Ford Develops an Innovative Suspension Design Using Adams
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Multibody Dynamics Ford develops an innovative suspension design using Adams By Andreas Carlitz, Sebastien Allibert, Thomas Schmitz, and Axel Engels (Ford Motor Company), Hemanth Kolera (MSC Software) 44 | Engineering Reality Magazine 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 Volume XI XI - - Summer Summer 2020 2020 | | mscsoftware.com | 45 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 4 : Disadvantages of a twist-beam Suspension: toe-out Exploring new suspension designs Ford’s twist-beam suspension basic constructive geometry used to Figure 6 : Standard twist-beam in Adams design overcame these challenges parameterize the model elements. with two innovations. To develop Adams simulations showed a 10 % the innovative suspension system, reduction in toe-out at the turn outer the Ford vehicle dynamics team side. created an Adams Car model with a flexible twist-beam. Adams The first innovation inclines the force Car is a template-based vehicle vectors on the rear spring. During modeling approach built on the cornering the outboard spring is Adams framework. Using Adams compressed, and the inboard spring is Car engineers can build virtual decompressed (Figure 8). prototypes of their vehicle systems or sub-systems and test their Simulations showed that increased performance via a library of vehicle lateral support is available on the events. In the twist-beam suspension turn outer side and decreased lateral model the hub, the front frame and support is available on the turn inner the front lower control arm were all side, thus counteracting the side forces Figure 7 : Force vectoring twist-beam in Adams modeled as flexible bodies. The team that lead to the oversteering effect. used Adams simulations extensively Besides optimizing the inclination in to review design concepts, and the springs, Ford also rethought the again to validate the force vectoring spring design itself. A comparison springs. between the standard cylindrical springs and the force vectoring springs Vehicle models with a standard is as shown in Figure 9. When the force spring and a force vectoring spring vectoring spring is compressed it (Figure 6 and Figure 7) creates a load in the desired direction was tested across a set of and this provides lateral support to the virtual events to compare the twist-beam. The end coil inclination performance. Inclination of the determined by the piercing location spring was achieved by shifting generates the force vector inclination Figure 8 : Force vectoring spring forces the hardpoint. Hardpoints are the (Figure 10). during cornering Figure 3 : Disadvantages of a twist-beam suspension: oversteer effect Figure 5 : Modeling the flexible bodies in Adams 46 | Engineering Reality Magazine The step steer vehicle test (Figure 11) Comparing the standard and the Force vectoring provides the involves imparting a rapid steering force vectoring springs, the Moose opportunity to improve the input while the vehicle is traveling in test variant shows a reduction of performance of the twist-beam a straight line. The goal of this test is the side slip angle step like the suspension without resorting to to gauge the vehicle response time step steer maneuver (Figure 16) measures that affect vehicle weight and any overshoots. and also the yaw rate (Figure 15). or complicate packaging and part These findings signify that a vehicle installation. The performance is like The benefits from the force variant with the force vectoring that of a multilink rear suspension at vectoring spring are evident springs would be more stable as a reduced cost. when the two vehicle variants are compared to the variant with the compared. For the variant with force standard cylindrical springs. These vectoring springs, the phase lag of simulation results confirmed similar the lateral acceleration after the conclusions from vehicle testing. 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 Figure 11 : The step steer test Moose test which is shown in Figure 14. Figures 12 : Lateral acceleration Figures 15 : Yaw rate- moose test Figures 13 : Side slip angle Figures 16 : Side slip single- moose test Figures 9 & 10 Figures 14 : Moose test variant Volume XI - Summer 2020 | mscsoftware.com | 47 .