Interpretation ID: nht74-4.22

DATE: 08/05/74

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

TO: Oshkosh Truck Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your July 2, 1974, question whether a truck complies with the dynamometer requirements of S5.4 of Standard No. 121, Air brake systems, if its brakes are actuated by smaller slack adjusters and brake chambers than those specified by the brake manufacturer to establish torque levels which comply with the standard. Your question arises under the interim requirements of S5.3.1.2 and the full requirements of S5.4.2 and S5.4.3 as they apply to the on/off-highway category.

Standard No. 121 is a vehicle requirement, and the NHTSA will conduct compliance testing for dynamometer requirements using the force levels applied to the brakes on the particular vehicle it is testing. This means that if you use 5.5-inch slack adjusters and 30-inch brake chambers on your truck, the NHTSA would use the force applied by these components in its compliance testing. There is no prohibition to your modifying the brake manufacturer's "recommended package" to suit your needs. However, you should be able to show that, in the exercise of due care, the brake assembly meets the requirements of S5.4 as modified.

If you believe that the established dynamometer requirements conflict with optimum handling and stopping performance, you may, under NHTSA procedural rules (49 CFR 563, copy enclosed) petition to modify the standard.

In response to your July 10, 1974, letter asking whether an air-over-hydraulic front brake is subject to certain requirements of Standard No. 105a, I enclose a preamble to that standard which states that air-over-hydraulic systems are regulated only by standard No. 121.