Rear SLA (2pc LCA)

A Short-Long Arm or SLA suspension is included in the vehicle library as both a front and rear suspension. This configuration is also known as a Double Wishbone suspension. This variant of the suspension uses a two-piece lower control arm. 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.

Figure 1. Rear 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 rear SLA suspension differs from the front SLA suspension in that it contains a toe link (which is shown in the image above).

Model Use

The rear SLA (2pc LCA) suspension 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 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 point at each other (like hinges on a door). If the rear bushing is moved, both bushings realign so that they have the same axis of rotation. The lower control arm bushings’ rotational axes are defined in the same manner.
• When the suspension is built as a kinematic suspension (using the option menu in the System/Assembly panel), the upper control arm bushings are replaced with a single revolute joint located at the forward bushing location, with a rotational axis directed along the line from the forward bushing to the rearward 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.

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. Rear-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 rear suspension model. The Rear SLA susp (2pc LCA) system has five “child” systems.

Figure 3. Browser View of a Rear-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 rear SLA (2pc LCA) suspension:

Figure 4. Right Side Principal Points – Rear SLA Suspension with a 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 rear SLA (2pc LCA) suspension is comprised of the bodies shown in the image below.

Figure 5. Right Side Bodies – Rear SLA Suspension with Two (2) Piece Lower Control Arm

Note:

• The wheel hub body has no associated graphics and therefore is not visible in the image above.
• Only right side bodies are shown (as the left side bodies are symmetrical), and the coil spring has been omitted from the image above for clarity.

Bushings and Joints

The table below describes the bodies, bushings, and joints for a rear 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 Bushing	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	Subframe, Vehicle Body or Ground	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.
Inner Toe Link Bush	Universal Joint	Toe Link	Subframe, Vehicle Body or Ground	Inner Toe Link Bush	When the Compliant option is set to No, this joint compliance is turned "Off" and behaves as a pure universal joint.
Outer Toe Link Bush	Spherical Joint	Toe Link	Knuckle	Outer Toe Link Bush	When the Compliant option is set to No, the compliance for this joint is turned "Off" and behaves as a pure spherical joint.

The following image shows the location of the joints and bushings in the suspension:

Figure 6. Right Side Joints and Bushings: Rear SLA Suspension with Two (2) Piece Lower Control Arm