“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.     

Stabilization of a quadruped locomotion system by side step control

Several control schemes have been proposed for stabilization of artificial quadruped locomotion systems, but attainment of adequate stability is still one of the barriers in designing a legged vehicle. A combination of continuous and discrete state feedback seems to provide a good way to maintain stable motion of such a system. In this paper, side step control, which is a discrete feedback technique, is studied by means of a linearized locomotion system. The results are verified by application to a nonlinear quadruped vehicle simulation.     

The development of a soil trafficability model for legged vehicles on granular soils

This paper extends previous research in planetary microrover locomotion system analysis at the University of Surrey through the development of a legged microrover mobility model. This model compares various two- and three-dimensional soil cutting models to determine the most applicable model to legged locomotion in deformable soils, and is flexible to use any of these models depending on the leg shape, sinkage and other conditions. This baseline draught force model is used for determining the soil forces available for legged vehicle locomotion, as well as the soil thrust available to the vehicle footprint. Empirical investigations were performed with a robotic arm in planetary soil simulants to validate a legged mobility model through determination of the draft force of a robotic leg pushing through soil at constant and varying sinkage levels. The resulting locomotion performance model will be used to predict the ability of the legged vehicle to traverse a specific soil. An introduction to the planetary soil simulants used in this study (SSC-1 quartz-based sand and SSC-2 garnet-based sand) and the process used to determine their mechanical properties is also briefly presented to provide a baseline for this research.     

Control of a compound wheeled vehicle with two steering wheels

The problem of synthesis of a control system for a wheeled robotic vehicle consisting of two links (driving and driven) is analyzed. The robot is considered to be a controlled system of rigid bodies with nonholonomic constraints and to have two steerable wheels. Additionally, it is necessary to decide where the second steerable wheel is (on the driving or driven link). In this connection, two models of a compound wheeled robotic vehicle with two steerable wheels are considered. For these models, the problem of synthesis of feedback is solved. They are compared by modeling Translated from Prikladnaya Mekhanika, Vol. 44, No. 12, pp. 111–120, December 2008.     

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.     

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.     

The applicability of the MMP concept in specifying off-road mobility for wheeled and tracked vehicles

A review is given of the use of mean maximum pressure (MMP) in specifying off-road performance of vehicles. The need to quote a single mobility criterion which is unbiased in favour of either wheeled or tracked vehicles is recognised. The difficulties which researchers have encountered in developing expressions for MMP for both wheeled and tracked vehicles which correctly describe their relative performance are highlighted. Predictions of MMP for wheeled vehicles are compared with ground pressure measurements for a number of vehicles and it is shown that the MMP parameter does not actually represent the ground pressure accurately. Finally it is argued that the only safe route for the specifier is to quote a range of soil types, conditions and gradients on which the vehicle is to operate. This shifts the responsibility to the designer but also clears the way for innovative design, beyond the constraints of the MMP formulae.     

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.     

Assessment of tracked vehicle suspension system using a validated computer simulation model

A non-linear, in-plane computer simulation model of a typical high-mobility tracked vehicle is developed for suspension dynamic analysis and ride quality assessment. The Langrangian model formulation of the tracked vehicle is derived considering an arbitrary rigid terrain profile and constant vehicle speed. The model incorporates detailed representations of a trailing arm suspension system and dynamic wheel-track-terrain interactions. The computer model predictions are validated against field measurements, which were gathered from tests of an armoured personnel carrier traversing a discrete half-round bump and a random course. A parametric sensitivity analysis was carried out using the validated computer model in order to assess the influence of conventional suspension parameters on the ride performance of the test vehicle. In addition, the ride performance potentials of an alternate hydrogas suspension configuration were investigated.     

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).     

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.     

Diversification and sophistication

In this report, progress and state-of-the-art of research on vehicle and machinery designs are described for vehicles such as boat tractors, wheel type vehicles, tracked vehicles and a new locomotion system. The development stages and aspects of off-road vehicles are different in countries/districts, showing many kinds of running devices. Reflecting a high-technology in some countries, the mechanism and the control system of off-road vehicles are becoming more and more sophisticated.     

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.     

A hybrid force model to estimate the dynamics of curved legs in granular material

Robot locomotion on rigid terrain or in fluids has been studied to a large extent. The locomotion dynamics on or within soft substrates such as granular material (GM) has not been fully investigated. This paper proposes a hybrid force model to simulate and evaluate the locomotion performance of a legged terrestrial robot in GM. The model incorporates an improved Resistive Force Theory (RFT) model and a failure-based model. The improved RFT model integrates the force components of individual leg elements over the curved leg portion submerged in GM at any moment during a full period of leg rotation. The failure-based model is applied in a bar drag model to yield the normal and the lateral forces of the individual RFT elements as functions of the locomotion depth and speed. The hybrid model is verified by the coincidence between the theoretical predictions and the experimental results. The hybrid model is used to analyze the effects of angular velocity and leg shape with high precision and can guide the design of the legs with any profiles. Our study reveals that the interactions between locomotor and substrate are determined by the locomotor structural characteristics, the nature of the substrate, and the control strategy.     

Semi-active rotary damper for a heavy off-road wheeled vehicle

The development, simulation and laboratory testing of two-state discrete adjustable semi-active rotary dampers for heavy off-road wheeled vehicles, which is a joint venture between the South African based company Reumech Ermetek and Horstman Defence Systems from the UK, is described. A brief history of semi-active damping and rotary dampers is given, after which the working principle and features of combining the two technologies is outlined. Three dimensional simulation is used to determine the ride comfort gains achievable with semi-active rotary dampers compared to the conventional translational dampers currently used on the vehicle under consideration. Simulations are performed over different terrains, including the APG track and discrete obstacles. Semi-active rotary dampers were integrated on the 6×6 GV6 Self-propelled Gun Howitzer in order to quantify the improvements in ride comfort, transient response and handling of the vehicle as indicated by the simulation results. The control system, control strategies and characterisation tests, which includes determination of on and off characteristics as well as valve response times, are discussed.     

Vehicle dynamics—Steering I

The consistent theme running through the Vehicle Dynamics—Steering sessions at the 1990 ISTVS Conference was more sophisticated analysis applied to off-road vehicle problems. Of particular note was the gradual progress being made with improvements to the modelling of tracked vehicle steering, including some promising work by Ehlert et al. in verifying model predictions against measured results. State-of-of-art computer software for multibody system dynamics problems was reviewed by Xie and Claar with reference to a tractor-trailer problem. Throughout the papers, in these sessions, it is clear that improved software packages for dynamic analysis are responsible for raising the level of analytical sophistication applied to off-road vehicle design.     

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.     

A method for predicting the internal motion resistance of rubber-tracked undercarriages,Pt. 1. A review of the state-of-the-art methods for modeling the internal resistance of tracked vehicles

This article summarizes the known methods for calculating the internal resistance of tracked undercarriages. The values of the coefficient of internal resistance for sample tracked vehicles are available in the literature and presented in this paper. Although they are suitable for simple computations, they cannot be used to optimize the energy efficiency of new generation tracked undercarriages. This problem might be solved by the models where every phenomenon leading to energy dissipation during vehicle motion is described by a separate submodel as a function of vehicle speed, track tension, undercarriage layout, design features of the undercarriage components, etc. This kind of model is still missing for vehicles with conventional rubber tracks. The article presents multiple state-of-the-art models describing rolling resistance of road wheels, bending resistance of rubber belts, etc., including the models of belt conveyors resistance. A vast majority of the phenomena discussed herein are described by several incompatible models whose parameters have not yet been determined for conventional rubber tracks. Consequently, in the second and the third part of the article, the authors have undertaken a theoretical and experimental studies on the methods for calculating and optimizing the internal motion resistance of vehicles with conventional rubber tracks.     

The brachistochronic motion of a wheeled vehicle

The paper considers the brachistochronic motion of a wheeled vehicle on a horizontal plane surface. The objective is to transfer the vehicle from the specified initial position with given initial kinetic energy to the specified terminal position in minimum time with conserved total mechanical energy of the vehicle. The problem is solved by applying Pontryagin’s maximum principle and singular optimal control theory. The projection of the reaction force of the horizontal plane applied on the front vehicle wheels onto the axis of the front vehicle axle is taken for a control variable. The cases of unbounded and bounded value of this projection are considered. The shooting method is used to solve the two-point boundary value problem arising from Pontryagin’s maximum principle and singular optimal control theory.     

Prediction of impacts of wheeled vehicles on terrain

Traffic of off-road vehicles can disturb soil, decrease vegetation development, and increase soil erosion. Terrain impacts caused by wheeled off-road vehicles were studied in this paper. Models were developed to predict terrain impacts caused by wheeled vehicles in terms of disturbed width and impact severity. Disturbed width and impact severity are not only controlled by vehicle types and vehicle dimensions, but also influenced by soil conditions and vehicle dynamic properties (turning radius, velocity). Field tests of an eight-wheeled vehicle and a four-wheeled vehicle were conducted to test these models. Field data of terrain–vehicle interactions in different vehicle dynamic conditions were collected. Vehicle dynamic properties were derived from a global position system (GPS) based tracking system. The average prediction percentage error of the theoretical disturbed width model is less than 20%. The average absolute error between the predicted impact severity and the measured value is less than an impact severity value of 12%. These models can be used to predict terrain impacts caused by off-road wheeled vehicles.