The present paper proposes a dynamical model of wheelchairs with their rear wheels locked in oscillatory environments, such as onboard passenger ships. The most dangerous situations of wheelchair use in oscillatory environment are forward sliding and tumbling down rearward. The authors tried to build a model of wheelchair motion so as to clarify the mechanism of these undesirable phenomena. The coefficients of static and/or kinetic friction between wheelchair rear wheels and the test floor play an important roll in determining the wheelchair motion in oscillatory environment. A series of experiments showed that the Coulomb's law of friction is not always satisfied in the case of forward sliding of wheelchairs. The coefficient of kinetic friction at zero speed is defined here in the place of the coefficient of static friction and was applied to the modelling of forward sliding. A comparison of some experiments and the computer simulation of wheelchair motion shows that the dynamical model using the coefficient of kinetic friction at zero speed can estimate the wheelchair motion in oscillatory environments qualitatively.