A new measuring system for continuous friction monitoring (CFM) of wet surfaces on a tire test bench
To improve and better understand tire wet grip mechanism, it is essential to perform test bench measurements under wet conditions. Analyzing for example different tread compounds concerning their force transmission potential, assume to maintain constant conditions and to monitor all changes during the test, such as the micro roughness of the track surface. As on public streets, on the inner drum test bench of the Karlsruhe Institute of Technology with increasing testing cycles we observe a drop in the friction level. This polishing effect on the test bench is found on artificial corundum surfaces as well as on real asphalt roadways.
When we perform wet measurements on our test bench, it is necessary to have knowledge about the grip level of the track surface in order to analyze the test results. For this purpose, we developed and build up a new measuring device called CFM-System (Continuous Friction Monitoring System). This measuring device was inspired by the GripTester from Findlay Irvine Ltd and allows fully automatic measurements in order to survey the friction level of the track surface. The measuring principle is a continuous braked 10 inches test tire whereby a longitudinal friction coefficient is determined. The aim of the presented investigations is to prove, that the new CFM system is suitable to replace the current measuring methods, such as determination of the British Pendulum Number (BPN) or performing braking measurements with a Standard Reference Test Tire (SRTT).
In order to establish a suitable measurement methodology with the CFM-System on the inner drum test rig, first a basic understanding of the longitudinal tire behavior of the test tire was created. For this purpose, μ-slip characteristics were determined in order to investigate the friction behavior of the tire under different slip conditions. Further, the influence of the wheel load and the driving speed were examined and analyzed. Based on these findings, a test method has been deduced. The studies presented in this paper have shown a good correlation between the results of the CFM-System and the results obtained from SRTT-tire measurements. We have shown that this new CFM-System is suitable to monitor the grip level of the track surface on our inner drum test bench when performing wet measurements.