| Basic layouts |
Front-wheel drive layout |
In front wheel drive vehicle layouts, the engine can be mounted transversely or longitudinally. Drive is transmitted to the front wheels through a transaxle.
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Rear-wheel drive layout |
In a conventional rear-wheel-drive layout, the engine and transmission are mounted longitudinally at the front of the vehicle and drive is transmitted to a rear axle assembly by a propeller shaft.
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Four-wheel drive layout |
In a conventional four-wheel-drive vehicle, propeller shafts connect a transfer case at the rear of the transmission to final drive units on both front and rear axles.
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All-wheel drive layout |
In a conventional full-time four-wheel drive vehicle, a third differential is located in the transfer case.
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4WD vs AWD
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Four-wheel drive (4WD) and all-wheel drive (AWD) are different types of automotive drive systems.
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Differential |
A device, usually consisting of gears, for supplying equal torque to the driving wheels, even as they rotate at different speeds. |
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| Front-wheel drive |
Front-wheel drive shafts |
In front-wheel-drive vehicles, the drive shafts transfer the drive directly from the differential to the front wheels.
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Front-wheel final drives |
In front-wheel-drive vehicles, the final drive gears are incorporated in the transaxle assembly.
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Front-wheel differentials |
When a vehicle is turning, a standard differential allows the two drive shafts and the driving road wheels to rotate at different speeds while still applying an equal turning effort to both. |
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| Rear-wheel drive |
Rear-wheel drive shafts |
The propeller shaft transfers the drive from the transmission to the final drive. It is normally of a hollow, tubular construction.
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Universal joints |
The most common type of universal joint fitted to propeller shafts is known as a Hookes joint or a cross and roller joint.
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Rear-wheel final drives |
In a conventional rear wheel drive vehicle, a crownwheel and pinion transfers the drive through ninety degrees and provides a final gear reduction to the driving road wheels.
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Salisbury axles |
In Salisbury axles the carrier is integral with the axle housing and a removable cover gives access to the final drive components.
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Rear-wheel drive differentials |
The differential gears are bevel gears which are mounted at right angles to each other inside the differential case.
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Limited slip differentials |
Limited slip differentials allow differential action under normal driving conditions but reduce or prevent differential action when conditions are not normal so that a wheel cannot spin. |
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| Four-wheel drive (part-time) |
Four-wheel drive shafts |
In a conventional four-wheel-drive vehicle, drive is transferred from the transfer case to the front and rear axle assemblies by propeller shafts. Drive shafts enclosed within the axle housings transmit the drive to the wheels.
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Four-wheel final drive |
In part-time four wheel drive vehicles, drive is transmitted through the rear final drive to the rear wheels, in all drive ranges. Drive is only transmitted through the front axle final drive, when a four wheel drive range is selected.
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Four-wheel drive transfer case |
In a conventional part-time four-wheel drive vehicle, the transfer case allows the vehicle to be operated in high range in 2 wheel drive and or 4 wheel drive, in low range in 4 wheel drive only and in a neutral condition for a power take-off.
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Free wheeling hubs |
On part time- four-wheel drive vehicles, free wheeling hubs can be installed on the front hubs, to disconnect the hubs from the axle shafts, and prevent rotation of the front axle components.
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Four-wheel drive differentials |
In part-time four wheel drive vehicles, differentials are fitted to both front and rear axle assemblies. |
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| All-wheel drive |
Full-time four-wheel final drives |
All wheel, or fulltime four wheel drive, vehicles, drive is transmitted to the front and rear wheels at the same time.
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All-wheel drive transfer case |
In a conventional full-time four-wheel drive vehicle, the transfer case allows the vehicle to be operated in high range or low range, or in a neutral condition for a power take-off.
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Transfer case differential action |
When operating in high range in a conventional full-time four-wheel drive vehicle, a third differential in the transfer case caters for the speed difference between front and rear propeller shafts.
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| Final drives procedures |
Checking free play
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The alignment relationship of the crown wheel and the pinion is very important. Misalignment will cause noisy operation and uneven wear.
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Checking drive shaft joints
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Make sure the vehicle is safely supported on a suitable vehicle hoist. The wheels need to be free to turn.
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Checking manual transaxle/differential oil |
Selecting and using the correct type and grade of lubricant is critical. The wrong selection can lead to premature failure of the internal components. |
