ABSTRACT

Rolling resistance has become one of the key parameters that the vehicle industry is focusing on in their efforts to make vehicles more energy efficient. Rolling resistance is generally measured in steady state on a test drum that results in a higher rolling resistance than flat track measurements for the same test settings due to the curvature of the drum, which deforms the tire more. Therefore, the drum steady-state rolling resistance is commonly converted with Clark’s formula, as suggested in the rolling resistance measurement standards. Freudenmann et al. suggest an adjustment of Clark’s formula, claiming that it would improve the accuracy for steady-state conversions. The aim of this work is to compare non-steady-state drum and flat track measurements, performed at the same inflation pressure and tire temperature, to investigate whether Clark’s or Freudenmann’s formula can be used to convert the drum measurement to a corresponding flat track level when not in steady state. Non-steady-state measurements have been performed on both a test drum and a flat track. As expected, Freudenmann’s formula is not good for the conversion at non-steady-state settings because it was empirically developed for steady state. Clark’s formula works for non-steady-state conversions of measurements performed at the same tire temperature and inflation pressure. However, the dependency of rolling resistance on temperature is not the same in the drum and flat track measurements, causing a difference between the results that increases as the tire temperature decreases. Further research to improve Clark’s formula for non-steady-state measurements by including the effects of tire temperature would be beneficial.