Tire slip state determination method

A tire slip state determination method for determining a slip state of a tire of a wheel relative to a road surface, wherein the wheel is connected to a drive source via a power transmission member, the method comprising:

• detecting a rotation fluctuation of the power transmission member and a rotation fluctuation of a wheel body of the wheel;
• determining, based on an amplitude ratio of a rotation fluctuation amplitude of the wheel body to a rotation fluctuation amplitude of the power transmission member and a phase delay of the rotation fluctuation of the wheel body relative to the rotation fluctuation of the power transmission member, whether a vibration mode of the wheel body and the tire is an elastic slip mode in which the vibration mode of the wheel body and the tire represents a frequency response in an elastic slip state or a sliding slip mode in which the vibration mode of the wheel body and the tire represents a frequency response in a sliding slip state;
• and
• determining that the tire is in the sliding slip state when the vibration mode of the wheel body and the tire is the sliding slip mode,
• wherein the amplitude ratio and the phase delay are calculated by using, as a tire driving radius, an effective rolling radius in a region in which a relationship between a dynamic load radius and the effective rolling radius is linear,
• the dynamic load radius is a distance between a central axis of the wheel and the road surface, and
• the effective rolling radius is a value obtained by dividing a distance traveled by the tire in one rotation by 2π.