Interpretation ID: nht78-1.39
DATE: 06/16/78
FROM: AUTHOR UNAVAILABLE; J. J. Levin, Jr.; NHTSA
TO: Blue Bird Body Company
TITLE: FMVSS INTERPRETATION
TEXT: This responds to your April 24, 1978, letter asking two questions concerning Standard No. 222, School Bus Passenger Seating and Crash Protection.
In your first question, you ask what the National Highway Traffic Safety Administration (NHTSA) means by the phrase "seat components shall not separate at any attachment point." This phrase is found in the forward and rearward loading performance tests. You suggest that the NHTSA interpret this to mean a complete separation of a seat component from another component. The NHTSA disagrees with this suggestion.
The standard as written clearly indicates that the agency has intended that seat components remain connected at all attachment points during testing. If the agency had intended a complete separation of seating components to be the test for separation, it would have used that language in the drafting of the regulation. Therefore, the agency declines to adopt the interpretation that you suggest and will require the seat to remain attached at all attachment points during testing.
Your second suggestion concerns a possible problem in the computation of loads during rearward testing. You state that occasionally the loading bar will become buried in the upholstery material and, therefore, distort the actual seat loads. The NHTSA has not noted the phenomenon to which you refer. However, if it were to occur in compliance testing the agency would be certain to factor out any aberrations in the test results that occurred owing to this loading bar problem.
SINCERELY,
April 24, 1978
Joseph J. Levin Chief Counsel National Highway Traffic Safety Administration
Dear Mr. Levin:
The purpose of this letter is to seek a clarifying interpretation on two issues relating to FMVSS 222.
1. S 5.1.3 (e) and S 5.1.4 (e) state that during the forward and rearward performance tests respectively that "seat components shall not separate at any attachment point."
It has been brought to my attention that this phrase is subject to extreme interpretation which we don't believe National Highway Traffic Safety Administration intended. For example, if 1 out of 100 spotwelds attaching a seatback panel to the frame failed, creating a local separation, would this be considered a non compliance per S 5.1.3 (e) or S 5.1.4 (e)?
We have not used this interpretation. The reasoning for this is that the seat component (back panel) has not separated from the frame.
This is only one example. The same problem occurs in attaching the seat riser to the main frame etc., etc.
Without a clear cut definition of "separate" it is impossible to deal with this phrase. For example, would a torn bracket at a bolt attachment point be a separation? If so, how long must the tear be?
Therefore, we believe the only workable interpretation of S 5.1.3 (e) and S 5.1.4 (e) is to define "separate" as complete separation of one seat component from another; i.e. separation of the seat foot from the riser from the main frame etc.
We solicit your concurrence with this interpretation.
2. The second item deals with S 5.1.4 Seat performance rearward and S 6.5 loading bar.
S 6.5 requires the loading bar to be 4" narrower than the seat width to insure that panel type members are secured in a manner adequate to transmit loads to main seat frame members. We agree with this philosophy.
However, in our testing and development program we have noted an intermitent problem which is directly related to the "narrow" load bar but does not affect seat performance in any way.
The problem is that occasionally the "narrow" load bar will bury itself in the upholstery, padding, and sheet metal and hang up. When this occurs, the load bar begins pulling the seat back in tension creating high apparent loads rather than sliding along the upholstery and only sensing seat back bending loads. Because these tension loads build rapidly, (and exceed 2200 pounds) this phenomenon could be misinterpreted as a seat back which is too rigid.
This phenomenon is intermitent and not always repeatable. It occurs on different seat designs and appears to be related to parameters difficult to define such as padding thickness, the hardness of the loading bar spherical ends, and the coefficient of friction of the upholstery.
One possible solution, if this should occur during compliance testing, is to use the test results from the "narrow" bar test to prove the integrity of the panel to frame attachment and use a wider bar to get true results of seat back performance.
The purpose of this discussion is to go on record as acknowledging this phenomenon and seek confirmation from National Highway Traffic Safety Administration that this would not be judged a non compliance if encountered during compliance testing.
We look forward to your early response on these issues.
W. G. Milby Manager, Engineering Services