High- and Ultra High-Strength Steel Crucial to Lightweight, Cost-Saving Suspension Designs

Holistic Engineering, Part Consolidation and Optimization, and State of Art Processing Technologies Also Key

DETROIT, MI, May 17, 2000 – In the global steel industry's latest study, UltraLight Steel Auto Suspensions (ULSAS), the mass and cost savings with no compromise in performance are the results of extensive use of high- and ultra high-strength steels, the latest in steel processing technology, thorough analysis and an iterative, holistic design approach.

The results also stem from close collaboration between Lotus Engineering Services, Inc., England, which conducted the study, and representatives of several of the world's leading steel companies, who provided expertise and advice in the use, cost and performance of high-tech steels for the project.

Results of the two-year design study of automotive suspensions show mass reductions up to 34 percent at no increase in cost for four steel-intensive designs, compared to a range of benchmarked steel suspensions. The study also includs a fifth steel design that shows a 30 percent cost reduction with a small mass reduction, compared to a current aluminum design.

All five designs meet or exceed a range of performance criteria including those for ride & handling and NVH (noise, vibration, harshness), manufacturability and packaging (the effect on underbody, occupant and luggage space). Lotus assumed a high-volume production scenario in its analysis for cost and other factors.

Sponsored by a consortium of 34 of the world's leading steel companies, the ULSAS study is a companion to the UltraLight Steel Auto Body (ULSAB) study released in 1998, the UltraLight Steel Auto Closure (ULSAC) study, which is nearly complete, and ULSAB-AVC (Advanced Vehicle Concepts), which will be complete in 2001.

The ULSAS project comprised two phases. First, Lotus engineers carried out a comprehensive benchmark study in which they assessed a variety of vehicles from North America, Europe and Asia. They tested vehicles on roads and tracks in the United States and the United Kingdom, and conducted state-of-the-art evaluations, detailed design reviews, and weight, cost and manufacturing studies.

The benchmarked vehicles represent five standard classes of automobiles, based on size, from B class (small cars) through C, D and E class (luxury) and the U.S. PNGV (Partnership for a New Generation of Vehicles).

Based on its 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 meet or exceed the aggressive mass, cost and performance targets.

Lotus used state-of-the-art tools for its design and analysis work, including linear and non-linear finite element analysis, dynamic analysis using ADAMS software, and CAD with Catia. Lotus's use of MathCAD for mathematical modeling of sectional properties, in particular, has advanced the standard normally associated with a concept level study.

Key to ULSAS is that while Lotus focused on mass reductions, it also optimized the designs for performance. Based on its analysis and experience, the Lotus team identified 16 of the most significant performance parameters. Following painstaking measurement of the performance characteristics of the benchmark vehicles, the team's engineers simulated values for their new lightweight steel suspension designs and compared them to the benchmarks. The performance parameters included several measures of vehicle ride and handling and refinement (noise, vibration, harshness).

Throughout the ULSAS program, the Lotus team used computer dynamic simulations of vehicle behavior to specify and evaluate a wide range of elasto-kinematic performance characteristics. This is important because Lotus not only had to study the motion of rigid links, it also had to consider implications of the vibration-absorbing rubber bushings many suspension joints include and which deflect elastically under load.

To predict and design for durability, Lotus used finite element analysis extensively to pinpoint the stress loads on all structurally significant components.

In pursuing design and material strategies for lightweighting, Lotus engineers found that they could selectively substitute hollow steel tubes for solid bars and sheet steel stampings or hydroformed parts for forged components to gain additional mass savings. The results of these strategies show up in the finished designs.

Details concerning design, materials and manufacture for each of the five suspension types follow:

Twistbeam

The use of iterative, holistic design optimizing a clever use of high-strength and ultra high-strength steel sheet, tubing, bar, and forgings, coupled with innovative manufacturing technologies such as hydroforming resulted in the ULSAS twistbeam design having achieved a mass reduction of up to 32 percent at no cost penalty.

The ULSAS Twistbeam design differs from conventional twistbeams because it features a unique U-shape swept section that provides continuity of structure from hub to hub. The high-strength steel tube employed is a constant section thin-wall tube, bent through a tight radius at each corner. The center section of the tube has a plasma-cut profile to reduce torsional stiffness, thus allowing the twistbeam to twist. The two forward facing arms are hydroformed in order to achieve appropriate geometry with the main tube. The forged wheel bearing mountings are MIG welded onto the main tube, but the hub units are detachable for ease of service. Component details follow:

  • Transverse beam made of 600 MPa ultra high-strength steel tube (2.8 mm to 4.1 mm wall thickness, depending on the vehicle class; required thickness is related to vehicle curb weight) with a plasma-trimmed cutout. Profile shape of the cutout was developed and optimized by CAE techniques to achieve stiffness and strength targets. Lotus carried out manufacturing feasibility studies to confirm that this beam part can be made. Bend radius is 125 mm using wiper die and ball mandrel.

  • The trailing arm is a 70 mm by 2.0 mm tube, 400 MPa high-strength steel, that can be MIG welded to the twistbeam. FEA of this component geometry demonstrated that a satisfactory part might be produced by hydroforming. However, to produce the full design geometry, an end flaring or punch point operation would be required.

  • The spring pan is stamped from a 3.2 mm to 4.0 mm (depending on the vehicle class; required thickness is related to vehicle curb weight), 500 MPa ultra high-strength steel blank.

  • Springs are 10.04 mm to 11.43 mm (depending on the vehicle class; required thickness is related to vehicle curb weight) diameter, 1300 MPa ultra high-strength steel rod.

  • The hub mounting plate is forged from 600 MPa ultra high-strength steel.

Strut and Links

Lotus reduced the mass of the strut and links system up to 25 percent, in two alternative design approaches, one using a hybrid knuckle (stamped) and one using a hydroformed tubing knuckle. The hybrid is appropriate for D, E, and PNGV class vehicles while the hydroformed tubular knuckle is for B and C class vehicles, although both can equally be developed across the whole range. Most other components for both designs are similar; all use ultra high-strength or high-strength steels. Lotus engineering used a holistic, iterative design approach to optimize components, reducing mass through use of the high-strength steels. Details follow:

  • The hybrid knuckle is made from 4.0 mm, 500 MPa high-strength steel. It is stamped in two halves and laser welded together. Two parts complete the knuckle design, a lower housing forged 500 MPa high-strength steel, and a lower bracket, blanked and folded from 250 MPa, 4.0 mm thick high-strength steel.

  • The hydroformed knuckle is made from 3.5 mm wall, 500 MPa high-strength steel tubing. Only one additional part is required to complete the knuckle: a lower housing forged from 500 MPa high-strength steel.

  • The knuckles are attached to the integral hub bearing unit by "through wall" laser welding, which penetrates the knuckle wall joining it to the hub.

  • The forward and rear lateral links are tubular components, 2.0 mm wall thickness, 250 MPa high-strength steel.

  • Springs are 1300 MPa high-strength steel, with a diameter ranging from 10.6 mm to 12.3 mm, according to vehicle class.

  • The damper is fitted with a hollow damper rod and has a high-strength steel housing, which is MIG welded to the knuckle in both concepts.

Double Wishbone

The double wishbone design enjoys an estimated 17 percent mass saving compared to the benchmark with no cost penalty. It features a stamped high-strength steel fore and aft arm and forged steel upright, versus a cast iron upright on the benchmark design.

Lotus created the initial package layout and developed sectional properties in CAD. Engineers then refined the designs using structural analysis optimization techniques to establish the required shapes, material gauges and grades for the main structural components. This approach ensured the designs meet both the stiffness and structural targets. Further, more detailed analysis was carried out to validate the design detail. All design features were fully investigated and validated.

Initial analysis indicated in that, in theory, the optimum solution for links was a cigar-shaped tubular member. However, the structural advantages were found to be only marginal and these are outweighed by the cost penalty of increased manufacturing complexity.

The optimum solution for the steel fore and aft arm and the forged knuckle were optimized by extensive use of CAE optimization techniques. Details follow:
  • The forged knuckle is 600 MPa ultra high-strength steel

  • The fore/aft arm is stamped from a 3.0 mm 500 MPa high-strength steel

  • Tubular upper link –13 mm diameter by 1.5 mm wall thickness, 250 MPa min.

  • Tubular lateral link – 14 mm diameter by 1.5 mm wall thickness, 250 MPa min.

  • Tubular lateral link – 25 mm diameter by 3.0 mm wall thickness, 250 MPa min.

  • Springs are 1300 MPa rod material, with a diameter ranging from 9.08 mm to 10.91 mm, according to vehicle class.

Multi-Link

The Multi-Link design, which is appropriate across the D, E, and PNGV classes, is the only Lotus Engineering design in this exercise whose benchmark is aluminum intensive. The steel design shows a 30 percent cost savings with a slight mass advantage over the aluminum benchmark. It features large hollow section lower arms, formed by welding together two 2.0 mm thick 300 MPa high-strength steel clamshell stampings. The upper and lower stampings are of symmetrical design. The links in this system are designed to be made of 16 mm by 1.5 mm and 17.5 mm by 1.5 mm diameter, 250 MPa high-strength steel tubes. The sub-frame is a stamped assembly manufactured of five major components and several reinforcements using steels ranging from 1.2 mm to 2.0 mm and yield strength levels between 200 MPa and 400 MPa.

  • The stamped control arm halves, 2.0 mm gauge, 300 MPa high-strength steel, are butt welded together by MIG and spot welding.

  • The forged knuckle is 750 MPa ultra high-strength steel.

  • Stamped steel sub-frame joined by combination of laser, MIG and spot welding.

  • Through minor dimensional changes to the sub-frame the concept is easy adaptable to smaller or larger cars.

  • Springs are 1300 MPa rod material, with a diameter ranging from 10.42 mm to 10.78 mm, according to vehicle class.

Lotus Unique

The ULSAS Consortium commissioned Lotus Engineering 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. Engineers minimized the number of parts and optimized the use of materials in terms of strength and gauge.

The Lotus Unique system demonstrates a 34 percent mass savings at a modest cost savings, compared to the double wishbone suspension system. The dominant feature is a large integral fore-aft arm hub carrier made up of inner and outer tailor welded blank stampings. The two stampings represent six different material grade/gauge combinations shown in the table below:

Part

Trailing Arm Outer Panel

Process

Stamping Tailor Welded Blank

Material Gauge (mm)

1.8

2.7

1.2

Material Grade (MPa)

400

200

200

 

Part

Trailing Arm Inner Panel

Process

Stamping Tailor Welded Blank

Material Gauge (mm)

2.3

2.3

1.2

Material Grade (MPa)

500

150

250

The hub mounting sleeve is an integral part of the fore-aft arm and consists of a tubular housing of 3.0 mm and has a yield strength of 300 MPa and stamped outer hub reinforcement of 2.5 mm thick steel with a yield strength of 400 MPa.

  • The upper and lower links are 250 MPa high-strength steel tubes, 25 mm in diameter, 1.5 mm wall thickness. Tube requirements would primarily be met with conventional welded tubing. Extreme requirements for combinations of high gauge/small diameters may need to be specified as cold drawn tubing.

  • Gussets, reinforcements, and mounting brackets range in gauge from 1.7 mm to 3.0 mm, and in material grade from 150 MPa to 400 MPa high-strength steel.

  • Springs are a 1300 MPa rod material, with a diameter ranging from 8.65 mm to 11.16 mm, according to vehicle class.

Anti-Roll Bar (Torsion Bar)

The tuning of the roll stiffness of the suspension systems is undertaken using anti- roll bars. Lotus Engineering explored tubular designs to demonstrate the material requirements and mass reduction potential associated with adopting high-strength tubular products. A basic form of anti-roll bar was developed with the operating parameters suitable for the Multi-Link system. The results are 36 to 48 percent lighter, requiring a 5 to 10 percent stronger material grade in tubular form.

Bar Type

Diameter
Od x ld mm
Mass
kg
Wheel Rate
N/mm
Roll Stiffness
Nm/Deg
Solid
10
0.894
2.24
45.7
12
1.287
4.6
94.6
14
1.752
8.6
174.6
Tube
11 x 8.3
0.466
2.22
45.2

12.9 x 9.1

0.747
4.6
95
14.7 x 9.55
1.116
8.6
174.5

The bar was initially assumed to be solid with diameters typical of such parts in volume production and sizes to cover the required range of stiffnesses. Following the analysis, further part designs were subsequently generated using tubular product to achieve similar stiffness properties. The results of these studies appear below, along with the mass reduction potential of adopting high-strength tubular product.

High-Strength Steel Springs

During the creative ideas process of the redesign program, Lotus Engineering identified a direct relationship between the yield strength of spring material and the mass of the spring. Then during the iterative design phase, Lotus Engineering further explored the advantages of high-strength spring steel, especially effects on both mass and package.

The bottom line was the realization that there exists opportunities for significant mass reduction in using ultra high-strength steel bar for springs. The choice was to use 1300 MPa steel.

Max Stress

Mass
Length
Dia
Volume
800
5.15
283.5
127.50
3620
900
4.10
270.0
117.80
2943
1000
3.35
258.9
109.72
2448
1100
2.78
249.4
102.94
2075
1200
2.35
241.3
97.10
1786
1300
2.02
234.2
92.01
1557
1400

1.75

228.1
87.54
1373
1500
1.53
222.6
83.56
1221

Cost Analysis

Cost analysis occurred in two parts, one for benchmarking and the second for the new Lotus designs. To benchmark indicative costs for components and tooling, Lotus used Bills of Materials with cost estimates for components and sub-assemblies in the rear suspension systems. For tooling, it used tooling cost estimates for each of the major components and sub-assemblies.

Lotus computed costs in U.S. dollars, based on 1998 economics (the year the study began), estimated production volumes (table A below), supplier base cost (not OEM in-house costs) and other assumptions. For the design cost analysis, Lotus followed identical assumptions and similar rationale to the benchmarking phase and assumed productions volumes as follows (table B):

Benchmark Vehicle Production Volumes (Table A)

Manufacturer

Model

Volume

Assumptions

Honda

Accord

415,000

(1)

Ford

Taurus

380,000

(1)

BMW

5 Series

215,000

(2)

Audi

A6

110,000

(2)

(1) = 1997 North America (2) = 1997 European

Lotus Design Production Volume (Table B)

Vehicle

Annual Volume

Small (B Class)

400,000

Lower Medium (C Class)

400,000

Upper Medium (D Class)

400,000

North American PNGV

400,000


ULSAS Consortium member companies:

Aceralia Transformados S.A.

Nippon Steel Corporation

Acme Metals Incorporated

NKK Corporation

AK Steel Corporation

Pohang Iron & Steel Co., Ltd.

American Iron and Steel Institute

Rautaruukki Oy

Bethlehem Steel Corporation

Rouge Steel Company

BHP Steel – Rod, Bar and Wire Division

Stelco Inc.

Böhler Uddeholm AG

Sumitomo Metal Industries

British Steel Engineering Steels

The Tata Iron and Steel Co., Ltd.

Cockerill Sambre R&D

Thyssen Krupp Stahl AG

Dofasco Inc.

Usinor

Georgsmarienhütte GmbH

US Steel Group

Hoogovens Staal B.V.

USS/Kobe Steel Company

Ispat Inland Inc.

Vallourec Group

Ispat Stahlwerk Ruhrort GmbH

Voest-Alpine Stahl Linz GmbH

Kawasaki Steel Corporation

VSZ Holding A.S. Kosice

Kobe Steel, Ltd.

WCI Steel Inc.

LTV Steel Company

Weirton Steel Corporation

National Steel Corporation

 

The Automotive Applications Committee (AAC) is a subcommittee of the Market Development Committee of AISI and focuses on advancing the use of steel in the highly competitive automotive market. With offices and staff located in Detroit, cooperation between the automobile and steel industries has been significant to its success. This industry cooperation resulted in the formation of the Auto/Steel Partnership, a consortium of DaimlerChrysler, Ford and General Motors and the member companies of the AAC.

American Iron and Steel Institute/
Automotive Applications Committee:
AK Steel Corporation
Bethlehem Steel Corporation
Dofasco Inc.
Ispat Inland Inc.
National Steel Corporation
Rouge Steel Company
Stelco Inc.
United States Steel Corporation
WCI Steel, Inc.
Weirton Steel Corporation