Investigation into suspension component stiffness and its effect on vehicle handling Anudeep D Patil PG Student, Department of Automotive Engineering Coventry University, Coventry Patila6@uni.coventry.ac.uk
Abstract--This thesis unfolds the understanding of a double wishbone suspension system and various factors of the vehicle which contribute to the cornering forces in a vehicle during a steady state constant radius turn and in turn to determine the level of handling changes with varying bushing stiffness’s. The analytical method is used for the calculation of cornering forces, then using FEA analysis software like Hypermesh for stiffness measurement in the double wishbone arms. ADAMS (multi-body system) analysis software is then used for understanding the effect on vehicles performance with varying bushing stiffness’s and criteria for better handling purposes. Keywords---Double wishbone, Handling, Suspension design, Analytical method Hypermesh, CATIA, and ADAMS
The use of software ADAMS, CATIA, and Hypermesh help in identifying the prime stiffness of the component. The software are used to create conditions almost similar to that on road, vehicle dynamics is an extremely complex and difficult part. In order to have a broad view, understanding of each component and its response are required to be studied. The report provides the optimisation of component stiffness with the effect of its on vehicle handling. (Michael K. McGuire 1993 OBJECTIVES 1. Study of the disturbing forces generated during cornering on the tires, which is reaction point in the vehicle. This point manages the distribution of the input forces through the spring, dampers and linkages of the suspension components.
INTRODUCTION The purpose of this report is to understand the dynamic behaviour of a vehicle during cornering. Suspension system has been the part of vehicles since the horse-drawn carriage with a leaf like structure. With the increase in need for speed and light structure the present structure and kinematics used are very complex. Present suspension are designed to isolate the vehicle body from road vibrations, resist lateral forces generated by the wheels while cornering and at that instant maintain the contact between the wheel and road for better handling performance. The load acting on the suspension system (spring, dampers, rubber mountings and linkages) can be categorised into two parts the road disturbances and unbalanced loads on vehicle. The roads are due to road characteristics and unbalanced loads induced by cornering, braking and accelerating. Therefore a good suspension system balancing both loads is at present concerned for better handling performance and ride comfort. The suspension system consists of wishbones, springs, bush, dampers and shock absorbers for the transfer of all the forces between the body and the road and vice versa. If one of the components fails to sustain the force the vehicle is not safe. The load acting on the system is always changing from road force to unbalanced loads sometimes together. The stiffness of the components should be maintained in such a way that it doesn’t flex to failure. Meeting the requirement always have some constrains to system. The system needs to be as light as possible as it plays an important role on the performance of the vehicle, since a heavy body can generate significant forces.
2. Breakdown of the input forces generated by the wheels on the suspension components according to their magnitude and vectors. 3. To design and generate setup of the suspension assembly in Hypermesh for static analysis. 4. Analysis of the setup and identifying the stiffness for the suspension mounting and linkages, later to implement it on the ADAMS model for verification. 5. To identify the regions where optimisation in mass and handling is required and work accordingly. 6. To understand the effect of variation in bushing stiffness on vehicle handling. 7. To investigate the parameters affecting the vehicle handling. DOUBLE WISHBONE (INDEPENDENT SUSPENSION) In this investigation the independent suspension system used is Double wishbone. Today nearly all the commercial cars, sports car use double wishbone because of the simplicity, light weight and compactness. The double wishbone is also known as “A-arms”. The advantage of this system is that it provides good roll resistance which is necessary for good handling and larger suspension deflections to absorb vibrations and bumps. These are the characteristics of the Double wishbone suspension. The double wishbone gives the freedom to change the camber angle by varying the length the links which is useful for controlling the wheel forces generated in the
system. In this paper we are going to study the effect of bushing stiffness on handling.
and try to execute a constant velocity turn and accelerate while leaving the turn. Hence for the analytical method the vehicle is assumed to be manoeuvring the turn at constant speed. (942523). the radius of turn is taken 18.25 m which is the requirement for a FSAE car to manoeuvre. Centrifugal equation is used for velocity calculation when the acceleration acting on the car is known which is given as 2g.
F c=
m.V 2 R
Front outer wheel reaction: The lateral loading on the outer wheels of the vehicle is given by 2
G RSdyn =
G V B [ H+ ] B g竏由 2
(
)
Figure (picture of an working independent suspension system)
HANDLING Handling is basically defined as the amount of available friction between the tire and the road or maximum lateral acceleration achieved by the vehicle (reff). . The constant radius cornering and lane change test are the most widely used to predict the handling of a vehicle. The most important in this test is the measuring of roll angle, yaw rate, lateral acceleration, side slip angle and steering wheel angle response. The main aim is to improve vehicles transient response during cornering, handling is a relationship between the tire and the surface. The wheel forces generated are the function of load, hence weight of the vehicle plays and important part. Namely there are three sources of load transfer,
Where G is the weight at the front, V velocity and B is Track width. The reaction at the front outside wheel was found out to be 2125.56 N. Hence the side force is given according to the frictional coefficient which is taken as 0.8. The side force calculated was
S F =0.8*2125.56 N =1700 N
The FBD (Free body diagram) generated to simplify suspension system for analytical calculation is shown below.
窶｢ Centrifugal force at the vehicle centre of gravity acting on a moment arm between the C.G and the vehicle roll centre. The forces and moments of which is balanced by suspension and its components. 窶｢ The sprung mass during cornering rotates due to centrifugal forces causing change in the centre of gravity 窶｢ During cornering the forces on the wheel acting on the vehicle roll centre, creates a moment about a part on the road. (Kenneth D. Kramer, 2014).
Figure (Free body Diagram for mechanism force calculations) Rear outer wheel reaction: Similar method was used for the calculation of outer rear wheel force. The calculated rear outer wheel reaction force was 2592.5 N. similarly the side force calculated in this section was 2074 N, taking frictional coefficient as 0.8. Figure (SAE Vehicle Axis system) ANALYTICAL PROCEDURE Wheel forces while steady state cornering: Assuming a constant velocity while cornering which is preferred by the driver taking a turn. Normally the race car drivers brake before entering into a turn
Transfer of forces through front suspension arms: Using the equilibrium method in a system the transfer of forces through the contact point to the arms is calculated. The FBD is used for the calculation of these forces. First taking moment at the joint in the upper arm and then at lower arm the forces were calculated. The forces in the front upper arm calculated was 563.4 N
Similarly forces transferred in the front lower arm were 5375.83 N. HYPERMESH SETUP FOR CALCULATING BUSHING STIFFNESS Proceeding to the analytical result were obtained for the front suspension system in hypermesh, the result for the sides and ground reaction forces during the cornering instant was used to apply for the analysis in hypermesh. The procedure followed for analysis is the most basic one.
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Pre-processing: Pre-processing involves fixing of the geometry imported if needed for free edges and reducing the model to a mathematical model by meshing and applying finite elements (770605). The suspension arms were meshed using 3-D elements. The setup for the
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Solution: In this section definition of material properties, geometry, boundary conditions and loading conditions were applied to the model. These are the most important steps in setting up the analysis in order to be followed with a solution needed
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Post processing: This part involves using of the graphical interface and a technique in Hypermesh to display results after the analysis is performed. Coloured spectrums are used to display the results for displacement and stresses as per the command given. In this the deflection of the both the arms in X, Y and Z direction are noted down. The side force divided by the displacement measured gives the bush rate in respective directions, later which will be applied in bushing property in ADAMS for investigation into vehicle handling.
The Setup for hypermesh analysis is shown in the figure below. A lot of replacements were made in the model. Instead of spherical joints MPC (Multiple joint constraints) were used. The upright and the wheel were constructed with the help of rigid joints for simplification. Constraints were given to the other end keeping allowing the rotation in X axis.
Similar procedure was followed for the static analysis of the front suspension system. The forces for the front calculation were applied at the contact point of the setup where wheels are imagined to contact the road. DYNAMIC ANALYSIS IN ADAMS ADAMS/CARS have a built-in-system and system templates with already defined virtual tests and manoeuvres for performing dynamic analysis of just suspension system or full-vehicle dynamics. In present the vehicle industry refers to ADAMS for the computer analysis (Automatic Dynamic Analysis of Mechanical System) (http://web.mscsoftware.com/Academia/Learn/LearnAdamsCar.aspx) .Multi-body simulation is the most powerful tool developed for the vehicle dynamics. ADAMS is one of the most powerful tools used for the dynamic analysis of the double wishbone. In this paper ADAMS is used for the study of lateral effect during cornering and investigation into handling by varying the bushing stiffness of the suspension arms (930764). These programs have general capabilities and perform all the vehicle dynamic testing, and this computer analysis is known as Multi-body-system analysis (MBS). (The influence of rubber bush compliance on vehicle Suspension movement).
Figure (picture of FSAE model used in ADAMS)
PRINCIPLES OF ADAMS MODELLING In ADAMS the following procedure is to be followed for the development of model for dynamic analysis. The steps were • Mass and inertia of rigid parts of the sprung and un-sprung mass • The geometric aspects of the system centre of mass of the parts, the location of the parts in the system with respect to the joints, specifying of motions and force path. • A virtual creation of external forces such as bumps and cornering regions. • To enable animation and visualisation of the system behaviour by inputting graphical attributes. • Type of connectivity between the parts. But for the model provided everything was predefined even the type of joint, mass and moment of inertia with the hard points to define the parts in the global system. There are different theoretical formulations which can be used for MSA (Multi-body system analysis). (Kenneth D. Kramer, 2014).
Figure (picture of rear hypermesh setup)
The bushing stiffness of the vehicle can be modified by changing the values given in the template which is used for defining the property of the parts or the joint. For bushing stiffness the stiffness in X, Y, Z direction should be specified for the analysis to be performed. The constant radius cornering and lane change test are the most widely used to predict the handling of a vehicle. The results are plotted for the parameters which define the handling behaviour of the vehicle. The most important in this test is the measuring of roll angle, yaw rate, lateral acceleration, side slip angle and steering wheel angle response. The lateral acceleration is often considered the most important parameter to determine the level of handling. The rest parameters are measured to support the decision of lateral acceleration (http://ac.els-cdn.com/S0022489804000916/1-s2.0S0022489804000916-main.pdf?_tid=8b7e9ca8-2401-11e4-808c00000aacb35e&acdnat=1408055158_b6db399f5ee0af704dcad5b61b 0d6c5e) (one more). So the graphs plotted in the result section for the simulation are as follows.
Roll Steer: the roll steer is assumed to be negligible, since the change in body roll causes change in suspension and steering geometry. Bump Steer: Bump steer results when the wheels strike an obstacle while cornering and causes deflection in suspension geometry, which then affects the steering input. Rolling Resistance: when a vehicle turns the weight is transferred from the inner to the outer wheels which results in an increase in tire friction on the outer wheel, which contributes understeer effect is not considered. Camber Changes: Chamber changes while manoeuvring a turn is neglected. (942523) Dynamic Analysis: Static analysis was performed fin Hypermesh. SIMULATION RESULT The above programs were used to get the results which helped in the investigation of the handling performance. The simulations as discussed were based on FSAE 2011 model. In constant radius cornering: In this the result were plotted in ADMAS post processor. The graph for the lateral acceleration while cornering for the three bushing stiffness is below. The plot shows a slight difference in the lateral acceleration achieved by the vehicles before they slip off the track. From the maximum lateral achieved column the reading for all the three settings were taken into account. The maximum lateral generated by the vehicle was 10701827 mm/sec^2 having twice the bushing stiffness.
Figure (Constant radius cornering in ADAMS) For Constant radius Cornering •
Lateral acceleration vs. Time
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Roll angle vs. Lateral acceleration
For ISO-Lane change •
Roll angle vs. time
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Side slip angle vs. time
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Yaw angle vs. steering wheel angle ASSUMPTIONS
Many assumptions were made for developing the analytical calculation base for the steady state cornering of the vehicle. They are explained as follows for reference. Body Roll Axis: The body roll axis is assumed to be parallel to the road, hence when calculating the reaction forces the front and rear weight distribution is assumed to be in line with C.G Turn Radius: While taking a turn the outer and the inner wheel are assumed to be parallel and taking a turn of same radii.
Figure (Lateral acceleration vs. Time) The plot for the roll angle and lateral acceleration shows that there is very little or no change in roll angle of the vehicle. The only determining factor is the lateral acceleration. From the plot it can be seen the lateral acceleration achieved by the vehicle having stiffness twice the original is more than the other two settings. So in this bushing with twice the stiffness is recommended for better handling.
Figure (Roll angle vs. Lateral Acceleration)
Figure (Side Slip angle vs. time)
For ISO-Lane change: The first plot for Roll Angle vs. Time was plotted for the three bushing settings in one graph. From the plot it can be seen that lowest roll angle is for the bushing having twice the stiffness. The lowest maximum body roll defines the best handling of vehicle. From the plot the vehicle having twice the bushing setting is the best among them.
CONCLUSION A 2-D FBD was created for the analysis of the cornering forces. These forces were used for the Hypermesh analysis of the front and rear suspension system. A 3-D model was developed for the FSAE vehicle of Coventry. This model was used for the investigation into the suspension component stiffness and handling parameters. Following are the conclusions drawn from the work.
1. A 2-D method used for the analytical derivation of the forces gave values from the actual readings measured from the vehicle.
2. Assumptions were made during the development of mathematical derivations due to their complexities. These assumptions were considered negligible for the sake of calculations
3. The type of steering system does not affect the lateral force that is developed in the vehicle. Figure (Roll Angle vs. Time) The plot for the side slip angle for all the three setting is plotted below. The reason for plotting side slip angle is basically for better understanding of handling and in short verification of the chosen bushing stiffness. We can understand that the amount of slip angle measured directly suggests how hard the vehicle is pushed on the ground. The more downward pressure on the tire the more slip angle and more will be the cornering force. The slip angle therefore suggests more friction is generated at the wheels hence more grip which stand for better handling vehicle. the side slip sometimes is the most distinguishing factor for analysing handling parameters.( http://www.pratperch.com/2011/05/tyre-side-slip-explained/). So from the plot the bushing having twice the stiffness is preferred for better handling.
4. The handling is affected by varying the bushing stiffness. 5. From the plots it is observed that there is almost no difference in the body roll after varying the bushing stiffness, which suggests that for the FSAE vehicle the lateral acceleration is the only distinguishing factor for understanding the handling of the vehicle.
6. The contribution of the different driver handling results should also be taken into account to understand the vehicle handling.
7. Dynamic analysis should be performed for more accurate results
8. The lateral forces generated depend upon the degree of body roll angle. Thus body roll indirectly has an impact on vehicle handling. Since in this paper dynamic analysis of the vehicle is not considered.
Thus it can be concluded from the simulation results that the most feasible bushing stiffness for the FSAE vehicle for better handling is achieved by changing the stiffness to twice its original value. References 1.
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