Mr. James Jacobsen
Reinke Manufacturing Company, Inc.
P.O. Box 566
Dreshler, NE 68340

Dear Mr. Jacobsen:

This responds to your letter requesting an interpretation of whether a proposed trailer configuration would be excluded from the National Highway Traffic Safety Administration's (NHTSA's) rear impact protection (underride guard) regulations. Your proposed design is a flatbed trailer with a piece of four inch square steel tubing welded to the bottom of the chassis I-beam flanges. Without the tubing, the bottom of the flange would be 25 inches above ground level. With the flange, the bottom of the flange would be 21 inches off the ground. You state that the piece of tubing would meet the dimensional requirements of the rule. You ask if NHTSA would consider the tube to be a part of the chassis and, if so, whether the vehicle would be excluded from our regulations as a low chassis vehicle. As explained below, this proposed trailer design would not be excluded as a low chassis vehicle, and a compliant underride guard would need to be provided.

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 piece of steel tubing welded to the bottom of the frame rail flanges, so the question becomes whether the tubing 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 welded-on piece of steel tubing, the agency concludes that it is not part of the chassis. The tubing does not meet the "load supporting" aspect of the chassis definition because it does not contribute to supporting cargo load. The tubing is also not part of the frame structure of the trailer. It does not define the outline, but projects down from beneath the frame structure. It is not locked into the structure strongly enough to be considered as one unit with a frame structural component. In consideration of these factors, we conclude that the approach plate is not part of the frame structure. NHTSA considers the tubing to be an attachment. Therefore, the proposed trailer design would not be considered a low chassis vehicle, and it would have to have a compliant underride guard attached.

We note, however, that the tubing already appears to fulfill the configurational requirements of a horizontal member of an underride guard. You would only need to assure that it meets the strength and energy absorption requirements in Standard No. 223 to be able to certify this vehicle design to our 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