Interpretation ID: nht90-4.25

TYPE: Interpretation-NHTSA

DATE: September 28, 1990

FROM: Lawrence W. Rusk -- Project Engineer, Drum Brakes, Allied-Signal, Inc., Bendix Automotive Systems

TO: Paul Jackson Rice -- Chief Counsel, NHTSA

TITLE: Re Ref: FMVSS105, Revision to Standard Effective September 1, 1991

ATTACHMT: Attached to letter dated November 7, 1990 from P.J. Rice to L.W. Rusk (A36; Std. 105)

TEXT:

This standard is being modified to no longer permit the "locking out" of drum brake automatic adjusters. In the past, release type automatic adjusters used on duo-servo rear drum brakes were made inoperative during the 105 test to prevent the tendency o f such mechanisms to overadjust during the test procedure. Overadjustment can cause poor cooling in the rear brakes, and eventually may lead to dragging rear brakes. An apply type auto adjuster is not subject to overadjustment during the 105, but most Bendix current production duo-servo drum brakes are fitted with the release type. Apply type adjusters have more piece parts than release type, and therefore do cost more in any brake they are used in.

Both current and revised standards allow for manual adjustment of the rear brakes after the initial burnish, and after each of three subsequent reburnishes. With inoperative adjusters, this is done to compensate for lining wear that occurs during the te st. Therefore, such manual adjustments were to expand the drum brake shoes to be closer to the drum. Since the revised standard still allows for adjustment at each burnish, with operative release type adjusters, manual adjustments could be made to back the shoes away from the drum and therefore counteract the release adjuster's tendency to overadjust.

In summary, it seems to us at Bendix Automotive Systems that duo-servo brakes with release type adjusters could be made to pass the revised 105 by making manual brake adjustments to offset overadjust tendencies. On September 26, 1990, I discussed this p roposal with Zach Fraser in the Enforcement Group. He suggested that I request a written interpretation from you on this proposal. Please consider this our formal request for an NHTSA position on the concept described herein.

If you or other representatives of NHTSA would like to discuss this issue further, I can be reached at (219) 237-2074. Thank you in advance for your assistance in this matter.