Interpretation ID: nht74-2.11

DATE: 08/19/74

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

TO: Dura Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Dura Corporation's July 24, 1974, questions whether 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 parking 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 rearmost nonliftable, nonsteerable axles on a vehicle with more than two nonsteerable axles.

Under this provision, if a vehicle has two nonliftable, nonsteerable 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,

July 24, 1974

Sidney F. Williams -- Office of Operating Systems, National Highway Traffic Safety Administration, Department of Transportation

Dear Sir:

Dura Corporation is presently involved in manufacturing add-on liftable axle/suspension assemblies which are marketed with OEM and body builders. Due to the impending effectivity dates of FMVSS 121 we are presently preparing to equip our products to comply. Our liftable airide axle/suspension assemblies are applicable to both pusher and tag situations.

The intent of this letter is to request an official interpretation of FMVSS 121. Our present understanding of the standard is as follows:

An anti-lock system will be required with the add-on axle/suspension if added to a single axle rig but not required if added to a tandem axle rig.

II. An emergency/parking spring brake will not be required with the add-on axle/suspension. This supposition is based on a loss of air pressure will automatically lift the axle rendering the spring brake inoperable.

Please indicate if the above statements are correct. Your consideration and cooperation in this matter is truly appreciated.

Sincerely,

Raymond E. Jones, Project Engineer -- DURA CORPORATION

cc: W. Locke