Knowledge Base

Article Number: 64 | VC4 | VC3 | Post Date: May 30, 2018 | Last Updated: May 30, 2018

I’m trying to simulate a sedan effectively engaging in an offset frontal collision with trailer tandems, but Virtual CRASH seems to simulate it as a sideswipe. What can I do to model this type of impact?

Whether it's an impact with a trailer's tandems or any other structure on the underside of another object, one may see similar results with the default settings of the Kudlich-Slibar collision model. In the exercise below, we’ll develop a similar collision sequence, diagnose the issue, and find a solution. It is highly recommended to review the various aspects of collision modeling with the Kudlich-Slibar impulse-momentum model, which is explored in great lengths in the following places:

First, we’ll download the 53’ trailer model given at the bottom of this Knowledge Base post:

To ensure the Kudlich-Slibar collision model is triggered (and the depth of penetration clock begins) when the polygons of the sedan first penetrate into the volume of the tandems, we’ll follow the steps shown in this post: This is shown in the video below:

Note, in the video, the lower half of the cylinders were removed to insure that the polygon mesh of the trailer does not accidentally interact with the ground terrain. 

Next, we remove physics from the trailer. Then simultaneously select the cylinders and trailer object, and use Project > Export Selected to save our mesh as a .vcm file. 

Next, we drag and drop the .vcm file onto our original trailer object to replace the geometry with the geometry including our cylinder polygons. As a final step, we fine-tune the ground clearance, since that must reflect the new cylinder polygons. 

Now that our trailer is finished, we can attach it to our tractor using the “pick leader” option. Once attached, we can begin working on our simulated collision. 

The sideswipe motion is occurring because of the way the vehicles are overlapping at the moment when impulses are exchanged. By default, the overlap geometry (overlap of the vehicle bounding boxes) looks like a typical sideswipe configuration, thus the normal axes are automatically aligned tangent to the trailer’s heading, and the friction plane is aligned parallel with the trailer’s heading. 

We see the same behavior in the sideswipe collision between two sedans shown below.

Since the normal component of the impulses delivered to each vehicle is directly proportional to the normal component of the closing-velocity vector at impact, the smaller the normal component of the closing-velocity vector is, the smaller the Delta-V components will be in the normal direction for each vehicle. Additionally, because the tangent components of the Delta-V vectors (associated with frictional effects) are directly proportional to the normal Delta-V components, the Delta-V components along the tangent plane direction will also be small if the normal components are small. Consequently, as in the case of impacts where the vehicle headings are nearly anti-parallel, it is unlikely for the relative tangential motion sliding contact to be completed arrested from the tangent Delta-V components with the normal axes being directed nearly orthogonally to the closing-velocity vector. 

The solution is therefore, to rotate the normal axes to achieve the desired result. If one wants the vehicles to come to a common velocity (in both the normal and tangent directions) at the point of contact, rotating the normal axes such that the impulse vector is inside the blue friction cone will ensure this condition is satisfied. Since we want to treat the tandem impact as essentially an offset frontal impact, in the video below, we include a small study of a simple offset frontal impact with a barrier object in order to better understand how we might want to orient the friction cone in the subject collision. Remember, if the displayed impulse vector is within the conical volume of the blue friction cone, then the vehicles reach a common velocity at the point of contact in both the normal and tangent directions. If however, the impulse vectors are on the conical boundaries of the friction cones, then the coefficient of friction value is too small the ensure the common velocity condition is satisfied along the tangent direction. Thus, using a combination of modifying the normal axis orientation and/or modifying the friction value, we can ensure the vehicles reach common velocity. In the video below, we experiment with both options. 

Finally, suppose we wish to have sliding contact as the passenger side A pillar makes contact with the lower edge of the trailer, leading to the offset frontal impact with the tandems. In this case, we’ll create a user contact near the tandem collision and again rotate the normal axis. Note, we also move the impulse centroid to ensure it’s placed within the overlapping volume of the sedan’s front-end and the wheel. We also enabled deformation. 

The final animation is shown below.

Tags: Tandems impact, tractor trailer, sideswipe, Kudlich-Slibar. 

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