Identification of a wheel--rail adhesion coefficient from experimental data during braking tests

The forces that occur in the wheel-rail interface significantly affect vehicle dynamics, especially in the longitudinal direction. Conventionally, the tangential component of the force exchanged between the rail and the wheel is expressed as the product of the normal component of the force, and the so-called adhesion coefficient. This ratio depends on several parameters that are usually summarized in the term 'adhesion conditions'. When the adhesion conditions are degraded (for example, in cases of rain, fog, ice, dead leaves, etc.), and the vehicle is accelerating or braking, pure rolling conditions between the wheels and the rails do not hold any more, and macroscopic sliding occurs on one or more of the wheels. The aim of this work is to identify a relationship between adhesion coefficient and some parameters, namely wheel sliding and train speed, starting from a set of experimental measurements, obtained from test runs conducted with artificially degraded adhesion conditions.