A Measuring System for Continuous Friction Monitoring on Wet Track Surfaces
To improve and better understand the tire wet grip mechanism, it is essential to perform test bench measurements under wet conditions. On both public streets and the track surfaces of the internal drum test bench of the Karlsruhe Institute of Technology (KIT), a drop in the friction level of the track surface can be observed with an increasing number of wet measurements. For this purpose, a new measuring device was developed and built: the continuous friction monitoring system (CFM-System). The measuring principle is a continuously braked standard test tire, whereby a longitudinal friction coefficient is determined. To establish a suitable measuring method using the CFM-System, a basic understanding of the longitudinal behavior of this standard test tire was achieved. For this purpose, μ-slip characteristics were determined to investigate the friction behavior of this tire under different slip conditions. Furthermore, the influence of wheel load and driving speed were examined and analyzed. Based on these findings, an adequate test method was derived. The investigations presented in this article show a good relation between the results of the CFM-System and the results obtained from measurements using the standard reference test tire (SRTT). The results show that this new CFM-System is suitable for monitoring the grip level of the track surface on the internal drum test bench when performing wet grip measurements. It is assumed that the results can be directly transferred to the real road or to proving grounds because real track surfaces are used on the test bench.ABSTRACT
Drop of the maximum longitudinal friction coefficient (80 kph, 4500 N, 2.5 bar) and the British Pendulum number due to a polishing procedure (average load, combined force conditions) on an artificial corundum surface on the internal drum test bench [30].
Schematic illustration of the internal drum tire test bench at the Karlsruhe Institute of Technology.
Schematic illustration of the continuous friction monitoring system.
Chronological sequence of the CFM-System's measuring process.
Measured and magic formula [37] fitted μ-slip characteristic of the ASTM Standard E1844-96 wheel at a load of 200 N and a velocity of 10 kph under braking slip.
Velocity influence of the ASTM Standard E1844-96 wheel at a load of 200 N and 30% slip. Points represent averaged measured data with the simple standard deviation as error bars. The solid line represents the linear regression y = −0.0061x + 0.7078 determined by the means of the indicated points. The dashed line represents the 90% confidence bounds.
Load influence of the ASTM Standard E1844-96 wheel at 10 kph and 30% slip. Points represent the averaged measured data with the simple standard deviation as error bars.
Correlation between friction coefficients of SRTT and CFM-System. Points represents the averaged measured data with the simple standard deviation as error bars. The solid line represents the linear regression y = a·x + b determined by the means of the indicated points. The dashed line represents the 90% confidence bounds.
Correlation between the British Pendulum number and friction coefficients of the CFM-System. Points represents averaged measured data, with the simple standard deviation (pendulum standard deviation according to [11]) as error bars. The solid line represents the linear regression determined by the means of the indicated points. The dashed line represents the 90% confidence bounds.
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