A significant number of studies address various aspects related to tire modeling; most are dedicated to the development of tire models for on-road conditions. Such models cover a wide range of resolutions and approaches, as required for specific applications. At one end of the spectrum are the very simple tire models, such as those employed in real-time vehicle dynamic simulations. At the other end of the spectrum are the very complex finite element models, such as those used in tire design. In between these extremes, various other models have been developed, at different levels of compromise between accuracy and computational efficiency. Existing tire models for off-road applications lag behind the on-road models. The main reason is the complexity added to the modeling due to the interaction with the soft soil. In such situations, one must account for the soil dynamics and its impact on the tire forces, in addition to those aspects considered for an on-road tire. The goal of this project is to develop an accurate and comprehensive, while also efficient, off-road tire model for soft soil applications. The types of applications we target are traction, handling, and vehicle durability, as needed to support current army mobility goals. Thus, the proposed approach is to develop a detailed semi-analytical tire model for soft soil that utilizes the tire construction details and parallels existing commercially available on-road tire models. The novelty of this project relies in developing a three-dimensional three-layer tire model employing discrete lumped masses and in improving the tire-soil interface model. This will be achieved by enhancing the resolution of the tire model at the contact patch and by accounting for effects and phenomena not considered in existing models. / Master of Science

Understanding the effects of various wheel parameters on tractive performance is not completely understood. In order to properly quantify the individual effects of wheel parameters on the mobility of rigid and flexible wheels in soft soil, tests were performed, in cooperation with NASA Glenn Research Center (NASA-GRC), using the terramechanics rig at the Virginia Tech Advanced Vehicle Dynamics Lab (AVDL). To conduct such a study, four different wheels were evaluated under similar normal loads, slip ratios, and soil density. The first wheel represents the baseline, against which all the others were benchmarked. The remaining three wheels included the following parameter changes: 1) same diameter as the baseline but wider, 2) same width as the baseline but smaller in diameter, and 3) same width and diameter as the baseline but with a longer contact length. For each test the normal load, drawbar pull, and driving torque were measured and recorded for further analysis. To measure the effect of the changes in the wheels' parameters on the contact patch under different loads, a pressure pad was embedded below the surface of the Lunar simulant to measure the contact patch shape, size, and pressure distribution. Analysis of the experimental results showed that the drawbar pull was more significantly affected by the wheel diameter than by the contact width, and that same trend followed suit for the driving torque. Overall, the greater contact patch area resulted in a higher drawbar pull and torque. / Master of Science

Thesis (M.Eng.(Mechanical and Aeronautical Engineering))--University of Pretoria, 2008. / Includes bibliographical references.

The main goal of this diploma thesis is to create a design of an off-road vehicle. The vehicle has become a concept vision of an ecological off-road of the future fueled by hydrogen fuel cells. The secondary goal is: to bring new innovative form elements into conventional design solution of off-road vehicles and to highlight the problem with ecology, environmental protection and limited resources of fossil fuels.

Este trabalho apresenta o estudo da dinâmica vertical de um veículo off-road de competição e do comportamento elastocinemático de suas suspensões primárias de duplo estágio com o auxílio da ferramenta computacional de simulação de sistemas multicorpos MSC-ADAMS. O modelo multicorpos do veículo inclui os modelos da suspensão dianteira, suspensão traseira, sistema de direção, pneus e massa suspensa. As análises elastocinemáticas das suspensões definiram geometrias que permitem longos cursos de trabalho das suspensões sem variações indesejadas de parâmetros de projeto. Com a análise modal do modelo de veículo completo foram definidos os valores de rigidez das molas das suspensões de duplo estágio que resultaram em valores de freqüência natural no primeiro estágio semelhantes aos indicados para carros de passeio e no segundo estágio, próximos dos encontrados em veículos de competição. A análise de conforto do veículo durante simulação de passagem por trecho de pista demonstrou que os elementos de força definidos na análise modal resultam em níveis de conforto raramente encontrados em veículos de competição. / This dissertation presents the study of a off-road competition vehicle ride and the double rate suspensions elastokinematic behavior using the multibody software MSC-ADAMS. The vehicle multibody model includes the rear and front suspensions, the steering system, the tires and the sprung mass. The suspensions elastokinematic analysis defined geometric configurations that allowed long jounce and rebound travel of wheel, without undesirable project parameters variations. In the modal analysis of the vehicle multibody model, the springs rates that results in natural frequencies values similar to passenger cars for the first suspension stage and, for the second stage, similar to competition vehicles, were defined. The ride analysis during the simulation of the vehicle running on a rough track showed that the force elements defined on the modal analysis resulted in a good ride quality, rarely found in competition vehicles.

Este trabalho apresenta o estudo da dinâmica vertical de um veículo off-road de competição e do comportamento elastocinemático de suas suspensões primárias de duplo estágio com o auxílio da ferramenta computacional de simulação de sistemas multicorpos MSC-ADAMS. O modelo multicorpos do veículo inclui os modelos da suspensão dianteira, suspensão traseira, sistema de direção, pneus e massa suspensa. As análises elastocinemáticas das suspensões definiram geometrias que permitem longos cursos de trabalho das suspensões sem variações indesejadas de parâmetros de projeto. Com a análise modal do modelo de veículo completo foram definidos os valores de rigidez das molas das suspensões de duplo estágio que resultaram em valores de freqüência natural no primeiro estágio semelhantes aos indicados para carros de passeio e no segundo estágio, próximos dos encontrados em veículos de competição. A análise de conforto do veículo durante simulação de passagem por trecho de pista demonstrou que os elementos de força definidos na análise modal resultam em níveis de conforto raramente encontrados em veículos de competição. / This dissertation presents the study of a off-road competition vehicle ride and the double rate suspensions elastokinematic behavior using the multibody software MSC-ADAMS. The vehicle multibody model includes the rear and front suspensions, the steering system, the tires and the sprung mass. The suspensions elastokinematic analysis defined geometric configurations that allowed long jounce and rebound travel of wheel, without undesirable project parameters variations. In the modal analysis of the vehicle multibody model, the springs rates that results in natural frequencies values similar to passenger cars for the first suspension stage and, for the second stage, similar to competition vehicles, were defined. The ride analysis during the simulation of the vehicle running on a rough track showed that the force elements defined on the modal analysis resulted in a good ride quality, rarely found in competition vehicles.

Elements of an individuals occupational exposure, such as their posture can affect their comfort during work, and also their long term musculoskeletal health. Knowledge as to the extent of the influence of particular aspects of the exposures can help in providing guidance on risk evaluation, and direct future technical design focus. In many situations the exposures interact, and even if the effects of individual exposures are understood, the interactions are often less so. This is certainly the case with off-road driving exposures. Specific investigations have focussed on the effects of vibration exposure, resulting in the development of international standards and guidelines on measurement and evaluation of exposure. Consideration of the posture of the operator can be accomplished through postural assessment tools, although none of the currently available methods are developed specifically for use within a vehicle environment. The issues of both the posture of the operator and the seated vibration exposure are particularly apparent in off-road agricultural driving environments, where the driving task dictates that operator is often required to maintain specific postures whilst also exposed to whole-body vibration. In agriculture, many of the tasks require the operator to maintain axially rotated postures to complete the task effectively. The analysis of the combined effects of the axial rotation of the operator and the whole-body vibration exposure has been limited to a few studies within the literature, and is currently poorly understood. The overall aim of the thesis was to assess the influence of axial rotation and whole-body vibration on the musculoskeletal discomfort of the neck and trunk, in order that the true extent of the exposure risk may be evaluated. A field study was conducted to determine the common characteristics of some typical exposures, to provide a basis for the laboratory studies. A survey of expert opinion was conducted, examining the knowledge and experience of experts in assessing the relative influence of axial rotation and whole-body vibration on operators musculoskeletal health. The main investigations of the thesis are focussed in the laboratory, where the objective and subjective effects of axial rotation (static and dynamic) and whole-body vibration were investigated. Objective measures included the investigation of muscular fatigue in response to exposures. The tasks investigated in the field study indicated that the exposures often exceed the EU Physical Agents Exposure Limit Value, and that the axial rotation is a large component of the postures required. The survey of expert opinion concluded that combined exposure to axial rotation and whole-body vibration would increase the risks of lower back pain, and that acknowledgement of combined exposures should be included when assessing for risk. The results of the laboratory studies indicated that the greatest discomfort was present when subjects were exposed to axial rotation in the neck and shoulders. Out of the 8 muscles investigated, at most 6 of the 8 indicated fatigue during an experimental exposure. The muscle group which was affected most by the exposures was the m. trapezius pars decendens. Findings demonstrated that when subjects were exposed to axial rotation and whole-body vibration they indicated discomfort and their muscles fatigued. However, there was poor correlation between the sites of discomfort and the location of muscular fatigue. The discomfort findings suggest that there is an increased risk of discomfort from experiencing axial rotation together with whole-body vibration. Investigations of muscular fatigue do not substantiate this finding.

Starting a few decades ago, the unmanned wheeled vehicle research has drawn lately more attention, especially for off-road environment. As the demand to use electric vehicles increased, the need to conceptualize the use of electrically driven vehicles in autonomous operations became a target. That is because in addition to the fact that they are more environmentally friendly, they are also easier to control. This also gives another reason to enhance further the energy economy of those unmanned electric vehicles. Off-road vehicles research was always challenging, but in the present work the nature of the off-road land is utilized to benefit from in order to enhance the energy consumption of those vehicles. An algorithm for energy consumption optimization for electrically driven unmanned wheeled vehicles is presented. The algorithm idea is based on the fact that in off-road conditions, when the vehicle passes a ditch or a hole, the kinetic energy gained while moving downhill could be utilized to reduce the energy consumption for moving uphill if the dimensions of the ditch/hole were known a distance ahead. Two manipulated variables are evaluated: the wheels DC motors supply voltage and the DC armature current. The developed algorithm is analysed and compared to the PID speed iii controller and to the open-loop control of DC motors. The developed predictive controller achieved encouraging results compared to the PID speed control and also compared to the open-loop control. Also, the use of the DC armature current as a manipulated variable showed more noticeable improvement over using the DC input voltage. Experimental work was carried out to validate the predictive control algorithm. A mobile robot with two DC motor driven wheels was deployed to overcome a ditch-like hindrance. The experimental results verified the simulation results. A parametric study for the predictive control is conducted. The effect of changing the downhill angle and the uphill angle as well as the size of the prediction horizon on the consumed electric energy by the DC motors is addressed. The simulation results showed that, when using the proposed approach, the larger the prediction horizon, the lower the energy consumption is.

The dissertation develops a framework for the visualization of dynamic terrains for use in interactive real-time 3D systems. Terrain visualization techniques may be classified as either static or dynamic. Static terrain solutions simulate rigid surface types exclusively; whereas dynamic solutions can also represent non-rigid surfaces. Systems that employ a static terrain approach lack realism due to their rigid nature. Disregarding the accurate representation of terrain surface interaction is rationalized because of the inherent difficulties associated with providing runtime dynamism. Nonetheless, dynamic terrain systems are a more correct solution because they allow the terrain database to be modified at run-time for the purpose of deforming the surface. Many established techniques in terrain visualization rely on invalid assumptions and weak computational models that hinder the use of dynamic terrain. Moreover, many existing techniques do not exploit the capabilities offered by current computer hardware. In this research, we present a component framework for terrain visualization that is useful in research, entertainment, and simulation systems. In addition, we present a novel method for deforming the terrain that can be used in real-time, interactive systems. The development of a component framework unifies disparate works under a single architecture. The high-level nature of the framework makes it flexible and adaptable for developing a variety of systems, independent of the static or dynamic nature of the solution. Currently, there are only a handful of documented deformation techniques and, in particular, none make explicit use of graphics hardware. The approach developed by this research offloads extra work to the graphics processing unit; in an effort to alleviate the overhead associated with deforming the terrain. Off-road ground vehicle simulation is used as an application domain to demonstrate the practical nature of the framework and the deformation technique. In order to realistically simulate terrain surface interactivity with the vehicle, the solution balances visual fidelity and speed. Accurately depicting terrain surface interactivity in off-road ground vehicle simulations improves visual realism; thereby, increasing the significance and worth of the application. Systems in academia, government, and commercial institutes can make use of the research findings to achieve the real-time display of interactive terrain surfaces.

The National Park Service (NPS) monitors off-road vehicle (ORV) use in National Seashores across the United States. The sediment disturbance that is caused by ORVs is believed to have a large impact on erosion (by wind or waves), which there by affects the morphology of the foredunes. With greater knowledge of ORV impacts, the NPS can better manage ORV use and minimize anthropogenic affects to the coastal environment. There remains considerable uncertainty about the disturbance and its larger-scale impact. This study quantifies the sediment disturbance made by tire tracks, as well as the tire track form, width, depth, and evolution with relation to the number of vehicle passes and location on the beach at Assateague Island National Seashore (ASIS), Maryland. To measure ORV impact, ground-based LiDAR was used to collect detailed profiles across a three by three meter test plot at each site. Based on the quantification of the displaced sediment and redistribution of that sediment from the tracks, a recommendation to the NPS can be made as to where along the beach traffic should be limited to, in order to minimize impact to the physical environment at ASIS. Tire tracks were found to widen after the first pass, as a result of the imperfections of driving. Compaction of the sediment in the center of the tire track accounts for only a minimal amount of the sediment lost from the tire tracks. Sediment removal accounted for greater than 75% of the sediment lost from the tire tracks at all sites. It was concluded that sediment removal is the most dominant factor in the creation and evolution of a tire track. The width, depth, and evolution of a tire track were also found to be controlled by the imperfections of driving. Despite the amount of sediment disturbance, it is found that there is no net downslope displacement of sediment. This conclusion counters previous ORV impact studies and suggests that ORVs are not directly responsible for beach erosion. It is also recommended that to minimize the impact of OVRs on the beach at ASIS, the NPS should limit driving to the backshore.