Forces at the Wheel
The forces acting on the contact surface between the tire and the road are also
subdivided into the three main directions.
The vertical force is fundamental. This acts vertically to the road and corresponds to the
load on the tire. The maximum transferable lateral and tangential tire forces are the prod-
uct of the vertical force and the adhesion coefficient.
The radius of
the Kamm's cir
cle shows this mathematical relationship graphically. It is
also possible to see the dependency between the tangential and lateral forces in the
Kamm's circle.
5
E70 Chassis Dynamics
z
x
y
K
F
RE
F
RR
F
S
F
U
F
V
TF04-5454
Index Explanation Index Explanation
K
Kamm’s cir
cle
F
v
V
ertical tire force
F
u
Tangential tire force
F
RE
Resulting force on surface
F
s
Lateral tire force
F
RR
Resulting force in space
Explanation of the Kamm's Circle using an Example
If a lateral tire force is acting on the wheel, a braking or accelerating force (tangential tire
force) can only build up in a longitudinal direction up to the maximum total force (resulting
force on surface). When this is reached, the wheel locks or spins.
Conversely, only a limited lateral cornering force (lateral tire force) can be achieved under
braking. If this is exceeded, the wheel slips in a lateral direction. This causes the vehicle
to skid. If a braking force takes effect, the full lateral cornering force can be established in
accordance with the radius of the Kamm's circle.
In the same way, the full braking or acceleration force can be established when the
vehicle is driving straight ahead (again according to the radius).
This relationship shows that acceleration or braking that is too rapid under cornering can
cause the vehicle to skid, as any longitudinal force on the wheel, whether it serves to
accelerate or brake, inevitably results in a failure of the lateral cornering forces.
The radius of the Kamm's circle depends on the friction coefficient between the tire and
the road, i.e. on the tire, the road surface and the road conditions. If the road is wet, for
example, the radius is considerably smaller than if the road is dry.
Interrelationships between the effects of the dynamic driving systems
The possible effectiveness of modern dynamic driving systems is based only on the
interrelationship between the tires and the road.
In order to classify and differentiate between the many systems in the E70, they are
described in three separate Reference Information documents:
• E70 longitudinal dynamics systems - The following dynamic driving systems act on
the tangential wheel forces:
– ABS
– ASC
– DSC
– MSR
These influence the translatory (longitudinal) movement along the x axis and the rotational
movement about the y axis.
• E70 lateral dynamics systems - Lateral wheel forces are primarily generated by the
steering angle, i.e. they are influenced by the power steering and Active Steering on
the front axle. The most significant effect occurs as a rotational movement about the
z axis.
• E70 vertical dynamics systems - The following essentially act upon the vertical
wheel forces and the wheel contact forces:
– VDC
– EHB
– ARS
6
E70 Chassis Dynamics
This affects the translatory movement along the z axis and, depending on the system, the
rotational movement about the x axis for ARS or about the y axis for EHC.
Furthermore, the rotational movement about the z axis due to altered wheel contact
forces is also influenced by ARS (actual dynamic significance of the anti-roll bar).
The complexity of the relationships and the reciprocal influencing of the tire forces and
therefore the vehicle movement should be made clearer by the following graphic.
7
E70 Chassis Dynamics
Index Explanation Index Explanation
1
Lateral tire forces/lateral dynamics
B
Ride comfort
2
Vertical tire forces/vertical dynamics
C
Traction
3
Tangential tire forces/longitudinal dynamics
D
Safety when braking and accelerating
A
Handling
Through intelligent design layout and optimum package space utilization on the E70,
the basis has been created for distinctly increasing the driving dynamics while improving
comfort and vehicle handling. At virtually identical wheel loads, a greater track width and
a larger wheelbase have been realized compared to the predecessor, the E53.
While essentially retaining the same center of gravity, the best prerequisites have been
created for meeting the target "Best in segment" with the new chassis and
suspension of the E70.
E70 Chassis and Suspension
8
E70 Chassis Dynamics
Index Explanation Index Explanation
1
Cent
er of gravity
3
Wheelbase
2
Track width, front
Comparison
9
E70 Chassis Dynamics
E53 E70
Front axle
Double pivot spring strut front axle Double wishbone front axle
Suspension/damping, front
Steel spring or air spring Steel spring
Anti-roll bar, front
Mechanical Mechanical or Hydraulic
Rear axle
Integral IV Integral IV
Suspension/damping, rear
Steel spring or air spring Steel spring or air spring
Anti-roll bar, rear
Mechanical Mechanical or Hydraulic
Brake, front
Brake disc diameter up to 356 mm Brake disc diameter up to 365 mm
Brake, rear
Brake disc diameter up to 324 mm Brake disc diameter up to 345 mm
Parking brake
Drum brake, mechanical
Drum brake, with electro-mechanical
parking brake (EMF)
Wheels/tires
Standard tires Run flat tires
Steering
Power steering or Servotronic Power steering or active steering
Track Width, General
The size of the track width at the front and rear has a decisive influence on the cornering
characteristics of the vehicle and its tendency to roll.
• The track width should be as large as possible, however, it cannot exceed a defined
value in relationship to the width of the vehicle.
• The fully deflected (spring compressed) wheel turned at full lock on the front axle
must not scrape or snag in the wheel arch cutout.
• A certain degree of clearance for fitting snow chains is required on the drive axle
(irrespective of whether this is the front, rear or both axles).
• The wheels must not make contact with any chassis or body parts when the
suspension springs fully compress and rebound.
Wheelbase, General
The wheelbase -measured from the center of the front axle to the center of the rear axle
has a decisive influence on the vehicle handling properties.
A large wheelbase compared to the length of the vehicle permits favorable accommoda-
tion of the vehicle occupants between the axles and reduces the influence of the vehicle
load on the overall load distribution. Short body overhang at the front and rear reduces
the pit
ching tendency.
A short wheelbase, on the other hand, provides favorable cornering characteristics, i.e. a
smaller turning circle at the same steering lock angle.
The outstandingly balanced values on the E70 result in safe, superior and agile vehicle
handling characteristics that represent the standard in the SAV class (SAV = Sports
Activity Vehicle) also for the future. These technical data are the prerequisite for achieving
the top position in its class. In terms of driving dynamics, the E70 will assume a leading
position without forfeiting driving and rolling comfort compared to the competition (with
comparable equipment).
10
E70 Chassis Dynamics
E53 E70
Unladen weight (kg)
2070 kg 2085 kg
Center of Gravity
678 mm 680 mm
Track width, front
1576 mm 1644 mm
Track width, rear
1576 mm 1650 mm
Wheelbase
2820 mm 2933 mm
Chassis and Suspension Overview
Front Axle
For the first time on a BMW vehicle, a double wishbone front axle is used on the E70.
The outstanding driving dynamics, the excellent driving comfort as well as the stable
straight-ahead running properties are factors of this design solution that contribute to a
high degree of driving pleasure and safety while making the vehicle ideal for every day
use and pr
oviding the most relaxing drive on long journeys.
Rear Axle
Compared to the E53, the further-developed integral IV rear axle in the E70 is character-
ized by further
improved driving dynamics without compromising comfort and driving
safety. This axle design on the E70 has made it possible to increase the width and depth
of the load area.
The result is a considerably larger and more functional load space (third row of seats)
particularly through the use of the single-axle air spring (rear axle air suspension). This
design layout guarantees brilliant road handling characteristics irrespective of the vehicle
load and at a constant ride height.
11
E70 Chassis Dynamics
Index Explanation Index Explanation
1
Spring/damper
4
Steering
2
Rear axle
5
Brakes
3
Wheels/tires
6
Front axle
Dampers/Suspension
In the E70, the range of spring/damper units extends from steel springs with conventional
dampers through to the new vertical dynamic control (VDC) that, in addition to the elec-
tronically controlled dampers, also allows a combination of a 1-axis air spring on the rear
axle. This 1-axis air spring is compulsory on vehicles with 8-cylinder engines and/or a
third row of seats.
Brakes
The brake system installed on the E70 is a further-developed high performance brake
system with newly adapted dimensions for the E70. The service brake is based on the
conventional design while in contrast to the E53 the parking brake features an
electro-mechanical parking brake system (EMF).
Steering
The E70 is available with two steering system variants:
• Hydraulic power steering
• Active steering (AL)
Both steering systems are adapted to the diverse and varied possible applications of the
E70 and the active steering is used for the first time in an all-wheel drive vehicle.
Wheels and Tires
The E70 is the first all-wheel drive vehicle (X-family) that is equipped with a run flat safety
package as standard.
12
E70 Chassis Dynamics
General
The chassis and suspension system is divided into the following main components:
• Front axle
• Rear axle
• Damping/suspension
• Brakes
• Steering
• Wheels/tires
Front Axle
13
E70 Chassis Dynamics
Index Explanation
1
Ride-height sensor
2
Mount
3
Spring strut
4
A-arm, top
5
Spring strut support
6
Swivel bearing
7
Wheel bearing
8
Stabilizer link
9
Tension strut with
hydraulic mount
10
Contr
ol arm, bottom
11
Spring strut fork
12
Anti-roll bar
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 McPherson
strut axle.
Components with special materials (see graphic on previous page):
• The forged aluminum swivel bearing (6) with the 3rd generation wheel bearing (7)
Semi-trailing arm connected via steel bushes/tapered screw connection to the swivel
bearing. Attention: Refer to special repair instructions!
• The A-arm at the top (4) is made from forged aluminum and the cylindrical joint pin
is clamped in the swivel bearing (6).
• Tension strut with hydraulic mount (9) and bottom control arm (10) are forged steel
components while the bottom control arm bears the spring strut (3) by means of the
cast steel spring strut fork (11).
• The front axle subframe is a welded steel structure with an aluminum thrust panel for
maximum lateral stiffness with service openings.
14
E70 Chassis Dynamics
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