Interpretation ID: nht76-4.27
DATE: 09/03/76
FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA
TO: Freightliner Corporation
TITLE: FMVSS INTERPRETATION
TEXT: This responds to your July 23, 1976, question whether the "no lockup" requirement of S5.3.1 of Standard No. 121, Air Brake Systems, requires wheel sensors on both axles of a tandem axle system in those cases where the "no lockup" performance is provided by means of an antilock system. Sections S5.3.1 (trucks and buses) and S5.3.2 (trailers) specify that the vehicle shall, under various load, road surface, and speed conditions, be capable of stopping
. . .without lockup of any wheel at speeds above 10 mph, except for:
(a) Controlled lockup of wheels allowed by an antilock system. . .
(b) * * * * *
This basic requirement is stated in performance terms, permitting a manufacturer to choose any brake system design that will ensure that the wheels do not lock up under the specified conditions.
The exception to the "no lockup" requirement set forth above permits "controlled lockup of wheels allowed by an antilock system." Manufacturers demonstrated, during the course of rulemaking, that properly functioning antilock systems might be designed to allow wheel lockup for a fraction of a second, and that antilock design should not be inhibited by a prohibition on all lockup. The agency made the "controlled lockup" exception a part of the standard (36 FR 3817, February 27, 1971) and has subsequently interpreted the term to permit manufacturers latitude in the design of their systems.
In compliance with the basic requirement, most manufacturers have equipped each axle of a vehicle with a valve to regulate the air pressure that applies the brakes, sensors at each wheel to send a signal when a wheel is locking up, and a logic module that receives the signals and instructs the valve when to release air pressure to prevent lockup ("axle-by-axle control"). Recently, some manufacturers have simplified their systems by utilizing only one valve and logic module to modulate the air supply to both axles of the typical tandem axle system found on many trucks and trailers ("tandem control"). Two approaches to wheel sensor placement have been used for tandem control systems. If it is possible to predict which of the two axles will lock first during braking, sensors may be placed on this axle only, knowing that reduced air pressure in response to a signal from the "sensed" axle will also release the brakes on the "unsensed" axle. In other cases, where it is not possible to predict which axle will lock first, tandem control systems may have sensors on all four wheels of the tandem.
In November 12, 1974, and March 7, 1975, letters of interpretation to Dana Corporation, the NHTSA confirmed that a manufacturer may choose the number of wheel speed sensors and logic modules that he includes in his antilock system. Thus, tandem control is not prohibited by the standard, regardless of the number of wheel speed sensors provided. When Dana asked if lockup on the unsensed axle of a single-axle sensor system would qualify for the "controlled lockup" exception of the requirement, the agency said that it would not, reasoning that the logic module would not exert effective control over the lockup of the unsensed axle without benefit of input signals from wheels on that axle. Therefore, according to the Dana interpretation, the unsensed axle in a single-axle sensor system could not be allowed to lock at all, even momentarily, during the service brake stopping test. No data of actual performance was submitted with the Dana letter.
Your letter argues that the NHTSA's interpretation of "controlled lockup" (to Dana Corporation) creates an anomalous and unjustified restriction on the use of "tandem control." Your submission, and data received by the agency from other interested persons, demonstrate that the Dana interpretation does not adequately reflect the degree of control which a single-axle sensor system actually can exert over the unsensed axle of a tandem system. Based on analysis of the submitted data, it appears that the amount of lockup permitted on unsensed axles is closely controlled by the permitted on unsensed axles is closely controlled by the available antilock systems. While there is a measurable difference in stopping performance between "axle-by-axle" control and "tandem control," the standard already permits either of these means to satisfy the requirements. When the narrower question of the performance difference between sensors on one or both axles is analyzed, it is apparent that virtually no difference exists in the stopping distance of vehicles equipped these two ways. The effective lateral stability available during a stop also appears comparable regardless of placement of sensors on one or both axles. A technical report summarizing these findings will be placed in the public docket as soon as possible.
For this reason, and based on review of test unavailable at the time of the Dana interpretation, the agency concludes that its interpretation of "controlled lockup" in response to the question posed by Dana should be, and is hereby, withdrawn. It is the agency's interpretation that the "controlled lockup" exception is not dependent on the number or location of sensors used in an antilock installation.
Sincerely,
ATTACH.
FREIGHTLINER CORPORATION
July 23, 1976
LEGAL COUNSEL -- National Highway Traffic Safety Administration
Re: Motor Vehicle Safety Standard No. 121 (49CFR571.121)
Dear Sir:
Freightliner Corporation, a manufacturer of light-weight heavy-duty air braked vehicles, is vitally affected by the requirements of FMVSS-121. Therefore, we are requesting a clarification of the exemption contained in S5.3.1, for "controlled lockup of wheels allowed by an antilock system." In reading the requirements of Standard No. 121, the exemption to the "no wheel lockup" requirement contained in S5.3.1 clearly applies to any wheel of a tractor, truck or bus which is subject to the control of an antilock system. But in a previous interpretation on this subject (March 7, 1975, letter of James C. Shultz to Harold D. Shall, Dana Corporation), the Acting Chief Cousel appeared to limit this exemption only to wheels which are equipped with antilock wheel speed sensors. Clearly, this interpretation is design restrictive in that it is stated in design rather than performance terms. Further, this interpretation does not take into consideration the possibility of alternate means for achieving controlled wheel lockup, such as mechanical drives which interlock both tandem axles.
The results of recent tests conducted by Freightliner show that the wheels on both tandem axles of a tandem axle vehicle can be effectively controlled through the use of one antilock system sensing wheel speeds from only one axle, and demonstrate that "controlled lockup" typical of that provided by axle-by-axle antilock systems can be achieved through a combination of suspension design, controlled brake actuation timing, and antileck system design. The test results (see attachment) indicate that the vehicle equipped with only one antilock controller meets the stopping distance requirements of FMVSS-121 with performance equivalent to that of vehicles equipped with the more complex and costly axle-by-axle antilock systems currently employed.
Since tractors and trucks, such as those manufactured by Freightliner Corporation, are subject to stopping distance requirements under a variety of road and load conditions, efficient utilization of available traction for braking is already a requirement of the standard for powered vehicles. Therefore, we believe that requiring the use of axle-by-axle antilock systems is unnecessary to ensure efficient utilization of available traction for braking of powered vehicles. Accordingly, we request interpretation of the requirements of S5.3.1 which is:
1. Stated in performance rather than design criteria.
2. Which does not impose a more strigent requirement for axles which are not equipped with wheel speed sensors (but subject to the control or an antilock system) than the "controlled lockup" requirement applicable to wheels which are equipped with wheel speed sensors.
If further information is necessary or desirable, we would be pleased make a technical presentation to you and appropriate members of the NHTSA staff, which would include a discussion of our test results along with movies of the tests.
Respectfully submitted,
Ray W. Murphy -- Director, Research and Development
Attach. FMVSS-121 STOPPING DISTANCE TEST DATA
Vehicle - Three Axle COE Tractor, 138" Wheelbase Control Axle Loads (lbs.) Steering Drive Trailer Loaded 10,480 34,110 33,290 Bob tail 7,550 6,040 -
Brakes Size Actuation Power Linings Steering Axle 15x4 14 degrees x9 ABB-551D Drive Axle 15x7 12 degrees x12 (Dual) ABB-551D
Average service brake stopping distances in ft., vehicle equipped with tandem control antilock system (numbers in parenthesis are values from tests of similarly configured vehicle equipped with axle-by-axle antilock control system)* Loaded Bobtail FMVSS-121 Requirement 20 MPH Dry Asphalt 33.6 27.6 35 (SN 75) Test Surface (34.0) (26.4) 60 MPH Dry Asphalt 259.8 251.2 293 (SN 75) Test Surface (253.7) (225.5) 20 MPH Wet Slippery 58.8 60.2 ** (SN 20) Test Surface (71.3) (57.3)
* Freightliner CTC Report No. T081-75/12, November 20, 1975
** 60 ft. on SN 30 test surface