ISO 18318:2026

This document establishes definitions and evaluation methods for the following wheel-rail contact geometry parameters influencing the vehicle running dynamic behaviour:

• the rolling radius difference between the two wheels of a wheelset (Δr-function) which serves as a basis for all further calculations;
• the equivalent conicity function from which are derived:
• a single equivalent conicity value for a specified amplitude, which is relevant for the assessment of vehicle running stability on straight track and in very large radius curves;
• the nonlinearity parameter, which characterizes the shape of this function and is related to the vehicle behaviour, particularly in the speed range close to the running stability limit;
• the rolling radii coefficient, which is used to describe the theoretical radial steering capability of a wheelset in a curved track.

Additional information is given about the relationship between the contact angles of the two wheels of a wheelset (Δtanγ-function) and about the roll angle parameter.

NOTE            Out of the presented parameters only those related to the contact angle are relevant for independently rotating wheels of wheel pairs.

Descriptions of possible calculation methods are included in this document. Test case calculations are provided to achieve comparable results and to check the proper implementation of the described algorithms.

To validate alternative methods not described in this document, acceptance criteria are given for the equivalent conicity function. This includes reference profiles, profile combinations, tolerances and reference results with tolerance limits.

This document also includes minimum requirements for the measurement of wheel and rail profiles as well as of the parameters needed for the transformation into a common coordinate system of right-hand and left-hand profiles.

This document does not define limits for the wheel-rail contact geometry parameters and gives no tolerances for the rail profile and the wheel profile to achieve acceptable results.

For the application of this document some general recommendations are given.