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 Definitions 4X4 = Four wheeled vehicle, four wheels can receive torque. 4X2 = Four wheeled vehicle, two wheels can receive torque. 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 4WD Definitions Transfer case - An auxiliary transmission mounted to the side or rear of the main transmission. No differential action is provided. 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 4WD Definitions 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 Components Drive 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 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. 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 Part-Time 4WD Older designs may have different driveaxle 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 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 electromagnetically 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 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 4WD Systems Full-time systems Cannot be selected out of four-wheeldrive 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 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. 4WD Definitions On-demand four-wheel drive 4WD Definitions Viscous coupling in a center differential 4WD Definitions Viscous coupling in a center differential 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 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. 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
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