Interpretation ID: nht74-1.46

DATE: 11/05/74

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

TO: Midland-Ross Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Midland-Ross' October 8, 1974, clarification of its February 8, 1974, petition for an amendment of S5.1.2.1 and S5.2.1.2 of Standard No. 121, Air brake systems, that would establish separate air reservoir volume requirements for several brake chamber types generally available in the air brake component market. You point out that Midland-Ross was referring to chamber stroke and not chamber diameter as the chamber dimension which could affect the safety of a brake system. You also requested that we adopt SAE Standard J10b instead of J10a as our specification of a reservoir that "withstands" certain internal hydrostatic pressure.

In our denial of your petition, we did understand your point that additional stroke could be discouraged by a reservoir capacity requirement based on chamber size at maximum travel of the piston or diaphragm. We found that the stopping distance requirements in effect mandate the installation of high performance components, and we do not anticipate a safety problem. If a safety problem does arise in the future, we would consider a modification of S5.1.2.1 and S5.2.1.2.

SAE Standard No. J10b is identical to J10a in its requirement that no rupture or permanent circumferential deformation of the reservoir exceed one percent. Therefore, for purposes of S5.1.2.2 and S5.2.1.3, we are adopting SAE J10b as our specification of "withstand" until we undertake further rulemaking.

Yours truly,

ATTACH.

POWER CONTROLS DIVISION Midland-Ross Corporation

October 8, 1974

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

Dear Mr. Gregory:

Subject: N40-30 (TWH)

Thank you for your response to our petition dated February 8, 1974 in regard to Section S5.1.2.1 and S5.2.1.2 of Standard 121 Air Brake Systems. We feel that our petition may not have been clear in regard to use of small volume chambers which apparently led to your misinterpreting our concern. We also believe you may have referred to the incorrect SAE Standard regarding air reservoirs.

In regard to our petition for clarification of air reservoir required volumes, we made mention of the fact that "current reservoir volume requirement based on maximum displacement encourages the use of small volume chambers". By this statement we did not imply smaller diameter chambers but shorter stroke chambers. It is quite easily determined that little chamber stroke is required if the foundation brakes are carefully adjusted with minimum liner to drum clearance. A chamber with 1.5" stroke could be adequate and will meet all of the standard's requirements. If a vehicle manufacturer would elect to go with this short stroke, he could reduce reservoir capacity by 25%. However, there would be very little safety factor to allow for drum expansion and liner wear. It is this condition of which we are concerned and feel it is wrong to penalize the vehicle manufacturer by requiring them to have larger reservoirs when they attempt to provide this additional safety advantage. We ask that you again review this matter and adopt one of the recommended changes to S5.1.2.1 and S5.2.1.2 as stated in our petition.

In the last paragraph of your response you mention the NHTSA has adopted the SAE Standard No. J10a in regard to the definition of "withstand". We assume you intended to refer to SAE Standard J10b and would appreciate your concurrence with this assumption.

Sincerely,

M. J. Denholm -- Director of Engineering