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01/06/2006
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Roger Bishop describes a how departing from traditional thinking looks set to bring the advantages of complex multi-link rear suspension systems to all classes of passenger cars
The elasto-kinematic performance of traditional multi-link suspensions is heavily geared to their part count and, over the past half century or so, they have increased in complexity to meet the increasing demands placed on their performance.
Now a major Tier One supplier, Magneti Marelli, is challenging traditional thinking with a design known as U-Link. This uses the known and predictable behaviour of materials and structures to replace the combinations of rigid links and bushes normally needed to achieve the required complex combinations of compliancy.
The architecture of U-Link consists of just a single lower control arm and a single upper control link – in theory the simplest mechanism capable of generating the kinematic behaviour required of a modern suspension system. In a traditional architecture, this simplicity introduces compromises into the compliant properties of the suspension that are impossible to resolve.
The design philosophy behind U-Link is very simple: incorporate a carefully calculated structural flexibility into the control arm that allows it to make a major contribution to the elasto-kinematic performance of the entire system without compromising its load-bearing capability.
In other words, by understanding compliant linkages from their first principles it has been possible to transfer the responsibility for elasto-kinematic performance from the configuration of the links to the structural stiffness properties of the control arm.
U-Link appears to represent a significant breakthrough in rear suspension design, allowing the architecture to be simplified and ensuring that the flexible control arm has the simplest form possible, guaranteeing savings on the module cost.
As well as low cost, U-Link has the potential to provide OEMs with performance improvements in terms of comfort and handling in an extremely compact package and with improved durability of the rubber bushes. It can be mounted on a single subframe with no additional attachments to the body in white and is compatible with under-floor spring-damper layouts.
According to Magneti Marelli, the design allows it to exhibit a high degree of longitudinal flexibility with large linear displacements at the wheel centre – widely known to be important for ride comfort – in combination with the very high lateral stiffness at the contact patch required for safe and predictable handling.
“Its ability to combine widely different compliant steer requirements under the various comfort and handling loads exceeds the vast majority of traditional multi-link. Of particular note is its capacity to generate significant stability enhancing toe-in during braking events,” says Barney Gerrard of Magneti Marelli Cofap Automotive Suspension.
The control arm can take an extremely simple form and is particularly suited to a stamped and welded construction. Its construction does not present an additional fatigue risk and the materials used are identical to the high-strength steels used for the majority of traditional control arms.
Providing the structural flexibility of the arm conforms to certain requirements, explains Gerrard, it can have a variety of forms and be manufactured in a number of ways according to the production volume required or the processes preferred by an OEM. Although flexible, only small displacements are needed so there is no fatigue risk.
The bushings used are said to be another source of cost-saving for an OEM. The concept removes emphasis from bushing performance, enabling them to be simple, durable and inexpensive. Of the seven bushes per suspension, six are simply required to be stiff and can, in principle, be identical. The seventh provides control over the longitudinal compliance of the suspension and needs to be softer.
Magneti Marelli says the U-Link architecture is in its most compact form when applied to front-wheel drive cars but can be used on rear- or all-wheel drive platforms with various options available for spring and damper mounting locations. It is also more suited to smaller passenger cars and is intended to sit in the cost gap that exists between the twist beam axle found on virtually all European A-B segment cars and the ‘control blade’ architecture adopted by many C segment vehicles.
However, OEMs in the B-C segments are currently responding to the market demand for increasing elasto-kinematic performance by substituting the twist beam rear suspension with multi-link designs – often the control blade type popularised by the Ford Focus. The Volkswagen Golf V is an example of this transition. The cost penalty is significant, however, and the U-Link was conceived to sit in this cost gap without sacrificing the performance expected from a multi-link design.
The concept has been analysed in the CAE environment and is now being prototyped and benchtested. A prototype vehicle will follow. Meanwhile, work is continuing on optimising the flexible control arm for volume production with preliminary case studies indicating “significant cost savings” over the benchmark control blade suspension. Patents are being pursued.
Independent and complaint ‘hinges’
Magneti Marelli believes any suspension architecture should be configured according to a clearly defined compliant mechanism. In the case of the U-Link the flexible control arm, together with the upper link, is designed to have two elastic axes of rotation – one transverse and the other horizontal.
These axes can be thought of as independent and compliant ‘hinges’ with the suspension system and they result in a high standard of elasto-kinematic behaviour. This internal mechanism of two compliant axes is said to provide the designer with much greater freedom to meet demand elasto-kinematic specifications. The orientation of and rotational stiffness around these axes are calculated from the elasto-kinematic specification itself and are then realised by the geometry and the stiffness properties of the flexible control arm.
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Author
Roger Bishop
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