Abstract
Traditionally, vehicle steering systems had a linear relation between steering wheel rotation and road wheel angle. Nonlinear steering racks arrived in the last decades of the 20th century with the aim to achieve better parking characteristics. In the current century, the development of variable steering ratios has become more and more important and not only for parking. The aim of these steering systems is to combine a low speed agility increase with safe high speed behaviour. Both mechanical solutions (variable rack) and mechatronic solutions (active front steering) have been implemented in production cars.
FEV has investigated the impact of these two approaches on the experienced steering behaviour.
Actual production vehicles with both systems have been tested to confirm the targeted improvement of agility whilst maintaining safety at high speed, including the feeling of linearity and transitional behaviour.
Secondly, the effect on objective test results was predicted using simulation models in which the different concepts were implemented. Here the advantage is that in this virtual vehicle several concepts and settings can be directly compared. For the system with the highest subjective impact (BMW 3 series), a series of objective tests was performed to confirm the simulation and subjective results.
The impact on expected test results is shown in relation to FEV’s scatter bands which show the current state of technology of traditional systems on the market. Here it becomes clear that the active mechatronic solutions deliver characteristics that are not possible with the current traditional steering systems. The mechanical solutions do reach the desired agility increase, but deliver characteristics that still fit into the scatter bands.
Finally, the impact of these results in relation to standardized test methods such as ISO 7401 is discussed. Here it is concluded that these test methods need some modification or extension to properly describe the behaviour of vehicles equipped with these nonlinear steering systems.
Keywords: vehicle dynamics, steering, agility, active front steering, variable rack