Lexus RX300 (MCU15). Service manual - part 54

CH

BO

CHASSIS — BRAKES

Direction of Travel of the Vehicle’s
Center of Gravity

Slip Angle

Movement
of Vehicle

151CH18

Making a Right Turn

Braking
Force

Understeering
Control Moment

Braking Force

161ES30

Braking Force

Making a Right Turn

Braking
Force

Oversteering
Control Moment

170CH07

89

2) Determining Oversteer

Whether or not the vehicle is in the state of over-
steer is determined by the values of the vehicle’s
slip angle and the vehicle’s slip angular velocity
(time-dependent changes in the vehicle’s slip
angle).  When the vehicle’s slip angle is large, and
the slip angular velocity is also large, the ECU
determines that the vehicle has a large oversteer
tendency.

Method of VSC Operation

When the skid control ECU determines that the vehicle exhibits a tendency to understeer or oversteer,
it decreases the engine output and applies the brake of a front or rear wheel to control the vehicle’s yaw
moment.

1) Dampening a Strong Understeer

When the skid control ECU determines that the
vehicle exhibits a strong tendency to understeer,
depending on the extent of that tendency, it con-
trols the engine output and applies the brakes of
the front wheels and inside rear wheel, thus pro-
viding the vehicle with an understeer control mo-
ment, which helps dampen its tendency to under-
steer.  Also, depending on whether the brakes are
ON or OFF and the condition of the vehicle, there
are circumstances in which the brakes might not
be applied to the wheels even if those wheels are
targeted for braking.

2) Dampening a Strong Oversteer

When the skid control ECU determines that the
vehicle exhibits a strong tendency to oversteer,
depending on the extent of that tendency, it con-
trols the engine output and applies the brakes of
the front and rear wheels of the outside of the
turn, thus generating an inertial moment in the
vehicle’s outward direction, which helps dampen
its tendency to oversteer.

CHASSIS — BRAKES

90

6. Outline of Brake Assist System

Brake Assist interprets a quick push of the brake pedal as emergency braking and supplements the braking
power applied if the driver has not stepped hard enough on the brake pedal.
In emergencies, drivers, especially inexperienced ones, often panic and do not apply sufficient pressure
on the brake pedal.
Brake Assist system measures the speed and force with which the brake pedal is pushed to determine whether
the driver is attempting to brake rapidly, and applies additional pressure to maximize braking performance
of both conventional brakes and ABS equipped brakes.
A key feature of Brake Assist is that the timing the degree of braking assistance are designed to ensure
that the driver does not discern anything unusual about the braking operation.  When the driver intentionally
eases up on the brake pedal, the system reduce the amount of assistance it provides.

— REFERENCE —
Effectiveness of the Brake Assist Operation:

a.

During emergency braking, an inexperienced driver, or a driver in a state of panic might not be able
to firmly depress the brake pedal, although driver can depress it quickly.  As a result, only a small
amount of brake force is generated.

b. The pedal effort of this type of driver might weaken as time passes, causing a reduction in the braking

force.

c.

Based on how quickly the brake pedal is depressed, the Brake Assist operation assesses the intention
of the driver to apply emergency braking and increases the brake force.

d. After the Brake Assist operation, if the driver intentionally releases the brake pedal, the assist operation

reduces the amount of Brake Assist in order to reduce the feeling of uneasiness.

d

170CH18

Braking
Force

With Brake Assist System

Without Brake Assist System

a

b

c

Time

CH

BO

CHASSIS — BRAKES

Loaded State

Normal State

181CH53

181CH54

91

7. Outline of EBD Control

General

The EBD control utilizes ABS, realizing the proper brake force distribution between front and rear wheels
in accordance with the driving conditions.
In addition, during cornering braking, it also controls the brake forces of right and left wheels, helping
to maintain the vehicle stability.
The distribution of the brake force, which was performed mechanically in the past, is now performed
under electrical control of the skid control ECU, which precisely controls the brake force in accordance
with the vehicle’s driving conditions.

Front/Rear Wheels Brake Force Distribution

If the brakes are applied while the vehicle is moving
straight forward, the transfer of the load reduces
the load that is applied to the rear wheels.  In this
case, if the same amount of brake force is applied
to the four wheels, the rear wheels with the smaller
load will become susceptible to locking.  The skid
control ECU determines this condition by way of
the signals from the speed sensor, and regulates the
brake actuator in order to optimally control the dis-
tribution of the brake force to the rear wheels.
For example, the amount of the load that is applied
to the rear wheels during braking varies whether
or not the vehicle is carrying a load.  The amount
of the load that is applied to the rear wheels also
varies in accordance with the extent of the decelera-
tion.  Thus, the distribution of the brake force to
the rear is optimally controlled in order to effective-
ly utilize the braking force of the rear wheels under
these conditions.

" EBD Control Concept A

181CH55

Rear
Brake
Force

Ideal Distribution
in Normal State

Ideal Distribution in Loaded State

EBD Control in Loaded State

EBD Control in Normal State

Front Brake Force

CHASSIS — BRAKES

181CH56

92

Right/Left wheels Brake Force Distribution (During cornering braking)

When the brakes are applied while the vehicle is
cornering, the load that is applied to the inner wheel
decreases.  In this case, if the same amount of brake
force is applied to the four wheels, the inner wheel
with the smaller load will become susceptible to
locking.  The skid control ECU determines this con-
dition by way of the signals from the speed sensor
and deceleration sensor, and regulates the brake ac-
tuator in order to optimally control the distribution
of the brake force to the inner wheel.

Brake Actuator (ABS with EBD & Brake Assist & TRC & VSC Actuator)

1) Construction

The brake actuator consists of 14 two-position solenoid valves, 1 motor 2 pumps, 2 reservoirs, 4 pressure
regurator valves ((1)) and master cylinder pressure sensor.
The 14 two-position solenoid valves consist of 4 master cylinder cut solenoid valve ((2), (3), (6), (7)),
2 reservoir cut solenoid valves ((4), (5)), 4 pressure holding valves ((8), (9), (10), (11)), and 4 pressure
reduction valves ((12), (13), (14), (15)).
Pressure regulator valve ((1)) is assembled into the master cylinder cut solenoid valve ((2), (3), (6), (7)).

2) Hydraulic Circuit

161ES35

Master
Cylinder

Master Cylinder
Pressure Sensor

(1)

(1)

(2)

(3)

(1)

(4)

(5)

(6)

(7)

(1)

(8)

(9)

(12)

(13)

(14)

(15)

(10)

(11)

Front Right
Wheel Cylinder

Rear Left
Wheel Cylinder

Rear Right
Wheel Cylinder

Front Left
Wheel Cylinder

Reservoirs

Pumps