Interpretation ID: 1983-2.17

TYPE: INTERPRETATION-NHTSA

DATE: 06/14/83

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Mack Trucks, Inc. -- S. Robson, Sr. Project Engineer - Vehicle Regulations

TITLE: FMVSS INTERPRETATION

TEXT:

NOA-30

Mr. S. Robson Sr. Project Engineer - Vehicle Regulations Mack Trucks, Inc. P.O. Box 1761 Allentown, Pennsylvania 18105

Dear Mr. Robson:

This responds to your recent letter requesting an interpretation concerning the test procedures of Safety Standard No. 207. You ask whether a seat in a heavy duty truck must remain in its adjusted position when tested with a load of 20 times the weight of the entire seat, if the seat has been tested and remains in its adjusted position when subjected to a load of 20 times the weight of the adjustable upper section of the seat.

This question arises because of the configuration of some heavy-duty truck seats which include suspension fixtures (pedestal structures) on which the upper seat section rests. Safety Standard No. 207 requires seats to withstand a load equal to 20 times their weight and requires the seats to remain in their adjusted position during the required loading (paragraph S4.2). The upper seat sections and adjusters of your truck seats are tested on rigid test beds by the seat manufacturer and reportedly comply with the adjuster provision of S4.2. However, when you test these seats in the actual vehicle (applying 20 times the weight of the entire seat, including pedestal), the center of gravity falls on the seat adjuster and the seat does not always stay in its adjusted position during loading.

In answer to your specific question, the seat must remain in its adjusted position when tested to 20 times the weight of the entire seat, as required by paragraph S4.2 of the standard. We would agree with you, however, that with a seat configuration such as you describe, the adjusters and upper seat section would never experience a loading of 20 times the weight of the entire seat in an actual crash. Your problem appears to arise because the center of gravity of this seat happens to fall on the seat adjusters and the standard requires the loading to be applied through the center of gravity. The purpose of this loading requirement, however, is to ensure the integrity of the entire seat as it is attached to the vehicle structure.

Therefore, it is our opinion that for a pedestal seat such as you describe, a manufacturer could establish due care through a combined test procedure which would load the seat adjusters and upper seat section to only 20 times the weight of those components to determine if the seat would remain in its adjusted position, and which would load the seat as anchored to the vehicle structure to 20 times the weight of the entire seat, including the pedestal.

Please contact Hugh Oates of my staff if you have any further questions (202-426-2992).

Sincerely,

Frank Berndt Chief Counsel

April 29, 1983

Office of General Council, NHTSA

Gentlemen:

Subject: Clarification of Seat Testing Procedure FMVSS 207

Mack Trucks, Inc., a major manufacturer of heavy duty trucks is requesting an interpretation and clarification of the seat test procedure appearing in Sections 4 and 5 of 49 CFR Part 571.207 -Standard No. 207; Seating Systems.

The typical structure of the vast majority of seats used in heavy duty trucks is considerably different than those used in passenger cars. Note the comparisons on the attached sheet. In a passenger car, the fore/aft (longitudinal) adjuster is typically at the bottom of the seat between the seat and the floor of the vehicle body (see Figure 1). In a heavy duty truck, the adjuster is typically above the suspension mechanism of the seat and below the upper seat section (see Figure 2). In an actual 20g acceleration, the adjuster on the typical passenger car seat would experience the force of the mass of the entire seat being accelerated (or decelerated). However, on a typical heavy duty truck seat the adjuster would experience the force of only the upper seat section (perhaps about 40% of the total seat weight) being accelerated (or decelerated) at 20g.

Mack Trucks, Inc. is not a manufacturer of the seats used on our vehicles; they are manufactured by outside vendors. These outside vendors test and certify the seats supplied to Mack Trucks, Inc. for compliance with FMVSS's 207, 210 and 302.

However, these certifications are made with the seats mounted to rigid test beds, not in an actual cab. Mack Trucks, Inc., in exercising due care in its installation of seats, performs its own compliance testing of each new seat or seat belt mounting in an actual truck cab. Because of the basic difference in seat structure between a passenger car seat and a truck seat, it is necessary that a fixture be welded to the seat as close as possible to the actual vertical c.g. (center of gravity) of the entire seat. This fixture is used to attach the hydraulic mechanism necessary to apply the 20g load to the seat. In many instances, the vertical c.g. of the seat is approximately at the same height as the seat adjusters. When attaching this fixture to the seat, it quite often becomes necessary to attach it to brackets that are part of the upper seat section. In doing so, the 20g load for the entire seat is being transmitted through the fore/aft adjusters. As mentioned earlier, the fore/aft adjusters would not "see" this much load in an actual 20g acceleration (or deceleration). It would "see" only 20 times the weight of the upper seat section. Therefore, in testing the seat with the fixture attached to the upper seat section, the adjusters would be greatly overloaded from their design load and may not remain in their adjusted position.

The question that Mack Trucks/ Inc. poses to the NHTSA is this: Would it be considered as not meeting the test requirements of FMVSS 207 if a seat, when tested with a load of 20 times the weight of the entire seat being applied through the adjusters for the upper seat section, does not remain in its fore/aft adjusted position, even if the adjusters have been tested and certified from the vendor for compliance when subjected to the load of 20 times just the weight of the adjustable upper section of the seat?

As you can appreciate, the answer to this question will affect our current test procedures and therefore, we request a timely response. If additional information is required or you wish to discuss the problem further, please contact this office.

Very truly yours,

MACK TRUCKS, INC.

S. Robson Sr. Project Engineer - Vehicle Regulations

SR:bh

Attachment Omitted.