SsangYong Stavic / SsangYong Rodius (2005 year). Manual - part 405

ABS/ESP SYSTEM

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ABS SYSTEM

1. GENERAL INFORMATION

The aim of the ABS is to mmaintain steerability and driving stability and to take the burden off the driver. If the stopping
distance is shorter on some road surfaces (carriageway conditions), this is a gift of physics and not a development aim.
ABS is a device which senses that one or more of the wheels are locking up during braking. It monitors the rotational
speeds of the wheels and reduces hydraulic pressure to any wheel it senses locking up. It is controlled by both
mechanical and electronic components. When you apply the brakes, the ABS will regulate the flow of brake fluid being
delivered to the brake calipers. By the use of electronic computers, the brakes rapidly alternate (at a rate of 30 times per
second) from full pressure to full release.

DRIVING PHYSICS

To give you a better understanding of the tasks and functions of ABS, we will first look at the physics principles.

The Stopping Distance

The stopping distance depends on the vehicle weight and initial speed when braking starts. This also applies for vehicle
with ABS, where ABS always tries to set an optimum brake force on each wheel. As great forces are exerted between
the tires and the carriageway when braking, even with ABS the wheels may scream and rubber is left on the road. With
an ABS skid mark one may be able to clearly recognize the tire profile. The skid mark of an ABS vehicle does not
however leave any hint of the speed of the vehicle in the case of an accident, as it can only be clearly drawn at the start
of braking.

Brake Force on a Wheel

The maximum possible brake force on a wheel depends on the wheel load and the adhesion coefficient between tire and
carriageway. With a low adhesion coefficient the brake force, which can be obtained is very low. You are bound to know
the result already from driving on winter roads. With a high adhesion coefficient on a dry road, the brake force, which can
be obtained, is considerably higher. The brake force, which can be obtained, can be calculated from below formula:

Maximum brake force

FB

max 

= Vehicle Weight

 

 x Adhesion coefficient

mh

The braking process cannot be described sufficiently accu-
rately with the brake forces calculated. The values calculated
only apply if the wheel is not locked. In the case of a locking
wheel, the static friction turns into lower sliding friction, with
the result that the stopping distance is increased. This loss of
friction is termed “slip” in specialist literature.

Brake force on a wheel

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Slip

The brake slip is the difference between the vehicle speed and the wheel circumference speed. If the wheel locks, the
slip is greatest, that is 100 %. If the wheel is running freely and unbraked, the slip is the lowest, equal to 0 %. Slip can
be calculated from the vehicle speed Vveh and the wheel speed V

w

. The equation for this is:

S =

         X 100 %

V

veh 

= 100 km/h, V

w 

= 70 km/h

S =

         X 100 %

The slip is 30 %.

V

veh

V

veh - 

V

w

100

100 - 70

Typical slip curves

For the various carriageway conditions the adhesion coeffi-
cients were plotted. The typical course of the curves is al-
ways the same. The only special feature is shown by the
curve for freshly fallen snow, for this curve increases at 100 %
slip. In a vehicle without ABS, the wheel locks on braking and
therefore pushes a wedge before it. This wedge of loose sur-
face or freshly fallen snow means and increased resistance
and as a result the stopping distance is shorter. This reduc-
tion in stopping distance is not possible with a vehicle with
ABS, as the wheel does not lock. On these surfaces the stop-
ping distance with ABS is longer than without ABS. The rea-
son for this is based in physics and not in the Anti-Lock
System. However, as mentioned before, ABS is not about the
stopping distance, but maneuverability and driving stability,
for with ABS you can steer round an obstacle. A device with
locking wheels without ABS cannot be steered. So what use
then is the shorter stopping distance if the vehicle has already
hit the car in front, because you did not have a chance to
steer round the obstacle?

Kamm circle

Before we go into the Kamm circle, you should know that a
tire offers a maximum of 100 % transmissibility. It is all the
same for the tire whether we require 100 % in the direction of
braking or in the direction of the acting lateral force, e.g. when
driving round curves. If we drive into a curve too fast and the
tire requires 100 % transmissibility as cornering force, the tire
cannot transmit any additional brake force. In spite of the ABS
the car is carried out of the curve. The relationship between
brake force B and cornering force S is shown very clearly in
the Kamm circle. If we put a vehicle wheel in this circle, the
relationship becomes even clearer. In this relationship: as long
as the acting forces and the resulting force remain within the
circle, the vehicle is stable to drive. If a force exceeds the
circle, the vehicle leaves the road.

Dry concrete

Wet concrete

Snow

Ice

Friction coefficient

<Typical slip curves>

Cornering

side force

Braking force

KAMM circle

Slip

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Brake and cornering force

- Brake force

When depressing the brake pedal the brake force increases
to the maximum, then the brake force decreases until the
wheel locks.

- Cornering force

The cornering force is a maximum when the wheel is
turning freely with zero slip. When braking the cornering
force falls to zero if the wheel locks (slip 100 %).

- ABS operating range

The operating range starts just before the maximum brake
force and ends in maximum, for the unstable range then
begins, in which no further modulation is possible. The
ABS controls the regulation of the brake pressure so that
the brake force only becomes great enough for a sufficient
proportion of cornering force to remain. With ABS we
remain in the Kamm circle as long as the car is driving
sensibly. We will leave driving physics with these
statements and turn to the braking systems with and
without ABS.

Basic ABS Controls

Applications of the ABS control unit

The signals produced by the wheel sensors are evaluated in
the electronic control unit. From the information received, the
control unit must first compute the following variables:

 - Wheel speed

 - Reference speed

 - Deceleration

 - Slip

Reference speed

The reference speed is the mean, I.e. average speed of all wheel speeds determined by simple approximation.

Simplified ABS control

If, during braking, one wheel speed deviates from the reference speed, the ABS control unit attempts to correct that
wheel speed by modulating the brake pressure until it again matches the reference speed. When all four wheels tend to
lock, all four wheels speeds suddenly deviate from the previously determined reference speed. In that case, the control
cycle is initiated again in order to again correct the wheel speed by modulating the brake pressure.

Selector low control

This control is used for regulating the brake pressure for rear axle during ABS operation. This control uses lower
adhesion coefficient to prevent the rear wheels from locking.

ABS-Control range

Stable

Unstable

Brake force

  Cornering force

Slip

Brake pressure

Reference speed

Vehicle speed

Wheel speed

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Front axle braking force

Rearaxle braking force

System Description

As an add-on logic to the ABS base algorithm, EBD works in
a range in which the intervention thresholds for ABS control
can not be reached.

EBD ensures that the rear wheels are sensitively monitored
for slip with respect to the front axle. If slip is detected, the
inlet valves for the rear wheels are switched to pressure hold
to prevent a further increase in pressure at the rear-wheel
breaks, thus electronically reproducing a pressure-reduction
function at the rear-wheel brakes.

ABS features an enhanced algorithm which includes control
of the brake force distribution between the front and rear axles.
This is called Electronic Brake Distribution. In an unloading
car condition the brake efficiency is comparable to the con-
ventional system but for a fully loaden vehicle the efficiency of
the EBD system is higher due to the better use of rear axle
braking capability.

The Benefits of EBD

• Elimination of conventional proportioning valve EBD utilizes

the existing rear axle wheel speed sensor to monitor rear
wheel slip.

• Based on many variables in algorithm a pressure hold,

increase and/or decrease pulsetrain may be triggered at
the rear wheels insuring vehicle stability.

• Vehicle approaches the ideal brake force distribution (front

to rear).

• Constant brake force distribution during vehicle lifetime.

• EBD function is monitored via ABS safety logic

(conventional proportioning valves are not monitorable).

• “Keep alive” function.

EBD (ELECTRONIC BRAKE FORCE DISTRIBUTION) SYSTEM

Service precautions

Observe the following general precautions during any
ABS/TCS service. Failure to adhere to these precau-
tions may result in ABS/TCS system damage.

1. Disconnect the EBCM harness connector before

performing the electric welding procedures.

2. Carefully note the routing of the ABS/TCS wiring

and wiring components during removal. The ABS/
TCS components are extremely sensitive to EMI
(eletromagnetic interference). Proper mounting is
critical during component service.

3. Disconnect the EBCM connector with the ignition

OFF.

4. Do not hang the suspension components from

the wheel speed sensor cables. The cables may
be damaged.

5. Do not use petroleum based fluids in the master

cylinder. Do not use any containers previously
used for petroleum based fluids. Petroleum
causes swelling and distortion of the rubber
components in the hydraulic brake system,
resulting in water entering the system and
lowering the fluid boiling point.