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3.1 Dynamic features

The evolution in the field of brakes for motorcars has lead to the realization of efficient and reliable braking systems, able to perfectly slow down the motorcars even at high speeds.
Therefore, a car can be braked and stopped in a safer and quicker way in the normal road traffic.
In critical run situations (wet or slippery roadbed, sudden reaction of the driver in front of a sudden obstacle, uncorrect behaviour of other road users, etc.) the driver can react causing the locking of the wheels during braking, so that the vehicle cannot be controlled any more and tends to go away from the roadway or even to veer.

The ABS anti-lock system intervenes in a situation of this type, identifying on time the tendency to locking one or more wheels and immediately providing for keeping constant or reducing the braking pressure.
In this way the vehicle responds to the controls of the steering, it remains stable and the braking is excellent.
Therefore, the ABS system offers a decisive contribution to the road safety because it helps the driver to overcome difficult braking situations (figures 25 and 26).

	Fig. 25: braking without ABS The tracks show that the wheels have locked and that the vehicle has veered.
	Fig. 26: braking with ABS The vehicle keeps control and excellent directional stability also in case of full braking.

ABS requirements
The ABS must satisfy many safety requirements, in particular those relevant to the braking dynamics and the technique of the braking systems:

the regulation of the braking must guarantee the stability and the control on every type of roadbed (from those dry and with good adherence up to the icy roadbeds).

The ABS must be able to use in an optimum way the braking capacity of the wheels on the roadbed, where the running stability and the control have a primary importance with respect to the reduction of the braking distance. On the contrary, no importance must have the fact that the driver suddenly presses the brake pedal or gradually increases the braking pressure up to the limit of locking.

The regulation of the braking must work in the whole field of speed of the vehicles, including the speed at walking pace; if the wheels lock at this low speed, the remaining trajectory of the vehicle is not critical up to stop.

The regulation of the braking must rapidly adapt itself to the different degree of adherence of the roadbed. For instance, on a dry roadbed, covered by isolated layers of ice, the locking of the wheels (if any) must be limited to times so short as not to jeopardize the run stability and the control. On the other side, the adherence on the dry part of the roadbed must be used at its best.

During braking on a roadbed with non uniform adherence (for instance, wheels on the right hand side on ice, wheels on the left hand side on dry asphalt) the torsional torques (torques around the vertical axis of the vehicle, that tend to make the car rotate perpendicularly to the run direction), that inevitably occur, must increase so slowly as to allow the normal driver to easily compensate them by countersteering.

On a bend the vehicle must remain stable and controllable during braking and have, also in this case, a braking distance as short as possible, provided that the speed of the vehicle be sufficiently lower than the speed limit on the bend (with speed limit on a bend we mean the speed of the vehicle, at which a bend of a given radius can still be taken without the vehicle moving from its trajectory).

Even on a rough roadbed run stability. control and excellent deceleration with any braking intensity must be preserved.

The braking regulation must detect the presence of aquaplaning (floating of the wheels on a roadbed covered by water) and react in an optimum way. Stability and straight run of the vehicle must be guaranteed.

The adaptation to the hysteresis of the braking (resumption of the braking after releasing the brake) and to the influences of the engine (if the braking is made during the engagement of the clutch) must happen as quickly as possible.

The oscillations of the vehicle due to the presence of vibrations must be avoided.

A circuit must permanently control the perfect operation of the anti-lock system. If it detects a fault that can jeopardize the braking behaviour, the ABS is disconnected. A warning lamp shows the driver that now only the basic braking system without ABS is working.

Dynamics of the braked wheel
Figures 27 and 28 show the physical relationships during braking with l'ABS, the ABS regulation fields are dotted.

	Fig. 27: coefficient of the braking force N_Bas a function of the slide S_C during straight braking with the ABS regulation fields. 1 Radial tyres on dry concrete 2 Diagonal winter tyres on wet asphalt 3 Radial tyres on snow 4 Radial tyres on wet ice Dotted zones = ABS regulation fields
	Fig. 28: coefficient of the braking force N_B and of the side force N_S as a function of the slide S_C during braking and of the drift angle a with ABS regulation fields Dotted zones = ABS regulation fields

The behaviour (figure 27) of the curves 1 (dry roadbed), 2 (wet roadbed) and 4 (icy roadbed) shows that with the ABS it is possible to obtain shorter braking distances with respect to a braking with locked wheels (Sc = 100 %).
In curve 3 (snow) the snow wedge formed in front of the wheels guarantees a supplementary braking effect with the locked wheels; in this case the advantage of the ABS is in the maintenance of the directional stability and of the control.

As shown by the two curves of the coefficient of the braking force NB and of the coefficient of the side force NS in figure 28, the field of regulation of the ABS must be widened for the drift angle of 10° (that is, high side force due to a strong transversal acceleration of the vehicle) with respect to the drift angle of 2°; in case of full braking on a bend, with a strong transversal acceleration, the ABS intervenes in advance and allows an initial braking slide not higher than 10 %. In fact, with drift angle equal to 10°, to the braking slide of 10 % corresponds a coefficient of the braking force equal to just 0.35, while the coefficient of the side force, 0.80, still practically has its maximum value.

Insofar as the speed and consequently the transversal acceleration decrease during a braking on a bend, the ABS allows higher and higher slide values, so that the deceleration increases while decreases the coefficient of the side force in correspondence to the reduction of the transversal acceleration.
During a braking on a bend the increase of the braking forces is such as to make the braking distance just a little longer of that during a straight braking in the same conditions.