Beneath the technical wizardry, the way in which a car works is fundamentally very simple. Energy generated by the engine is transferred to the wheels, which in turn, forces them to rotate. Due to the friction between the tyres and the ground, this rotation then drives the entire car forwards.
However, when rigging a model of a car for animation, 3D artists traditionally approach the problem the other way around. It would be far too complicated to derive the motion of the entire car from the rotation of the wheels; instead, hierarchies and/or constraints are used for the overall motion, while expressions are used to make the wheels rotate accordingly. But while this approach gets the job done, it isn't particularly intuitive. If the road surface is anything but perfectly flat, the components of the car fail to react to this vertical motion, adding metaphorical as well as literal bumps to the workflow. But instead of going through the lengthy process of animating these different parts manually, we can make use of XSI's dynamics engine. By adding Rigid Body Dynamics (RBD) to the animation rig, we can recreate the same essential behaviour as a real car.
In this tutorial, we'll be illustrating this technique on the toy car above. Working with simplified geometry enables you to interact intuitively with the components of the scene without losing the accuracy of the simulation, so we'll be using an animation rig made up of simple primitives to simulate the workings of its suspension. Creating a separate primitive rig eliminates any uncalled-for calculation and enables you to adjust elements such as the body or wheels later in production, more or less on the fly. In this way, one underlying rig can be used for many different cars.