ID: nht74-2.1

DATE: 01/17/74

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

TO: Aeroquip Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your request of January 8, 1974, made to Mr. Herlihy of this office, for an interpretation of S5.1.2.3 of Standard No. 121, Air brake systems, as it applies to a check valve which is designed to bleed back no more than 5 psi pressure from the service reservoir toward the source of air pressure.

The check valve described in the specification you enclosed would satisfy the requirements of S5.1.2.3 as long as there is no air loss in excess of the designed bleed off, and that total air loss never lowers the pressure at any time below the air compressor "cut in" pressure.

Yours truly,

ATTACH.

AEROQUIP CORPORATION/INDUSTRIAL DIVISION

January 4, 1974

John Womack and Richard Dyson -- Chief Council, National Highway Traffic Safety Administration

Gentlemen:

This letter is a request for approval of our XFF9260-50 check valve for use in truck and bus air brake systems as detailed in the Motor Vehicle Safety Standards #121, Paragraph S5.1.2.3.

The function of the subject check valve is to allow air bleed back of no more than 5 psi pressure from the maximum pressure setting on the primary reservoir. The purpose of the air bleed back is to increase the service life of our Aerofiner Air Purifier (P/N FF9260-01). The air volume used off the #1 air reservoir (pre-dried air) is used to back flush the desiccant bed in the Aerofiner at approximately 3 cfm until the pressure in the reservoir drops to the relief valve setting, (5 psi below maximum reservoir operating pressure) at which time the valve is completely closed. In all other respects, it functions as a typical safety check valve.

Attached is a layout sketch of the subject check valve showing the seal surfaces and flow paths under various conditions.

Also attached, is a copy of Bulletin #5212, describing our Aerofiner brake system air purifier.

Please supply me in writing, the council's approval of our XFF9260-50 check valve in truck and bus air brake systems.

If there are any questions, feel free to contact me.

Sincerely yours

Donald T. Fischer -- Project Engineer, Coupling Section

ID: nht74-2.10

DATE: 11/07/74

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

TO: Columbia Body & Equipment Co.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to CBE's October 24, 1974, question whether a trailer equipped with a telescoping drawbar that varies the distance between axles to conform to various State regulations would qualify as a "Heavy hauler trailer" as that term is defined in Standard No. 121, Air brake systems:

"Heavy hauler trailer" means a trailer with one or more of the following characteristics:

(1) Its brake lines are designed to adapt to separation or extension of the vehicle frame; or

(2) Its body consists only of a platform whose primary cargo-carrying surface is not more than 40 inches above the ground in an unloaded condition, except that it may include sides that are designed to be easily removable and a permanent "front-end structure" as that term is used in @ 393.106 of this title.

The answer to your question is no. The extension of the drawbar is not an "extension of the vehicle frame." This characteristic is intended only to describe trailers whose function necessitates a highly specialized configuration that significantly interferes with brake design. In contrast, the telescoping drawbar is not related to the trailer's function, but simply permits the vehicle to conform to State highway regulations.

Yours truly,

ATTACH.

October 24, 1974

Acting Chief Consel -- U.S. Dept. of Transportation

Attn: Richard Dyson

Dear Mr. Dyson;

Does the Pony Trailor on our drawing number T65-497 (enclosed) qualify as a heavy hauler trailer and exempt from the S121 or any other antiskid requirement until Sept. 1, 1976 as Mr. Tadd Herllhy defined in our phone conversation Oct. 23, 1974. Please reply as soon as possible.

Sincerely yours,

COLUMBIA BODY & EQUIPMENT COMPANY

John C. Hansen -- Sales Manager

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

ID: nht74-2.12

DATE: 11/14/74

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

TO: Truck Trailer Manufacturers Association

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your October 23, 1974, question in behalf of Steadman Containers, Ltd., asking if Standard No. 121, Air brake systems, applies to the Steadman "liftainer", which is described as a semi-trailer frame capable of carrying 20- and 40-foot cargo containers that are loaded by means of cranes mounted at each end of the semi-trailer. The vehicle is used to move containers in terminal areas and is used on the highway only in relocation to another terminal.

The "liftainer" is a motor vehicle as that term is defined in the National Traffic and Motor Vehicle Safety Act of 1966, because it uses the highway on a necessary and recurring basis to move between work sites. A discussion of the motor vehicle definition is enclosed for your information. As a motor vehicle, it is subject to the requirements of Standard No. 121 for trailers, effective January 1, 1975.

I have also enclosed a discussion of Standard No. 121's applicability to vehicles purchased by Canadians for use between Canadian and United States points. The standard would apply to Steadman trailers if they are purchased for use on U.S. highways.

Yours truly,

ATTACH.

October 23, 1974

James B. Gregory -- National Highway Traffic Safety Administration, Department of Transportation

Dear Dr. Gregory:

Could you please furnish us with an interpretation in connection with the enclosed described equipment and its relationship with the anti-skid devices to be placed on equipment by January 1, 1975?

You will note that Steadman containers indicates that "The Liftainer Container handling unit is not intended, nor used, as a highway unit, but is used in terminals where it is moved by means of a conventional highway tractor. Its highway use is limited only to possible occasional re-positioning from one yard to another. As its width in the closed, unfolded condition, is in excess of 8', road movement always involves use of special permits."

We will appreciate your consideration and response to the above.

Sincerely yours, Charles J. Calvin -- President

ID: nht74-2.13

DATE: 09/30/74

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

TO: Nabors Trailers, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your August 21, 1974, question whether a "logging pole trailer", which consists of a beam to which an axle-mounted bolster can be clamped at different points to accomodate different log lengths, qualifies as a heavy hauler trailer as that term is defined in Standard No. 121, Air brake systems:

"Heavy hauler trailer" means a trailer with one or more of the following characteristics:

(1) Its brake lines are designed to adapt to separation or extension of the vehicle frame; or

(2) Its body consists only of a platform whose primary cargo-carrying surface is not more than 40 inches above the ground in an unloaded condition, except that it may include sides that are designed to be easily removable and a permanent "front-end structure" as that term is used in @ 393.106 of this title.

This also acknowledges receipt of your September 5 and September 17, 1974, letters on the same subject.

The logging pole trailer you describe is a heavy hauler trailer, and as such, Standard No. 121 does not apply to this trailer until September 1, 1976. The beam or "reach", together with the bolster, constitutes the frame of the trailer, and the brake lines are designed to adapt to extension of the bolster element along the beam.

This arrangement differs from the standard highway van which has a one-piece frame with an adjustable tandem axle. The purpose of this sliding arrangement is unrelated to an extension of the frame itself to accomodate the transportation of heavy or oversize loads.

Yours truly,

ATTACH.

NABORS TRAILERS, INC.

Sept. 17, 1974

Sidney Williams -- National Highway Traffic Safety Administration

Dear Mr. Williams:

It should be of interest and concern to the National Highway Traffic Safety Administration that we have not had the sight nor the specifications nor the price of a brake equipped axle assembly that can be certified to meet the requirements of Federal Motor Vehicle Safety Standard 121. We are wholly dependent upon the one source for our brake axle assemblies. From that source we have been told that they have encountered one delay after another in their program to develop and test brakes that will meet the new requirements. Their best hope now is that by the end of October they will be able to give us technical information and sometime in November they will be able to begin shipping certifiable brakes.

The same situation prevails with respect to wheel and drum assemblies. Our supplier of drums have told us that they are conducting tests now and that they should be able to tell us in November what drums will be required for compliance with the new Safety Standard.

You can guess from this information the predicament in which we find ourselves when trailers that we put into production in December scheduled for completion in January will be required by law to be in compliance with SS 121. We are certain that there are many other trailer manufacturers who are dependent upon the same sources of supply that we are who are in the same predicament. There may be other sources of supply for brake axle assemblies and for wheels and drums who are ready to describe and certify their products to meet the new Safety Standard, but we, for one, could not at this time get in schedule with new suppliers.

Sincerely yours, B. H. Smith -- President

NABORS TRAILERS, INC.

Sept. 5, 1974

Sidney Williams -- National Highway Traffic Safety Administration

Dear Mr. Williams:

On November 15, 1973 I sent a collection of our pole trailer catalogs and of photographs of logging pole trailers in service to the Truck Trailers Manufacturers Association in Washington. I thought that these had been sent over to your offices. Perhaps they never did get there. My letter went to Mr. M. L. Higgins who at that time was handling these matters for TTMA, and very shortly after that he resigned and Mr. Burt Weller took his place.

With this letter I am sending you two each of our Catalog 196 and our Form 198 describing our standard types of pole trailers.

The logging pole trailers described in Catalog 196 are those about which we are primarily concerned with respect to MVSS 121. Those trailers are designed and used exclusively in logging operations and move logs from the woods to the saw mills or to storage areas. Logging pole trailers are subjected to unbelievably difficult conditions, or at least unbelievable to those who have never witnessed difficult logging operations. Maintaining brakes and lights on logging pole trailers when the very minimum of equipment is used is difficult and often impossible. When the hardware that is required for compliance with MVSS 121 is added to braking equipment as now being used, the difficulties of maintenance will be multiplied and the likelihood that the brakes will be kept operational is reduced.

On the inside of Catalog 196 you will see good views of the rear assembly of our two popular models, the Logmaster and the SPR. These two trailers are identical in capacity and in usage, and the two models are built simply because some loggers like one type and some like the other. In both models you will see that the rear bolster sits on a frame that sits on the two-spring suspension that sits on the axles. A pole or drawbar connects the front bolster assembly to the rear assembly. The rear assembly can slide along the pole to change the distance between the front and rear bolsters to accommodate various log lengths. If you look closely you will see that the rear assembly is locked into place on the pole by means of clamping collors around the coupling pole ahead of and behind the rear assembly. Also, for safety purposes a pin is dropped through the pole at the rear to avoid the possibility that the trailer might slide off the pole.

You will see in the illustrations that the two hoses for the brakes and the cable for the lighting on the rear assembly are connected to the truck or to the front bolster assembly at the front and are connected to the rear assembly at the rear with quick detachable couplings. You will see that when the distance between the bolsters is relatively short these brake and light lines must be coiled up and supported by the pole. If the trailer should be extended in length, the full length of these lines would be required. It is a characteristic of these trailers that the brake lines are designed to adapt to permitting the distance between the front and rear bolsters to contract and to expand.

Another common practice in the use of logging pole trailers is to load the rear frame assembly on the back end of the truck when the trailer and truck are being carried unloaded back into the woods. This practice is for both economic and safety reasons. When the rear assembly is loaded on the truck the brake lines must be disconnected. Also, no part of the brake system can extend below the bottom of the axles because the axles must rest on the truck frame or on the runners that are put on the truck frame to carry the trailer.

When you look closely at these pictures of these logging pole trailers you will observe that the space between the axles and the load-carrying bolster is limited and is filled with equipment with current types of brakes. The trailers shown have only one air reservoir. In order to comply with MVSS 121 another larger air reservoir will have to be added. There simply is no place to put it on these logging pole trailers without interfering severely with their intended usage. If in addition to this other air reservoir we must add spring-loaded power chambers and the valves and logic boxes and electrical connections required for the anti-skid brake system, it is up to now inconceivable to us how to add that hardware in such a way that it could reasonably be expected to stay on the trailers when they are used in the log woods.

The Models TP and TPC Pole Trailers illustrated in our Form 198 are designed and used primarily for hauling pipe and poles and reinforcing rods and long steel beams and other, long and self-supporting objects. The same difficulties in adding the anti-skid brake equipment and the spring loaded power chambers apply to these trailers as to the logging pole trailers. However, since these two models are used primarily on improved roads the difficulties of maintaining the equipment would not be as great. Trailers of these two models constitute no more than perhaps 50 units a year for us. They do have the same characteristic of logging pole trailers in that the brake lines are designed to be adapted to permitting the distance between the front and rear bolsters to change. There would be the same difficulties on these trailers in getting the speed of brake application and release that there would be on other pole trailers.

I can understand the difficulty of writing an exemption for any one class or category of trailers without letting the exemption be subject to unintended interpretations. However, it has seemed to me since we first became concerned with the problems of MVSS 121 that pole trailers as defined in Part 371.3 of the Federal Motor Vehicle Safety Standards could be exempted without any reasonable misunderstandings. Pole trailers as defined in Part 371.3 were exempted from the requirements of MVSS 108 and have been exempted from certification requirements. I have not understood why they could no also be exempted from MVSS 121, and I think that their categorical exemption would be beneficial rather than detrimental to the highway safety program. I have made all of these statements previously in letters to the National Highway Traffic Safety Administration and to the Truck Trailer Manufacturers Association.

We appreciate very much your consideration of our problems and I thank you again for your telephone call on September 3rd in response to our letter of August 21st.

If it might be helpful to giving you and others concerned a better understanding of our problems, which we think are shared by many others, I would be willing to come to Washington for a conference.

Sincerely yours, B. H. Smith -- President

Enclosures

NABORS TRAILERS, INC.

August 21, 1974

Sidney Williams -- National Highway Traffic Safety Administration

Subject: FMVSS 121 - Air Brake Systems, Docket #74-10, Notice 5

Dear Mr. Williams:

We are unable to determine from the subject Notice 5 and from the discussions and considerations that preceded it whether or not it is the intention of NHTSA that Logging Pole Trailers be considered "Heavy Hauler Trailers" within the definition of that category.

In one way of looking at it nearly all Logging Pole Trailers come within Characteristic (1) of the definition because the brake lines are designed to adapt to permitting the rear frame assembly of the pole trailer to slide on the pole or drawbar and thereby to adapt to extension or contraction of the effective length between the load bearing members. In another way of looking at it, pole trailers would not come within Characteristic (1) because there is no actual separation or extension of the vehicle frame. Actually, in pole trailers there is no frame as such connecting the front and rear load bearing members.

The one consistent and definitive characteristic of pole trailers, as we understand them, is that they consist of a front bolster that rests on the towing tractor and a rear bolster that rests on the trailer suspension and axles with the two connected with a pole on which the rear assembly can slide forward and backward.

In some logging operations the length between the front and rear bolsters is changed frequently, maybe daily. In some operations the length between the bolsters is changed only as the species or types of logs being hauled change. In some operations, on every unloaded trip the rear assembly is moved forward on the pole and is loaded on the truck to be hauled back into the woods. Since logging conditions and practices require different effective lengths between the front and rear bolsters and different problems with respect to the brake lines, practically all of our Logging Pole Trailers are rigged out with quick disconnectors for the brake and light lines at the rear assembly, and to that extent the rigging is similar to that on extendable platform trailers. The difficulties of maintaining good air line connections and unrestricted flow of air are certainly as great on pole trailers as on extendable frame trailers.

There are other inherent problems in applying and maintaining brakes on Logging Pole Trailers that in our opinion should be considered before there is a requirement upon trailer manufacturers to add anti-skid devices, spring loaded power chambers, dual or triple air reservoirs, and the other hardware that goes with the system. For example, up to now we have not been able to devise a way to put spring loaded power chambers on the axles of Logging Pole Trailers without swinging them under the axles. We definitely do not have sufficient clearance for putting the spring loaded power chambers over the axles with the suspensions and construction that we are using now. To put the (Illegible Word) chambers under the axles on pole trailers that are destined for the logging woods is, in our opinion, impracticable. The operators will not be able to keep the power chambers in place. The net result will be that they will knock them off or take them off the trailers and run the trailers without brakes. We run into similar but not insoluble problems with respect to installing additional air reservoirs and all of the other wiring and hardware required in an anti-skid system, but the trailers will have to be redesigned, they will be made substantially more expensive, and they will be less suited to their purpose.

Opinions have been expressed that we should simply assume that the definition of Heavy Hauler Trailers does embrace Logging Pole Trailers and that we should bother ourselves and our customers no more about the problem. We are not willing to leave it there because we cannot afford to be found in violation of the law and to be required to go back and add equipment which we did not furnish when the trailers were built. If we must put the anti-skid equipment on Logging Pole Trailers we must be getting ready for it in our designs and in our production methods, and we must be preparing our customers for the added cost and for the maintenance problems that will be inevitable.

Pole Trailers make up hardly more than 1% of the truck trailers that are manufactured. According to the Bureau of the Census Industrial Report for May, 1974, 189 pole trailers were shipped in May 1974, 214 in April 1974, and 177 in May 1973. For ourselves, the total volume in pole trailers is less than 300 units per year, but that is a substantial portion of our total volume and we are one of the largest suppliers of such trailers to the Southern lumber industry. Could you tell me exactly where we stand with respect to Logging Pole Trailers and MVSS 121?

Sincerely yours, B. H. Smith -- President

ID: nht74-2.14

DATE: 09/30/74

FROM: R. B. Dyson; NHTSA

TO: Royal Industries

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your August 28, 1974, question whether a "logging pole trailer", which consists of a beam to which an exle-mounted bolster can be clamped at different points to accommodate different log lengths, qualifies as a heavy hauler trailer as that term is defined in Standard No. 121, Air brake systems:

"Heavy hauler trailer" means a trailer with one or more of the following characteristics:

(1) its brake lines are designed to adapt to separation or extension of the vehicle frame; or

(2) Its body consists only of a platform whose primary cargo-carrying surface is not more than 40 inches above the ground in an unloaded condition, except that it may include sides that are designed to be easily removable and a permanent "front-end structure" as that term is used in @@393.106 of this title.

The logging pole trailer you describe is a heavy hauler trailer, and as such, Standard No. 121 does not apply to this trailer until September 1, 1976. The beam or "reach", together with the bolster, constitutes the frame of the trailer, and the brake lines are designed to adapt to extension of the bolster element along the beam.

ID: nht74-2.15

DATE: 03/20/74

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

TO: Kelsey-Hayes Company

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your February 1, 1974, request for interpretation of the language "maximum temperature of 500 +/- 50 degrees F" appearing in S6.1.8.1 of Standard No. 121, Air brake systems, 49 CFR 571.121.

The language is intended to specify a temperature range within which to conduct the burnishing procedure in the event brake applications cause the hottest brake on a vehicle to reach 500 degrees F. The word "maximum" is inappropriate and will be deleted in an upcoming notice responding to petitions that raised the same point.

Yours truly,

ATTACH.

KELSEY-HAYES COMPANY

February 1, 1974

National Highway Traffic Safety Administration, U. S. Department of Transportation,

Attention: Lawrence Schneider -- Chief Counsel

RE: Request for Interpretation FMVSS-121; Air Brake Systems Docket 73-13; Notice 3 @ 6.1.8.1 Brake Burnish Procedure

Gentlemen:

We would appreciate an interpretation of the following sentence which appears in the above cited section of Standard 121:

"If during any of the brake applications specified in Table IV, the hottest brake reaches 500 degrees F, make the remainder of the 500 applications from that speed except that a higher or lower speed shall be used as necessary to maintain a maximum temperature of 500 degrees F +/- 50 degrees F."

We believe the meaning of this sentence to be that the temperature of a brake may not exceed 550 degrees F at any time during a brake application and that the speed limitations specified in Table IV may be waived to maintain the temperature below 550 degrees F only if the temperature of the hottest brake reaches 500 degrees during any stop.

We are puzzled by the words ". . . maintain a maximum of 500 degrees F +/- 50 degrees F", as the tolerance appears to be inappropriate if our interpretation is correct. Conversely, if the intent is to limit the temperature range to 500 degrees F +/- 50 degrees F once a temperature of 500 degrees F is reached, the word "maximum" is inappropriate.

A prompt reply would be very much appreciated.

Very truly yours,

John F. McCuen -- Attorney

cc: W. T. Birge; D. Renner

ID: nht74-2.16

DATE: 11/14/74

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

TO: Hall and Myers

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your October 23, 1974, question whether the language of S5.3.1(b) and S5.3.2(b) in Standard No. 121, Air brake systems, exempts all liftable, nonsteerable axles from the "no lockup" requirements of the standard. You specifically ask whether a liftable, nonsteerable "tag" axle and "pusher" axle would be exempt if they were both mounted on a vehicle equipped with a single nonliftable, nonsteerable axle or with tandem nonliftable, nonsteerable axles.

The sections in question permit "lockup of wheels on nonsteerable axles other than the two rearmost nonliftable, nonsteerable axles on a vehicle with more than two nonsteerable axles." This language is limited to vehicles which have more than two nonsteerable axles and therefore a liftable axle on a vehicle with only one other nonsteerable axle would not be exempt. Such combination can be found on some intercity buses.

In both of the examples you described the vehicle has more than two nonsteerable axles, and therefore the language of S5.3.1(b) and S5.3.2(b) would exempt the tag and pusher axles from the "no lockup" requirements of the standard. I would like to emphasize, however, that our language is intended to require "no lockup" performance on not less than two nonsteerable axles of any vehicle with at least two nonsteerable axles. We did not contemplate the unlikely configuration of a single fixed axle and two liftable axles which you cite as an example. If a safety problem arises with this configuration, we would consider an amendment of the standard to require "no lockup" performance of two of these axles.

Yours truly,

ATTACH.

ID: nht74-2.17

DATE: 08/12/74

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

TO: Walter Case

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your June 13, 1974, request for approval of your "park-lock" device under the parking brake system requirements of Standard No. 121, Air brake systems, 49 CFR @ 571.121. After a trailer's emergency air supply applies the brakes through the service brake chamber in response to a low service brake air supply, your device locks the brakes in the applied position.

The relevant provision of the standard states:

S5.6.3 Application and holding. The parking brakes shall be applied by an energy source that is not affected by loss of air pressure or brake fluid pressure in the service brake system. Once applied, the parking brakes shall be held in the applied position solely by mechanical means.

The arrangement described would not meet this requirement because the energy source to apply the brakes (the emergency air supply) would be affected by loss of air pressure in the service brake system. For example, any failure in the service brake piston diaphragm would cause a loss of air pressure that would in turn "affect" the energy source that applies the parking brake. The brake chamber housing assembly is an element which is not considered to be part of the service brake system for this requirement.

Standard No. 121 does not specify the design of brake system components. Therefore we neither approve nor disapprove the use of particular components like the "park-lock" device. It may be used in any parking brake system which meets the requirements of the standard.

I would like to point out that the standard becomes effective January 1, 1975, for trailers, and that it does not regulate air brake systems on trailers manufactured before that date.

Yours truly,

ATTACH.

June 13, 1974

Larry Schneider -- Chief Counsel National Highway Admn.

Dear Mr. Schneider:

This is being written at the suggestion of Mr. Sid Williams, made to me during a telephone conversation only a few minutes ago.

I am the inventor of a brake locking device for air brake system equipped trailers. This device which weighs barely two pounds has been pronounced by leaders of the Motor Freight industry as the greatest safety device offered the industry in thirty-five years.

Some eight or so weeks ago, B. F. Goodrich Company examined and tested this brake lock and informed us of their intention to use it on their new safety systems, these being readied for market. This followed careful examination of the requirements for complying with NVSS 121 by both B. F. Goodrich and ourselves.

Our device does not alter the regular braking system in any manner. This brake lock works in conunction with the long required and approved Reserve Emergency Valve and trailer emergency air supply system. With the improved (modulated) RE 4 Valve plus our (Park-Lock) brake holding device, safety involving trailers has been increased many fold whether on the highway, parking lot or at the loading dock.

Should the service brake air supply be broken intentionally or by accident the trailer emergency air supply would take over and bring the vehicle to a stop. In such an instance, the Park-Lock would lock the set brakes automatically when the air pressure dropped to 20 lbs.

We submit that our brake locking device meets all requirements of NVSS 121 and of particular reference to (e) paragraph of Div. 12-369-26508 which prompted B. F. Goodrich's bringing to our attention they had been informed there was some question about Park-Lock meeting one requirement. The question concerns a brake chamber diaphragm failure.

Conversations with two large Motor Freight Companies (Transcon - E.T.M.F.) answered our inquiry this way: Concerning failure or malfunction of a diaphragm was, according to their records the rarest of their problems. The only maintenance reports are that of slow leaks reported by the driver. Neither Company could recall a burst diaphragm. In the event of such leak there is a warning device in the cab calling such to the driver's attention.

Particular care was given to explain to us that should the service system fail, the trailers system would be activated by the RE 4 Valve and the vehicles would be brought to a stop, even with a leak in a diaphragm.

Should the trailer system malfunction, the tractor system, protected by the tractor protection valve, would provide the necessary braking power to bring the vehicles to a safe stop. Either way, they emphasize, adequate braking is there to bring vehicles to a stop and all that is really needed for safety is a satisfactory brake lock to fit the tried and proven system now in use.

The Park-Lock brake lock has been proven in regular service for over a year. It is small enough to hold in your hand. The device is simple of construction, has three moving parts making for little wear, or requirement of maintenance.

We will appreciate your careful examination and checking out of our claims. The same, we are confident, can be said for the industry who has hailed our safety device.

Sincerely yours, Walter Case -- PARK-LOCK, INC.

ID: nht74-2.18

DATE: 06/07/74

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

TO: Certain-Teed Products Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your April 5, 1974, question whether S5.6.1 or S5.6.2 of Standard No. 121, Air brake systems, requires parking brakes on all axles other than front steerable axles.

S5.6.1 requires "parking brakes on an axle other than a steerable front axle" to have certain static retardation force values. "An axle" refers to any axle other than steerable front axles and therefore S5.6.1 requires parking brakes on all axles other than steerable front axles. A tandem axle consists of two separate axles for purposes of this requirement.

S5.6.2 has no specific axle-by-axle requirements. Its performance standard may be met by any means which hold the vehicle stationary and conform to S5.6.3 and S5.6.4. It should be emphasized that this requirement cannot be met simply be equipping the vehicle with parking brakes which hold to the limit of tractive ability but permit vehicle movement.

Yours truly,

ATTACH.

CERTAIN-TEED PRODUCTS CORPORATION

April 5, 1974

Chief Council -- National Highway Traffic Safety Adm., U.S. Department of Transportation

RE: Federal Vehicle Motor Safety Standards No. 121-Air Brake System Docket No. 74-10; Notice 1

Dear Sir:

We are requesting clearification of Paragraph S. 5.6.1 and S. 5.6.2.

Is the intent of Paragraph S. 5.6.1 to require parking brakes on all axles other than a steerable front axle, and is a tandum axle from the viewpoint of this regulation considered to be made up of two (2) separate axles?

In Paragraph S. 5.6.2, is it necessary to have parking brakes on each axle of the vehicle, or would a vehicle be acceptable if parking brakes mounted on only a portion of the axle were adequate to meet the grade holding requirements.

There appears to be questions in the minds of vehicle manufacturers, who are our customers, as to whether a trailer or a truck and bus with a modulated emergency system would require parking brakes on all non-steerable axles or whether only a portion of the axle requires parking brakes. We would appreciate an early reply to this inquiry.

Yours very truly,

B. D. Sibley -- Chief Product Engineer, Brake Products