Interpretation ID: nht75-4.9

DATE: 05/20/75

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Oshkosh Truck Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your April 1, 1975, request for confirmation that the emergency braking stopping distance requirements in S5.7.2.3 of Standard No. 121, Air brake systems, specify that, when stopped six times for each configuration of weight and speed specified in S5.3.1.1 on a road surface with a skid number of 75 (with a single failure introduced in the service brake system), the vehicle must stop at least once within the distances specified in Column 3 of Table II and no part of the vehicle must leave the 12-foot roadway. You also request confirmation that modulation of the service brake control during the stop is not prohibited.

With certain exceptions, the statements in your letter are correct. Your interpretation only sets out the basic stopping distance requirements for those vehicles which the manufacturer has chosen to make conform to S5.7.2 of the standard. Thus, your interpretation does not include any of the requirements of the emergency braking capability option found in S5.7.1. Additionally, your interpretation does not include the requirements for a truck-tractor at unloaded vehicle weight plus 500 pounds, or for the trucks and buses which qualify for the interim requirements of S5.7.2.3.1 and S5.7.2.3.2.

Section S5.7.2 does not prohibit modulation of the emergency braking capability, and modulation by means of the service brake control is therefore permissible.

YOURS TRULY,

April 1, 1975

Richard Dyson Acting Chief Counsel U.S. Dept. of Transportation National Highway Traffic Safety Admin.

Please review our interpretation of Sections 5.7.1 and 5.7.2 of FMVSS 121 which pertain to emergency brake system operation and performance.

Our interpretation is such that:

1) During an acceptable emergency braking stop, the stopping distance must not exceed the values specified in Column 3 of Table II and no part of the vehicle must leave the 12 foot roadway.

2) When stopped six times, for each combination of weight and speed specified in 5.3.1.1, with a skid number of 75, at least one acceptable emergency braking stop must occur.

3) During an emergency braking stop, the emergency braking system may be modulated. Modulation occurs by actuation of the service brake control.

OSHKOSH TRUCK CORPORATION

Danny J. Lanzdorf Supervising Engineer