Double Pivot Spring Strut Front Axle

Fig 1: Identifying Double Pivot Spring Strut Front Axle

Courtesy of BMW OF NORTH AMERICA, INC.

The double pivot system establishes a geometrical "axis" that is formed by the suspension links, which in turn create pivot points, one upper and two lower. Two lower pivot points (double pivot) established by the control arm and tension strut create a "imaginary" pivot point, that is extended further out on the wheel carrier. This design allows pivot points on the wheel carrier to be selected in order to effectively accommodate larger brakes.

Fig 2: Locating Double Pivot Spring Strut Front Axle/Determining Lower Pivot Point

Courtesy of BMW OF NORTH AMERICA, INC.

Additional advantages of double pivot system are:

Ability to reduce body roll while cornering.
Reduces front end dive tendencies during severe braking situations.
Utilization of a small positive steering offset, which offers improved handling if friction levels while braking is different on both wheels.
Improved caster position, improving straight line stability at higher speeds, plus better steering return after small steering inputs.

On the double pivot spring strut front axle the position of the control arm and tension strut, with respect to each other, determines the size of the Steering/Kingpin Offset (Steering Roll Radius) as well as the Steering Axis Inclination/kingpin angle.

The Steering Offset (R0) is the distance between the projected line of contact for the steering axis at the road surface and the center line of a tires contact patch at the road surface.

Fig 3: Measuring Tire Center Line

Courtesy of BMW OF NORTH AMERICA, INC.

A positive Steering Offset (R0) exists when the steering axis line is inside the center line of the tires contact patch.
A negative Steering Offset (R0) exists when the steering axis line is outside the center line of the tires contact patch.
A zero Steering Offset (R0) exists when the steering axis line is exactly on the center line of the tire contact patch.

NOTE: When using the double pivot spring strut front axle, a slightly positive Steering Offset is defined.

Fig 4: Measuring Steering Axis Inclination

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 5: Identifying Components Of E90 Front Axle

Courtesy of BMW OF NORTH AMERICA, INC.

Both tension struts are mounted with hydraulic mounts in the front axle carrier.

The greater the distance of the tension strut and control arm pivot points at the swivel bearing the greater the steering "reset" force, force required to return wheels to straight head/center position.

TECHNICAL DATA (FRONT AXLE)

Explanation	Value	Explanation	Value
Total Toe-in	14'	Steering/Kingpin Offset	6,1 mm
Track Width	1500 mm	Wheel lock	41° 5'
Camber	-18'	Wheel lock, outer	33° 18'
Kingpin/Steering Axis Inclination	14° 7'	Offset	34 mm
Caster Angle	7° 5'	Tire Size	205/5 R 16
Caster	19.8 mm