Page 361 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 2. 1 Braking systems

1 Brake circuit 1 such documents forms a very important part of the brake
2 Brake circuit 2
II variant X variant engineer’s database of information.
There are many arguments both for and against legis-
Figure 12.1-1 Common brake system layouts. lation. However, despite some inevitable drawbacks
standards and legislation form a necessary and desirable
part of today’s society and they are here to stay. Poorly
the rear axle then these are typically of the Simplex type written legislative documents may smother initiative and
which employs a leading and trailing shoe configuration restrict technical progress by enforcing unrealistic stan-
to generate the required brake torque. The torque output dards and by failing to recognize the advanceof technology.
of this type of drum brake is not sensitive to change in The phrasing of the documents is somewhat complex
vehicle direction. On vehicles fitted entirely with disc which can lead to difficulties in understanding their con-
brakes, then a small drum unit is often employed to act as tent. Thisis,toalargeextent,unavoidable because theyare
a parking brake on the rear axle of the vehicle. Issues legal documents and they must attempt to cover all
surrounding the selection of the materials used to man- eventualities, prevent ambiguity and close any loopholes.
ufacture both discs and drums together with their fric- By default, proof of compliance with a national or in-
tion material partners are discussed in more detail later in ternational standard generates an overhead which is
the text. transferred to the consumer as an added cost. It is essential
that this process does not inhibit either new or small
manufacturers from entering the marketplace. Finally,
12.1.1.3.6 Brake system layouts
national legislation can be used as an economic weapon
Legislative requirements demand a dual circuit trans- (termed a technical barrier to trade), particularly by those
mission system to be installed on all road vehicles. Of countries operating an approval system, to protect their
the five possible configurations, two have become stan- industry from worldwide intrusion into the local market.
dard and these are known as the II and X variants shown The job of the importer is made very difficult through the
in Figure 12.1-1. The II design is characterized by sep- use of standards and test procedures that favour the home
arate circuits for both the front and rear axles whilst in industry or by withholding interpretations, by allowing the
the X configuration, each circuit actuates one wheel at test authority the use of subjective judgement and by
the front and the diagonally opposed rear wheel. The II showing a lack of co-operation in test scheduling and in the
design is often found on vehicles that are rear heavy and issue of approval documents.
the X layout has application on vehicles that are front Technical standards and legal requirements need to
heavy. be kept under review in order to force quality upwards.
They must set realistic standards for new and in-service
vehicles that result in real improvements in safety and
12.1.2 Legislation environmental protection. Bearing in mind the necessary
legal constraints, they should be straightforward to
Without exception, motorized road vehicles, whether understand and interpret as well as be universally
cars, buses or lorries, represent a potentially lethal hazard acceptable to encourage free trade and so prevent the
to other road users and pedestrians. Also the rise in production of trade barriers. Standards and legislation
‘green thinking’ during the past decade has led to serious must also be applicable to all types of vehicle and should
consideration of the impact of road vehicles on the en- not preclude innovative design by being so inflexible as
vironment in which we all must live. It is one of the many to limit technical advance; ideally they must actively
responsibilities of a government to ensure that all road encourage the use of new technology. Given time,
vehicles are as safe as possible and that any adverse ef- companies incorporate the formal test procedures in
fects of the vehicle on the environment are minimized. their design programmes and develop close working
This task is achieved through legislation which, in so far relationships with the national approval bodies. In
as the brakes are concerned, primarily sets the minimum principle this leads to improved export performance
standards for the performance of the systems and their since approval obtained in the country of manufacture is
components that combine to arrest the motion of a automatically valid for all others. Also the clearly
vehicle in a controlled manner. A design engineer has to defined ground rules quite often act as an aid to product
take into account many factors associated with the development.
mechanics of braking when designing a new brake With regard to the braking system, legislation first
system. In addition to these elements, conformity to the appeared in the form of the Motor Cars Order of 1904.
legislative requirements of the country or countries in Since this time, the range and complexity of the vehicles
which the vehicle is to operate is absolutely essential. that populate the road network has markedly increased.
Thus a working knowledge of the content and scope of Inevitably this has been accompanied by a similar

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