Magneti Marelli Redefines Rear Suspension Design
Multi-link independent rear suspensions have numerous advantages, but cost isn't one of them. A flexible new design out of Italy, however, promises all of the benefits and none of the compromises at a lower cost.
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Italy's Magneti Marelli (Mila, Italy; www.magnetimarelli.com) is best known for its electronics–the company is both a supplier to and sponsor of Ferrari's F1 team–and the fact that Enrico Fermi was its first director of Science and Research in 1919. It's less well known that its Cofap Automotive Suspension (CAS) unit designs, develops, manufactures, and supplies shock absorbers and complete automotive suspension systems globally. Or that CAS's new rear suspension design claims the ride and handling of an expensive multi-link design at a cost closer to that of a twist beam.
"Automakers can justify the cost of a multi-link design in the upper ranges of the C- and D-class segments, but not for their lower price vehicles or those in the B-class segment," says Wayne Jaslow, program manager, Suspension, CAS (Troy, MI). "We set out to change that." However, whittling away at a typical multi-link suspension would have only a marginal effect on cost. A radical rethink was necessary.
The fruit of that development is "FLECS" (Flexible Link Elevated Compliance Suspension), a design based around a lower control arm with flexible longitudinal blades. Sharing the elasto-kinematic duties between the bushings and the links means the number of suspension control arms can be reduced, which results in a direct cost savings. In addition, the bushings themselves–except for the single compliant bushing–are relatively simple off-the-shelf components.
The only limitation to the FLECS design is that the flexible lower control arm cannot be used as a spring platform. However, it can be packaged with a McPherson strut or other coil-over-damper style units, a remote coil spring, or combined with a torsion bar. High lateral stiffness of the design allows the upper control link to be lowered without a significant loss in cornering performance so packaging height can be significantly reduced, if necessary. "The packaging possibilities meet most every need with little effect on cornering performance," says Jaslow, "and the available wheel travel makes it possible to use the same unit for all of the vehicles built off a single platform, including SUVs and crossovers." However, longitudinal stiffness at the wheel center is an important consideration in rear- or all-wheel-drive vehicles, as is packaging the halfshafts, springs and dampers. For these drive layouts, joined upper links–in which one of the links is the compliant one–or an upper A-arm with a compliant bushing are preferred.
The materials needed to build the FLECS lower link are prosaic. Grades of high-tensile-strength steel normally employed to build suspension components are utilized in the flexible link. For the prototype system these include FeE 490 for the flexible blades and FeE 510 for the tube that connects them. High-volume units can be stamped as long as the stiffness of the arm meets requirements, while the remaining links (longitudinal compliance and camber control) have no specific design requirements beyond structural integrity.
The prototype unit is fitted to an existing Alfa Romeo platform and uses as many of the production attachment points as possible, "We custom-fabricated a cross member that used the original mounting points and the stock upper damper attachment points. It bolts right in," says Jaslow. Well, almost. The fuel tank and exhaust pipe routing were modified slightly to accommodate the new subframe. One big advantage for CAS engineers is that the damper noise path was maintained so on-road evaluations are comparable. Initial testing showed a 33% reduction in peak-to-peak longitudinal acceleration at the driver's seat for an impact at a 30-kph road speed, and a 24% reduction for a 50-kph impact. Noise transmission into the passenger compartment, it's claimed, also is reduced. As for FLECS's production readiness, Jaslow says, "I'm sure we could do a 2008 vehicle if they asked us today, and supply everything from the design to the complete module." Given the interest currently being shown in the system by OEMs, this prediction may prove conservative.
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