Interpretation ID: aiam1580
Project Engineer
Dura Corporation
Fords Mill Road
Paris
KY 40361;
Dear Mr. Jones: This responds to Dura Corporation's July 24, 1974, questions whethe S5.6 of Standard No. 121, *Air brake systems*, requires parking brakes on air suspension liftable axles, and whether the 'no lockup' requirements of the standard apply to a liftable axle on a 'tandem axle rig'.; The parking brake performance options of S5.6 do not require parkin brakes on an air suspension liftable axle such as you describe. S5.6.2 requires only that the parking brakes installed on a vehicle meet minimum performance levels. S5.6.1 requires parking brake retardation force on 'an axle other than a steerable front axle'. We do not consider this requirement to apply to an axle which is not on the ground when the parking brake system is activated.; The standard's 'no lockup' requirement (S5.3.1) applies to >>>'any wheel at speeds above 10 mph except for . . . (b) Lockup of wheels on nonsteerable axles other than the two rearmos nonliftable, nonsteerable axles on a vehicle with more than two nonsteerable axles.<<<; Under this provision, if a vehicle has two nonliftable, nonsteerabl axles at the rear which do not lock up (such as an antilock-equipped tandem axle rig) it may be equipped with a liftable nonsteerable axle which does not meet the 'no lockup' requirements.; Yours truly, Richard B. Dyson, Acting Chief Counsel.