Front SLA (2pc LCA)

A Short-Long Arm (SLA) suspension configuration is also known as a Double Wishbone suspension. The SLA name is derived from the length of the control arms. The lower arm is typically long to provide a good spring lever ratio. The upper arm is typically short to provide the proper camber curve. This suspension is widely used on cars, light trucks, and on independent suspension heavy trucks. The two-piece lower control arm is the only difference between the two front SLA suspensions.

Figure 1. Front SLA Suspension with Two (2) Piece Lower Control Arm and Optional Coil Spring, Shock Absorber, and External Jounce Bumper Subsystems


In conventional designs the coil spring (or optionally a torsion bar), shock absorber, and jounce bumper act between the lower control arm and chassis. The lower control arm carries most of the load and defines the motion of the lower ball joint location on the knuckle. The upper control arm carries smaller loads and defines the motion of the upper ball joint location on the knuckle. The steering motion of the knuckle is controlled by the tie rod and the motion of the steering system. In the vehicle library the steering systems contain the tie rod.

Model Use

The SLA system can be used in either a half car or a full vehicle analysis. The default geometry and mass is that of a passenger car or light truck, however the model and data can be revised to reflect any size vehicle, from a large truck to a scale model car.
Note:
  • The wheel body represents the mass and inertia of the tire and the rim.
  • The wheel hub body represents the mass and inertia of other rotating bodies such as a brake rotor, but not the half-shafts if the suspension is driven. The wheel hub and brake rotor have no associated graphics.
  • The wheel and wheel hub parts use the Wheel CG location as the center of gravity.
  • The upper control arm bushings are defined so their axes are parallel. If the front bushing is moved, both the front and rear bushing realign so they have the same axis of rotation. The lower control arm bushings’ rotational axes are defined in the same manner.
  • When the suspension is switched from Compliant to Kinematic, the upper control arm bushings are replaced with a single revolute joint located at the forward bushing, with a rotational axis directed along the line from the forward bushing to the rear-ward bushing. The lower control arm bushings are replaced with a revolute joint in the same fashion.
  • Each body’s Center of Gravity (CG) is estimated from the body’s geometry. The formulas are coded into the point panel and can be seen via the graphical user interface. If more accurate CG locations are available they should be used.
  • The point giving the location of the ball joint between the suspension knuckle and steering tie rod point (outer tie rod ball) resides in the steering system.
Tip:
  • A wide variety of combinations of suspensions and subsystems can be built using the Assembly Wizard. You are encouraged to build systems and understand the resulting model using the graphical user interface.

    When building a new suspension model, build the model with all of the optional systems (stabilizer bar, etc) included in the model. Immediately turn off the systems using the Project Browser and run an analysis on the base suspension to ensure it solves properly. As data becomes available for the optional systems; activate those systems and populate them with data.

Figure 2. Front-Half-Vehicle Model Employing a SLA Suspension with Two (2) Piece Lower Control Arm


The image below shows the Project Browser view of the systems in a fully populated front suspension model. The Frnt SLA susp (2 pc LCA) system has five “child” systems.
Figure 3. Browser view of a Front-Half-Vehicle Model Systems and Subsystems Employing a SLA Suspension with Two (2) Piece Lower Control Arm


Points

Points locate the joints and bushings that connect the suspension bodies to one another. The image below shows the principal points for the front SLA (2pc LCA) suspension :
Figure 4. Right Side Principal Points – Front SLA Suspension with Two (2) Piece Lower Control Arm


Note: The image above omits the left side of the suspension, points locating body centers of mass, and points that locate the optional subsystems (springs, dampers, bump stops and stabilizer bar) for clarity.

Bodies

The front SLA (2pc LCA) suspension is comprised of the bodies shown in the image below:
Figure 5. Right Side Bodies – Front SLA Suspension with One (1) Piece Lower Control Arm


Note: The wheel hub body has no associated graphics and therefore is not visible in the image above.

Optional subsystems may add bodies to the suspension, for example the shock absorber adds two bodies: a shock rod and shock tube.

Bushings and Joints

The table below describes the bodies, bushings, and joints for a front SLA (2pc LCA) suspension:
Note: The table omits the left side joints for clarity.
Label Type Body 1 Body 2 Point Notes
Lower Ball Joint Spherical Knuckle Lower Control Arm Lower Ball Joint
Upper Ball Joint Spherical Knuckle Upper Control Arm Upper Ball Joint
Wheel Spindle Revolute Wheel Hub Knuckle Wheel Center
Wheel Hub

Fix Jt

 Fixed Joint Wheel Wheel Hub Wheel Center When Spindle compliance is set to Yes, the joint type changes to universal and a bushing is superimposed to model the wheel bearing compliance.
Tension Strut Bush Universal Tension Strut Subframe, Vehicle Body, or Ground Tension Strut Bush When the Compliant option is set to No, this joint compliance is turned "Off" and behaves as a pure universal joint.
LCA Bushing Bushing Lower Control Arm Subframe, Vehicle Body, or Ground LCA Rear Bush When the Compliant option is set to No, this joint compliance is turned "Off" and behaves as a pure universal joint.
UCA Front Bush Bushing Upper Control Arm Upper Control Arm UCA Front Bush When the Compliant option is set to No, this bushing becomes a revolute joint.
UCA Rear Bush Bushing Upper Control Arm Subframe, Vehicle Body or Ground UCA Rear Bush When the Compliant option is set to No, this bushing is deactivated.
The following image shows the location of the joints and bushings in the suspension:
Figure 6. Right Side Joints and Bushings: SLA Suspension with Two (2) Piece Lower Control Arm


Similar Suspensions

Front SLA (1pc LCA)

Rear SLA (1pc LCA)

Rear SLA (2pc LCA)