Tractive performance of a tread design for improved flexibility

The tractive performance of an 18.4R38 radial-ply tractor tire with increased flexibility in the tread area was compared to that of a standard tread design. Normal soil-tire interface stresses were measured at four locations on the lug surfaces of both tires operating on Decatur clay loam and Norfolk sandy loam soils. There was a tendency for the increased flexibility in the tread area to provide a higher net traction ratio at the same tractive efficiency as the standard tread design, especially on Decatur clay loam soil. The more flexible tread design reduced the magnitude of peak normal contact stresses across the tire width, which may have implications for reducing soil compaction without compromising tractive performance. The more flexible tire reduced the average normal contact stress by approximately 15% in the sandy loam soil and 23% in the clay loam soil for the range of operating conditions investigated.     

Advantages of all-season versus snow tyres for off-road traction and soil stresses

Tractive performance, as well as soil stresses under a vehicle equipped with two types of tyres, was investigated in this study. All-season and snow tyres were installed in a 14 T 6 × 6 military truck and the vehicle was driven over sandy and loess soil for drawbar pull tests. Simultaneously, the stress state was determined in the ground surface under the driving wheels. Effects of tread pattern on both traction curves and soil stress were analyzed for three different levels of vehicle loading. All-season tyres provide slightly better traction for both terrain surfaces, at all three loading levels, or the differences between traction measures are not significant. Soil stress analysis showed that the difference between the two tread patterns is not significant. Generally, on soft surfaces all-season tyres performed no worse than snow tyres, while they are pronouncedly better for highway use.     

Updated Standards of the International Society for Terrain-Vehicle Systems

The last version of the ISTVS standards was published in the Journal of Terramechanics in 1977. Since then, the document has not been updated, although new concepts, techniques, testing procedures, and technology have been developed in the last 40 years, which renders some content of the 1977 ISTVS standards outdated and in-complete. The ISTVS identified as a priority the need to develop a set of standards for terminology and testing for modern day research on off-road mobility. This paper, for which the work has been funded in part by ISTVS, is an updated version of the 1977 ISTVS standards and covers a range of aspects in off-road mobility for: vehicles, tires, tracks, soil, wheels, modelling approaches, test methods, and equipment.     

Prediction of tractive response for flexible wheels with application to planetary rovers

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.     

Terramechanics and its influence on the concepts of tractors, tractor power development, and energy consumption

Through the foundation and work of ISTVS, a forum and a megaphone have been provided to allow an interchange of terramechanics ideas among the Society's members. The important achievements in the science of terramechanics, which have been assisted by the Society's membership, are reviewed.The role of terramechanics in machine design has an effect on vehicle component geometry and selection. As an example, five agricultural tractor types are compared with respect to their tractive performance, based on an analysis of soil properties tire design, and ground pressure distribution. An analysis is also presented concerning traction-slip curves of radial and diagonal tires by accounting for the various componets of traction force and rolling resistance.Tractor development in the U.S.A. and Germany is discussed, together with the factors that have influenced this development since 1950. Consumption of energy in agriculture is analyzed, and the need for conservation of energy through more efficient fuel use, cultivation, and stabilization of energy consumption per worker is developed. The contribution that terramechanics can make to this effort by improving traction efficiences, optimal tractor design, soil cultivation practices, and off-road transportation is identified.     

Tire tread pattern design trigger on the stress distribution over rigid surfaces and soil compaction

Studies comparing the structural differences of tires have not qualitatively or quantitatively considered the effects of tread geometry on tire behaviour or the interactions of the tire with the surface. Therefore, to determine the effects of different tire tread patterns on the stress distribution of the tire and soil compaction, we compared the structural behaviours of a high-flotation tractive-tread (TT) tire and a smooth-tread (ST) tire. The experiments were conducted over a rigid and over a deformable surface. The results from the rigid surface shows the influences of the tread pattern and sidewalls is dependent of the loads. Over the deformable surface, the contact area of the TT tire was larger than that of the ST tire. The inflation pressure (IP) was mainly responsible for the load support before the soil reached its maximum deformation. Next, the tread and sidewalls exhibited the same behaviour as observed on the rigid surface. In addition, we observed alterations in the balloon point with the tread geometry and the type of surface due to changes in the contact pressure. With carcass deformation, the volume of the tire was visibly reduced, which indicated that the IP could increase.     

Multi-objective traction optimization of vehicles in loose dry sand using the generalized reduced gradient method

A work optimization strategy is combined with algorithms within the vehicle-terrain interface (VTI) model to maximize the traction of a four-wheel vehicle operating on loose dry sand. The optimization model distributes traction among the steered and non-steered wheels with the work optimum coefficient (WOC) of each wheel treated as an independent design objective. Drawbar pull (DBP), motion resistance (MR), longitudinal traction coefficient (LTC), lateral force coefficient (LFC), tire deflection, and wheel slip are key parameters that appear in the VTI model for traction performance analysis. The analysis includes wheels of different diameters, widths, heights, and inflation pressures, under variable wheel slips. A multi-objective optimization problem is formulated over a thirteen-dimensional search space bounded by eight design constraints. The generalized reduced gradient method is used to predict optimal values of the design variables as well as ground and traction parameters such as DBP, MR, LTC, and LFC for maximum slope climbing efficiency. The WOCs are maximized for lateral slip angles between 0° and 24° to find a set of Pareto optimal solutions over a wide range of weight factors. A method to apply the optimization results for predicting vehicle performance and traction control on dry sand is presented and discussed.     

Thrust and slip of a track determined by the compression–sliding approach

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.     

Traction data analysis with the traction prediction equation

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.     

Design of manned lunar rover wheels and improvement in soil mechanics formulas for elastic wheels in consideration of deformation

Most of the current lunar rover vehicle wheels are inconvenient for changing broken wheels and have poor shock absorbing in driving, so they cannot be used to carry people on the moon. To meet the demands for manned lunar transportation, a new wheel possessing a woven metal wire mesh tire and using hub-rim combination slide mechanism is designed in this article. The characteristics of the new wheel is analyzed by comparing with the same-size conventional rover wheels after demonstrating the validity of FEM simulation. The new wheel possesses lighter structure and superior shock absorbing. It also provides stronger traction because the deformation of the designed wheel increases the contact area between the tire and lunar terrain. In order to establish an on-line soil parameter estimation algorithm for low cohesion soil, the stress distribution along a driven deformable wheel on off-road terrain is simplified. The basic mechanics equations of the interaction between the wheel and the lunar soil can be used for analytical analysis. Simulation results show that the soil estimation algorithm can accurately and efficiently identify key soil parameters for loose sand.     

A study of soil thrust exerted by a tracked vehicle

This paper deals with soil thrust exerted by a tracked vehicle. Measurements of the ground pressure beneath the tracks of a tracked vehicle were carried out and it was shown that the ground pressure distribution is approximately represented by discontinuous triangles which have their maxima under the roadwheels. The relationship between soil shear curve (shear stress or force-deformation curve) obtained from shear test and thrust curve (soil thrust-slip ratio curve) of the tracked vehicle is analyzed by using the above mentioned ground pressure distribution, and it is shown that there is a transformation law between both curves. Namely, the thrust curve due to soil shear under any wheel portion is given as a function of soil and vehicle parameters. Further, the reliability of the above method is confirmed experimentally.     

A simplified method for the estimation of soil thrust exerted by a tracked vehicle

A simplified method for estimating the soil thrust exerted by a tracked vehicle is proposed. The relationship between the soil shear torque curve (shear torque-deformation curve) obtained from ring shear test and the thrust curve (soil thrust-slip ratio curve) of a tracked vehicle is analyzed and it is shown that there is a transformation law between these curves. A simplified analytical method for estimating the soil thrust exerted by a tracked vehicle is developed by using the above-mentioned transformation law. Soil thrust can be estimated by using the soil shear torque curve, shear ring and vehicle parameters. It is experimentally confirmed that the soil thrust can be easily estimated by using the proposed method.     

Analytical model of longitudinal tire traction in snow

An analytical model to estimate longitudinal traction of a tire in snow was developed and verified to have good predictability in comparison with measurements. Snow traction of a tire is composed of four kinds of forces in this model: braking force attributable to snow compression, shear force of snow in void (space between tread blocks), frictional force, and digging force (edge effect generated by sipes and blocks). The mechanical characteristics of snow were considered in the prediction of braking force and shear force, but were not considered in the prediction of other forces. The contribution of shear force of snow in void and the frictional force was large in static traction (traction just before a tire slips). On the other hand, the contributions of digging force and frictional force were large in situations involving high slip ratios.     

Soil compaction and tires for harvesting and transporting sugarcane

Four tire types (A, block-shape tread; B, rib-shape tread; C, low-lug tread; D, high-lug tread) used to harvest and transport sugarcane were compared regarding the compaction induced to the soil. Tires were tested at three inflation pressures (207, 276, 345 kPa) and six loads ranging from 20 to 60 kN/tire. Track impressions were traced, and 576 areas were measured to find equations relating inflation pressure, load, contact surface and pressure. Contact surface increased with increasing load and decreasing inflation pressure; however, the contact pressure presented no defined pattern of variation, with tire types A and B generating lower contact pressure. The vertical stresses under the tires were measured and simulated with sensors and software developed at the Colombian Sugarcane Research Center (Cenicaña). Sensors were placed at 10, 30, 50 and 70 cm depth. Tire types A and B registered vertical stresses below 250 kPa at the surface. These two tires were better options to reduce soil compaction. The equations characterizing the tires were introduced into a program to simulate the vertical stress. Simulated and measured stresses were adjusted in an 87–92% range. Results indicate a good correlation between the tire equations, the vertical stress simulation and the vertical stress measurement.     

Compression–Sliding approach: Dependence of transitional displacement of a driving element on its size and load

Every element of a pulling traction device (e.g. track shoe with grouser or tire section with lug) exhibits increasing rearward displacement during its engagement with soft ground. Compression–Sliding (CS) approach states in agreement with experimental evidence that on common soft ground this displacement starts due to longitudinal soil compression by the grouser or lug, which steadily increases up to the transitional displacement when the soil segment beneath a driving element fails in shear. Further displacement of a driving element is marked by forced slide of a sheared off soil block, which may eventually collapse. There was justified reasoning that the transitional displacement depends not only on the grouser (lug) contact pressure but also on the area and load of the respective traction element. The presented article reports on experiments designed to test this premise. The measurements applying the novel double plate (DP) meter technique were carried out in a laboratory soil bin containing loam charge of uniform bulk density and moisture content. Three sizes (proportions 1:2:4) and two mean vertical contact pressures (ratio1:2) of DP meter main plate were tested. The analysis of performed experiments confirmed the existence of dimensional and loading relationship “main plate – transitional displacement”, which bears upon the evaluation of thrust–slip relationship of any traction device by the CS approach or by any other method observing the existence of displacement.     

Dynamic load and inflation pressure effects on contact pressures of a forestry forwarder tire

A Trelleborg Twin 421 Mark II 600/55-26.5 steel-reinforced bias-ply forwarder drive tire at inflation pressures of 100 and 240 kPa and dynamic loads of 23.9 and 40 kN was used at 5% travel reduction on a firm clay soil. Effects of dynamic load and inflation pressure on soil–tire contact pressures were determined using six pressure transducers mounted on the tire tread. Three were mounted on the face of a lug and three at corresponding locations on the undertread. Contact angles increased with decreases in inflation pressure and increases in dynamic load. Contact pressures on a lug at the edge of the tire increased as dynamic load increased. Mean and peak pressures on the undertread generally were less than those on a lug. The peak pressures on a lug occurred forward of the axle in nearly all combinations of dynamic load, inflation pressure, and pressure sensor location, and peak pressures on the undertread occurred to the rear of the axle in most of the combinations. Ratios of the peak contact pressure to the inflation pressure ranged from 0 at the edge of the undertread for three combinations of dynamic load and inflation pressure to 8.39 for the pressure sensor on a lug, near the tire centerline, when the tire was underinflated. At constant dynamic load, net traction and tractive efficiency decreased as inflation pressure increased.     

Numerical investigation of snow traction characteristics of 3-D patterned tire

Traction and braking performances of automobile tire on the snow road are quite distinct from those on the dry or wet road, because of the complicated snow deformation caused by the complex tread blocks. In fact, the mathematical formulation of the snow deformation is extremely difficult, because not only it depends on the loading condition but its material properties are significantly dependent on the icing state (i.e. the snow density). The purpose of the current study is to introduce a numerical simulation of the snow–tire interaction by making use of Lagrangian finite element method and Eulerian finite volume method. The interaction between the tire tread blocks and the snow deformation is implemented by the explicit Euler–Lagrangian coupling scheme. The multi-surface yield model is adopted to describe both the softening and yielding of snow, and the associated material properties are chosen based upon the existing data in literature and the preliminary verification simulation. The numerical experiments are carried out by MSC/Dytran to investigate the parametric characteristics of the snow traction to the snow hardness, the block depth and the tread pattern.     

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.     

Effect of variations in front wheels driving lead on performance of a farm tractor with mechanical front-wheel-drive

Most previous researches indicate that about 20–55% of available tractor power is lost in the process of interaction between tires and soil surface. Vertical wheel loads and tire performance are parameters that play a significant role in controlling slip and fuel consumption of a tractor. Tractor’s slip is adjusted by attaching additional weights and/or reducing tire pressures, and this may have an impact on driving lead of front wheels. Mechanical Front-Wheel-Drive (MFWD) tractors work efficiently when driving lead of front wheels is 3–4% in soft soil and 1–2% in hard soil. This research was aimed to experimentally determine such tire pressures that allow adjusting tractor’s slip without deviating from set value of driving lead of front wheels. The research was also aimed to determine the effect of driving lead of front wheels on MFWD tractor’s slip and fuel consumption. Experimental results showed that front/rear tire pressure combinations that generate a well-targeted driving lead of front wheels have no effect on slip on hard soil; however, it significantly affect fuel consumption. Results show that when air pressures in front/rear tires varied within 80–220 kPa, driving lead of front wheels varied in the range from +7.25% to −0.5%.     

Evaluation of an empirical traction equation for forestry tires

Variable load test data were used to evaluate the applicability of an existing forestry tire traction model for a new forestry tire and a worn tire of the same size with and without tire chains in a range of soil conditions. The clay and sandy soils ranged in moisture content from 17 to 28%. Soil bulk density varied between 1.1 and 1.4g cm⁻³ with cone index values between 297 and 1418 kPa for a depth of 140 mm. Two of the clay soils had surface cover or vegetation, the other clay soil and the sandy soil had no surface cover. Tractive performance data were collected in soil bins using a single tire test vehicle with the tire running at 20% slip. A non-linear curve fitting technique was used to optimize the model by fitting it to collected input torque data by modifying the coefficients of the traction model equations. Generally, this procedure resulted in improved prediction of input torque, gross traction ratio and net traction ratio. The predicted tractive performance using the optimized coefficients showed that the model worked reasonably well on bare, uniform soils with the new tire. The model was flexible and could be modified to predict tractive performance of the worn tire with and without chains on the bare homogeneous soils. The model was not adequate for predicting tractive performance on less uniform soils with a surface cover for any of the tire treatments.