AMERICAN RECREATIONAL VEHICLE MANUFACTURER USES FINITE ELEMENT ANALYSIS SOFTWARE TO IMPROVE CONTEMPORARY DASHBOARD DESIGN

Coachmen's 1999 Sportscoach is more modern, aerodynamic and ergonomic than previous motorhome models.

September 4, 1998, Pittsburgh, Pennsylvania -- Recreational Vehicle (RV) camping ranks second only to wildlife viewing in satisfaction among participants in outdoor activities, exceeding hiking, biking, boating and other sports, according to a 1997 survey conducted by the Go RVing Coalition. To stay competitive in the growing RV market, RV manufacturers must design safe, aesthetic vehicles that require minimal maintenance. They must road test RVs for safety and durability to verify all components' functionality. A leading American RV manufacturer, Coachmen Recreational Vehicle Company, recently used finite element analysis (FEA) software by Algor, Inc. to test an innovative new dashboard under loading conditions experienced in an RV on the road. Analyzing the dashboard on the computer saved the company time and money on physical prototype testing.

Coachmen Recreational Vehicle Company in Middlebury, Indiana recently introduced its Sportscoach model motorhome, a contemporary, affordable vehicle that has an aerodynamic design and more features and ergonomic considerations than any other Coachmen product. One of the innovations in the new Sportscoach is a plastic-molded, automotive style dashboard to replace the outdated, vinyl dashboards of older motorhomes. Coachmen used Algor's linear static stress analysis software to analyze the new dashboard's steel support structure after subjecting it to a static load using the computer. The company optimized the structure's geometry by locating and strengthening its weak areas and shortened the length of its design cycle in time to introduce the Sportscoach at the biggest RV industry trade show of the year.

Modernized Dashboard Must Withstand Weight of Human

Coachmen Recreational Vehicle Company designs and manufactures RVs including motorhomes, travel trailers, fifth wheels and truck campers for sale primarily in the United States and Canada. Coachmen's Class A Motorhome Series is marketed to those who wish to explore the great outdoors for months at a time, but who also want to travel with the comforts of home.

Coachmen's top-of-the-line Sportscoach was constructed more efficiently and ergonomically than previous models. Its new ABS plastic dashboard is tool molded in one piece rather than assembled from numerous pieces like the conventional vinyl dashboard. The dashboard is smaller and lighter, increasing cabin space and RV capacity, which enables customers to purchase additional features. It also gives drivers greater road visibility and its gauges and instrument panel are larger and placed in the driver's line of sight.

Because the dashboard can be up to 96 inches wide, it requires a support structure beneath it. The steel support bracket that Coachmen designed for the plastic dashboard consists of four parallel bars that extend approximately 18 inches into the cabin from the front of the vehicle. They are connected by two perpendicular bars, one is mounted to the front bulkhead of the vehicle and the other extends across the width of the dashboard.

Coachmen needed to test the strength of the new dashboard support structure to locate potential stress because the dashboard's collapse could cause extensive injury or maintenance costs. Coachmen realized that the RV's low, wide dashboard invites people to sit or lean on it. The company determined that simulating the weight of a person sitting on the dashboard's midsection and the weight of a person leaning on the dashboard's side section above the glove box would represent the most severe stress profile experienced by an RV dashboard. Designing towards this profile would ensure that the dashboard could withstand other RV experiences, such as vibration during road travel.

"Producing and testing a dashboard prototype can cost $40,000 so the entire dashboard design, including the steel support structure, must be as close to the final design as possible when a prototype is constructed," said Jim Keough, engineering design manager. "Using FEA software to test the support bracket allowed us to quickly pinpoint problems with our design and saved us the time and cost of two prototype tests."

Coachmen had little time to spare because they intended to introduce the new Sportscoach at the Recreational Vehicle Association's annual trade show, which was quickly approaching.

Preparing for the Stress Analysis

To prepare for the support structure's stress analysis, Scott Schrock, product designer at Coachmen, designed a model of one-half of the dashboard support bracket using AutoCAD 14. He modeled the passenger side in order to analyze stress results for the area above the glove box. Jim Keough imported the model into Algor to prepare for a finite element analysis that would replicate the application of weight on the support bracket and reveal resulting stress. Keough used Algor's Supergen meshing tool to create a four-node quadrilateral mesh.

"Algor's Supergen automatic meshing engine quickly made the elements symmetrical in non-uniform areas, which saved me a lot of time," said Keough. "In the past, I created a mesh around critical features like holes by hand and made certain that the elements matched."

Keough chose 3-D plate elements to best represent the steel bracket's thin-walled material and orientation. "It was convenient for me to quickly alter the material's thickness to optimize the design when it became necessary," said Keough.

To determine the steel material's yield point, Keough chose the Tresca stress yield criterion because of its conservative yield calculation for combined stress. Keough then applied steel material properties to the model based on standard steel purchasing requirements. He applied boundary conditions at the interface between the bracket and the front bulkhead of the vehicle.

Keough applied a 200-pound static load to the left bar of the support bracket model, or "arm", that represents the center of the dashboard support and a 100-pound static load above the glove box. The 200 pounds represented the average weight of a person sitting on the midsection of the dashboard and the 100 pounds represented stored material in the glove box as well as a person leaning on the passenger side of the dashboard. The goal of the analysis was to determine if these heavy cantilevered loads would exceed the yield stress of the steel bracket. If its yield stress was exceeded in the analysis, the bracket would fail to support the plastic dashboard covering under similar real-world conditions.

Analysis Results

In the first analysis, Keough found excessive stress in the cross bar above the glove box. He increased the stiffness of the entire bracket support by increasing its material thickness. The stress remained excessive in the same area in the second analysis so Keough added a one-inch flange to the cross bar above the glove box to increase its strength and reduce stress.

Coachmen's Algor model was one-half of the original design. The analysis results to the left follow the addition of a one-inch flange to the cross bar above the glove box (lower right). Stress remains excessive in the cross bar and begins to appear in the arm of the support bracket (left).

The third analysis revealed a new problem as a result of the thickened cross bar; some of the stress from the cross bar had shifted to the arm of the bracket support model. It also showed that the one-inch flange did not eliminate excessive stress above the cross bar. Keough added another one-inch flange to the top of the arm to compensate for the shifted load and also added a Z channel to the length of the cross bar to stiffen the entire bracket assembly.

The final analysis showed that stress levels had been reduced below Coachmen's limit of 25,000 PSI except at the tip of the arm, which Keough attributed to point loading over a small area. Keough had successfully reduced stress by strengthening the support bracket in critical areas. He had completed crucial redesign steps before creating one physical prototype.

The final analysis showed that stress levels decreased below the required limit of 25,000 PSI except at the tip of the arm, which was attributed to point loading over a small area. Engineering design manager Jim Keough had removed stress by strengthening the support bracket in critical areas.

"The weakest point in the model was identified in each analysis. I was able to easily work the weak point out of the design in each iteration to arrive at the final configuration," said Keough.

Coachmen's advanced development department proceeded to manufacture a dashboard prototype. They tested the prototype in their laboratory, checking its fit, function and response to loading. The results correlated closely to Algor's software results. A final RV prototype was created and subjected to a lifetime durability test at an automotive test ground. The test simulates actual road events such as railroad track crossings and other rough terrain. The road test also confirmed Algor's software analysis results.

Coachmen Saves Time and Money with Algor Software

"The virtual manipulation of the design was much more cost effective and timely than the trial and error method of prototyping and testing each concept," said Keough. "The evaluation of the bracket design would have taken weeks longer without the Algor analysis."

Keough also appreciated being able to share the visual results with his company with minimal effort. "To locate an area of interest, I don't have to scroll through tables with elements and node numbers," he said. "I can obtain a colored stress output, convert it to a bitmap file and place it in a word processing document for easy presentation to my colleagues."

Coachmen did not study the effects of dynamic stress on the dashboard, such as vibration, because the company designed it with the most severe static load in mind. The support bracket inherently handles lighter loads like everyday vibration from road conditions. But Coachmen is expanding their design processes in the future to include composites, nonlinear and vibration analysis.

An Algor customer for eight years, Coachmen reports good correlation between the analytical and practical results. Coachmen used Algor's linear static stress analysis software to improve other parts of the Sportscoach including the RV's sidewall that is 38 feet long and only 1 ½ inches thick to ensure that it could withstand the handling and assembly process. They also used Algor to analyze the RV's bumper attachments and various brackets for mounting appliances.

The Sportscoach model achieved the highest number of new product sales ever recorded at the Recreational Vehicle Association trade show in Louisville, Kentucky last December 1997. "We cannot make them fast enough," reports its designer, Eric Johanson.

The Industrial Designers Society of America, an organization sponsored by Business Week magazine, recently awarded Coachmen a 1998 Bronze Industrial Design Excellence Award (IDEA) in the transportation category for the Sportscoach. The Sportscoach was judged on its design innovation, user benefits, ecological responsibility, aesthetics and appeal. It competed against 1,031 entries in 47 categories.

"Algor finite element analysis has become a requirement for all structural evaluations in Coachmen Recreational Vehicle Company's design processes," said Keough. "We are training additional personnel in the use of Algor to further support the growing demand for the analysis tool. Our commitment to providing quality products to our customers begins with Algor software at the early stages of design."