Chapter 6 | Modifying Vehicle Properties
Introduction
In Virtual CRASH all vehicle models are customizable. The user has the option to change vehicle dimensions, inertial properties, axle locations, suspension model parameters, and tire model parameters. In this write-up we demonstrate how to modify vehicle properties that are commonly adjusted during an accident reconstruction analysis. Notice, as you change the various properties of your vehicles, the Virtual CRASH system updates the simulation results in real time, allowing you to instantly see the effects of your new inputs. You can watch a video about this topic online below:
Select a Vehicle from the Database
Let’s start by placing a vehicle into our simulation environment. Left-click on “vehicles” in the left side control panel. Next, left-click on the “+” symbol next to “car” to reveal the car selection library. Left-click on the desired make to reveal the various models. Finally, left-click and hold on the desired model label, and drag the mouse into the environment, and then release the mouse button. Your vehicle will appear in the scene:
The Properties Menu
Place your cursor in “Select And Move” or “Select, Move And Manipulate” mode, and then left-click on your vehicle model to select it. Next, left-click on “edit” in the left side control panel to reveal the properties menu options.
You should now see the properties menu in the left side control panel (see below). This displays all properties of your vehicle which can be modified.
Axles
Left-click on “axles” to reveal the axle and tire properties. Left-click to select the axle you wish to modify (see below). You can modify properties common to both axles simultaneously by using ctrl+left-click to select multiple axles at once.
Overhang
The “overhang” parameter will adjust the distance from the front axle to the front-most point of the vehicle front-end as measured along the local (vehicle frame) x-axis. Below, one sees the effect of increasing this value from the initial value of 3.543 feet to 6 feet. Note both the front and rear axles are shifted backward in order to maintain a constant wheelbase length.
COG to Front Axle
The “cog to front axle” parameter will adjust the distance between the front axle and the local x-coordinate of the center-of-gravity. The center-of-gravity position will be indicated by a small red dot. The figure below illustrates an example of the center-of-gravity being shifted backward along the local x-axis direction such that it is displaced 5 feet from the front axle, whereas its initial displacement was 3.77 feet from the front axle. Note in the previous figure, the “cog to front axle” value was automatically updated to reflect the changing distance between the center-of-gravity and front axle as the front axle was shifted backward.
Track Width
The “track width” parameter will adjust lateral distance between the contact patch centers of each tire as measured along the local y-axis. The figure below illustrates an example of increasing the track width of the front axle from the vehicle’s default value of 4.987 feet to 7 feet.
Wheelbase
The “wheelbase” parameter will adjust the vehicle’s wheelbase length. Note that adjusting this parameter moves the rear axle while keeping the front axle fixed at its current position.
Tire
The “tire” parameter can be used to modify tire sizes. The figure below illustrates the front tires being changed from the vehicle default values of 195/65R15 to 285/50R18 for the sake of demonstration. Note in this figure a green bounding box is included to show the size of the default tire.
To learn more about entering custom tire sizes please see Knowledge Base Article 77, found here: https://www.vcrashusa.com/kb-vc-article77.
Tire Model
Virtual CRASH comes with three tire force models. Left-clicking on the box to the right of“tire model” brings up the tire model selection box.
Note “constant” is the default model used in Virtual CRASH. The user can use different tire force models for the left and right tires by deselecting “symmetry left-right” by left-clicking on the “x” (see below). By deselecting this option, a separate selection box for the right and left tires is revealed.
Suspension
In Virtual CRASH the user can modify various suspension properties. Vehicle suspensions are modeled as independent damped harmonic oscillators for each wheel. The suspension spring force is linearly dependent on the displacement away from the each wheel’s equilibrium position, and is directly proportional to the spring constant. By default, Virtual CRASH uses the “normal” suspension type, which refers to the suspension spring constant values. By left-clicking on “suspension: normal,” a pull down menu is revealed with more options.
By left-clicking on “user,” a new set of input boxes is revealed, which allows the user to set the spring stiffness coefficients as well as the damping factors. Note the right and left side suspensions can be adjusted independently.
Each wheel is free to move independent of the others along the local z-axis; however, suspension stop limits are enforced for both jounce and rebound. “limits-upper” controls the stop distance from equilibrium for jounce, whereas “limits-lower” controls the stop distance from equilibrium for rebound.
Note, in cases where the wheel displacement reaches the stop distance either in jounce or rebound, an effectively infinite force deflection model is assumed, where the wheel instantly comes to common velocity with the vehicle, after which the wheel suspension will return to equilibrium due to spring loading up to the stop value.
LIMITS
MAX STEERING ANGLE
The steering angle is defined at the axle; the angle itself can be thought of as the angle of the wheel heading with respect to the local x-axis for an axle with zero-track-width. This will yield a specific turning radius which is directly dependent upon this angle as well as the vehicle wheelbase. The actual angles of the wheels are automatically adjusted such that the equivalent turning radius is maintained (assuming no sideslip) for any track width. Generally the difference between the angles at each wheel (turning angle) is negligible. The exact angle for each wheel is calculated assuming an Ackermann angle geometry. The “max steering angle” parameter controls the maximum allowed steering angle.
MASS PROPERTIES
WEIGHT
To change your vehicle’s weight or moment-of-inertia values, left click on “mass properties” in the left side control panel. You can quickly and easily change the vehicle weight by using the “mass” entry field (see figure below). Depending on your units settings, the units will either be displayed in mass units or force units.
Moment-of-Inertia
Initially, you’ll notice the moments-of-inertia are fixed and unchangeable. These values are set according to the vehicle weight and size, using a functional relation appropriate for your vehicle type. Users will generally want to use values from third-party sources such as Expert Autostats. To insert custom values, left-click on “type:car” to reveal the object type pulldown menu. Left-click on “user” which will unlock the entry fields and allow you to enter your own values.
Adding Passenger and Cargo Weight
Virtual CRASH allows you to increase vehicle weight to account for front or rear occupants as well as cargo.
The occupant and cargo options add additional mass to the vehicle system centered at the following locations depending on the option used:
front occupant: \( \left( 0.5 \cdot OAL + 0.15 \cdot WB , y_{cg}, z_{cg} \right) \)
rear occupant: \( \left( 0.5 \cdot OAL - 0.2 \cdot WB , y_{cg}, z_{cg} \right) \)
roof cargo: \( \left( x_{cg}, y_{cg}, OAH \right) \)
trunk cargo: \( \left( x_{RA} - 0.1\cdot WB , y_{cg}, z_{cg} \right) \)
where \(x_{cg}\), \(y_{cg}\), and \(z_{cg}\) are the vehicle’s center of gravity components before accounting for additional masses, \(OAL\) is the vehicle’s overall length obtained from “length” input field in the “size” menu, \(OAH\) is the overall height obtained from the “height” input field in the “size” menu, \(WB\) is the wheelbase derived from the “axles” menu, and \(x_{RA}\) is the rear axle location derived from the “axles” menu.
Adding Custom Weight
You can also add weight at arbitrary locations by first left-clicking on “add,” then left-click on the cargo label, which will reveal the mass and location options. The figure below illustrates a large amount of cargo being added to the rear of the passenger car causing obvious weight shifting effects.
You can modify the position of your extra weight within the environment by first going to the upper toolbar and changing your selection type to “Cargo.” This will cause weight icons to appear in the environment.
You can now left-click on the weight icon and drag it around in the scene. You’ll notice the (x,y,z) position data update in the left control panel in real time as you move the weight around.
After you finish positioning your additional cargo, place the selection type back to “Object” so you can resume editing your simulation scenario.
Note, for additional masses that are added using the front occupant, rear occupant, roof cargo, trunk cargo, and add cargo features, the system center of gravity is updated such that:
$$ \bar{R}^{\prime}_{cg} = {m_{i} \cdot \bar{R}_{cg,i} + \Sigma m_{k} \cdot \bar{R}_{cg,k} \over m_{i} + \Sigma m_{k}} $$
where \(\bar{R}_{cg,i} \) is the center of gravity position before additional mass is added, \(\bar{R}^{\prime}_{cg} \) is the center of gravity position after accounting for all additional mass, \(m_{i}\)is the vehicle curb weight derived from the “mass” input field in the “mass properties” menu, and \(k\) is the index over all additional masses.
The \(x\), \(y\), and \(z\) moments of inertia are each rescaled such that:
$$ I^{\prime} = I_{i} \cdot \left( 1 + { \Sigma m_{k} \over m_{i} } \right) $$
where \(I_{i}\) is the \(x\), \(y\), or \(z\) moment of inertia taken from “inertia-x”, “inertia-y”, or “inertia-z” field in the “mass properties” menu, and \(I^{\prime}\) is the \(x\), \(y\), or \(z\) moment of inertia after accounting for all additional masses.
The total system mass is increased to:
$$ m^{\prime} = m_{i} + \Sigma m_{k} $$
SIZE PROPERTIES
Geometrical Dimensions
One of the many features that makes Virtual CRASH remarkable is its ability to modify the polygon meshes of the 3D objects within its library. It is common to need a vehicle for a given subject case only to not find it in a commercial accident reconstruction simulator’s model library. Virtual CRASH has many vehicle types that will likely match most use cases. The meshes themselves can be resized to match any vehicle dimensions. To resize the dimensions of a mesh, left-click the “size” button in the left control panel to reveal the dimensions’ properties. You’ll find fields for length, width, height, and ground clearance. The length input specifies the physical distance from first to last vertex in the polygon mesh as measured along the vehicle’s x-axis, without regard to the simulated wheels (simulated wheels are not a part of a vehicle’s polygon mesh). For most vehicles, this will make no difference and values from sources such as Expert Autostats can be used without any changes; however for some vehicles, such as motorcycles, where the simulated front and rear wheels may extend beyond the polygon mesh fenders (see image below), care should be taken to ensure that the subject vehicle’s length value excluding protruding wheels is used. The height input value specifies the highest vertex’s position above the x-y plane, while the ground clearance input specifies the lowest vertex’s position above the x-y plane (see below).
The width input value specifies the physical lateral distance from first to last vertex along the vehicle’s y-axis (see below). Note, the exclude overlapping input can be used to exclude vertices from the width calculation that are associated with side mirrors or other objects that should not be included in a vehicle’s overall width. See the following Knowledge Base post for more information on the exclude overlapping feature: https://www.vcrashusa.com/kb-vc-article31.
The figure below illustrates an extreme case of resizing the vehicle mesh.
CG Height and Lateral Offset
The user can set the vehicle center-of-gravity height and center-of-gravity lateral offset (y-coordinate in the local frame) in the “size” menu. The figure below illustrates a case where the center-of-gravity position was laterally displaced such that the passenger vehicle became destabilized and rolled over.
Tags: Change cg height, change weight, change length, change width, change overhang, change trackwidth, cg to front axle, wheel location, tire location, change wheelbase, add passenger weight, change moment of inertia, change tire size, cg location, center of gravity location, cog location, suspension properties, spring constant, suspension constant, damping coefficient.
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