Chapter 14 | Staging Bicycle & Motorcycle Impacts
Introduction
Virtual CRASH provides the user a fast and innovative way to model motor vehicle accidents. It is particularly useful for studying pedestrian, motorcycle, and bicycle accidents due to the speed with which an accident scenario can be set up and simulated. In this chapter, we show how to stage a bicycle impact scenario in the Virtual CRASH software environment. These same lessons are applicable to setting up motorcycle collisions as well. You can find more information about modeling bicycle and motorcycle impacts at this link.
Place Bicycle into Scene
We’ll start by left-clicking “assets” to enter the assets browser (learn about assets >). Next, left-click on vehicles 3d > medium. Use the “keyword” dropdown to select “bicycle”.
Hold+left-click to drag and drop a bicycle into the scene from the assets browser.
In this example, we will choose the “bicycle kid” object. Using the left mouse button, hover the cursor over “bicycle kid” and press and hold down the left mouse button. Continue holding the left mouse button as you drag the cursor into the environment editor window. Once in the environment editor, release the left mouse button to “drop” your bicycle into the scene. Remember to set the keyword back to “.”; otherwise, it will continue filtering future asset searches.
You should now see your bicycle object in the scene. We’re going to keep the bicycle aligned with the global x-axis (yaw = 0 degrees) to take advantage of the Group tool.
Like any other vehicle object, you can easily modify the bicycle’s geometrical and inertial properties. Learn about more about modifying vehicle properties >
Insert Multibody Model
Following the steps from Chapter 13, place a multibody into the scene. After the bicycle and multibody are in the scene, left-click the large “edit” button to exit the assets browser. Following the steps shown at the bottom of Chapter 23, modify the clothing as needed. In the example below, the multibody’s height and mass were also modified.
Position Multibody on the Bicycle
To position the rider on the bicycle, first disable physics by pressing [P] on the keyboard. You should see the gears icon become highlighted to indicate that physics has been paused. This will prevent objects from moving while you are trying to place the rider on the bicycle.
Next, go to the multibody’s “misc” menu and disable “auto align to plane”. This will allow you to freely elevate the multibody above the terrain mesh.
Since our setup is on a flat plane, with yaw, pitch, and roll set to 0 degrees, we can easily align our rider to the bicycle along the y-axis direction by simply ensuring that the position-local y value is the same for both the bicycle and the rider.
Select the multibody and open the “poses” menu. Choose the pose that most closely matches the multibody configuration needed for your case. In this example, we select “cyclist5right”.
Next, press [F2] to change mouse cursor control to “Select And Move” and press [1] to switch to “Restrict To X” mode. Then, select the multibody. Using the translation control grips or position-local (x, y, z), reposition the multibody relative to the bicycle as needed. Switching your camera view to top-down [NumPad5] may help with this process.
With the multibody selected, go to the object selection type dropdown menu and select “Joints”. You will then see joint selectors appear on the multibody. Ensure you are in one of the “Restrict To” modes (press [1], for example). Select “Axis Local” so that the rotation grips are defined relative to the local joint frame rather than the global frame. Left-click on each joint and adjust its (yaw, pitch, roll) interactively using the rotation control grip. You may find it necessary to adjust the multibody rotation-local (yaw, pitch, roll) as well.
In the example shown below, the pitch angle was adjusted by 16 degrees so that the multibody could reach the handlebars while standing on the pedals. Note that rotating the whole multibody will realign the multibody’s local x-axis such that it is not parallel with the global x-y plane. The initial velocity vectors of all objects point along their local x-axis direction, so you must correct the body’s initial vnz value to account for any initial pitch rotation (see http://www.vcrashusa.com/kb-vc3-article24). If the correction is not made, the body’s initial velocity vector will not be parallel with the global x-y plane; however, working with group objects helps solve this issue automatically (see below).
Note that there is also an option to open the multibody’s “joint” menu and set the selected joints' (yaw, pitch, roll) values through keyboard entry.
Once you are satisfied with the joint angles, switch back to “Object” selection type [Shift+O] and “Move And Rotate” cursor control [0].
Group Multibody and Bicycle
At this stage, you may find it beneficial to place the multibody and bicycle into a group object. Working with a single group object allows you to set the position, orientation, and initial velocity vector of the group object itself, without needing to do so for each individual group member.
To group the multibody and bicycle, select each, then go to Group > Group.
Note that a group has its own local x-axis; however, unlike other objects in Virtual CRASH, a group object always has its initial velocity vector aligned with the global x-axis. As the group’s orientation (yaw, pitch, and roll) is changed from (yaw=0, pitch=0, roll=0), its vni and vnz values are automatically adjusted to give the desired initial velocity vector direction, but only if the initial speed is first set to a value greater than 0.
Since both the bicycle and multibody were facing along the global x-axis direction when the group object was formed, they will also face along the group’s local x-axis direction, which starts at yaw = 0. Once the group object is created, the group’s velocity and orientation can then be modified like any other object. The bicycle’s initial orientation and velocity vector, as well as the rider’s, will properly reorient accordingly as the group is modified, but only if the group’s initial speed is first increased from 0. After the initial speed is increased from 0, the group’s orientation can be modified. If the group’s initial speed is 0 and the orientation is first modified, you may see unintended behavior. You can, of course, expand the group’s folder and set the initial velocity and orientation of the rider and bicycle separately.
Once the bicycle and rider are contained in a group object, set the initial speed to any non-zero value (0.001 mph is used below), then reposition and reorient the group object as needed. In this case, we position the bicycle+rider group in front of the bullet vehicle SUV just prior to the moment of impact. This is often the fastest way to optimize simulations rather than simulating long pre-impact trajectories.
Un-pause physics by pressing [P] on your keyboard. Adjust the simulation input parameters as needed for your study until the simulation converges to an output condition that is acceptable within your analysis tolerance.
Add Kinematics
When starting a simulation near the moment of impact, you can use the Kinematics Tool to model pre-impact vehicle behavior. The Kinematics Tool links a simulated object to a trajectory, where time-distance behavior is determined by the tool’s sequence menu settings. At time = 0 s, the object will take the exact speed, vni, and vnz used for the simulated object's initial settings in the dynamics menu.
Use path follow on 3D terrains
In our simulation, which occurs on a 3D terrain mesh, we included pre-impact trajectories drawn with smooth polylines, starting from each object’s CG and working backward. We’ll use the Kinematics Tool’s path follow feature to ensure the vehicles follow the terrain mesh.
Add Kinematics To Selection
Open the bicycle+rider group object. Select the bicycle and the rider using Ctrl+left-click (do not select the group object itself). Next, left-click on Create > Physics > Add Kinematics To Selection. Because both objects were individually selected, a single kinematics trajectory will be created to handle the pre-impact trajectory for both.
Next, select the SUV and left-click Create > Physics > Add Kinematics To Selection.
To ensure the kinematics trajectory segments follow the terrain contours, select the kinematics object. Then, in the sequences menu, left-click “pick path” and left-click on the object’s polyline. We perform this process once for the bicycle+rider and again for the SUV. Note that to do this, you must either be in “Select And Move” or “Select, Move And Manipulate” mode, and your polyline cannot be frozen.
Next, set the kinematics sequence list entries as needed to model the pre-impact vehicle behavior for your case.
Below we show a video of the final simulation.
Alternative Workflows in Forward-time Evaluation
Instead of initiating your simulation at or near the point of impact, Virtual CRASH provides enhanced workflows that enable the rider to stay on or inside the vehicle for a significant period prior to the collision. The details of these workflows are outlined below.
Attach RIGID mesh rider to vehicle
With a multibody selected, go to tools > convert.
The option “to mesh” freezes the multibody into a solid mesh object. All joints are removed as well as rigid body properties. One use of this feature is to merge a multibody shape with a vehicle. For example, below, we converted the multibody to a mesh. Then, we removed physics from the scooter and grouped together the multibody and scooter meshes. We then made the group into a rigid body object, added front and rear axles, and modified the wheel data to match the original scooter’s data. We then increase the roll and pitch moments of inertia to 1e6 to ensure the scooter+rider system remain upright as it moves forward (see below). Using this same method, a stationary occupant can be placed in an occupant cabin of a vehicle model for a more realistic looking visual aid.
Alternatively, the “Link” feature can be used to attach a converted mesh multibody to a simulated vehicle object. Just select the mesh, go to the “misc” menu, left-click on “link” then left-click on the vehicle.
CONVERT MULTIBODY TO RIGID BODY OBJECTS
Multibodies can also be converted to “rigid bodies.” When this option is selected, Virtual CRASH will build a multibody-like object from hyperellipsoids and ragdoll joints which are placed in an identical pose as the original multibody object. A new group object will be placed in the project menu consisting of the hyperellipsoid objects, joint objects, and the multibody skin “envelope”. In the figure below, the envelope is hidden and joints revealed.
We can now connect the rigid body objects to the scooter or other rigid body objects using the joint tools. In the case below, we attach the hands, feet, and pelvis to the scooter. Here we give the scooter a third axle with small wheels in order to move forward and remain upright.
The joints can then broken at the moment of impact so the rider can move independent of the scooter.
Note, the rigid body object created with the “to rigid body” feature cannot be posed in the same manner as the parent multibody object after creation. The multibody global weight and size input functions are also lost, though individual hyperellipsoid and joint properties can be configured. The rigid body object is also not optimized to deliver fast simulation results as the parent multibody object is, and therefore you may experience increases in CPU time needed to complete simulations using these objects. You may also see differences in simulation scenario outcomes where the rigid body object is used rather than the parent multibody. Note, “auto align to plane” and “optimize” are also unavailable for the rigid body object. Learn more about this workflow in this Blog Post.
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