[Auricom]

Great new owner question on IAS in the G07 forums and since everyone is accustomed to my technical essays... errr... lengthy replies... I figured I should cross post here since the G05 shares technical features with G06 and G07:

Originally Posted by sjwr

Hi All,

I have a X7 on order via PCD delivery that among many other options, includes Integral Active Steering (IAS).

I haven't test driven a X7, only a X5 and not sure whether it was with or without IAS so could someone who has actually driven the X7 with and without the IAS explain the experience of having IAS?

I know that theoretically the rear wheels turn in one direction at low speeds and a different direction at high speeds, but I am looking for a more practical "what you experience" and "what you actually get" insight. Thanks.

Regards,
sjwr

There are two parts to "what you experience" and "what you actually get" question

TL;DR

What you experience - it's all in the steering
What you experience varies for different drivers - some may notice the variable steering ratio changes (direct / indirect steering feel) and others don't notice the change but sense that during turns steering wheel travel / feel's reduced or elongated. I'm in later category in regards to steering. Otherwise, you're not going experience IAS.

What you actually get
Slow speeds - from 5mph to 37mph, reduced turning turning by approximately 1m / 3.28ft in comparison with vehicles without rear axle slip angle control (HSR)

High speeds - from 37mph and up, directional stability while driving and changing lanes


THE LONG LONG LONG INTEGRAL ACTIVE STEERING EXPLANATION

Electromechanical Power Steering with Variable Rack
There's a component that's a part of IAS that is very rarely discussed which is the electromechanical power steering system with variable rack (front axel) as well as the rear axle slip angle control (HSR).

The variable steering gear ratio in IAS is accomplished by the geometry/pitch of the toothed gear of the rack. Around the center position of the steering gear, the steering system behaves with steady directional stability. As the steering angle moves away from the center position, the ratio becomes increasingly more direct.



Variable Steering Gear Ratio Front axle




A) More indirect steering gear ratio in the center position of the variable rack (0° - 45°)
B) Transitional range (45° - 130°)
C) Direct steering gear ratio outside the center position of the variable rack (> 130°)
X) Steering angle in degrees
Y) Steering gear ratio in mm/steering wheel turn
1) Variable rack geometry (IAS)



Steering Angle of IAS on the Front Axle




A) More indirect steering gear ratio (variable rack geometry)
B) Direct steering gear ratio (variable rack geometry)


Rear Axle Slip Angle Control (HSR)




The rear axle slip angle control (HSR) mounted on the rear angle permits a maximum steering angle of the rear wheels of +/- 2.5°.

As a result, it was possible to reduce the turning circle by approximately 1m / 3.28ft in comparison with vehicles without rear axle slip angle control (HSR).

The rear axle slip angle control is available from a speed of approximately 5 km/h / 3 mph up to the maximum speed.

The request to adjust the steering angle at the rear axle is issued by the Dynamic Stability Control (DSC). A spindle drive inside the component is rotated via the electric motor. This produces a linear movement at the two track control arms. Using the position sensor, the control unit of the rear axle slip angle control (HSR) can determine the linear displacement and thus calculate the steering angle of the two rear axle wheels.

The spindle drive of the rear axle slip angle control (HSR) is self-inhibiting. This means that the X7 has the identical drivability to a vehicle without rear axle slip angle control (HSR) in the event of system failure.


SPECIAL NOTE WHEN GETTING ALIGNMENTS



A special fixture is required for alignment in order to assemble the carrier plates. For this reason, the carrier plates on the rear axle slip angle control (HSR) must not be undone. If this is not observed, the wheel alignment on the rear axle will become incorrectly adjusted.


Operation of Integral Active Steering




A) Opposite steering
B) Parallel steering

In the speed range up to approximately 60 km/h / 37 mph, the rear axle slip angle control (HSR) steers in the opposite direction to the steering angles of the front axle steering. This permits increased vehicle maneuverability.

As from the speed range of approximately 60 km/h / 37 mph, the rear axle slip angle control (HSR) steers in the same direction. This improves the directional stability of the vehicle.



Driving control interventions when cornering

When changing lanes quickly, all vehicles have a tendency to produce a significant yaw response and can sometimes start to oversteer. If the Dynamic Stability Control (DSC) detects a deviation between the driver wish and vehicle response, the vehicle is stabilized by means of a steering intervention at the rear axle. The speed of the stabilizing intervention is such that it is hardly discernible by the driver. It is possible to largely do without decelerating DSC brake interventions. As a result, the vehicle is more stable and the driving dynamics are preserved.

If the driver underestimates a bend when driving rapidly, he may be surprised by sudden understeer. The rear axle slip angle control (HSR) can also perform a corrective intervention in understeer situations and thus additional increases active safety.


Driving control on different surfaces
Hard braking maneuvers on roadways that are slippery on one side make the vehicle yaw to the vehicle side with more grip. In the event of emergency braking, the driver of a conventional vehicle must take corrective action. With this so-called µ-split braking, the Dynamic Stability Control (DSC) provides a stabilizing yaw moment via a steering intervention at the rear axle.





Further to A) Without DSC
When braking with a vehicle without DSC, maximum brake force is supported on the dry side of the road and only low brake force on the wet or icy side. An anti-clockwise yaw moment is produced which can lead to the vehicle skidding to the right.

Further to B) With DSC
When braking with a vehicle equipped with DSC, the brake forces at the wheels are metered so that the yaw moment acting on the vehicle is reduced. The vehicle therefore remains controllable. The stopping distance may be increased slightly.

Further to C) with DSC and rear axle slip angle control (HSR)
When braking with a vehicle equipped with DSC and rear axle slip angle control (HSR) (optional equipment Integral Active Steering SA 2VH), the DSC control unit calculates the steering angle for the rear wheels. The actuators of the rear axle slip angle control (HSR) convert the calculated steering angle into an active steering angle at both rear wheels. The resultant stabilizing yaw moment allows the maximum brake forces to be applied in order to achieve the shortest possible stopping distance. The perfect interaction of steering and brake intervention therefore increases active safety and enhances the driving dynamics of the vehicle.