Rear Semi-Trailing Arm

A Rear Semi-Trailing Arm suspension is included in the Vehicle Library. This suspension is an independent rear suspension system similar to the pure Trailing Arm suspension, with the primary difference being that the bushing axis (or instant axis) runs at an angle in all X, Y, and Z directions. The Semi-Trailing Arm suspension combines the advantages of the trailing arm and double-pivot swing axle suspension principles, without the associated disadvantages. The semi-trailing arms resemble triangulated wishbones, with the pivots arranged at an angle to the vehicle's transverse axis and either horizontal or slightly towards the vehicle's centerline.

rear_semi_trailing_arm_mv

Rear Semi-Trailing Arm Suspension

System Options

The Rear Semi-Trailing Arm suspension 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 what bodies the suspension system attaches to. The Rear Semi-Trailing Arm includes attachments for the trailing arm, the upper link, and the lower link:

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The trailing arm, the upper link, and the lower links attach to the sub-frame by default. When a sub-frame is not present in the model, the Assembly Wizard attaches the trailing arm and both links to the vehicle body. If the vehicle body is not present, for example in a half vehicle model, then the Assembly Wizard will attach them to ground. You can set the attachment for the trailing arm and the upper and lower links using the Attachment Wizard from the Model menu, or by selecting the Rear Semi-Trailing Arm suspension system in the Project Browser and revising the attachments within the Attachments tab on the System/Assembly panel.

Compliant Option

The Rear Semi-Trailing Arm suspension like all Vehicle Library suspension systems includes a Compliant option. When the Rear Semi-Trailing Arm suspension’s Compliant option is set to No, the toe link disappears and the bushings attached to the toe link get deactivated.

Toe Link Option

The Rear Semi-Trailing Arm suspension like all Vehicle Library suspension systems includes a Toe Link option. When the Rear Semi-Trailing Arm suspension’s Toe Link option is set to No, the toe link disappears.

Static Wheel Alignment

The Rear Semi-Trailing Arm suspension 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. 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. In some cases 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 Semi-Trailing Arm suspension. The table below shows the optional subsystems available with the Rear Semi-Trailing Arm 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 Jounce Bumper	External Jounce Bumper
Rebound Bumper	None	Internal Rebound Bumper	External Rebound Bumper

Notes

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

•	External jounce and rebound bumpers act between the lower lateral link 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 bodies to one another in the suspension. The image below shows the principal points for the Rear Semi-Trailing Arm suspension:

rear_semi_trailing_arm_points_mv

Right Side Principal Points – Rear Semi-Trailing Arm Suspension

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 Semi-Trailing Arm suspension is comprised of the bodies shown in the image below:

rear_semi_trailing_arm_bodies_mv

Right Side Bodies – Rear Semi-Trailing Arm Suspension

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 the Rear Semi-Trailing Arm suspension:

Note	The table omits the left side joints for clarity.

Label	Type	Body 1	Body 2	Point
Trailing Arm Wheel Revolute Joint	Revolute	Wheel Hub	Semi-Trailing Arm	Wheel Center
Wheel Hub Fix Joint	Fixed	Wheel	Wheel Hub	Wheel Center
Inner Toe Link Bush	Universal	Toe Link	Vehicle Body	Inner Toe Link
Outer Toe Link Bush	Ball	Toe Link	Semi-Trailing Arm	Outer Toe Link
Trailing Arm Outer Bush	Bush	Semi-Trailing Arm	Vehicle Body	Outer Trail Arm Bushing
Trailing Arm Inner Bush	Bush	Semi-Trailing Arm	Vehicle Body	Inner Trail Arm Bushing

The figure below shows the location of the joints and bushings in the suspension:

rear_semi_trailing_arm_jnts_bushings_mv

Right Side Joints and Bushings: Rear Semi-Trailing Arm Suspension

Use of this Model

The Rear Semi-Trailing Arm 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 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.

•	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.

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, build the model with all of the optional systems (stabilizer bar, etc) included in the model. Immediately turn off the systems via 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_semi_trailing_arm_half_car_mv

Rear-Half-Vehicle Model Employing a Semi-Trailing Arm Suspension

The image below shows the MotionView Project Browser view of the systems in a fully populated rear suspension model. The Rear semitrail arm susp system has five “child” systems.

rear_semi_trailing_arm_browser_mv

Browser view of a Rear-Half-Vehicle Model Systems and Subsystems Employing a Semi-Trailing Arm Suspension