Interpretation ID: nht76-4.28

DATE: 09/03/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Frehauf Corporation

TITLE: FMVSS INTERPRETATION

TEXT: 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 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.

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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 available antilock systems. While there is a measurable difference in stopping performance between "axle-by-axle"

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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 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.

FRUEHAUF DIVISION / FRUEHAUF CORPORATION

August 17, 1976

Chief Counsel NHTSA Gentlemen:

RE: 49 CFR 571.121

In our evaluation of anti-lock systems, Fruehauf has become convinced that a considerable economic improvement can be made in the system provided for FMVSS 121. A system involving sensors on one axle, a logic controlling that axle, and a second axle of the suspension controlled by the same logic, performs identically to a system on the same suspension that uses sensors on each wheel and a logic for each axle. The economic gains for trailer users is very attractive. This system conforms with the 121 standard in its entirety and no change in the standard is requested.

We have exhibited in many tests, that our suspension provides mechanical control of lock-up. The momentary lock-up of the second axle of the suspension is always initiated at a slightly later time than the front axle. When we use an air and electrical system as described above, we can assure that the release of air to the two axles is simultaneous. Therefore, the second axle performance in lock-up is equal to or slightly better than the first axle. See charts in Appendix B of typical tests.

Both axles are under the control of an anti-lock system. The control is such that any momentary lock-up on the axle without sensors is equal to or shorter than momentary lock-up with sensors. This is possible through the geometry of the suspension. The suspension has equal loads on the axles at rest. However, with the application of brakes, one axle reduces load, transferring load to the second axle momentarily. This transfer assures that the first axle always locks or reduces speed faster than the second axle. The effect of this system of anti-lock is that the average of the two wheels of the front axle controls both axles. No wheel is allowed more than momentary lock-up.

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The suspension described above is different from that on which Dana Corporation requested an interpretation some time ago. However, the letters of interpretation in answer to Dana are general enough that they should be clarified so they do not restrict our use of this system. Those interpretations say that an axle must have sensors if it is to be allowed to have momentary lock-up. This requirement does not seem justified in this instance since it requires the axle without sensors to perform better than the axle with the complete sensor and logic system. Certainly, we agree that the axle-by-axle complete control system adequately controls the braking function and stability of a trailer. Wheel-by-wheel systems were not significantly better. We believe that a "slave" axle that performs as well as those controlled axles in all respects, should be adequate even though mechanical control is used rather than sensors in each wheel.

While stopping distances is not a test requirement on trailers, it is an important safety factor. Considerable testing with measured stopping distances has been done with both bogie control and axle-by-axle control. Best stopping distances are secured with systems which allow momentary zero wheel speed intervals on low mu surfaces. A system tuned to give no lock-up or zero wheel speed would give longer stopping distances. While there are differences in some instances with one system having shorter distance than the other, the variations are not as great between bogie control and axle control of a given anti-lock system on a given trailer, as they are between two manufacturers' anti-lock systems. See Appendix A. Therefore, the use of bogie control does not change the stopping distance beyond the ranges already exhibited by variations in anti-lock systems.

In all of the testing that has been done with bogie control, we've had no indication of lack of stability.

Air consumption, which is not a test requirement but is a factor in the selection of systems, is not significantly different.

Systems on which we have seen test data using four sensors with a single logic, do not perform better than the bogie control described here.

In the interest of providing the most economical system that meets the requirements and intents of FMVSS 121, Fruehauf is considering production of the described system of anti-lock where any momentary lock-up on any wheel is of equal or shorter time than any momentary lock-up on the wheel using electrical speed sensors.

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We request your prompt confirmation that the described system complies with FMVSS 121. If NHTSA has any questions about this systems' compliance, Fruehauf would request an early response to avoid any economic loss or inconvenience to the trailer customers.

Sincerely,

A. F. Hulverson Vice President Engineering

(Enclosures Omitted.]