Rollover of trucks is a major problem. In The Netherlands, it occurs about twice a week, and the number is increasing. A factor contributing to this might be the maximised speed of commercial vehicles, not stimulating the driver to reduce this speed in potentially critically conditions.

Several causes can be identified, one of them is excessive yaw behaviour of truck or trailer posing major difficulties for the driver, and likely to occur at lateral accelerations far below the static tilting boundary.

This paper first treats yaw-stability for a single heavy vehicle in a fundamental way. The analysis will be based on the full non-linear vehicle behaviour where critical (stationary) points are discussed within a phase plane representation. Local behaviour of trajectories around these points will be analysed yielding the type of (in-)stability, for all possible combinations of effective tyre behaviour (incorporating suspension characteristics).

These effects will also be presented in terms of the handling-diagram representation, showing the lateral acceleration under stationary conditions in terms of steering angle and road curvature. The shape of this diagram can be interpreted in terms of steering sensitivity of the vehicle.

The results will be extended to tractor-trailer combinations, where the previous results applied to each of the articulations together lead to an understanding of stability of the full vehicle combination.

The different types of excessive yaw modes (jackknifing, trailer swing) of the combination will be discussed as related to the steering characteristics of the separate parts, and with reference to various vehicle parameters.