A stipulation of the 1974 Federal Motor Vehicle Safety Standard (FMVSS) 215 is that no portion of the test vehicle is permitted to contact planes A or B of the test pendulum. Planes A and B are surfaces located, respectively, below and above the impact ridge of the federally designed pendulum.
One situation in which the bumper could contact plane B, and therefore fail the test, occurs while the bumper and pendulum are separating after impact. The return stroke of the energy absorber can delay bumper-pendulum separation and influence test results.
This report presents the technical procedure used to establish energy absorber rebound performance characteristics and the modifications made to the General Motors hydraulic-pneumatic design to obtain rebound control on certain 1974 car models.
FMVSS 215 also necessitated the application of energy absorbers to the rear bumper systems of most 1974 vehicles. One real world use of the rear bumper is for trailer towing with bumper mounted hitches. Accordingly, tests were conducted and load values on the hitch ball and energy absorbers for various trailer weights, road conditions, and speeds are presented.