rigid cargo vehicle to demonstrate the destabilizing effects of liquid load shift. It is concluded that during a steady steer input, the distribution of cornering forces caused by the liquid load shift yields considerable deviation of the path followed by the tank vehicle. The lateral load shift encountered in a partially filled tank vehicle during lane change and evasive type of highway manoeuvres gives rise to roll and lateral instabilities.

Dynamic behavior of a partly-filled liquid cargo vehicle subject to simultaneous application of cornering and braking maneuvers is investigated through computer simulation. A three-dimensional quasi-dynamic model of a partly-filled tank of circular cross-section is developed and integrated into a comprehensive three-dimensional model of an articulated vehicle to study its directional response under varying steering and braking inputs, fill volumes and road surface friction. The liquid load movement encountered under combined steering and braking is expressed in terms of variations in the instantaneous c.g. coordinates and mass moments of inertia of the liquid bulk, assuming negligible influence of fundamental slosh frequency and viscous effects.

Relative rollover conditions of a heavy vehicle are analyzed to establish an array of potential dynamic rollover indicators towards development of an early warning device. A relative roll instability indicator defined as Roll Safety Factor (RSF) is proposed and shown to be a highly reliable indicator regardless of vehicle configurations and operating conditions. The correlation of various potential rollover indicators with the roll safety factor are then investigated for a 5-axle tractor semi-trailer combination using a comprehensive directional dynamic analysis model to determine the reliability of the proposed indicators over a range of operating conditions. The indicators are further examined in terms of measurability, lead time, and potential for application in an early warning system. The study shows that the trailer lateral acceleration and axle roll angles are closely correlated with the RSF.

Baffles are known to help reduce the amplitude of fluid slosh in partly filled tanks, particularly during braking and acceleration. The transient fluid slosh approach is proposed to evaluate the effectiveness of baffles designs. A computational fluid dynamic (CFD) fluid slosh model is developed using the VOF (volume of fluid) technique coupled with a Navier-Stokes solver. The validity of the model is demonstrated using the experimental data acquired with a scale model tank. The validated CFD model is subsequently formulated for a full scale tank and simulations are performed under excitations idealizing the straight-line braking maneuvers to investigate the anti-slosh role of four different transverse baffles concepts. The fluid slosh responses are analyzed in terms of the fundamental slosh frequency, and the resulting forces and moments under different fill volumes of liquid cargos of constant load.