Interpretation ID: nht95-2.67

TYPE: INTERPRETATION-NHTSA

DATE: May 3, 1995

FROM: Doug Russell -- Design Engineer, Advance Engineered Products

TO: John Womack -- Acting Chief Counsel, NHTSA

TITLE: FMVSS 121 Paragraph S5.4.2.2 Brake Power

ATTACHMT: ATTACHED TO 06/29/95 LETTER FROM JOHN WOMACK TO DOUG RUSSELL (A43; STD. 121)

TEXT: Dear Sir

Recently, I have been reviewing test data used to certify brake components to FMVSS - 121. Could you please answer two questions which arose as a result of this review:

1. Paragraph S5.4.2.2. of FMVSS - 121 requires that a deceleration be made from 20 mi/hr to a stop at an average rate of 14 ft/s<2>. A maximum service line brake pressure is not specified for this deceleration. Thus, in order to meet the deceleration requirement, a service line pressure exceeding 100 lbs/in<2> could be used despite the fact that tractor - trailer brakes are usually configured to operate at a maximum nominal brake pressure of 100 psi. Are there any limitations on the pressure which c an be used to achieve the required deceleration rates?

2. Test results from one of Advance's suppliers show that the actual deceleration rate was approximately 15% below the 14 ft/s<2> specified in paragraph S5.4.2.2. I have discussed these results with NHTSA's Richard Carter, and according to him the mini mum requirements of FMVSS-121 have not been met. However, according to the supplier, this stop must be done as part of the whole dynamometer test procedure but not meeting the deceleration requirements for this particular stop does not constitute a fall. Have any interpretations been made which allow a reduced deceleration rate?

Please call if you have any questions. Thank-you