Four-wheel drive

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 DesignsRWD 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 4WDLocking Hubs

Manual or automatic operation


manual operation


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 4WDLocking Axles - Toyota’s Automatic

Disconnecting Differential (ADD)

Transfer Case Modes of Operation

Ranges available with a part-time transfer case: Neutral


Ranges available with a part-time transfer case: Two-wheel-drive—high


Ranges available with a part-time transfer case: Four-wheel-drive—high


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 SystemsFull-time systems

Cannot be selected out of four-wheel-drive

May have an open or limited-slip center differential.

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.

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

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 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.

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.

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

Automotive

Four-wheel drive