Interpretation ID: nht75-3.50

DATE: 06/10/75

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

TO: Volvo of American Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Volvo of America Corporation's May 9, 1975, request for reconsideration of the NHTSA's March 31, 1975, determination that a Volvo brake system that employs air pressure modulated by the vehicle operator to provide the energy used to actuate the brake is an air brake system subject to Standard No. 121, Air brake Systems.

Having reviewed all of the data submitted with your letter, it is concluded that the Volvo system is an air brake system subject to Standard No. 121. In the development of separate air brake and hydraulic brake system standards, the NHTSA had to make a determination of the status of brake systems which employ both air and hydraulic fluid as a means of transmitting force to the vehicle brakes. The agency decided that use of air as a means of power and transmission of the brake force would quality the system as an air brake system. This decision permits manufacturers to determine with certainty whether a standard applies to their products.

Since the withdrawal of applicability to trucks of Standard No. 105-75, Hydraulic brake systems, our decision has had the beneficial effect of ensuring that "air over hydraulic" systems are subject to a braking standard. If you are aware of any adverse safety consequences of our decision, I would appreciate hearing from you.

Sincerely,

May 9, 1975

James Schultz, Acting Chief Counsel National Highway Traffic Safety Administration

Re: Request for interpretation, FMVSS 105-75 - Hydraulic Brake Systems, and FMVSS 121 - Air Brake Systems

This correspondence is a follow-up to our request for interpretation dated March 3, 1975, your response dated March 31, 1975, and a subsequent telephone conversation between the undersigned and Mr. Richard Dyson, Esq. of your office.

In our March 3rd request we described a somewhat unique truck service brake system employing both air and hydraulic subsystems and asked for your concurrence that the described system was in fact a "hydraulic brake system" with a "brake power unit" as defined in 571.105-75 S4 and, therefore, not subject to the requirements of 571.121. In your March 31st response you disagreed with Volvo's proposed interpretation, stating in part that the system in question appeared to be an "air over hydraulic" system and cited an earlier interpretation contained in the preamble to 571.121 which defines the term "air brake system" as including brake configurations commonly referred to as "air over hydraulic", in which failure of either medium can result in complete loss of braking ability.

Having reviewed your response very carefully, we are presently of the opinion that your classification of the described Volvo brake system as "air over hydraulic" may be somewhat arbitrary and it appears you may have overlooked some important characteristics of the system. This could be in part due to overly simplified technical information supplied with our original request. It is, therefore, our intention to submit more detailed technical information as well as additional arguments to support our position at this time, in hopes that you will review and revise your original interpretation.

It is Volvo's contention that the brake system in question is a "Hydraulic brake system" as defined in 571.105-75 S4 in that it uses hydraulic fluid as the primary medium for transmitting force from the service brake control to the service brake and in that it incorporates a "brake power unit" as also defined in S4. The "brake power unit" provides the energy required to actuate the brakes, with operator action consisting of only modulating the energy application level. It is our belief that the subject Volvo system, to be described more fully, is in principal identical to currently used hydraulic brake systems in all major respects, except that it is a power brake and that the energy source which the driver modulates happens to be derived from compressed air rather than vacuum or hydraulic pressure. This single factor is not, in our opinion, decisive for classifying the Volvo system as an "air brake system", inasmuch as failure of a single air or hydraulic subsystem will not result in complete loss of braking ability, as you imply is the case with "air over hydraulic" systems.

A complete set of specifications and working description of the Volvo brake system in question, including illustrations and braking performance characteristics, is provided as Attachment No. I. Additionally, we are providing as Attachment No. II a group of pictorials depicting the actual layout of the components on the truck chassis.

The subject Volvo brake system has the following major characteristics in common with other truck "hydraulic brake systems":

1. The main chassis plumbing consists of hydraulic lines connecting the master cylinders to the wheel brake cylinders.

2. The medium transmitting force between the master cylinders and the wheel brake cylinders is hydraulic brake fluid.

3. The brake shoes are activated by hydraulic wheel cylinders.

In addition to the mentioned similarities, the subject Volvo brake system offers the following features and characteristics which we believe are superior to most current truck hydraulic brake systems:

1. Volvo offers a "split service brake system" with completely independent hydraulic circuits for the front and rear axles. plus split "brake power" (air) circuits which are isolated from each other and other air powered equipment in case of leakage.

2. We are confident that Volvo trucks equipped with the subject system offer stopping performance which is superior to most competitive trucks with hydraulic system in compliance with Federal Motor Carrier Safety Regulations, 393.40.

3. Volvo offers an air controlled parking brake system which can be effectively modulated and used for emergency braking.

4. The Volvo system includes a load sensitive valve which proportions hydraulic pressure to the rear axle.

The air portion of the subject Volvo brake system, as described in principal in Attachment No. I, will meet all applicable requirements of 571.121 S5.1. Additionally, the hydraulic fluid used meets the requirements of 571.116, and all brake hoses, both air and hydraulic, meet the requirements of 571.106.

We hope that the foregoing information and discussion will enable your office to revise its previous interpretation rendered to us on this topic. Before reaching a final decision, however, we would welcome an opportunity to meet with your technical and legal personnel to discuss the Volvo braking system in detail, and to further clarify our position in this matter. Due to pressing time constraints, we would appreciate your cooperation in arranging such a meeting before the end of May, 1975.

We thank you in advance for your cooperation.

VOLVO OF AMERICA CORPORATION Product Engineering and Development

Donald W. Taylor Manager, Product Safety & Quality

cc: B. Klingenberg/Truck Div.