Non-linear rolling friction of a tyre-caster system: analysis of a rugby wheelchair

Tyre-caster systems such as wheelchairs consist of several components with different bearing and rolling friction, with the latter depending on the tyre pressure. The aim of this study was to determine the rolling friction of a rugby wheelchair with deflated and maximally inflated tyres. The rolling friction was determined with coast-down tests by instrumenting the wheelchair with an accelerometer. As the energy loss of coasting down comes primarily from the rolling friction and aerodynamic drag, the latter (including the lift) was determined using wind tunnel experiments. The ratio of the sum of horizontal forces (drag and inertial) to the sum of vertical forces (lift and gravitational) determined the rolling friction coefficient. The rolling friction coefficient expressed as a function of the velocity was found to be highly non-linear, consisting of an initial viscous spike at low velocities, a constant component, and a parabolic component increasing with velocity. The rolling friction coefficient of the wheelchair with deflated tyres was on average three times higher than the one with maximally inflated tyres.