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SAE J1489

May 1, 2000

Complete Document

(R) Heavy Truck and Bus Retarder Downhill Performance Mapping Procedure

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Product Details:

  • Revision: May 1, 2000
  • Published Date: May 2000
  • Status: Active, Most Current
  • Document Language: English
  • Published By: SAE International (SAE)
  • Page Count: 17
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:

The procedure covers the estimation of the total retardation capability available to a specific vehicle from:

a. Natural retardation (rolling resistance, aerodynamic drag, etc).

b. Engine drag

c. Engine, integral automatic transmission, driveline or trailer-axle retarders

It assumes that foundation brakes are not used for maintaining speed on long mountain descents. Retardation is rated in terms of the maximum grades on which stable control speeds can be maintained for each gear over the range of highway speeds appropriate to that gear. For each gear, the calculation procedure determines maximum grades for at least four values of control speed ranging from the vehicle velocity corresponding to full load governed engine rpm, to the vehicle velocity corresponding to the engine rpm at minimum (idle) speed. In addition, the calculation procedure provides information on the total retarding power available for each gear.


This procedure provides a uniform method for calculating and reporting the steady-state control speed maintainable by either engine, integral automatic transmission, driveline, or trailer-axle retarders employed by heavy trucks and buses operating on downgrades. It provides a practical method of predicting retarder performance in a specified vehicle installation using data describing retarder performance characteristics and other vehicle design variables. It is intended to assist in the proper selection of a retarder by matching stated retarder performance capabilities with particular vehicle functional requirements.
AIA/NAS Aerospace Standards