Rear Underride Guard Passes Test; Trailer Engineers Pose Questions

Oct. 1, 1997
A DEMONSTRATION of rear impact guard testing proved to be a learning experience for both regulators and those who will be subject to the regulations.The

A DEMONSTRATION of rear impact guard testing proved to be a learning experience for both regulators and those who will be subject to the regulations.

The demonstration was geared to validate the procedure currently being developed for compliance testing to the requirements of FMVSS 223, "Rear Impact Guards." It was conducted by MGA Research Corporation, the research lab that the National Highway Traffic Safety Administration has commissioned to develop a test procedure for underride guards.

The test attracted 47 people, including 33 members of the Truck Trailer Manufacturers Association Engineering Committee. Manufacturers of van trailers, tanks, platforms, and heavy haulers all watched as an underride guard was slowly squeezed on a rigid test stand.

The TTMA Engineering Committee met August 14 at the MGA Research Corporation test facility in Burlington, Wisconsin. The test of a Stoughton rear impact guard proved that manufacturers can comply with FMVSS 223. But a question-and-answer session following the demonstration drove home the fact that several key issues remain to be resolved during the few months left until the January 26, 1998, effective date of FMVSS 223 and FMVSS 224.

The actual test proceeded smoothly. With the force-application device positioned on the horizontal member at the intersection of the streetside vertical member, the guard moved forward five inches when placed under load, absorbing energy as required by the regulation. NHTSA representatives considered the test successful, and an MGA researcher commented how complying with the rear impact guard requirements is relatively easy to achieve.

Following the demonstration, NHTSA encouraged input from TTMA members in the subsequent question-and-answer session. It was at this point that it became clear that both the regulators and the regulated have work remaining before everyone is ready for the underride standard.

Concerns, Issues Among the issues:

* What specifically is the underride guard? The device that MGA Research tested included not only the horizontal member that the rules address, but also the rear sill and seven crossmembers, slider rails, and enough laminated flooring to cover the entire assembly.

* How much detail should installation instructions include? NHTSA requires the guard manufacturer to have written instructions that explain how to mount the guard on the trailer. But if the guard consists of more than just two vertical members and a horizontal beam, will the manufacturer be required to explain how to assemble such items as the floor, cross sills, and slider rails?

* Is it better to test the guard on a rigid test stand or on a trailer? NHTSA wrote the regulation around compliance testing based on the use of a rigid stand. The agency chose to allow a quasi-static test on a rigid test fixture because such a method would be simpler and less costly than dynamic crash testing. However, several manufacturers expressed apprehension that test-stand certification would be more difficult to defend in the event of a product liability suit.

* If the guard is tested on a trailer, how should the trailer be secured during the test? One manufacturer pointed out that when a platform trailer is held in place by the kingpin during the test, the trailer will bow up in the middle when the force level specified in the regulation is applied. Because the trailer itself absorbs the force, the guard does not deform. As a result, the guard fails the energy absorption part of the test. If the same trailer were held in place by the axles, the guard would be forced to deform and could pass the test. In response, NHTSA said that this is an area that the agency must address.

* What about the certification of replacement guards? While admitting that aftermarket issues are complex, NHTSA said the replacement guards must be certified.

Element of Trust Following some of the "what if" questions, NHTSA personnel responded that the underride regulations will require mutual trust between the agency and the industry. The questions centered on scenarios in which NHTSA tests for compliance in a different manner than the one on which the manufacturer based his certification.

"We are not out to trap anyone," one NHTSA representative said. "You may test the guard on a trailer or in a fixture. If we test your guard, we will do it the same way you did. We want to be able to duplicate your test method."

Some of the trailer manufacturers attending the demonstration wanted to know if the agency might test a guard on a trailer not suited to accept it. "A strong guard mounted on a cardboard box will fail," one engineer said.

The agency responded by pointing out that under the terms of FMVSS 223, the manufacturer specifies how and to what the guard will be attached. The guard manufacturer may dictate the type of trailer on which the guard is to be mounted, along with the trailer structure, and the specifications of the mounting surface.

Conducting the Test MGA Research demonstrated how a rear impact guard can be tested on a rigid fixture. The complete test system (including all equipment and instrumentation) used at MGA cost approximately $50,000, but it has far more capacity than required for testing rear impact guards. Researcher David Winkelbauer estimates that a system costing half that amount would be adequate.

The MGA equipment combined a rigid stand for holding the assembly in place, a hydraulic system for applying the specified force at the required rate, load cells and instrumentation for measuring the forces, and computer hardware and software for evaluating the results.

Under the MGA test, massive steel beams held the assembly in place. To hold the guard assembly in the fixture, crossmember end clips were bolted directly to the beams using 3/8" Grade 5 fasteners. The beams in turn were secured to a frame on which the force application device was mounted.

The MGA stand allowed the force application device to be positioned at any of the five designated test points. For the test of the Stoughton guard, MGA located the device on the horizontal member of the guard where it intersects the vertical member (position P3 of the standard).

An eight-inch hydraulic cylinder with 24" stroke applied the force. A hydraulic pump rated at 2000 psi maximum pressure powered the cylinder with 1000 psi operating pressure.

A control system is required to deliver the load at the precise rate (1.25 mm per second) over a distance of 125 mm (5"). To achieve this, MGA uses a servo valve linked to an electronics box and a computer.

The computer and a software program written in-house regulate the entire test procedure. Technicians simply type in the desired parameters.

Test Results The vertical and horizontal guard members sustained only minor visible damage during the test. However, as the force application device pushed the guard forward, the cross sills were deformed. The deformation increased with proximity to the impact.

"We have energy being absorbed all over the place," one trailer engineer observed.

MGA test equipment recorded data throughout the test process. Results of the test will be available to the public. The NHTSA web site ( will list the test report number when the report is ready (sometime later this year). When complete, copies of the report will be available by calling NHTSA's Information Resources Management office at (202) 366-4941.

NHTSA personnel encouraged those attending the meeting to send comments about the test procedure. The agency specifically is looking for technical input. The Office of Vehicle Safety Compliance will consider practical suggestions designed to clarify the procedure while still achieving the objectives of the standard. Comments should be mailed to: National Highway Traffic Safety Administration, Office of Vehicle Safety Compliance NSA-30, Room 6111 400 Seventh Street SW Washington DC 20590 Phone (202) 366-2832.

"The demonstration gave an indication about what will happen during compliance testing," said Dick Bowling, TTMA president. "The discussions that followed made it clear how important it will be for manufacturers to have detailed installation instructions so that NHTSA can duplicate them."

How to Help Toward the end of the question-and-answer session, one trailer engineer asked NHTSA what the industry can do to help get ready for the coming regulation.

"Give us your recommendations for securing trailers during the test procedure," replied Jim Gilkey, chief of NHTSA's Equipment Branch.

Beyond that, NHTSA said, trailer manufacturers simply need to comply.

"No major changes are coming," said Marilynne Jacobs, director of NHTSA's Office of Vehicle Safety Compliance. "Some technical amendments may be forthcoming, but the basic rule is there. The guard in today's test passed with flying colors. If you know what the rule says, you can comply."