See Also: Suspension Concepts
The Axle Rigidity keeps the wheel aligned with the axle of the vehicle. It is used to avoid having to make a complex structure in order to hold the wheel in place. In fact, it keeps the two wheel nodes and the defined node in a straight line. This is intended to use with solid non-steerable axles. However, you probably can devise a way to use this as independent suspension.
Normally a wheel needs some sort of pyramidal hub to support it, such as the image to the right. Axle rigidity allows you to avoid making this by doing some internal magic to keep the nodes supported. Wheels need at least 2 nodes to define their width, and the node used for axle rigidity is the innermost node of the wheel.
Here, the black beam are normal beams. Red beams are a crude suspension set up. Light-blue nodes are the outermost wheel nodes, while green nodes are axle rigidity nodes.
For the left wheel (defined by nodes 1 and 2), the special rigidity node will be 3, and for the right wheel (defined by nodes 3 and 4), the special rigidity node will be 2.