Peterbilt 579 - Page 198

OPERATING THE BRAKE SYSTEM
4
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 sufcient 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 inuence 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
(in addition to potentially applying
the trailer brakes), 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.
A Real
World Example of How Yaw
Control Operates
Excessive speed
exceeds the
threshold, creating a situation where a
vehicle is likely to spin and jackknife.
The Bendix®
Yaw Control system
reduces engine throttle and selectively
applies brakes to reduce the vehicle
speed, thereby reducing the tendency
to jackknife.
Yaw Control Example
4-26 Y53-6047–2C (04/13)