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Four-Wheel-Drive
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
4X4 = Four wheeled vehicle, four wheels can receive torque.
4X2 = Four wheeled vehicle, two wheels can receive torque.
Definitions
4WD Designs
RWD vehicles equipped with an added transfer case, a front drive shaft, a front differential, and front drive axles
4WD Designs
FWD vehicles with an added transfer case, a rear drive shaft, and a rear axle
Transfer case - An auxiliary transmission mounted to the side or rear of the main transmission. No differential
action is provided.
4WD Definitions
4WD Components Transfer case
Transmits power from the transmission to the front and rear drive shafts
4WD Components Transfer case
Will typically be a gear or chain type
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
Types of Transfer Cases
Gear-type Uses only gear-sets to transfer power Stronger & more durable
Center differential - serves the same purpose as a transfer case but allows for differential operation.
4WD Definitions
Limited-slip center differential. Controls differential operation through a disc or cone-type clutch. Later designs are
computer operated.
4WD Definitions
4WD ComponentsDrive shafts
Connect to the front and rear differentials
4WD Components
Universal joints or CV-joints Are used to connect front axles to the wheel hubs
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
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
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
Part-Time 4WD Shift Controls
Manual Shift lever
Some vehicles have a low-speed range that can be selected
Part-Time 4WD
Locking Hubs Manual or automatic operation
Part-Time 4WD
Locking Hubs manual operation
Part-Time 4WD
Locking Hubs automatic operation - vacuum or
electro-magnetically operated.
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
Part-Time 4WD
Locking Axles - Toyota’s Automatic Disconnecting Differential (ADD)
Transfer Case Modes of Operation
Ranges available with a part-time transfer case: Neutral
Transfer Case Modes of Operation
Ranges available with a part-time transfer case: Two-wheel-drive—high
Transfer Case Modes of Operation
Ranges available with a part-time transfer case: Four-wheel-drive—high
Transfer Case Modes of Operation
Ranges available with a part-time transfer case: Four-wheel-drive—low
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
4WD Systems
Full-time systems Cannot be
selected out of four-wheel-drive
May have an open or limited-slip center differential.
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.
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 (2:1 ratio) for slow, high torque driving
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
On-demand four-wheel drive
4WD Definitions
Viscous coupling in a center differential
4WD Definitions
Viscous coupling in a center differential
4WD Definitions
On-demand four-wheel drive
Ford’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.
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 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.
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
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
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