Computational Analysis of a Concept of Rear

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COMPUTATIONAL ANALYSIS OF A CONCEPT OF REAR SUSPENSION SYSTEM FOR OFF-ROAD VEHICLE

DIEGO DAVID SILVA DINIZ RAPHAEL SOUSA SILVA

ARTHUR FERREIRA AZEVEDO ANTONIO ALMEIDA SILVA

WANDERLEY FERREIRA DE AMORIM JNIOR

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2012-36-0425

Computational Analysis of a Concept of Rear Suspension System for Off- Road Vehicle

Diego David Silva Diniz

Federal University of Campina Grande

Arthur Azevedo Ferreira Federal University of Campina Grande

Raphael de Sousa Silva


Antnio Almeida da Silva Federal University of Campina Grande

Wanderley Ferreira de Amorim Junior


Copyright 2012 SAE International

ABSTRACT

The studies in the field of vehicle dynamics have been performed since the beginnings of the automobile industry, however in relation to advancements in embedded technologies for greater security and performance of vehicles in adverse conditions, research on this subject, gained a greater intensity in recent decades. Several research groups have sought to understand and model the real dynamic behavior of vehicles subject to the conditions imposed by the irregularities of the soil, in order to obtain suspensions of innovative concepts for vehicles that have high reliability and dynamic performance car. This paper presents a study for a cinematic concept of independent rear suspension, applied to off-road vehicles (off road). Thus, the analysis was performed using the software MSC Adams car, a type of vehicle suspension traditional double wishbone front with direction of tendency geometry to the oversteer effect and rear suspension arms overlaid with guiding bar. In these simulations allows perform an analysis of the behavior of the change in camber and convergence of the rear wheels during work vertical of the suspension by the method of kinematic analysis, using the theory of instantaneous centers of rotation. The concept developed is a modification of suspension of overlapping arms

(double wishbone), with an addition of a guiding bar. The simulation results show the combined effect of the variation in camber and especially the variation in the effect of convergence in the oversteer of the vehicle, which was verified in experimental tests on a vehicle off-road mini-baja. Furthermore it has been found beneficial effect at the rolling of the chassis on the difference of change at the convergence of the wheels, which involves a tendency of exit from the rear of the vehicle. Based on these results verified the validity of the applicability on the suspension of arms superimposed with guiding bar for off-road vehicles, especially the mini-baja, which one of main characteristics is sought to obtain the ability to perform at the lowest curves possible radius.

INTRODUCTION

The prediction of performance of a vehicular suspension system before the physical construction of a prototype, allows the design of the components prior to integrate more precisely by optimizing it, thus the final cost of the product and the resulting cost and time changes to solve problems that would be detected only in the physical prototype testing (Freitas, 2005). Given this, the auto industry has been seeking the aid of computer modeling in order to achieve the above factors,


where new concepts are applied, which, through this tool are simulated and optimized, obtaining a virtual project that meets the prerequisites of engineering and consumer preference, can be implemented in any vehicle including off-roads.

Figure 1: Off-road vehicles.

The off-road vehicles have different characteristics from traditional cars, such as increasing the range of the car, heavy-duty suspension, transmission system 4x4 or 4x2, brakes more effective, components of unsprung mass and have a tendency more oversteer. Usually, vehicles in this segment are classified as off-road light (like Fiat Palio Weekend Adventure 4x2), 4x2 pickups (Chevrolet S-10, F-1000 and Ford F-350), 4x4 Pickups (Mitsubishi L-200, Ford Ranger, Chevrolet S-10 and Ford F-1000), 4x4 SUVs (Pajero Mitsubishi, Jeep Cherokee and Grand Cherokee, Land Rover Range Rover), Jeeps 4x4 (Ford, Willys, Suzuki Samurai and Jimny, Troller) and also the baja style of vehicle. These cars are designed to overcome obstacles and walk on unstable land without presenting great difficulties.

Most high performance cars and off-road uses independent front suspension, usually choose to type suspension SLA (Short Long Arm) also known as Double-Wishbone or Portuguese term double "A", this suspension shows great advantages when compared to other types of suspension is required in applications where high performance, robustness, ease of adjustments and fine adjustments (Costa, 2006). In rear suspensions of vehicles off-roads, the Double-A is not used much, knowing that their dynamic stability, the tendency to interfere oversteer, characteristic in this vehicle segment, so it is customary to use other types of suspension such as swing- axle, semi-trailing, multi-link axle drive shaft and Dion since these allow variations in the angles of the wheel, as the angle of convergence which significantly influence the oversteer tendency.

The objective of this study is to analyze how a new type of kinematic rear suspension for off-road vehicles, called the overlapping arms with guiding bar. This new concept emerged as a way to associate the qualities of the Double-A with the dynamic instability required rear suspensions in off-road vehicles. The study was done by using the commercial software MSC ADAMS CAR (Automatic Dynamic Analysis of Mechanical Systems), through two simulations. The first considered only the suspension under study, analyzing variable camber and convergence in relation to the vertical displacement at the wheel and the body scrolling, respectively. The second simulation used the complete model of a baja type vehicle with oversteering geometry, with

doubleA front suspension and suspension of arms superimposed with guiding bar at the rear. Through the second simulation, it was possible to analyze the variation of yaw movement of the vehicle in cornering..

With this methodology it is expected that this type of suspension characteristics oversteering will be shown on the vehicle and keep the good qualities of overcoming obstacles of traditional suspension double tray, allowing their development in the application as the rear suspension for off-road vehicles.

DOUBLE-A SUSPENSION

Known as SLA (Short Long Arm), Double Wishbone or double A, this suspension is independent, can be used on front or rear of the vehicle, ensures optimum stability control as well and camber angles of convergence. The same is composed of two transverse control arms, the suspension type is widely used in high performance vehicles such as racing cars, sports cars, etc.

The model of the geometry of a Double-A suspension requires a careful refining in order to achieve good performance, as illustrated in FIG. 2, which can check the typical geometry of this suspension.

Figure 2: Geometry of a Double-A front suspension

(Gillespie, 1992).

The point referred to as pole in Figure 2 this is the IC (instantaneous center of rotation of the suspension), its location is extremely important since it influences directly the change in the camber angle of the wheel during the vertical movement of the suspension thus the greater the distance between the IC and the pole or plane of the tire, the smaller the variation in camber of the wheel. The position of the IC also interferes with the location of the suspension roll center, a point of fundamental importance as it affected the transfer of load between the wheels and the variation of the same gauge.

Double-A suspension has some disadvantages such as increased weight and a larger occupation when compared for example with the MCPherson suspension, factors that influence the non-use in vehicles. In addition, Double-A has limited its application to act as rear suspension, it presents a high dynamic stability, which is a negative factor in the oversteer behavior desired in a off-road vehicle, when they perform a curve. However, this suspension with an appropriate


behavior and vertical geometry can be readily obtained in projects, which allows good control of parameters such as camber, caster, roll center height, kingpin inclination, camber change as a function of steering.

BASIC CONCEPTS

Neutral steering, Understeer and Oversteer

The effects of understeer, oversteer and neutral steer in a vehicle can be seen in Figure 3 and defined as the route traveled by a car when subjected to a lateral force acting at the center of gravity (Landgraf, 2004).

Figure 3: Behavior of steering in a curve. OS (Oversteer), NS (Neutral Steer), US (Understeer), (Milliken & Milliken, 1995).

In the neutral steer, the way that varies the steering angle is equal setting the variation of the angle Ackerman, considering that in a curve of constant radius, the vehicle does not need to change the steering angle with the change in velocity. In fact, the case of neutral steering corresponds to an equilibrium in the vehicle so that the "strength" of the CG lateral acceleration causes a similar increase in the angle of slip on both the front and rear wheels. (Leal, 2008).

In understeer, lateral acceleration on the CG of the vehicle causes the front wheels slip side and a track traverse larger than the rear wheels. Therefore, to promote a front wheel lateral force sufficient to maintain the radius of the steering angle should be greater (Gillespie, 1992).

For oversteer, lateral acceleration on the CG of the vehicle causes the rear wheels slip more than the front. The slip to the outside of the curve of the rear wheels indicates the front wheels toward the inside of the curve decreasing the curve radius. The increase in lateral acceleration which follows causes the rear wheels slip and further continuously unless the steering angle is reduced in order to maintain the curve radius (Gillespie, 1992).

The configuration of how the steering angle varies with speed in a given constant radius curve in cases previously discussed is shown in Fig.4. When the vehicle steering is neutral steer, the angle to maintain the curve at any speed is simply angle Ackerman. With understeer, the angle increases with the square of the speed reached twice the initial angle the speed characteristic. In the case oversteer, the steering angle decreases with the square of the speed and reaches the zero value of the critical speed (Leal, 2008).

Figure 4: Graphic of the steering angle variation of the speed. (Gillespie, 1992).

Convergence Angle The convergence angle (toe angle) is defined as the angle

between the XY plane between the longitudinal axis of the vehicle and the plane of the wheel.

Figure 5: Convergence of the wheels (adapted from Soares,

2005).

In a design of a vehicle suspension is to obtain the minimum possible variation of the angles of convergence in order to reduce the drag and loss tire lateral stability, but in racing vehicles especially off-road is desirable to have a vehicle with a certain characteristic oversteer when it performs curves to obtain a variation of this toe - out (divergence) in the outside rear wheel, where the curve would be ideal. In Figure 6 you can see the effect of varying the angle of convergence of the outer rear wheel to turn.


Figure 6: Behavior on under/oversteer as variation and

divergence of the rear wheel outside the curve.

Steering due to the scroll (rollsteer)

The location of the internal point of the steering bar influence on the behavior of the vehicle to be over/understeer or neutral. If it is located below the ideal point (ge) determined by the geometry (Fig. 7) by performing a curve, for example, to the left, the car body scrolls to the right, causing the closure of the respective side of the suspension, then the right suspension closed describes an arc greater than the steering bar, generating a steer to the right of this. On the left, the steering bar describes an arc greater than the suspension also led to a steer to the right, thus giving the car a understeer characteristic, if the internal point of the steering bar is above the ideal point determined by the geometry will be given the car a oversteer characteristic. (Madureira apud Fernandes, 2005).

Figure 7: Location of the internal point of the steering bar in the front (Gillespie, 1992).

Coordinate System

In the study of vehicle dynamics, it is important to know

the number of vehicle movements that influence the behavior of the same. SAE (Society of Automotive Engineers) then developed a standard coordinate system located in the CG (Center of Gravity) of the vehicle (fig. 8), defining the most important movements to be considered.

Figure 8: Vehicle coordinate system defined by SAE (Jazar, 2008).

As fig. 8, the movement of rotation around the X axis is called roll, the rotation around the Z axis is called yaw motion and the rotation about the Y axis is called pitch. (Soares, 2005).

METHODOLOGY

This work was performed at the Laboratory of Modeling and Computer Simulation of the Academic Unit of Mechanical Engineering at Federal University of Campina Grande - PB in partnership with the project SAE Baja UFCG, using the commercial software MSC Adams car. The first simulation model consists only of the rear suspension under study, which was applied in a vertical movement of the wheels in order to analyze the variation in camber and convergence. In the second simulation, we used the complete model of the Baja vehicle, with traditional Double wishbone suspension in front with geometry tendency direction oversteer and rear suspension arms overlaid with guiding bar, with this simulation was possible to obtain the Yaw motion of the vehicle, characterizing the dynamic behavior of handling it.

Figura9: MSC Adams Car Models: Complete vehicle assemble.

Oversteer Neutral Understeer Neutral


Figure 10: MSC Adams Car Models: Model of the rear suspension.

The new concept of the rear suspension used in this work

for the suspension kinematic study is a modification of the conventional Double-A, with an addition bar, called the handlebar arm (Fig.11). This change resulted from the need to adapt the Double-A on the rear of a baja vehicle, so that the dynamic behavior of the vehicle to provide an oversteer characteristic during the curves.

Figure 11: Double A-arm suspension with handlebars.

The addition of the handlebar arm aims to add a variation in convergence of the rear wheels as a function of the roll frame, which, according to the literature, it is an influential factor in oversteer characteristic of the vehicle, particularly in curves medium and high speed, where there is significant roll variation in compare to low speeds curves. With this new configuration of the suspension is expected, get features more oversteer, keeping the good qualities of overcoming obstacles of traditional Double-A suspension.

For this study, the method of instantaneous centers of rotation, shown in Figure 12, from the location of anchor points of the suspension and the location of the IC is possible to obtain the optimal positioning of the connection point of the handlebar arm and chassis (C), from where the variations in position of the same, it is possible to obtain variations in the

angle of convergence of the wheel. From the reason shown in item 3.3, using the point C2 as anchorage arm handlebar with the chassis, it is possible to obtain during the scroll certain divergence in the curve outer wheel, since it turns out to cover an arc less than respective side of the suspension, adding to the vehicle, an oversteer effect, as seen in Figure 6. Using the point C1 as a starting point for the handlebar arm, the effect generated when scrolling becomes reverse, giving the wheel a certain convergence and therefore an understeer characteristic to the vehicle. When the arm is anchored in the handlebar point C (shown by the ideal geometry) the behavior in relation to the variation of convergence becomes neutral, since the arm travels along the arc described by the same suspension. Point C is widely used in the conventional double-A front suspension with the steering system, it provides small variations of convergence, not affecting the handle of the vehicle.

Figure 12: Geometry of suspension in study. RESULTS

To validate the methodology, two simulations were performed, with the first model of the suspension under study and the other with the model of the whole vehicle. The parameters used in the simulations were based on the 2011 Baja prototype from the Federal University of Campina Grande (Table 1).

Table 1: Parameters used.

Wheel database (mm) 1400 Gauge (mm) 1500 Tire Radius (unloaded) (mm) 266.7

Vehicle Weight (kg) 220 Roll angle 5 Bump travel (mm) 100 Rear Height roll center (mm) 243

Vehicle speed (km / h) 40

Rebound (mm) 50

With these parameters and with the geometry of the suspension shown in FIG. 12, the suspension and the whole vehicle was modeled in software MSC Adam Car. In the

Guide Arm


model of the baja vehicle, was added all the major subsystems of a vehicle such as chassis, powertrain, steering system, front and rear suspension. These subsystems were modeled based on the actual prototype.

In figure 13 it is possible to see the variation in camber, the settings using C1 and C2 of Fig. 12, using the model of suspension. For this analysis was applied to a course bumptravel of 100 mm and a rebound of 50 mm for both configurations. See in curves there is no apparent change in camber angle of the wheels, if the position changing of point C. That is, the additional arm did not influence significantly on the variation of the camber angle of the wheels. This happens because the camber in a Double-A suspension is influenced only the anchor points of the suspension arms. Thus we can consider that the modified Double - A and traditional Double-A behaves in the same way for this variable.

Figure 13: Camber variation (deg) for points C1 and C2.

Figures 14 and 15 represent the behavior of the variation of toe-in of the scroll according to the configuration with the points C1 and C2, respectively. The variation of the chassis roll angle was determined experimentally through tests applied in baja vehicle, which revealed an average angle of 5 variation of the scroll. It is important to mention that the variation of this angle is influenced by several factors, such as height of the center roll, hardness of the springs, inclination of the shock absorbers, height of the center of gravity, etc. Thus, it was simulated the operation of the adapted Double-A in a turn to the right using the two configurations to the handlebar arm. Figure 14 shows the convergence variation curves of the inside and outside wheels, as seen in the left wheel (outer curve), there is a convergence ranging from 0 to a maximum of 3.5 , the right wheel (inner curve), there is a divergence ranging from 0 to 4.1 , this behavior can be seen in Figure 16. Figure 15 shows the curves of variation of geometry with convergence towards the point C2, where the right wheel (inner curve) converged at 0 to 3.069 and the left wheel (outer curve) diverged from 0 to 3.93, this behavior is illustrated in Figure 17. With these results it can be seen that there is a great influence the position of point C in the convergence behavior of the variation, as shown in Figure 16 at the curve outer wheel geometry convergence had a gain on C1 and on C2 geometry occurred gain of divergence, that allows to modify the form of driving the vehicle may be oversteer or understeer, as shown in Figure 6.

Figure 14: Variation of the TOE (deg) configuration with C1 point.

Figure 15: Variation of the TOE (deg) configuration with C2 point.

Figure 16: Variation of convergence and divergence in the left wheel outside the curve to the point C1.

Figure 17: Variation of convergence and divergence in the left wheel outside the curve to the point C2.

Another significant influence on the oversteer behavior

of a vehicle is the height of the center roll, since it directly affects the transfer of load between the inner and outer wheel during a turn, in Figure 18 can be seen that there is an increase on roll angle as the vertical height of the center roll to the soil also increases, favoring thus a greater load transfer to the


internal and external wheel rear suspension and consequently an increase in the tendency of vehicle oversteer.

Figure 18: Roll Center height (mm) variation of the rear suspension roll x Variation in the angle of roll.

Figure 19 shows the variation of yaw motion on time

using the full vehicle model and the two geometric configurations for the points C1 and C2. To set the simulation, was considered that Baja vehicle would be in a constant speed of 40 km/h, during a curve, turning the steering so that the inner curve wheel reached 45 of steering. The curve indicated a understeer is blue light, representing the point indicated as C1 and aggressive oversteer for red one, representing the geometry of the point C2. You can see that the same conditions, the rotational motion around the Z axis (yaw) of the chassis was higher for the geometry of the point C2 than in C1 point, this is only possible if there is a greater tendency to get out the rear wheels, characterizing the oversteer. In the blue curve you can see a Yaw angle smaller than the other curve, and this was influenced by the geometry of point C1, giving to the car a understeer tendency.

Figure 19: Variation of Yaw motion for a complete model of the vehicle.

CONCLUSIONS

The study showed that the suspension concept proposed requirements imposed reaches its applicability in off-road vehicles and on competition making its use feasible for use as a rear suspension, since this concept provided a variation in convergence, that has created a dynamic instability, modifying the behavior of driving the vehicle without changing the good ability to overcome obstacles of the conventional Double-A suspension.

The simulations showed that depending on the position of point C on the geometry, the vehicle may have changed their driving behavior; it is possible to implement mechanisms of regulation that allows the driver to choose the best option for a given situation. Furthermore, in vertical behavior, it was found that there is no influence on the handlebar arm camber variation, which allows the control it in the same way the traditional Double-A suspension.

In implementing the UFCG Baja prototype, the concept of suspension proposed reached the needs of a off-road racing vehicle, showing satisfactorily the same results obtained in this study. You can see a qualitative change of vehicle dynamic behavior when changing the position of the handlebar arm, thereby achieving an outcome results.

The results also showed that it is possible to apply the concepts of rollsteer, convergence, instant centers, roll to analyze suspensions from their traditional and disadvantages, making modifications that allow them to adapt so they can meet the requirements laid down in the projects.

REFERENCES

Fernandes, MA, 2005. Study on vehicle steering systems. 99p. Dissertation (professional master's degree in automotive engineering). Polytechnic University of Sao Paulo, Sao Paulo - SP;

Freitas, Luis M. 2005. Study of vehicle suspension vertical dynamics type Macpherson. 122p. Thesis (Master). School of Engineering of Sao Carlos, USP, Sao Paulo - SP;

Gillespie, TD 1992. Fundamentals of Vehicle Dynamics. R SAE - 114, Society of Automotive Engineers, Warrendale, PA, USA;

Landgraf, HL 2004. A proposal for fuzzy logic control with braking and steering of a vehicle. Campinas - SP;

Leal, LC M., Rose, E.; Nicolazzi, LC, 2008. A introduction to modeling quasi-static motor vehicle wheel. Thesis (Free lecture). Federal University of Santa Catarina. Florianpolis - SC;

Leal, TiberioJoppert Alexandre de Castro. Effect of reducing the stiffness of the tires on the vehicle stability. London: School of Engineering of Sao Carlos, 2008. Dissertation submitted as part of the requirements for the degree of Master in Mechanical Engineering from the School of Engineering of Sao Carlos daUniversidade of Sao Paulo.

Jazar, Reza N., 2008. Vehicle Dynamics: Theory and

Applications, Springer, New York, USA Milliken, WF, Milliken, DL 1995. Race Car Vehicle

Dynamics. R SAE - 146, Society of Automotive Engineers, Warrendale, PA, USA.;


Moreira, HS, 2008. The influence of SMAG welding parameters and the morphology of the cord on the fatigue behavior of welded joints of a rear axle vehicle. Dissertation (MA). Polytechnic University of Sao Paulo, Sao Paulo - SP;

Neto, C. 2006. Scale and calibration of suspension type double A to Mini Baja vehicle. Completion of course work. Polytechnic University of So Paulo, SP-SoPaulo;

Soares, LLP, 2005. Analysis elastic-kinematic comfort and suspensions of dual-stage of a competition off-road vehicles multibody environment. San Carlos, 2005. 89p. Thesis (Master). School of Engineering of Sao Carlos, USP, Sao Paulo - SP;

CONTACT INFORMATION

Diego David da Silva Diniz, email address: [email protected];

Arthur Azevedo Ferreira, email address: [email protected];

Raphael de Sousa Silva, email adress: [email protected];

Antonio Almeida da Silva, email adress: [email protected];

Wanderley Ferreira de Amorim Junior, email adress: [email protected];

ACKNOWLEDGMENTS

The authors are very grateful to the Laboratory of Modeling and Computer Simulation/UFCG and Parahybaja for helping the development of this research.


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Computational Analysis of a Concept of Rear

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