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 (LCA). 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.

rear_sla_2pc_lca_mv

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).

System Options

The rear SLA suspension with a two piece lower control arm offers a standard set of attachments, options, and properties that you can set by selecting the suspension system in the Project Browser to display the System/Assembly panel.

Attachments

Attachments determine how the suspension connects to the remainder of the model. The SLA suspension with two (2) piece lower control arm includes attachments for the lower control arm (LCA), upper control arm (UCA), toe link, and tension strut:

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The lower control arm attaches to the sub-frame by default (if a sub-frame is present). When a sub-frame is not present in the model, the Assembly Wizard attaches the lower control arm to the vehicle body. If the vehicle body is not present, for example in a half vehicle model, then the Assembly Wizard will attach the lower control arm (LCA) to ground. You can set the attachment for the lower control arm using the Attachment Wizard or by selecting the Rear SLA (2pc LCA) suspension system in the Project Browser and revising the attachments within the Attachments tab on the System/Assembly panel.

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The upper control arm (UCA) to the vehicle body by default. If the vehicle body is not present, then the Assembly Wizard will attach the upper control arm (UCA) to ground. You can set the attachment for the upper control arm using the Attachment Wizard or by selecting the Rear SLA (2pc LCA) suspension system in the Project Browser and revising the attachments within the Attachments tab on the System/Assembly panel.

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The toe link attaches to the sub-frame by default. When a sub-frame is not present, then the toe link attaches to the vehicle body. When a vehicle body is not present, for example in a half vehicle model, the Assembly Wizard attaches the toe link to ground. The toe link attachment can be set to any body using the Attachment Wizard or by selecting the Rear SLA (2pc LCA) suspension in the Project Browser and revising the attachments within the Attachments tab on the System/Assembly panel.

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The tension strut attaches to the sub-frame by default. When a sub-frame is not present in the model, the Assembly Wizard attaches the tension strut to the vehicle body. If the vehicle body is not present, for example in a half vehicle model, then the Assembly Wizard attaches the tension strut to ground.

Compliant Option

The Rear SLA (2pc LCA) suspension system includes a Compliant option. When the SLA suspension’s Compliant option is set to No, the upper control arm bushings are replaced with revolute joints. To set the value of the Compliant option, select the Rear SLA (2pc LCA) suspension in the Project Browser and click the Options tab on the System/Assembly panel.

When the Compliant option is set to No, both of the upper control arm bushings are replaced with a single revolute joint located at the front bushing of each arm, with a rotational axis directed along the line from the front bushing to the rear bushing.

Spindle Compliance Option

The Rear SLA (2pc LCA) suspension system includes a Spindle compliance option. When the Spindle compliance option is set to Yes, the wheel body connects to the wheel hub via the universal joint and bushing. The universal joint allows camber and toe deflection of the wheel relative to the wheel hub. The wheel hub bushing’s Kx and Ky rates determine the amount of spindle compliance introduced. To set the Spindle compliance option, select the Rear SLA (2pc LCA) suspension in the Project Browser and click the Options tab on the System/Assembly panel.

Static Wheel Alignment

The Rear SLA (2pc LCA) suspension system includes a static alignment dataset and form holding toe and camber variables that determine the orientation of the wheel relative to the knuckle and body in the Global coordinate system. The toe and camber variable values control the location of the spindle align point relative to the wheel center point through parametric expressions. You can view these expressions by selecting the spindle align point and examining its X, Y, and Z locations.

To set the values for toe and camber, select either the static alignment dataset or form and alter the values of the toe and camber variables. It is important to note that the left and right wheels’ toe and camber values must be symmetric. You cannot set the toe and camber angles independently for the two wheels.

Subsystem Options

Select options using the Assembly Wizard to include stabilizer bar, spring, shock-absorber, jounce bumper, or rebound bumper subsystems in the model. You can also select where the subsystem acts, for example whether the rebound bumper is internal to the shock absorber or external.

When you finish creating your model using the Assembly Wizard, the subsystems you have chosen show in the Project Browser as children of the Rear SLA (2pc LCA) suspension. The table below shows the optional subsystems available with the SLA suspension:

Subsystem

Options

Stabilizer Bar	None	Stabilizer Bar with Links	Two (2) Piece Bar with Links
Spring	None	Coil Springs	----
Shock Absorber	None	Two (2) Inline Joints	One (1) Cylindrical Joint
Jounce Bumper	None	Internal to Shock	External Jounce Bumper
Rebound Bumper	None	Internal to Shock	External Rebound Bumper

Notes

•	Coil springs act between the lower control arms (LCA) and the vehicle body. The SLA suspension system contains points for the upper and lower spring seats to locate coil springs.

•	Internal jounce and rebound bumpers act between the shock absorber piston and tube.

•	External jounce and rebound bumpers act between the lower control arm and the vehicle body.

•	Suspensions created without a spring or a shock absorber subsystem will not function in a full vehicle analysis.

•	You can alter how optional subsystems attach to the suspension by selecting the subsystem in the Project Browser and altering the subsystem’s attachments.

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 suspension with a two piece lower control arm:

rear_sla_2pc_lca_points_mv

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 suspension with two (2) piece lower control arm is comprised of the bodies shown in the image below.

rear_sla_2pc_lca_bodies_mv

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.

Optional subsystems may add bodies to the suspension, for example the shock absorber adds two bodies: a shock rod and shock tube. Any bodies added by optional subsystems have been omitted from the image above for clarity.

Bushings and Joints

The table below describes the bodies, bushings, and joints for a Rear SLA suspension with a two (2) piece lower control arm (LCA):

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 the Spindle compliance option is set to Yes, the joint type changes to universal.
Tension Strut Bushing	Universal	Tension Strut	Subframe, Vehicle Body, or Ground	Tension Strut Bush	When the Compliant option is set to No, the compliance for this joint 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, the compliance for this joint 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, the 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, the compliance for this joint 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:

rear_sla_2pc_lca_jnts_bushings_mv

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

Use of this Model

The Rear SLA 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.

Notes

•	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. There are no graphic entities for the wheel hub and brake rotor.

•	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.

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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.

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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 viewed using the graphical user interface. If exact CG locations are available (from a solid model or testing), they should be entered as X, Y, Z coordinates instead of the estimated number.

Tips

•	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.

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When building a new suspension model, be sure to 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.

rear_sla_2pc_lca_half_vehicle_mv

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

The image below shows the MotionView Project Browser view of the systems in a fully populated rear suspension model. The Rear SLA susp (2pc LCA) system has five “child” systems.

rear_sla_2pc_lca_browser_pic

Browser view of a Rear-Half-Vehicle Model Systems and Subsystems Employing a SLA Suspension with Two (2) Piece Lower Control Arm

Similar Suspensions:

•	Front SLA (1pc LCA)

•	Front SLA (2pc LCA)

•	Rear SLA (1pc LCA)