|Twist beam assembly mass comparison. Click to enlarge.|
Results of a new design study for a lightweight steel proof-of-concept twist beam (used in suspensions) by the Steel Market Development Institute’s (SMDI) Automotive Applications Council indicate that the preferred U-Beam Design based on 22MnB5 tubular construction with DP780 and SPFH540 sheet achieves a 30.0% mass reduction relative to the baseline assembly, at a 12–15% premium in manufacturing cost at production volumes of 30,000 to 250,000 vehicles per year, respectively.
An S-Beam Design based on 22MnB5 sheet, DP780 tube and HSLA550 materials was predicted to have a 14.9% mass reduction relative to the baseline assembly. The objective of the study was to develop a twist beam design achieving a 15–25% mass reduction with equivalent structural and elasto-kinematic performance relative to the baseline design at a ≤ 10% cost premium.
Durability (Max Twist load case) and strength (Max Vertical load case) were the primary design drivers for both designs.
Both designs were deemed manufacturable based on expert manufacturing assessment and relevant production application examples. A twist beam assembly in commercial use today was selected for the baseline packaging, performance, mass and cost.
The initial design concepts were developed based on size and shape optimization of the available design space; an iterative optimization strategy was used to minimize the mass of each design, while meeting the specified structural requirements.
Without manufacturing constraints, the optimization output was a truss structure with a distinct “U” shape in plain view. This result was interpreted into the concept “U-Beam” design. An extruded constraint resulted in a second initial concept: the “S-Beam” since the cross-section developed into an “S” shape as additional optimization was performed.
The U-Beam design utilizes UHSS and AHSS to enable aggressive gage and mass reductions. The design features hot-formed tubular transverse and swept longitudinal members, all from 22MnB5 material with a constant 2.5 mm thickness.
The transverse member has a closed inverted “U” cross section to provide the desired shear center location for roll steer performance. The roll steer can be tuned if required with this design by adding a rear-view sweep to the beam. The final design was tuned to achieve the OEM baseline roll steer with an unswept design for simplicity.
|U-beam design. Click to enlarge.||S-beam design. Click to enlarge.|
The S-Beam design features a hot-stamped main beam and an associated hot-stamped lower reinforcement, all from 22MnB5 material. The stamped beam design provides the “S” cross section derived from optimization. The overall shape of the beam is also a “U” in the plan view, reflecting the optimization results consistently observed during development.
With fuel economy regulations increasing rapidly, automakers are looking for every way they can to take weight out of vehicles. The lightweight steel twist beam project is a great example of how currently available advanced steel technologies enable aggressive weight savings in the vehicle’s suspension, thus enabling better fuel economy for the automaker and the consumer.—Ronald Krupitzer, vice president, automotive market, SMDI
Funded by members of SMDI’s Automotive Applications Council and members of Chrysler Group, LLC, Ford Motor Co. and General Motors Co., the lightweight twist beam project was conducted by Ontario, Canada-based Multimatic Inc. SMDI is a business unit of the American Iron and Steel Institute (AISI). AISI comprises 24 member companies, including integrated and electric furnace steelmakers, and approximately 125 associate members who are suppliers to or customers of the steel industry. AISI’s member companies represent over three quarters of both US and North American steel capacity.