This paper describes experiments and methodology employed in an attempt to develop a system for the grading of wet tire traction. Data from instrumented vehicle experiments and the University of Michigan Highway Safety Research Institute mobile tire tester are used to support the conclusions. The vehicle tests include J‐curve cornering and diagonal, locked‐wheel braking. The maneuvers are used to generate information on tire tractive properties and their dependence on tread depth, rim size, suspension, and surface conditions. Three tire force measures obtained with the mobile tire tester, the peak longitudinal, peak free‐rolling lateral, and locked‐wheel longitudinal coefficients of friction, are presented for a group of 90 percent of the original equipment tire sizes in use on American cars and are rank correlated with vehicle results. The results indicate the feasibility of defining minimum wet traction levels for various tire‐road interface properties.

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