Four wheel drive layouts
Note: in the United States the term "four-wheel drive" usually refers only to transmissions which are primarily two-wheel drive with a part-time four-wheel drive capability, as typically found in older designs of pickup trucks, while the term "all-wheel drive" is used to refer to full time four-wheel drive systems found in performance cars and off-road vehicles. This section uses the term four-wheel drive to refer to both, as per the rest of the world.
Most 4WD layouts are front-engined and are derivatives of earlier front-engined, two-wheel drive designs. They fall into two major categories:
Longitudinal-engined 4WD systems are typically (but not always) derived from front-engined, rear-drive layouts, for example
Mercedes-Benz 4-matic
Transverse-engined 4WD systems are derived almost exclusively from front-engined, front-drive layouts, for example in the
Mitsubishi 3000GT VR-4
For a full explanation of 4wd engineering considerations, see the main article on
four-wheel drive In terms of handling, traction and performance, 4WD systems generally have most of the advantages of both front-wheel drive and rear-wheel drive. Some unique benefits are:
Traction is nearly doubled compared to a two-wheel drive layout. Given sufficient power, this results in unparalleled acceleration and driveability on surfaces with less than ideal grip, and superior engine braking on loose surfaces. The development of 4WD systems for high performance cars was stimulated primarily by
rallying.
Handling characteristics in normal conditions can be configured to emulate FWD or RWD, or some mixture, even to switch between these behaviours according to circumstance. However, at the limit of grip, a well balanced 4WD configuration will not degenerate into either understeer or oversteer, but instead break traction of all 4 wheels at the same time into a four-wheel drift. Combined with modern electronic driving aids, this flexibility allows production car engineers a wide range of freedom in selecting handling characteristics that will allow a 4WD car to be driven more safely at higher speeds by inexpert motorists than 2WD designs.
- 4WD systems require more machinery and complex transmission components, and so increase the manufacturing cost of the vehicle and complexity of maintenance procedures and repairs compared to 2WD designs
4WD systems increase power-train mass, rotational inertia and power transmission losses, resulting in a reduction in acceleration performance in ideal dry conditions and increased fuel consumption compared to 2WD designs
The handbrake cannot be used to induce over-steer for maneuvering purposes, as the drivetrain couples the front and rear axles together. To overcome this limitation, some custom prepared stage
rally cars have a special mechanism added to the transmission to disconnect the rear drive if the handbrake is applied while the car is moving.
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