Sometimes when engineers fix one problem, they end up with another.
That was the case at French carmaker Renault. Engineers designing the new Vel Satis luxury car found that the large interior and high center of gravity posed problems of stability and handling. They solved that problem by redesigning the MacPherson front suspension, strengthening 80% of its components, and developed a new multi-armed rear suspension. Problem solved? Yes and no.
The Vel Satis uses the same platform as Renault's Espace and Laguna. Engineeers did extensive computer simulations to optimize the suspension to improve the ride.
Their solution resulted in a rough ride for passengers when the car went over bumps, so they went back to their computer screens and their suite of software (Nastran and ADAMS, from MSC.Software, and PAM-CEM, from ESI).
Designed to compete with the Mercedes E-Class, the BMW 5-series, and the Audi A6, the Vel Satis (which is not available in the U.S.) has a long hood, a windshield that extends up into the roof, and a compact back end with a panoramic rear window. It's built on the same platform as the Renault Espace and Laguna.
"When we created this new structure on a common platform, we had to research potential problems and find specific solutions that would not involve changing the platform," said Fabrice Galinier, who is responsible for information systems of the common platform. "The Vel Satis has a much larger interior than the average sedan; it was the same size as the Espace and not at all the standard shape." The Renault engineers could therefore take nothing for granted, including the fact that their redesign of the front suspension to improve stability would be the end of their problems.
They had to analyze the vehicle structure, resistance, deformations and acoustics, and perform extensive studies on crash behavior. The company also performed electric and electronic simulations using PAM-CEM to verify the numerous electronic systems in the car. "All engineering required increased levels of simulation, which also enable us to reduce design time through optimization," said Galinier.
Renault engineers developed a patented trigonal rear suspension for the Vel Satis which gives better handling and stability.
For the rear of the Vel Satis, engineers at Renault developed a patented trigonal suspension to offer better under-steering and stability despite the car's higher center of gravity. The suspension is multi-armed with two overlapping forms connected to the axle spindle. The lower triangular-shaped arm supplies the required rigidity, and has a flexible connection to the subframe to give a more supple movement along a virtual axis. It's virtual in the sense that, unlike with most multi-link suspensions, the lower arm is not physically joined to the subframe but connected through an intermediate link. The joint action of both the upper and the lower arms over the spindle axle controls the toe and camber angles to accomodate micro-steering in curves or during braking, an effect similar to a slight rotation of the steering wheel.
The arrangement of the trigon-type linkage allows engineers to program micro-steering effects according to the gravity and braking loads on each wheel. To filter out all noise, engineers flexibly mounted the suspension arms with the structure of the rear side member.
Fine-tuning. "Early in the design cycle, we realized that the rear suspension was a crucial element because it would control the level of center roll, ensure good wheel traction, and function as an important vibro-acoustic filter," says Olivier Molins, suspension design engineer for simulation and virtual testing at Renault.
However, the new suspension concept was both brand new and complex, including a wide degree of liberty. During testing, Renault performed tests on the seats and discovered a problem with vertical comfort for the passenger due to micro-sticking of the rear suspension. The increased number of articulations produced friction that was affecting the vertical comfort level of the passenger. The Renault engineers then performed additional testing on the prototypes to identify the cause of the over-stiffening of the suspension under low levels of demand, and compared the results with other benchmarks.
"Through testing, we were able to identify the role of friction in the suspension and the joints and bushings," says Molins. The engineers then created a semi-static mechanical model of the suspension using ADAMS to calculate each joint's contribution to the friction. "This simulation was quite simple to set up, based on a clever use of the model of the suspension," Molins asserts. Engineers performed a feasibility study on each joint to judge the possible reduction of both friction and spurious stiffness. The engineering team then sorted possible solution variations by simulation, and confirmed the final solution via prototype testing.
Since numerous joints of the suspension contributed to the problem, the team used a variety of approaches. This included: lowering the friction from the studs by using a special mix of rubbers; optimizing of the ball-joints of the suspension to avoid sticking; adjusting the damping characteristics of the shock absorber; and applying a surface treatment for lubrication for the main bearings of the anti-roll bar.
The simulation played a key role in helping the engineers understand the relationship between the problem's parameters and the design of the components. "The simulation work offered multiple advantages," says Molins. "The analysis of each joint indicated the most effective direction to find a solution, and the ability to compare different solution scenarios as they evolved ensured that we could choose the optimum modifications. We also reduced the number of physical prototypes of the suspension's joints and bushings."
Molins credits the efficiency of the engineering work to the good communication between the analysis near Paris and testing departments in Normandy. Engineers met regularly, exchanged e-mails, and used common data-storage areas. "Our ability to work together and consult on the solutions being developed enabled us to rapidly find the best solution possible," Molins concludes.