Completed ULSAS concepts show the way forward for a new generation of safer, lighter vehicles

29 Oct 2000

Concept designs resulting from the recently completed UltraLight Steel Auto Suspension (ULSAS) study reveal significant mass savings of up to 34 per cent compared with benchmarked steel suspensions. Remarkably, the study also shows that a steel multi-link design can be produced that is 3 per cent lighter than a benchmarked aluminium suspension.

What is more, the study also discloses that all of these mass savings can be achieved without cost penalty, with the steel ULSAS multi-link design yielding a substantial 30 per cent cost saving over the aluminium multi-link suspension.

Commenting on the findings of this comprehensive two year design study, Peter Rawlinson of Corus Automotive Engineering and ULSAS programme director said: "The results explode the myth that the substitution of aluminium is the only answer to mass savings, and will help automotive engineers to better understand and exploit the latest steel technologies for more sophisticated and cost-effective mass-efficient designs."

Suspension systems account for a significant proportion of vehicle mass (typically 12 per cent), alongside the main body structure (20 per cent) and powertrain (18 per cent). It is another major area in the vehicle where mass and cost can be addressed to good effect.

As an added benefit, reducing the unsprung mass directly influences vehicle dynamic performance by increasing the potential grip available from the tyres - a distinct advantage in emergency manoeuvres - thereby enhancing the active safety of the vehicle. Therefore mass savings identifiable in this area are of particular importance to carmakers.

The ULSAS study is a clear primer for automotive engineers to appreciate all-new processes such as hydroforming and laser welding and to make the most of the properties of modern high-strength steels such as understanding the performance trade-off between absolute strength and formability.

"This balance is the key to liberating vehicle design engineers to realise their design ideals," says Rawlinson. "Traditionally, automotive engineers have relied on the excellent formability of mild steels, associating high strength steels with formability limitations that unduly restrict design freedom. Today, predictive formability tools are available to the engineer, which coupled with the innovation of the tailor welded blank and progressive advances in material properties collectively make the widespread deployment of high strength steels a realistic proposition for achieving safe, mass efficient solutions."

"An important point, which is often overlooked is that these new high strength steels are in themselves no stiffer than mild steel," he added. "They are, of course, markedly stronger. Both attributes are important and as the ULSAS study has shown it is possible to create stiff lightweight components by using good design practice to make the best use of the materials."

The programme's objective was to demonstrate the potential for cost effective mass reduction through the application of the latest steels and steel based technologies, which have recently become available to carmakers. The exclusive focus on rear suspensions recognises their relative diversity in the marketplace and significant influence upon vehicle characterisation and layout. The technologies explored and demonstrated are deemed equally applicable to front suspensions.

An initial, benchmark study assessed a comprehensive range of vehicles drawn from world markets, including both steel and aluminium-based suspension systems. This served to define four mainstream suspension types, suitable for a range of high volume cars, which were considered to afford the best potential for future development.

These designs were then applied to three main classes of vehicle comprising small (B class), lower medium (C class) and upper medium (D class) vehicles. A fourth vehicle class (P or PNGV class) was selected representing the US Partnership for a New Generation of Vehicles. This forward-looking vehicle research programme is pursuing advanced engineering solutions to achieve a full size US sedan with the kerb weight of a small European family car.

Eight key attributes of suspension design were also identified, against which the vehicles were assessed subjectively, on roads and tracks both in the US and the UK, as well as objectively, using Lotus' world-class 'kinematics and compliance' test rigs. The assessments included detailed design reviews as well as mass, cost and manufacturing studies.

Lotus proceeded to create a series of new, steel intensive suspension designs for each of these systems. In addition, a fifth, Lotus unique design was created further to explore the potential for steel.

The ULSAS designs were created using state-of-the-art computer engineering tools, including predictive software for ride and handling simulation. Significantly, ULSAS served as a catalyst to catalogue, probably for the first time in the industry, sixteen key suspension characteristics that collectively determine vehicle dynamic behaviour.

The strictures of mass and cost were of particular significance to the ULSAS programme. On the other hand, the study has also helped the steel consortium to better understand other constraints important to the carmaker - namely design, vehicle dynamics, manufacturing and packaging - and their significance in defining the character of a vehicle. The philosophy of the programme was to recognise and not compromise these other parameters, thereby leaving room for each carmaker to create their individual blend of unique vehicle characteristics.

Of overriding consideration, however, was that for each suspension type the key attributes with which the steel industry can assist the carmaker are cost and mass. Conventionally, these factors are considered to be mutually exclusive, with savings in mass incurring increases in cost or visa versa.

"A mindset has existed linking mass saving with cost penalties," said Rawlinson. "The ULSAS programme will help to explode another myth by demonstrating that significant mass savings are achievable without the anticipated cost penalty."

"ULSAS establishes that the intelligent application of the latest steel technologies can match the mass savings normally associated with substitution of alternative materials, while offering significant cost benefits. It also demonstrates this can achieved without compromising performance attributes such as vehicle dynamic behaviour, spatial efficiency and the practicalities of manufacture."

The four mainstream types of suspension identified by the ULSAS study comprised twist-beam, strut and links, double wishbone and multi-link systems. The twist-beam system excels in the area of interior space, system packaging and manufacturing, which makes it ideally suited to smaller vehicles. However, these attributes are achieved at the expense of vehicle dynamic performance and, in particular, ride quality.

The strut and links, double wishbone and multi-link suspensions are all fully independent systems. In the case of the strut and links system, low cost is the dominant attribute. Yet this system still provides a well rounded overall compromise ideally suited to medium-sized sedans. The double wishbone system has better dynamic performance and design, but occupies more space in the vehicle and is more expensive. This system, therefore, is well suited to a sporty vehicle. The multi-link system significantly compromises space in the vehicle as well as cost to achieve excellence in ride and handling and vehicle refinement. These attributes make it better suited to executive class saloons.

Most striking achievement is the 'twist-beam' design

The most impressive design achievement in the ULSAS program is that of the twist-beam suspension, which reduces mass by 32 per cent with no cost penalty. Moreover, the design also improves many other performance parameters -- to an extent such that it could encourage many carmakers to reconsider their rear suspension options for future product.

With the exception of some aspects of ride, a traditional weakness of twist-beam suspensions, the ULSAS design offers excellent performance, yet retains all the traditional virtues of spatial efficiency, cost effectiveness and modularity -- a compelling combination. It potentially widens the scope of twist-beam application into vehicle segments requiring superior dynamic performance coupled with enhanced practicality and cost effectiveness.

Perhaps the most important consideration when designing a twist beam is achieving high structural stiffness. This is because the hubs are mounted directly onto the main structure of the beam. Therefore stiffness of the structure has a major influence upon wheel control and geometry, and hence the handling characteristics of the vehicle. The other important design consideration is achieving high strength, which is necessary for durability and to withstand occasional abuse loadings such as striking a kerb without incurring permanent damage.

The ULSAS study contemplates high strength steel being applied extensively to achieve both superior strength as well as a massive improvement in stiffness. For a number of key metrics, such as lateral force steer and lateral compliance for example, calculations show that stiffness has typically been doubled.

The ULSAS twist-beam differs from conventional designs because it has a unique feature. This is a constant section thin-wall tube in high-strength steel, which is bent through a tight radius at each end, providing continuity of structure from hub to hub. The centre section of the tube has a plasma-cut profile design to relieve torsional stiffness thus allowing the beam to twist. To achieve the correct geometry, both two forward-facing arms would be hydroformed to connect precisely with the main tube. The forged wheel bearing mountings are also welded onto the main tube, and the hub units are detachable for ease of service.

Strut and links design

The 'strut and links' design saves 25 per cent in mass versus the benchmark, at slightly lower cost, and with performance exceeding the benchmarks. A unique feature of the structure is a knuckle that is integrally welded onto both the hub-bearing unit and also directly to the base of the shock absorber. The design contemplates the process of "through-wall laser welding" to connect the hub unit.

Double wishbone design

The 'double wishbone' design shows an estimated mass saving of 17 per cent with no cost penalty. It too exceeds the performance targets. The most significant features of this design are a stamped high-strength steel fore-aft arm and a forged steel upright (versus a cast iron upright on the benchmark). As is the case with all the ULSAS designs, the double wishbone is carefully optimised for function and formability, with sections developed to meet specific load requirements.

Multi-link design

The multi-link suspension is the only design with an aluminium-intensive benchmark, and the only one to include a sub-frame. The steel-intensive design reveals a huge cost saving of 30 per cent and a slight mass advantage over aluminium. It matches all target criteria. The ULSAS 'multi-link' design contemplates large hollow-section lower arms, formed by welding two high-strength steel clamshell stampings together. The upright proposes a process-optimised steel forging into which the hub-bearing unit would be mounted. True to ULSAS design philosophy, the links in this system feature tubular high-strength steel.

Lotus unique design

The ULSAS Consortium also asked Lotus to propose a unique rear suspension design in order to demonstrate just what can be achieved with the freedom of a clean sheet approach, and specifying the latest range of steel materials and technologies. Therefore no direct benchmark comparison to this particular configuration exists, although it is nominally of the double wishbone type.

The design put forward is a fully independent system, which is compatible with both driven and non-driven drivetrain layouts. The dominant design feature is a large integral fore-aft-arm-cum-hub-carrier, onto which the hub bearing unit and brake calliper are directly mounted. Lateral wheel location and camber control is provided by upper and lower lateral links. A co-axial spring damper unit mounts from the rear of the arm directly to the body structure.

The Lotus unique design is an excellent example of component integration achieved through multiple functionality; a design philosophy adopted throughout the ULSAS study. This simply means that wherever possible, a component or combination of components is designed to do more than one job.

So, for example, the large arm is designed to mount the hub unit, which would be press fitted to the tubular insert to provide fore-aft wheel location, toe control to the wheel and to transmit brake torque reaction.

Compared with a conventional double wishbone design, the Lotus unique concept shows a mass saving of 34 per cent as well as significant cost advantages. It is predicted to perform well with a relatively efficient package, and would be easy to manufacture and assemble.

Throughout the ULSAS project the steel consortium worked in close support of the Lotus design team reviewing manufacturing and material requirements. Near-reach high-strength steels were specified wherever it was beneficial to mass and cost improvements. To satisfy performance requirements, the consortium suggested combinations of high-strength and ultra high-strength steel sheet and forging grades.

Large, thin-wall sections, for example, are contemplated because of the unique properties of high-strength steel. There is widespread use of tubular components. Some of these are hydroforms. Uniquely, the Lotus design employs a tailored blank solution comprising two clamshell pressings to form the main fore-aft arm of the suspension. Laser welding is assumed in many cases, both at the component level and in final assembly, where it can add significant strength. An essential feature of the double wishbone and multi-link suspensions is the knuckles, which are lightweight process-optimised high strength steel forgings.

Some elements of a suspension system can be considered truly generic inasmuch as they feature in the majority of suspensions. Classic examples are springs, dampers and wheel bearings. Springs are an excellent example of how the deployment of high strength steel can have a direct bearing on mass saving -- an advantage that is fully exploited in the ULSAS study.

"ULSAS and its companion studies establish that intelligent application of the latest steel technologies can often match the mass savings offered by more expensive materials while offering significant cost advantages," added Peter Rawlinson. "That is because while steel is a relatively dense material, it also is very stiff and strong, and appropriate engineering can take advantage of these properties to produce lightweight, cost-effective solutions."

Further information:

automotive@corusgroup.com

Note to editors:

The UltraLight Steel Auto Suspensions (ULSAS) study is a two-year concept design programme aimed at helping the automotive industry find affordable lightweighting solutions to enhance vehicle safety while improving fuel efficiency and minimising emissions.

Companion programmes include UltraLight Steel Auto Closures (ULSAC) and Advanced Vehicle Concepts (ULSAB-AVC) studies. All three studies build on the success of the original UltraLight Steel Auto Body programme. Both ULSAB and ULSAC studies have progressed from concept designs into a validation phase in which actual hardware has been built and demonstrated. It is possible this precedent will be followed by the ULSAB-AVC and ULSAS initiatives.

A consortium of 33 of the world's major steel producers from 15 countries, which funded the programme, commissioned Lotus Engineering, a world-class consultant in vehicle engineering, to undertake the study of automotive rear suspension systems. In addition to supplying its vehicle engineering expertise, Lotus acted as systems integrator, identifying and working closely with industry leaders with a proven record of technical excellence in the design of components such as dampers, brakes and hub-bearing units. The steel consortium played a crucial role in providing expertise in the latest steel processes and materials - and in this manner a truly holistic approach was achieved.

ULSAS consortium members include:
Aceralia Transformados S.A.
Acme Metals Incorporated
AK Steel Corporation
American Iron and Steel Institute
Bethlehem Steel Corporation
BHP Steel - Rod, Bar and Wire Division
Böhler Uddeholm AG
Corus Engineering Steels
Cockerill Sambre R&D
Dofasco Inc.
Georgsmarienhütte GmbH
Ispat Inland Inc.
Ispat Stahlwerk Ruhrort GmbH
Kawasaki Steel Corporation
Kobe Steel, Ltd.
LTV Steel Company
National Steel Corporation
Nippon Steel Corporation
NKK Corporation
Pohang Iron & Steel Co., Ltd.
Rautaruukki Oy
Rouge Steel Company
Stelco Inc.
Sumitomo Metal Industries
The Tata Iron and Steel Co., Ltd.
Thyssen Krupp Stahl AG
Usinor
US Steel Group
USS/Kobe Steel Company
Vallourec Group
Voest-Alpine Stahl Linz GmbH
VSZ Holding A.S. Kosice
WCI Steel Inc.
Weirton Steel Corporation