Interpretation ID: nht74-2.7
DATE: 07/24/74
FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA
TO: The Bendix Corporation
TITLE: FMVSS INTERPRETATION
TEXT: This responds to Bendix's June 10, 1974, request for interpretation of the Standard No. 121 requirement in S5.5.1 that "on a vehicle equipped with an antilock system, electrical failure of any part of the antilock system shall not increase the actuation and release times of the service brakes." You ask whether this language permits use of a separate device that senses electrical failure in the antilock system and automatically reduces pressure to the front brakes as a safety measure. The device would increase brake actuation and release time.
S5.5 is addressed to antilock systems and S5.5.1 mandates that antilock systems be designed not to interfere with air brake performance if they fail electrically. S5.5.1 does not prohibit separate safety devices which are designed to operate in the event of antilock electrical failure to compensate for that failure. Therefore the Bendix automatic front axle limiting system, as we understand its operation, is not prohibited by S5.5.1 simply because it operates when it senses an antilock electrical failure.
Sincerely,
ATTACH.
BENDIX HEAVY VEHICLE SYSTEMS GROUP
James B. Gregory -- Administrator, National Highway Traffic Safety Administration
June 10, 1974
Subject: Request for Interpretation
Re: S5.5.1 (Antilock System Failure) Federal Motor Vehicle Safety Standard No. 121
Gentlemen:
As a result of the adoption of the stopping distance provisions of FMVSS No. 121, higher torque brakes on front axles were required. On certain vehicles, these higher torque brakes created wheel lockup/stability problems which were to be solved or controlled by the use of an antilock system. The Automotive Industry concerns with vehicle stability, as expressed in petitions submitted to NHTSA, are directed towards antilock reliability and stability problems that may be amplified by an antilock system failure.
The concern with vehicle stability due to high torque front brakes are recognized by NHTSA in Docket 74-10, Notice 1, wherein a manual control for limiting front axle braking was proposed. Such proposal was, however, subsequently withdrawn in Docket 74-10, Notice 2, primarily, we suspect, because of the manual control feature.
The Bendix Corporation, Heavy Vehicle Systems Group, has developed an automatic front axle limiting system, separate from the antilock system itself, that will sense an electrical failure of the front axle antilock system and "automatically" reduce front axle braking to 50% of that normally provided. Attention is invited to the performance comparison (Enclosure 1) wherein the automatic limiting curve reflects that 120 psi rear axle pressure is required to reach 60 psi at the front axle. While Bendix' system does not introduce an air line restriction, it does limit the level of front axle braking pressures so that the 60 psi and 95 psi pressure levels set forth in Section 5.3.3 (Brake Actuation Time) and Section 5.3.4 (Brake Release Time) respectively, are not achieved, nor are the corresponding actuation and release times realized.
Section 5.5.1 (Antilock System Failure) provides that an ". . . electrical failure of any part of the antilock system shall not increase the actuation and release times of the service brakes." Bendix is of the opinion that since the affect on response time is caused by a system separate from the antilock system itself, it is not within the purview of, and hence not in conflict with Section 5.5.1.
An official interpretation is requested from NHTSA as to whether it concurs with Bendix' opinion that the Bendix Automatic Front Axle Limiting System is not covered by the provisions of, and is not in conflict with Section 5.5.1.
We would be happy to discuss the details of this matter if you feel additional information is necessary.
Very truly yours,
R. W. Hildebrandt -- Group Director of Engineering
Attachment
PERFORMANCE COMPARISON
Standard (Non Limiting) System Vs: System With Automatic Limiting Upon Antilock Failure
(Graphics omitted)
Rear Axle Brake Pressure (PSI)