Virtual CRASH 3.0 Staging a Car Crash

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Virtual CRASH 3.0 | Staging a Car Crash

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Staging a Car Crash

Virtual CRASH

Virtual CRASH 3.0 | Staging a Car Crash

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Staging a Car Crash

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Staging a Car Crash

Virtual CRASH 3.0 | Staging a Car Crash

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Staging a Car Crash

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Staging a Car Crash

Introduction

In Virtual CRASH 3.0 one can setup and refine a collision sequence with incredible speed. Virtual CRASH automatically updates simulations in real time as input parameters are tuned and adjusted.

In this write-up we will explore how to simulate a collision between two automobiles. The exact same techniques will be applicable to collisions involving pickup trucks, commercial vehicles, or any other vehicle type. We will attempt to reproduce results from a staged collision test from the Research Input for Computer Simulation of Automobile Collisions (RICSAC) series. In this reconstruction, we will attempt to solve for the pre-impact speeds given knowledge of the pre- impact orientations and post-impact rest positions and orientations. The purpose of this write-up is to show the Virtual CRASH user the typical workflow used during an accident reconstruction analysis; it is not intended to be a refined study of RICSAC collisions, as such studies have been performed by other researchers. We recommend reading references (1), (2), and (3) for more information on the RICSAC tests and test conditions.

A video of the resulting simulation can be found online at: https://youtu.be/l3FagVLz96M

RICSAC 1

In the RICSAC 1 collision, a 1974 Chevrolet Chevelle Malibu impacted a 1974 Ford Pinto. Figure 1 depicts the impact configuration, as well as pre and post-impact trajectories.

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Staging a Car Crash

Figure 1

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Staging a Car Crash

Import Scale Diagram

Using the procedure outlined in reference (4), import the diagram shown in Figure 1 into the Virtual CRASH environment (Figure 2). You can copy and paste the diagram directly from this pdf file by simply copying a screen capture, pasting into Window’s Paint application, and using the crop tool. After you have imported, scaled, and oriented your diagram, it is recommended you freeze the image object to prevent accidental modification as you build your simulation.

Figure 2

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Staging a Car Crash

With the scale diagram as your environment’s backdrop, you can easily place additional graphical elements into your Virtual CRASH scene to suit your needs. In this example, the scale diagram was rotated and positioned so that it properly aligned with the impact and rest positions described in the “Summary Form” of page 2-8 of reference (2); this form is shown in Table 1 below. Note the data shown in this table is assuming SAE conventions, with the z-axis running parallel with gravitational acceleration, whereas in Virtual CRASH, the z-axis is aligned anti-parallel with gravity. This implies one must take y  -y and yaw  -yaw when reading data from this table for use in Virtual CRASH. The diagram was further refined using the “Axes” tool, which allows one to place a coordinate axis system within the environment. This tool also allows the user to place points within the x-y plane. This feature was used in order to place crosshairs at the impact and rest positions of each vehicle (Figure 3). The “Text” tool was used to annotate the diagram.

Figure 3

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Staging a Car Crash

Table 1

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Staging a Car Crash

Place Vehicles into Scene

Now that we’ve finished building our simulation environment, we are ready to place our vehicle models into the scene. Looking through reference (5), one can look for reasonable exemplar vehicle shells for the subject vehicles. In this case, a Skoda Felicia seems to reasonably match the body style of the Pinto. To place a vehicle into the scene, left-click on the “+” symbol next to

“cars” in the left side control panel to reveal the car library. Next left-click on the “+” symbol next to “Skoda,” and then left-click and hold “Felicia.” Drag the mouse into the scene and release the left mouse button. You should see the vehicle model in the scene (Figure 4).

Figure 4

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Staging a Car Crash

Using this same procedure, we will next place an AMC Matador into the scene as our exemplar for the Malibu (Figure 5).

Figure 5

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Staging a Car Crash

Modify Vehicle Data

Using the methods described in reference (6), modify your vehicle properties to match the geometric size of each vehicle, center-of-gravity positions, and wheel placement (Figure 6). In this case, the weights were taken from the “Crash Test Summary” on page 7-4 of reference (3) (shown as Table 2 below), whereas the wheel position (Table 3 and Table 4) and size data (Table 5 and Table 6) were taken from Expert Autostats. On page 7-20 of reference (3), the front/rear weight distributions are given for both vehicles. Using this distribution, we can solve for a more precision value for the distances between the centers-of-gravity and the front-axles. Doing this, one obtains 3.81 ft between the cg and front axle for the Pinto, and 4.24 ft for the Malibu. Note, when the cg location is adjusted after a vehicle has already been placed in the scene, take care to reposition the initial start positioning to the desired location as it will shift when the cg location is modified.

Figure 6

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Staging a Car Crash

Table 2

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Staging a Car Crash

Table 3

Table 4

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Staging a Car Crash

Table 5

Table 6

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Staging a Car Crash

Touch-up the Vehicle Mesh

Now that the vehicle wheels have been repositioned to match the Autostats data, the Pinto’s vehicle mesh needs to be touched up to correct the awkward relative position of the rear wheels and the rear wheel wells (Figure 7). Using the method described in reference (7), the vertices can be easily adjusted to correct this problem (Figure 8).

Figure 7

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Staging a Car Crash

Figure 8

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Staging a Car Crash

Place Vehicles at Pre-Impact Positions

Using the data in Table 1, we can place the vehicles at their pre-impact positions. The Malibu should be given coordinates (x,y,yaw) = (-10.8 ft, y=-1.0 ft, -30 deg) and the Pinto should be placed at (0.0, 5.5, 90 deg). The vehicle positions can be precisely controlled using the “position-local”

menu in the left side control panel, and the orientations can be controlled using the “rotation-local”

menu (Figure 9).

Figure 9

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Staging a Car Crash

Modify the Tire Adhesion Values

Go to the left side control panel and click on the name of your Malibu vehicle. Hold the left mouse button and drag it down so that you also select the name of your Pinto vehicle; this is another way to select multiple objects at once. You can use the left control panel menus to modify values that are common to simultaneously selected objects. You should notice both vehicles have red boxes in the simulation environment indicating they are both currently active selections (Figure 10).

Next, go to the “contact” menu and change the locked-wheel drag factor “adhesion” value to

=0.87 (Figure 11) as indicated by the tire pavement drag factor in Table 1.

Figure 10

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Staging a Car Crash

Figure 11

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Staging a Car Crash

Enter Rolling Resistance Values

With both vehicles still selected, go to the left side control panel and left-click on “sequences” to access the braking data. Next, left-click on the empty box to the left of “wheels separately” to adjust the braking at each wheel as a percentage of the adhesion value. Table 1 indicates that the rear wheels of both vehicles had a drivetrain resistance of 20%  whereas the front wheels had a rolling resistance of 1% . Set the “brake lag” value to 0 seconds so that the braking is enabled at time = 0 seconds (Figure 12).

Figure 12

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Staging a Car Crash

Begin Iterating the Pre-Impact Speeds

With the pre-impact configuration set, we are now ready to optimize our simulation. Here we assume no initial knowledge of the pre-impact speeds. In order to converge upon a final solution for our reconstruction, we must iterate across our physics model’s input parameters, which must be kept within a physically meaningful range. It is possible that given a final post-impact state which we are trying to match, there can be multiple solutions which do equally as well to match our intended goal; in this case our goal is have the vehicles come to rest as close as they can to the measured rest positions.

Let’s start by increasing the pre-impact speed of the Malibu. Go to the “dynamics” menu and slide the speed up until you see the Malibu stop near the final position marked on the diagram (Figure 13). You can use your mouse’s scroll wheel to move the “v” slider. Note, at about 15 mph, the Malibu comes close to the final position.

Figure 13

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Staging a Car Crash

Now adjust the Pinto’s initial speed. You’ll notice that the Virtual CRASH system automatically updates and plays the simulation based on your current inputs (Figure 14). At 15 mph pre-impact speed for both vehicles, the Pinto is about a car-width short of the final target, implying there is not enough initial kinetic energy for the Malibu in the current simulation. Iteratively increase the speeds of each vehicle until a better match is found.

Figure 14

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Staging a Car Crash

We will stop this first round of iterating with v[Malibu] = 19.9 mph and v[Pinto] = 16.8 mph. In this case, both vehicles are much closer to their rest points (Figure 15).

Figure 15

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Staging a Car Crash

Open the Report

To see how well we’re doing so far with our simulation, go to the left side control panel and left click on “report dynamics” in the “tools” menu (Figure 16). We only need to look at the final positions of the vehicles, so set the “time” increment setting to 1 second. This will display the vehicle data every second in the report table. Now we can read off the final positions and orientations of the vehicles. The final (x,y,yaw) configuration for the Malibu is (-0.660 ft, 1.535 ft, -15.234 deg) and is (8.274 ft, -1.653 ft, 1.628 deg) for the Pinto.

Figure 16

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Staging a Car Crash

Collision Data

Using the methods shown in reference (8), access the first impact collision data by left-clicking on

“auto-ees” and then selecting “next contact” in the left control panel. In the “defaults” menu, we can see the collision model parameters used. Under “object 1” we see that the Pinto’s v = 16.470 mph and the Malibu’s v = 11.019 mph (Figure 17). These values could also have been obtained in the report, by setting the output time increment value to a much smaller value and reading off and analyzing the velocity vector data just before and just after impact. The results for the current state of our simulation are shown in Figure 16 below. The results are summarized in Table 7.

Thus far, the largest discrepancy appears to be the final position and orientation of the Malibu. Our initial velocity estimates are already to within 16% of the measured values, and we were able to determine this in just minutes. Note, we compared the Pinto’s simulated cg position to an estimated true final cg position, by assuming the cg’s relative position with respect to the Pinto’s wheels remained unchanged after impact – our estimated position is shown by a large blue circle (without crosshairs) in the diagram. You will notice the reported cg value is shifted about 9 inches closer to the passenger side in the Table 7.

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Staging a Car Crash

Table 7

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Create User Contact

Depending on the needs of your case, you may already wish to declare victory with this reconstruction project and stop; however, we will try to do some further optimization to better match the final positions and orientations of our vehicles. To do this, we need to adjust the parameters of the impulse vector for the first impact. With the initial contact already selected from the previous step, left-click on “create user contact” under the “selection” menu (Figure 18). This will create a new object in the left side control panel with its own collision properties which you can now modify (Figure 19).

Figure 18

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Figure 19

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Adjust the Impulse Centroid Position

Next, go to the objects window in the left side control panel, and hide the “auto-ees” objects to make things easier to see in the diagram. Freeze the Malibu and Pinto objects so that they are not accidentally selected in the next step (Figure 20). Deselect “auto-position” so that we may move the centroid of the impulse vectors, and finally enable crush damage by selecting “deform.” Make sure your mouse cursor is on “Select And Move” or “Select, Move And Manipulate.”

Figure 20

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Staging a Car Crash

Virtual CRASH will initially automatically determine the centroid position for the impulse vectors based on the intersection of the vehicle bounding boxes at the collision time, which controlled by the “depth of penetration” parameter. This centroid position can be adjusted based on your accident reconstruction analysis to yield more consistent results for your case. You can reposition the centroid either by using the “position-local” menu or by left-clicking, hold, and dragging your mouse (Figure 21). Though you can place the impulse centroid anywhere in the scene, obvious care must be taken to ensure it is being correctly positioned with respect to both vehicles such that it is consistent with the physical evidence.

You’ll note in this case, dragging the centroid further up on the y-axis will tend to decrease the torque on the Malibu, thus causing less rotation in its final state.

Figure 21

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Adjust a Few Other Parameters

Now that you are comfortable controlling the impulse model parameters, with your first collision object selected, increase the coefficient of restitution in order to increase the separation velocity between the two vehicles after the first impact (Figure 22). You’ll notice this gives greater distance for the Pinto to decelerate before the secondary impacts.

Figure 22

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Staging a Car Crash

Continue fine-tuning the impulse centroid, pre-impact speeds, and restitution values until you are satisfied with the final position and orientations of the vehicles. In this case, we were able to converge on a reasonable solution after a few more minutes, where v[Pinto] = 18.4 mph, v[Malibu]

= 19.3 mph,  = 0.19, and the impulse was located at (x,y,z) = (-2.476 ft, 1.970 ft, 1.507 ft). Indeed, there are more parameters that could have been modified to further improve accuracy if needed.

The final results can be seen in Table 8. Our final rest positions are simulated to better than 0.6 ft for the Pinto and 2.5 ft for the Malibu. The final heading angles are simulated to better than 7 degrees. The pre-impact speeds estimates are accurate to better than 7.5% for both vehicles. Our largest inaccuracy is in the v value for the Pinto; however, we note that a similar value was found using an automatic optimizer tool from other simulation packages (see SAE 2001-01-0507). In our study, we were quickly and easily able to optimize our simulation by hand because of the Virtual CRASH system’s ability to visualize the simulation trajectories as inputs are updated in real time, and our optimization was based on matching the final rest positions and orientations.

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Table 8

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References

(1) “Examples of Staged Collisions in Accident Reconstruction,” R. Smith and J. Noga, NHTSA, US DOT.

(2) “Research Input for Computer Simulation of Automobile Collisions, Volume IV. Staged Collision Reconstructions,” NHTSA, US DOT, DOT HS 805 040.

(3) Research Input for Computer Simulation of Automobile Collisions, Volume II. Staged Collision Reconstructions,” NHTSA, US DOT, DOT HS 805 040.

(4) “Virtual CRASH 3.0 | Scaling Images,”

http://www.vcrashusa.com/s/VirtualCRASH_30_Scaling_Images.pdf (5) “Current Vehicle & Object Library,”

http://www.vcrashusa.com/s/VirtualCRASH_ObjectLibrary.pdf (6) “Virtual CRASH 3.0 | Modifying Vehicle Properties,”

http://www.vcrashusa.com/s/VirtualCRASH_30_Modifying_Vehicle_Properties.pdf (7) “Virtual CRASH 3.0 | Touching Up the Polygon Mesh,”

http://www.vcrashusa.com/s/VirtualCRASH_30_Touching_Up_the_Polygon_Mesh.pdf (8) “Virtual CRASH 3.0 | Reading Collision Data,”

http://www.vcrashusa.com/s/VirtualCRASH_30_Reading_Collision_Data.pdf

Virtual CRASH 3.0 Staging a Car Crash

Virtual CRASH 3.0 | Staging a Car Crash

Virtual CRASH

Virtual CRASH 3.0 | Staging a Car Crash

Virtual CRASH 3.0 | Staging a Car Crash

Contents

Introduction

RICSAC 1

Import Scale Diagram

Place Vehicles into Scene

Modify Vehicle Data

Touch-up the Vehicle Mesh

Place Vehicles at Pre-Impact Positions

Modify the Tire Adhesion Values

Enter Rolling Resistance Values

Begin Iterating the Pre-Impact Speeds

Open the Report

Collision Data

Create User Contact

Adjust the Impulse Centroid Position

Adjust a Few Other Parameters

References