Search

This research is spearhead for longitudinal slippage of robot platform wheels traversing on solid balls based uneven terrain. For this purpose, effects of solid ball diameter (0.0127–0.0508 m) accompanying with tire inflation pressure (20.68–55.16 kPa) and platform forward speed (0.17–0.50 m/s) on the slippage were practically quantified. Robotic experiments were executed in controlled test bed prepared by solid ceramic balls. Tentative data collected from the experiments indicate that the wheels experienced low ( 60%) under effects of independent variables. Results demonstrated that the increasing effect of the dominant variable of solid ball diameter on the slippage was 3.6 and 2.6 times greater than that of platform forward speed and tire inflation pressure, respectively. Meanwhile, the increasing linear effect of tire inflation pressure on the slippage was prevailed (1.4 times) than the decreasing linear effect of platform forward speed. Hence, to minimize the slippage as well as power loss of the platform traversing on each solid ball diameter, adjustment of tire inflation pressure should be enumerated as first priority. Overall, analytical results obtained in this research lead to open a new prospection for comprehending the slippage trends of off-road wheeled robots traversing on an uneven terrain composed of solid balls. Practicability of this type of the robots is remark in various industrial robotic operations, especially for process towers and reactors of oil, gas, petrochemical, and chemical industries. Hence, quantitative and qualitative information of this research are a rich source for relevant industrial investigators and experts who involved in this realm.

A microcontroller-based embedded digital display and warning system was developed for measuring wheel slippage, velocity ratio, PTO torque, and draft requirement of active tillage machinery. The hardware system included magnetic pickup sensor for measuring the engine speed, load cells and amplifiers to measure and amplify the sensing unit signals of the draft, proximity sensors for wheel slip, and PTO torque transducer for measuring the torque requirement. It was provided with buzzers and LEDs to warn the operator, whenever slip and velocity ratio were not in the desired range based on the algorithm, for maximum fuel efficiency and tractive performance. It measured slippage, velocity ratio, torque and draft with a maximum absolute variation of 12.90%, 7.92%, 8.99% and 11.57%, respectively. The developed system can be easily adaptable to any combination of tractor and tillage implements, and guide the operator for better soil tilth with lesser energy input.

Application of rotary tillage has been increased due to less tillage passes required, reduced draft, and greater efficiency through reduction in wheel slippage. Early failure of the bearing of tractor power take-off (PTO) shaft was observed in tractors of power range 30–35 horsepower during rotary tillage. An instrumentation setup involving an extended octagonal ring transducer (EORT) was developed and installed at the bottom of the bearing to measure the axial load and the vertical component of the radial load. The horizontal component of radial load was measured by strain gauges. Based on measured loads, the bearing life was assessed. Independent variables were: operating depth, number of blades, gear setting, engine speed, and tyre size. The average axial and radial loads varied from 786–3869 N, and 134–430 N, respectively. However, bearing experienced very high peak loads during each trial. The peak axial and radial loads was recorded between 1081–7534 N and 566–1794 N, respectively. The estimated bearing life based on peak loads was 171.98–28341.39 h. Based on the findings, it may be concluded that the average loads were not sufficient to cause quick failure of PTO bearing, rather sudden peak loads might be the root cause of early failure.

Terramechanics plays an important role in determining the design and control of autonomous robots and other vehicles that move on granular surfaces. Traction capabilities, slippage, and sinkage of a robot are governed by the interaction of a robot’s appendage with the operating terrain. It is important to understand how the terrain flows under this appendage during such an interaction. In this work, dynamics of soil performance and locomotion performance of a lugged wheel travelling on soft soil are numerically investigated. Studies are conducted with a two-dimensional model by using the discrete element method to analyze the interactions between a lugged wheel and the soil. The soil performance is studied by examining the force distribution and evolution of force networks during the course of the wheel travel. For two different control modes, namely, slip-based wheel control and angular velocity-based wheel control, the performance parameters such as, sinkage, traction, traction efficiency, and power consumption of the wheel are compared for various wheel configurations. The findings of this work are expected to be useful for optimal design and control of the lugged wheel travelling on deformable surfaces.

This paper presents a comprehensive comparison of well-known machine learning algorithms for estimating discrete slip events associated with individual wheels in planetary exploration rovers. This analysis is performed with various tuning configurations for each algorithm (55 setups). This research also shows the key role that environment plays in the performance of the learning algorithms: rover speed (0.05–0.25 [m/s]), type of terrain (gravel vs. sand), and tire type (off-road tires vs. smooth tires). These contributions are validated by using a broad data set collected using a planetary rover equipped with proprioceptive sensing. This work not only identifies the best algorithm to be deployed for discrete slip estimation, but it also helps with the selection and the mounting position of the sensing systems to be employed in future robotic planetary missions.

This paper investigates the traveling and abrasion characteristics of rigid wheels for a lunar exploration rover at atmospheric pressure and in a vacuum. For this investigation, a traveling test system that enables the wheel to continuously travel over a long distance was developed. Using this system, tests on traveling performance and abrasion were conducted with the wheel on a lunar regolith simulant surface. In the initial tests, various wheels traveled over different ground conditions and their performances were evaluated based on the relationship between the drawbar pull and slippage. In the later tests, a wheel with grousers traveled a distance of 3 km and the abrasion was analyzed at various intervals. From the traveling performance tests, it was found that for a soft ground condition, the traveling performance of the wheels in vacuum was slightly lower than that in atmosphere. This indicates that ground tests performed in atmosphere overestimate the actual performance on the lunar surface. The abrasion tests suggested that the scratching of wheels occurs more easily in vacuum than in atmosphere. These experiments confirmed that the abrasion of the wheels do not cause any critical problem for a traveling distance of up to 3 km in a simulated lunar environment.

There is a need to radically increase mobility of terrain vehicles through new modalities of vehicle locomotion, i.e., by establishing a new technological paradigm in vehicle dynamics and mobility. The new paradigm greatly applies to military vehicles for the radical improvement of tactical and operational mobility. This article presents a new technological paradigm of agile tire slippage dynamics that is studied as an extremely fast and exact response of the tire–soil couple to (i) the tire dynamic loading, (ii) transient changes of gripping and rolling resistance conditions on uniform stochastic terrains and (iii) rapid transient changes from one uniform terrain to a different uniform terrain. Tire longitudinal relaxation lengths are analyzed to characterize the longitudinal relaxation time constants. A set of agile characteristics is also considered to analyze agile tire slippage dynamics within a time interval that is close to the tire longitudinal relaxation time constants. The presented paradigm of agile tire slippage dynamics lays out a foundation to radically enhance vehicle terrain mobility by controlling the tire slippage in its transient phases to prevent the immobilization of a vehicle. Control development basis and requirements for implementing an agile tire slippage control are also analyzed and considered.

The traveling performance of off-the-road vehicles, such as construction machinery and exploration rovers, significantly depends on the interaction between the ground and the traveling mechanism, since inelastic ground deformation and frictional sliding phenomena are induced by the vehicle’s movement. In general, a tread surface causes anisotropic frictional interaction behavior on a macroscopic scale. In this study, an acceptable frictional interaction model was implemented to finite element method to rationally examine the anisotropic frictional interaction behavior between the tire and the ground. Finite element analysis of the single tire traveling performance, including certain slippage and side slip (skid), was then carried out to examine the effect of the anisotropic frictional interaction on the numerical results for the drawbar-pull and side force.

On the Moon or Mars, typical target environments for exploration rovers are covered with fine sand, so their wheels easily slip on such weak ground. When wheel slippage occurs, it is hard for the rover to follow its desired route. In the worst case, the rover gets stuck in loose soil and cannot move anymore. To reduce the risk of the rover getting stuck, analysis of the contact mechanics between the soil and wheel is important. Various normal stress distribution models for under the wheel surface have been proposed so far. However, classical models assume a uniform stress distribution in the wheel’s width direction. In this study, we measured the two-dimensional normal stress distribution of a wheel in experiments. The results clarified that the stress distribution in the wheel’s width direction is a mountain-shape curve with a peak located at the center of the wheel. Based on the results, we constructed a stress distribution model for the wheel’s width direction. In this paper, we report our measurements for the two-dimensional stress distribution of a wheel on loose soil and introduce our stress distribution model for the wheel’s width direction based on our experimental results.

In recent years, various numerical simulation methods have been applied to vehicle terrain interaction problems. In this study, the formation of periodic ripples under a rigid wheel was investigated within the framework of finite element (FE) analysis. First, the capability of FEM to analyze periodic ripple formation was demonstrated under relatively low slippage. Then, a hypothesis for the generation mechanism of periodic ripples was provided: periodic ripples are formed under conditions in which clear local failure zones occur and soil is translated vertically via wheel shear deformation action. Furthermore, the characteristics of ripple periodicity (i.e. ripple pitch and period) were discussed. It was also revealed that ripple formation has an impact on the multi-pass scenario for lightweight small vehicles.

In order to perform vehicle motion analysis, a model of the interaction between the traveling tire and the ground surface is necessary. The interaction model or the tire model needs to adequately obtain the forces exerted on the tire based on the ground characteristics, besides being simple and easy to use. This research aims to construct a simple tire model for the motion analysis of a vehicle traveling on dry sand. A single small rubber tire was driven on evenly flattened dry Toyoura standard sand to measure the longitudinal and lateral forces on the tire. During the wheel drive test, the tire was dug into the sand to keep the sinkage constant. In addition, the tire was given a sideslip angle, which is the attack angle to its travel direction. The collected data demonstrated that the sand–tire interaction produces a tire force in the same direction as the tire slippage, and the definition of the tire force norm for each sinkage value well approximates the data using a single curve independent of the sideslip angles. By using those characteristics, the tire model can be expressed by relatively simple functions of the sideslip angle and sinkage. A turning motion experiment on a four-wheel model vehicle on evenly flattened sand was conducted, and the result was compared with the result from the numerical simulation with the tire model, which confirmed the model’s accuracy.

In a planetary exploration mission, a rover’s slippage badly limits its control performance. This paper presents an online method to modify the predictive errors for the slippage caused by the variability of the soil parameters and limitation of the predictive models. To modify the slippage appropriately, all of the soil parameters related to the predictive models are analyzed to estimate their sensitivities to slippage, and tests indicate that φ and K have the greatest influence on the slippage. Based on the differential principle, φ and K are decoupled and solved using the predictive model errors. To eliminate the errors in the modification process caused by the models’ nonlinearity, an effective coefficient is proposed, and its definition is given based on Taylor’s formula. To implement a more stable modification, some necessary constraints are presented. The validity of this method is verified using simulations and experiments. The results show that approximately three steps are needed to reach the convergence points; it has a good response with a fluctuation in the input slippage; a reasonable initial value will be beneficial for the modification. In summary, this method is effective at eliminating predictive slippage errors; the proposed effective coefficient and method are helpful at boosting the modification’s convergence speed and maintaining its stability.

Planetary rovers are typically developed for high-risk missions. Locomotion requires traction to provide forward thrust on the ground. In soft soils, traction is limited by the mechanical properties of the soil, therefore lack of traction and wheel slippage cause difficulties during the operation of the rover. A possible solution to increase the traction force is to increase the size of the wheel-ground contact area. Flexible wheels provide this due to the deformation of the loaded wheel and hence this decreases the ground pressure on the soil surface. This study focuses on development of an analytical model which is an extension to the Bekker theory to predict the tractive performance for a metal flexible wheel by using the geometric model of the wheel in deformation. We demonstrate that the new analytical model closely matches experimental results. Hence this model can be used in the design of robust and optimal traction control algorithms for planetary rovers and for the design and the optimisation of flexible wheels.

It has been known from empirical equations that soil strength can be determined if wheel sinkage and slip of a vehicle can be measured on a soil surface. In this study, field data of wheel sinkage and slip were collected from two platform tractors of different sizes on gravely sandy and sandy loam soils. Using an empirical equation, the rating cone index was determined using the measured wheel sinkage and slip data. The data demonstrated that the same rating cone index can be obtained although the measuring platforms are different. It was also noted that the rating cone index can be estimated in real time by measuring the sinkage and slippage of a driving wheel.

To investigate influences of gravity on mobility of wheeled rovers for future lunar/planetary exploration missions, model experiments of a soil–wheel system were performed on an aircraft during variable gravity maneuvers. The experimental set-up consists of a single rigid wheel and a soil bed with two kinds of dry sands: lunar soil simulant and Toyoura sand. The experimental results revealed that a lower gravity environment yields higher wheel slippage in variable gravity conditions. In addition to the partial gravity experiments, the same experiments with variable wheel load levels were also performed on ground (1 g conditions). The on-ground experiments produced opposite results to those obtained in the partial gravity experiments, where a lower wheel load yields lower slippage in a constant gravity environment. In low gravity environments, fluidity (flowability) of soil increases due to the confining stress reduction in the soil, while the effect of the wheel load on sinkage decreases. As a result, both of these effects are canceled out, and gravity seemingly has no effect on the wheel sinkage. In the meantime, in addition to the effect of wheel load reduction, the increase of the soil flowability lessens the shear resistance to the wheel rotation, as a result of which the wheel is unable to hold sufficient traction in low gravity environments. This suggests that the mobility of the wheel is governed concurrently by two mechanisms: the bearing characteristics to the wheel load, and the shearing characteristics to the wheel rotation. It appears that, in low gravity, the wheel mobility deteriorates due to the relative decrease in the driving force while the wheel sinkage remains constant. Thus, it can be concluded that the lunar and/or Mars’ gravity environments will be unfavorable in terms of the mobility performance of wheels as compared to the earth’s gravity condition.

Driving gear of a vehicle (here a track) generates thrust as a reaction to the opposite force taken by the ground. This force causes rearward soil deformation, which is associated with vehicle slippage. The presented compression–sliding (CS) approach, based on field measurements with the original double plate (DP) meter, states that the soil deformation as a consequence to the increasing thrust occurs in two principal stages: (a) primary horizontal soil compression, which steadily increases towards the back of the contact length forming virtual soil segments among neighboring grousers and (b) secondary slide of these sheared off segments referred to as soil blocks, which may collapse under specific conditions. These two stages are separated by a transient situation when both the compression and sliding occur simultaneously. The respective compression–sliding (CS) approach enables to analyze the effect of track arrangement, design and loading on its thrust–slip characteristics. The paper also deals with situation of the existing shear plane theory in view of the CS approach and finally suggests a practical thrust–slip function complying with the CS logic.

Prediction of wheel performance by analysis of normal stress distribution under the wheel-soil interface was reported by one of our research members. In this study analysis of both normal and tangential stress distributions are included for the prediction of wheel performance. A visco-elastic soil model based on a three-element Maxwell model is used to evaluate normal stress distribution under a wheel running on soft ground. The values of the parameters characterizing the visco-elastic behavior of the soil can be derived from plate penetration tests. A rigid wheel-soil interface model is used to evaluate the tangential stress distribution under the wheel-soil interface. Shear deformation modulus, cohesion and angle of internal shearing resistance of the soil are derived from shear-displacement tests. Test results indicate that both maximum normal and shear stress occur in front of the wheel axle, and the location of peak normal stress shifts backwards towards the wheel axle while that of tangential stress shifts forwards when slippage is increased from a low value. Increasing slippage also causes a decrease in normal stress and an increase in tangential stress. Coefficients of traction and tractive efficiency are low at low slippage, increase with an increase in slippage, and level off at higher slippage.

In order to understand clearly the characteristics of the soil reaction forces on a single movable lug, the resultant of measured soil reaction forces was determined and presented along with its position on the lug plate. The resultant of soil reaction forces acting on the movable lug increased gradually and reached the maximum value when the lug was on about its lowest position in the soil, then it decreased without offering any downward resistance to the lug till the lug left the soil. The maximum resultant force of the movable lug was higher than that of a fixed lug. The point of action of the resultant force on the movable lug shifted in a similar way in all test cases, that is, it moves to the center of the lug from the outer tip until it reaches the position where it becomes the maximum, then it moves to the outer tip till the lug leaves the soil. The inclination angle of the resultant force increased with the decrease of lug inclination angle. The bigger lug sinkage of the movable lug produced bigger soil reaction forces and shifted the point of action of the resultant force from the tip part to the central part of the lug. However, there was no significant effect of the lug sinkage on the direction of the resultant force. The increase in lug slippage from 25% to 50% brought bigger soil reaction forces on the movable lug, but did not influence the direction and point of action of the resultant force.

The assumption of “zero true slippage at zero net traction” by Upadhyaya has given rise to a heated argument of wide interest for ISTVS engineers. This article discusses this argument. Using the traction prediction equations presented by Upadhyaya, traction data obtained from experiments with a 4WD tractor in tilled Kanto loam soil were analyzed. It was impossible to evaluate the unique values for all the parameters of the traction prediction equations independently. We proposed a regression model by reforming Upadhyaya's equations, and the regression model fitted well with the traction data plotted. Implications of a zero condition in the traction data analysis are also discussed.

The advantages of using a four-track steering (4TS) vehicle are less slippage and sinkage of the tracks in turning, compared with conventional skid-steering tracked vehicles. This paper describes the steerability and the mobility of the 4TS vehicle for on- and off-road conditions. Mathematical models have also been developed to predict the effects of various types of differential torque transfer on the handling behavior of a vehicle. A turning vehicle motion was simulated and compared with the experimental data. The results demonstrate that the proposed mathematical model could accurately assess the steering performance of a 4TS vehicle.

A bow-shaped enamel coated float attachment was developed for a 4.74 kW power tiller and fixed underneath the power tiller between the two cage wheels. The performance of the power tiller with and without the float was studied in flooded and puddled soil conditions. During the general performance test it was observed that the float prevented the cage wheels from digging deeper into the soil when slippage occurred. The float-cage wheel combination eased the operator's work. The statistical analysis showed that there was no significant difference in the pull and power delivered by the power tiller with and without the float in the flooded soil condition. However, in a puddled soil condition the power tiller with the float showed lower performance, in terms of pull and power delivered, than the power tiller without the float, as the cage wheels were prevented by the float from reaching the hardpan layer lying below the top puddled layer to develop enough pull. The drag test of the power tiller with and without the float showed that the presence of a float on a power tiller did not increase the drag force required to pull it.

The purpose of this paper is to analyse the several factors affecting the critical wear life of off-road tyres of wheel loaders, and to estimate the wear life of the tyre. Here, the variation of tread wear depth with service meter hour was measured for two kinds of OR tyre, 37.25-35-30/36PR and 45/65-45-50PR which were used for bucket capacity 7.7 and 9.1–12.0 m 3 of wheel loader, respectively. As a result, the critical wear life could be expressed by the initial tread depth, the tyre diameter, the real contact pressure, the roughness of terrain surface, the loading cycle and the amount of slippage. To elongate the wear life, the terrain surface should be controlled to be as smooth as possible and the amount of tyre slippage should be decreased by means of increasing the rear axle load.

Using a forced-slippage test apparatus and a small three-axial force transducer attached to a tire surface, the distribution of normal, longitudinal and lateral stresses along the tire-soil contact area were precisely, simultaneously and independently measured on a standard sand in the laboratory and processed by a personal computer. Stress distribution patterns, positions of the maximum stresses, relations between thrust and sside force with parameters of slippage and side slip angle and so on are shown in both cases of straight and slant runs.

To determine the tractive performance of a bulldozer running on weak ground in the driven state, the relations between driving force, drawbar pull, sinkage, eccentricity and slip ratio have been analysed together with each energy balance; effective input energy, sinkage deformation energy, slippage energy and drawbar pull energy. It is considered that the thrust is developed not only on the main straight part of the bottom track belt but also on parts of the front idler and rear sprocket, and the compaction resistance is calculated from the amount of slip sinkage. For a given vehicle and soil properties, it is determined that the drawbar pull increases directly with the slip ratio and reaches about 70% of the maximum driving force. The compaction resistance reaches about 13% of the maximum driving force. The sinkage of the rear sprocket, the eccentricity, and the trim angle increase with the increment of slip ratio due to the slip sinkage. These analytical results have been verified experimentally. After determining the optimum slip ratio to obtain a maximum effective tractive power, it is found that a larger optimum drawbar pull at optimum contact pressure could be obtained for a smaller eccentricity of vehicle center of gravity and a larger track length-width ratio under the same contact area.

This paper presents a theoretical analysis of steerability of tracked vehicles during uniform turning on level pavement. Considering all possible factors related to steering problems such as track slippage, centrifugal force and vehicle configuration, equations for uniform turning motion have been developed in order to analyze and predict steering dynamics, and steerability in plane motion of vehicles. These equations have been numerically solved by a digital computer in terms of turning radius, side slip angle, shift of instantaneous center of vehicle and track slippage.

This paper presents a theoretical analysis of non-stationary motion of a tracked vehicle on level ground. A practical model that includes track slippage, inertia force and the moment of inertia was developed to analyze and predict steering dynamics and steerability on the subject examined. The system of differential equations was programmed and numerically solved on a digital computer, where the inputs are circumferential velocities of right and left drive sprockets. The simulations for J-turn maneuver disclose the effects of initial forward velocities on the transient responses of the track slip velocity, side slip angle, yaw rate, and acceleration of the center of gravity of a tracked vehicle.

Track forces for outer and inner tracks have been calculated for a military tracked vehicle in a skid-steer situation. The present work is an attempt to improve the understanding of track force variation with turning radius. Furthermore, a reasonable estimate of transmission loads is required for the design of steering transmission for turning a tracked vehicle. This may also be obtained from the track forces. The understanding of track force variation with turning radius has been rather poor. In literature, the reason for lower track force at larger turning radius has been explained in terms of the deflection of the various suspension components like the track shoes, bushings, etc., which are associated with steer action. Deflection of the suspension components does not seem to be an adequate explanation for the variation of track forces with turning radius. In the present work, track forces have been obtained from the dynamics of the moving vehicle. The variation of tractive coefficient (coefficient of friction) due to lateral track slippage has also been considered. This is where the present work differs from the conventional track force estimation where a constant value of coefficient of lateral friction has been used. The estimation of tractive coefficient is made by using pull-slip equation found in literature. The explanation of decreasing track force with increasing radius is given in terms of variation of slip with speed and turning radius. It is found from the study that the concept of variation of coefficient of friction (tractive coefficient) is very important and probably a realistic one in the prediction of track forces. The results of the calculations compare reasonably well with the trends of test result plots obtained in the literature.

A mathematical model was developed to describe the motion resistance of rigid wheels under self-propelled conditions where no drawbar pull is developed. The modeling was based on the concept that the work required to overcome the motion resistance is equal to the energy dissipated in compacting the soil beneath the wheel. Slippage of the wheel was also considered in the modeling. Finally the validity of the proposed model was discussed.

This paper presents theoretical and experimental analysis of steering performance of articulated tracked vehicles on level ground. A mathematical model for predicting the steerability of articulated units has been developed and computerized for numerical application. The accuracy of the analog has been verified by scale model tests. From the results of the simulation and scale model tests it was found that steerability was significantly improved and required sprocket torques for steering and track slippage were considerably decreased in articulated tracked vehicles when compared with a single and coupled tracked vehicles.

The failure phenomena associated with blade action in soil have been observed with two-dimensional glass box apparatus for different blade configurations and different soil types. The three soils used were (1) a medium-dense, dry Ottawa sand, (2) a silty clay, and (3) a plastic clay. The situation examined were (1) a vertical blade in all soils, (2) a blade inclined 45 deg to the rear in silty clay and plastic clay, (3) an L-shaped blade with horizontal cutting edge in all soils, and (4) a blade inclined 20 deg forward in sand. A series of photographs were taken to show the soil deformations and mode of failure in each case. In sand the failure patterns generally resemble those prescribed by ‘passive earth pressure’ theories, but the failure surfaces did not intersect the soil surface at the angle of 45 - o/2 deg indicated in the theories. After each failure occurred addition blade movement was required before the next one formed. The inclination of these successive failure surfaces decreased due to build up to overburden with blade movement. The soil particle adjacent to the blade remained stationary, the slippage occurring in the soil. In silty clay and plastic clay, passive shear failure occurred for the vertical blade and a tensile failure or splitting due to the uplift for the inclined blade. The passive shear failure sursfaces were approximately plane and slippage of the soil occurred along the blade. The failures were discontinuous and the inclination changed as for the sand. The L -shaped blade initiated a tensile crack and then a shear failure through the rupture zone after the vertical part of the blade came into contact with the soil.

The steady two-dimensional flow benetath a driven rigid cylindrical wheel moving in a soft saturated clay is solved by assuming that the soil behaves like a rigid-plastic material. The general equations of plastic flow are discussed, with ephasis on the intertial effects. A plastic flow pattern is suggested and an approximate solution providing the magnitude of the recovery angle, the vertical and horizontal forces and torque acting on the wheel as functions of wheel radius, sinkage and shear strees along the wheel (assumed constant on the blow portion) is found. A minimum slippage necessary to maintain the shear strees on the wheel is found. The theoretical results are compared with some existing measurements and the agreement is generally satisfactory.

INTRODUCTION IN JANUARY, 1972, the International Society for Terrain-Vehicle Systems, Inc., and the Society of Automotive Engineers, Inc., published a paper entitled Load Support Capability of Flat Plates of Various Shapes in Soils, by Dr. I. R. Ehrlich and Colonel A. D. Sela [1]. In this work, Dr. Ehrlich and Colonel Sela presented a significant improvement upon the long used methods of M. G. Bekker for predicting the loadbearing capacity of flat plates sinking into soft soils. In a subsequent paper, Prediction of Rigid Wheel Performance from Plate Sinkage Tests [2], prepared for the Department of Defense under Project Themis, Dr. Ehrlich and Colonel Sela carried their work one step further and developed equations for predicting the performance of rigid wheels in soft soils under "no slip" conditions. In this paper, experimental validation was performed in sand by Captain F. G. Smith [3]. It was the objective of this research project to provide further experimental validation for Ehrlich and Sela's wheel performance prediction equations in a clay soil. A secondary objective was to provide additional support for the use of the system constant as a means of comparing wheel performance.

A study was made of the effects of various testing techniques on the performance of a test wheel mounted with a pneumatic tire operating in an air-dry desert sand. Tests employing both controlled-slip and controlled-pull techniques were conducted with a single-wheel, dynamometer-equipped carriage. The results are presented in the form of graphs with emphasis placed upon the pull-slip and torque-slip relations. For the conditions tested, wheel performance was found to be independent of testing techniques with one exception: Consistent differences were noted when comparing results of programmed-increasing and programmed-decreasing slip tests. Although this disagreement occurred only for a narrow range of slip (approximately -10 to +10 percent), it can be important. A brief analysis of the horizontal forces acting on the dynamometer is given to amplify certain relations, particularly difference in pull-slip and drawbar pull-slip responses. It is shown that because of the inability to always attain a stable condition, controlled-slip tests are better than controlled-pull tests for defining pull-slip and torque-slip relations.