BMW Technology - Active Front Steering (AFS)

BMW is the first manufacturer to introduce a mechanism to enhance steering systems through the use of new-age electronics. The Active Front Steering (AFS) system introduced as an option on the new BMW 5 and 6 Series models performs like other of BMW's highly advanced electronic driving aid systems.

BMW has developed Active Front Steering (AFS) that maintains a mechanical link between the front wheels and the steering wheel. AFS is a system that is able to adjust both the steering ratio and the amount of power assist for optimum feel and control under varying driving conditions.

At higher speeds, for example on the Autobahn, the steering transmission ratio becomes increasingly indirect up to the level of a conventional steering (or even beyond), giving the driver a significantly higher standard of driving stability. AFS was first announced for the E60 5 Series and E63 6 Series models as optional equipment (about 1,200 Euros).

With AFS, you won't be crossing your arms over each other on the steering wheel as often during sharp cornering, and parking requires far fewer turns of the wheel.

AFS varies the steering transmission ratio electronically in direct relation to the style and speed of driving and road conditions. Under normal road conditions at low and medium speeds, the steering becomes more direct, requiring less steering effort of the driver, increasing the car's agility in city traffic or when parking. At high speeds the steering becomes less direct offering improved directional stability. When cornering at high speeds, or when undertaking sudden movements, the steering stiffens up by monitoring increases in the yaw rate.

AFS works hand-in-hand with Dynamic Stability Control (DSC) by monitoring the yaw rate and changing the steering angle accordingly. This reduces the number of DSC interventions, providing more control to the driver and increased comfort for the passengers.

Here's how it works

AFS is a "by-wire" approach, using sensors and replacing the mechanical linkage between the steering wheel and the road wheels with electronics. Here's a representative schema of how it works:

AFS works by inserting an electric motor-driven planetary gear set between the steering rack and the steering column

In order to not lose steering "feel", BMW uses a rack and pinion steering system enhanced with a variable ratio device controlled by electronics. When the system's sensors detect driver input at the steering wheel (turning motion), a computer analyzes the data and then sends the information to an electric motor and linkage. Based on this input, the front wheels are turned the appropriate amount.

AFS uses an electric motor-driven planetary gear set between the steering rack and the steering column. The DC Motor is controlled electronically and works in conjunction with the Servotronic variable power steering system. At lower speeds the system dials in a more direct steering ratio (with variable power assist tailored to match anything from parking to medium speeds). Thus, a small movement of the steering wheel results in a greater movement at the road wheels.

More features:

• AFS actually changes the steering ratio (the number of turns of the steering wheel required to turn the road wheels from lock to lock) while you drive.
• AFS can intervene in an instant to provide a correction if the rear end starts to break away.
• AFS is different than variable assist power steering, which only varies the amount of effort, not the actual steering ratio.
• AFS works with Dynamic Stability Control (DSC) to prevent a skid. When the yaw sensors detect an oversteer situation (the back end beginning to step out), it clicks in a few degrees of opposite lock.