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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