Interpretation ID: nht76-2.16

DATE: 07/30/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Thomas Built Buses, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Thomas Built Buses' June 4, 1976, question whether the requirements in S5.1.3 and S5.1.4 of Standard No. 222, School Bus Seating and Crash Protection, to "Apply additional force . . . through the . . . loading bar until (a specified number) of inch-pounds of energy has been absorbed in deflecting the seat back . . ." can be satisfied in part by the energy that is returned to the load bar as it is withdrawn from the seat back. You also ask if there are minimum or maximum time limits on withdrawal of the loading bar from the seat surface.

The requirement for the absorption of a minimum amount of energy in (Illegible Word) the seat back in the forward and rearward directions is calculated to provide adequate measurement of the energy involved in the impact between the bus occupants and the seating in a percentage of school bus crashes. The agency calculated the amount of energy to be consumed by the seat back that would result in adequate protection. The specification requires the seat to "absorb" (i.e., receive without recoil) a specific amount of energy. This value is represented by the amount of energy that is not returned to the loading bar as it is withdrawn. Described graphically, the area that represents returned energy under the seat back force/deflection curve must be subtracted from the entire area that lies under the curve in order to calculate the energy "absorbed" by the seat back.

With regard to your second question, no time limits have been established for withdrawal of the loading bar. The agency intends to utilize a withdrawal time that is not more than five minutes so that creep will not be a significant factor in determining energy absorption. Because the time is not specified, the manufacturer is free to use any reasonable time that does not significantly affect the elastic and plastic components of the seat back loading.

thomas BUILT BUSES, INC.

June 4, 1976

Mr. Bob Krause Office of Standards Enforcement Motor Vehicle Programs U.S. Dept. of Transportation National Highway Traffic Administration

Re: Part 571 - Federal Motor Vehicle Safety Standards - Para. 571.222 S5.1.4.1 & S5.1.4.2

Our engineering department is in the process of developing testing and analysis techniques for demonstration of compliance with the referenced section of FMVSS. To ensure that these techniques will meet all requirements, we are in need of further clarification of the paragraphs pertaining to seat back load application. Accordingly, we would appreciate answers from your department on the following specific questions:

(1) Para. 571.222 S5.1.4.2 specifies energy absorption of the seat back deflection during load application. What is the significance of or treatment required of the energy returned during the backing off of the loading bar?

(2) Are there any requirements regarding elapsed time interval for the load back-off?

(3) Same questions re Para. 571.222 S5.1.3 thru S5.1.3.4 - Seat Performance Forward.

Thank you for your prompt assistance in answering these questions.

Malcolm B. Mathieson Engineering Manager