Interpretation ID: aiam2379
Vice President
Engineering
Fruehauf Division
Fruehauf Corporation
10900 Harper Avenue
P.O. Box 238
Detroit
MI 48232;
Dear Mr. Hulverson: This responds to your August 17, 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 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 permit '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 hav 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 t 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 'controlle 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 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 data unavailable at th 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, Frank A. Berndt, Acting Chief Counsel