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Interpretation ID: nht72-2.44

DATE: 05/26/72

FROM: AUTHOR UNAVAILABLE; E. H. Wallace for E. T. Driver; NHTSA

TO: Wagner Electric Corporation

TITLE: FMVSS INTERPRETATION

TEXT: The following interpetations are submitted in response to your letter of March 16, 1972, concerning Federal Motor Vehicle Safety Standard No. 121, Air Brake Systems and are numbered as the questions were in your letter.

1. The 30 skid number surface referred to in the Standard is wet and measured by the ASTH E-274-65T procedure omitting water delivery as specified in paragraph 7.1 of that procedure.

2. In S5.1.6 total electrical failure" means any electrical failure within the antilock electrical system circuitry which would cause loss of antilock control of every wheel on the vehicle.

3. S5.1.6 does not at present require the activation of the antilock warning device so long as proper antilock control remains on at least one wheel or axle.

4. The requirement for an antilock warning device is not limited to a failure in the power supply at the antilock electrical connection. However, an electrical failure in a vehicle which causes failure of the whole vehicle electrical system is not expected to activate the warning system since there would be no power to energize it.

5. In S5.1.7, for air over hydraulic brake systems, the 6 psi pressure is measured in the power air chamber coupled to the master cylinder used to convert air pressure to hydraulic pressure.

6. The wording of S5.1.7 is not intended to exclude the use of any auxiliary hand application valves for controlling the trailer brakes as long as there is also a foot treddle valve which controls the brakes of the towing vehicle and any towed vehicle.

7. The stoplight on a trailer is to be actuated whenever the service brakes on the trailer are applied. S4.5.3 of FMVSS No. 168 states "the stoplamps on each vehicle shall be activated upon the application of the service brakes."

8. The intent of S5.2.1.2 is for the service reservoir capacity, to be eight times the combined volume of all of the service brake chambers.

9. In S5.3.2 the 90 psi pressure is to be fully applied to the trailer at the start of the (Illegible Words).

10. In S5.3.2, the air compressor and air supply system of the towing vehicle are expected to be operating normally.

11. In S5.3.3, the towing vehicle brakes may be by passed by any convenient means so long as it does not cause the air pressure applied to the trailer to fall below 90 psi.

12. In S5.3.2, item 4 and 5 of Table I are not applicable.

13. A truck tested brake may be run on a dynamometer by a manufacturer for his own purposes but compliance with S5.4.1, S5.4.2 and S5.4.3 of the Standard will be determined by the Government by testing a new brake assembly identical to the one on the vehicle.

14. Same answer as for 13 above.

15. The intent of S.5.4 is for a new brake assembly identical to the one on the vehicle to be tested on a dynamometer for conformance with S5.4.1, S5.4.2 and S5.4.3

16. Same (Illegible Word) answer for 15 above.

17. In S5.4.1, S5.4.2 and S5.4.3, for a air (Illegible Word) hydraulic brake systems, the "brake chamber air pressure" is the air pressure in the power chamber coupled to the master cylinder used to convert air pressure to hydraulic pressure.

18. Same answer as for question 17.

19. The Standard does not consider tandem ratings, it addresses only the GAWR of each individual axle.

20. Vehicles conforming to S5.6.1 must have a parking brake on each individual axle of a tandem axle arrangement.

21. In S5.6.2(a) "Gross vehicle weight rating is correct for semi-trailers as well as trucks and buses; gross axle weight rating is not meant.

22. Semi-trailers are not excluded from the meeting of the alternate requirement of S5.6.2.

23. A dolly is classified as a trailer and is a separate vehicle.

24. In S5.6.2, the unloaded dolly weight does not include an unloaded semi-trailer.

25. 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 pounds as the rating for each axle, then the dynamometer inertia would be at 8,500 pounds for each brake assembly.

Sincerely,

ATTACH.

WAGNER ELECTRIC CORPORATION WAGNER DIVISION

March 16, 1972

Elwood T. Driver, -- Director, Office of Operating Systems, NHTSA Gentlemen:

We recently provided our customers with copies of the original FMVSS-121 with the Notice 3 amendments noted thereon. This action has resulted in much discussion on product availability and system simplicity. Certain portions of FMVSS-121 have prompted this request for clarification and interpretation in order to better direct our test and development efforts toward customer solutions for the more exacting compliance investigations that will be carried on in the immediate future.

Each of these items is identified individually in the event a partial response can be made immediately - should a reply to some items be deferred.

The S4 definition for determination for skid number states ". . . omitting water delivery . . ." from the ASTM method. The summary statement for the Federal Register Notice 3 emphasizes the need to test on wet pavement. Sections S5.3.1.1 and 5.3.1.2 state ". . . on a wet surface with a skid number of 30 . . .". Table II has a column headed Wet Skid No. 30. Table I has a reference to ". . . skid number of 30. . ." which does not disringuish between wet and dry, and lacks consistency in the instructions.

Question 1. Is the 30 skid number determined with or without water delivery? (We would assume it to be with water delivery in view of the specific references cited. A skid number of 30 on a dry surface and subsequent testing with that surface wetted does not control the friction level of the wetted surface for uniformity in compliance testing).

There is still some ambiguity in S5.1.6 concerning ". . . a total electrical failure . . ." although the Notice 3 amendments have significantly improved this section.

Question 2. What is meant by the phrase ". . . total electrical failure . . ."? (The next two questions reflect some of our thoughts).

Question 3. In multi-axle systems where each axle has a separate control package, is it permissible for one or more axle sections to be inoperative and no signal announced as long as one axle section of the total system remains electrically intact?

Question 4. Or should the requirement for warning be limited to a failure in the power supply at the antilock electrical connection? (If power fails at battery terminals the warning device will not work).

Air over hydraulic systems employ a variety of power cylinder or power chambe types coupled to master cylinders for the pressure conversion.

Question 5. Are we correct in interpreting S5.1.7 that the 6 psi control pressure is measured in the air powered component of an air over hydraulic system even though it is not specifically a ". . . service brake chamber"?

Question 6. A popular auxiliary service brake control is the hand actuated application valve that requires rotary motion and is not depressed (Ref. S5.1.7). Is it intended to allow or exclude this option for manual actuation?

Question 7. Assuming the option of Question 6 is permitted - for tractor use to control the trailer of a combination vehicle - is it correct that the service brake stop lamp switch should be actuated by either control means and not by the foot control only?

Question 8. Is it the intent of S5.2.1.2 for only the "service" reservoirs to be used in calculating total volume, that is, exclude the isolated reservoirs provided for parking brake release?

In S5.3.1 Stopping distance - trucks and buses, there is a procedural requirement of 6 stops for each test phase in Table I. This is not permitted in S5.3.2 - Stopping capability - trailers. Hence for each single stop in the sequence specified in Table I we ask:

Question 9. Is the 90 psi the starting pressure for each stop?

Question 10. Is there intended to be any allowance for maintaining normal replacement of air pressure from the tractor compressor during the stopping tests?

Question 11. Are we at liberty to use any convenient means we choose to bypass the application of the towing vehicle brakes?

Since the items 4 and 5 of the Table I stopping sequence refer only to S5.7.2.3 conformance (Emergency stopping distance - trucks and buses) and S5.8, Emergency braking capability - trailers brakes no provision for hydraulic (Illegible Word) of the trailer emergency system we ask:

Question 12. In S5.3.2 should the instructions limit ". . . each combination of weight, speed, and road conditions . . . specified in Table I . . ." to only items 1,2 and 3 of Table I?

In S5.4 Service brake systems - dynamometer, the second sentence is still ambiguous.

"A brake assembly that has undergone a road test pursuant to S5.3 need not conform to the requirements of this section."

S5.4.1.1 requires burnish per S6.2.6 before conducting the procedure that establishes the Brake Retardation Force Curve. Therefore these inferences:

Question 13. A truck tested brake may be run on a dynamometer and if it does not conform to S5.4.1, S5.4.2 and S5.4.3, it does not disqualify the brake; correct?

Question 14. May a truck tested brake later be run on a dynamometer procedure to develop the characteristics required in S5.4.1, S5.4.2 and S5.4.3? (That is, benefit from a double burnish).

Question 15. If a truck tested brake does not conform, a different brake may be tested on the dynamometer to qualify the assembly according to S5.4.1, S5.4.2 and S5.4.3. Correct?

Question 16. If some other interpretation is intended, what is that intent? (The sentence under discussion only seems a valid part of the text if Question 14 is answered in the affirmative).

As in Question 5, there are special conditions applicable for hydraulic disc and drum brakes used in air over hydraulic systems. Therefore the "brake chamber air pressure" reference in S5.4.1 may be construed to be the pressure in the air powered element of air over hydraulic systems.

Question 17. Is this a correct interpretation?

Question 18. Assuming the answer to 17 is in the affirmative, is it correct that similar pressure sensing is applicable to the instructions of S5.4.1.1, S5.4.2.1, S5.4.2.2, S5.4.3, and Table III?

There appears to be a need to provide an NHTSS definition for axle rating when establishing the GAWR value to S5.6.1 requirements for the S5.6.1 or S5.6.2 options contained in S5.6.

Question 19. Is the conventional tandem rear axle considered to be a pair of single axles, each having a rating equal to one-half the tandem rating?

If the answer is in the affirmative, then the calculations of S5.6.1 permit a vehicle with a tandem rear axle to have a parking brake system on only one of the individual axles of that tandem. There appears to be no requirement for "a parking brake system acting on each axle except steerable front axles" as previously stated in S5.4 of the original FMVSS-121 (Notice 2).

Question 20. Is this a valid interpretation?

In S5.6.2 Grade holding - the term gross vehicle weight rating is appropriate for trucks, buses, tractors and full trailers. It does not seem appropriate to semi-trailers - especially when considered in conjunction with S6.1.9 that permits an unbraked dolly to support the front end of a semi-trailer. The GVWR of a semi-trailer is not defined as explicitly as any other vehicle and is not adequately covered in the general definitions section of Part 571, 49 CFR 571.3(b).

Question 21. Does S5.6.2(b) mean the GAWR of the axles on a semi-trailer rather than ". . . the gross vehicle weight rating. . ."?

Question 22. If the answer to 21 is negative, was the intent to eliminate the Grade Holding option permitted in S5.6.2 as one of the S5.6 options for semi-trailers?

Question 23. Is a converter dolly classed as a separate vehicle and classified as a "trailer"?

Question 24. If 23 is answered in the affirmative, is the converter dolly empty loading to be specified with an unloaded semi-trailer attached?

It has been an industry practice to combine a pair of axles (each rated at one value for single axle duty) and give the resultant tandem axle a lesser rating: e.g., two 18,500-pound single axles become a 34,000-pound tandem.

Question 25. Is it correct for dynamometer test conditions of S6.2.1 that the dynamometer inertia for each wheel be one-fourth the tandem axle rating?

(e.g., for the above illustration a wheel load of 8,500 pounds).

If we can expedite a response by discussing these points by telephone, the writer may be reached at (314) 432-5800.

We can appreciate the task that your Office faces in developing meaningful interpretations and hope that our discussions have given you an insight into the problems of semantics that the industry must resolve to avoid costly delays in testing or design activity. We will greatly appreciate the attention your staff must direct to this lengthy submission.

Very truly yours,

John W. Kourik -- Chief Engineer, Automotive Products

cc: L. R. Schneider, Code 40-30 -- Chief Counsel