Drivers
Because of the constraints we added to five of the six wheel objects, we need only concern ourselves with the remaining pair of wheels at the front of the train, Wheels A. However this wheel spins, the other wheels behind it will do so as well.
So how are we to make it spin? The X Euler Rotation value of Wheels A should be a simple multiplicative function of the Evaluation Time property of Track. This should remind you of our unicycle wheel in the previous chapter. This time, however, we don’t need to copy and paste any keyframes. Doing so only suggests a relation between two properties. By using a driver, we can encode this relation so that it works automatically.
A driver is less of an object relationship and more of relationship between a property and one or more other properties. We’ll use a driver on the X Euler Rotation property of Wheels A and use the Evaluation Time property of Track as the input variable that will drive it.
