Interpretation ID: nht73-3.27

DATE: 02/15/73

FROM: AUTHOR UNAVAILABLE; Richard B. Dyson; NHTSA

TO: Kelsey-Hayes Company

TITLE: FMVSS INTERPRETATION

TEXT: This is in reply to your letters of October 23, 1972 and January 25, 1973, concerning the intent of the antilock performance requirements of Motor Vehicle Safety Standard No. 121. I apologize for our delay.

Your question arises from an amendment to S5.3.1 of the standard. As originally adopted in February 1971, the section required that the vehicle be capable of stopping without wheel lockup except for "momentary" lockup allowed by an antilock system. As amended in February 1972, the word "controlled" is used in place of "momentary", so that the section now provides that stops are made

without lockup of any wheel at speeds above 10 m.p.h. except for controlled lockup of wheels allowed by an antilock system . . . .

In making this change, the agency had in mind the type of antilock system that was designed to permit one wheel on an axle to lock under some circumstances while the other wheels continued to turn. It was thought that adequate control could be attained by such systems, and the standard was amended accordingly.

The question you raise is whether a system could be designed in which all wheels could be permitted to lock for substantial periods, so long as they are "controlled" by an antilock system. As you correctly indicate, the term "controlled", unlike "momentary", is not a time-related word. Our answer, therefore, is that such an antilock system would be permitted under the standard as it now stands.

It is our present opinion, however, that such a system would probably not provide an acceptable level of performance. If it appears that such a system would be installed, it is likely that we would undertake rulemaking action to prohibit it.