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, May 17 /PRNewswire/ -- 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
includes 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 35 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
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
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' 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
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:
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
* 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.
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
* 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
* Tubular lateral link -- 14 mm diameter by 1.5 mm wall thickness, 250 MPa
* Tubular lateral link -- 25 mm diameter by 3.0 mm wall thickness, 250 MPa
* Springs are 1300 MPa rod material, with a diameter ranging from 9.08 mm
to 10.91 mm, according to vehicle class.
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
* Through minor dimensional changes to the sub-frame the concept is easily
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.
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 MASS WHEEL RATE ROLL STIFFNESS
Od x ld mm kg N/mm 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 11x8.3 0.466 2.22 45.2
12.9x9.1 0.747 4.6 95
14.7x9.55 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 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
The American Iron and Steel Institute (AISI) is a non-profit association
of North American companies engaged in the iron and steel industry. The
Institute comprises 46 member companies, including integrated and electric
furnace steelmakers, and 175 associate and affiliate members who are suppliers
to or customers of the steel industry. For more news about steel and its
applications, view American Iron and Steel Institute's website at
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.
This release and other steel-related information are available for viewing
and downloading at American Iron and Steel Institute/Automotive Applications
Committee's website at http://www.autosteel.org .
Automotive Applications Committee member companies:
AK Steel Corporation Rouge Steel Company
Bethlehem Steel Corporation Stelco Inc.
Dofasco Inc. US Steel Group, a unit of USX Corporation
Ispat Inland, Inc. WCI Steel, Inc.
LTV Steel Company Weirton Steel Corporation
National Steel Corporation
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.
Bohler 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
Georgsmarienhutte 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
SOURCE American Iron and Steel Institute
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