44 | Engineering Reality Magazine

Ford develops an innovative suspension

design using Adams

Multi b ody Dynamics

By Andreas Carlitz, Sebastien Allibert, Thomas Schmitz, and Axel Engels (Ford Motor Company), Hemanth Kolera (MSC Software)

44 | Engineering Reality Magazine

Volume XI - Summer 2020 | mscsoftware.com | 45

The design of a vehicle suspension system influences the customer’s perception of vehicle handling and ride comfort. The art of suspension design deals with making the right tradeoffs and compromises between handling and comfort. As an example, lowering the center of gravity helps handling but also reduces the ground clearance. This in turn limits suspension travel, requiring stiffer springs and hurting ride comfort.The Ford motor company is focused on providing its customers with a vehicle that is world-class in both drivability and comfort.

These critical vehicle attributes are in turn impacted by the suspension design. Ford recently invented and patented a new twist-beam rear suspension system which received widespread media acclaim. In its review of the Fiesta ST in July 2018, AutoCar UK said, “Perhaps it’s to do with the bent ‘force vectoring’ springs, but during the compression stroke the rear seems to help the car pivot through the bend. It’s a sudden but subtle effect and gives the car stunning agility”. In September 2018, in its review of the Ford Focus Top Gear called it the best drive in class and said, “Focus feels agile, pointy, dexterous and actually quite playful”. Adams vehicle dynamics simulation played an integral role in the development of the novel suspension design.

A twist-beam suspension is a semi-independent suspension system that is most often used on the rear wheels. It combines the effects of a dependent and an independent suspension. It allows individual wheels to twist similar to an independent suspension but also permits a wheel to have some effect on another like a dependent suspension.

The twist-beam suspension consists of two trailing arms attached to the chassis and the wheels. Connecting the arms is a torsion beam, forming a typical H-shaped suspension architecture, Figure 1. The front of the H connects to the car body via the rubber bushings, Figure 2. When a wheel undergoes an impact, the beam will twist and some of the shock is absorbed, reducing its transmission to the opposite wheel. The twist-beam suspension provides several advantages as compared to a multi-link suspension such as efficient packaging, lower weight and cost effectiveness. However, it comes with a few disadvantages that can affect customer ride and comfort.

As seen in Figure 2, the wheel center in a twist-beam suspension resides below the bushing. This results in the wheel moving forward with a bounce as the vehicle travels. Vehicles with twist-beam suspension also exhibit an oversteer effect where the rear of the car drifts outwards while cornering (Figure 3). The oversteer effect during cornering is caused by the lateral forces on the tyre creating a toe-out effect where the front of the wheels are farther apart from the rear. (Figure 4)

Past solutions to correct these issues have involved complex reinforcements or an additional Watt’s linkage. This drives up costs, increases weight and results in NVH issues. Modifications such as inclining the bushing attachment to the vehicle body to reduce the toe-out angle result in increased lateral compliances and less agility.

Volume XI - Summer 2020 | mscsoftware.com | 45

46 | Engineering Reality Magazine

Exploring new suspension designs

Ford’s twist-beam suspension design overcame these challenges with two innovations. To develop the innovative suspension system, the Ford vehicle dynamics team created an Adams Car model with a flexible twist-beam. Adams Car is a template-based vehicle modeling approach built on the Adams framework. Using Adams Car engineers can build virtual prototypes of their vehicle systems or sub-systems and test their performance via a library of vehicle events. In the twist-beam suspension model the hub, the front frame and the front lower control arm were all modeled as flexible bodies. The team used Adams simulations extensively to review design concepts, and again to validate the force vectoring springs.

Vehicle models with a standard spring and a force vectoring spring (Figure 6 and Figure 7) was tested across a set of virtual events to compare the performance. Inclination of the spring was achieved by shifting the hardpoint. Hardpoints are the

basic constructive geometry used to parameterize the model elements. Adams simulations showed a 10 % reduction in toe-out at the turn outer side.

The first innovation inclines the force vectors on the rear spring. During cornering the outboard spring is compressed, and the inboard spring is decompressed (Figure 8).

Simulations showed that increased lateral support is available on the turn outer side and decreased lateral support is available on the turn inner side, thus counteracting the side forces that lead to the oversteering effect.Besides optimizing the inclination in the springs, Ford also rethought the spring design itself. A comparison between the standard cylindrical springs and the force vectoring springs is as shown in Figure 9. When the force vectoring spring is compressed it creates a load in the desired direction and this provides lateral support to the twist-beam. The end coil inclination determined by the piercing location generates the force vector inclination (Figure 10).

Getting ready for the road

To test the performance of the twist-beam suspension the team built full vehicle models in Adams Car with the standard spring and force vectoring spring variants. The vehicle models were tested on two key vehicle events; the step steer and double lane change (Moose Test).

Figure 1 : A Twist-beam Suspension Top View

Figure 2 : A twist-beam suspension side view

Figure 6 : Standard twist-beam in Adams

Figure 8 : Force vectoring spring forces

Figure 7 : Force vectoring twist-beam in Adams

Figure 4 : Disadvantages of a twist-beam

Figure 3 : Disadvantages of a twist-beam

Figure 5 : Modeling the flexible bodies in Adamssuspension: oversteer effect

Suspension: toe-out

during cornering

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The step steer vehicle test (Figure 11) involves imparting a rapid steering input while the vehicle is traveling in a straight line. The goal of this test is to gauge the vehicle response time and any overshoots.

The benefits from the force vectoring spring are evident when the two vehicle variants are compared. For the variant with force vectoring springs, the phase lag of the lateral acceleration after the transient steering input is reduced (Figure 12). Additionally, due to the reduced lateral compliance, the side slip angle is smaller as well (Figure 13).

The double lane change test is used, which is used to evaluate the stability of the vehicle and the agility of its dynamic response. In this case, the simulation is based on a Double Lane Change variant called the Moose test which is shown in Figure 14.

Comparing the standard and the force vectoring springs, the Moose test variant shows a reduction of the side slip angle step like the step steer maneuver (Figure 16) and also the yaw rate (Figure 15). These findings signify that a vehicle variant with the force vectoring springs would be more stable as compared to the variant with the standard cylindrical springs. These simulation results confirmed similar conclusions from vehicle testing.

Figure 11 : The step steer test

Figures 9 & 10

Figures 13 : Side slip angle

Figures 12 : Lateral acceleration Figures 15 : Yaw rate- moose test

Figures 16 : Side slip single- moose test

Figures 14 : Moose test variant

Force vectoring provides the opportunity to improve the performance of the twist-beam suspension without resorting to measures that affect vehicle weight or complicate packaging and part installation. The performance is like that of a multilink rear suspension at a reduced cost.