Chassis means the load supporting frame structure of a motor vehicle.
Horizontal member means the structural member of the guard that meets the configuration requirements of S5.1 of this section when the guard is installed on the vehicle according to the installation instructions or procedures required by S5.5 of § 571.223 , Rear Impact Guards.
Low chassis vehicle means 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.
Outer or Outboard means away from the trailer centerline and toward the side extremities of the trailer.
Pulpwood trailer means a trailer that is designed exclusively for harvesting logs or pulpwood and constructed with a skeletal frame with no means for attachment of a solid bed, body, or container.
Rear extremity means the rearmost point on a trailer that is above a horizontal plane located above the ground clearance and below a horizontal plane located 1,900 mm above the ground when the trailer is configured as specified in S5.1 of this section and when the trailer's cargo doors, tailgate and other permanent structures are positioned as they normally are when the trailer is in motion, with non-structural protrusions excluded from the determination of the rearmost point, such as:
Road construction controlled horizontal discharge trailer means a trailer or semitrailer that is equipped with a mechanical drive and a conveyor to deliver asphalt and other road building materials, in a controlled horizontal manner, into a lay down machine or paving equipment for road construction and paving operations.
Rounded corner means a guard's outermost end that curves upward or forward toward the front of the vehicle, or both.
Side extremity means the outermost point on a vehicle's side that is located above a horizontal plane 560 mm above the ground, below a horizontal plane located 190 cm above the ground, and 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 when the vehicle is configured as specified in S5.1 of this section. Non-structural protrusions such as taillights, hinges, rubber bumpers, and latches are excluded from the determination of the outermost point.
Special purpose vehicle means a trailer or semitrailer that:
Wheels back vehicle means a trailer or semitrailer whose rearmost axle is permanently fixed and is located such that the rearmost surface of tires of the size recommended by the vehicle manufacturer for the vehicle on that axle is not more than 305 mm forward of the transverse vertical plane tangent to the rear extremity of the vehicle.
49 C.F.R. §571.224