Rubber tires
One of the gnarly features that sets the awesome original Transformers from the weak and lame modern Transformers is that a great many of them featured rubber tires.
Rubber tires on a real vehicle provide cushioning and friction to its wheels; on a Transformer, they are usually ornamental shells over the wheels, though they do roll nicely on the table, and on some toys (such as the Powerdashers with their pull-back motors) they serve mechanical purposes. In most cases, they are not actually rubber, but PVC plastic, which has been plasticized to improve its flexibility.
The plasticizers used can break down over time, causing the plastic to become harder. Furthermore, the plastic shrinks as this happens, causing the cracks and splits sometimes seen in those cool rubber-tired toys.
