Page 362 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Braking systems C HAPTER 12.1

1.0
0.9 k = z + 0.07
k = 0.8
0.85
0.8

0.7
Adhesion utilization (k) 0.5 k = z z = 0.8
0.6

0.4

0.3
k = z + 0.05
0.2
k = 0.2
0.1

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Deceleration (z)
Figure 12.1-2 Adhesion utilization diagram for a category M 1 vehicle.

increase in the size and complexity of the regulations that must be satisfied by a road vehicle relates to its use
pertaining to braking. The first major change away from of available tyre–ground adhesion. Manufacturers have to
the self-certification process came when the UK com- submit adhesion utilization curves that demonstrate
menced along the road of Type Approval, favoured by compliance with the limits defined in figures IA and IB of
continental Europe, by the incorporation of the Eco- EEC Directive 71/320 Annex II. The adhesion utilization
nomic Commission for Europe (ECE) Regulations. These diagram that refers to a category M 1 vehicle (passenger
voluntary regulations attempt to harmonize vehicle leg- vehicle with seating capacity up to eight including the
islation and they provide for the reciprocal acceptance driver) is shown in Figure 12.1-2, and is derived from
and notification of vehicle systems and component ap- that contained within EEC Council Directive 71/320/
proval. The relevant braking legislation is contained in EEC as last amended by 91/422/EEC.
Regulation 13. The next major change occurred when the The Directive uses the letter k to indicate adhesion
UK became a member of the Common Market in which utilization and z for deceleration and it states that for all
the acceptance of EEC Directives is binding on all categories of vehicle for values of adhesion utilization
member states. Member states are not allowed to impose between 0.2 and 0.8,
more stringent standards than those contained in the
Directives. They are, however, free to demand additional z 0:1 þ 0:85ðk 0:2Þ
standards with regard to matters not covered by EEC
legislation. EEC Directives differ from the corresponding For category M 1 vehicles, the adhesion utilization
ECE Regulations in one major aspect: Approvals issued curve of the front axle must be greater than that of the
by one member must be accepted by all others. The rear for all load cases and values of deceleration between
objective of either is, however, the same, namely the 0.15g and 0.8g. Between deceleration levels of 0.3g and
harmonization of differing technical requirements. In 0.45g, an inversion of the adhesion utilization curves is
1978, it became mandatory in the UK for all new cars to allowed provided the rear axle adhesion curve does not
be type approved to the EEC Braking Directives. A exceed the line defined by k ¼ z by more than 0.05. The
similar exemption was granted in 1980 for vehicles ap- above provisions are applicable within the area defined by
proved to ECE Regulation 13. The current EEC Di- the lines k ¼ 0.8 and z ¼ 0.8.
rective on braking is Directive 71/320/EEC as last Compliance of the braking system to the constraints
amended by Directive 91/422/EEC. defined in Figure 12.1-2 ensures that the rear wheels do
Of the many requirements laid down in the EEC not lock in preference to the front wheels and that the
regulations, perhaps the single most important aspect proportion of braking effort exerted at the front of

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