Measurement of Road Friction Coefficient with Strain at Tire Sidewall: Verification by Actual Vehicle Driving Experiments

<div>If road friction coefficient can be measured in a car driving, the performance of advanced driver-assistance systems (ADAS) such as antilock braking system (ABS) and automatic braking systems can be improved. Generally, ADAS uses information obtained from wheel speed sensors, acceleration sensors, and the like. However, it is difficult to measure accurately road friction coefficients with these sensors. Therefore, many studies measured road friction coefficients from strain or deformation in the bottom of a tire (tread), which is the only place to contact with a road surface. However, a sensor installed on the bottom of a tire is easy to peel or damage because greater deformation occurs locally on the bottom of a tire. Therefore, this study develops a method of measuring the road friction coefficient from the strain induced in a tire sidewall. If the tire sidewall can be used, stable measurement can be expected because the sidewall is harder to deform locally than the bottom of a tire. It has be previously confirmed that the triaxial direction loads acting on a ground contact surface of a tire and the strain induced in the tire sidewall have almost a linear relationship. By determining the experimental formulas about the relationship, we can measure road friction coefficient during car driving. This article describes the method to determine appropriate formulas with determining the optimal measurement condition of the strains induced in the tire sidewall and confirms the availability with actual driving experiments.</div>