Four-Wheel-Drive

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Four-Wheel-Drive
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Why Four-Wheel-Drive?

• Advantages
• Improved traction and handling
• Able to transmit torque to all four wheels
• Disadvantages
• Higher initial cost and weight
• Requires special service and maintenance

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Definitions

• 4X4 Four wheeled vehicle, four wheels can
  receive torque.
• 4X2 Four wheeled vehicle, two wheels can
  receive torque.

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4WD Designs

• RWD vehicles equipped with an added transfer
  case, a front drive shaft, a front differential,
  and front drive axles

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4WD Designs

• FWD vehicles with an added transfer case, a rear
  drive shaft, and a rear axle

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4WD Definitions

• Transfer case - An auxiliary transmission mounted
  to the side or rear of the main transmission.
• No differential action is provided.

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4WD Components

• Transfer case
• Transmits power from the transmission to the
  front and rear drive shafts

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4WD Components

• Transfer case
• Will typically be a gear or chain type

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Types of Transfer Cases

• Drive chain design
• Has less weight to improve fuel economy
• Is usually used with planetary gearsets
• Is the most common design

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Types of Transfer Cases

• Gear-type
• Uses only gear-sets to transfer power
• Stronger more durable

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4WD Definitions

• Center differential - serves the same purpose as
  a transfer case but allows for differential
  operation.

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4WD Definitions

• Limited-slip center differential. Controls
  differential operation through a disc or
  cone-type clutch.
• Later designs are computer operated.

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4WD Components

• Drive shafts
• Connect to the front and rear differentials

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4WD Components

• Universal joints or CV-joints
• Are used to connect front axles to the wheel hubs

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4WD Definitions

• Part-time 4WD - Typically systems that operate in
  two-wheel drive until the driver commands
  four-wheel drive operation. Torque is then
  divided (50/50 fixed) between the front and rear
  drive-shaft.
• They do not corner well on dry pavement.
• Selection of two- or four-wheel-drive is done by
  a shifter, electric switch and/or locking hubs.

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Part-Time 4WD Shift Controls

• Electric switch or shift lever
• Allows the driver to select which axles receive
  power
• Power can be directed to
• all four wheels
• two wheels
• none (neutral)
• Some vehicles have a low-speed range that can
  be selected

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Part-Time 4WD

• Older designs may have different drive-axle gear
  ratios.
• This will result in a push-pull type action if
  the vehicle is driven on dry pavement.
• This phenomenon is called drive-line windup.

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Part-Time 4WD Shift Controls

• Manual Shift lever
• Some vehicles have a low-speed range that can
  be selected

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Part-Time 4WD

• Locking Hubs
• Manual or automatic operation

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Part-Time 4WD

• Locking Hubs
• manual operation

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Part-Time 4WD

• Locking Hubs
• automatic operation - vacuum or
  electro-magnetically operated.

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Part-Time 4WD

• Locking Axles - final differential will be of an
  open design. When one axle is disconnected from
  the wheel the spider gears rotate freely, neither
  axle receives torque.
• Used with an open differential

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Part-Time 4WD

• Locking Axles - Toyotas Automatic Disconnecting
  Differential (ADD)

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Transfer Case Modes of Operation

• Ranges available with a part-time transfer case
• Neutral

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Transfer Case Modes of Operation

• Ranges available with a part-time transfer case
• Two-wheel-drivehigh

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Transfer Case Modes of Operation

• Ranges available with a part-time transfer case
• Four-wheel-drivehigh

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Transfer Case Modes of Operation

• Ranges available with a part-time transfer case
• Four-wheel-drivelow

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4WD Definitions

• Full-time four-wheel drive (4wd) - Constantly
  provides power to all four wheels. These systems
  typically do NOT have a provision for low
  operating range.
• Axles and differentials are constantly locked.
• Usually equipped with a center differential
  instead of a transfer-case.
• All-wheel drive - see above

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4WD Systems

• Full-time systems
• Cannot be selected out of four-wheel-drive
• May have an open or limited-slip center
  differential.

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Full-time 4WD

• Open Center Differential - If the wheels on
  either final differential loose traction, maximum
  torque is delivered to the axle with the LEAST
  traction.
• For this reason, limited-slip center
  differentials are far more common today.

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Full-time 4WD

• Newer computer-controlled designs allow for
  different operational modes.
• Ford Automatic Four-wheel-drive (A4WD)
• Navigator and Expedition
• 4WD AUTO - full power to the rear wheels. The
  front wheels receive power as required.
• 4WD High - Full power to both wheels, no center
  gear-reduction.
• 4WD Low - Low gear ratio in center differential
  for (21 ratio) for slow, high torque driving

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4WD Definitions

• On-demand four-wheel drive - Automatically
  controlled 4WD. One axle is driven until slip is
  detected or anticipated.
• Computer controlled
• uses wheel speed sensors and/or axle speed
  sensors to detect slippage.
• When slippage is detected, torque is sent to the
  opposite axle. The amount sent depends on the
  amount of slippage.

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4WD Definitions

• On-demand four-wheel drive

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4WD Definitions

• Viscous coupling in a center differential

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4WD Definitions

• Viscous coupling in a center differential

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On-demand four-wheel drive

• Fords control-trac system
• normal operation - 96 of torque is sent to the
  rear wheels.
• Upon slippage a computer energizes an
  electro-magnetic clutch in the transfer case. Up
  to 96 of torque can be sent to the front wheels.

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4WD Systems

• Four-wheel drive systems without a center
  differential.
• Tire sizes must all be the same (this holds true
  for any 4WD application)
• Tread wear must be similar
• Tire changes on an all-wheel drive must be done
  in sets of four
• Gear ratios should be the same in the front and
  rear differentials.

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Front Axle and Hub Service Guidelines

• Most front axles are serviced like those in 2WD
  systems
• Follow the recommended procedure for servicing
  4WD hubs
• Some hubs are not serviceable and must be
  replaced

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Effects of Vehicle Modifications

• Installing larger tires and lift kits
• Alters vehicle handling by changing unsprung
  weight
• Increases steering effort
• Causes a rough ride
• May cause vehicle to wander and vibrate
• Possible driveshaft working angle issues
• Possible Vehicle Speed sensor issues
• May necessitate computer programming

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Maintenance Tips

• Check the fluid level often
• Check for fluid contamination any time the
  vehicle has been driven through deep water
• Use only the recommended lubricant
• Always follow the proper towing procedures