In this standard, directional terms such as bottom, center, height, horizontal, longitudinal, transverse, and rear refer to directions relative to the vehicle orientation when the guard is oriented as if it were installed on a vehicle according to the installation instructions in S5.5 of this section.
Chassis means the load supporting frame structure of a motor vehicle.
Guard width means the maximum horizontal guard dimension that is perpendicular to the longitudinal vertical plane passing through the longitudinal centerline of the vehicle when the guard is installed on the vehicle according to the installation instructions in S5.5 of this section.
Ground clearance means the vertical distance from the bottom edge of a horizontal member to the ground.
Horizontal member means the structural member of the guard that meets the configuration requirements of S5.1.1 through 5.1.3 of § 571.224 , Rear Impact Protection, when the guard is installed on a vehicle according to the guard manufacturer's installation instructions.
Hydraulic guard means a guard designed to use fluid properties to provide resistance force to deformation.
Load path means a route of force transmission between the horizontal member and the chassis.
Rear impact guard means a device installed on or near the rear of a vehicle so that when the vehicle is struck from the rear, the device limits the distance that the striking vehicle's front end slides under the rear end of the impacted vehicle.
Rigid test fixture means a supporting structure on which a rear impact guard can be mounted in the same manner it is mounted to a vehicle. The rigid test fixture is designed to resist the forces applied to the rear impact guard without significant deformation, such that a performance requirement of this standard must be met no matter how small an amount of energy is absorbed by the rigid test fixture.
49 C.F.R. §571.223