Based on the presentation from Martinrea International @ Great Designs in Steel 2019
Martinrea engineers conducted a thorough benchmarking study of engine cradles (also known as front chassis subframes) with different geometries and material systems. Upon concluding their benchmark analyses of steel intensive, aluminum intensive, and hybrid steel / aluminum cradles; they used state-of-the-art gauge and grade optimization tools to develop a steel intensive solution which:
Resulted in a 15 percent reduction in mass from the selected full perimeter baseline cradle while exceeding performance requirements within the same packaging space. This was achieved by gauge and grade optimization of select components.
- Limited the cost premium of the optimized solution to an estimated 11.3 percent over the baseline, resulting in around $3.36 / kg saved which is considerably less than the cost of saving mass using aluminum intensive or hybrid executions.
OEMs have been continually pursuing lightweighting of their vehicles’ body and chassis structures to improve fuel economy and enhance vehicle performance. They have demonstrated noticeable success in achieving their targets by leveraging the broad selection of advanced high-strength steel (AHSS) grades and implementing disciplined geometry, gauge, and grade design optimization methods.
A recent study commissioned by the automotive program for the American Iron and Steel Institute (AISI) found that AHSS grades replaced high-strength steel (HSS) and high-strength low alloy (HSLA) grade applications in seat tracks, hinges, interior support structure, mounts, suspension and closures. 1
With that insight, Martinrea engineers used a similar approach to redesign a current production cradle and achieved a manufacturable lighter weight version of the baseline at a modest increase in overall cost.
The Martinrea presentation reviewed examples of the various cradle designs considered in the benchmarking study. The cradles were categorized by size (small, medium and large) as well as material system (steel, aluminum, and steel / aluminum hybrid). Observations from the study include:
- Potential mass reduction enablers used by different OEMs included X-brace designs, lightening holes, and tubes.
- Cradle sizes and designs are heavily influenced by what they are intended to support, i.e.:
- Suspension and steering subsystems only
- Suspension, steering, and powertrain subsystems
- All of the above plus the condenser-radiator-fan module (CRFM)
- Aluminum intensive executions were primarily used for large cradles with an average weight of 16.8 kg.
- The average weight of large steel cradles was 29.4 kg.
- The average weight of small steel cradles was similar to that of large aluminum cradles.
- The average weights of medium steel cradle and hybrid cradles were similar.
Optimized Engine Cradle Baseline Design
The engineering team selected a large steel intensive production cradle as a baseline for their redesign and optimization study. Figure (1) is an illustration of the baseline cradle showing the gauges and steel grades used for the different cradle components. As seen, all the parts were predominantly designed using High-Strength Low Alloy (HSLA) steel grades.
Figure (1): Baseline Production Cradle
A number of lightweighting measures were considered including replacing lap welding with butt welding, replacing HSLA grades with 3rd Gen 1180 for strength-driven components, gauge optimization, tailored blank rings and light front cross members. The final design reflected all these measures as shown in figure (2).
Figure (2): Optimized Design Concept
A comparison between the baseline and optimized designs is summarized in the table below.
In conclusion, the Martinrea team created a steel-intensive cradle design concept that was 15 percent lighter with a modest cost premium when compared with aluminum intensive executions. The structural performance of the optimized cradle exceeded that of the baseline design within the same packaging constraints. The broad spectrum of AHSS grades coupled with sound design principles and state-of-the-art optimization methods can help reduce the weight of automotive body and chassis structures by up to 25 percent.
For more information or to view the entire presentation, visit www.autosteel.org