This report reviews the problem of truck air brake adjustment and specifically the use of automatic slack adjusters (ASAs) to maintain proper brake adjustment on S-cam drum brakes. The report reviews the importance of proper brake adjustment for effective functioning of air brake systems. It describes methods of improving brake adjustment, with emphasis on ASAs. The report presents findings from a large field test of ASAs (with comparative data on manual slack adjusters; MSAs) used on heavy commercial vehicles. Project data indicated that the use of ASAs can be an efficient and effective means of maintaining brake adjustment. For the 36 ASA/fleet applications tested, the median percentage out-of-adjustment was 4.0%. The median number of out-of-adjustment observations per brake per 10,000 miles was 0.016. ASAs were, in general, significantly more effective than MSAs in similar application. The ASAs tested were models from the early to mid-1980s. There have been extensive design improvements since that time. Thus, the data presented here for these earlier ASA designs most likely understate the performance levels that can be expected from ASAs purchased today. However, in spite of the demonstrated effectiveness and reliability of ASAs, they operate in a hostile environment. On occasion, they fail to adjust brakes either because of a mechanical ASA failure or a failure in other parts of the foundation brakes. Thus, brake adjustments still need to be checked regularly.

  • Supplemental Notes:
    • Based in part on research performed by BBN Laboratories Inc., under NHTSA Contract Number DTRS-57-83-C-00066.
  • Corporate Authors:

    National Highway Traffic Safety Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Authors:
    • Williams, S F
    • Knipling, R R
  • Publication Date: 1991-2

Media Info

  • Features: Appendices; Figures; Photos; References; Tables;
  • Pagination: v.p.

Subject/Index Terms

Filing Info

  • Accession Number: 00622648
  • Record Type: Publication
  • Source Agency: National Highway Traffic Safety Administration
  • Report/Paper Numbers: HS-807 724
  • Files: HSL, TRIS, USDOT
  • Created Date: Jun 30 1992 12:00AM