Interpretation ID: aiam4418
Director
Government Technical Affairs
Mercedes-Benz Truck Company
Inc.
4747 N. Channel
P.O. Box 3849
Portland
OR 97208;
Dear Mr. Rossow: This responds to your request for an interpretation of Federal Moto Vehicle Safety Standard No. 121, *Air Brake Systems*. Section S6.2.1 of that standard specifies for certain tests conducted on a dynamometer that '(t)he dynamometer inertia for each wheel is equivalent to the load on 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 stopping 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 Safet 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 all 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 wa 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 Standard No. 121 of the Mercedes-Benz model L-1317, a two axle straight truck. You responded to that request by submitting a compliance certification and interpretation concerning section S6.2.1. 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 S5.4.1 (brake retardation force--relevant only to towed vehicles), S5.4.2 (brake power), and S5.4.3 (brake recovery), under the conditions of S6.2. One of those conditions, set forth in S6.2.1, is as follows:; >>>S6.2.1 The dynamometer inertia for each wheel is equivalent to th 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 load of a decelerating vehicle vary under different deceleration conditions, i.e., as a vehicle travelling forward decelerates, the load on the axles shifts so that the front axle load rises and 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 view 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 determination of whether the load carrying 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 S6.2.1, setting dynamic test conditions, indicate 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 language 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 S5.4. Thus, the static GAWR is permitted to be linked to dynamic conditions by the word 'equivalent.'<<<; We disagree with your suggested interpretation, which we believe i 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 indicated above, S6.2.1 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 'loads,' to show that the dynamometer inertia has only a single value. By itself, this suggests that S6.2.1 was not intended to provide multiple options for the dynamometer inertia setting, depending on the dynamic conditions simulated.; Further, the overall language of S6.2.1 shows how the singl 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 axle 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 th issue raised by your letter, we also note that agency guidance in the form of past interpretation letter and OVSC's laboratory procedures for Standard No. 121 are also clear. In an interpretation letter to Wagner Electric, dated May 26, 1972, the agency stated:; >>>In the dynamometer test conditions of S6.2.1, the dynamomete 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 pounds as the rating for each axle, then the dynamometer inertia would be at 8,500 pounds for each brake assembly.; <<>>Section S5.1.1 does not specify whether or not the vehicle is movin 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 affect the outcome of testing.<<<; We do not agree that this letter supports your suggeste 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 testing.; We agree that a manufacturer must take into account the transfer o 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 normal 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 driving. 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 brake applications. The agency therefore referred to GAWR in section S6.2.1, 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 requirements 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 brak 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 out 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 follows:; >>>. . . In the empty driver best effort stops the driver was also abl 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 the same pedal effort level. In the loaded case, however, the front axle tended to lock prematurely and it was 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 it investigation concerning the compliance of your vehicles with Standard No. 121.; Sincerely, Erika Z. Jones, Chief Counsel