GM takes indoor rollover testing in house

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A 2006 Buick Rainier drives off a ramp at the new GM rollover test facility.

A sport utility vehicle skids sideways until it abruptly stops and two wheels lose contact with the pavement. In short order, the vehicle topples on its side and the belted occupants make hard contact with parts of the interior.

That grisly occurrence happens every day on U.S. highways, which is why engineers replicate rollovers and capture the resulting mayhem under bright lights and high-speed imagers to develop and refine safety systems.

Of course, airbags help protect vehicle occupants during a rollover accident. But rollover-activated airbags—which stay inflated for several seconds longer than do traditional head-curtain and side-impact airbags—provide even more protection. And when it comes to vehicle crashes, safety is ultimately measured in lives saved.

Approximately 40% of all vehicle accident fatalities in the United States involve rollovers, even though those crashes account for only about 2% of all motor vehicle incidents. "We're going to be able to make cars and trucks safer because of what we'll be doing at the rollover test facility at the Milford Proving Ground," said Robert Lange, General Motors Executive Director for Structure and Safety Integration.

Until December 2006, indoor vehicle rollover tests for GM were conducted at safety system supplier Autoliv's Auburn Hills, MI, facility. While an average 125 vehicle rollover tests were done each of the past three years at Autoliv's facility, GM engineers will spearhead between 150 and 200 indoor vehicle rollover tests inside a newly opened $10 million GM facility. Although SUVs and vans typically are used for GM rollover tests, all vehicle types eventually will undergo tethered and nontethered tests. "We're able to reuse a vehicle for additional rollover tests if it's tethered because vehicle damage isn't as severe as it is with a nontethered test," said Lange.

Engineers can use computer tools to simulate a frontal vehicle crash, but mimicking vehicle rollover crashes via math models to develop sensor calibrations remains a murky undertaking. "We can't use simulations to develop a rollover sensor calibration because the simulations are conducted with a rigid body model, and vehicles inherently are not perfectly rigid. Vehicles are flexible because they're made of rubber and metal and other materials that flex," said Bridget O'Brien-Mitchell, GM's lead engineer for airbag sensor development.

Computer programming does facilitate real-world rollover tests by handling tasks such as getting the test vehicle to a speed that prompts a rollover or falls short of prompting a rollover, as both scenarios are important to the process of developing an airbag-deployment sensor. "The purpose of the rollover-sensing system is to discriminate between deployment and nondeployment rollover events," said O'Brien-Mitchell. "The overall goal is to improve the containment of occupants in rollover crashes by knowing precisely when to deploy safety systems such as curtain airbags and seatbelt pretensioners."

The U.S. government does not mandate a specific vehicle rollover test, but U.S. federal motor vehicle safety standard 216 requires that a vehicle undergo a static roof crush test. A proposal, if approved, would increase that crush standard from 1.5 times to 2.5 times the vehicle's weight. At GM's new rollover facility, four different tests—each designed to replicate an actual rollover—represent the in-house tests. The proving ground, especially off-road areas and rough road surfaces, complement the tests being done in a 38,500-ft² (3577-m²) facility adorned with 1728 8-in (200-mm) light globes that produce 1000 W per lamp.

"Each 20-ft bank of lights is suspended from an overhead wench-drive platform that is computer-controlled to lift the lights from floor level to 27 ft high, and each light bank can be tilt-adjusted from 10° off vertical to 80° off vertical," said Brian Coughren, Vice President of Seattle Safety, the light system supplier for GM's new rollover test facility. The$1 million lighting system illuminates a zone 120 ft (37 m) long by 15 ft (4.6 m) wide by 9 ft (2.7 m) tall. "This is absolutely the largest-capacity safety test lighting system that Seattle Safety has ever built," Coughren said.

That 40,000-lb (18,140-kg) tungsten-halogen lighting system, as well as in-vehicle-mounted lighting built by GM's Vehicle Safety and Crashworthiness Laboratory personnel, provides the illumination for high-speed imagers. The off-board-mounted digital cameras, supplied by Redlake, can capture 1000 pictures per second at approximately two megapixels per picture. The cameras, however, can provide 100,000 pictures per second at reduced resolutions. Onboard cameras, also supplied by Redlake, can be placed in more confined spaces, such as under a seat, in the footwell, or between an occupant and the door.

"GM designed in the ability to run a maximum of 25 cameras for offboard imaging of their rollover events, and a maximum of eight cameras for recording the onboard event. The number of cameras used will depend on the actual test requirements and can be as few as four or five offboard and two to four onboard," said Michael Meyer, Redlake Business Manager for Vehicle Impact and Testing.

A key information source point for any live rollover test is the crash-test dummy. At GM's new facility, each of the four dummies supplied by First Technology Safety Systems represents a 50th percentile male fitted with internal data acquisition. To gain useful information from a rollover test, "you need to see the dummies' kinematic responses," said James Flachsmann II, Technical Services Manager for First Technology.

Crash-test dummies essentially help engineers fashion a safety system roadmap. "The onboard and offboard images are analyzed after a test for several uses, including dummy movement during the crash," noted O'Brien-Mitchell. A rollover test enables engineers to develop rollover sensor calibrations "to determine the conditions in which a rollover sensor-commanded deployment is necessary and conditions for which the rollover sensor should not command deployment," O'Brien-Mitchell said. A test evaluation also helps engineers understand how occupant injury occurs as well as how the safety restraint system performs during a vehicle rollover.

More than 20 GM vehicles—representing Chevrolet, Pontiac, Saturn, Buick, GMC, Cadillac, Saab, and Hummer brands—will offer rollover-activated airbags in the 2007 and 2008 model years. GM expects to be the first automaker to offer rollover-activated airbags on all of its vehicles and has set the 2012 model year as the target deadline.

Kami Buchholz