Truck Steering System and Driver Interaction

Licentiate thesis, 2014

This thesis presents a compilation of methods to consider when mapping steering functions and results as vehicle dimensions change. It is concluded that some final tuning inevitably will be required as major changes are performed. It is however possible to create qualified starting points using a set of simple rules as presented. Considered properties are steering wheel size, wheelbase, steering ratio, and understeer gradient. An investigation of driver behaviour when a sudden yawing disturbance is acting on the vehicle is also presented. Two examples, automatic braking on split friction and front tyre blow-out, are studied in detail. For automatic braking of a heavy truck it is concluded that current legal requirements and technology for split friction conditions are sufficient for an alert driver, but may create some problems for a driver being distracted. Most heavy trucks have positive steering-axis offset at ground, also known as kingpin offset at ground. This can induce a destabilising steering wheel torque when a front tyre is damaged. The effect from this is investigated using a qualitative approach. For an active driver it is found that elimination of the destabilising steering wheel torque has a small, yet statistically significant, effect on lateral deviation. And furthermore that the lateral deviation increases as the driver exhibits higher admittance. A general conclusion from the analysis, on driver behaviour at yawing disturbances, is that lateral deviation will reduce substantially when driver reaction time is reduced. This can be achieved by warning the driver prior to the incident. Hence, the warning phase, that commonly precedes automatic brake activation, is of high importance. Another method is to use steering support in the initial phase of the incident.