Interpretation ID: nht76-1.9

DATE: 08/09/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Clayton Dewardre Company Limited

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Clayton Dewandre Company's May 20, 1976, request for confirmation that its "Dual Brake Booster" system is designed to conform to the definition of "split service brake system" and the requirements of S5.1.2 and S5.1.3 specified in Standard No. 105-75, Hydraulic Brake Systems. In unimpaired braking, the Dual Brake Booster system supplies two separate brake circuits, the primary circuit which is initially powered by the driver's application of pedal force and subsequently supplemented by pressurized fluid from the pump accumulator system, and the secondary circuit which is powered by pressurized fluid from the pump accumulator system. In the event of a primary circuit failure, a mechanical connection unimpaired by a loss of reservior fluid continues to modulate the secondary circuit. In the event of the a secondary circuit failure, the driver's pedal application continues to actuate the primary circuit by muscular effort alone. A single master cylinder reservoir is provided to supply the primary circuit. A single pump reservoir supplies the pump, accumulator, and secondary circuit.

As you are no doubt aware, the National Traffic and Motor Vehicle Safety Act does not authorize a "type approval" of vehicle design as the basis for certification (15 U.S.C. @ 1397(a)(1)(A)). Our comments on the description of your system do not relieve the vehicle manufacturer of its responsibility to design a system for each of its products that actually complies with the standard's requirements.

From your description of the system, it would appear to qualify as a "split service brake system" as that term is defined in S4 of the standard. You state that, in the event either subsystem is failed, the other subsystem is capable of indefinite operation. This conforms to the

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NHTSA's September 14, 1973, letter to Citroen on the meaning of "unimpaired operation" of a subsystem.

With regard to partial failure performance, you state that the vehicle can meet the requirements of S5.1.2 (inadvertently designated as S5.1.1 in your letter) using either of the two subsystems.

With regard to the requirements of S5.1.3 (for inoperative brake power assist unit or brake power unit), you indicate that the vehicle is capable of stopping within the specified distances of column IV of Table II "purely by muscular effort of the driver". By this we assume you mean that the vehicle conforms to the condition required for testing under S5.1.3.1, i.e., with one power unit inoperative and deleted of all reserve capability. Please note that the NHTSA regards the "pump and accumulator" energy source to constitute a "brake power unit" and not a "brake power assist unit", because the described unit "provides the energy required to actuate the brakes, either directly or indirectly through the auxiliary device, with the operator action consisting only of modulating the energy application level." (S4 definition of "brake power unit").

We are assuming that Clayton Dewandre does not object to making public the designs described in your May 20 letter. The NHTSA will place the materials in the public docket three weeks after the date of this letter unless we hear otherwise from you.

Yours truly,

ATTACH.

Clayton Dewandre Company Limited

May 20, 1976

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

Dear Sir,

Request for advice on the conformities of New Design equipment to the revised Hydraulic Brake System Standard FMVSS 105-75

Clayton Dewandre would like to take this opportunity to introduce to you our newly developed Hydraulic Brake Booster and associated system, (Illegible Word) on behalf of our potential customers, we seek clarification from you regarding its compliance with the revised Hydraulic Brake Standard FMVSS 105-75.

We would refer to section S4 and the definition of a "Split Service Brake System" which includes a statement that a failed sub-system, quote "shall not impair the operation of any other sub-system".

Our interpretation of the expression "impair" is "reduce (Illegible Word) prescribed effective level", and that same implied level is quantified under requirements S.5.1.1. Partial Failure and S.5.1.3. In-operative Brake Power Assist Unit. Both the latter clauses call for the same level of performance i.e. stopping distances from a vehicle speed of 60 m.p.h. without exceeding a pedal force of 150 lbs. Vehicle Cars Trucks<10000 lbs. trucks>10000 lbs. Stopping 456' 517' 613' distance (.263g) (.233g) (.195g)

We would like to show, with reference to the attached description of the Dual Brake Booster, that with a power failed situation or rear brakes failed, we have the same residual secondary braking i.e. the front brakes, being activated purely by muscular effort of the driver, with chamber (a) acting as a conventional master cylinder. The designed performance for this condition in the case of a 2000 Kg (4,400 lbs) gross weight vehicle would be .3g for a pedal force 71 lbs (316N). This meets the prescribed performance tabled above.

Should a failure occur in the other sub-system (Illegible Words) mechanically by further movement of the pedal (approximately 12 mm.) for full output to "knee point". Fluid under pressure is delivered by the valve in proportion to pedal effort. Again in this case, the system is tuned to provide an acceptable performance. The attached figures show a retardation of .3g for 52 lbs (231N) pedal force, which is well within the prescribed limits.

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(Illegible Text)

Yours faithfully,

W. M. PAGE -- Senior Engineer - Systems Engineering

[Attachments Omitted.]