How The System Works For Understeer; How The System Works For Oversteer; Emergency Brake Assist (Eba) - Ford Transit 2002 Owner's Manual

Rear wheel sensor
11
12 Rear wheel sensor
13 Front wheel sensor
14 ESP switch
15 Rear brakes
16 Front brake
The ABS monitors the different wheel speeds of the vehicle with the aid of wheel speed
sensors. Using the data from all of the wheel speed sensors, the ABS module calculates the
so-called reference speed, which is a measure of the actual road speed. The ABS module
compares the individual circumferential wheel speeds with this reference speed when the
driver initiates braking. If one or more of the circumferential wheel speeds deviates too far
from the reference speed, this means that slip at the affected wheels is so great that
steering stability of the vehicle is no longer ensured. The ABS module actuates electro-
mechanical valves which influence the brake pressure at the relevant wheels.
Like the traction control system (TCS), the ESP system uses a large proportion of the ABS
components. In addition, there are sensors which pick up the steering angle, the
acceleration forces acting on the vehicle and the yaw rate or yaw moment. The sensors
transmit these signals to the combined ABS/ESP module. Using the wheel speed and
steering angle data, the ABS/ESP module calculates the direction of travel planned by the
driver and determines the corresponding speed-dependent lateral acceleration and yaw
moment. These values are compared with those actual measured. If the actual lateral
acceleration and the yaw moment deviate excessively from the target values (unstable
driving characteristics), the ABS/ESP module actuates individual brakes selectively via the
HCU (hydraulic control unit). In addition, the engine speed is reduced by intervention in the
engine management system.
How the system works for understeer: In the event of understeer, brake intervention
occurs at the wheels on the inside of the curve. The rear wheel is braked heavily, so that a
high amount of slip is caused. In this way, the cornering force of the rear axle is heavily
reduced and the centrifugal force that now becomes effective turns the rear of the vehicle
back into the curve. The front wheel is not braked as hard. The braking force that is
transmitted via the front wheel to the road surface generates a torque with the aid of the
lever arm (vertical tire force to the vehicle's centre of gravity), which supports the yaw
moment of the vehicle. Both measures together result in the vehicle reverting back to the
curved path intended by the driver.
How the system works for oversteer: In the event of oversteer the wheels on the
outside of the curve are braked. This time, the front wheel is subjected to a high level of
slip so that the cornering force at the front axle is reduced. The rear wheel is not braked as
heavily and, together with the effective lever arm, results in a reduction in the vehicle yaw
moment. Both measures together result in the vehicle being stabilized and reverting back to
the curved path intended by the driver.
If ESP control occurs, possible ABS interventions will be overridden as the ESP works at
higher slip rates than the ABS.
Emergency brake assist (EBA): The emergency brake assist helps drivers in emergency
braking situations by automatically applying the brakes with the maximum possible braking
force.
If the brake pedal is pressed very suddenly, the ABS module increases the hydraulic
pressure to all of the brakes until the threshold for ABS intervention is reached. This action
applies the maximum braking effort for the available traction. The ABS control unit monitors
inputs from the brake pedal switch and from the pressure sensor within the HCU to check
for sudden actuation of the brakes. With the brake pedal pressed, the ABS module triggers
emergency braking if the rate of increase of hydraulic pressure exceeds the predetermined
limit.
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