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Interpretation ID: nht73-1.7

DATE: 09/14/73

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

TO: U.S. Technical Research Corp.

TITLE: FMVSS INTERPRETATION

TEXT: This is in reply to your letter of July 24, 1973, asking for an interpretation of several aspects of Standard No. 105a.

You have asked the following questions

"1. Paragraph S.5.1.2. 'Partial failure.'

It is required that '. . . the remaining portion(s) of the service brake system shall continue to operate . . .' What is the exact meaning of 'continue to operate'? Does it mean that the remaining portion of the brake system must be permanently fed or does it mean that it is required to have temporary braking with the operative portion reserve capability? In this latter case, how many brake applications are required?"

"Continue to operate" means that the portion of the brake system that has not failed continues to operate indefinitely, i.e., to the point that it wears out or until a second failure occurs in the brake system. It requires a permanent feed and does not depend upon the reserve capability of operative portion of system.

"2. Paragraph S.5.1.3.3 'Brake power units':

What exactly constitutes the power source? On the Citroen D and S models, the front brake circuit is fed by the pressure prevailing in the rear suspension. The brake accumulator and the rear suspension are fed from the high pressure source (which includes an HP pump, a pressure regulator and a main accumulator) . . . What is meant by 'inoperative brake power unit'? Does that mean that the high pressure pump only is inoperative or also the other components of the power source (main accumulator and regulator)?. . . What is meant by 'when the inoperative unit is depleted of all reserve capability' (paragraph S.5.1.3.3.(ii))? Are we correct in assuming that it means that only the main accumulator is depleted of reserve capability? (It is obvious that if one considers that not only the main accumulator, but also the brake accumulator and the rear suspension are depleted, no braking is possible)."

The power source consists of pumps, accumulators and/or back up systems such as a separate electric or hydraulic pumps, etc. A primary power source would be the pump, while the accumulators would constitute a secondary source and would be the portions used in optional test. A high pressure source would include the pump, regulator and, in Citroen's case, the main accumulator.

"Inoperative brake power unit" could mean that the (1) main pump is out, but the accumulators are functioning, (2) the main pump is operating, but only one brake accumulator is operating, (3) the pump and brake accumulator are operating, but the suspension accumulator is out, (4) the pump or accumulator is out, and the system is operating on reserve or backup pump. This list is meant to be illustrative rather than exhaustive.

"When the inoperative unit is depleted of all reserve capability" means that one of the units (pump, accumulator, etc.) is completely non-functional, e.g., the pump has failed, the accumulator has failed, the check valve has failed, etc.

"3. Paragraph S.7.10.2 'Optional procedures'

We believe that subparagraph 'b' (vehicles with brake power unit) applies to our vehicles.

The test procedure mentioned in paragraph S.7.10.2(a) cannot be applied to our vehicles since, if the system is depleted of 'any residual brake power reserve capability', it is obvious that no braking is possible. We believe that the power source only should be depleted of any residual reserve (HP pump inoperative, main accumulator depleted), but not the entire brake system. Since, by definition, a 'brake power unit' is a unit where the operator action consists 'only of modulating the energy application level,' but not of supplying energy to the system, it is obvious that no braking is possible if all internal residual energy left in the brake system is depleted (since, in this case, there would be no energy available for braking from either the HP source, the driver or the system).

S7.10.2(b) does apply to Citroen. Your comments on S7.10.2(a) are correct.

Yours truly,

July 24, 1973

National Highway Traffic Safety Administration

Attention: Chief Counsel

Gentlemen:

In relation to Motor Vehicle Safety Standard No. 105a "Hydraulic Brake System", as published in the Federal Register vol. 38 No. 96 of May 18, 1973, we would like clarification of the following points.

1. Paragraph S.5.1.2 "Partial failure."

It is required that ". . . the remaining portion(s) of the service brake system shall continue to operate . . ." What is the exact meaning of "continue to operate"? Does it mean that the remaining portion of the brake system must be permanently fed or does it mean that it is required to have temporary braking with the operative portion reserve capability? In this latter case, how many brake applications are required?

2. Paragraph S.5.1.3.3 "Brake power units":

- What exactly constitutes the power source? On the Citroen D and S models, the front brake circuit is fed by a brake accumulator while the rear brake circuit is fed by the pressure prevailing in the rear suspension. The brake accumulator and the rear suspension are fed from the high pressure source (which includes an HP pump, a pressure regulator and a main accumulator). (see enclosed sketches)

We understand that the high pressure source is constituted by the HP pump, the pressure regulator and the main accumulator only. Are we correct?

- What is it meant by "inoperative brake power unit"? Does that mean that the high pressure pump only is inoperative or also the other components of the power source (main accumulator and regulator)?

We believe it should be considered that several failures cannot simultaneously happen to the same system. (as it is considered in equivalent European regulations).

- What is it meant by "when the inoperative unit is depleted of all reserve capability" (paragraph S.5.1.3.3.(ii))? Are we correct in assuming that it means that only the main accumulator is depleted of reserve capability? (It is obvious that if one considers that not only the main accumulator, but also the brake accumulator and the rear suspension are depleted, no braking is possible).

3. Paragraph S.7.10.2 "Optional procedures"

We believe that subparagraph "b" (vehicles with brake power unit) applies to our vehicles.

The test procedure mentioned in paragraph S.7.10.2(a) cannot be applied to our vehicles since, if the system is depleted of "any residual brake power reserve capability", it is obvious that no braking is possible. We believe that the power source only should be depleted of any residual reserve (HP pump inoperative, main accumulator depleted), but not the entire brake system. Since, by definition, a "brake power unit" is a unit where the operator action consists "only of modulating the energy application level," but not of supplying energy to the system, it is obvious that no braking is possible if all internal residual energy left in the brake system is depleted (since, in this case, there would be no energy available for braking from either the HP source, the driver or the system.)

We remain, of course, at your disposal should you need more information on the operation of our braking system, and, awaiting your answer, we are,

Very truly yours,

By Bernard Belier -- U.S. Resident Engineer for CITROEN S.A.

Enclosures: 1 print "Citroen SM braking"; 1 sketch "Citroen high pressure source"; 1 sketch "Citroen brake system"; 1 sketch "Suspension"; 1 plate No. 21 "Citroen brake accumulator"