SsangYong Actyon Sports II. Manual — part 149

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5. SYSTEM OPERATION

1) Block Diagram of ABS HECU

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2) Basic Theory of ABS Function

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

principles.

(1) Stopping distance

(2) 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

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FBmax = wheel load FR x coefficient of

frictionMh

The braking process cannot be described

sufficiently accurately 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.

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.

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Slip

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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 un-braked, the slip is the

lowest, equal to 0 %. Slip can be calculated from the vehicle speed Vveh and the wheel speed Vw. The

equation for this is:

Vveh = 100 km/h, Vw = 70 km/h

Slip ratio (S) =

X 100%

S = 30%

Vveh - Vw

Vveh

Typical Slip Curves

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For the various road conditions, the friction

coefficients were plotted. The typical course of

the curves is always 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 surface or freshly fallen

snow means and increased resistance and as

a result the stopping distance is shorter. This

reduction in stopping distance is not possible

with a vehicle with ABS, as the wheel does not

lock. On these surfaces the stopping distance

with ABS is longer than without ABS. The

reason 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 the

vehicle with locking wheels without ABS cannot

be steered.

Ex)

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KAMM circle

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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.

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.

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Brake and cornering force

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