Interpretation ID: aiam0705
Chief Engineer
Automotive Products
Wagner Electric Corporation
11444 Lackland Road
St. Louis
MO 83141;
Dear Mr. Kourik:The following interpetations (sic) are submitted i 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 an measured by the ASTM B-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 failur 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 antiloc 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 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 pressur 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 an 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 servic brakes on the trailer are applied. S4.5.3 of FMVSS No. 108 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 b 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 traile at the start of the stop.; 10. In S5.3.2, the air compressor and air supply system of the towin vehicle are expected to be operating normally.; 11. In S5.3.3, the towing vehicle brakes may be by-passed by an 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 manufacture 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 th 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 as answer for 15 above. 17. In S5.4.1, S5.4.2 and S5.4.3, for air or hydraulic brake systems the 'brake chamber air pressure' is the air pressure in the power chamber coupled to the master cylinder usedto (sic) convert air pressure to hydraulic pressure.; 18. Same answer as for question 17. 19. The Standard does not consider tandem ratings, it addresses onl the GAWR of each individual axle.; 20. Vehicles conforming to S5.6.1 must have a parking brake on eac individual axle of a tandem axle arrangement.; 21. In S5.6.2(a) 'Gross vehicle weight rating' is correct fo semi-trailers as well as trucks and buses, gross axle weight rating is not meant.; 22. Semi-trailers are not excluded from meeting the alternat 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 unloade semi-trailer.; 25. 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.<<<; Sincerely, E. T. Driver, Director, Office of Operating Systems, Moto Vehicle Programs;