32 Steering And Wheel Alignment: Introduction

Active Steering is the most recent BMW development in the innovative steering systems sector.

Active Steering varies the steering gear ratio from direct to indirect and vice versa as a function of the vehicle's speed.

The benefits of Active Front Steering are:

• direct steering with no more than minimal movements of the steering wheel at the low end of the speed range
• More indirect steering at higher speeds in conjunction with yaw-rate control

Active Steering, including Servotronic, is available as an option (option 217).

Active Steering gives the vehicle an even sportier character.

New for the E70 is the 3rd generation Active Steering: 

• Yawing moment compensation has been fully integrated as a function in the Active Steering control unit
• Cumulative steering angle sensor discontinued
• Digital motor-position sensor
• Commissioning exclusively via service function in BMW diagnosis system

On the X5, Active Steering is available for the first time on an all-wheel drive vehicle.

In 03/2007 the actions described for the E70 will also be implemented on the BMW 5-Series. Other model series will follow later.

Option 216 "Servotronic" is not  available without Active Steering.

Active Steering is not available in conjunction with  option 3AC "Trailer coupling with folding ball head".

New for the BMW 3-Series was: 

• Yaw moment compensation as additional function
• Output stages for the Servotronic valve and the ECO valve are integrated into the Active Steering control unit
• Only 1 DSC sensor (redundant)

New for the BMW 5 and 6-Series from 09/2005 was: 

• Active Steering control unit taken from BMW 3-Series
• This allows the Servotronic valve and the ECO valve to be directly actuated by the Active Steering control unit
• Only 1 DSC sensor as on BMW 3-Series

With option 216 "Servotronic", the body gateway module (KGM) actuates the Servotronic valve.

A conventional steering system inevitably constitutes a compromise:

on the one hand, the steering cannot be too direct as otherwise it would be overly sensitive when the vehicle is travelling at high speed. On the other hand it is practical to have much more direct steering for maneuvering at low speeds and for parking. The new Active Front Steering system is the ideal trade-off. The directness of the steering of a vehicle fitted with AFS varies with speed.

This results in three major benefits:

• enhanced agility
• enhanced comfort
• enhanced safety

Enhanced agility 

Up to about midway through the speed range (about 100 km/h), the directness of the steering means that the driver perceives the vehicle as more agile and easier to handle. A driver who has to avoid an unexpected obstacle, say, has much better control, plus significantly enhanced steering precision and less work at the steering wheel. There is no loss in feedback from the road surface right through the steering wheel.

Enhanced comfort 

Current BMW models need more than three full turns of the steering wheel in order for the wheels to describe the arc from full lock in one direction to full lock in the other.

When the vehicle is travelling at low speed, Active Steering reduces this to less than two full turns of the steering wheel from lock to lock.

The advantage: The driver's task is easier when turning corners in city traffic or when maneuvering into parking slots where space is at a premium. On twisty roads such as mountain passes, moreover, the reduced steering angle means that the driver's hands remain in the ideal position on the wheel. There is no longer any need for the driver to feed the wheel through his or her hands, or to cross hands in a tight bend. The multifunction buttons on the steering wheel and the paddles for the sequential manual transmission (SMG) are always perfectly positioned relative to the driver's hands, regardless of the driving situation.

Enhanced safety 

A completely different set of conditions applies when the vehicle is travelling at high speed:

With the assistance of indirect steering there is increased stability when travelling straight ahead at high speeds (such as on motorways) compared with conventional steering. At the same time, the yaw-rate control is in the position to stabilize the vehicle by correcting the steering angle when the vehicle is oversteering.

This supports the Dynamic Stability Control (DSC) function.

Like conventional systems, BMW's Active Front Steering features a steering column with a permanent connection from the steering wheel to the front wheels. This mechanical link ensures the operability of the steering system at all times, even in the event of disruption to or the complete failure of the system. The mechanical steering linkage, what is more, is essential in sustaining the authentic "feeling of steering" as perceived by the driver. True steer-by-wire systems are intrinsically unable to simulate the realism of this feedback from the road to the driver.

The fully electric steering-column adjustment allows the driver to set the steering wheel to the best possible position. This is done with a switch on the left-hand side of the steering column. The angle and the reach of the steering column are electrically adjusted.

The steering column adjustment is actuated by the center console switch cluster (SZM).

One motor drives the tilt and reach adjustment. The second motor switches adjustment gears to select the adjustment direction.

The steering column adjustment is a constituent component of option 459 "Electric seat adjustment". Options 456 "Comfort seats, front" and 481 "Sports seats" automatically include option 459 "Electric seat adjustment".

Vehicles with electric steering-column adjustment have a steering-column memory (in conjunction with seat memory and mirror memory).

New from 09/2005: 

• The steering column switch cluster (SZL) is directly connected to the PT-CAN.
• The body gateway module (KGM) forms the gateway between the PT-CAN and the K-CAN.