Comparing the NRMM (VCI), MMP and VLCI traction models

A military vehicle’s ability to traverse soft soil is a significant aspect of its performance. It is therefore important to be able to accurately predict and compare the soft soil performance of wheeled and tracked vehicles. The paper compares the vehicle cone index (VCI) model used in the NATO Reference Mobility Model (NRMM) with the mean maximum pressure (MMP) and vehicle limiting cone index (VLCI) models. The wheeled vehicle models compare reasonably well but the VLCI model for tracked vehicles indicates markedly higher limiting go/no-go soil strengths compared to the VCI and MMP models. Some of the relationships used in the VCI model appear rather irrational and could be significantly simplified. There is a considerable lack of reliable experimental data for tracked vehicles especially in the low traction region.     

“Wheels vs. tracks” – A fundamental evaluation from the traction perspective

The issue of wheeled vehicles vs. tracked vehicles for off-road operations has been a subject of debate for a long period of time. Recent interest in the development of vehicles for the rapid deployment of armed forces has given a new impetus to this debate. While a number of experimental studies in comparing the performances of specific wheeled vehicles with those of tracked vehicles under selected operating environments have been performed, it appears that relatively little fundamental analysis on this subject has been published in the open literature, including the Journal of Terramechanics. This paper is aimed at evaluating the tractive performance of wheeled and tracked vehicles from the standpoint of the mechanics of vehicle–terrain interaction. The differences between a tire and a track in generating thrust are elucidated. The basic factors that affect the gross traction of wheeled and tracked vehicles are identified. A general comparison of the thrust developed by a multi-axle wheeled vehicle with that of a tracked vehicle is made, based on certain simplifying assumptions. As the interaction between an off-road vehicle and unprepared terrain is very complex, to compare the performance of a wheeled vehicle with that of a tracked vehicle realistically, comprehensive computer simulation models are required. Two computer simulation models, one for wheeled vehicles, known as NWVPM, and the other for tracked vehicles, known as NTVPM, are described. As an example of the applications of these two computer simulation models, the mobility of an 8 × 8 wheeled vehicle, similar to a light armoured vehicle (LAV), is compared with that of a tracked vehicle, similar to an armoured personnel carrier (APC). It is hoped that this study will illustrate the fundamental factors that limit the traction of wheeled vehicles in comparison with that of tracked vehicles, hence contributing to a better understanding of the issue of wheels vs. tracks.     

Clarification of vehicle cone index with reference to mean maximum pressure

The US Army developed Vehicle Cone Index (VCI) as a metric for directly quantifying the ability of vehicles to traverse soft-soil terrain. In order to ensure minimum soft-soil performance capabilities for their new military vehicles, the US Army has used VCI for many years as a performance specification. The United Kingdom’s Ministry of Defence (UK MOD) has used the Mean Maximum Pressure (MMP) parameter for many years as a performance specification. It has been demonstrated that the MMP parameter relates to soft-soil performance capabilities, and hence, the UK MOD has ensured minimum performance capabilities for their new military vehicles by using MMP specifications. Both the VCI and MMP specification approaches have served their users well, but fundamental differences in the two specification approaches have produced some misunderstandings concerning what VCI really is and how it relates to MMP. This article clarifies that VCI is a performance metric, not a set of predictive equations, explains how VCI is measured, and compares different methods of predicting VCI for one-pass performance (i.e., VCI₁) of wheeled vehicles in fat clay soils. It is further clarified that MMP should not be compared with VCI but instead with Mobility Index (MI), which is the principal parameter used by the US Army for predicting VCI. Relationships are presented for using MMP to predict VCI₁ for wheeled vehicles in clay, and the resulting relationships allow comparison between MMP and MI in terms of their ability to predict VCI. Seventy-nine VCI₁ performance measurements were used for the comparison, and they demonstrate that MI describes the historical performance data somewhat better than MMP.     

An investigation of pressure under wheeled vehicles

This paper reviews methods currently used to predict the pressure exerted by a wheeled vehicle on the ground. It describes a programme of experiments designed to measure the pressure at a certain depth in a soil mass, the surface of which is traversed by a number of different vehicles in a range of loading conditions. Methods for inferring the surface pressure from underground measurements are described and compared. The inferred surface pressures are then compared with predicted values. A discussion is given on the usefulness and validity of ground pressure characterisation for wheeled vehicles.     

A skid steering model with track pad flexibility

The paper describes a model for predicting the skid-steering performance of tracked vehicles that allows for the flexibility of the track pads. It thus accounts for the reductions in friction moment that are observed as the radius of the turn is increased. The pad model computes a compound slip function and takes account of the shear stiffness of the pad and the limiting friction between the pad and the ground. Vehicle dimensions and the equations of motion are entered into a Microsoft Excel spreadsheet. The equations are solved using the Excel Solver routine. This avoids the need for specialised software or programming skills. It also gives good insight into the mechanics of steering and the factors affecting performance. Predicted sprocket torques for a Jaguar vehicle turning at different radii show good agreement with experimental measurements. The steering performance of an example six axle 24 tonne vehicle is computed and compared with that using the early Merritt/Steeds model that ignored track pad flexibility. The flexible pad model generally shows the vehicle to be slightly oversteer, whereas the Merritt/Steeds model predicts the vehicle to be understeer. At higher speeds the maximum cornering acceleration is likely to be limited by available power at the sprockets. Altering the static weight distribution of the vehicle shows that a forward weight distribution tends to cause a more oversteer response with reduced limiting lateral acceleration. With a rearward weight distribution, the vehicle response tends towards neutral to slight understeer. This is in contrast to Ackerman steered wheeled vehicles with pneumatic tyres where moving the CG forward tends to a more understeer response. Using the concept of static margin as applied to wheeled vehicles, it is suggested that a uniform or slightly forward weight distribution would make tracked vehicles less sensitive to external disturbances (cambered roads for example).     

Development of a tracked vehicle to study the influence of vehicle parameters on tractive performance in soft terrain

This paper describes a new special tracked vehicle for use in studying the influence of different vehicle parameters on mobility in soft terrain; particularly muskegg and deep snow. A field test in deep snow was carried out to investigate the influence of nominal ground pressure on tractive performance of the vehicle. The vehicle proved useful for studying vehicle parameters influencing the tractive performance of tracked vehicles. The tests show that the nominal ground pressure has a significant effect on the tractive performance of tracked vehicles in deep snow. The decrease in drawbar pull coefficient when the nominal ground pressure is increased and originates at about the same amount from a decrease of the vehicle thrust coefficient, an increase of the belly drag coefficient and an increase of the track motion resistance coefficient.     

Long-term effects of off-road vehicle traffic on tundra terrain

Traffic tests were conducted at two sites in northern Alaska with an air cushion vehicle, two light tracked vehicles, and three types of wheeled Rolligon vehicles. The traffic impact (surface depression, effect on thaw depth, damage to vegetation, traffic signature visibility) was monitored for periods of up to 10 years. Data show the immediate and long-term effects from the various types of vehicles for up to 50 traffic passes and the rates of recovery of the active layer. The air cushion vehicle produced the least impact. Multiple passes with the Rolligons caused longer-lasting damage than the light tracked vehicles because of their higher ground contact pressure and wider area of disturbance. Recovery occurs even if the initial depression of the tundra surface by a track or a wheel is quite deep (15 cm), as long as the organic mat is not sheared or destroyed.     

An approach to computer control for legged vehicles

Observation of the locomotion of animals and human beings in difficult terrain makes it quite obvious that legged locomotion offers substantial mobility advantages over conventional wheeled or tracked systems. However, effective adaptation of legged locomotion principles to off-road vehicles has to date been frustrated by the complexity of the joint coordination control problem and by the lack of suitable sources of power for individual leg joints. This paper is addressed to the first problem and is intended to show that some of the techniques used in aircraft autopilots can be adapted to legged vehicle control. The main results presented are derived from a computer simulation study of a system in which vehicle speed and direction are determined by a human operator while individual joint commands are generated automatically by a digital computer. Present indications are that such a vehicle might be as easy to control as a conventional wheeled or tracked automotive system.     

Calculating fractal parameters from low-resolution terrain profiles

Driver comfort on rough terrain is an important factor in the off-road performance of wheeled and tracked ground vehicles. The roughness of a terrain has typically been quantified by the U.S. Army as the root-mean-square elevation deviation (RMS) of the terrain profile. Although RMS is an important input into many mobility calculations, it is not scale invariant, making it difficult to estimate RMS from low resolution terrain profiles. Fractal parameters are another measure of roughness that are scale invariant, making them a convenient proxy for RMS. While previous work found an empirical relationship between fractal dimension and RMS, this work will show that, by including the cutoff length, an analytic relationship between fractal properties and RMS can be employed. The relationship has no free parameters and agrees very well with experimental data - thus providing a powerful predictive tool for future analyses and a reliable way to calculate surface roughness from low-resolution terrain data in a way that is scale invariant. In addition, we show that this method applies to both man-made ride courses and natural terrain profiles.     

Triaxial-in-a-plane soil stress gage for vehicle mobility applications

The approach used here consists of an axisymmetric gage with a tapered exterior. Raw outputs are normal stress in three directions in the plane perpendicular to the gage axis of symmetry. From these outputs, the time history of the complete state of stress in this plane can be determined. Of special interest is the plane of symmetry which is the vertical plane centered on one side of a wheeled or tracked vehicle proceeding in a straight line. The gage was placed from a berm on the side of the vehicle path, approximately 4 ft horizontally and at approximately 9 in. below the ground surface. Test vehicles were a 4 × 4 wheeled vehicle and a M113A2 armored personnel carrier. The measured stress results are largely consistent with expectations.     

Etude de la direction des véhicules articulés à chenille (l analyse théorique et expérimentale)

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.     

Military terrain vehicles

The author gives a survey of the historical background, current developments and future aspects of the most important wheeled and tracked military terrain vehicle systems focusing, in particular, on their automotive technological performance as well as on their tactical features. Weapon systems, radio and other military equipment are not part of this survey.This paper does not cover niche products such as over-snow and amphibious vehicles which are also used for military purposes. However, hybrid propulsion systems are included.     

Improved sinkage algorithms for powered and unpowered wheeled vehicles operating on sand

Modeling and simulation of vehicles in sand is critical for characterizing off-road mobility in arid and coastal regions. This paper presents improved algorithms for calculating sinkage (z) of wheeled vehicles operating on loose dry sand. The algorithms are developed based on 2737 tests conducted on sand with 23 different wheel configurations. The test results were collected from Database Records for Off-road Vehicle Environments (DROVE), a recently developed database of tests conducted with wheeled vehicles operating in loose dry sand. The study considers tire diameters from 36 to 124 cm with wheel loads of 0.19–36.12 kN. The proposed algorithms present a simple form of sinkage relationships, which only require the ratio of the wheel ground contact pressure and soil strength represented by cone index. The proposed models are compared against existing closed form solutions defined in the Vehicle Terrain Interface (VTI) model. Comparisons suggest that incorporating the proposed models into the VTI model can provide comparable predictive accuracy with simpler algorithms. In addition to simplicity, it is believed that the relationship between cone index (representing soil shear strength) and the contact pressure (representing the applied pressure to tire-soil interface) can better capture the physics of the problem being evaluated.     

A simulation model for the prediction of the ground pressure distribution under tracked vehicles

A computer based simulation model for the prediction of the ground pressure distribution beneath tracked vehicles under static conditions has been developed. The model can differentiate between various track designs and is based on an analytical method developed and described by Garber and Wong. Simulating the model with the parameters of a rubber tracked forestry vehicle (FARMI TRAC 5000) led to several conclusions. The road wheel arrangement has a considerable effect on the ground pressure distribution: increasing the number of road wheels reduces the maximum ground pressure and improces the uniformity of the pressure distribution. The radius of the road wheel, the stiffness of the suspension and the stiffness of the track tensioning device have an insignificant effect on the ground pressure distribution. In contrast, the initial track tension and the width of the track have a significant effect on the ground pressure distribution: increasing the initial track tension reduces the maximum ground pressyre and improves the uniformity of the pressure distribution. The same conclusions are valid for an increase of the track width. This model can be used as a tool to assist in the design of off-road vehicles, and is currently being used in the design of forestry vehicles in Ireland.     

Slip sinkage effect in soil-vehicle mechanics

The paper presents an analysis and quantitative evaluation of the slip sinkage and its effect on the tractive performance of wheeled and tracked vehicles in different soils. The results of this study indicated that to accurately predict the sinkage and motion resistance of a vehicle in a given soil and operating conditions, the slip sinkage effect should be taken into account. An effective analytical formula that takes into consideration the slip sinkage effect on sinkage of plates and vehicles is developed. The formula was validated in different soil conditions and compared with other formulae used in terramechanics for slip sinkage effect predictions.     

Influence of age-hardening and strain-rate on confined compression and shear behaviour of snow

The problem of assessing mobility performance of tracked or wheeled vehicles over snow is addressed in terms of the capability of the snow to provide flotation and traction capabilities. To obtain input into analyses for energy transfer from the traction element to the supporting snow cover, it is necessary to describe confined compression and shear performances of the snow—recognizing that a large density increase occurs under initial attack of the vehicle traction element. This study provides experimental input and diagrams of energy surfaces describing confined compressive and shear effects related to snow age and initial density.     

An optimum design method for robotic tracked-vehicles operating over fresh concrete during straight-line motion

This paper addresses the general problem of the design of tracked base travel systems for special purpose vehicles and/or robotic machines that may be required to move over weak surfaces or over a lightly bonded terrain composed of fresh concrete. For the special case of a vehicle travelling on a very soft fresh concrete during construction, the paper presents detailed comparative studies of the tractive performance of several tracked vehicles with alternative slump values and mean contact pressure configurations. To complete these studies a detailed simulation-analytical method was used. From this, it was established that the simulation analysis method is useful for predicting land locomotion performance of specially designed small tracked vehicles running over fresh concrete of different consistencies during driving and braking action. This work was done for straight-line motion. Some possibilities for the real-time optimum control method of the tractive and braking performance of automated and robotic vehicles are also outlined.     

Analytical simulation as a support tool in the decision process “wheels or tracks”

In the development of a weapon system, the maximization of demands concerning features like fire power, armor and mobility often leads to incompatibilities within the overall systems, to an imbalanced design and excessively high cost. The consequence is a decrease in mission availability. Therefore, system analysis utilizing simulation techniques is being used more and more in the early stages of the development process. This paper describes the application of such simulation techniques to practical cases. First, the development of a system is described by which to select wheeled or tracked running gears for combat vehicles considering the compatibility of fire power, armor, mobility and cost. Secondly, the possible upper weight ranges for wheeled combat vehicles are discussed, using analytical modeling of vehicle mobility as a basis. The methods used in these cases can also be applied to non-military problems.     

Track-terrain modelling and traversability prediction for tracked vehicles on soft terrain

Skid-steered tracked vehicles are the favoured platform for unmanned ground vehicles (UGVs) in poor terrain conditions. However, the concept of skid-steering relies largely on track slippage to allow the vehicle to conduct turning manoeuvres potentially leading to overly high slip and immobility. It is therefore important to predict such vulnerable vehicle states in order to prevent their occurrence and thus paving the way for improved autonomy of tracked vehicles. This paper presents an analytical approach to track-terrain modelling and a novel traversability prediction simulator for tracked vehicles conducting steady-state turning manoeuvres on soft terrain. Traversability is identified by predicting the resultant track forces acting on the track-terrain interface and the adopted models are modified to provide an analytical generalised solution. The validity of the simulator has been verified by comparison with available data in the literature and through an in-house experimental study. The developed simulator can be employed as a traversability predictor and also as a design tool to test the performance of tracked vehicles with different vehicle geometries operating on a wide range of soil properties.     

How to calculate the effect of soil conditions on tractive performance

The paper presents an analysis and quantitative evaluation of the effect of soil conditions on tractive performance of off-road wheeled and tracked vehicles. The results of this study indicated that to accurately calculate the tractive performance of a vehicle in a given soil condition, soil properties and parameters and their changes as functions of soil moisture content and density should be taken into account. An effective Tractive Performance Analytical (TPA) model which takes into consideration the effect of soil conditions on tractive performance of the vehicles is developed. The TPA model uses invariant soil parameters that can be given or measured before the calculations by routine methods of classical soil mechanics. Soil parameters can also be obtained by recommended empirical equations using four physical soil parameters measured in the field with hand held instruments without time consuming and costly plate or vehicle tests. The model was validated in different soil conditions and compared with other models used in terramechanics for tractive performance predictions. The paper includes also an analysis of capabilities and limitations of the observed models.