Effects Of Change In Road Geometry Design On Vehicle Driving Dynamics

Despite traditional alignment design method such as determination of horizontal or vertical location and drawing on papers using a pen manually has been replaced by CAD technique, dimensions of horizontal, vertical and cross section are still determined separately. Due to the unique of each alignment fitting the topography, the procedure of "design-prototype-test-redesign" commonly used to manufacturing is not suitable for highways designers. Therefore, highway designers are hard to make target modification, and quality of alignment is determined by the early experience of designer's. Numbers of design drawbacks preserved into operation phase always result in the mismatch of highway alignment, modern vehicle and driver behavior, and accident clusters everywhere. So, to this day, driving safety and riding comfort can not be reached for highway design.If virtual roadway test like used to machinery production can be apllied in alignment design, let vehicle dynamics model run on 3D road models and log the dynamics/kinematics response, can also a good testing instrument, and it is the main objective of this paper. In this context, the virtual driving system of "roadway-driver-vehicle-enviroment" is develpoed in the paper. According to input date of alignment parameters,3D roadway models can be obtained rapidly, if a full vehicle model inducted, parameters of driving behavior and enviroment impact defined togeter, vehicle model driving on 3 D roadway model can be relized and the driving process is visible. Through conducting virtual driving test, designers can identify the location of driving instability or difficulty in vehicle control and modify its design value based on vehicle response and steering input, can see the effects of improment. By means of virtual roadway test, we can parameterize the design value of hignway alignment and obtain the relationship among highway geometry features, vehicle kinematics and driver behavior, such as the effect of change in alignment parameter on vehicle kinematics and driver behavior, the effect of change in vehicle parameter and driver behavior on highway alignment values, etc. which all can result in highway design compatible with modern vehicle design and driver behavior.In the paper, creation and coupling of subsystems of "roadway-driver-vehicle-enviroment" are firstly completed, then the simulation system is applied in evaluation of alignment design, driving simulation under under adverse conditions, mechanism analysis of single vehicle run-off-road, and vehicle motion performance and driving behavior on curved segment, which can be described as follows:1. Creation of 3D roadway models.The roadway module in the second section of the paper can generate 3D road-surface used to contact with tires, and can deal with arbitrary complex alignment in current design. For different conditions of use, we design two formats of input date and their corresponding algorithm. One input is the design values of horizontal/vertical alignment and cross-section, conception of "typology element" is put forward in the paper, which takes a highway alignment as the sequence of "typology element". A horizontal "typology element" is composed of four elements of "tangent+spiral 1+ circular+spiral 2". A vertical "typology element" is composed of a straight grade and its adjacent vertical curve. The other input is spatial coordinate of sampling point in analyzed roadway, which suitable for alignment date absent. We select multi-quadric radial basis function as the interpolation function, and put forward reconstruction algorithm of "local, overlapping" and "exchange of range and independent variable". These algorithm can prevent ill-conditioning of coefficient matrix caused by huge scale, belt distribution, and non-injection of date, and can obtain smooth and continued 3D road-surface.2. Creation of full vehicle dynamics models, tire-road contact models, and environment impact.In the third section of the paper, the topology of suspension, steering gear, anti-roll bar, brake, drive axle, driving-line and frame with different configuration are analyzed, and a database of vehicle models is developed. Which include a microbus, two passenger cars, and a truck built in ADAMS. Tire models come from an edition of magic formula Pacejke'94 or Pacejke'89. To let tire contact with road-surface, we design an algorithm of defining friction coefficient that can define a half roadway or several segments a different coefficient with others. Environment impact IS simulated through tire-road contact model and vehicle models, such as define a small friction coefficient for a given segment to simulate the effect of ice or water gathered, and act a lateral force on vehicle body to simulate the process of driving in lateral wind.3. Steering and speed models.Our innovative work in this aspect is developing a prediction model of desired speed on a given highway, which can take in account highway geometry features, vehicle dynamics, and driving behavior. Principle of determination of desired speed is that, the desired speed on curve areas should meet the condition:the lateral acceleration of vehicle bodies no more than the tolerated value aytoal; speed change between two adjacent curves is subject to the acceleration rate ax and deceleration rate ab, and the desired speed is always no more than the environment speed Vxmax. According to speed measurement date on real road, we developed the models of aytoal, ab, ax, and Vxmax·Vxmax is a function of average change rate of curvature and total roadway width, aytoal, ab and axare all functions of curve radii and lane width, so they change along roadway. Currently, models developed by foreign researchers are often aiming at several rural roads in level or level-hilly areas, despite how modify model parameters, the model can not suitable for highways of different design speed in China. Therefore, we put forward a method of calibrate model parameters for design speed separately, by this way, we can use a unitive model to deal with highways of different design speed.4. Evaluation of highway alignment based on speed.In the fifth section of the paper, we analysis the speed along given roads. Our conclusions are as follows:When driving at a constant speed, the frequency of driver change pedals is equal to the use of sharp curves, therefore, we can control highway's driving workload by alignment design. When driving vehicle reaches such a situation of lose control, the profile of lateral speed will has a catastrophe in magnitude and cause discontinuity in longitudinal speed, so we can use the continuity of speed to evaluate the vehicle stability when driving at a constant speed, and further do a judgement whether exists threaten to driving vehicle in difficult segment and whether to refine design.For free driving, our observation indicates that, deceleration rate while entering curve is always more than acceleration rate while exiting curve; beginning point of slow down and ending point of speed up are always beside the spirals, so the suppose of speed change in spirals is wrong. In addition, for freeway, acceleration rate of 0.5m/s2 is too high. The best method to control speed fluctuation is let the curve radii and tangent length of adjacent element similar, and tangent should be short. Besides alignment, the reason cause speed fluctuant frequently on complex and trivial roads in mountainous area is the exchange between kinetic energy of rotation and translational energy, so, we should take in account the factor. Criteria of deference between operation speed and design speed less than 20km/h may not suitable for lower standard rural roads, while for freeway design in high standard, operation speed method also not suitable.5. Evaluation of highway alignment based on vehicle driving stability and steering workload.In the sixth section of the paper, rollover probability of alignment is evaluated by indicator of vertical force of tires; effect of superelevation/reverse superelevation on steering control is analyzed by measurement of tire slip angle, steering force and steering input; and steering workload of alignment is measured by medium of steering input and its angular speed. Our main conclusions are as follows:within curve areas, rollover probability increases as design speed of the road decreases when driving speed around design speed. Exposure can change load sharing effect among front/middle/rear axle, and further change the rollover probability of upgrade and downgrade, the results indicate downgrade is favorable to rollover. Superelevation can reduce tire slip angle when driving on curve, therefore, stability of driving vehicle is improved. In addition, superelevation also can reduce steering input and the force acting steering wheel and make handing easy. But its disadvantage is increasing vehicle's lateral inclination when traveling on curves. Reverse superelevation make against stability of running vehicle on curves, it can increase tire slip angle, steering force, and steering input.Lateral acceleration of running vehicle will no more than tolerated limit, but it exceeds the comfort limit, if the highway alignment meet the design standard recommended in Chinese policy. If spiral no exists in the fourth-class roads, change rate of lateral acceleration when entering/exiting curve will larger than 1.0 m/s3. There does not exist difference between steering on curve and steering on tangent if curve radii exceeds a certain value, but they all need steering corrections. Even in mountain terrain, steering workload of freeway alignment is very small; although riding comfort of secondary roads is not as good as freeways, alignment of secondary roads can not lead to drivers stress; third-class roads may cause drivers who like high speed stress. Egg-shape curves are in favor of steering control, because steering wheel angle can change in the middle spirals, whereas, convex curves call for continuous steering input. When point of circular to tangent overlap point of tangent to spiral, the skip of horizontal curvature will result in difficulty in steering, so, this overlapping should be eliminated and change this alignment combination to an egg-shape curve.6. Vehicle driving stability under adverse conditions.In the seventh section of the paper, the effect of crosswind, water gathered on tangent, and tunnel entrance/exit on driving vehicle are analyzed, our main findings as follows:To assure the straight line performance of driving vehicle, drivers should let passengers and goods close to rear end of their vehicles, by this mean, vehicle'cg can locate behind wind pressure center. Vehicle's straight line performance increases as its cg reduces, vice versa, so drivers should control the height of center of gravity of laden vehicle. Drivers decrease their speed before entering crosswind areas can reduce the lateral displacement and deflection angle.Mechanism of crash on tangent segment gathered water is that, unbalanced loading, road crown, difference in spring stiffness of both side→cg of vehicle departures its longitudinal axis→unbalanced wheel load between right to left→the tire with larger load has larger friction forcee→it has larger linear rolling speed→it has larger displacement in wheel center→side skidding occurs. If tires of two sides simultaneously contact with water, vehicle will deflect toward the side of lighter tire load. If one side tires contact with water, vehicle will deflect toward the water areas.The abrupt change in adherence at location of pavement transition of curved tunnel entrance can cause additional deflection of driving car, and cement pavement lower adherence in sequence can lead to instability of braking vehicle. Yaw motion of driving vehicle at location of pavement transition increases as speed increases, so we can layout the transition location of entrance behind the end of brake and the transition location of exit before the beginning of acceleration. The abrupt change in adherence at location of pavement transition is the factors that mostly contribute to the occurrence of accidents near tunnel opening, to reduce the yaw motion caused by adherence change, and assure adherence of inside tunnel more than 0.35.7. Mechanism of several typical run-off-road crashes on horizontal curves.In the eighth section of the paper, we simulate the process of avoiding on curved segment and recur the process of vehicle running off roadway on spiral of S-shape curve, our findings are as follows:When vehicle travels at a special speed, lateral deformation of tire generated on circular will not release adequately on the adjacent spiral, residual deformation of tire will be maintained to the spiral jointed another circular and will release suddenly under increasing lateral force cause by spiral, which will result in lateral slip between tire and pavement and cause vehicle instability. The crashes caused by hysteresis effect of tire more often occur on the detection of sharp curve to flatter curve, because sharp circular curves often joint to shorter spirals, which results in residual deformation. The best method to reduce these crashes is releasing tire's lateral deformation adequately, so, increase the spiral length or insert a tangent at point of reversing curvature can reach this effect. When Avoiding on curved segment, increase/decrease in curvature of track results in change in driving safety of two running vehicle in opposite. To reduce the additional increment in track curvature while recovering, the driver in outside curve should prolong the recover track of vehicle. The driver inside curve should begin his lane change earlier and recover smoothly, because the additional curvature always occurs on the phase of avoiding beginning and recovering.8. Effect of change in bend geometry on vehicle kinematics and driving behavior.In the ninth section of the paper, we analyzed the effect of change in bend features on two benefits resulting from corner cutting driving pattern, speed increment and radii flatting on curves, consequently, answer the question of where drivers cut the curve. Current alignment design assume track the same as road centerline, but when driver choose corner cutting pattern, radii of track will exceeds design value of curve radii a lot, at this time, how can we achieve our design control? According to our findings, designers can know in which kind of parameters combination the curve can really influence driver's speed choice and track radii equal to curve design radii; and in which kind of parameters combination drivers will cut the curve, how much the track radii and traveling speed when corner is cut.In this section, we also analyzed the steering behavior while vehicle driving on simple curves. Steering time, steering distance and steering characteristic point are obtained, therefore, steering behavior on simple curves can be depicted clearly. Profile of steering length versus curve radii in this section can provide design control for spiral length, because a typical viewpoint currently believes desired spiral length equal to the distance traveled during the steering time. Profile of steering time depending curve radii in this section can provide another control for spiral design, because spiral length recommended in design policies of several countries no less than steering time multiply design speed. In addition, profile of advanced steering length versus curve radii can help designers install curve alignment markers rightly, because we can know where drivers begin their steering.