Page 17 - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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2 Automobile mechanical and electrical systems

Figure 1.3 Rear-engine, rear-wheel drive Figure 1.4 Mid-engine, rear-wheel drive

Table 1.1 Common abbreviations
FWD Front-wheel drive
RWD Rear-wheel drive

AWD All-wheel drive
4WD Four-wheel drive

A common layout for a standard car is the front-engine, front-wheel drive vehicle.
This is because a design with the engine at the front driving the front wheels has
a number of advantages:

It provides protection in the case of a front-end collision.
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Engine cooling is easier because of the air fl ow.
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Cornering can be better if the weight is at the front.
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Front-wheel drive adds further advantages if the engine is mounted sideways-
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on (transversely).
There is more room in the passenger compartment.
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The power unit can be made as a complete unit.
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The drive acts in the same direction as the steered wheels are pointing.
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Rear-wheel drive from a front engine was the method used for many years. Some
manufacturers have continued its use, BMW for example. A long propeller shaft
from the gearbox to the fi nal drive, which is part of the rear axle, is the main
feature. The propshaft has universal joints to allow for suspension movement.
This layout has some advantages:
Weight transfers to the rear driving wheels when accelerating.
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Complicated constant velocity joints, such as used by front-wheel drive
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vehicles, are not needed.
Four-wheel drive combines all the good points mentioned above but makes the
vehicle more complicated and therefore expensive. The main difference with
four-wheel drive is that an extra gearbox known as a transfer box is needed to
link the front- and rear-wheel drive.