Interpretation ID: nht87-3.41

TYPE: INTERPRETATION-NHTSA

DATE: 12/10/87

FROM: AUTHOR UNAVAILABLE; Erika Z. Jones; NHTSA

TO: Mercedes-Benz Truck Company, Inc.

TITLE: FMVSS INTERPRETATION

TEXT:

Mercedes-Benz Truck Company, Inc. 4747 N. Channel P.O. BOX 3849 Portland, Oregon 97208

Dear Mr. Rossow:

This responds to your request for an interpretation of Federal Motor Vehicle Safety Standard No. 121, Air Brake Systems. Section 56.2.1 of that standard specifies for certain tests conducted on a dynamometer that "(the dynamometer inertia for each wheel is equivalent to the load an the wheel with the axle loaded to its gross axle weight rating." According to your letter, you have interpreted the term "equivalent" in this section to "authorize compliance testing by reference to axle loads under actual st opping conditions." You requested confirmation of this interpretation. As discussed below, we disagree with your suggested interpretation.

By way of background information, the National Highway Traffic Safety Administration (NHTSA) does not provide approvals of motor vehicles or motor vehicle equipment. Under the National Traffic and Motor Vehicle Safety Act, it is the responsibility of the manufacturer to ensure that its vehicles or equipment comply with applicable standards. The following represents our opinion based on the facts provided in your letter.

As indicated in your letter, your request for an interpretation was submitted in light of recent correspondence between your company and NHTSA's Office of Vehicle Safety Compliance (OVSC). OVSC requested you to submit information on the compliance with S tandard No. 121 of the Mercedes-Benz model L-1317, a two axle straight truck. You responded to that request by submitting a compliance certificate and interpretation concerning section 56.2.1. In letter dated April 9, 1987, OVSC informed you that it did not agree with your interpretation.

Standard No. 121's dynamometer tests are set forth in section S5.4. That section specifies that brake assemblies must meet the requirements of 55.4.1 (brake retardation force-relevant only to towed vehicles), 55.4.2 (brake power), and 55.4.3 (brake recov ery), under the conditions of 56.2. One of those conditions, set forth in 56.2.1, is as follows:

S6.2.1 The dynamometer inertia for each wheel is equivalent to the load on the wheel with the axle loaded to its gross axle weight rating. For a vehicle having additional gross axle weight ratings specified for operation at reduced speeds, the GAWR used is that specified for a speed of 50 mph, or, at the option of the manufacturer, any speed greater than 50 mph.

In support of your suggested interpretation, you noted that axle loads of a decelerating vehicle vary under different deceleration conditions, i.e., as a vehicle traveling forward decelerates, the load of the axles shifts so that the front axle load rise s and the rear axle load falls. You stated that it is your reading of Standard No. 121 that the manufacturer "can assess compliance by either using a static load value or determining which of the varying values of the axle load should be considered in vi ew of actual vehicle behavior." With respect to gross axle weight rating (GAWR), you suggested that when used in the context of Parts 567 and 568, the GAWR is properly measured in a static manner, to permit a static determination of whether the load carr ying capacity of a vehicle axle in actual use has been reached. For dynamometer tests of service brakes under dynamic conditions, however, you argued that such tests should properly take into account the dynamic effects of deceleration.

You then stated the following:

The language of 56.2.1, setting dynamic test conditions, indicates that the dynamometer inertia for each wheel is to be set at the "equivalent" to the load on the wheel, when the axle is loaded to its GAWR (i.e., its load-carrying capacity). This languag e is not restrictive and grants a manufacturer the flexibility of determining an "equivalent" loading in consideration of the dynamic phenomena in conducting the texts required by 55.4. Thus, the static GAWR is permitted to be linked to dynamic condition s by the word "equivalent."

We disagree with your suggested interpretation, which we believe is inconsistent with the language of S6.2.1, past interpretations of that provision, and the compliance test procedures the agency has long followed with respect to this provision. As indic ated above, 56.2.1 specifies that the dynamometer inertia for each wheel is "equivalent to the load on the wheel with the axle loaded to its gross axle weight rating." The phrase "equivalent to the load" uses the singular "load," instead of the plural "l oads," to show that the dynamometer inertia has only a single value. By itself, this suggests that 56.2.1 was not intended to provide multiple options for the dynamometer inertia setting, depending on the dynamic conditions simulated.

Further, the overall language of 56.2.1 shows how the single dynamometer inertia setting is to be determined. The term "GAWR" is defined in 49 CFR Part 571.3 as "the value specified by the vehicle manufacturer as the load-carrying capacity of a single ax le system, as measured at the tire-ground interfaces." When an axle is loaded to its load-carrying" capacity, there is one "load on the wheel," at whose "equivalent" the dynamometer inertia must be set.

While we believe that the language of section S6.2.1 is clear on the issue raised by your letter, we also note that agency guidance in the form of a past interpretation letter and OVSC's laboratory procedures for Standard No. 121 are also clear. In an in terpretation letter to Wagner Electric, dated May 26, 1972, the agency stated:

In the dynamometer test conditions of S6.2.1, the dynamometer inertia for each brake assembly is based on 1/2 the GAWR of the axle. The rating for each axle is required to be stated separately. If, in the example you give, you choose to give 17,000 pound s as the rating for each axle, then the dynamometer inertia would be at 8,500 pounds for each brake assembly.

That interpretation explicitly states that the dynamometer inertia is set with regard to the assigned GAWR, and makes no reference to the varying values of axle load during braking. This agency interpretation has been a matter of public record for the la st 15 years. Moreover, as OVSC noted in its letter of April 9, 1987, its test procedure TP-121-02 provides a specific formula for calculating "dynamometer inertia equivalent to the GAWR for the applicable vehicle axle." This test procedure has been used by the agency since March 163 1978, and has been available to the public since that date.

You asserted in your letter that the static GAWR is "permitted to be linked to dynamic conditions by the word 'equivalent'." We find no basis in the word "equivalent" for your suggestion that the load on the wheel- should somehow be calculated during bra king. Section S6.2.1 uses the term "equivalent" to account for the fact that the terms "load" and "inertia" are different without the same dimensions and are not numerically equal; an axle's tire rolling radius must be considered in determining the prope r inertia as well as the load. We note that section S5.4.2.1 of Standard No. 121 uses the term "equivalent" in the same manner. That section specifies for dynamometer testing that the drum or disc be "rotating at a speed equivalent to 50 m.p.h." Since t he drum or disc is obviously not moving along longitudinally, the word "equivalent" in that section is used to bridge the gap between longitudinal and rotational movement.

Your letter also argued that a February 18, 1976, interpretation letter to Oshkosh supports your suggested interpretation of 56.2.1. Oshkosh had asked whether a vehicle that meets section S5.1.1's requirements for air Compressor capacity when it is movin g must also comply when the vehicle is stationary. The agency stated:

Section S5.1.1 does not specify whether or not the vehicle is moving as a test condition for the requirement. In view of the absence of this test condition, the NHTSA will resolve differences in this test condition in the manufacturer's favor if they aff ect the outcome of testing.

We do not agree that this letter supports your suggested interpretation. The letter addressed only the issue of how a requirement should be read in view of the absence of a particular test condition. As explained at length above, we conclude that section S6.2.1 clearly specifies the particular test conditions to be followed for this section. Therefore, the Oshkosh letter is not relevant to requests for interpretation of S6.2.1.

You also argued that in order to provide an appropriate braking system, with proper distribution of brake forces between the axles, its design must take into account the transfer of weight from the rear axle to the front axle during normal and emergency braking conditions. You stated that such a design and compliance test leads to a significant reduction in premature lockup of the rear axle. You also argued that NHTSA has recognized your braking system as "a safe and effective system" in its research te sting.

We agree that a manufacturer must take into account the transfer of weight from the rear axle to the front axle when designing an appropriate braking system. This is necessary to provide safe brake performance during varying loading conditions, for norma l and emergency brake applications on varying road conditions: and it is so for all kinds of vehicles. However, the requirements of Standard No. 121 do not require vehicles to have too much rear braking, as you appear to imply. The requirements of S5.4.2 (Brake Power), and S5.4.3 (Brake Recovery), are minimum performance requirements intended to help ensure that brakes retain adequate stopping capacity during and after exposure to conditions caused by prolonged or severe use, such as long, downhill driv ing. In practice, in order to perform well in such conditions, both front and rear brakes must have a minimum capacity, and this capacity is related to GAWR despite the fact that the actual loads borne by the front and rear axles vary during different br ake applications. The agency therefore referred to GAWR in section 56.2.1, because this is an objective value that is readily ascertainable for every vehicle, and performance based on this value meets the particular safety need provided for by the requir ements of section S5.4. These minimum requirements are not intended, nor do they operate, as a restriction on the design decisions that manufacturers must make independently to distribute braking capacity to meet anticipated load distributions.

Contrary to your assertion, NHTSA has not concluded that your brake system is "safe and effective." We also note that the quotations of the agency's research report cited in your letter address only limited aspects of braking performance and are taken ou t of context. We note that you stated that "(t)he Agency reported finding that the subject vehicle's front and rear axles were '. . . well balanced and tended to lock at close to the same pedal effort level.' (p. 19)." A more complete quotation is as fol lows:

. . . In the empty driver best effort stops the driver was also able to utilize this peak friction, although not as effectively as the antilock, because the brakes on front and rear axles of the vehicle were well balanced and tended to lock at close to t he same pedal effort level. In the loaded case, however, the front axle tended to lock prematurely and it Has not possible for the driver to maintain all four wheels near the peak friction level. He could keep the front tires near the peak but when this occurred rear braking was relatively low. If he applied more braking, the front axle locked and he lost steering control due to lack of lateral traction at the front tires."

Based on the information before the agency, OVSC is continuing its investigation concerning the compliance of your vehicles with Standard No. 121.

Sincerely,

Erika Z. Jones Chief Counsel

May 22,1987

Ms. Erika Z. Jones, Chief Counsel Department of Transportation National Highway Traffic Safety Administration 400 Seventh Street, SW Washington, D.C. 20590

RE: Request for an Interpretation Concerning FMVSS 121, Air Brake Systems

Dear Ms. Jones:

On behalf of our parent company, Daimler-Benz Aktiengesellschaft (DBAG), Mercedes-Benz Truck Company, Inc. (MBTC) requests an interpretation of Federal Motor Vehicle Safety Standard (FMVSS) 121, Air Brake Systems. This standard specifies that for tests c onducted on a dynamometer, "The dynamometer inertia for each wheel is equivalent to the load on the wheel with the axle loaded to its gross axle weight rating.

49 C.F.R. S 571.121.56.2.1. DBAG has interpreted the term "equivalent in this regulation to authorize compliance testing by reference to axle loads under actual stopping conditions. It is this interpretation which we are asking be confirmed by your offic e.

The need for such an interpretation is the result of recent correspondence between MBTC and NHTSA. NHTSA's Office of Vehicle Safety Compliance has asked MBTC, as the manufacturer of trucks bearing the trade name of Mercedes-Benz, to submit information on the compliance with FMVSS 121 of Mercedes-Benz truck model L-1317. MBTC filed a response to this request containing the DBAG compliance certificate and the foregoing interpretation. In a letter dated April 9, 1987, NEF-31 HTS CIR 2879.1, the Office info rmed MBTC that it does not agree with the DBAG interpretation of FMVSS 121 used to assess compliance.

In determining compliance with FMVSS 121, DBAG recognizes, as do all automotive engineers, that axle loads of a decelerating vehicle vary under different deceleration conditions. That is, as a vehicle traveling forward decelerates, the load on the axles shifts so that the front axle load rises and the rear axle load falls. (See DBAG Compliance Certificate, section 2.16) Under Standard 121, the manufacturer must specify an axle load in connection with the tests conducted to assess compliance. It is DBAG' s reading of Standard 121 that the manufacturer can assess compliance by either using a static load value or determining which of the varying values of the axle load should be considered in view of actual vehicle behavior.

As noted above, Standard 121 refers to the "gross axle weight rating" (GAWR) of the vehicle. GAWR is defined generally as "the value specified by the vehicle manufacturer as the load-carrying capacity of a single axle system, as measured at the tire-grou nd interfaces."

49 C.F.R. S 571.3(b). When used in the context of Parts 567 (Certification) and 568 (Vehicles Manufactured in Two or More Stages), the GAWR is properly measured in a static manner, to permit a static determination of whether the load carrying capacity of a vehicle axle in actual use has been reached. The nominal GAWR value on the certification label therefore must be used for such a determination.

A test to represent a dynamic procedure such as braking presents quite different requirements. Instead of a static measurement, such a test should properly take into account the dynamic effects of deceleration. Standard 121 mandates dynamometer tests of service brakes under dynamic conditions. Thus, the question is whether the standard is specific in requiring a GAWR determined on a static test or whether language would permit the type of interpretation utilized by DBAG.

The language of 56.2.1, setting dynamic test conditions, indicates that the dynamometer inertia for each wheel is to be set at the "equivalent" to the load on the wheel, when the axle is loaded to its GAwR (i.e., its load-carrying capacity). This languag e is not restrictive and grants a manufacturer the flexibility of determining an "equivalent. loading in consideration of the dynamic phenomena in conducting the tests required by 55.4. Thus, the static GAWR is permitted to be linked to dynamic conditions by the word "equivalent."

The foregoing interpretation is supported by prior interpretative guidelines of the Agency. The standard itself does not specify that the "load on the wheel" must be evaluated in a static manner. In fact, it specifically uses the word "equivalent," a wor d not used in other sections of the standard. (See for comparison 5.5.6.1) In the context of braking, a manufacturer could, therefore, reasonably conclude that the dynamics of wheel loads under deceleration can be considered. In a letter to the Oshkosh T ruck Corporation, the Chief Counsel's Office has supported such an interpretation. In the Oshkosh case, the Agency indicated that, where the standard is silent as to an issue, the manufacturer may exercise its discretion. Oshkosh had asked whether a vehi cle that complies with S5.1.1 of the standard (air compressor capacity) when it is moving must also comply when the vehicle is stationary. The Chief Counsel replied:

"Section S5.1.1 does not specify whether or not the vehicle is moving as a test condition for the requirement. In view of the absence of this test condition, the NHTSA will resolve differences in this test condition in the manufacturer's favor if they af fect the outcome of testing." Letter from Richard B. Dyson to Oshkosh Truck Corporation (February 18, 1976).

For these reasons also, DBAG relies on the conclusion that Standard 121 does not specifically restrict the test procedure and permits a manufacturer to assess compliance by reference to the dynamics of braking for an actual vehicle. DBAG has concluded th at in order to provide an appropriate braking system, with proper distribution of brake forces between the axles, its design must take into account the transfer of weight from the rear axle to the front axle during normal and emergency braking conditions . Such a design and compliance test leads to a significant reduction in premature lockup of the rear axle.

The Agency's own testing of the L-1317 supports this Conclusion. In August 1986, the Agency issued a report entitled, "Performance Evaluation of a Production Antilock System Installed on a Two Axle Straight Truck (NHTSA's Heavy Duty Vehicle Brake Researc h Program Report #6)" which included dynamic testing of this vehicle with and without the use of its ABS system. The Agency reported finding that the subject vehicle's front and rear axles were "... well balanced and tended to lock at close to the same p edal effort level." (p. 19). Further, the Agency reported that "... in the empty condition the vehicle has a relatively high braking efficiency over a broad range of road friction levels." The report explains that efficiency is a measure of the vehicle's ability to use available friction before lockup and loss of control occurs (p. 19). Finally, the report generally notes "if loss of control of the overbraked axle prevents the driver, no matter how skilled he is, from utiliz ing the full capability of the underbraked axle .." (p. 22). Accordingly, not only does FMVSS 121 provide sufficient breadth to allow the interpretation utilized by Daimler-Benz, its use results in a braking system which the Agency has recognized as a sa fe and effective system.

Based on the foregoing request, we would appreciate your office responding with respect to the appropriateness of this interpretation. If you require any additional information, please do not hesitate to contact me.

Sincerely,

Gary W. Rossow Director, Government Technical Affairs

cc: Mr. George L. Parker