Interpretation ID: 19825-1.pja
Jason Backs, Vice President
Travis Body and Trailer
13955 F.M.. 529
Houston, TX 77041
Dear Mr. Backs:
This responds to your letter asking whether a flexible rear apron on the rear of a trailer that your company is interested in building constitutes a "nonstructural protrusion" within the meaning of our rear impact protection (underride guard) regulations. Our answer is no.
You provided a drawing and a description of a trailer your company is thinking about manufacturing. The steel body of this trailer extends eight inches behind the rear most point of the rear tires. However, bolted on to the rear of the steel body is a flexible rear apron extending 27 inches behind the rearmost point of the rearmost tires. The apron's purpose is to support asphalt as it is transferred backward out of the trailer body and into a paving machine. You state that the apron could be composed of inch thick plastic. You state that the apron would be "substantial enough to support the asphalt load, yet would be extremely flexible in a rear impact." As explained below, the flexible rear apron would not be considered a nonstructural protrusion.
Federal Motor Vehicle Safety Standard No. 224, Rear impact protection, requires most trailers and semitrailers with a gross vehicle weight rating of over 10,000 pounds to be fitted at the rear with an underride guard complying with Standard No. 223. Paragraph S5.1.3 of the standard specifies that "the rearmost surface of the horizontal member of the guard shall be located as close as practical to a transverse vertical plane tangent to the rear extremity of the vehicle, but no more than 305 mm [about 12 inches] forward of that plane." S4 defines the rearmost extremity, in pertinent part, as
the rearmost point on a vehicle that is . . . below a horizontal plane located 1,900 mm above the ground . . . when the vehicle's cargo doors, tailgate, and other permanent structures are positioned as they normally are when the vehicle is in motion. Nonstructural protrusions such as taillights, rubber bumpers, hinges and latches are excluded from the determination of the rearmost point.
Merely because something is attached to the body does not mean that an object is nonstructural. The definition of rear extremity refers to the "rearmost point on the vehicle" (emphasis added), not the rearmost point of the chassis, or the rearmost point of the steel structure. The attributes that the examples of nonstructural protrusions listed in S5.1.3 have in common are that they are relatively small and localized and would not have a major impact on a colliding passenger vehicle.
We do not consider your apron design to be nonstructural. We have previously determined that "[a] 0.19 inch thick aluminum (or 7 gauge steel) [deflector] plate extending across the entire width of the trailer is part of the vehicle, and is not a "nonstructural protrusion." (1) We believe that your proposed plastic apron is indistinguishable from a deflector plate, for the purposes of the regulation. As with the deflector plate, your apron is rigidly attached to the rear of the trailer. It seems to wrap around the tailgate, so it is probably full-width and could not be considered relatively small or localized. The distinction that you seem to be urging upon us, that an apron constructed of inch thick plastic is more flexible than steel and thus not harmful if impacted by a colliding vehicle, is not likely to be true in most highway crashes. Plastics can vary greatly in their rigidity and strength. If your flexible rear apron did not contact any metal structure of the colliding passenger vehicle but instead penetrated the windshield, it could be harmful if its lower edge struck the head or neck of the front seat occupants as they are thrown forward by the force of the crash. We conclude that the rear edge of the apron would be considered the rear extremity of the vehicle, and an underride guard would have to be mounted no more than 12 inches forward from it.
If you have any further questions, please feel free to contact us at (202) 366-2992.
Sincerely,
Frank Seales, Jr.
Chief Counsel
Enclosure
ref:224
d.10/7/99
1. Letter of October 20, 1997 to Mr. Michael L. Ulsh. That letter addressed a similar situation in which a full width deflector plate helped to transfer the trailer's load outboard from the vehicle.