Interpretation ID: nht76-2.17

DATE: 12/14/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: School Bus Manufacturers Institute

TITLE: FMVSS INTERPRETATION

TEXT: This is in response to your letter of November 2, 1976, in which you ask for an interpretation of the term "absorbed" as it is used in Standard No. 222, School Bus Passenger Seating and Crash Protection. Further, you request that the NHTSA withdraw its earlier interpretation of the same term made on July 30, 1976, to Thomas Built Buses.

In your letter, you outline data showing that a seat may meet the energy absorbtion requirements of S5.1.3 when recoil energy is included, while failing those same requirements when recoil energy is subtracted from the total energy. You further argue that the NHTSA interpretation of July 30, 1976, which explained the subtraction of recoil energy, is at variance with the wording of the standard, because the standard does not explicitly require the subtraction of recoil energy and speaks only to the application of force upon the seat. Moreover, you suggest that plotting the recoil energy results in insufficient area under the force/deflection curve to meet S5.1.3. For these reasons, you request that the term "absorbed" be defined as the total energy received by the seat without subtracting energy that is returned through recoil.

The NHTSA declines to adopt the interpretation that you suggest. The dictionary definition of the term "absorbed" is "to receive without recoil." This definition, when applied to energy absorbed by a seat, contemplates the subtraction of recoil energy in the computation of absorbed energy. The NHTSA intentionally chose the term "absorbed" to denote exactly this meaning. Therefore, according to the common usage of the term "absorbed," the standard does require the subtraction of recoil energy even though those express words are never used.

Your assertion that plotting the recoil energy results in a force/deflection curve that falls within the prohibited zones indicates a misunderstanding of the force/deflection zone requirements. The force deflection zone requirements (S5.1.3(a), S5.1.3(b), S5.1.4(a), and S5.1.4(b)) prescribe limits within which the seats must perform only during the force application phase of the test procedure.

SINCERELY,

SCHOOL BUS MANUFACTURERS INSTITUTE

November 2, 1976

Frank A. Berndt Office of Chief Counsel National Highway Traffic Safety Administration

On July 30, 1976, the National Highway Traffic Safety Administration issued an interpretation to Thomas Built Buses, Inc. (Item 1) defining the term "energy absorbed in deflecting the seat back" as it relates to the new FMVSS 222 School Bus Passenger Seating (Item 2). The Agency's definition of this term is based on the concept that the absorbed energy equals the amount of energy received less the energy associated with recoil.

The School Bus Manufacturers Institute representing the six major manufacturers of school buses takes exception to this terminology being applied to the present configuration of FMVSS 222. We do not disagree with the semantics but we do believe that there is a definite conflict between the definitions interpretation and the test procedures outlined within the standard.

Our disagreement is not just a recent development. As early as September 1974 through discussions with National Highway Traffic Safety Administration Legal and Engineering, the SBMI indicated that a seat demonstrating purely elastic properties could be constructed to meet the then proposed FMVSS 222.

On a number of occasions since that time, this question has been reviewed by National Highway Traffic Safety Administration personnel. Nevertheless, on January 22, 1976, the final draft of FMVSS 222 was issued without any reference to rebound or recoil adjustments to the test procedure.

Based on the FMVSS 222 test criteria, the SBMI members have designed, developed and tested an entirely new generation of school bus seats. The Thomas Interpretation drastically changes the test criteria used in compliance calculation.

FMVSS 222 Section S5.1.3 states:

Seat performance forward. When a school bus passenger seat that has another seat behind it is subjected to the application of force as specified in S5.1.3.1 and S5.1.3.2 and subsequently, the application of additional force to the seat back as specified in S.5.1.3.3 and S5.1.3.4:

(a) The seat back force/deflection curve shall fall within the zone specified in Figure 1;

(b) Seat back deflection shall not exceed 14 inches; (for determination of (a) and (b) the force/deflection curve describes only the force applied through the upper loading bar, and only the forward travel of the pivot attachment point of the upper loading bar, measured from the point at which the initial application of 10 pounds of force is attained.)

(c) The seat shall not deflect by an amount such that any part of the seat moves to within 4 inches of any part of another school bus passenger seat or restraining barrier in its originally installed position;

(d) The seat shall not separate from the vehicle at any attachment point; and

(e) Seat components shall not separate at any attachment point.

In order that we may more clearly define our objection to the Thomas Interpretation, a typical force/deflection seat test is enclosed (Item 3).

The shaded areas above and below the acceptable zone indicate a seat that is too rigid (upper shaded zone) or too limber (lower shaded zone) to manage the accident induced impacts. Therefore, the force/deflection characteristics properly designed seat will fall within the unshaded area.

Line A plotted on the force deflection curve (Item 3) indicates the amount of seat back deflection for a given loading.

Prior to the Thomas Interpretation line A would be a satisfactory test.

S5.1.3

(a) The curve fell within the specified zone

(b) The seat back deflection did not exceed 14"

(c) The seat did not encroach to within 4" of an adjacent seat

(d) The seat did not separate from the vehicle

(e) The seat components did not separate

The area below line A was above the minimums set by the National Highway Traffic Safety Administration.

Should the Thomas Interpretation be applied to this same seat test, the results are entirely different (Item 4). The new interpretation will require that the recoil of the seat after testing be measured and plotted on the graph-line B.

S5.1.3

(a) The curve fell within the shaded area

The area included within lines "A" and "B" is less than the limit allowed by the National Highway Traffic Safety Administration.

This example outlines one area of conflict between FMVSS 222 and the Thomas Interpretation. FMVSS 222 makes no mention of measuring and plotting rebound, as a matter of fact the test criteria requires only forward motion of the loading bar during the forward test and rearward motion during the rearward test.

The SMBI is now to the point of product verification based on the final draft of FMVSS 222. To revise the test levels at this late date will place an unjust economic burden on this industry.

If it is the Agency's intention to have school bus seats that "eat up" a given amount of energy during a crash, then this requirement should be spelled out within the standard and not within a private interpretation.

Because of the wide reaching effects of this interpretation, we ask that the National Highway Traffic Safety Administration withdraw the Thomas Interpretation and in its place introduce a proposal to revise FMVSS 222 to include the Agency's definition of energy absorption.

If we can be of any assistance in clarifying this matter please feel free to contact me.

Byron A. Crampton Manager of Engineering Services

ITEM 2

(Illegible Line) and Crash Protection FMVSS 222 Effective April 1, 1977

(Regulation Omitted)