Receipe: Preparing a street vehicle

  • After the Blender model has reached a good state, you want to import the road vehicle into LOTUS for the first time. You can find out how to do that here.

    First of all, it should be emphasized again that the height of the body and axles must be selected correctly before importing! To do this, follow this thought experiment: Lift the bus (not by the wheels but e.g. on a lifting platform) until the wheels are just "sprung out" but still just touching the ground. In this position, both the body and the wheels must be in front of the export in Blender, where Z = 0 corresponds to the height of the floor.

    Now start the ContentTool. If you haven't installed it yet, you can learn how to do so here.

    In the main menu of ContentTool, select "Objects and vehicles", then "Create/import new object":

    Then road vehicle:

    In the now empty ContentTool, the X3D file created in Blender is imported by pressing the Mesh Import button:

    Two questions are then asked:

    The answer to the first question depends on whether you have already taken care of the corners and curves in Blender. You can find more information here. For tests, the question is usually answered with "Yes".

    The second question is usually answered with "No" for vehicles, since the vehicle should be imported exactly as it is in Blender.

    Now your model is visible in the ContentTool. First of all, the vehicle should be able to drive and for that LOTUS needs to know where the axles are located and how they behave. On the left side, under the tab "Road Vehicle", two axles are added for a standard test vehicle:

    Double-click to open the associated settings:

    The Y-position can be read out directly in Blender if the wheels have been built accordingly (object center on the axle). The diameter means the diameter of the wheel including the tire. The width means the width of the entire axle, i.e. measured from the left outer edge of the tire to the right outer edge of the tire. The specifications for spring stiffness and damping can initially be adopted, as can the value for rotational inertia. The index of the trailing arm indicates to which part of the bus (if it is an articulated bus) the axle is attached. For a solo trolley, a 0 is always entered, for the trailer a 1, and for a double-articulated bus then a 2 for the last part.

    Now the animations have to be created. The hierarchy should look like this in the end (for a two-axle solo car); the names are of course only recommendations:

    The animations are created by selecting the first higher hierarchy level and then clicking on "Add" under the hierarchy display. Depending on the hierarchy animation, the following type is selected:

    • HR_spring, HL_spring, VR_spring and VL_spring, the respective animations of the suspension, are of the type "Displacement along an axis".
    • VL_steering and VR_steering, the respective animations of the steering, are of the type "rotation around an axis
    • HR red, HL red, VR red and VL red, the respective animations of the wheel rotation, are also of the "rotation around an axis" type

    Next, the meshes should first be assigned to the animations. This has the advantage that the ContentTool then already offers to take over the Blender object center point, which saves a little work in the following animation configuration. However, only the wheels must be assigned to the respective hierarchically lowest level, i.e. to the respective wheel rotation animations.

    To do this, click on the respective wheel rotation animation and activate the "Add/Remove Meshes" button. The mesh associated with this wheel is then marked (clicked) in the ContentTool and then the action is ended by clicking the "Add/Remove Meshes" button again. This step is then repeated for the remaining three wheels.

    The properties of all animations can then be edited by double-clicking on them. The properties of the suspensions can be selected as follows:

    The properties of the steering animation look as follows - here, however, the pivot point must be adjusted to that of the imported bus:

    And this is what the wheel rotation animations look like - here, too, the pivot point must be adjusted:

    In the properties of the "Main" animation there are coordinates of the center of gravity and the empty mass of the bus in kg.

    In the end, it can't hurt to check the tree structure again!

    Before proceeding with the import and configuration, it is important to test in the simulator whether the vehicle behaves as desired, especially with regard to the suspension. This is influenced both by how heavy the vehicle is and where the center of gravity lies, but also by the spring properties of the axles, whereby hard springs ensure that the bus sinks less into the springs, but the frequency increases. Particularly if it then comes to bouncing in poor performance, a lower value should be selected here. Damping, on the other hand, ensures that the bus does not "bounce" for as long.

    Last but not least, we should mention the object settings, at the bottom of which the following parameters appear:

    • y-coordinate of the non-steered axle: This decides the point around which the vehicle turns when it goes around the curve. In fact, it is simply the coordinate of the rear axle on a solo two-axle. On three-axle vehicles, the third axle usually steers passively, so the y-coordinate is at the second axle. In articulated buses, too, it is the second axle, i.e. the rear one of the front carriage, that must be measured here.
    • Reciprocal of the tightest curve radius: Simply take the tightest curve radius (measured from the center of the unguided axis) in the reciprocal (i.e. 1 divided by R).


    Everything that has been configured here should now be checked again before completion, because misconfigurations have a decisive effect on the first test attempts.