Interpretation ID: nht76-4.49
DATE: 07/09/76
FROM: AUTHOR UNAVAILABLE; R. L. Carter; NHTSA
TO: Chrysler Corporation
TITLE: FMVSS INTERPRETATION
TEXT: This is in response to your March 16, 1976, request for assurance that paragraph S4.3(f) of Standard No. 209, Seat Belt Assemblies, does not apply to the mechanism in a continuous loop seat belt system that limits (prevents) transfer of belt webbing from the shoulder to the pelvic portion of the belt when the buckle is engaged. Your letter recommended that Standard No. 209 be amended to specifically exempt such adjustment mechanisms from the requirements of paragraph 4.3(f) or that separate, appropriate requirements for those mechanisms be established.
The National Highway Traffic Safety Administration (NHTSA) has determined that the adjustment mechanism described in your letter is not a "tilt lock" within the meaning of paragraph S4.3(f) of the standard, although the two mechanisms are superficially similar. Therefore, this mechanism does not have to comply with the requirements of paragraph S4.3(f).
We would like to emphasize our comments to you of June 13, 1975, regarding the requirement that a continuous loop assembly have a sufficiently low level of friction at the buckle mechanism to ensure that the lap portion of the belt is automatically adjustable. The friction in the buckle must be low enough that the normal motion of the occupant against the shoulder belt tightens the lap portion of the belt to prevent excessive slack and possible submarining of the occupant.
The NHTSA has also considered Chrysler's petition to amend Standard No. 209 to establish "separate appropriate requirements" for the adjustment mechanism in question and decided that it should be granted. A notice proposing such modifications of the standard is anticipated in the near future.
As you are aware, the commencement of a rulemaking proceeding does not signify that the rule in question will necessarily be issued. A decision concerning the issuance of a rule is made on the basis of all available information developed in the course of the rulemaking proceeding, in accordance with statutory criteria.
SINCERELY,
ATTACH.
CHRYSLER CORPORATION
March 16, 1976
James B. Gregory -- Administrator, National Highway Traffic Safety Administration
Subject: REQUEST FOR INTERPRETATION AND PETITION FOR AMENDMENT OF @ 4.3(f) OF MVSS 209 - SEAT BELT ASSEMBLIES
Dear Dr. Gregory:
Recently, the U.S. Testing Company, Inc. conducted a test on our continuous loop (unibelt) seat belt system for the American Safety Equipment Corporation (our supplier). They are questioning whether an adjustment mechanism in a unibelt system utilizing a tilt-lock design superficially similar to the tilt-lock buckle of a Type 1 lap belt assembly needs to meet the requirements of @ 4.3(f) of MVSS 209, Seat Belt Assemblies.
In our opinion, these requirements should not apply to a unibelt adjustment mechanism, since its function is quite different from that of a tilt-lock buckle in a Type 1 lap belt assembly. In a Type 1 lap belt only assembly it is important that the tilt-lock buckle hold the webbing firmly in place without excessive slippage under load. Accordingly, the tests prescribed in @ 4.3(f) of MVSS 209 are appropriate for that application. In a continuous loop system, however, it is the retractor (not the adjustment buckle) which adjusts the belt webbing to the proper length and holds the webbing at that length under load. In a loop system, the primary purpose of the adjustment mechanism is to permit easy or automatic tightening of the total belt system by the retractor. Several of the continuous loop belt systems currently being used provide for this adjustment by use of a simple slip ring design which allows free movement of the webbing between the lap and shoulder belt portion of the system. This design does not have a buckle adjustment mechanism which holds the lap belt taut. Obviously, in these cases, the requirements of @ 4.3(f) do not apply. It is our opinion that this section is equally inapplicable if the adjustment device uses a tilt mechanism superficially similar to a tilt-lock of a Type 1 lap belt assembly in place of the slip ring.
On May 16, 1975, Chrysler engineers demonstrated our unibelt system to NHTSA personnel in the garage of the NHTSA headquarters. At that demonstration we pointed out the tilt-type adjustment feature designed into our system. In addition to allowing automatic tightening of the total belt system by the retractor our tilt-type buckle adjustor holds the lap belt taut once the user has tightened it. The feature was provided for the convenience of and added safety of belt users who like a tighter fitting lap belt than can be provided by a slip ring. Coupled with our window shade tension relief mechanism for the shoulder belt, it allows the belt user to have a snug fitting lap belt and a comfortable shoulder belt. This adjustment feature is accomplished by the use of a tilt-lock mechanism superficially similar to a tilt-lock buckle used with a Type 1 lap belt assembly. As noted above the design and function are significantly different, however.
We recognize that NHTSA does not give certification approvals or otherwise prejudge the compliance of a device. However, in a letter dated June 13, 1975 to S. L. Terry from the Administrator, it was noted that NHTSA personnel saw no evidence of design deficiency related to our unibelt system. The letter gave two important requirements for such systems: (1) the level of friction in the assembly must be low enough to allow "automatic adjustment" of the lap belt portion with normal motion of the occupant against the shoulder belt, and (2) devices that limit retractive action may only be used with seat belt assemblies that have an "individually adjustable lap belt." It is our understanding from the NHTSA representatives who participated in the demonstration that our system meets both of those requirements.
In view of questions concerning our understanding and interpretation of the standard, we request that NHTSA provide us assurance that @ 4.3(f) of MVSS 209 does not apply to the adjustment mechanism of a unibelt system using a design superficially similar to a tilt-lock buckle commonly used in Type 1 lap belt assemblies.
Secondly, we recommend that the standard be modified to specifically exempt such adjustment mechanisms from those requirements or that separate appropriate requirements for these mechanisms be established.
Very truly yours, S. L. Terry -- Vice President, Public Responsibility and Consumer Affairs