Rear Twist Beam

A rear twist beam suspension is also known as “torsion beam axle” suspension. They are named twist axles, because the axle must twist when the vehicle rolls. The beam holds the two trailing arms together, and provides the roll stiffness of the suspension by twisting as the two trailing arms move relative to each other. This suspension is widely used on small inexpensive passenger cars.



Figure 1. Rear Twist Beam Suspension
There are three types of twist beam suspension systems. The difference is in the location of the cross beam:
  • Beam at the bushing center.
  • Beam partway between the bushings and the wheel center.
  • Cross beam at the wheel centers.

The figure shown above is a rear wheel twist beam suspension system of the second type. The model can be easily modified to represent all three types of axles.

Model Use

The rear twist beam 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 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.
  • The model contains beams to represent the twist portion of the axle. This is a good representation of the axle early in a vehicle program. As a design is refined, the axle twist should be represented by a flexible body.
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 Twist Beam Suspension and a Powertrain
The image below shows the Project Browser view of the systems in a fully populated rear suspension model. The Rear twistbeam susp system has five “child” systems.


Figure 3. Browser View of a Rear-Half-Vehicle Model Systems and Subsystems Employing a Twist Beam Suspension

Attachments

Attachments determine how the suspension system attaches to the remainder of the vehicle system. The rear twist beam includes attachments for the trailing arm and the optional track bar:
Entity Attaches To
Trailing Arm, Track Bar Default: Sub-frame

No subframe: Vehicle body

No vehicle body: Ground
You can also set any attachments using the Attachment Wizard, or by selecting the subsystem in the Project Browser and revising the attachments on the System/Assembly panel.

Points

Points locate the joints and bushings in the suspension. The image below shows the principal points for the rear twist beam suspension:


Figure 4. Right Side Principal Points – Rear Twist Beam 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 twist beam suspension is comprised of the bodies shown in the image below:


Figure 5. Right Side Bodies – Rear Trailing Arm Suspension
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 the rear twist beam suspension:
Note: The table omits the left side joints for clarity.
Label Type Body 1 Body 2 Point Notes
Wheel Spindle Joint Revolute Wheel Hub Knuckle Wheel Center  
Wheel Hub Fixed Joint Fixed Wheel Wheel Hub Wheel Center When the Spindle compliance option is set to Yes, the joint type changes to universal.
Trailing Arm Bush Revolute Knuckle Vehicle Body Trail Frame  
The following image shows the location of the joints and bushings in the suspension:


Figure 6. Right Side Joints and Bushings: Rear Twist Beam Suspension

Beams

The rear twist beam suspension has two beams that simulate the behavior of a single beam in the suspension. Two beams are used so that the shear center can be defined differently from the area centroid. Beams shaped in a U typically require the shear center to be different from the area centroid. If a single beam is desired, delete one of the beams and enter in complete beam properties.


Figure 7. Beams used in the Rear Twist Beam Suspension