Using Vibrational Properties of the Wheel to Identify Tyre-Road Friction

This paper describes the use of vibrations in a single passenger car wheel to identify road fiction. The peak in the transfer function of wheel speed to brake torque variations at the lowest resonance frequency (~32 Hz) is strongly influenced by the 'constraint' provided by the road surface. A model of the wheel with its rotational inertia in contact with the road via the tyre is developed. The inflated tyre carcass provides the stiffness in all directions, here mainly the rotational motion of the wheel, combined with the forward motion of the vehicle is considered. The tyre to road interaction includes the effect of saturation in the force versus brake slip characteristic. After linearization it can be proven that the local derivative of the slip characteristic has a consistent influence on the dynamic behaviour of the tyre. Where the local derivative tends to zero, the maximum achievable brake force is reached. Therefore, assessing the local derivative of the brake force versus slip curve is vital for optimal braking. The phenomenon is also studied in a laboratory environment, with a single braked wheel, running on an electrically driven steel drum of 2.5 m diameter. Special attention is needed for the instrumentation of the wheel rotational speed. To reach the required sample rate and resolution in wheel speed information, the data-acquisition counts the duration of an encoder period. Since this wheel speed signal becomes available at encoder flanges, it needs to be resampled to create data that is synchronous with the input brake pressure and -torque and -force data, typically sampled at 512 Hz.For some conditions and road surfaces (bare steel and 'safety walk paper') both the relation between local derivative and resonance gain and the application of this relation as a friction identification methodology will be assessed.