Iveco Daily. Manual — part 387

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Base - May 2004

The braking of a vehicle in motion and the consequent

deceleration and stopping space mainly depend on the grip

between the tyre surface and the type of road surface.
With a perfectly efficient braking system, further

improvement of braking can be obtained only acting on the

tyre friction characteristics or on the quality of the road

surface.
Even in these optimum conditions, absolute braking safety

is not however guaranteed when needing to cope with

particular critical situations, such as low grip due to the

conditions of the wet or icy road surface: this compels the

driver to moderate the braking action to prevent one or

more wheels from partially locking, with the possibility of

dangerous skidding.
The friction between the tyre and the road surface does not

correspond to the friction between rigid bodies but to the

skidding (or slipping) between the tyre and the road in the

contact area.

The figure shows the indicative trend of the longitudinal µl"

and transversal µt" rip coefficients in relation to the

percentage of slipping "S".

The diagram reveals that the maximum value of the

longitudinal grip coefficient is not when the wheel is locked,

but for a much lower slipping value.

Therefore, the road-tyre grip can be exploited to optimise

longitudinal or transversal control of the vehicle. Namely, it

is possible to utilise the grip trying to make the braking

distance as short as possible or to ensure the best possible

handling.

An excellent compromise is obtained using slip rates near

the point S* in which there is a high µl value which ensures

optimum braking and a µt value that offers good lateral

roadholding.

TREND OF WHEEL LONGITUDINAL AND TRANSVERSAL GRIP IN RELATION TO SLIPPING

WHEEL

VEHICLE

TRANSVERSAL GRIP

LONGITUDINAL GRIP

STABLE

AREA

OPTIMUM AREA

UNSTABLE

AREA

Figure 111

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In order to better understand the logic of the system and the

parameters that govern it a few basic concepts should be

briefly examined.
Due to the effect of the braking action the tyre, that before

was rolling freely, undergoes a deformation, called "braking"

deformation, in its area of contact with the road and slows

down rotation reducing its peripheral speed, to a higher

extent than the linear speed of the vehicle.
At the limit, with the wheels completely locked under the

braking action, and thus with a wheel peripheral speed of

zero, there is the maximum deviation between the wheel

rotation speed and the linear speed of the vehicle.
Slipping varies its values within the limits set by the two

extreme conditions in which the wheel and vehicle speed

may be in.
When the wheel is free, not braked, thus turning at the same

speed as the vehicle, the slipping coefficient is 0%.
When the braked wheel is completely locked and the

vehicle continues moving forwards due to inertia, the

skidding coefficient amounts to 100%.

Experimentation has made it possible to establish that the

most effective braking condition is obtained for optimum

slipping values contained between about ∼ 5% and 20%.

The need to contain the slipping values within precise limits

is imposed by the behaviour of the tyres under the braking

action, during which the braking friction coefficient comes

into play.

The higher this coefficient, the more braking is effective.

If the relation between slipping S and the friction coefficient

U is represented on Cartesian axes µ, we see how there are

the highest friction coefficient values for slipping values

between an average of 5% and 20%.

As the friction coefficient is directly proportionate with the

applicable braking force, the result is that the "ABS" device

acts in such a way as to apply the maximum braking force

exactly in correspondence with the best friction coefficient,

and this system tends to bring any type of vehicle within this

sector.

FRICTION COEFFICIENT

S. Slipping % - A. Dry asphalt - B. Wet asphalt - C. Ice

Figure 112

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Base - May 2004

on the brake pedal, (anyway higher than allowed by the grip)

can cause loss of steering control of the vehicle if both front

wheels lock, skidding, resulting in about-facing, if both rear

wheels lock.
The need for abrupt braking on a bend, compels the driver to

immediate action to avoid losing control of the vehicle,

resulting in skidding.
Lastly, the so-called panic braking (caused by a sudden

obstacle), compels the driver to press the brake pedal as

forcibly as possible in the desperate attempt to stop in time..
So what is the solution even for the most expert drivers?
Being able to avail of a braking system capable of fully

exploiting all the grip available without locking the wheels,

except at a minimum pre-established speed.
The ABS Antibrake Locking System has been developed

exactly to obtain this.
A device inserted in the braking system, with the task of

preventing wheel locking when the brake operating pressure

is too high in relation to the grip of the tyre on the ground.
Therefore, the purpose of the "ABS" device is to ensure

vehicle stability (under all braking conditions) preventing

locking of the wheels regardless of the conditions of the road

surface, thereby ensuring total use of the grip available.
Also in the vent of emergency braking, the system makes it

possible to maintain "steerability" of the vehicle, i.e. acting on

the steering to avoid obstacles, without the danger of skidding.
Keeping the rotation and grip of the wheels within the

optimum parameters, the system makes it possible to obtain

those braking distances that only an expert driver would be

able to approach; this way even the less expert driver is able

to act like one of the best.
The diagram opposite shows some examples of braking

without ABS and with ABS.
In order to be able to intervene effectively the system must

not only be precise in response, but also very quick.
This is now possible thanks to electronic information which

warrants reliability, precision and rapidity, with a minimal

number of components and lower system cost.

VEHICLE BRAKING TREND

WITHOUT AND WITH ABS

A. Vehicle without ABS - B. Vehicle with ABS

A

A

B

B

Figure 113

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In addition to the ABS, the system in question also

incorporates the EBD and ABD functions.
EBD - Electronic Brake Force distribution
This device replaces and optimises the function of the

present hydraulic braking action proportioning valve, better

controlling the braking force on the rear wheels.
It is obtained by adding specific software to the ABS and it

acts in a determinate interval before the cutting in of the

ABS.
It ensures control on any locking of the rear wheels in

relation to the front ones, optimising the braking force in the

different load, driving and conditions of use of the vehicle.

ABD - Automatic Brake Differential
This device comes into action automatically braking the

driving wheel that has less grip when moving off or travelling

and tends to skid transferring the torque through the

differential to the wheel with higher grip.

It cuts in up to a speed of about 40 km/h and does not

interfere with the normal braking system.
It cuts in up to a speed of about 40 km/h and does not

interfere with the normal braking system.
The driver is informed when this system is engaged by the

flashing of th yellow warning light on the dashboard.

SYSTEM WARNINGS ON DASHBOARD

A. ABS failure warning light 58703 - B. EBD failure warning light 58734 - C. ABD failure warning light 58704

A

C

B

ABS

EBD

Figure 114