Peterbilt 220, Operator'S Manual

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the system will intervene to assist the
driver.
Electronic Stability Control may reduce the
vehicle speed automatically. To minimize
unexpected deceleration and reduce the
risk of a collision the operator must:
• Avoid aggressive driving
maneuvers, such as sharp turns or
abrupt lane changes at high
speeds, which might trigger the
stability system.
• Always operate the vehicle safely,
drive defensively, anticipate
obstacles and pay attention to
road, weather, and traffic
conditions. ABS, ATC, and ESC
stability systems are no substitute
for prudent, careful driving.
Roll Stability
Roll Stability, an element of the overall
ESC system, addresses rollover
conditions. In the case of a potential roll
event, the system will override the throttle
and quickly apply brake pressure at all
wheel ends to slow the vehicle
combination. The system will apply proper
levels of braking and should be
proportional to roll risk.
A Real World Example of How the
System Operates
Excessive speed for road conditions
creates forces that exceed the threshold at
which a vehicle is likely to rollover on a
higher-friction surface. The system
automatically reduces engine torque and
applies the service brakes (based on the
projected rollover risk) to reduce the
vehicle speed, thereby reducing the
tendency to roll over.
Yaw Stability
Yaw stability counteracts the tendency of a
vehicle to spin about its vertical axis.
During operation, if the friction between the
road surface and the tires is not sufficient
to oppose lateral (side) forces, one or more
of the tires can slide, causing the truck/
tractor to spin. These yaw events are
referred to as either "under-steer" (where
there is a lack of vehicle response to
steering input due to tire slide on the steer
axle) or "over-steer" (where the tractor's
rear end slides out due to tire slide on the
rear axle) situation. Generally, shorter
wheelbase vehicles (tractors, for instance)
have less natural yaw stability, while longer
wheelbase vehicles (straight trucks, for
instance) have greater natural yaw stability.
Factors that influence yaw stability are:
wheelbase, suspension, steering geometry,
weight distribution front to rear, and vehicle
track width.
Yaw Control
Yaw Control responds to a wide range of
low- to high-friction surface scenarios
including rollover, jackknife and loss of
control. In the case of vehicle slide (over-
steer or understeer situations), the system
will reduce the throttle and then brake one
or more of the "four corners" of the vehicle,
thus applying a counter-force to better
align the vehicle with an appropriate path
of travel. For example, in an over-steer
situation, the system applies the "outside"
front brake; while in an under-steer
condition, the "inside" rear brake is applied.
DRIVING - ABS Brakes
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