For many a car rig would seem a simple task. Simply make a control with joint and bind it for each wheel and then the body of the car. Simple, right? Simply close Maya and enjoy the rest of your day? Nope! There's something very special that I added to this rig to ensure that it would be easy to animate and easy to control. I created a control called the "AutoDrive_CTRL". Just like names says, it automatically drives the car! Here is a video demonstration.
By simply dragging the control the dragging will automatically moves the wheel forward and back based on which direction it's facing. This allows the animator to easily animate the car without having to key the wheels. Without giving too much away, this is how I made it.
I connected the AutoDrive_CTRL to the bind joints driving the wheels. While it may seems simple I needed to ensure that that translation wouldn't spin the wheels like crazy due to the scale of the rig being enormous. Which is why I used plusMinusAverages to ensure that, regardless of scale, the values will spin at average rotation. You may have noticed that I used an expression. That allowed to make it easy for me to manage the values used in translation to the bind joints so I don't have to adjust the values on each PMA, I can simply adjust it on the expression which applies to all the connected nodes.