Chapter 6 | Modifying Vehicle Properties
Introduction
In Virtual CRASH, all vehicle models are fully customizable. Users can modify vehicle dimensions, inertial properties, axle locations, suspension parameters, and tire model characteristics. This guide demonstrates how to adjust key vehicle properties commonly modified during accident reconstruction analysis. As you update these properties, Virtual CRASH dynamically recalculates the simulation in real time, providing immediate feedback on the effects of your adjustments.
The Properties Menu
To begin, left-click “assets” to open the assets browser (learn about assets >) and add a vehicle to the scene. Once the vehicle is placed, left-click the large “edit” button in the bottom-left corner to exit the assets browser.
You will now see the properties menu in the left-side control panel (see below), which allows you to adjust various vehicle properties.
Axles
Left-click “axles” to display the axle and tire properties. Then, left-click to select the axle you want to modify (see below). To adjust properties for multiple axles simultaneously, hold ctrl and left-click to select them.
Note: When selecting two or more axles at the same time, the wheelbase will not be displayed, as it is not an attribute of axle 1 (see the wheelbase section below).
Overhang
The “overhang” parameter represents the distance from the front axle to the front-most vertex of the vehicle's mesh, measured along the vehicle’s local x-axis.
COG to Front Axle
The “cog to front axle” parameter adjusts the distance between the front axle and the local x-coordinate of the center of gravity.
Track Width
The “track width” parameter adjusts the lateral distance between the contact patch centers of each tire, measured along the local y-axis. Note that each axle can have its own unique track width.
Wheelbase
The “wheelbase” parameter controls the distance between the selected axle and the axle directly in front of it. As a result, axle 1 does not have a wheelbase input, and the wheelbase parameter will be hidden if axle 1 is selected. Note that adjusting this parameter moves the rear axle while keeping the front axle fixed in its current position.
Tire
The “tire” parameter allows you to modify tire sizes. Select the axle or axles you want to adjust. Tire sizes can be chosen from the available options in the dropdown menu, or users can manually enter a custom tire size.
To learn more about entering custom tire sizes please see Knowledge Base Article 77, found here: https://www.vcrashusa.com/kb-vc-article77.
Texture
Users can apply custom wheel covers by left-clicking “texture” and loading an image file through the Windows file explorer or by dragging and dropping the image directly onto the wheel object. Example wheel covers can be found in assets > textures > wheels. To revert to the default wheel geometry, left-click the “x” to the right of the “texture” button.
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 (see Appendix 4).
Note that “constant” is the default tire model used in Virtual CRASH. Users can apply different tire force models to the left and right tires by deselecting “symmetry left-right” by left-clicking the “x” (see below). When this option is deselected, separate selection boxes for the left and right tires will appear.
Suspension
In Virtual CRASH, users 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 from each wheel’s equilibrium position and is directly proportional to the spring constant (see this KB post for a detailed discussion on suspension modelling).
By default, Virtual CRASH uses the “normal” suspension type, which defines the suspension spring constant values. To explore other options, left-click “suspension: normal” to reveal a dropdown menu with additional suspension types.
By left-clicking on “user,” a new set of input boxes is revealed, allowing users to customize the spring stiffness coefficients and damping factors. Note that the right and left side suspensions can be adjusted independently.
Each wheel can move independently along the local z-axis; however, suspension stop limits are enforced for both jounce and rebound. The “limits-upper” parameter controls the stop distance from equilibrium for jounce, while the “limits-lower” parameter controls the stop distance from equilibrium for rebound.
Note that when the wheel displacement reaches the stop distance in either jounce or rebound, an effectively infinite force deflection model is assumed. In this case, the wheel instantly matches the vehicle's velocity. Afterward, the suspension returns to equilibrium due to spring loading, up to the stop value.
Steering type
Use the “steering type” dropdown menu to select and change the steering axle.
Drive wheel
Use the “drive wheel” dropdown menu to adjust the drive axle. Note that all vehicles in Virtual CRASH default to front-wheel drive. If necessary, you must manually change the drive axle to match your vehicle's configuration. This may be important when applying forward-directed tire forces through simulation sequences.
LIMITS
MAX STEERING ANGLE
The steering angle is defined at the axle and represents the angle of the wheel heading relative to the local x-axis for an axle with zero track width. This angle determines a specific turning radius, which depends on both the steering angle and the vehicle's wheelbase.
For any given track width, the actual wheel angles are automatically adjusted to maintain the equivalent turning radius (assuming no sideslip). In most cases, the difference between the angles at each wheel (turning angle) is negligible. The exact wheel angles are calculated based on Ackermann geometry (see this KB post for more detail).
The “max steering angle” parameter controls the maximum allowable 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 adjust the vehicle's curb weight using the “mass” entry field (see figure below). The displayed units will depend on your unit settings, showing either mass units or force units accordingly.
Moments-of-Inertia
Initially, the moments of inertia are fixed and unchangeable. These values are determined based on the vehicle's weight and size using a functional relationship appropriate for the vehicle type. Users typically refer to third-party sources, such as Expert Autostats, for more precise values.
To enter custom values, left-click on “type: car” to open the object type dropdown menu. Then, select “user” to unlock the entry fields, allowing you to input 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.”
To adjust cargo placement interactively, select the “cargo” option from the object selection dropdown menu, use the select and move tool ([F2]), and manipulate the gizmo (press [1] at the top of the keyboard). By default, the cargo object is positioned along the vehicle’s x-axis at the forward-most point of the vehicle, with a height equal to the ground clearance.
After positioning your additional cargo, set the selection type back to “Object” ([Shift+O]) and turn off the gizmo by pressing [0] at the top of the keyboard.
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 menu
Vehicle Geometry
One of the standout features of Virtual CRASH is its ability to easily modify the polygon meshes of 3D objects within its library. Often, users need a specific vehicle for a case but may not find it in a commercial accident reconstruction simulator’s model library. Virtual CRASH offers a wide selection of vehicle types that will likely suit most use cases, and its meshes can be resized to match any vehicle’s dimensions.
To adjust the dimensions of a mesh, left-click the “size” button in the left control panel to reveal the dimension properties. Here, you will find fields for length, width, height, and ground clearance. The length input specifies the physical distance from the rear-most vertex to the front-most vertex of the polygon mesh, measured along the vehicle’s x-axis. This measurement does not include the simulated wheels, as they are not part of the vehicle’s polygon mesh.
For certain vehicles, such as motorcycles—where the simulated front and rear wheels may extend beyond the polygon mesh fenders (see image below)—it is important to ensure that the subject vehicle’s length value excludes protruding wheels.
height
The height input specifies the position of the highest vertex above the x-y plane, while the ground clearance input specifies the position of the lowest vertex above the x-y plane.
The width input specifies the physical lateral distance from the left-most to right-most vertex along the vehicle’s y-axis (see below).
width
Note that the exclude overlapping input can be used to remove vertices from the width calculation that are associated with side mirrors or other objects that should not be included in the vehicle’s overall width. See (this KB post for more information on the exclude overlapping).
CG Height and Lateral Offset
cg height specifies the center-of-gravity's local z-position, assuming the vehicle is initially at rest on a flat plane.
cg offset y specifies the center-of-gravity's local y-position.
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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