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News date: August 14, 2000, Updated September 5, 2000. News


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Steel refuses to bend before aluminium's challenge.

The new UltraLight Steel Auto Suspension (ULSAS) study (see separate story May 18) reflects a major effort by the steel industry to fight back against the inroads that aluminium is making in vehicle manufacturing. The ULSAS study has enabled Lotus Engineering to come up with impressive new technology for car suspension systems that promise substantial savings in weight and cost and improvements in vehicle performance compared to traditional systems. The research was undertaken as an initiative by a global consortium of 34 steel companies from 15 countries, who united to communicate the attributes of modern steel to their automotive customers.

In this special illustrated feature we go into the background for the research and detail and illustrate the main findings. "ULSAS and its companion studies establish that intelligent application of the latest steel technologies can match the weight savings of so-called exotic materials while offering significant cost advantages,'' according to Peter Rawlinson, the ULSAS programme director. "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.'' Rawlinson maintains that the challenge for the steel industry will be to change the mindset of engineers who are actually designing suspensions for tomorrow's cars.
"We must convince them to think of steel when considering ways to reduce mass without increasing cost, and to learn to exploit the incredible affordability of high strength steel," he says.
The two-year ULSAS project involved Lotus engineers carrying out a comprehensive benchmark study in which a variety of vehicles from North America, Europe and Asia were assessed. They tested vehicles on roads and tracks in the US and the UK, conducted state-of-the- art evaluations and detailed design reviews, and carried out weight, cost and manufacturing studies.

Then, based on those assessments, Lotus undertook a holistic review of suspension system requirements, and identified opportunities for application of new steel technologies. This exercise enabled the engineers to establish an extensive range of targets for the design phase of the ULSAS project.

The design phase encompassed five types of steel suspension systems across a range of vehicle sizes, resulting in the creation of a comprehensive range of suspension system designs that met or exceeded the aggressive mass, cost and performance targets.
The objective was to reduce the mass of a new steel design by at least 20% versus benchmarked conventional steel suspensions without a cost penalty, and to match the mass of a benchmarked aluminium system with a steel suspension system while demonstrating a cost saving of at least 20%.

In the process, the ULSAS study opened new avenues in suspension design, material applications and technology.
ULSAS transcends a weight reduction study and moves into the realm of reference work for vehicle dynamic factors and priorities, according to Nick Sampson, chief engineer for chassis design at Lotus.
"In fact, the study is so thorough and definitive that Lotus plans to use it as an aid to train young engineers on suspension system design,'' he says. "While much of the technology applied in ULSAS could not be considered ground breaking by itself, the application of the technology and the mindset for using it and advanced steels effectively are key messages from the study,."
Lotus used such state-of-the-art tools as finite element analysis - both linear and non-linear - dynamic analysis using ADAMS software, and CAD (Catia).

"The use of analysis in the degree to which it has driven some of the designs - and particularly the mathematical modelling of sectional properties through MathCAD - have progressed the standard normally associated with a concept level study,'' according to Sampson. The Lotus Engineering team covered performance based aspects of the new suspensions through 16 detailed parameters that their analysis and experience identified to be the most significant. They measured the performance characteristics of the benchmark vehicles, and then simulated values for their new lightweight steel suspension designs and compared these to the benchmarks. The performance parameters included several measures of vehicle ride and handling as well as refinement (Noise, Vibration, Harshness).

Because the main purpose of the suspension is to hold the tyre firmly on the road, stiffness of the suspension's structural elements is a primary design objective. Throughout the ULSAS program, a wide range of elasto-kinematic performance characteristics also were specified and evaluated, based on computer dynamic simulation of vehicle behaviour. In addition to studying the motion of rigid links, the Lotus team took into account the fact that many suspension joints include vibration-absorbing rubber bushings, which deflect elastically under load.
They also used finite element analysis extensively to pinpoint the stress loads on all structurally significant components. Strength is also a requirement, providing the durability required during a vehicle's life cycle.

Perhaps the most impressive design achievement in the ULSAS program is that of the Twistbeam suspension. This is a package-efficient system used most often in smaller cars in which ride quality is of less importance than passenger and luggage space, which are critical. It is not the natural choice for a performance-oriented vehicle.

Lotus developed a design which reduces mass by 32% with no cost penalty. The design also improves many other performance parameters - to such an extent that it could encourage many OEMs to reconsider their rear suspension options for future product. The ULSAS Twistbeam promises great performance (with the exception of some aspects of ride, a traditional weakness of Twistbeams), yet should retain all the traditional virtues of spatial efficiency, cost effectiveness and modularity - a compelling combination.

It potentially widens the scope of Twistbeam application into vehicle segments requiring superior dynamic performance coupled with enhanced practicality and cost effectiveness.

The ULSAS Twistbeam design differs from conventional Twistbeams because it features a unique sweep in a U-shape to provide continuity of structure from hub to hub. The high strength steel tube has a constant section, thin-wall tube, which is bent through a tight radius at each corner. The two forward- facing arms are hydroformed, however, in order to achieve appropriate geometry with the main tube. The forged wheel bearing mountings are welded onto the main tube, and the hub units are detachable for ease of service.

Strut and Links suspension systems are normally a low-cost design with some compromises in ride and handling and in vehicle refinement. Strut and Links is widely used by the large North American automakers - exemplified by the benchmark vehicles, where were the Chevrolet Lumina, Dodge Intrepid and Ford Taurus.

The Strut and Links design from Lotus saved 25% in weight 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 process of "through-wall laser welding'' is used to connect the hub unit. The laser beam penetrates directly through the relatively lower carbon content of the hydroform on to the body of the hub bearing unit beneath. The damper body also is MIG welded to the knuckle.

Double wishbone suspensions are generally regarded as a good overall compromise suspension often used on sporty cars. The Double Wishbone design from Lotus showed an estimated mass saving of 17% with no cost penalty and exceeded the performance targets.

The most significant features of the Lotus 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 suspensions are most often used in the luxury segment because of their superior dynamics and ride quality. It is the only design in this study with a benchmark that is aluminium-intensive, and the only one to include a subframe. The Lotus steel-intensive design showed a huge cost saving of 30% and a slight mass advantage over aluminium. It matched all the other target criteria.

The ULSAS Multi-link features large hollow section lower arms, formed by welding two high-strength steel clamshell stampings together. The upright is a process-optimised steel forging into which the hub-bearing unit is mounted. True to ULSAS design philosophy, the links in this system feature tubular high-strength steel.

The ULSAS Consortium commissioned Lotus to craft a unique rear suspension design 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 genre.

The system is fully independent, and compatible with both driven and non- driven drivetrain layout. The dominant feature is a large integral fore-aft- arm-cum-hub-carrier, onto which the hub bearing unit and brake calliper are mounted directly. Lateral wheel location and camber control are 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 example of component integration through multiple functionality - a design philosophy that has been adopted throughout the ULSAS program. That 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 is press fitted to the tubular insert; provide fore-aft wheel location; provide toe control to the wheel; and transmit brake torque reaction.

Compared with the conventional Double Wishbone design, the Lotus Unique concept showed a mass saving of 34%, and significant cost advantages. It is predicted to perform well with a relatively efficient package, and should be easy to manufacture and assemble.

Steel suppliers worked in close support of the design team.

"As the study progressed, it became clear that there are great benefits to automakers of getting materials suppliers involved proactively at the advanced vehicle engineering stages of new vehicle programs - especially in areas such as materials, cost analysis and forming simulation,'' Sampson said.

"Review of manufacturing feasibility of each design included extensive input by Consortium members, analysis of assembly requirements and assembly time estimates.

"Throughout the design process, Consortium members reviewed material requirements. Where it was beneficial to mass and cost improvements, we specified near-reach high-strength steels. To satisfy performance requirements, we used combinations of high-strength and ultra high-strength steel sheet and forging grades.''

For example, large, thin-wall sections were possible because of the unique properties of high-strength steel. Hydroforming was proposed for some components, but there was little tailored blanking, which is more appropriate for larger sheet metal sections. Laser welding was assumed in many cases, both at the component level and in final assembly, where it can add significant strength.

Lotus deployed a small team working to tight deadlines on the ULSAS project. It was no more than 12 members at any stage, but as requirements changed relevant specialists were brought in.

"It was the kind of effort you might associate with an OEM's approach to a new suspension design,'' says Sampson. "In addition, we had to do our work in a relatively tight time frame, also similar to OEM product development gateway process.''

The ULSAS consortium members included 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, Bohler Uddeholm AG, British Steel Engineering Steels, Cockerill Sambre R&D, Dofasco Inc., Georgsmarienhutte GmbH, Hoogovens Staal B.V., 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., and Weirton Steel Corporation

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