Mr. Dale Dierks
Engineering Manager
1909 S. Rowley
Mitchell, SD 57301

Dear Mr. Dierks:

This responds to your letter requesting an interpretation of whether several different trailers your company manufactures would be excluded from the National Highway Traffic Safety Administration's (NHTSA's) rear impact protection (underride guard) regulations. In a May 14, 1998, telephone conversation with Paul Atelsek, one of our attorneys, you explained that you had changed the designs of your trailer line so that you are confident that all but one of the trailers now comply with the rule. You asked Mr. Atelsek to address only the Model DHT Series Hydraulic Tail trailers. As explained below, this trailer is an excluded low chassis vehicle, to which our underride guard regulations do not apply.

These trailers are essentially a flatbed design, with an eight foot long, full width tail section extending rearward behind the rear tires. The rear edge of the tail section can be raised or lowered using hydraulic cylinders located beneath the front part of the tail. The tail section tilts down to permit loading of wheeled vehicles. At the rear of the tail section is a four foot long, full width approach plate. During loading, the approach plate extends farther rearward (12 feet total length from the front edge of the tail section) and downward, bridging the gap between the tail section and the ground. Its purpose is to allow construction equipment to transition across its triangular cross section from the ground up onto the bed without encountering the "bump" of the edge of the tailpiece.

During transit, this approach plate folds under the tail section and stows against the bottom of the tail section. You added further detail in a March 27, 1998, telephone conversation with Mr. Atelsek. Although the tail section can be tilted up and down, it is designed so that it can not be transported in any position other than the horizontal position, and that the tail section and the approach plate both lock into that position. In this position, you state that the combined rear surface of the tail section and the approach plate meets the configuration requirements our regulations. However, without counting the tucked-under approach plate, the lower rear of the tail section does not meet the configuration requirements because it is too far above the ground. The approach plate is attached by a large full-width piano-type hinge, fits into an indentation on the bottom of the tail section, and locks in place there during transit.

Federal Motor Vehicle Safety Standard No. 224, Rear impact protection, requires most trailers and semitrailers weighing over 10,000 pounds to be fitted at the rear with a rear impact (underride) guard meeting the requirements of Standard No. 223, Rear impact guards (49 CFR 571.223 and 571.224, published on January 24, 1996, at 61 FR 2004). However, certain kinds of vehicles are excluded. The only excluded category that is relevant for the purposes of this letter is low chassis vehicles.

Low chassis vehicles are defined in S4 of Standard No. 224 as "a trailer or semitrailer having a chassis that extends behind the rearmost point of the rearmost tires and a lower rear surface that meets the configuration requirements of S5.1.1 through 5.1.3 of this section." In other words, the chassis itself must satisfy the configuration requirements applicable to a guard when the vehicle is outfitted for transit. The only part of your vehicle that may meet these configuration requirements is the rear edge of the folded approach plate itself, so the question becomes whether the approach plate is considered to be part of the "chassis" of the vehicle. Chassis is defined in S4 as "the load supporting frame structure of a motor vehicle." There are two elements to this definition that must be satisfied: "load supporting" and "frame structure."

To be considered "load supporting," the frame structure has to support load when the trailer is performing its function. Generally, this means that the structure would have to contribute to supporting the cargo load when the trailer is in transit.

To be considered part of the frame structure, a structural member must be either an integral part of the overall frame structure, or be connected with other frame structural members in a way that is necessary to the structural integrity of the trailer. One factor the agency considers in deciding whether a structural member is part of the frame is its size and strength. Frame structural components often are the major structures defining the shape of the trailer. Although frame structure is not limited to the largest frame components (i.e., the frame rails for most trailers), generally frame components are substantial and have strength similar to other frame components. Moveable components may "lock" into the frame structure sufficiently that they are integral with other frame members-in this situation NHTSA may consider the combined components to be one frame unit. However, the agency also looks at the purpose and function of the structural member in supporting the trailer and its load.

Applying these principles to your approach plate, we conclude that it is part of the chassis. The folded approach plate conforms with the outline of the underside of the trailer bed, maintaining a constant bed thickness and helping to define the outline of the bed. Your approach plate is hinged along the entire rear of the trailer and, when folded, locks at its forward edge into a place fitted for it on the underside of the chassis. NHTSA considers the approach plate on your trailer to be "locked" into the frame of the vehicle sufficiently that it is considered integral with it, as one unit, and therefore a part of the frame structure. By contributing to the structural integrity of these frame members, NHTSA considers the approach plate to be supporting load. Therefore, the approach plate is part of the chassis, and the trailer is a low chassis vehicle, excluded from the underride guard requirements.

If you have any further questions, please feel free to contact Paul Atelsek of my staff at this address or by telephone at (202) 366-2992.

John Womack
Acting Chief Counsel