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Msc Adams car for Vehicle Dynamics - Suspension Analysis .

Data Hierarchy in Msc Adams Car MSc Adams Car provides a large variety of subsystems, templates and assemblies. Group of templates becomes…

Amith Ganta
updated on 23 Jan 2021

Data Hierarchy in Msc Adams Car

MSc Adams Car provides a large variety of subsystems, templates and assemblies. Group of templates becomes a subsystem. similarly, a group of subsystems forms an Assembly.

Templates act as the skeleton of the suspension system. It defines the topology or the Architecture like how different parts fit together (Joints, Bushings, Gears etc). Subsystem starts from where the template left. It forms one of the major systems of the model like front/rear suspension, steering or Powertrain. This is a specific instance of the template where the user can modify parameters. One template can form multiple subsystems. Example having a single double-wishbone suspension template can help to utilize it in both front and rear ends. Assembly is a combination of one or more subsystems and a test rig. Testrig helps in providing actuation.

Adams Files

After running the simulations inside the working directory all the above-mentioned files in the working directory are created. These files can be edited manually in a 'Notepad' document as well.

There are two major types of front suspension systems widely used in passenger cars.

1. MacPherson strut

2. Double wishbone suspension systems.

This project mainly focuses on front suspension architecture starting from building an assembly from templates, performing suspension analysis and finally post-process the results obtained after modifying various hardpoints.

The Graphical User Interface is user friendly and options like zoom, translate, rotate, left view, right view, Iso view all are available just after clicking right-click button.

By selecting each part in the GUI the name of the part can be known individually. Bodies are mounted on hardpoints. The position of the individual bodies can be changed by changing the coordinates of the hardpoints. The above MacPherson strut represents the following main parts

1. Lower control arm

2. Knuckle (Upright)

3. Steering Wheel

4. Rack and Pinion steering

5. Tie rod

6. Subframe

7. Bushings

8. Springs

9. Tyre

10. Test rig (Actuators)

Suspension Analysis of MacPherson Strut - Parallel Wheel Travel

The purpose of this suspension analysis is to study the behaviour of suspension parameters like camber angle, toe angle, Ackermann steering etc with respect to wheel travel. Adams car provides inbuilt suspension systems like this which can be modified according to the requirement of the final product that is used.

The first step is to run the suspension analysis with the given design without changing any kind of Hardpoints in the MacPherson strut. Later the hardpoints are modified to make a comparative study of change in toe angle, camber angle with respect to wheel travel in jounce and Rebound.

The suspension is made to travel 100mm in Jounce (Bump travel) and 100mm in the rebound from the wheel centre. A virtual test rig supports the suspension system. Both wheels travel up and down at the same time to determine toe angle change.

From the above graph, it can be observed that when the wheel centre travels 100mm in jounce the suspension experiences about 2 degrees toe-out. Similarly when the wheel centre travels 100mm in Rebound the suspension experiences 2 degrees toe-in. Also at zero wheel travel, the slope is -0.0182 $\frac{d e g}{m m}$ $(deg)/(mm)$ . Which means 18 degrees per metre. Which is impossible to construct.

The second step is to modify the hardpoints of any given individual parts and compare the results of both modified and unmodified graphs. Here in this case 'Tie rod inner' hardpoints local z coordinates have been changed from 300mm to 320mm

Similar to the above case the suspension is made to travel 100mm in Jounce (Bump travel) and Rebound. Finally, overlay the graph on the previous plot in the Adams post-processor to check the difference in toe change.

From the above graph(plot), it can be observed that there is toe-in in Jounce (Bump travel) and toe-out in Rebound. Also, the slope is about 0.0059 $\frac{d e g}{m m}$ $(deg)/(mm)$ which is equal to 5.9 $\frac{d e g}{m}$ $(deg)/(m)$ toe-in . But for proper design, it is preferred to have toe-out for better understeering instead of toe-in in Jounce.

Note: Shape of the curve changes with respect to change in hardpoints.

MSc Adams Car Post Processor

Suspension Analysis of Double Wishbone Suspension - Parallel Wheel Travel

Test_1

Post-processing Results :

Independent Axis (X-axis = Time), Dependent Axis (Y-axis = Length)

Independent Axis (X-axis = Wheel travel), Dependent Axis (Y-axis = Camber angle)

Test_2:

The hardpoints have been modified at the upper control arm and the second test was conducted.

Suspension Analysis of Double Wishbone Suspension - Steering analysis

For performing steering test the steering was set to rotate 400 degrees to the left and 400 degrees to the right (in opposite direction). Adams post-processor is used to post-process the results and all the results can be exported externally to visualize the outputs in other systems as well. The links for the output files can be found here.

https://drive.google.com/drive/folders/1fDYtXRnICAocpTmG-hMnhuh8BkUrXYMd?usp=sharing

Animation file of Steering Analysis

Detailed Study of Double Wishbone Suspension-Building from hardpoints to performing Suspension analysis

Hardpoints: Hardpoints are the very basic elements of any suspension model in Adams car. They define the skeleton of the suspension. They are responsible for the kinematics primarily. Hardpoint can be modified according to the requirements of the user. Almost all suspension systems are symmetric in nature. Multiple hardpoints can also be modified at a time from the 'Hardpoint Modified Table' column available. It is also possible to export all the coordinates of the suspension subsystem to a file in order to build the same in other systems for reference. Hardpoints are very crucial in determining the exact kinematics of the suspension system like Front view instant centre, Side view Instant centre etc.

Parts: Combination of parts together form a complete assembly. In a Double wishbone suspension lower control arm, upper control arm, spring damper, joints, bushings together form the assembly. These parts are defined by hardpoints and connected to each other with bushings/joints. In the Double wishbone suspension system, lower control arm and subframe are two individual parts and are joined by bushings. Similarly, the upper control arm and knuckle are connected using a joint. Adams car software usually focusses on the hardpoints and the centre of mass of each individual part. The shape/dimensions of the geometries which are observed can be of any form in a suspension system unless the change of CG is not affected. Geometries can be built only in the template builder. but the hardpoints can be modified according to the requirements. Part modification with hardpoint change and inertial properties will have a change in the overall behaviour of the system.

All parts/bodies in the Adams car are rigid in nature which means there will be no deformation in shape upon application of loads. This makes no sense in real-world applications. So a rigid body can be replaced with a flexible body to study the deformation and associated stresses and strains. The flexible body is a .mnf file which can be created using FEA software like Ansys, Optistruct, Ls Dyna implicit etc.

Bushing: Bushing or rubber bushing is a rubber element and also a type of vibration isolator. It provides an interface between two parts, damping the energy transmitted through the bushing. Bushings and Joints are the constraints that connect different parts together. Rubber is highly non-linear and Adams allows the user to capture the non-linearity. It is also possible to add linear preload, Torsional preload, Linear offset and Rotational offset values. Inside the data file (Property file) of the bushing used in this suspension analysis, it provides the functional values for Damping measured in $(\frac{N - m m}{\sec})$ $((N-mm)/sec)$ , Damping also has three translational stiffnesses ( $F_{x}$ $F_x$ , $F_{y}$ $F_y$ , $F_{z}$ $F_z$ ) measured in ( $\frac{N}{m m}$ $N/(mm)$ ) and three rotational stiffnesses ( $T_{x}$ $T_x$ , $T_{y}$ $T_y$ , $T_{z}$ $T_z$ ) measured in ( $\frac{N - m m}{d e g}$ $(N-mm)/(deg)$ ). $F_{z}$ $F_z$ represents Axial stiffness, $F_{y}$ $F_y$ and $F_{x}$ $F_x$ represents radial stiffness. $T_{x}$ $T_x$ and $T_{y}$ $T_y$ represents radical torsional stiffness, $T_{z}$ $T_z$ represents axial torsional stiffness. These are forces and moments. Bushings are extremely important for compliance numbers like lateral force compliance, camber, Aligning torque etc. That is how the kinematics and compliance (K & C) are defined by hardpoints and bushings.

Joints: Joints are used to connect different parts together. Bushings have stiffness curves whereas joints are completely contained to certain degrees of freedom. So bushings are considered as compliance elements whereas joints are considered as kinematics elements. Example, a spherical joint connects the upper and lower control arms with the knuckle. It restricts all 3 degrees of translation and the remaining 3 rational degrees of freedom are completely free. Similarly, the revolute joint is placed in between the spindle and the Knuckle (Upright) which allows wheel rotation about one axis and the remaining 5 degrees of freedom is restrained. The translational joint allows only one degree of freedom (translation) is placed in the Rack. Links can be parametrized to the hardpoints so that when the hardpoints change it changes its shape. But the change in shape does not affect anything in results because it is purely for visualization purposes.

It is possible to run the analysis with either kinematic or compliant. Selecting kinematic will replace all bushings with joints.

Bumpers: Bumpers or Bump stop are Bump and Rebound stoppers. It is also another type of bushing which is highly non-linear in nature. These are used to stop the suspension from crashing into the frame of the body by arresting the movement beyond a certain point. Clearance can also be set which is the amount of bump stop distance before it engages.

With a clearance of 10 mm reduced from 200mm, for brief understanding, it is necessary to study the behaviour of bump stopper by performing suspension analysis parallel wheel travel.

From the above graph, it can be observed that for 10mm clearance the force starts to peak faster compared to 20mm clearance. This means that the bump stop engages earlier with less clearance. lower the value of clearance faster will be the engaging time.

Springs: Springs are the primary compression - expansion elements in the suspension. Free length defines the actual length of the spring before it was set inside the vehicle. When the spring is installed into the vehicle there will be an initial compression of spring which defines the preload. When the applied load exceeds this preload only then the spring starts to get compress. In a Double wishbone suspension, the springs are placed in between lower strut and strut to the body. Motion ratio is the relation between wheel travel and spring rate / Spring displacement.

The slope defines the ratio of spring travel to wheel travel. Which is approximately 0.5 in this case

Dampers: The primary difference between springs and dampers are springs control the displacement whereas dampers control the velocity. Springs have displacement vs force curves whereas Dampers has Velocity vs force curves. Since parallel wheel trave and opposite wheel travel suspension analysis are Quasi-static simulations so it is impossible to predict the behaviour of dampers (velocity change) in these simulations. In order to predict the realtime behaviour of the suspension system, it is necessary to perform dynamic analysis. Dynamic analysis is time-dependent and the input will be time. In this dynamic analysis, two different velocities vs Force graphs are taken for study

Test data provides similar data for velocities with different forces. So the velocity graphs re same for both test cases.

Tires: Tires in full car models or full assembly models uses Pacjeka magic tire formula which has a lot of coefficients and ultimately determines the relationship between

Lateral force to Slip angle
Aligning moment to slip angle
Longitudinal force to longitudinal slip ratio

Different tyre models are provided by Adams car which is physically tested and the coefficients are determined by performing curve fitting. The normal force $F_{z}$ $F_z$ cannot be negative or zero which means it loses contact with the road.

The Magic Formula

$Y (X) = D \cdot \sin (C \arctan (B x - E (B x - \arctan (B x))))$ $Y(X) = D*sin(Carctan(Bx-E(Bx - arctan(Bx))))$

$Y (X) = y (x) + S_{v}$ $Y(X) = y(x) + S_v$

$x = X + S_{h}$ $x = X + S_h$

Graphical representation of the magic formula

Whereas

$Y (X)$ $Y(X)$ = Cornering force, braking force or self-aligning moment

$X$ $X$ = Slip angle or skid ratio

$B$ $B$ = Stiffness factor

$C$ $C$ = Shape factor that controls stretching in the x-direction.

$D$ $D$ = Peak value or Peak factor

$X_{m}$ $X_m$ = Slip angle at peak value

$E$ $E$ = Curvature factor that affects transition in the curve and the position $X_{m}$ $X_m$ at which the peak value occurs

$S_{v} and S_{h}$ $S_v and S_h$ = Offsets to account for camber thrust, Conicity, Plysteer or rolling

$y_{s}$ $y_s$ = Steady state value

$S l o p e = B \cdot C \cdot D$ $Slope = B*C*D$

$B = \frac{S l o p e}{C \cdot D}$ $B = (Slope)/(C*D)$

values of B

Lateral force - 1.3

Longitudinal force - 1.65

Aligning moment - 2.4

$y_{s} = D \cdot \sin (\frac{π \cdot C}{2})$ $y_s = D*sin((pi*C)/2)$

$E$ $E$ = $\frac{B X_{m} - \tan (\frac{π \cdot C}{2})}{B X_{m} - a \tan (B X_{m})}$ $(BX_m - tan((pi*C)/2))/(BX_m - atan(BX_m)$

Whereas for a half vehicle analysis tyre will be rigid tire normal parameters which are a user-defined like Tire unloaded radius and Tire stiffness is enough to study the behaviour of the suspension.

This graph explains the difference between a Rigid tyre and a realtime Pacjeka model tyre. It can be clearly observed that during the simulation the tyre loses contact with the testrig and it does not take any negative forces. whereas the Rigid tyre which has just normal stiffness can take negative forces.

After an increase in the braking ratio from 50 percentage to 85 percentage the change in anti-dive braking. Other vehicle parameters like wheelbase, Sprung mass, CG height can also be changed to determine the overall behaviour of the system

Roll bars: Roll bars are modelled as discrete beam elements in Msc Adams car. Bushing connects the rollbar to the body. The roll bar diameter can be increased to restrict the body roll. This study shows the behaviour of roll bar on body roll with increased (38) and reduced (28) roll bars. Total roll rate defines its effect on tyres, springs etc.

Steering :

Steering in Msc Adams provides both Power and Manual type. The property file provided in double-wishbone suspension property files gives the amount of force applied on the rack after applying a unit degree (angle) of the steer. Power steer has hydraulic pressure applied and this inturn converts into force.

Types of Analysis

Displacement Driven

$\to$ $rarr$ Parallel wheel travel

$\to$ $rarr$ Opposite wheel travel

$\to$ $rarr$ Roll and Vertical force

$\to$ $rarr$ Single wheel travel

$\to$ $rarr$ Steering

Force Driven

$\to$ $rarr$ Static loads

$\Rightarrow$ $rArr$ Longitudinal

$\Rightarrow$ $rArr$ Lateral

$\Rightarrow$ $rArr$ Aligning moment

$\Rightarrow$ $rArr$ Vertical

$\Rightarrow$ $rArr$ Overturning Moments

$\Rightarrow$ $rArr$ Rolling resistance Moments

$\Rightarrow$ $rArr$ Combined Loading

$\Rightarrow$ $rArr$ External loadcase files

$\to$ $rarr$ Dynamic loads

$\to$ $rarr$ Parallel wheel travel

In parallel wheel travel both the wheels travel in the same phase. The wheel travel could be either relative to the contact patch or the wheel centre. The bump travel (Jounce) and rebound travel distance need to be mentioned before running the analysis. In order to compare results, it is necessary to perform multiple tests with and without a change in hardpoints until the desired result is achieved.

*It is necessary to maintain toe out in jounce in the front suspension in order to maintain kinematic understeer.

$\to$ $rarr$ Opposite wheel travel:

Opposite wheel travel is quite contradictory compared to parallel wheel travel. Here the wheels move in opposite phase to one another wheel goes in rebound and the other wheel goes in Jounce. This analysis is important to study the behaviour of vehicle when it travels in potholes.

The above graphs show the change in camber angle with respect to change in roll angle to determine roll camber and toe angle VS roll angle to determine roll steer. The slope of the curve determines the roll steer also camber angle change per degree change in the toe angle. From the above graph, the total roll stiffness is about 2.9 $e^{- 05}$ $e^(-05)$ ( $\frac{N - m m}{d e g}$ $(N-mm)/(deg)$ ) The roll angle change from the wheel centres or from contact patch can be selected.

$\to$ $rarr$ Roll and Vertical force:

In roll and vertical force suspension analysis instead of using wheel travel in both jounce and rebound, roll angle and vertical force in compression are defined. Instead of force, distance can also be set. Due to this instead of zero, the wheel travel starts from a certain offset distance.

$\to$ $rarr$ Single wheel travel:

In single wheel travel, only one wheel is set to travel in bump and Rebound.

$\to$ $rarr$ Steering:

Steering analysis helps in studying various parameters like Kingpin, Ackermann steering etc.

Force Driven suspension analysis: There are two types of suspension analysis in which the force that is applied either static load or dynamic load. In static load, the following forces/moments can be applied either at the contact patch or at the wheel centre. The applied load does not wary with respect to time. These moments and forces can be applied at a time (combined loading) or individually. If the applied load is at the contact patch it creates moments. Cornering force, braking forces are applied at the contact patch. Traction force is applied at the wheel centre. If the lateral force is applied at the wheel centre then Damage force is used.

$\Rightarrow$ $rArr$ Longitudinal

$\Rightarrow$ $rArr$ Lateral

$\Rightarrow$ $rArr$ Aligning moment

$\Rightarrow$ $rArr$ Vertical

$\Rightarrow$ $rArr$ Overturning Moments

$\Rightarrow$ $rArr$ Rolling resistance Moments

$\Rightarrow$ $rArr$ Combined Loading

$\Rightarrow$ $rArr$ External loadcase files

Lateral force steer compliance is about 0.05 deg/KN. A static condition, the force is about 1600N but due to cornering, there is a shift in the wheel load from one side to another. The graph explains that one side loads and the other side unloads.

The longitudinal compliance is about 0.85 KN/mm

Wheel vertical force applies a total vertical force of 5000N whereas Wheel vertical force delta applies an additional force 5000N to the system. This was illustrated in the above graph.

rarr` Dynamic loads

In static loads the effect of the damper is zero whereas in Dynamic loads the velocity of the damper for the isolation of vibrations is very much important to study the behaviour of overall suspension system. Instead of force, time and Amplitude is specified in Dynamic analysis. the steering analysis, cornering analysis, Roll everything can be studied in the Dynamic analysis. Dynamic analysis is also used to simulate vehicle in road profiles in order to study the behaviour of the suspension system. whereas quasi-static simulations are purely kinematics and compliance-based study.