A method and apparatus derives a change amount in the cumulative value of
displacement for the unit of time as a change ratio in tire deformation
by using the cumulative value of displacement derived present time and
the cumulative value of displacement derived last time, and calculate a
rate of change ratio using the change ratio in tire deformation
previously calculated as a reference value and the change ratio
calculated present time, and determine a cornering stability of the wheel
that represents a degree of a margin in which the wheel keeps no sliding
and beyond which the wheel start sliding based on the rate of change
ratio.