Comparison Between Tire Rolling Resistance Measurements on a Flat Track and a Test Drum Under Non-Steady-State Conditions
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.
Tire temperature sensors [14]. The left sensor measures the outer tread temperature and the right sensor right measures the inner tread temperature and the inflation pressure.
Schematic illustration of tread area temperature changes through the tire body cross section at steady state.
Tire inner tread temperatures in real traffic [15]. The tire temperature is the average of the two free-rolling rear tires on a passenger car.
Temperature and inflation pressure during one measurement series on the drum at operational condition I. The vertical lines indicate the start and stop for the 15-s-long rolling resistance measurements, and the shaded areas show the time periods when the tire was not in contact with the drum.
Tire temperature and inflation pressure during rolling resistance measurement in the flat track. The inflation pressure is regulated, but shows spikes when lifting or loading the tire, and the shaded areas show the time period that the tire was separated from the drum. The heating commenced at the first spike (at 11.3 min), stopped at the second spike (at 20.3 min), and finally the measurement was performed at the third spike (at 33.5 min).
Measured relationship between rolling resistance and tire temperature by using the VTI flat track.
Rolling resistance measured at three to four different speeds. The three colors represent three different combinations of inflation pressure and tire temperature.
Rolling resistance measurement results on test drum and VTI tire test facility. The plot also includes adjusted results using both Clark’s and Freudenmann’s formulas. The values that are averages of four to five measurements have their 95% confidence interval shown as error bars.
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