The present authors recently gave an analytical method for estimating three spring constants Kr, Ks, and Kt, for sidewall stiffnesses of radial tires. These represent the radial, lateral, and in-plane rotational directions respectively. The method is based on netting theory with special consideration to stiffness of the rubber matrices in the sidewall These theoretical results were verified by experiment to have sufficient accuracy. In order to confirm the availability of these spring constants, the twisting stiffness Rt of a radial tire has been analyzed in the present paper by using a spring-supported ring model. An explicit formula for Rt, expressed in terms of the three components of the spring constant, was obtained. Experiments were conducted on a 175SR14 radial tire by increasing the inflation pressure while keeping the tread circumference constant. The theoretical results agreed well with the experimental results. A related problem is also referred to; this is the forced lateral vibration with fundamental eigen-modes of the inflated sidewall-rim system when the tread is fixed. Eigen-frequencies calculated by using those spring constants coincide well with the experimental results.