Different steering responsiveness at different speeds can explain active steering in short. It consists of a planetary gear set included into the steering column with an electric motor at the joint. The motor adjusts the angle of the front wheels in proportion to the vehicle’s current speed.
At slow speeds through city traffic, while manoeuvring into tight parking spaces or negotiating ghat sections, Active Steering increases the size of the steering angle. The front wheels react immediately to small steering inputs, enabling the driver to manoeuvre without excessively turning the steering wheel. It also reduces effort during parking as the car feels sprightlier. The steering is easy at medium speeds as well, and at high speeds, it gets indirect to reduce the change in steering angle for every movement of the steering wheel.
BMW’s Active Steering system keeps direct connection between the steering wheel and front wheels, so that in case of an electronic system failure, the car can still be controlled.
Variable Ratio Steering
A steering system in which the rack-and-pinion gearset uses a different tooth pitch. The teeth at the centre are closer to each other and as you go away from the centre, the distance between them increases. This means that the car is more responsive while turning as the rack is near the centre.
Rear-Wheel and Four Wheel Steering
Four-wheel steer (4WS) systems have been offered on a handful of Japanese models for more than a decade, but have made little headway in other markets. In Japan, the much tighter turning circle which can be achieved by steering the rear wheels in the opposite direction to the front wheels has a strong appeal for drivers who must frequently manoeuvre in extremely confined spaces. Elsewhere, the cost, weight, complication and possible safety implications of 4WS have proved too much of a discouragement.
The main technical question with 4WS is whether, or rather, when, the rear wheels should steer in the same sense as the front wheels. As already pointed out, ‘opposite rear’ steering allows much tighter turning circles for manoeuvring in crowded car parks. On the other hand, similar-sense steering provides better behaviour at high speeds, especially in lane-change manoeuvres, where it enables the car to ‘move sideways’ without having to change direction so much; it is the change of direction which can lead to control problems in extreme situations. It is now generally accepted that to fulfil its potential, 4WS needs the ability to steer in either sense according to speed, with ‘opposite-sense’ rear steering at parking speeds, same-sense steering at high speeds, and a cross-over between the two at around 30kph.
Components giant Delphi began developing its Quadrasteer system from 1999, applying the same 4WS principles of operation, but aiming at the very large American market for luxury-equipped light trucks and big off-road vehicles. Even in large American car parks, these vehicles can be awkward to manoeuvre and opposite-sense rear steering would help matters. At the same time, same-sense rear steer would improve their stability in lane-changes, especially when towing a heavy trailer. Delphi developed Quadrasteer for the rear axles of such vehicles. An interesting aspect of Quadrasteer is that steering signals to the electric motor which operates the rear wheel steering are themselves transmitted electrically — making this one of the first true examples of ‘steer-by-wire’ as well.