Cal Poly Student Designers Propose Solutions to Bumper Height Incompatibility Issues

Detroit, MI, August 24, 2005 - California Polytechnic State University, San Luis Obispo, released three engineering student designs that serve to normalize the difference in bumper height among cars, sports utility vehicles (SUV) and light trucks.  The three concepts are identified by intriguing names:

  1. Torsion Bar Connected to Firewall Support (Figure 1)
  2. Reinforcing Rails with Leaf Spring Bumper (Figure 2)
  3. Collapsing Cow Catcher. (Figure 3)

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Figure 1:
Torsion Pendulum Firewall Solid Model

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Figure 2:
Reinforcing Rails and Spring Bumper

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Figure 3:
Collapsing Cow Catcher Solid Model

The project, which continues with the build of concept models, is sponsored by the Bumper Task Force of the American Iron and Steel Institute (AISI), which in recent years has become more aggressive in its problem-solving role for the North American automotive industry.

Bumper height incompatibility, which is the difference in height between light trucks or SUVs and passenger cars, has been the focus of a concerned effort by automakers worldwide looking to minimize the repair cost for damage that occurs when two such vehicles meet in a minor “fender bender.”  Because of the difference in heights, cars don’t always bump together bumper to bumper, and excessive damage can occur to the vehicle with a lower bumper.

The Cal Poly student design team began by creating thirty idea, which were reduced to twenty-one.  Using a decision matrix, the team again reduced the field to eight concepts.  Basic analysis of the eight yielded discards and consolidation, resulting in the final three.

Engineering Specifications

The engineering specification puts numbers to the key factors in the design allowing the team to assess how a particular design stacks up with the important parameters of the customer. The Cal Poly team determined that a successful design will meet 11 specifications.

Table 1 lists the specification number, description requirements and the importance of meeting the particular specification with an associated weight factor. In addition, the table shows meeting the specification, which the student design team addresses.

Table 1: Engineering Specification
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Loads

As this project was primarily concerned with the generation of concepts, the loads are not as important as they would be in, perhaps, a deflection-driven final design. The loads used to analyze the proposed concepts are garnered from crash test data at 5 mph. The force necessary to yield the bumper is 64,314 N. This force was used as the low-speed impact force. The force necessary to start frame rail buckling is 446,976 N. This force is used to simulate the impact of a high-speed crash. The forces available were sufficient to compare the concepts, but accurate high-speed crash data would be needed before analysis is complete.

Concept Table

The Decision Matrix, Section 2, includes all the concepts and the most important four design specifications, (5 mph crash, 35 mph crash, feasibility, and made of >90% steel).  The design specifications were used to rule out over half of the ideas before any analysis was necessary. Each specification was evaluated for every concept and a value was given based upon the chances of the concept meeting the specification. The green highlighting indicates an idea that meets all the specifications and those eight concepts were analyzed.

Table 2: Decision Matrix
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For the theoretical case, the Cal Poly team used the front bumper of a Ford F-150 pickup.  The concepts focused on a bumper drop of 150 mm (~6 in.) and efforts were made to make a majority of the new structures from steel.  The analysis remains valid for different geometries, but the parameters would need to be changed to garner an accurate solution to a different configuration. 

The student team based its designs using a yield strength of 400 MPa. For Finite Element Analyses, concepts were rendered in a Pro/Mechanical FEA model.  This was followed by tuning the geometry, size and cross section of the various design elements. 

Even with Finite Element Modeling, the analysis completed is not sufficient for a final design. Many more factors must be considered including: dynamic loading, plastic deformation, and packaging constraints within the vehicle. Actual high-speed crash data would also be invaluable for future analysis. These designs are general concepts for a theoretical problem. Any of these concepts must be specifically adapted to the vehicle in which they would be implemented.

A complete discussion of the concepts and their analyses are presented in a 93-page report available on line.

The Automotive Applications Committee (AAC) is a subcommittee of the Market Development Committee of American Iron and Steel Institute 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 key to its success. This industry cooperation resulted in the formation of the Auto/Steel Partnership, a consortium of DaimlerChrysler Corporation, Ford Motor Company and General Motors Corporation and the member companies of the AAC. For more news or information, view the American Iron and Steel Institute /Automotive Applications Committee's website at www.autosteel.org.

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 key to its success. This industry cooperation resulted in the formation of the Auto/Steel Partnership, a consortium of DaimlerChrysler Corporation, Ford Motor Company and General Motors Corporation and the member companies of the AAC. For more news or information, view the American Iron and Steel Institute /Automotive Applications Committee's website at www.autosteel.org.

American Iron and Steel Institute/
Automotive Applications Committee:
AK Steel Corporation
Dofasco Inc.
Mittal Steel USA
Nucor Corporation
Severstal North America Inc.
Stelco Inc.
United States Steel Corporation

Bumper Project Group Member Companies:
A.G. Simpson
Benteler Automotive
Cosma International, Inc.
DaimlerChrysler Corporation
Flex-N-Gate
Ford Motor Company
General Motors Corporation
Meridian Automotive Systems
Shape Corporation
SKD Automotive Group