The plastics used on that kinda steering wheel were slightly plasticized to
help them deal with the differential expansion between the metal (expands
slightly) and the plastic (expands a LOT!). The plastic gets compressed when
it gets really hot and then when it cools down it is not stretchy enough to
stay together when it shrinks, so you get cracks and (maybe)even fairly large
openings. I've seen them separate ¼". Also as the plastic gets older the
plasticizer evaporates out and the plastic also crosslinks, both of which make
it more brittle. I suspect that keeping it together without cracking would
probably require near-constant temperature storage.
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George Downs Bartlesville, Heart of the USA!