Method of detecting longitudinal force of tire and longitudinal force detecting apparatus used therein

An apparatus for detecting a longitudinal force of a tire which detects the longitudinal force in a circumferential direction applied to the tire, comprising:

• at least one mark provided in the tire on one circumferential line around a tire axis;
• a first sensor fixed to a vehicle body and capable of sequentially detecting a passage of said mark rotating together with the tire;
• a second sensor capable of detecting a rotation angle θ of an axle;
• and
• a computing means for computing a longitudinal force applied to the tire on the basis of an information from said first and second sensors,
• wherein said computing means comprises:
• a rotation angle information memory portion sequentially storing an information of a rotation angle θr of the axle detected by said second sensor when receiving the information of the passage signal from said first sensor according to the passage of said mark;
• a no-load rotation angle information memory portion previously storing an information of a no-load rotation angle θo of the axle detected by said second sensor when receiving the information of the passage signal from said first sensor according to the passage of said mark as the tire rotates in a no-load state;
• a first computing portion sequentially computing a tire warp angle αr corresponding to a difference (θo−θr) between the rotation angle θr of said rotation angle information memory portion and the no-load rotation angle θo of said no-load rotation angle information memory portion;
• and
• a second computing portion sequentially computing a longitudinal force Fr of a tire applied during the running from said warp angle αr computed by said first computing portion, using a previously determined relational equation between the warp angle α and the longitudinal force F applied to the tire.