Interpretation ID: nht87-2.82

TYPE: INTERPRETATION-NHTSA

DATE: 08/28/87

FROM: BARRY NUDD -- SENIOR PROJECT ENGINEER, ATWOOD MOBILE PRODUCTS

TO: ERIKA JONES -- CHIEF COUNSEL NHTSA

TITLE: NONE

ATTACHMT: ATTACHED TO LETTER DATED 08/03/88 TO BARRY NUDD FROM ERIKA Z. JONES, REDBOOK A32(3), STANDARD 207, VSA 108; LETTER DATED 07/14/88 EST TO S. ROBSON FROM FRANK BERNDT; STANDARD 207

TEXT: Dear Erika Jones,

Atwood Mobile Products is a manufacturer of seat adjusters serving the recreational and heavy duty vehicle markets. A majority of our customers use our product in an assembly consisting of a bucket seat mounted to a pair of seat adjusters which are in t urn attached to a pedestal that elevates the seat above the floor of a vehicle and generally incorporates the seat belt anchorages. The inclusion of the pedestal in the seating systems causes considerable confusion with some customers when testing to St andard No. 207. The main issue is whether the weight of the pedestal should be added to the weight of the seat when calculating the force to be applied to the seat back for testing the compliance of the seat adjusters when mounted between the seat and p edestal.

A letter from the Chief Counsel of NHTSA to Mack Trucks Inc. dated July 14, 1983 (copy enclosed) states [a combined test procedure that first tests the seat adjuster for its ability to stay in the adjusted position when subject to a force of 20 times the upper seat and adjuster weight and then subjects the entire seat assembly (seat, adjusters and pedestal) to a 20 g force as anchored to the vehicle structure] establishes due care in testing to FMVSS Standard No. 207.

We would conclude from this opinion that the first portion of the procedure outlined above, establishes that the seat adjusters meet the requirement of Standard No. 207 S4.2. (The second portion of the test then establishes the compliance of the seat st ructure from the seat adjusters down to the pedestal mounting in the vehicle to the general requirements of S1 that seat attachment assemblies and their installation meet the standard as well as the seat itself.)

We request your interpretation with regard to the test procedures required by Standard No. 207 for seat assemblies consisting of a trimmed seat mounted on seat adjusters which are in turn mounted to a pedestal which incorporate seat belt anchorages (S4.2 (c)), specifically as outlined in the two part test set forth (in figures 1 & 2).

Our questions are specific to the Mack Truck letter with the addition of the seat belt loads.

Question No. 1: Does a load applied as in figure 1 satisfy the requirements of Standard No. 207 concerning the seat adjusters remaining in their adjusted position?

Figure 1 tests the ability of the seat adjusters to remain in their adjusted position when subjected to 20 times the weight of all seat components mounted above the adjusters plus the Standard No. 210 belt load.

Question No. 2: Can the seat adjuster, having passed figure 1 loads be locked for the second part of the test as in figure 2.

The second portion of the test (figure 2) established the compliance of the seat structure from the bottom of the seat adjusters down to the connection of the pedestal to the floor. Because the forces in figure 2 do not accurately reflect the forces act ually imposed on the seat adjusters in an actual crash, the adjusters can be welded or otherwise locked together (the adjusters having been tested in figure 1, see paragraph three of the Mack Truck letter" ... the adjusters and upper seat section would n ever experience a loading of 20 times the weight of the entire seat in an actual crash.") while the rest of the seat assembly is tested to the requirements.

Question No. 3: Does the two part test procedure shown in figures 1 & 2 establish due care in meeting Standard 207 when applied to an upper seat and adjusters mounted on a pedestal assembly which incorporates seat belt anchorages?

There are several other questions of a more general nature that arise when applying Standard No. 207 to pedestal mounted seats as shown in figure 1.

Question No. 4: Can the 20 times seat weight load as required in Standard No. 207 S4.2 be applied at the center of gravity of the seat as more accurate alternative to application from a point outside the seat frame in the horizontal plane of the seats ce nter of gravity?

Section S5.1.1 of Standard No. 207 illustrates a bracing system apparently intended to insure that the force applied to the seat back remains in the horizontal plane of the center of gravity of the seat. If the seat frame were excessively flexible and t he braces not used, the deformation of the seat frame would raise the load application line above the plane of the center of gravity of the seat resulting in a larger moment being placed on the seat attachment. This condition is especially evident in buc ket seats mounted on pedestals which incorporate seat belt anchorages.

A simultaneous application of a type I seat belt load and a 20 times seat weight force causes deflections to the entire seat system resulting in the seat force acting significantly above the plane of the seat center of gravity. This condition can consid erably overstate the moment load on seat attachments that would occur in an actual 20g impact. To more accurately simulate a 20g impact load (which appears to be the intent

of the standard) the seat force should be applied at the seat center of gravity not at the seat back through the plane of the center of gravity. The current technique as outlined in S5.1.1 applied to pedestal mounted bucket seats is contrary to elem entary mechanical principles when significant deflections take place.

Another question related to testing for compliance to Standard No. 207 concerns paragraph S4.2(c), the simultaneous application of Standard No. 210 seat belt loads and the 20 times seat weight load.

Question No. 5: Can the seat belt load in figure 1 be eliminated based on the reasoning that an identical floor mounted seat and belt anchorage are not required to sustain the seat belt load and that the second part of the test, figure 2, subjects the se at assembly to the load to check the seat attachment assembly (pedestal) for compliance with Standard No 207?

On installations as previously described and shown in the attached figures, the seat belt anchorages are a part of the seat pedestal on which the seat adjusters and seat are fastened. If a bucket seat were to be mounted on the floor, and the seat belt a nchorages mounted to the floor with separate attachments, the seat and adjusters would not be subject to paragraph S4.2(c). Because the seat pedestal seems to be included in the definition of a seat, the pedestal mounted belt anchorages require the seat to be subjected to paragraph S4.2(c) loads with the seat remaining in its adjusted position during application of each force specified (S4.2). It can be shown that identical bucket seats can be mounted on a floor of a vehicle or on a pedestal and their respective belt anchorages can be located in identical positions, in relation to the seat adjusters.

A typical seat pedestal installation involve the seat belts wrapping around the sides of the seat. As the belts are loaded, considerable transfer of force occurs from the forward stretch of the belts to the seat frame and hence into the seat adjusters. Accepted lab techniques for applying the seat belt load include using steel cables to simulate belts and untrimmed or bare seat frames for applications of loads. The steel cable can bite into a bare seat frame and transfer a large load into a seat adju ster. This force is variable and subject to extremes when testing techniques vary within the limits set forth in Standard Nos. 207 and 210. In case of an identical bucket seat and adjusters mounted on the floor of a vehicle with the floor mounted belt anchorages located in the same position relative to the seat, this force is totally ignored (S4.2(c) is not required).

It is certainly essential that the 20 times seat weight load be applied simultaneously with the Standard No. 210 seat belt load to verify compliance with general requirements of S1 that the chance of failure of the seat attachment assemblies and their in stallation by forces acting on them as a result of vehicle impact be minimized. However, if the seat adjusters of an identical seat installation mounted on the floor are not subjected to loads induced by seat belts wrapping around the seat frame, a pede stal

mounted seat assembly should be permitted to have the tracks welded or locked together while the loads of 4.2(c) check the seat attachment assemblies to the general requirement of Standard No. 207. Of course if the seat belt anchorages were on the s eat frame itself (above the seat adjusters) then the seat adjusters would definitely have to remain in their adjusted position when subject to the simultaneous application of Standard No. 210 seat belt anchorage load and the 20 times seat weight load.

Respectfully Submitted,

TEST PROCEDURE PART 571 S207 SECTION S4.2

ALL LOADS TO BE APPLIED SIMULTANEOUSLY; FIGURE 1 AND 2

(DRAWINGS OMITTED)