Off-road Vehicle Performance Research Articles

Speed-made-good: Mobility map generation for wheeled vehicles on soft terrain

The Nepean Wheeled Vehicle Performance Model (NWVPM) software has been in use for decades as a tool for engineers in industry and governmental agencies to predict the performance of off-road wheeled vehicles. It is based on the Bekker-Wong wheel-terrain interaction model which takes into account both the normal pressure and shear stress distributions at the wheel-terrain interface and uses the Bekker-Wong terrain parameters to characterize the terrain behavior. Using terrain and vehicle parameters as inputs, NWVPM computes various traction indicators such as drawbar pull, tractive effort, sinkage, and external motion resistance as functions of wheel slip by solving a set of equilibrium equations. This paper describes a novel method for predicting vehicle speed-made-good mobility maps and fuel consumption through an analytical approach using NWVPM. To evaluate the proposed method for predicting speed-made-good, a comparison is made with predictions obtained using Vortex, a commercial multibody dynamic simulation software that performs dynamic simulations of vehicles on soft terrain. In addition, a comparison of the capabilities of NWVPM and Vortex in predicting off-road vehicle performance over a range of terrains is presented and the results of the drawbar pull traction test from both software are compared with experimental data.

Tyre-terrain interaction modelling and analysis: literature survey

Depending on whether the vehicle is used off-road or on-road, the terrain on which it operates can range from hard surfaces to deformable surfaces, such as soil and snow. It is well known that the soft terrain characteristics have a significant impact on off-road vehicle performance, therefore modelling and analysing the soils and tyres are critical. This paper reviews the available published work related to tyre-terrain interaction modelling and testing. The tyre mechanics fundamentals, as well as the modelling and validation methods used for developing the finite element analysis (FEA) tyres, are discussed in detail. The techniques used for soil modelling and calibration such as FEA, and smoothed particle hydrodynamics (SPH) are also discussed. In addition, the tyre-terrain contact algorithm is addressed.

Experimental analysis of mechanical properties of coastal terrain via bevameter tests

The driving performance of off-road vehicles is determined by the interaction between the terrain and the driving device. This study measured the mechanical properties of coastal terrain using a bevameter to predict the driving performance of vehicles driving along the coast. The results of the pressure-sinkage test on the dry terrain confirmed that there could be a relative error of approximately 10% between the test value and the predicted value of the pressure-sinkage characteristic equation. Thus, it was confirmed that an error of approximately 10% should be taken into account when predicting the driving performance of the vehicle. Shear tests on the dry terrain and saturated terrain confirmed that even the same soil texture had different shear behaviors depending on saturation. In addition, the maximum shear stress showed a significant difference, but the shear stress after the critical state showed no significant difference. Finally, the shear test was carried out by varying the number and height of the grouser to confirm the grouser effect. The grouser effect test results confirmed that the number of grousers does not have a significant effect on the shear stress, but the height of the grouser does have an effect on the shear stress.

Performance analysis and optimized design of a certain frame

In recent years, energy shortages and environmental pollution have become increasingly prominent, and automobile lightweight has become an important field in the development of the automobile industry. This paper analyses the frame performance of an all-terrain single-seat off-road vehicle and optimizes the structure design. The weight of the new structure has been reduced by 4.084 kg, and the weight reduction rate has reached 12.83%. The statics analysis and modal analysis of the new frame structure show that the new structure still meets the requirements of usage.

Quantification of the soil stiffness constants using physical properties of paddy soils in Yangtze Delta Plain, China

The prediction of soil sinkage under wheels and tracks is of significant importance for determining the off-road vehicle performance and level of compaction of agricultural soils. Bekker's theory for pressure and sinkage relationship has been widely applied to estimate the vehicle mobility performance for designing off-road vehicles for deformable terrains. However, the Bekker's constants, which reflect the soil sinkage and behaviour of soil under load, vary significantly among soils. There is a paucity of studies on paddy soils in the plastic state. Furthermore, the determination of soil stiffness constants via pressure–sinkage tests is usually time-consuming. In this study, the impact of three soil physical properties, including soil water content, clay content, and density, on the soil stiffness constants for sinkage behaviour in paddy soils was assessed based on experimental results. Subsequently, numerical models were established to quantify the Bekker's stiffness constants measured from soil bin tests using soil physical properties. Three quadratic polynomial regression models were established and evaluated to quantify the three constants using soil water content, clay content, and density. The P values from goodness of fit tests suggested significant fit (p 0.05) between the simulated and experimental results. The established models were further validated using the soil samples with different physical properties. The comparisons between soil stiffness constants predicted using the model with laboratory-measured values indicated satisfactory model performances (R2 > 0.9). In this study, a simple and promising method was established to estimate soil stiffness constants using easy-to-measure soil physical properties. The model can provide a sound basis for the design and development of agricultural machinery in muddy and wet paddy fields.

Numerical analysis on tractive performance of off-road tire on gravel road using a calibrated finite element method–discrete element method model and experimental validation

The interaction between off-road tires and granular terrain has a great influence on the tractive performance of off-road vehicles. However, the finite element method or the discrete element method cannot effectively study the interaction between off-road tires and granular terrain. The three-dimensional combined finite element and discrete element method is applied to handle this problem. In this study, a calibrated finite element method–discrete element method model is established, in which the finite element model of off-road tire is validated by stiffness tests, while the discrete element model of gravel particles is validated by triaxial compression tests. The calibrated finite element method–discrete element method model can describe the structural mechanics of the off-road tire and the macroscopic mechanical properties of the gravel road. Tractive performance simulations of the off-road tire on gravel road under different slip conditions are performed with the commercial software LS-DYNA. The simulation results are basically corresponded with the soil-bin test results in terms of granular terrain deformation and tractive performance parameters versus the slip rates. Finally, the effects of tread pattern, wheel load, and tire inflation pressure on tractive performance of off-road tire on granular terrain are investigated. It indicates that the calibrated finite element method–discrete element method can be an effective tool for studying the tire–granular terrain interaction and predicting the tractive performance of off-road tire on granular terrain.

Optimization and testing of suspension system of electric mini off-road vehicles.

Mini vehicles with a small wheelbase are extremely sensitive to road roughness. The aim of this study is to explore the influence of road information on ride comfort and enhance the ride comfort of mini vehicles. According to the 5-degree-of-freedom vibration model of an electric mini off-road vehicle, the partial differential matrix equation of system motion is established using the Lagrange method, and the frequency response characteristic of the system is analyzed. The input matrix of pavement unevenness is obtained by considering the mutual power spectrum density between front and rear wheels. Road surface roughness information is obtained using an instrument for measuring road roughness. A comprehensive objective function and a constraint condition are established for comfort and safety. Based on the parameters obtained through the optimum design of a 1/4 vehicle model, the optimized stiffness and damping coefficient of suspension are obtained using a MATLAB optimization program. The law of the vibration performance of off-road vehicles with respect to suspension stiffness and the damping coefficient is obtained through the analysis of optimized results. A driving simulation and a test are conducted on the electric mini off-road vehicle. The results show that the use of the measured pavement data as simulation input is closer to the actual situation and provides higher accuracy compared to the simulated pavement model. According to the optimization and test results, the parameters optimized by a 1/2 vehicle (5 degrees of freedom) vibration model are better than those optimized by the 1/4 vehicle vibration model. The optimization results confirm reduction in acceleration, acceleration power spectrum density, and the root mean square of the weighted acceleration of the seat. This shows that the electric mini off-road vehicle provides better ride comfortability after optimization.

Components Sizing and Performance Analysis of Hydro-Mechanical Power Split Transmission Applied to a Wheel Loader

The powertrain efficiency deeply affects the performance of off-road vehicles like wheel loaders in terms of fuel economy, load capability, smooth control, etc. The hydrostatic transmission (HST) systems have been widely adopted in off-road vehicles for providing large power density and continuous variable control, yet using relatively low efficiency hydraulic components. This paper presents a hydrostatic-mechanical power split transmission (PST) solution for a 10-ton wheel loader for improving the fuel economy of a wheel loader. A directly-engine-coupled HST solution for the same wheel loader is also presented for comparison. This work introduced a sizing approach for both PST and HST, which helps to make proper selections of key powertrain components. Furthermore, this work also presented a multi-domain modeling approach for the powertrain of a wheel loader, that integrates the modeling of internal combustion (IC) engine, hydraulic systems, mechanical transmission, vehicle(wheel) dynamics, and relevant control systems. In this modeling, an engine torque evaluation method with a throttle position control system was developed to describe the engine dynamics; a method to express the hydraulic loss of the axial piston hydraulic pump/motor was developed for modeling the hydraulic transmission; and a vehicle velocity control system was developed based on altering the displacement of a hydraulic unit. Two powertrain models were developed, respectively, for the PST and HST systems of a wheel loader using MATLAB/Simulink. The simulation on a predefined wheel loader drive cycle was conducted on both powertrain models to evaluate and compare the performance of wheel loader using different systems, including vehicle velocity, hydraulic displacement control, hydraulic torque, powertrain efficiency, and engine power consumption. The simulation results indicate that the vehicle velocity controller developed functions well for both the PST and HST systems; a wheel loader using the proposed PST solution can overall save about 8% energy consumption compared using an HST solution in one drive cycle. The sizing method and simulation models developed in this work should facilitate the development of the powertrains for wheel loaders and other wheeled heavy vehicles.

Research on differential performance of four‐wheel independent steering of a hydraulic wheel‐driving off‐road vehicle

Steering differential performance is an important factor affecting the driving performance of the vehicle. A hydraulic control method is proposed for the differential system of the hydraulic wheel-driving off-road vehicle, and the driving force equalisation technique is adopted to realise the adaptive steering differential under the different speed modes. Based on the analysis of the working principle of static pressure driving steering differential, the mathematical model of the steering differential system is derived by using kinematics and dynamics theory. The hydraulic driving system and multibody dynamics model are established by AMESim and LMS Virtual.Lab Motion, respectively, and joint simulation is carried out. The experimental scheme is designed for the typical working conditions in the simulation, and the rotation speed and the pressure differential of the motor in the steady state are analysed. The experimental results are consistent with the theoretical analysis and simulation results, which indicate that the system can achieve a better steering differential performance. This research provides a new idea for the study of the steering performance of the hydraulic wheel-driving walking system, which is of great practical significance to improve the driving performance of off-road vehicles under the complex terrain.

Application of Terramechanics in Off-road Vehicle Performance Prediction

This paper is an introduction of terramechanics and its application in off-road vehicle mobility prediction. Firstly the concept of terramechanics is introduced. Terramechanics is an important branch of applied mechanics in which the interaction between the vehicle and its operating terrain is studied. Secondly, many well-known techniques in terramechanics, such as Cone Penetrometer technique, Bevameter technique are introduced. By comparing the RCI of soil with VCI of vehicle, trafficability of Off-road vehicle can be determined which predicts the “go/no go” property of vehicle. Furthermore, to predict the speed behavior of off-road vehicle over trafficable terrain, three types of analyzing methods, empirical, semi-empirical, computer-aided, are presented in detail. It is evident that computeraided method is much more efficient in simulating terrain-vehicle interaction and produces more realistic prediction in vehicle mobility. Therefore, it is meaningful to develop computer-aided method to provide engineers with powerful parametric analysis in vehicle performance and design.

Improving accuracy of vehicle-terrain interface algorithms for wheeled vehicles on fine-grained soils through Bayesian calibration

The Vehicle-Terrain Interface (VTI) model for off-road vehicle performance is often used in virtual prototyping to support preliminary vehicle design. Five VTI algorithms for wheeled vehicles on fine-grained soils are calibrated in this study using a Bayesian calibration technique and the Database Records for Off-road Vehicle Environments (DROVE): powered and unpowered sinkage, drawbar pull, motion resistance, and gross traction. These algorithms are improved through a two-stage Bayesian calibration technique utilizing the Metropolis algorithm with three separate calibration strategies. The VTI and proposed algorithms are compared for performance in coefficient of determination and root-mean square error. The results from each algorithm were validated through k-fold cross validation with five folds. The final algorithms from the best performing strategy after accounting for increased complexity for each of the five performance parameters are reported as calibrated algorithms. Validated improvements in coefficient of determination are recorded for all five parameters: 7.8% for powered sinkage, 1.4% for unpowered sinkage, 6.4% for drawbar pull, 0.9% for motion resistance, and 12.5% for gross traction. Improvements are also seen in the normalized root-mean square error performance: 13.4% for powered sinkage, 1.9% for unpowered sinkage, 7.6% for drawbar pull, 17.5% for motion resistance, and 23.2% for gross traction.

Driving force distribution based on tyre energy for independent wheel-drive vehicle on rough ground

The driving force of a wheel is an important factor to determine the travelling performance of vehicles on rough grounds. An excessive driving force induces a large slip, whereas insufficient driving force prevents vehicles from overcoming obstacles. Hence, an optimum driving force distribution is necessary to improve the travelling performance of off-road vehicles on rough grounds. In this study, the driving force distribution method of four independent wheel-drive vehicles based on the energy-loss ratios of the wheels is proposed. The squared sum of the energy-loss ratios is minimized in the proposed method. To validate the proposed method, travelling tests were conducted using a model vehicle on a rough and rigid ground and a soft ground. Moreover, simulation tests were performed on a rough and rigid ground. The results obtained using the proposed method were compared with those of the distributions based on the tyre workload, even force, and wheel-velocity control. The experimental and simulation results confirm that the proposed method is the most efficient for vehicles moving on a rigid ground. The movement of the vehicle on the soft ground was the most stable when the proposed method was employed.

ANCF Continuum-Based Soil Plasticity for Wheeled Vehicle Off-Road Mobility

This technical brief describes the procedure and demonstrates the feasibility of integrating soil/tire models using the absolute nodal coordinate formulation (ANCF). The effects of both the soil plasticity and the tire elasticity are captured using ANCF finite elements (FEs). Capturing the tire/soil dynamic interaction is necessary for the construction of higher fidelity off-road vehicle models. ANCF finite elements, as will be demonstrated in this paper, can be effectively used for the modeling of tire and soil mechanics. In this investigation, the soil model is developed using ANCF hexahedral finite elements, while the tire model can be developed using different ANCF finite elements including beam, plate, or solid elements; ANCF plate elements are used in this investigation for demonstration purposes. The Drucker–Prager plastic material, which is used to model the behavior of the soil, is appropriate for the simulation of a number of types of soils and offers a good starting point for computational plasticity in terramechanics applications. Such higher fidelity simulations can be fruitfully applied toward the investigation of complex dynamic phenomena in terramechanics. The proposed ANCF/Drucker–Prager soil model is implemented in a multibody system (MBS) algorithm which allows for using the ANCF reference node (ANCF-RN) to apply linear connectivity conditions between ANCF finite elements and the rigid components of the vehicle. This new implementation is demonstrated using a tire of an off-road wheeled vehicle. The generality of the approach allows for the simulation of general vehicle maneuvers over unprepared terrain. Unlike other approaches that implement force or superelement models into an MBS simulation environment, in the approach proposed in this paper both the soil material and vehicle parameters can be altered independently. This allows for a greater degree of flexibility in the development of computational models for the evaluation of the off-road wheeled vehicle performance.

A Measuring Instrument for Off-road Vehicle Performance Testing

An overall description of a measuring instrument, wheel force transducer (WFT), developed for off-road vehicle performance testing is presented in the paper. For the research of off-road vehicle performance, it is important to determine the forces and torques as a response of the interaction between the wheel and the terrain. The WFT developed provides a solution of the direct measurement of the physical quantities. The WFT was designed as an automatic real-time data acquisition device with the wireless transmission technology. In the aspect of mechanical structure, only minor modifications of the wheel rim and two flanges were required for the installation of the WFT. The elastomeric of the sensor was designed with excellent mechanical strength for wheeled vehicles of weights ranging from 1.2 to 15 ton. Field tests were conducted after static calibration and dynamic calibration. The WFT was installed on the two front wheels of a 1.6-tone weight car. In preliminary field test, various vehicle maneuvers were executed for the validation of the validity, reliability and accuracy of the whole system, which was demonstrated by the test results. Six kinds of special testing pavements were selected to simulate the off-road situations. From the results, great potential of the WFT system is indicated in the application of off-road vehicle performance testing.

Mobility algorithm evaluation using a consolidated database developed for wheeled vehicles operating on dry sands

A substantial number of laboratory and field tests have been conducted to assess performance of various wheel designs in loose soils. However, there is no consolidated database which includes data from several sources. In this study, a consolidated database was created on tests conducted with wheeled vehicles operating in loose dry sand to evaluate existing soil mobility algorithms. The database included wheels of different diameters, widths, heights, and inflation pressures, operating under varying loading conditions. Nine technical reports were identified containing 5253 records, based on existing archives of laboratory and field tests of wheels operating in loose soils. The database structure was assembled to include traction performance parameters such as drawbar pull, torque, traction, motion resistance, sinkage, and wheel slip. Once developed, the database was used to evaluate and support validation of closed form solutions for these variables in the Vehicle Terrain Interface (VTI) model. The correlation between predicted and measured traction performance parameters was evaluated. Comparison of the predicted versus measured performance parameters suggests that the closed form solutions within the VTI model are functional but can be further improved to provide more accurate predictions for off-road vehicle performance.

Design of Transmission System Optimization Software for Off-Road Vehicle

Based on a technique of mixed programming with MATLAB and C#, the transmission parameters optimization software was developed to improve the power performance of off-road vehicle. The power loss rate and the starting acceleration time were chosen as objective function, and the requirements of the vehicle performance and the gearbox ratio interval was used as constraint conditions. The result of verification example showed that both objective function values are reduced after optimization and the effect of optimization with power loss rate is more significant.

Analysis of soil compaction and tire mobility with finite element method

Soil compaction, as a form of soil degradation, accounts for the increase of soil strength and the reduction of soil production capability. Two finite element tire models were developed based on the real geometry and structure of a Bridgestone bias tire. With a single wheel tester, field experiments were performed to validate the finite element tire–soil interaction models. The influence of axle load and inflation pressure on the soil compaction was studied with a finite element tire model using coarse meshes, which improves the computation efficiency without degrading the simulation accuracy. Tire mobility such as drawbar pull, rolling resistance and lateral force generated inside the tire–soil contact patch is an important index for the performance of off-road vehicles. The relationship between the slip ratio, the slip angle and the tire mobility was investigated through the simulations carried out with the other finite element tire model using fine meshes.

On the multi-wheeled off-road vehicle performance and control

For off-road vehicle applications, the construction of drivetrain system has usually been largely dominated by the mobility requirements. With the growing demand to have a multi-purpose on/off road vehicle with improved manoeuvrability over deformable soil, particularly at higher speed, the challenges opposing vehicle designers have become more complex. In addition to the conventional mechanical torque distribution devices, active torque distribution (ATD) systems utilise active differentials to control the drive torque independently distributed to each driving wheel and accordingly provide active control of traction and yaw moment. In this paper the multi-wheeled off-road vehicle performance and control is presented and different critical aspects are summarised and critically discussed.

Comparison of gradability performance of fuel cell hybrid electric and internal-combustion engine vehicles

Fuel cell vehicles are currently undergoing extensive research and development due to their potential for low emissions and high efficiency. Fuel cells are rapidly appearing not only in on-road vehicles but also in off-road vehicles. Off-road vehicles require special geometrical and performance parameters to be able to work in different road conditions. Gradability is one of the most important parameters in off-road vehicle performance which is defined as vehicle’s ability to climb a grade at a given speed. This paper presents a comparison study between four-wheel drive (4WD) fuel cell hybrid electric vehicle gradability performance and internal-combustion engine (ICE) 4WD vehicle using Matlab/Simulink software.

PREDICTION OF WHEEL NUMERIC FOR SANDY-TIRE

ABSTRACTThe various techniques for evaluating the wheel numeric and vehicle performancehave been studied. The wheel numeric depends on many parameters; vehicleweight, tire geometry, and soil strength in terms of cone index. The tire geometryincludes tire width, tire diameter, section height and tire deformation. The tiredeflection is very difficult to accurately measure. So the main purpose of this paper isto develop a new wheel numeric formula with a new parameter depending on theinflation pressure inside the tire instead of the tire deflection.A theoretical study to investigate the effect of soil cone index, tire geometry andvehicle weight on different wheel numeric has been carried out and so their effectson the vehicle performance have been carried out. The performance of off roadvehicle on soft soil in terms of rolling resistance, drawbar pull and tractive efficiencyhas been studied and investigated using variety of methods. These include;theoretical, semi-empirical and empirical methods based on wheel mobility number.The results showed that the new numeric formula gives a good correlation withpublished results at different soil cone index, vehicle weight, tire width, tire diameterand tire inflation pressure. Also the predicted performance has a good agreementwith published experimental results at same conditions.