Underriding concerns

A NEW ERA in underride regulation will be ushered in September 1.

Currently new trailers are allowed to have rear impact guards that comply with U S standards but fall short of the higher performance requirements of Canadian Motor Vehicle Safety Standard 223, a regulation that Transport Canada announced September 30, 2004.

“Transport Canada published its proposed regulation in the Canada Gazette in 2004,” said Jeff Patten, a registered professional engineer who is the manager of test and evaluation engineering with the National Research Council's Center for Surface Transportation Technology. “It included a three-year grandfather clause for underride guards meeting the existing standard. But September 1, a big line is drawn in the sand. You must comply.”

Effective September 1, all newly manufactured trailers with a gross vehicle weight rating of 4,536 kg (10,000 pounds) or more will be required to comply. Exceptions: some low-chassis trailers, some specialized trailers, and trailers whose wheels or structure prevents or limits underride.

How much more higher are Canadians setting the bar? CMVSS 223 includes the same P1 and P2 tests as the U S standard. However, it also requires that the guard be able to withstand a full-width crush test of 350 kN and absorb 20 kJ of energy. This CMVSS requirement is more than three times the strength requirement of the U S standard, and the energy absorption is nearly four times that called for in FMVSS 223 and 224.

Another major difference is the ground clearance requirement. FMVSS 223 specifies that the bumper be no more than 560 mm above the ground and the bottom of the guard prior to the test. The new CMVSS 223 requires the same distance — but after the impact.

Trailer manufacturers can skip the energy absorption requirement by building a substantially stronger guard. If the guard can withstand a force of 700 kN, no energy absorption test is required. Manufacturers have the option of building to either set of specifications.

The changes brought about by CMVSS 223 have required the CTEA to modify the generic rear impact guard program that the association has developed.

CTEA contracted the Center for Surface Transportation Technology, part of the National Research Council of Canada, to perform design and test efforts and to develop application packages for CTEA program stakeholders.

CTEA believes that this program with CSTT has produced not just a single answer, but several. While the project is not yet complete, the designs have been developed, and physical testing is now underway for three new generic designs.

Manufacturers' criteria

While Transport Canada has set its performance objectives, CTEA has its own criteria — from a trailer manufacturer's perspective — of what a generic guard needs to be like. Among the considerations:

  • The guard must use readily available material. The association is evaluating guards made of steel, aluminum, or stainless steel.

  • The design should distribute the forces, be as strong as possible while still absorbing energy, and has to be suitable for practical welding.

  • It should bolt directly onto the frame rails or some type of integration kit.

CSTT developed the designs for strength and energy absorption with the help of finite element analysis and a physical testing program.

Once the testing was finished, finite element analysis was used to evaluate and fine tune the design.

Validating the designs

While finite element analysis is a valuable tool, the key to proving the validity of the designs is in physical testing. Through CSTT, the association came up with a dedicated test fixture that the CTEA could call its own. It is capable of pushing 300,000 pounds (1,335 kN). It has three load cells, each rated at 100,000 pounds.

The pusher assembly is restrained to move horizontally only and does not rotate. It is designed to move at a rate of 40 mm per minute.

To test each guard, CSTT installed it on the test stand along with the pull cord transducers to measure movement. After starting data acquisition and waiting five seconds, the pusher exerted force on the guard until it moved forward 125 mm. Researchers then halted the process, analyzed the guard, and took pictures. They repeated the cycle, pushing ahead an additional 125 mm as many times as required until the guard failed.

What they learned

In an effort to expand the capabilities of the CTEA's existing generic underride guard, CTEA and its researchers made several discoveries, according to John Billing, senior research officer with NRC-CSTT.

“Previously, minimum height of the support was 18",” Billing said. “At 18", we were getting at the edge of the capacity of the welding. We have a real risk of it tearing off under the new forces to which the guard will be subjected.

“With the new design, we have the same working height, but we have increased the height of the support to minimize the risk of tear-off.”

Billing also said that the project discovered ways to mount the guard that do not necessarily involve the bottom of the frame rails.

The design uses the same bumper — 4" × 4" × 3/16", but the total system is stronger than the previous generic rear impact guard.

“If someone accidentally backs into a dock, or if the guard is subjected to other incidental battle damage, you will not need to do as much repair,” Billing said. “In the event of a nuclear war, stand underneath this guard.”

The overall strength of the guard simplifies some of the testing for compliance with U S standards.

“We can say definitively that if you pass the CMVSS 223 test, you will automatically pass the FMVSS 223 test.

The only FMVSS test really needed to be done is the full width.

“Any guard with this general configuration that passes the full-width push test at 350 kN automatically qualifies under the FMVSS rule and can be certified for use on vehicles operated in the United States. It is double the energy and about 125,000 pounds stronger than the standard requires.”

A generic upgrade

The designs also will be substantially stronger than CTEA's generic rear impact guard that members have been using for several years.

“A goal of the CTEA is to initiate consortium-testing programs that could lower the cost of meeting compliance regulations,” says Eddy Tschirhart, CTEA's director of technical services. “Because CTEA members sell vehicles to customers in the USA or vehicles their Canadian customers purchase travel to U S destinations, it was necessary for many CTEA members to comply with the U S regulation.”

The idea for a generic rear impact guard was initially conceived by CTEA to assist smaller Canadian trailer manufacturers and exporters to certify their trailers to meet the requirements of FMVSS 223 and 224 introduced in 1996. FMVSS 224 established the dimensional tolerances and required this mandatory safety device on most new trailers manufactured and/or operating on U S federal highways, while FMVSS 223 established the strength and minimum energy absorbing characteristics and certification test procedures.

In 2002, Transport Canada concluded that rear impact guards meeting FMVSS 223 and 224 were not strong enough. As a result, the agency wrote higher performance requirements into its CMVSS 223.

In February 2003, CSTT undertook a project for Transport Canada to develop a recommended practice for generic rear impact guard design concepts that could meet the proposed standard. The result was a recommended practice for three additional generic designs, in steel, all based on the previous CTEA generic design.

“Manufacturers Compliance Procedures and the requirement are quite straightforward, under the current practice for installation of a CTEA generic rear impact guard that has been shown to meet FMVSS 223,” says Don Moore, executive director for CTEA. “However, different (and higher) forces are required for new rear impact guards that comply with CMVSS 223, and that may affect the way the new guards are attached.”

According to NRC-CSTT, the guard must be mounted in a manner that allows the attachment and vehicle structure to withstand the largest force that can be imposed. That force occurs just as the guard begins to yield.

Manufacturers who follow the appropriate steps using the package provided by NRC-CSTT and evaluate the attachment of the generic rear impact guard on their trailers will have a guard that complies with CMVSS 223, and will have the supporting documentation to allow a regulatory inspector to determine compliance without need for a further test. The basic procedure involves designing the rear impact guard attachment, making assembly drawings, manufacture, putting on the compliance sticker, and keeping records.

According to NRC-CSTT, if a trailer manufacturer can show its membership in the CTEA CMVSS 223 rear impact guard program, a regulatory inspector will be able to verify compliance by checking that:

  1. The manufactured rear impact guard corresponds to the base drawing.

  2. The rear impact guard is manufactured from materials meeting the minimum specifications.

  3. The manufacturer's attachment design is based on appropriate calculations.

Trailer manufacturers that participate in CTEA's rear impact guard program must consistently manufacture the guard based on the recommended package and ensure that the rear impact guard is attached to the trailer in a way that complies with CMVSS 223.

“The calculation required is a straightforward static stress calculation, and manufacturers should be able to design the attachment of a rear impact guard to a specified static load, if they have already designed the primary structure to resist all service loads for the life of the trailer,” Patten says. “A manufacturer without an engineer on staff could use an outside engineering service, such as NRC-CSTT, to do the work. NRC-CSTT is available to work with interested CTEA members on these or other issues.”

To date, NRC-CSTT has completed the design and construction of the test fixture and is currently testing rear impact guard designs in steel, stainless steel, and aluminum. Program packages will be available to stakeholders early in 2007.

For more information, contact the CTEA offices at (519) 631-0414 or email [email protected].

About CSTT

CSTT is part of Canada's NRC-a federal government organization with a mandate to conduct leading edge research and development in support of Canadian industry. NRC-CSTT has been a fully cost-recoverable Technology Centre since 1995, and provides consulting engineering services to national and international clients in the highway and railway sectors of the transportation industry. Web Site: cstt.nrc.gc.ca

For more information regarding NRC-CSTT's services for CTEA members, including testing of unique rear impact guard designs and help with rear impact guard attachment issues, please contact Rick Zaporzan, Business Development, Road Vehicle & Military Systems Division at (613) 990-7249 or via email at [email protected]

Underride: a brief background

1969 — NHTSA proposes requiring underride guards capable of withstanding 333 kN.

1996 — The US introduces FMVSS 223 and 224

1999 — CTEA reports on its generic underride guard.

2000 — CSTT crush tests; CSTT develops a generic underride guard for use in Canada that exceeds FMVSS 223 requirements.

2002 — Transport Canada conducts crash tests indicating that small vehicles will experience extensive damage when impacting guards that were built to the standard at the time.

2003 — CSTT issues a recommended practice calling for stronger guards capable of absorbing more energy than those currently in use.

2004 — Transport Canada announces CMVSS 223 for trailers.

TAGS: Trailers
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