Descending A Grade - Mack Granite GU Series Operator's Handbook Manual

As grades and vehicle weight increase, it will be necessary to use the HIGH
position.
1. High
2. Low

Descending a Grade

DANGER
The MACK PowerLeash™ Engine Brake assists you in establishing faster
downhill descent speeds than in a similarly configured and loaded vehicle without
an engine brake; nevertheless, even with an engine brake, there are limits to the
maximum vehicle speed at which you can travel on downhill grades and still
keep your vehicle safely under control. Always approach these situations with
caution, and gain the experience necessary for each grade and driving condition
to determine the appropriate downhill descent speed.
For a certain vehicle and load condition, a "control speed" may be established
for a given descent. The control speed is the vehicle speed at which the
retarding forces of air drag, rolling resistance and engine brake power are
equally balanced by the natural force of gravity that causes the vehicle to
accelerate down the hill, resulting in a steady, controlled vehicle speed. The
additional vehicle slowing power offered by the MACK PowerLeash™ Engine
Brake allows the driver to descend the hill in a higher gear than normal (at a
faster control speed), without overspeeding the engine.
For example, consider that you are descending a grade with a specific vehicle
and an assumed total vehicle weight. You will find that without an engine
brake, you must descend this grade in third gear to maintain a steady 10 mph
at 2,000 engine rpm, without the need to apply the service brakes.
OPERATION 155
3. Off
C0029332