PURPOSE: To prevent production of discomfortable riding sensation through bolding of frequency at a specified value even if the load of a vehicle is fluctuated, by a method wherein, according to an instruction outputted from a controller, air from an air feed source is fed to a pressure varying bag to correct a spring constant.

CONSTITUTION: A wheel 22 is secured to a hub 23 through a rim 22 for holding a tyre 21 by means of a nut 24, and the hub 23 is brought into slide contact with an axle 25 to be held thereto. An air passage 26 is formed in the axle 25, air from an air feed source, flows through a passage 26, flows through a groove 25a in the axle 25, flows through an annular groove 23a in the hub 23 adjoining to the groove 25a, and flows in a tyre 21 through a passage 27. A pressure varying bag 21', communicated with the passage 27, is located in the tyre 21, and the air from the air feed source is fed in the air pressure varying bag 21'. The feed is effected according to an instruction outputted from a controller, and a spring constant is corrected to maintain the frequency of a spring at a specified value.