Mr. Jason Backs
VP Operations
CPS Trailers
Highway 77, P.O. Drawer K
Oran, MO 63771

Dear Mr. Backs:

This responds to your letter requesting an interpretation of whether the bottom dump trailer that your company manufactures with a large push block at the rear would be excluded from the National Highway Traffic Safety Administration's (NHTSA's) rear impact protection (underride guard) regulations. As explained below, your trailer is not excluded but, assuming you are attaching an underride guard to the push block in the location that you drew on your diagram, your vehicle may already comply with our regulations.

Your letter attached a diagram of your bottom dump trailer. Attached to the rear and extending 25 inches behind the rear of the trailer is a push block, used by bulldozers to push the trailer out of soft ground at construction sites. Viewed from above, the push block shaped like a trapezoid with its long side attached to the trailer. It tapers to a width of 28 inches at its rear, which is 28 inches off the ground. Eleven inches forward of its rear a guard-shaped structure, which you refer to as the horizontal member of the push block, is attached to the bottom of the push block. The bottom of the guard-shaped structure is 19 3/4 inches above the ground. Based on a conversation with David Coleman of NHTSA's Office of Vehicle Safety Compliance,(1) you believe that the trailer is excluded from our regulations because it meets the definitions of two classes of excluded vehicles: the low chassis vehicle and the special purpose vehicle.

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 categories that are relevant for the purposes of this letter are low chassis vehicles and special purpose 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 meets these configuration requirements is the guard-shaped structure attached to the underside of the push block(2), so the question becomes whether this is considered to be part of the "chassis" of the vehicles. 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. For most trailers that carry things, this means that the structure would have to contribute to providing underlying support for 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. 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 the horizontal member of your push block, we find that it is not part of the chassis. Although the push block and presumably also the guard-like structure are strongly attached to the chassis, they do not contribute to supporting cargo load. The push block itself is not part of the frame structure of the trailer, it is an attachment. Although you imply that the guard-like structure attached to the bottom of the push block is part of the push block, we consider it to be another attachment (to the push block). It does not define the outline of the trailer, but projects downward from an attachment to the rear of the chassis. Therefore, it is not part of the chassis, and the bottom dump trailer is not a low chassis vehicle.

We turn now to the question of whether the bottom dump trailer is excluded as a special purpose vehicle. A special purpose vehicle is defined in S4 of FMVSS No. 224 as "a trailer or semitrailer having work-performing equipment that, while the vehicle is in transit, resides in or moves through the area that could be occupied by the horizontal member of the rear underride guard, as defined by S5.1.1 through S5.1.3."(3) Again, the guard-like structure on the underside of the push block is the only part of your trailer that, while the vehicle is in transit, resides in the area that could be occupied be the rear underride guard. Therefore, it would have to be considered work-performing equipment for the trailer to be excluded.

There is no definition in the standard for "work-performing equipment." In determining the meaning of regulatory language, the first place the agency looks is the plain meaning of the words. In the context which is relevant to this safety standard, "work" is defined as "the transfer of energy from one physical system to another; especially, the transfer of energy to a body by the application of force . . ." "Perform" is defined as "to begin and carry through to completion; do." American Heritage Dictionary of the English Language, 1971. Taken together, NHTSA interprets the words "work-performing" to mean that the equipment must actively perform its function, and that the function must involve exerting force or moving something else. The guard-like structure does not perform work in this sense. Its function is unclear. Even the push block does not perform work. Its function is to merely transmit the force of the bulldozer blade to the chassis of the trailer. Therefore, the guard like structure is not work-performing equipment, and the bottom dump trailer does not meet the definition of a special purpose vehicle. An underride guard would have to be provided on this vehicle.

However, your bottom dump trailer may already comply with the rule. You made several correct observations in your letter regarding the determination of the rear and side extremities. The rear extremity is defined in S4 of Standard No. 224 as " the rearmost point on the vehicle . . ." (not the chassis). Therefore, it coincides with the rearmost point on the push block. The side extremity is defined in S4 as "the outermost point on the vehicle's side that is located . . . between a transverse vertical plane tangent to the rear extremity of the vehicle and a transverse vertical plane located 305 mm forward of that plane." Since your push block tapers toward the rear, the side extremity coincides with the outer edges of the push block in a transverse plane one foot forward of the rear extremity. The bottom of the guard-like device extends to the side extremities and the 21 inch ground clearance meets the vertical height requirements of S5.1.2. Assuming the face of the horizontal member of the guard-like structure is at least 100 mm high, as required by S5.1 of Standard No. 223, the guard-like structure would meet all the configuration requirements of an underride guard. If it can pass the strength and energy absorption requirements as well, the guard like structure itself could be labeled and certified as a guard under Standard No. 223.

If you have difficulty meeting these requirements, the agency would consider a petition for temporary exemption from Standard No. 224. Under one of our regulations (49 CFR Part 555), vehicle manufacturers may apply for a temporary exemption from the Federal motor vehicle safety standards. Under Sec. 555.6(a), a manufacturer whose yearly production is not more than 10,000 units may ask for an exemption of up to three years on the basis that compliance would cause it substantial economic hardship and that it has attempted in good faith to comply with the standard from which it has asked to be excused. We have enclosed a copy of Part 555 for your information. We have also enclosed a copy of our regulations relating to the protection of confidential business information. Most of the trailer manufacturers submitting petitions for temporary exemption have requested that their financial information remain confidential.

Please note Part 555 requires the agency to publish a notice in the Federal Register seeking public comment on each exemption petition before a decision can be made on such a request, and then publish a second notice either granting or denying the petition. This process normally takes three to four months from the date of submittal.

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

1. We note that the Chief Counsel's Office is the only office in the agency that can issue interpretations of our regulations.

2. The push block itself is too high to meet the maximum height requirement of S5.1.2.

3. Note that this definition, as quoted, reflects an amendment made in response to petitions for reconsideration of the final rule. An unnecessary reference to pipe equipment containing hazardous materials was eliminated. See 63 F.R. 3654 (January 26, 1998).