Interpretation ID: nht76-4.2

DATE: 03/30/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Motor Coach Industries, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Motor Coach Industries' February 20, 1976, letter asking whether Standard No. 121, Air Brake Systems, requires the installation of parking brakes on all non-steerable axles of a bus, including a lightly-loaded axle, in satisfaction of the emergency braking provisions of S5.7.1. Once parking brakes are applied on the non-driving, lightly-loaded axle on some Motor Coach Industries (MCI) buses in cold, wet weather, the linings can freeze to the drums and "lock" the wheels so that they will not turn even after the parking brake is released. You suggest that parking brakes are inappropriate on a lightly-loaded axle, citing an interpretation of the standard that stated parking brakes are not required on an air-lift axle which lifts off the ground when the vehicle is parked.

MCI raised the same question of parking brake requirements for lightly-loaded axles in an April 17, 1972, letter requesting interpretation of the provision of S5.7.1 that requires automatic application of parking brakes. At the time, the NHTSA had just issued S5.7.1, expanding the methods for meeting performance levels for emergency braking performance. The question of whether parking brakes should be required on all axles under S5.7.1 was left open pending formal rulemaking.

What was not raised in MCI's April 1972 letter was whether both methods for meeting S5.7.1 performance levels necessitate parking brakes on all non-steerable axles. Section S5.7.1.2 permits reliance on retardation force capabilities of each non-steerable axle or, in the alternative, reliance on vehicle stopping capability using the vehicle's available parking brakes. In the second case, the NHTSA does not interpret S5.7.1 to require installation of parking brakes on an axle if it is not necessary to meet the stopping performance of S5.7.2.3 specified under S5.7.1.2. To the degree the language of S5.7.1 does not specifically address this method of satisfying the requirement, we regret that the agency's July 1972 response was not more clear.

An interpretative amendment of S5.7.1 would be appropriate in view of the difficulties that its misinterpretation has caused. However, in view of the short time remaining before the automatic application option will no longer be available, the NHTSA does not expect to undertake rulemaking to formalize this interpretation.

YOURS TRULY,

Motor Coach Industries, Inc.

February 20, 1976

Office of the Chief Counsel National Highway Traffic Safety Administration

SUBJECT: Tag Axle Parking Brake - MVSS 121 S 5.7.1.

Since March 1, 1975 Motor Coach Industries, Inc. of Pembina, N.D. had to add a parking brake (piggy back spring brake) to the tag axle wheels on MC-8 model coaches sold with the optional "Parking brake with automatic application" in order to comply to MVSS #121, S 5.7.1. The option for automatic application is a requirement for the States of New Jersey and Massachusetts and constitues about 20% of the MC-8 yearly production. One hundred and seventy five coaches have been equipped with this braking system since March 1, 1975. The addition of the piggy back parking brake is solely to meet the requirement of S 5.7.1 "parking brake on each axle, except steerable front axles" and is not needed to meet the parking brake performance.

During winter operation a problem has developed and quick action had to be taken to provide protection to the bus passengers due to possibility of tire fire. A copy of the Defect Information Report initiated by Motor Coach is attached for your review. A copy of a letter from a bus operator is also attached together with a picture of two damaged tires. In the letter of Mr. W. Owens of Capitol Bus Company, you will notice that a State trooper stopped the driver of bus CP 861 after noticing the wheels not turning.

M.C.I. is approaching NHTSA to obtain an interpretation of S5.7.1. which would differenciate between axle loading. Already an interpretation to the Dura Corporation ref - N40-30 (TWH) by your office states "the requirement for parking brake retardation force does not apply to an axle which is not on the ground when the parking brake system is activated." The tag axle wheels give a reading of 3,000 lbs per wheel at the ground level while the drive axle will carry to 11,000 lbs per duo wheel.

The parking brake retardation force is negligeable as the deterioration of the tire shows it. MCI has built 2,550 MC-7 model coaches and 1,500 MC-8 model coaches for the last 8 years without the need for parking brakes on the tag axle. We have a clear record as to the operation of the parking brake on our vehicles and do not understand the agency arbitrary requirement for a parking brake on each non-steerable front axle without consideration of axle loading. Our certification nameplate shows a G.A.W.R. of 22,000 lbs for the drive axle and a G.A.W.R. of 6,000 lbs. for the tag axle. This difference in axle rating on a same vehicle with the same tire size explains why the parking brake (DD3) on the drive axle will free under motion while the tag axle will drag along.

Effective September 1, 1976, MVSS 121 will prohibit the automatic application vs. a modulated control of the parking brake and no requirement for parking brakes on each non steerable front axle is maintained.

It is unfortunate to penalize the small number of coaches involved, to raise the cost of the vehicle without added return and to expose the bus operators to obvious danger.

George A. Hunt Engineering Manager