ID: nht76-4.11

DATE: 12/27/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Jack's Tire Company, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This is in response to your November 23, 1976, letter concerning retreaded tires and Federal Motor Vehicle Safety Standard No. 117.

Your understanding that you are required to retain for three years certain records described in your letter is mistaken. While such a requirement was proposed in a Federal Register notice published on March 5, 1970 (35 FR 4136), it was never adopted. The NHTSA strongly recommends, however, that retreaders retain information on the materials and processes that they use, so that in the event of a defect or noncompliance they will be able to determine which tires are involved.

To assign you a retreader's identification mark, we need more information from you. 49 CFR Part 574.6 specifies that:

To obtain the identification mark required by @ 574.5(a), each manufacturer of new or retreaded motor vehicle tires shall apply after November 30, 1970, in writing, to "Tire Identification and Record- keeping," National Highway Traffic Safety Administration, 400 Seventh Street, S.W., Washington, D.C., 20590, identify himself as a manufacturer of new tires or retreaded tires, and furnish the following information:

(a) The name, or other designation identifying the applicant, and his main office address.

(b) The name, or other identifying designation, of each individual plant operated by the manufacturer and the address of each plant, if applicable.

(c) The type of tires manufactured at each plant, e.g., passenger car tires, bus tires, truck tires, motorcycle tires, or retreaded tires.

Enclosed for your convenience is an information sheet entitled "Where to Obtain Motor Vehicle Safety Standards and Regulations.

SINCERELY,

Frank A. Berndt Acting Chief Counsel

MEMO

TO: Motor Vehicle Safety Performance Service National Highway Safety Bureau Federal Highway Safety Administration U.S. Dept of Taxation

FROM: JACK'S TIRE COMPANY INC

SUBJECT: D O T. REg. No. DATE: 11/23/76

MESSAGE

Please Issue a Code # to identify retreads produced by My Company. I understand that it is required of me to keep the following records for at least a period of three years. (1) records of the material used in theretreading process) (2) records of performance test, (3) records of reported defects and failures, with associated causes.

Also requesting a Current copy of "Federal Motor Vehicle Safety Standard No. 117.

Jack D. Cooper, PRESIDENT

ID: nht76-4.12

DATE: 08/04/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Blue Bird Body Company

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Blue Bird Body Company's June 21, 1976, question whether S5.3.1 of Standard No. 121, Air Brake Systems, requires the installation of an antilock system, or whether the stopping distance and "no lockup" requirements may be met using modulation of the service brake control by the vehicle driver.

Section S5.3.1 requires that vehicles subject to it must be capable of stopping under specified conditions, within the stopping distance set forth in Table II without leaving a 12-foot-wide lane and without "uncontrolled" lockup of certain wheels of the vehicle. There is no requirement for the installation of an antilock system. Also, there is no test condition that specifies a full brake application and modulation of the service brake may be used to reduce or eliminate lockup.

Yours truly,

ATTACH.

BLUE BIRD BODY COMPANY

June 21, 1976

Thomas Herlihy -- Office of Chief Counsel, National Highway Traffic Safety Administration

Dear Mr. Herlihy:

SUBJECT: FMVSS 121 - AIR BRAKE SYSTEMS

It is our understanding that the subject standard does not currently specify maximum allowable stopping distances. We also understand that maximum allowable stopping distances are specified for future vehicles.

Our question is, when and if the stopping distance requirements of Paragraphs S5.3.1 and Table 2 are reinstated for buses, will it be permissible to meet those stopping requirements without an anti-lock system, provided we can meet the requirements stated in S5.3.1 ". . . . without any part of the vehicle leaving the roadway and without lock up of any wheel at speeds above 10 MPH . . . .?" Obviously, the driver would have to modulate the braking force to prevent wheel lock up on a system not equipped with an automatic anti-lock system.

We would appreciate your early reply.

Thank you.

Yours very truly, W. G. Milby -- Staff Engineer

c Ben Newberry

ID: nht76-4.13

DATE: 06/29/76

FROM: AUTHOR UNAVAILABLE; John Womack for F. Berndt; NHTSA

TO: Peterbilt Motors Company

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Peterbilt Motor Company's June 9, 1976, questions whether Standard No. 121, Air Brake Systems, contains a permanent exclusion for "auto transporters" and whether "auto transporter" would include a truck-trailer vehicle combination that includes a dromedary forward of the fifth wheel to hold empty tin cans that are loaded by means of the trailer. I would like to note that I am unaware of "discussion and interpretive rulings suggested through telephone contact" with this office.

Your interpretation is not correct that the exclusion for "auto transporters" is permanent. Some confusion may arise from the words used in S3 to describe the exclusion until September 1, 1977. The phrase "or to any vehicle which" that appears at the end of the second sentence in S3 will shortly be modified to "or that" to improve the structure and clarity of the sentence.

Your question whether a "can hauler" qualifies as an "auto transporter" appears to be based on the proposed wording of this definition that was modified in final form. As defined in the standard, "auto transporter" means:

. . . a truck and a trailer designed for use in combination to transport motor vehicles, in that the towing vehicle is designed to carry cargo at a location other than the fifth wheel and to load this cargo only by means of the towed vehicle.

It is clear from this definition that a "can hauler" would not qualify as an "auto transporter" subject to the exclusion.

Sincerely,

ATTACH.

Peterbilt MOTORS COMPANY

June 9, 1976

OFFICE OF CHIEF COUNSEL -- National Highway Traffic Safety Administration

Re: FMVSS 121 Air Brake Systems

Gentlemen:

Peterbilt Motors Company is the manufacturer of the custom built, heavy duty, Class 8 trucks and truck-tractors identified by the "Peterbilt" nameplate. As such, we have received inquiries regarding the constructing of specialized vehicles concerning which we find conflicting information stipulated in FMVSS 121.

This letter is to substantiate and record the substance of discussion and interpretive rulings suggested through telephone contact with your office.

As we interpret the wording of S3. Application, vehicles referred to as "auto transporter" are exempt from the requirements of FMVSS 121. However, S5.3 Service Brakes - Road Tests specifies that auto transporters manufactured after September 1, 1976 shall meet the requirements of S5.3.1.

Please clarify the total applicability of FMVSS 121 to vehicles defined as "auto transporter".

Additionally, we wish to have clarified the scope of vehicles to which the definition "auto transporter" can be applied.

Section S4. Definitions defines "auto transporter" as a truck and trailer designed for use in combination, in that the towing vehicle, with fifth wheel mounted, is designed to carry cargo at a location other than the fifth wheel and that this cargo is loaded by means of the towed vehicle.

This definition describes a vehicle combination which is commonly referred to as a "can hauler". This vehicle combination consists of a truck and trailer, which truck has a fifth wheel mounted at the end of the frame and has a van body (dromedary) mounted forward of the fifth wheel. This forward cargo area is loaded by means of the towed vehicle, which is a van body that opens at both ends. The truck and trailer remain connected in combination for both loading and unloading through the rear of the towed vehicle.

The application of the "can hauler" vehicle combination further parallels that of an "auto transporter" in that the cargo being carried is a low density, high bulk commodity (empty "tin cans").

We, therefore, respectfully request an interpretation from your office regarding the applicability of the definition of "auto transporter" for inclusion of the "can hauler" as a thereby controlled identity.

We will appreciate your thoughtful consideration of these questions.

Sincerely,

Arlen E. Riggs -- Executive Engineer

ID: nht76-4.14

DATE: 07/23/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: White Motor Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to White Motor Corporation's April 26, 1976, question whether S5.3.3 and S5.3.4 of Standard No. 121, Air Brake Systems, require minimum brake chamber air pressures of 60 psi and 95 psi, respectively, or whether these air pressures are included in the sections only as "bench marks" on which to base specifications for minimum actuation and release timing in brake systems. Section S5.3.3 specifies in part:

. . . With an initial service reservoir system air pressure of 100 psi, the air pressure in each brake chamber shall, when measured from the first movement of the service brake control, reach 60 psi in not more than 0.45 seconds. . . .

Your understanding that S5.3.3 and S5.3.4 only specify the air pressures of 60 psi and 95 psi as the basis for timing requirements is correct. Neither value is intended as a requirement that the vehicle be designed to provide a certain level of brake chamber air pressure. The values were based on an understanding of the typical configuration of existing air brake systems at the time the final rule was issued.

In response to your request for interpretation of these sections in view of White's intent to use a lower air pressure than was commonly used in the past, the agency will utilize the stated 60-psi value or a value that is 70 percent of the maximum air pressure (measured by the NHTSA at the brake chamber), whichever is lower. In the case of release, the stated 95-psi value or the value that represents maximum air pressure (measured by the NHTSA at the brake chamber), whichever is lower, will be used. For purposes of this determination, the maximum air pressure in the brake chamber is that obtainable with full brake application when the pressure in the service reservoir is at 100 psi. Use of the maximum air pressure application timing would be unreasonable because of the decreased rate of air pressure build-up that occurs as the brake chambers reach maximum pressure.

The agency will issue an interpretive amendment to S5.3.3 and S5.3.4 to reflect this interpretation.

ID: nht76-4.15

DATE: 09/30/76

FROM: AUTHOR UNAVAILABLE; R. L. Carter; NHTSA

TO: Rockwell International

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Rockwell International's February 23, 1976, request for confirmation that the deactivation of automatic adjusters is acceptable during burnish procedures of Standard No. 121, Air Brake Systems, at the option of the manufacturer.

The procedure that you recommend is not permitted by any provision of Standard No. 121. The NHTSA would consider some provision to deal with the overadjustment of automatic adjusters upon receipt of technical data showing justification for such action. Based on consideration of the data received and a petition for amendment, the agency could commence a rulemaking proceeding in accordance with established procedures.

It should be noted that the option in FMVSS 105-75 that you describe requires that the automatic adjusters be deactivated for the entire test sequence, not just the burnish procedure.

Sincerely,

ATTACH.

February 23, 1976

National Highway Traffic Safety Administration Office of Crash Avoidance

Gentlemen:

Subject: Air Brakes - Automatic Adjusters

Rockwell International requests an interpretation of FMVSS #121 testing procedures as it relates to Rockwell cam and wedge brakes with automatic adjustment features.

Automatic adjusting air brakes are premium brakes having significant safety advantages in day to day operation. The automatic adjustment feature is offered on Rockwell cam and wedge brakes, single and dual actuated air and single and dual air/hydraulic units. The automatic adjusting feature enables vehicles to maintain their stopping capability when an otherwise manually adjusting brake would exceed available adjustment or "stroke." At the same time the automatic adjustment feature keeps brakes within a favorable operating range of the air chamber. This becomes more important with the improved brake performance required by FMVSS #121. It is noted that NHTSA has made proper allowance for automatic adjusting hydraulic brakes in the test procedures of FMVSS #105-75.

The accelerated brake burnishing required by FMVSS #121 results in rapid expansion of the brake drum and to a lesser extent brake lining, thereby preventing proper operation of the adjusters. It is, therefore, necessary to manually adjust the brakes for the test procedure.

Enclosed are copies of our recommended procedure for Rockwell International brakes with automatic adjusters when tested for FMVSS #121. It is requested that NHTSA review this procedure and advise that it is acceptable for testing under FMVSS #121. Secondly, it is requested that NHTSA allow for deactivation of automatic adjusters in the FMVSS #121 similar to the provisions of FMVSS #105-75 at the next opportunity.

Very truly yours,

ROCKWELL INTERNATIONAL, CORPORATION AUTOMOTIVE OPERATIONS;

William R. Rodger -- Vice President - Advanced Programs

Encls.

ID: nht76-4.16

DATE: 11/10/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Midland-Ross Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Midland-Ross' September 30, 1976, question whether the "method" specified by Compliance Testing, Inc., in its December 8, 1975, Technical Proposal for Solicitation NHTSA-6-A212 is consistent with the laboratory procedure contemplated by the NHTSA for its test contractors in evaluating the compliance of air-braked vehicles with Standard No. 121, Air Brake Systems. The NHTSA laboratory procedure for compliance contractors in the case of Standard No. 121 states in relevant part:

PROCEDURE:

A. Physically locate check valve or equivalent device.

B. Following manufacturer's recommendation, check the check valve or equivalent device for proper function without disconnecting any air line or fitting. Describe method and technique used.

The Compliance Testing, Inc. (CTI) solicitation was evaluated along with other proposals and has been accepted by the NHTSA. The "method" set forth by CTI as its intended course of action in evaluating the compliance of valves in accordance with the requirements of S5.1.2.3 will not appear in the manual produced for NHTSA compliance testing.

I would like to note in closing that this letter does not constitute an interpretation of the requirements of Standard No. 121. Although the laboratory procedure sets forth the method by which contractors satisfy NHTSA contracts, it does not mean that a vehicle need not meet the requirements of the standard when tested according to its terms by other methods.

MIDLAND-ROSS

September 8, 1976

Salvatore J. Nicolosi -- Office of Contracts and Procurement, National Highway Traffic Safety Administration

Gentlemen:

Subject: Laboratory Procedures for Air Brake Systems, Federal Motor Vehicle Safety Standard No. 121, Report #TP-121-00

Midland-Ross Corporation is a manufacturer of air brake actuation equipment for heavy duty trucks. We therefore manufacture components which are used by truck manufacturers for compliance with FMVSS 121. We are asking for interpretation of the requirements for check valves.

In the subject procedures, issued by the U.S. Department of Transportation, National Highway Traffic Safety Administration, Office of Standards Enforcement Motor Vehicle Programs, Section 1.3.2 states:

"This procedure must be followed by any contractor conducting FMVSS 121 tests for the National Highway Traffic Safety Administration."

Section 5.7 REQUIREMENT S5.1.2.3 -- SERVICE RESERVOIR SYSTEM AIR LOSS TEST states"

"Each service reservoir system shall be protected against loss of air pressure due to failure or leakage in the system between the service reservoir and the source of air pressure, by check valves or equivalent devices whose proper functioning can be checked without disconnecting any air line or fitting."

Under the same section PROCEDURE A and B:

"A. Physically locate check valve or equivalent devices.

B. Following the Manufacturer's recommendation, check the check valve or equivalent device for proper function without disconnecting any air line or fitting. Describe method and technique used." (Underscoring added.)

In the submission by Compliance Testing Inc., which we understand was accepted, the Contractor states:

"Method:

1. Build up air in system to maximum

2. Open bleed valve in supply tank

3. Monitor for 10 minutes pressure in primary and secondary tanks - no loss of air allowed"

We submit that on the surface this appears to be not in compliance with requirements of TP121.00 and we would like to have a clear interpretation of what the contractor plans to do with these valves in checking them for leakage. We submit that zero leakage is a physical impossibility and must be interpreted in terms of the accuracy of the equipment being used for measurement.

Further we submit that the contractor is obligated to test for compliance "following the Manufacturer's recommendation" which we in turn would consider to be the engineering specification for the appropriate valve.

We request that this question be clarified and the NHTSA position be stated so that erroneous claims of non-compliance can be avoided. An early response would be appreciated.

Sincerely,

M. J. Denholm -- Director of Engineering, Power Controls Div.

cc: J. Brewster, M-R; D. Gross, M-R; F. Nawalanic, M-R; A. Beier, IHC; R. MADISON

ID: nht76-4.17

DATE: 12/10/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Great Dane Trailers, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Great Dane Trailer's November 4, 1976, question whether the "controlled lockup" exception in S5.3.2 of Standard No. 121, Air Brake Systems, is available to a manufacturer who equips a "tri-axle" trailer suspension with one set of wheel speed sensors on the center axle (or, alternatively, on the front axle) and one antilock valve and logic module that meters air pressure to the two rearmost axles.

Your question appears to be similar to questions about tandem-axle suspensions that prompted an interpretation of the "controlled lockup" exception to the "no lockup" requirements of S5.3.1 and S5.3.2. A copy is enclosed for your information. You will note that the closing statement in the interpretation states that ". . . the controlled lockup exception is not dependent on the number or location of sensors used in an antilock installation." It is left to the manufacturer to decide what placement of the sensors will provide the best performance.

Sincerely,

Enclosure

Great Dane Trailers, Inc.

November 4, 1976

Thomas Herlihy Office of Chief Counsel National Highway Traffic Safety Administration

Subject: FMVSS 121-Air Brake Systems

Dear Mr. Herlihy:

In Section S5.3.2 of the 121 requirements, there are indications that anti-lock controls be applied to the two rearmost, nonliftable, nonsteerable axles. In the more recent interpretation, there are indications that if it is possible to predict which of the two axles will lock first during braking, sensors may be placed on this axle only; however, they both must be controlled.

My question involves a tri-axle unit. In this case, where must the sensors be used? By the first interpretation, controls need not be on the front axle of this suspension, only on the last two. Of these last two, the center axle will always lock before the rear axle, meaning that the control sensors should be installed on this center axle. However, in a tri-axle suspension, the front axle will lock first, the center axle will lock second and the rear axle will be the last to lock.

We would appreciate your early reply.

Sincerely,

Dudley E. DeWitt Manager/R & D

ID: nht76-4.18

DATE: 03/31/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Adams County

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your March 5, 1976, request for written permission to deactivate the "121 brake system" of a school bus manufactured from a Ford chassis with a Kelsey-Hayes antilock system.

From the description of the problems you have encounted with this vehicle, I assume that you do not intend to disconnect the entire "121 brake system", but only one or more antilock systems installed in satisfaction of the "no lockup" requirements of S5.3.1 of Standard No. 121, Air Brake Systems. The National Highway Traffic Safety Administration (NHTSA) has made a finding that early models of the antilock system supplied on transit and intercity buses is characterized by malfunction that warrants its deactivation until a correction is fully developed. This finding was not made with regard to the Kelsey-Hayes system that equips your vehicle.

Section 108(a)(2)(A) of the National Traffic and Motor Vehicle Safety Act (15 U.S.C. @ 1397(a)(2)(a)) prohibits, with one exception, knowing disconnection of the antilock system by manufacturers, distributors, dealers, or repair businesses. A person that does not fall into those categories is not prohibited from disconnection of the system. Other State or Federal requirements, such as those of the Bureau of Motor Carrier Safety, may prohibit disconnection. In any case, the NHTSA urges that you not disconnect safety devices without consulting the vehicle manufacturer with regard to the safest configuration of the vehicle.

SINCERELY,

School District No. 12

MEMO NO: B-0093

DATE: MARCH 5, 1976

TO: CHIEF COUNCIL NATIONAL HIGHWAY TRAFFIC ADMINISTRATION

FROM: RON STACY SUPERVISOR OF TRANSPORTATION

RE: DE-ACTIVATE 121 BRAKES

A letter to Dr. Neil McCormick, (attached), telephone conversation with Brad Marks, "National Highway Safety Administration", Colorado, Dr. Steve Sacks, "National Highway Traffic Safety", Washington, D.C., explaining existing problems with the 121 brake, (Kelsey-Hayes, Ford Chassis).

December 1975, Kelsey-Hayes representative performed an inservice workshop. Problems still exist!

Refer, Fleet Maintenance Magazine, February 1976 issue, page 61. Washington Report; QUOTE: "NHTSA has also tentatively determined, at this writing, that virtually the only anti-lock system currently available for buses "is characterized by malfunction that warrants its deactivation on all vehicles on which it is installed while a correction is fully developed." UNQUOTE.

Request permission, in writing, to deactivate the 121 brake system, until such time corrections are complete.

Your cooperation and expedience is greatly appreciated.

RON STACY

School District No. 12 Adams County

TRANSPORTATION DEPARTMENT

MEMO NO: B-0085

DATE: NOVEMBER 13, 1975

TO: DR. NEIL MCCORMICK SUPERVISOR OF SUPPORTING SERVICES UNIT COLORADO DEPARTMENT OF EDUCATION

FROM: ROW STACY

REF: 121 BRAKES

1. Frequently one brake will grab at low speed.

2. Occasionally brakes will fade.

3. Occasionally all four wheels will lock-up on panic stop.

4. Occasionally brakes will fade completely: gradual stop, below 15 m.p.h.. Then, brakes will grab as if panic stop condition existed.

If I can be of further assistance, please do not hesitate to call.

ID: nht76-4.19

DATE: 02/12/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Hon. Robin Beard - H.O.R.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your January 29, 1976, question whether an owner of a vehicle manufactured to comply with Standard No. 121, Air Brake Systems may legally disconnect portions of the brake system after a vehicle has been delivered for use in his business.

Section 108(a)(2)(A) of the National Traffic and Motor Vehicle Safety Act (15 U.S.C. @ 1397(a)(2)(A)) prohibits, with one exception, knowing disconnection of safety components by manufacturers, distributors, dealers, or repair businesses. Thus, there is no prohibition on disconnection by an owner of his own vehicle's system under the Traffic Safety Act. However, State statutes, or the regulations of the Bureau of Motor Carrier Safety may prohibit disconnection. In any case, the NHTSA urges that owners not disconnect safety devices without consultation with the vehicle manufacturer with regard to the safest configuration of the vehicle.

SINCERELY,

Congress of the United States House of Representatives

January 29, 1976

Robert L. Carter Associate Administrator Motor Vehicles Program U. S. Department of Transportation

I have been contacted by a constituent regarding the anti-skid devices on trucks and trailers.

I am informed by my constituent that he has encountered a great deal of expense due to the failure of these devices, such as damaged tires, tow bills, and time spent on repairs. My constituent would like to know if there would be a fine imposed or any type of penalty if these devices were disconnected.

I appreciate your checking on this matter and sending me a reply.

Robin Beard, M. C.

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