Active Steering — Variable Ratio Systems

Active steering — not to be confused with steer-by-wire — is a system that adds or subtracts steering angle using a planetary gear set or similar mechanism in the steering column. The mechanical connection between the steering wheel and the rack is maintained, but the system can change the effective ratio. This means the steering wheel and the front wheels are not always in a fixed 1:1 relationship — the system can turn the front wheels more or less than the driver's input.

BMW's system (used on some 5 Series, 6 Series, and 7 Series models) places a planetary gear set and electric motor in the steering column. At low speeds, the motor adds steering angle — a small turn of the wheel produces a larger turn of the front wheels, making parking easier. At high speeds, the motor subtracts steering angle — the steering ratio becomes slower and more stable. The system can also make independent steering corrections for stability control — turning the front wheels slightly without moving the steering wheel to correct a skid.

Audi uses a simpler approach called progressive steering — the rack itself has a variable-ratio gear pattern cut into it. Near center (straight-ahead driving), the ratio is slower for highway stability. As you turn the wheel further, the ratio quickens for parking and tight turns. This is a passive mechanical system with no electronics — the variable ratio is built into the rack gear teeth. It provides some of the benefits of active steering without the complexity.

BMW Active Steering failures typically involve the planetary gear motor or the angle sensor in the steering column. DTCs will point to specific faults. A common complaint is a steering center-point that drifts — the steering wheel is slightly off-center — which requires recalibration of the active steering module. After any work on the steering column, the active steering system needs to be initialized with a scan tool so it relearns its center position and operating range. On Audi progressive steering, there are no electronic components to fail — the only service consideration is that the rack has a unique gear pattern, so you must replace it with the correct variable-ratio rack.