four-wheel drive
TRANSCRIPT
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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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4X4 = Four wheeled vehicle, four wheels can receive torque.
4X2 = Four wheeled vehicle, two wheels can receive torque.
Definitions
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4WD DesignsRWD 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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Transfer case - An auxiliary transmission mounted to the side or rear of the main transmission. No differential
action is provided.
4WD Definitions
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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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Center differential - serves the same purpose as a transfer case but allows for differential operation.
4WD Definitions
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Limited-slip center differential. Controls differential operation through a disc or cone-type clutch. Later designs are
computer operated.
4WD Definitions
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4WD ComponentsDrive 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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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.
4WD Definitions
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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 wheelstwo wheels none (neutral)
Some vehicles have a low-speed range that can be selected
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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.
Part-Time 4WD
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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 4WDLocking Hubs
Manual or automatic operation
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Part-Time 4WDLocking Hubs
manual operation
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Part-Time 4WDLocking 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 4WDLocking Axles - Toyota’s 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-drive—high
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Transfer Case Modes of Operation
Ranges available with a part-time transfer case: Four-wheel-drive—high
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Transfer Case Modes of Operation
Ranges available with a part-time transfer case: Four-wheel-drive—low
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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
4WD Definitions
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4WD SystemsFull-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 4WDOpen 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 4WDNewer 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
(2:1 ratio) for slow, high torque driving
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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.
4WD Definitions
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On-demand four-wheel drive
4WD Definitions
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Viscous coupling in a center differential
4WD Definitions
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Viscous coupling in a center differential
4WD Definitions
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On-demand four-wheel driveFord’s 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 SystemsFour-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 similarTire changes on an all-wheel drive must be
done in sets of fourGear 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