Vehicular steering device

A vehicular steering device having steering means, a steering transmission system, a steering system, and a steered wheel, wherein the steering transmission system includes an electric motor and couples the steering means and a steered wheel, the steering system includes the steering means, a suspension, and components therebetween, and the device comprising:

• change means which is provided in the steering transmission system, and is used for changing a transfer ratio between a steering angle of the steering means and a steered angle of the steered wheel by the drive of the electric motor;
• state quantity detection means for detecting vehicle state quantity which represents a vehicle state;
• vehicle speed detection means for detecting vehicle speed;
• control means for controlling the electric motor and changing the transfer ratio according to the vehicle speed and the vehicle state quantity;
• steering index detection means for detecting at least one steering index among steering indices containing a steering torque and a steering force which are applied to the steering system;
• self-aligning torque estimation means for estimating a self-aligning torque generated in the steered wheel on the basis of the steering index;
• steered wheel index estimation means for estimating at least one steered wheel index among steered wheel indices including a side force and a steered wheel slip angle to the steered wheel on the basis of the vehicle state quantity;
• grip factor estimation means for estimating a grip factor at least to the steered wheel on the basis of a change of the self aligning torque to the steered wheel index;
• and
• judging means for judging whether the grip factor is less than a first predetermined threshold value, wherein the control means controls the transfer ratio according to the vehicle state when the grip factor is less than the first predetermined threshold value.