Subaru 2002 Impreza Service Manual page 311

ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
C: PRINCIPLE OF ABS CONTROL
When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle
speed does as well. The decrease in the vehicle speed, however, is not always proportional to the
decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle
speed is called "slip" and the magnitude of the slip is expressed by the "slip ratio" which is defined
as follows:
Slip ratio
=
Vehicle speed
-
Wheel speedNehicle speed x 100%
When the slip ratio is 0%, the vehicle speed corresponds exactly to the wheel speed; when it is
loo%, the wheels are completely locking (rotating at a zero speed) while the vehicle is moving.
The braking effectiveness is represented by the "coefficient of friction" between the tire and road
surface. The larger the Coefficient, the higher the braking effectiveness. The diagram below shows
the relationship between the coefficient of friction and the slip ratio for two different road surface
conditions (asphalt-paved road and icy road), assuming that the same tires are used for both the
conditions and the vehicles are moving forward. Although the braking effectiveness (coefficient
of friction) depends on the road surface condition as shown and also on the type of the tire, its peak
range generally corresponds to the 8
-
30% range of the slip ratio.
The ABS controls the fluid pressure to each wheel so that a coefficient of friction corresponding
to this dip ratio range is maintained.
Control range by ABS
Asphalt-paved road
Icy road
--
I
*
loo(%)
Slip ratio
0
ABS-5
NF0199