There is an obvious fetish in the motorcycle community. Although skulls are the most common expression of this fetish, it goes a little wider than just simply skulls.

Skulls are painted onto motorcycle parts, and they are incorporated into parts that are cast.

They can be rather straight forward depictions, or more stylized.

They are universal in the culture.

But, I believe the skull is just a convenient icon for a broader involvement.

The skull fetish is really an expression of a wider fetish with the morbid.