Passing the money test

  • 06-Aug-2008 09:57 EDT
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The Millbrook dipping sled is described as not only “the simplest, most cost-effective solution to a problem the motor industry faced but also the most accurate” by Crashworthiness Manager Phil Glyn-Davies.

Testing is not just about proving or improving vehicle mechanical integrity and capability. It is also about saving money for OEMs and suppliers. A significant example of that is the outcome of the initial program for SUV crash testing using a newly designed “dipping sled” system (previously described by AEI) at the Millbrook research and development facility in the U.K.

Millbrook has revealed that the program was so successful that its first European customer for the system estimates that the results of the test work recently completed could save up to €2 million in late design changes on every relevant new SUV or pickup program.

The outcome is significant because of changes in U.S. fuel economy incentives that are affecting large pickups and SUVs, many of which are constructed on a chassis that has “swan neck” front structural members, stated Millbrook’s Crashworthiness Manager Phil Glyn-Davies: “Safety engineers are aware that for vehicles of this construction, there is a consistent anomaly in the correlation between whole-vehicle frontal (and to some extent rear) impact results and those simulated on the test sled. This is significant because it means that the design can only be finalized after whole-vehicle crash testing right at the end of the vehicle program, leading to delays and substantially increased costs.”

Many new types of physical crash simulation have been developed to address the problem, but until recently all had required costly specialist sleds for a significant but far from complete improvement in correlation, added Glyn-Davies. The breakthrough came when a major European manufacturer of SUVs commissioned the crashworthiness laboratories at Millbrook to develop a fast, accurate, and affordable derivative of proven sled-based simulations.

“Our client found that a number of critical interactions between the occupants and the restraint systems, particularly the ride-down performance of the steering column, could be compromised by the cabin pitching forward during the impact,” explained Glyn-Davies. “This dipping motion was not simulated by the traditional test sled.”

The problem is caused by the design characteristics of many SUVs, pickups, and other off-highway vehicles that have high ground clearance at the front to allow good ramp access. To achieve this, the chassis rails are high at the front and then “swan neck” down to allow a lower cabin floor that improves cabin space and occupant access. When the rails are impacted at the front, the swan neck causes them to bend like a hinge at the base of the A-pillars, tilting the cockpit down. The Millbrook team found that due to this effect, traditional laboratory sleds (which only simulate movements in the horizontal plane) could underestimate peak chest acceleration values by as much as 20%.

“Typical vertical movement at the A-pillar may only be 100 mm in a 56-km/h front impact, but the resulting changes in angles can have a substantial effect on the ability of the steering column to collapse as designed—and consequently also on the positioning of the airbag,” said Glyn-Davies. “Many of the other interactions between occupants and the vehicle are also affected, including contact with the pedals, seatbelts, and knee bolsters, and the occupants’ movement in the seat.”

The Millbrook dipping test system is mounted on the sled carriage via a clevis pin at the rear. The front of the dipping carriage is supported at the appropriate height and angle by aluminum honeycomb blocks. During the impact, the moment of forces at the pivot creates a downward force on the dipping carriage, crushing the honeycomb blocks to allow a controlled downward movement. Up to 120 mm (4.7 in) of vertical pitch down is possible at the A-pillar position.

Calibration of the system is achieved by adjusting the size, density, and position of the blocks and by adjusting the height of the combined center of gravity of the dipping sled and test piece. This facilitates tuning of the onset time, rate, and maximum vertical displacement to ensure a close correlation with real-world impacts. The vehicle cockpit can be positioned at different distances from the pivot to change the ratio of A- and B-pillar movement.

“We believe this is not only the simplest, most cost-effective solution to a problem the motor industry faced but also the most accurate,” added Glyn-Davies. Millbrook may consider building similar systems for customers’ in-house test laboratories.

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