BMW 7 Series: Functional Areas of Integrated Active Steering

Low Speed Range

The variable steering-gear ratio of the Active Steering component reduces steering effort to approximately 2 turns of the steering wheel from lock to lock. In the low speed range up to approximately 37 mph, the variable steering-gear ratio for the front wheels is combined with a degree of opposite rear-wheel steer. The effect is to increase vehicle agility.

Fig. 89: Identifying Conventional Steering System Variable Steering-Gear Ratio - Low Speed Range

INDEX REFERENCE CHART

A - Conventional steering system B - Integrated Active Steering M1 - Momentary axis 1 M2 - Momentary axis 2

1. Center of vehicle

When the steering wheels of a vehicle are turned, it follows a curved path around what is called the momentary axis "M".

In the case of conventional vehicles, that momentary axis is positioned at a point along the extension of a line passing through the center of the rear wheels.

Active Steering intervention turns the rear wheels in the opposite direction at speeds up approximately 37 mph.

Fig. 90: Identifying Integrated Active Steering Variable Steering-Gear Ratio - Low Speed Range

INDEX REFERENCE CHART

A - Effective wheelbase reduction
B - Integrated Active Steering
M2 - Momentary axis 2

1. Center of vehicle
2. Straight line through Center of rear wheels
3. Axis of rotation closer to center of vehicle

The consequence of the rear-wheel steering intervention is that the axis of rotation moves closer to the center of the vehicle with the same amount of steering effort.

In terms of agility and dynamic handling, that is equivalent to a vehicle with a shorter wheelbase.

High Speed Range

As the vehicle speed increases, the degree of steering angle amplification by the Active Steering component is reduced. The steering-gear ratio becomes less direct.

At the same time, the steering strategy adopted by the Integrated Active Steering changes. Whereas, at low speeds, the rear wheels are steered in the opposite direction to the front wheels, at higher speeds the rear wheels are steered in the same direction as the front.

The momentary axis moves further back, equivalent to a vehicle with a longer wheelbase, producing more stable straight-line handling. The radius of the curve becomes longer.

By the combination with the Active Steering, an additional amount is added to the steering angle of the front wheels so that the radius of the curve and the required amount of steering lock remain at the familiar level.

Fig. 91: Identifying Variable Steering-Gear Ratio - High Speed Range

INDEX REFERENCE CHART

M1 - Momentary axis 1
M2 - Momentary axis 2

1. Center of vehicle

All in all, co-ordination of the steering interventions at front and rear makes lane changes and steering maneuvers considerably easier to negotiate without sacrificing agility or balance.

Combination of the Active Steering with the new rear-wheel steering system offers benefits for the driver at all speeds.

Fig. 92: Identifying Variable Steering-Gear Ratio - High Speed Range (Effective Wheelbase Increase)

INDEX REFERENCE CHART

M1 - Momentary axis 1
M2 - Momentary axis 2
A - Effective wheelbase increase

1. Center of vehicle
2. Straight line through center of rear wheels
3. Axis of rotation further from center of vehicle

Handling Stabilization by Integrated Active Steering When Understeering

When changing lanes quickly, all vehicles have a tendency to produce a significant yaw response and can sometimes start to oversteer.

If the ICM dynamic handling controller detects a difference between the response desired by the driver and the reaction of the vehicle, it initiates co-ordinated steering interventions on the front and rear wheels. The speed of the stabilizing intervention is such that it is hardly discernible by the driver.

Braking interventions by the DSC, which have a decelerating effect, can be largely dispensed with.

The end result is that the vehicle is more stable and more effectively damped.

Fig. 93: View Of Possible Dynamic Handling Interventions When Understeering

INDEX REFERENCE CHART

A - Prevention of understeer by individual brake modulation (DSC)
B - Prevention of understeer by rear-wheel steering intervention (IAL)

1. Individual brake modulation (DSC)
2. Rear-wheel steering intervention (IAL)
3. Course of an understeering vehicle
4. Course of a vehicle with neutral handling

M - Yaw force acting on the vehicle as a result of dynamic handling system intervention

If the driver underestimates how sharp a bend is when driving quickly on a country road, he/she can be caught out by sudden understeer.

By virtue of its inherent features, Active Steering was only able to react to vehicle oversteer.

Integrated Active Steering incorporating active rear-wheel steering is now also able to make corrective interventions when the vehicle is oversteering and thus further increases active safety.

Handling Stabilization by Integrated Active Steering Under Split Surface Braking Conditions

Hard braking on road surfaces which provide less grip for the wheels on one side of the vehicle than on the other causes the vehicle to yaw towards the side with more grip.

Fig. 94: View Of Handling Stabilization By Integrated Active Steering Under Split Surface Braking Conditions

INDEX REFERENCE CHART

A - Vehicle without DSC
B - Vehicle with DSC
C - Vehicle with DSC and AL (yaw force compensation on E90)
D - Vehicle with DSC and Integrated Active Steering

Under emergency braking, the driver of a conventional vehicle then has to correct the vehicle's course.

Under such split surface braking conditions, the dynamic handling controller generates a stabilizing yaw force by opposite steering interventions on the front and rear wheels.

A) Without DSC

In the case of a vehicle without DSC, maximum braking effect is achieved by the wheels on the dry side of the road, while those on the wet or icy side produce very little retardation.

As a result, a very substantial yaw force acting in an counterclockwise direction is produced, causing the vehicle to swerve to the right.

B) With DSC

A vehicle equipped with DSC brakes the individual wheels more sensitively in order to keep the yaw force within manageable limits for the driver, which however, slightly increases the braking distance.

C) With DSC and AL

The additional "yaw force compensation" function represents a significant safety feature.

When braking on road surfaces with differences in frictional coefficient between one side of the vehicle and the other (tarmac, ice or snow), a turning force is generated around the vehicle's vertical axis (yaw force) rendering the vehicle unstable. In such cases, the DSC calculates the required steering angle for the front wheels and the Active Steering implements it by actively applying opposite lock.

As a result, an opposing yaw force around the vertical axis is generated, "compensating" for the original yaw force (cancelling it out, i. e. the vehicle is stabilized by intelligent coordination of DSC brake modulation and AL steering, constituting a safety feature unique in this class of vehicle).

D) With DSC, dynamic handling controller and Integrated Active Steering

Under such split surface braking conditions, the dynamic handling controller generates a stabilizing yaw force by opposite steering interventions on the front and rear wheels.

That counteracts movement of the vehicle caused by the uneven braking forces.

At the same time, maximum braking force can be applied in order to achieve a short braking distance.

Integrated Active Steering is a logical development from the Active Steering systems. The functions of the systems complement each other perfectly, taking the driving experience to a new dimension.

Integrated Active Steering Special Function

Quite obviously, Active Steering systems must not be capable of being switched on or off by the driver.

In the case of Integrated Active Steering, there is a special feature in that regard because if snow chains are fitted to the rear wheels, Active Steering of the rear wheels must be disabled.

When snow chains are fitted, the rear-wheel steering is deactivated in order to ensure that the wheels are always free to rotate.

Automatic snow-chain detection assists the driver and indicates the detected status on the Control Display. This does not remove the responsibility for manually changing the setting.

Fig. 95: Control Display - Tire Chains Installed

When show chains are used, the setting on the iDrive Settings menu must be changed to "Show chains fitted".

If the maximum speed of 50 kph (31mph) for driving with snow chains is exceeded, the rear-wheel steering is reactivated regardless of the "Snow chains fitted" setting.

Automatic snow chain detection

It is possible to detect from the wheel-speed sensor signals a characteristic pattern produced by the motion of the wheel when show chains are fitted (only with BMW-approved show chains). From that characteristics signal pattern, the control unit is able to detect whether show chains are fitted on each individual wheel.

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