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 strictly ornamental shells over the wheels, though they do roll nicely on the table. 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.

From Teletraan I: The Transformers Wiki, a Wikia wiki.