Prediction of ground pressure distribution under tracked vehicles—II. Effects of design parameters of the track-suspension system on ground pressure distribution

The effects of vehicle design parameters on ground pressure distribution and track tension are evaluated quantitatively using the analytical method described in Part I of this paper. The ground pressure distribution of a vehicle is closely related to vehicle sinkage and external motion resistance, while the track tension affects the internal resistance of the track-suspension system. It is found that the number of road wheels on the track has a significant effect on both ground pressure distribution and track tension. On the other hand, the diameter of the road wheels has only a moderate influence on track tension and a rather insignificant effect on ground pressure distribution. The stiffnesses of the suspension and the track tensioning springs have varying degrees of influence on ground pressure distribution and track tension dependent upon terrain stiffness. It is also found that the initial track tension has a significant effect on the actual track tension over a wide range of terrain and a slight to moderate influence on ground pressure distribution, dependent upon terrain stiffness.It should be noted that although individually some of the design parameters have only a slight or moderate effect on ground pressure distribution and track tension, their combined effects may be significant. Therefore, in the selection of design parameters of the track-suspension system, their combined influence must be carefully examined.     

Development of high-mobility tracked vehicles for over snow operations

This paper describes a detailed investigation into the effects of some of the major design features on the mobility of tracked vehicles over snow. The investigation was carried out using the latest version of an advanced computer simulation model, known as NTVPM, developed under the auspices of Vehicle Systems Development Corporation (VSDC), Ottawa, Ontario, Canada. Results show that the road wheel system configuration, initial track tension (i.e., the tension in the track system when the vehicle is stationary on a level, hard ground) and track width have significant effects on vehicle mobility over snow. On deep snow where the vehicle belly (hull) contacts the snow surface, the location of the centre of gravity (C.G.) of the sprung weight in the longitudinal direction has a noticeable effect on vehicle mobility, as it affects the attitude of the belly and the belly–snow interaction. Based on the investigation, a conceptual high-mobility tracked vehicle for over snow operations is discussed. Results of this study will provide the vehicle designer with guiding principles for the development of high-mobility tracked vehicles. It also demonstrates that NTVPM is a useful and effective tool for design and performance evaluation of tracked vehicles from a traction perspective.     

Prévision de la répartition de la pression sous des véhicules à chenilles Ilème partie - effects des paramètres de conception du système de suspension des chenilles sur la répartition de la pression sur le sol

The effects of vehicle design parameters on ground pressure distribution and track tension are evaluated quantitatively using the analytical method described in Part I of this paper. The ground pressure distribution of a vehicle is closely related to vehicle sinkage and external motion resistance, while the track tension affects the internal resistance of the track-suspension system. It is found that the number of road wheels on the track has a significant effect on both ground pressure distribution and track tension. On the other hand, the diameter of the road wheels has only a moderate influence on track tension and a rather insignificant effect on ground pressure distribution. The stiffnesses of the suspension and the track tensioning springs have varying degrees of influence on ground pressure distribution and track tension dependent upon terrain stiffness. It is also found that the initial track tension has a significant effect on the actual track tension over a wide range of terrain and a slight to moderate influence on ground pressure distribution, dependent upon terrain stiffness.It should be noted that although individually some of the design parameters have only a slight or moderate effect on ground pressure distribution and track tension, their combined effects may be significant. Therefore, in the selection of design parameters of the track-suspension system, their combined influence must be carefully examined.     

A spatial motion analysis model of tracked vehicles with torsion bar type suspension

A spatial motion analysis model for high-mobility tracked vehicles was constructed for evaluation of ride performance, steerability, and stability on rough terrain. Ordinary high-mobility tracked vehicles are equipped with independent torsion bar type suspension system, which consists of road arms and road wheels. The road arm rotates about the axis of torsion bar, and rigidity of the torsion bar and cohesion of damper absorb sudden force change exerted by interaction with the ground. The motion of the road arms should be considered for the evaluation of off-road vehicle performance in numerical analysis model. In order to obtain equations of motion for the tracked vehicles, the equations of motion for the vehicle body and for the assembly of a road wheel and a road arm were constructed separately at first. Two sets of equations were reduced with the constraint equations, which the road arms are mechanically connected to the vehicle body. The equations of motion for the vehicle have been expressed with minimal set of variables of the same number as the degrees of freedom for the vehicle motion. We also included the effect of track tension in the equations without constructing equations of motion for the tracks. Numerical simulation based on the vehicle model and experiment of a scale model passing over a trapezoidal speed bump were performed in order to examine the numerical model. It was found that the numerical results reasonably predict the vehicle motion.     

Computer-aided methods for the optimization of the mobility of single-unit and two-unit articulated tracked vehicles

In the past decade, a series of computer-aided methods (computer-simulation models) have been developed for design and performance evaluation of tracked vehicles, particularly those with short track pitch designed for high speed operations. The latest version, known as NTVPM-86, developed under the auspices of Vehicle Systems Development Corporation, Nepean, Ontario, Canada, takes into account all major vehicle design parameters and terrain characteristics. The basic features of the model have been validated by field tests over a variety of terrains, including mineral, organic and snow-covered terrains. It has been gaining increasingly wide acceptance by industry and governmental agencies in the development and procurement of new vehicles in North America, Europe and Asia. In this paper, the effects of suspension characteristics, initial track tension, track width and ground clearance on the mobility of single unit and two-unit articulated track vehicles over deep snow are systematically evaluated using the computer simulation model NTVPM-86. It is found that these parameters have noticeable effects on vehicle mobility over marginal terrain. The approach to the optimization of tracked vehicle design from the mobility point of view is also examined. It is shown that the simulation model can play a significant role in assisting the procurement manager to select the appropriate vehicle candidates and the design engineer to optimize vehicle design for a given mission and environment.     

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 new contact & slip model for tracked vehicle transient dynamics on hard ground

This study presents a new general transient contact and slip model for tracked vehicles on hard ground which is simple, accurate, and in agreement with the test results to a satisfactory level. Simulating zero track speed instances become possible with the new contact/shear model which is the major proposed improvement in addition to more accurate results for transient steering and tractive inputs. The model represents a general tracked vehicle having rear or front sprockets, with parameters for center of gravity, wheel positions, number of wheels, and track-pretention. To calculate longitudinal and lateral forces, a transient shear model is used. Shear stress under each track pad is assumed to be a function of shear displacement. The contact time formulation used in shear displacement calculation is improved to gain accuracy for transient and zero track speed conditions.The model is implemented on the Matlab/Simulink platform and verified with a comprehensive program of road tests composed of transient steering and tractive/braking scenarios. The results of the simulations and the road tests are satisfactorily similar for both constant and transient input maneuvers. Moreover, sensitivity simulations for vehicle parameters are conducted to show that the model responses are inline with the expected vehicle dynamics behaviours.     

A method for predicting the internal motion resistance of rubber-tracked undercarriages,Pt. 2. A research on the motion resistance of road wheels

In spite of an increasing number of rubber-tracked crawlers, the literature provides few guidelines and calculation models suitable for minimizing their internal motion resistance. This article presents a model where the internal resistance of double-flanged road wheels for rubber-tracked vehicles is calculated as a sum of the losses resulting from the indentation of the wheels into the track surface and friction of the wheels against the track guide lugs. The model allows for vertical and lateral load of the wheels, the non-uniform distribution of the wheel pressure on the track, and the relationship between the friction coefficient and normal reaction force in the interface between the wheel and track guide lugs. The model has been verified by experiments. According to the results of model computations and experiments discussed in the article, the internal losses of a given rubber-tracked undercarriage might be reduced if: the road wheels are covered with a material that exhibits low friction coefficient and mechanical hysteresis, the vehicle suspension system features oscillating bogie wheels, the undercarriage is fitted with the largest possible number of road wheels, and the vehicle weight is evenly distributed to all of the road wheels.     

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.     

Numerical simulation and testing verification of the interaction between track and sandy ground based on discrete element method

Tractive effort of tracked vehicles plays an important role in military and agricultural fields. In order to solve the problem of low precision in numerical simulation of the interaction between track and sandy ground, a systematic and accurate discrete element modeling method for sandy road was proposed. The sandy ground was modeled according to the mechanical parameters measured by soil mechanics tests. The interaction coefficients of sandy soil were measured by the repose angle test and triaxial compression test combined with the corresponding simulation. On this basis, a discrete element interaction model of track-sandy ground was established, which can be used to test the tractive effort of track. Numerical simulation calculation of track model at different speeds was carried out, and the simulation results were compared with the results of indoor soil bin test for verification. The verification results show that the interaction between track and sandy ground based on DEM simulation is consistent with the actual soil bin test. The discrete element modeling method in this paper can be used to model the track and sandy ground accurately, and the simulation model can be used to test the tractive effort of tracked vehicle.     

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).     

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 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.     

Study on steerability of articulated tracked vehicles — Part 1. Theoretical and experimental analysis

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.     

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.     

Advanced mobility in difficult terrain

The development and success of the Swedish Combat Vehicle CV90 has demonstrated the abilities of the author in the field of terramechanics related to tracked military vehicles. The honour of the Bekker–Reece–Radforth Award 2002 has been granted in recognition of these achievements made during the author's employment at Hägglunds Vehicle AB since 1975. Hägglunds Vehicle AB has been a producer of military vehicles since the late 1950s, although the first years concentrated on production only. From the early 1960s, Hägglunds developed a number of its own tracked vehicles, all of which were influenced by the mobility demands dictated by their intended use in severe terrain conditions, such as those found in Northern Scandinavia. This paper presents a brief history of the advancement of tracked vehicle technology at Hägglunds Vehicle AB. The concepts discussed include: ground pressure, the number of road-wheels, articulated steering, track tension, track attack angle, sinkage, belly effects, and the use of terramechanic simulation. The success of the CV90 demonstrates that the combination of practical experience, terrain knowledge, and terramechanic simulations can effect substantial improvements in vehicle mobility. Evaluation of the CV90 versus other modern combat vehicles of the same class has shown that the CV90 possesses considerably higher mobility and speed under severe terrain conditions. These two attributes provide CV90 with the ability to access terrain that similar vehicles cannot, thus giving the military user greater mobility options.     

Prediction of the tractive performance of a flexible tracked vehicle

In this study, we describe a mathematical model designed to allow for the determination of the mechanical relationship existing between soil characteristics and the primary design factors of a tracked vehicle, and to predict the tractive performance of this tracked vehicle on soft terrain. On the basis of the mathematical model, a computer simulation program (Tractive Performance Prediction Model for Tracked Vehicles; TPPMTV) was developed in this study. This model took into account the characteristics of the terrain, including the pressure-sinkage, the shearing characteristics, and the response to the repetitive loading, as well as the primary design parameters of the tracked vehicle. The efficacy of the developed model was then confirmed via comparison of the drawbar pulls of tracked vehicles predicted using the simulation program TPPMTV, with those determined as the result of traction tests. The results indicated that the predicted drawbar pulls, with the change in slip, were quite consistent with the ones measured in the traction test, for the changes in the weight of the vehicle, the initial track tension, and the number of roadwheels within the entire slip range. Thus, we concluded that the simulation program developed in this study, named TPPMTV, proved useful in the prediction of the tractive performance of a tracked vehicle, and that this system might be applicable to the design of a vehicle, possibly enabling a significant improvement in its functions.     

Prediction of ground pressure distribution under tracked vehicles—I. An analytical method for predicting ground pressure distribution

An analytical method for predicting the pressure distribution beneath a tracked vehicle under static conditions is presented. In the analysis, the track-suspension system which consists of the track, the suspension and the track tensioning device, is considered inits entirety. All major design parameters of the vehicle, as well as terrain characteristics, are taken into consideration. It is shown that the analytical method proposed can provide a means whereby the effects of vehicle design parameters and terrain conditions on ground pressure distribution can be assessed quantitatively.     

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.