Influence of test conditions on tyre/road noise measured by the CPX method

The CPX method (ISO/DIS 11819-2) was developed to measure the influence of tyre/road noise on different road surfaces, using one or two reference tyres mounted on a trailer/vehicle. In recent years, the CPX trailer has been used to compare the noise behaviour of different passenger car tyres using the tyre load (3200 N) and inflation pressure (200 kPa) specified in the standard. Several experts on tyre/road noise question whether this method is appropriate for such use, and specifically whether measured values can be compared with the labelling values. The labelling values are based on coast-by conditions as defined in ECE Reg.117, with a loading and tyre pressure that differs from the CPX method. To investigate the influence of the test conditions on the measured noise levels, two tests were conducted: 1) A test according to the specifications of ISO 11819-2 and 2) A test according to the specifications of ECE Reg.117, but with a CPX trailer instead of a vehicle. A total of 11 passenger car tyres have been tested, including two tyres designed for electric vehicles. Measurements have been done on 7 different SMA road surfaces. The paper presents both A-weighted overall noise levels and third octave band spectra. The main conclusion is that the change in loading and tyre pressure did not improve the correlation with the label values