Fleet operators and the repair shops they patronize face a paradox with respect to trailer rear underride guards. When a guard is damaged, it must be repaired but not strengthened.

To determine the proper procedure for underride guard repair, The Maintenance Council, an affiliate of the American Trucking Associations, has established a task force to write a recommended practice to be used by repair personnel. A draft of that recommended practice was presented to TMC members during the Fall Meeting in Kansas City on September 14, 1998. The TMC S.7 task force for Repair Techniques for Rear Impact Guards is chaired by Joe Fleming, vice-president of Compass Enterprises Inc.

The recommended practice applies to the rear bumpers that serve as underride guards on trailers produced after January 26, 1998. It includes refrigerated trailers, dry vans, and platform trailers and excludes trailerswith work-performing equipment mounted at the rear.

Two Federal Standards Rear impact guards are regulated by Federal Motor Vehicle Safety Standards FMVSS 223 and FMVSS 224. These standards establish the strength, location, dimensions, and energy absorption requirements for trailer rear bumpers. In addition to specifying bumper performance, the standards require underride guards to carry a certification plate that identifies the manufacturer. The label must provide the name and address of the guard manufacturer, state the month and year of manufacture, and carry the letters "DOT." This label must be mounted on leading edge of the horizontal bumper, 12 inches from the right edge.

The Federal Highway Administration has a rule pending that requires fleets to ensure that underride guards conform to their original configuration throughout the service life of a trailer. The rule also proposes to require the certification label to be present for the entire trailer life. One of the purposes of the TMC draft recommended practice is to influence the content of a federal regulation that has not yet been adopted, meeting participants said.

The TMC draft recommended practice proposes regular inspections for rear underride guards. Inspectors are to look for cracked welds, cracked or damaged vertical support members, or cuts and tears in any part of the guard structure. In addition, the inspections should confirm that the rear impact guard conforms to its original dimensions. If necessary, trailer manufacturers may add bracing to the guard such as diagonal struts running from the horizontal bumper to the vertical supports.

Bracing Placement The most logical placement of diagonal struts would be to attach them at the middle of the horizontal bumper and at the upper limit of the vertical supports. However, this placement could interfere with the operation of dock locks, some TMC members said.

The primary goal of any inspection regime would be to ensure that impact guards are kept in the same condition and configuration as they had on delivery from the manufacturer.

Bent horizontal bumpers are the most common damage to rear impact guards. Typically, the horizontal bumper is a four-inch square tube made from 3/16-inch steel. Vertical deflection of the horizontal bumper in the space between the vertical supports is a particular problem. This damage is commonly caused by the dock locks which hold trailers against the dock during loading or unloading. If the vertical deflection is less than three inches, no repair is required. TMC believes that three inches of deflection in any direction is acceptable unless the impact guard has been moved from its required position under the trailer.

Reposition Bent Bumpers

TMC's recommended practice suggests that bent horizontal bumpers should be repositioned if the deflection is more than three inches. The same is recommended for deflection of the outer ends of the bumper. Deflection should be corrected by bending the bumper back into place. However, TMC does not recommend heating the bumper to aid in the process. Only cold bending should be attempted, because heating the metal changes its strength and, as a result, alters impact guard performance.

Inspections must include more than just the impact guard, TMC says. In some cases, the horizontal bumper can be bent toward the nose of the trailer. The impact that caused such damage could also damage the trailer structure. When the impact guard has been forced forward, maintenance personnel should inspect trailer crossmembers and the rear six feet of trailer floor. Damage to these areas should be repaired when repairs are made to the impact guard. TMC recommends that the entire horizontal bumper be replaced when damage of this type is discovered.

In some cases, the entire guard becomes canted toward the trailer nose without physical damage to the structure. Repair in such cases can performed by bending the guard back into position. Repair shops must take care to ensure that the horizontal bumper is 22 inches above the ground and that the vertical plane of the remains 12 inches from the rear face of the trailer.

Ignore Minor Damage The draft recommended practice concedes that impact guards are subject to extensive physical contact during normal use. These nicks and bumps pose no cause for alarm. The draft says that minor cuts, tears, dimples, bends, or other minor damage do not impact underride guard effectiveness. However, the draft directs immediate repair of cracked welds.

Replacement parts and repair procedures remain a subject of some controversy. Trailer manufacturers favor repair that requires the use of original equipment parts purchased from and installed by the original manufacturer. Many fleets are seeking bolt-on replacement underride guards. Fleet operators voice a desire for generic replacement parts that will fit any trailer or allow repairs to any impact guard.

The recommended practice for rear impact guard repair is still under consideration. For the present, TMC suggests that underride guards should be repaired using original replacement parts installed with techniques equivalent to the original manufacturing procedures.

Maintain Energy Absorption Repair parts and procedures require a delicate balancing act, because structural integrity of the repaired impact guard is not the only issue. If strength were the only concern, repairing rear impact guards wouldn't be a problem, but energy absorption must be considered as well, said one of the trailer engineers attending the task force meeting.

The task force was in general agreement that an entire underride guard should be replaced if the vertical supports become significantly damaged. However, the horizontal bumper is subject to the most abuse. Fleet operators seem to want trailer manufacturers to adopt designs that use the same horizontal bumper throughout the industry. This would allow fleets to purchase replacement parts from a wide variety of sources. Trailer manufacturers insist that each design is unique. The task force left the issue of generic replacement parts for debate at future meetings.

Resolving the issue of the federal certification label also got pushed forward to future meetings. In general, the label must be considered permanent. However, regulations require the label to be located on the horizontal bumper. The label must remain on the underride guard for the life of the trailer. The task force must find a way to comply with this requirement while allowing for horizontal bumper replacement