BMW 7 Series: System Components

General

The chassis and suspension is subdivided into the main components that are described in more detail in the following:

• Front axle
• Rear axle
• Damping/suspension
• Brakes
• Steering
• Wheels/tires.

Front Axle

Design Layout

Fig. 2: Identifying F01/F02 Front Axle Components

COMPONENT REFERENCE CHART

1. Spring strut
2. Transverse control arm, top
3. Swivel bearing
4. Wheel bearing
5. Stabilizer link
6. Transverse control arm, bottom
7. Stabilizer bar
8. Tension strut with hydraulic mount
9. Front suspension subframe

The introduction of a second control arm level for wheel control, which is arranged above the wheel, results in additional degrees of freedom for the kinematics of the front axle as well as for the suspension/damping compared to other designs such as a spring strut front axle.

Components with special materials:

• The forged aluminum swivel bearing (3) with the 3rd generation wheel bearing (4).

NOTE: The arms and links are bolted by means of ball/disc connections to the swivel bearing and, similar to the track rod heads, no longer have tapered screw fittings.

• The transverse control arm at the top (2) is made from forged aluminum and the cylindrical joint pin is clamped in the swivel bearing (3).
• Tension strut with hydraulic mount (8) and lower transverse control arm (6) are forged aluminum components while the lower control arm bears the spring strut (1) by means of a forged steel mount.
• The new front axle subframe (9) is a welded aluminum structure which, as the standard axle, does not require the familiar aluminum thrust panel with service openings for increasing stiffness. This is made possible by the solid transverse section in the front axle subframe.

NOTE: The design layout of the front axle subframe makes it possible to lower the complete steering gear for service purposes.

Fig. 3: Identifying Kingpin Offset At Hub

Kingpin offset at hub

INDEX REFERENCE CHART

1. Steering pivot axis
2. Wheel center plane
3. Kingpin offset at hub

The steering pivot axis of the wheel suspension is now formed by a joint at the top A-arm and the virtual pivot point of the lower arm level as known from the spring strut or McPherson front axle.

The steering pivot axis is therefore freely selectable and can be positioned such as to produce a small kingpin offset at hub with sufficient weight recoil.

This kingpin offset at hub is decisive for transmitting the irregularities on the road surface to the steering wheel.

The lower and upper arm levels now move simultaneously in response to wheel deflection. As a result, as the spring compresses, the wheel pivots in such a way that the negative camber to the road does not decrease as much as is the case with a spring strut front axle.

Since the two control arm levels undertake the wheel control, the damper is virtually no longer subjected to transverse forces and rotational motion.

This makes it possible to do without a roller bearing assembly (conventional strut mount) on the spring strut support. Instead of this conventional roller bearing a damping and support unit is installed that takes up all three load paths. The load paths are the damper piston rod, the inner auxiliary spring and the bearing spring. This damping and support unit is still referred to as the "strut mount".

Due to the lack of transverse forces, the piston rod can be made thinner, resulting in a similar displacement volume in the push and pull direction of the damper. This serves to improve the design layout of the damper and is the prerequisite for the innovative damper control system - vertical dynamics control (VDC).

Due to the substantially lower friction at the circumference of the piston rod, the damper can respond more sensitively.

By connecting the stabilizer bar via the stabilizer link to the spring strut, the torsion in response to body roll motion is equivalent to the total wheel lift from the inside to the outside of the curve (in other suspension setups, the stabilizer bars are connected to a transverse control arm and therefore achieve only a fraction of the torsion angle). Despite being highly effective, this high degree of torsion allows for the stabilizer bar to be made relatively thin which has a favorable effect on driving comfort and dynamics as well as saving weight.

Comparison of front axle technical data

FRONT AXLE TECHNICAL DATA COMPARISON CHART

Cast aluminum spring support (body side)

On the E70, a cast aluminum spring support was used for the first time on the front end of the X Series. This assembly is now also used on the F01/F02. It offers the following advantages:

• Reduced weight through intelligent lightweight construction
• Improved driving dynamics thanks to higher degree of stiffness
• Less components therefore reduced manufacturing expenditure.

The cast aluminum spring support takes up the forces from the chassis and suspension and directs them into the car body. Both the spring strut as well as the upper transverse control arm are secured to the cast spring support.

The component must exhibit a high degree of stiffness for this purpose. This is achieved by optimum material distribution by ensuring material is only accumulated where necessary. The spring support therefore represents an important contribution to controlling driving characteristics as it takes up both static and dynamic wheel forces. Since, with the cast construction, it is possible to integrate many individual functions and components in one single component, compared to the conventional shell construction, this setup is distinctly more compact while making a significant contribution to reducing weight.

• The cast aluminum lightweight construction reduces the weight by approx. 50 % compared to the conventional sheet steel construction
• More useful package space compared to conventional sheet steel construction -80 mm shorter front end
• Function-compliant design with specific local stiffening points adding to lightweight construction
• Integration of various brackets for mounting units etc. in the cast aluminum spring support with add-on parts.

The cast aluminum spring support is connected to the neighboring steel components (e. g. engine support) by means of a rivet-adhesion structure. The structure is of lower weight while making it possible to reduce the number of parts (no additional sheet metal brackets). Nevertheless, the vehicle body is more stable and torsionally rigid while increasing local stiffness. This design arrangement has a positive effect on improved driving dynamics.

Service

NOTE: The camber can also not be adjusted on the double wishbone front axle. As for the E70/E71, two replacement upper transverse control arms are available for the F01/F02 should the camber need to be changed (e. g. after an accident).

These replacement upper transverse control arms enable positive (+5 mm) and negative (-5 mm) correction.

Front axle wheel alignment is required during service when:

FRONT AXLE WHEEL ALIGNMENT REFERENCE CHART - SCREW CONNECTION REPLACED

FRONT AXLE WHEEL ALIGNMENT REFERENCE CHART - SCREW CONNECTION RELEASED

Rear Axle

Highlight in F01/F02

The integral-V rear axle is a revolutionary further development of the integral IV rear axle now installed in many BMW models.

The integral IV rear axle fulfils the primary function of the running gear and wheel control in a unique way while making a significant contribution to driving dynamics characteristic of a BMW.

Safety functions are defined by the superior vehicle control characteristics. Effective decoupling of the road and drive train guarantees outstanding levels of acoustic and vibration comfort.

The further developed integral-V rear axle in the F01/F02 also provides these properties. In addition, the new rear axle has been specifically tuned to the new requirements of the F01/F02:

• Larger vehicle dimensions
• Greater total weight
• Greater drive output
• Higher drive torque
• Runflat tires.

In addition, the demanding objectives relating to driving dynamics and comfort have been correspondingly adapted while the new system integrates driving dynamics systems required for this purpose.

The integral-V rear axle primarily fulfils the driving dynamics functions of the mechanical chassis and suspension, i. e. define elastokinematic wheel control in all relevant driving situations.

The particularly innovative BMW development of the integral active steering (IAL), however, makes specific demands in terms of the elastokinematics of the integral-V rear axle: To a certain extent, the wheels on the rear axle must be able to execute steering movements.

Kinematics and elastokinematics

The spatial arrangement of the pivot points or pivot axes of the arms and links is known as kinematics. This term applies to components that are assumed to be non-deformable.

Elastokinematics takes into account the flexibility at least of the rubber-metal mounts, often of the ball joints and rarely of the components.

Various arms define the horizontal plane of the rear axle wheel suspension at the axle carrier and the wheel carrier. These arms are mounted such that they can rotate about an approximately horizontal axis of rotation and therefore allow vertical movement of the wheel carrier.

Kinematics is primarily of significance in terms of vehicle handling. The kinematics is arranged such that defined camber and toe-in angles are achieved between the wheel and road surface in response to the suspension and steering.

Kinematics is superimposed by elastokinematic effects. These elastokinematic effects occur as the movement points and movement axes are spatially displaced by the effect of the forces at the wheel.

New challenge for the integral rear axle

In terms of the F01/F02, the new integral active steering (IAL) as a BMW driving dynamics innovation, posed a completely new challenge to the engineers and the tried and tested integral IV rear axle. The integral active steering is made up of the active steering and the rear axle slip angle control (HSR).

Fig. 4: Identifying Integral V Rear Axle Components With Integral Active Steering

COMPONENT REFERENCE CHART

1. Actuator, rear axle slip angle control (HSR)
2. Track rod, left
3. Transverse control arm, top
4. Wheel carrier
5. Wheel bearing
6. Integral link
7. A-arm (swinging arm)
8. Thrust strut
9. Rear axle carrier

The principle of the integral-V rear axle makes it possible to resolve the conflict between driving dynamics and comfort. The dynamic and drive forces applied through the wheel contact point into the wheel suspension are taken up by the wheel carrier, rear axle carrier, three links and an A-arm (swinging arm).

Fig. 5: Identifying Integral V Rear Axle Components Without Integral Active Steering

COMPONENT REFERENCE CHART

1. Track rod, right
2. Bearing assemblies, track rod
3. Track rod, left

The design layout reduces the flexible pulling action in the wheel carrier and therefore enables lengthways damping of the wheel control, which is important for rolling comfort, by means of axially soft front link mounts on the rear axle carrier.

Thanks to the position of the spring on the wheel carrier, it is no longer necessary to support the weight of the vehicle on the rubber mounts on the rear axle carrier.

This optimum spring position in conjunction with specific lengthways control guarantees effective isolation of rolling and drive noise while significantly contributing to the refined smooth and quiet vehicle running characteristics.

The main criteria that governed the selection of materials included component weight, production process (cold forming, casting properties, welding properties), strength and deformation characteristics as well as corrosion resistance.

Two versions of the integral-V rear axle are available. Bearing assemblies are fitted on the two track rods if the vehicle is not equipped with integral active steering.

The revolutionary further development of the integral IV rear axle culminates in the BMW patented integral-V rear axle. The new arrangement of the arms and links as well as the use of ball joints facilitates a rear axle with steering capabilities.

Arm arrangement, E65 integral IV rear axle

Fig. 6: View Of Arm Arrangement, E65 Integral IV Rear Axle

INDEX REFERENCE CHART

1. Top view (forward direction x)
2. Bottom side view

1. Angle joint
2. Angle joint
3. Rubber mount
4. Rubber mount
5. Rubber mount
6. Rubber mount
7. Rubber mount
8. Rubber mount
9. Ball joint

Viewing the arrangement of the arms and links in the integral IV rear axle of the E65 it is difficult to imaging that defined steering movement of the rear wheels about the Z-axis could be realized.

Theoretically, i. e. kinematically, the design of the integral IV rear axle could facilitate steering capabilities, however a large actuator would be required that could not be accommodated in the package space available on the F01/F02. This would have to be designed considerably longer and would therefore be decisively heavier and more expensive.

Arm arrangement, integral-V rear axle in the F01/F02

Summary of the design layout:

Fig. 7: View Of Arm Arrangement, Integral-V Rear Axle, F01/F02

INDEX REFERENCE CHART

1. Top view (forward direction x)
2. Bottom side view

1. Ball joint
2. Ball joint
3. Rubber mount
4. Ball joint
5. Rubber mount
6. Rubber mount
7. Rubber mount
8. Rubber mount
9. Ball joint

The system consists of a wheel carrier that is controlled from below by a torsionally rigid A-arm (swinging arm).

At the bottom, the wheel carrier is connected directly by means of a first bearing mount and indirectly by means of a second bearing mount, in connection with an integral link arranged vertically with respect to the plane of the A-arm (swinging arm), to the wheel carrier.

The two rubber mounts on the inside of the vehicle are connected to the rear axle carrier such that they are torsionally soft and can be displaced axially.

The upper transverse control arm lies approximately in the vertical plane of the drive shaft and therefore also at the center point of the wheel.

The rear track rod arranged approximately at the center point of the wheel is either mounted on the rear axle carrier or connected to the actuator of the integral active steering.

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