Interpretation ID: 1983-3.3

TYPE: INTERPRETATION-NHTSA

DATE: 09/06/83

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: BMW of North America, Inc.

TITLE: FMVSS INTERPRETATION

TEXT:

September 6, 1983 NOA-30

Mr. Karl-Heinz Ziwica, Manager Safety & Emission Control Engineering BMW of North America, Inc. Montvale, New Jersey 07645

Dear Mr. Ziwica:

This responds to your letter requesting an interpretation of Standard No. 105, Hydraulic Brake Systems. Your request was in regard to a type of brake reservoir you are considering producing which would contain common fluid for the brake circuits and the brake hydraulic power assist unit. The issue raised by your letter was whether section S5.4.2 of the stnadard permits the common fluid to be counted as part of the minimum capacity required for the braking system. As discussed below, the answer to that question is no.

The first sentence of section S5.4.2 states:

Reservoirs, whether for master cylinders or other type systems, shall have a total minimum capacity equivalent to the fluid displacement resulting when all the wheel cylinders or caliper pistons serviced by the reservoirs move from a new lining, fully retracted position (as adjusted initially to the manufacturer's recommended setting) to a fully worn, fully applied position, as determined in accordance with S7.18(c) of this standard.

As noted by your letter, the agency has previously interpreted this section with respect to a brake reservoir servicing both the braking system and the clutch. In an October 9, 1981, letter to Toyota, we explained:

This section specifies the total minimum fluid capacity that a vehicle's braking system reservoirs must have. That amount is determined by reference to the vehicle's braking system, i.e., by the fluid displacement which results when all the wheel cylinders or caliper pistons serviced bythe reservoirs move from a new lining, fully retracted position to a fully worn, fully applied position. The purpose of this requirement is to assure that a vehicle's braking system reservoirs have adequate fluid capacity to service the brakes.

The agency interprets section S5.4.2 to require that the minimum fluid capacity requirements be met by fluid which is solely available to the brakes. If fluid is available to both the brakes and the clutch, some of that fluid will be used by the clutch in normal service and thus be unavailable to the brakes. In the event of clutch failure, all of the common fluid may be used by the clutch. Therefore, while Standard No. 105 does not prohibit manufacturers from producing master cylinders with reservoirs that have some fluid available to both the braking system and the clutch, none of that common fluid may be counted toward meeting the minimum requirements of section S5.4.2.

This same rationale applies to a reservoir which contains common fluid for the brake circuits and brake hydraulic power assist unit. As indicated in our October 1981 letter, the minimum fluid capacity requirements were determined by reference to the vehicle's braking system for the purpose of assuring that a vehicle's braking system reservoirs have adequate fluid capacity to service the brakes, i.e., the wheel cylinders and pistons. More specifically, the requirement for minimum capacity equivalent to the fluid displacement resulting when all the wheel cylinders or caliper pistons serviced by the reservoirs move from a new lining, fully retracted position to a fully worn, fully applied position, was based upon maintaining a sufficient supply of fluid to enable a vehicle to stop even when there was complete brake lining wear-out in the service brakes. Put another way, the requirement assures an adequate supply of brake fluid over the lifetime of the brake linings, even if a driver fails to add fluid as part of routine maintenance.

This purpose would not be met if fluid available to both the brake circuits and brake hydraulic power assist unit was counted toward meeting the minimum fluid capacity requirements. Some of the common fluid would be used by the brake hydraulic power assist unit in normal service and thus be available to the brake circuits. Moreover, in some instances of brake hydraulic power assist unit failure (e.g., a failure in the brake hydraulic power assist unit return line), all of the common fluid might be used by that unit.

Sincerely,

Frank Berndt Chief Counsel Enclosure

May 11, 1983

Mr. Frank Berndt Chief Counsel National Highway Traffic Safety Administration U.S. Department of Transportation 400 Seventh Street S.W. Washington DC 20590 RE: Request for Interpretation - FMVSS 105-75

Dear Mr. Berndt

For future models, BMW is considering a new braking system with hydraulic power assist. Included in this system is a brake fluid reservoir which would contain common fluid for the brake circuits and the brake hydraulic power assist pump. This design is shown in the attached drawing, where X and Y are the individual reservoir compartments for the brake circuits and Z is the compartment for the brake hydraulic power assist pump. The area marked W represents fluid available to both the brake circuits and the brake power assist pump.

We request an interpretation of S 5.4.2 (reservoir capacity) of FMVSS 105-75, with respect to the proposed brake fluid reservoir described above.

We have reviewed the various interpretations given by NHTSA, and are unable to find any opinions which apply to our specific reservoir. Among the interpretations applicable to "multi-purpose" reservoirs is one given to Toyota in a letter from Mr. F. Berndt, dated October 9, 1981. The response to Toyota presented the agency's position that "the minimum fluid capacity requirements for brake reservoirs be met by fluid which is not available to the clutch, either during normal use or in the event of clutch failure".

We believe that the agency's position is appropriate with respect to a common brake/clutch system. A leak in the line to the clutch would not necessarily cause failure of the clutch itself; hence, there would be no warning that a leak existed and that the fluid level in the reservoir was being depleted. Thus, a driver could go through a complete set of brake linings without recognizing the leak. It is therefore brake and clutch systems should not be counted towards meeting the reservoir capacity requirements of S 5.4.2.

If, however, a leak should occur in the brake power assist subsystem of BMW's proposed design, the braking power assist ("boost") fails after a few brakings, Because the Boost pump compartment (Z) of the reservoir has been emptied. In this case, the fluid level warning lamp would be activated, due to the fact that the fluid level warning point is located above the walls of the boost compartment. Additionally, the warning lamp provided specifically for power assist unit pressure will be activated. Vehicle braking can be achieved through the application of increased pedal pressures, in compliance with S 5.1.2.1, S 5.1.3.4, S 6.13, and S 7.9 of FMVSS 105-75. Further, the amount of fluid remaining in compartments X and Y of the BMW reservoir is sufficient to meet the requirements of the second sentence of S 5.4.2., which is as follows:

"Reservoirs shall have completely separate compartments for each subsystem except that in reservoir systems utilizing a portion of the reservoir for a common supply to two or more subsystems, individual partial compartments shall each have a minimum volume of fluid equal to at least the volume displaced by the master cylinder piston servicing the subsystem, during a full stroke of the piston."

With regard to the total minimum capacity requirements of S 5.4.2 (full lining wear) under the condition of a power assist unit leak, neither the sum of the two brake circuit compartment volumes (X,Y) nor either of them are equivalent to the fluid displacement resulting when all wheel cylinders move from a new lining condition to a fully worn position.

We believe it highly unlikely that, under the conditions just described (lack of power assist, 100 - 150 pound pedal force for deceleration, and activation of two warning lights) a driver would wear down a complete set of brake linings over a typical range of 20,000 to 40,000 miles. In such a case, it would be reasonably expected that a driver would seek repair at a dealer or service station immediately upon loss of brake power assist and long before the brake linings were fully worn.

We respectfully submit, therefore, that the power assist unit should be considered an integral part of the brake system and should be recognized as a subsystem of the brake system for which the same requirements applying to brake subsystem leakage should also be valid. In our view this is a reasonable assumption because, as detailed earlier, the driver will receive immediate warning of a fluid leak, in the form of a loss of brake power assist, as well as activation of two warning lights. Similarly, a loss of fluid from a brake circuit would also be obvious to the driver, since pedal effort would increase noticeably and a warning light would be activated. As a further example, the proposed BMW system would comply with the following sections of FMVSS 105-75, if the power assist unit is considered a subsystem and if a leak developed in that subsystem (compartment Z):

S 5.1.2 Partial Failure S 5.4.1 Master Cylinder Reservoir S 6.13 Control Forces S 7.9 Service Brake System Test Partial Failure

We believe a final point which should be considered by the NHTSA regarding BMW's proposed design is international harmonization. As an exporter of vehicles to a number of markets throughout the world, our goal is to design components (including brake fluid reservoirs) to comply with as many different national regulations as possible.

In the case of our proposed reservoir, the U.S. and Japanese requirements conflict with regard to total reservoir capacity. FMVSS 105-75, based on previous NHTSA interpretations, refers to the fluid volume available exclusively to the brake system. On the other hand, Japanese requirements apply to the total reservoir capacity, including every and all subsystems. If the volumes of X and Y were increased to provide sufficient fluid within them to meet the requirements of S 5.4.2., we would be forced, in order to comply with Japanese regulations, to increase the volume W as shown on the attached drawing. The total reservoir capacity, already a significant amount (700 cc), would have to be increased dramatically (43 ?) to approximately one liter in order to meet NHTSA's interpretation of brake/clutch reservoirs. Further, we would be required to raise the position of the switch point for the fluid level warning light. This could result in unnecessary activations of the warning signal. Through normal use, the fluid could drop to a level which would switch-on the warning light, but this level of fluid would still meet S5.4.2 requirements and would not represent a "low" fluid level condition.

Interpretation of the power assist unit as a subsystem of the proposed BMW brake system will allow BMW to market a common reservoir worldwide, rather than being forced to manufacture a unique reservoir exclusively for the U.S. market.

To summarize, the proposed BMW brake fluid reservoir would contain the S 5.4.2 total minimum capacity fluid requirement when the brake power assist unit is considered to be a subsystem of the total brake system. We believe that inclusion of the power assist unit circuit as a brake subsystem is valid for the following reasons. First, in the event of a fluid leak in the power assist unit circuit, the driver would receive multiple warnings (increased pedal effort, warning lights) in a manner analogous to the warnings received when a brake circuit leak occurs. Second, the proposed BMW system, including the power assist unit subsystem would be accurately described by the S 4. definition of "split service brake system", which "...means a brake system consisting of two or more subsystems actuated by a single control designed so that a leakage-type failure of a pressure component in a single subsystem (except structural failure of a housing that is common to two or more subsystems) shall not impair the operation of any other subsystem."

Accordingly, we believe that the proposed BMW brake fluid reservoir described in this letter would fulfill the requirements of S 5.4.2 with regard to total minimum reservoir capacity. We ask that you confirm our interpretation at your earliest convenience.

Very truly yours,

Karl-Heinz Ziwica, Manager Safety & Emission Control Engineering

WS/fw 0104 - 83

Encl.

BMW PROPOSED BRAKE FLUID RESERVOIR

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