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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.

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Four-Wheel-Drive

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  1. Four-Wheel-Drive

  2. 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

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

  4. 4WD Designs • RWD vehicles equipped with an added transfer case, a front drive shaft, a front differential, and front drive axles

  5. 4WD Designs • FWD vehicles with an added transfer case, a rear drive shaft, and a rear axle

  6. 4WD Definitions • Transfer case - An auxiliary transmission mounted to the side or rear of the main transmission. • No differential action is provided.

  7. 4WD Components • Transfer case • Transmits power from the transmission to the front and rear drive shafts

  8. 4WD Components • Transfer case • Will typically be a gear or chain type

  9. 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

  10. Types of Transfer Cases • Gear-type • Uses only gear-sets to transfer power • Stronger & more durable

  11. 4WD Definitions • Center differential - serves the same purpose as a transfer case but allows for differential operation.

  12. 4WD Definitions • Limited-slip center differential. Controls differential operation through a disc or cone-type clutch. • Later designs are computer operated.

  13. 4WD Components • Drive shafts • Connect to the front and rear differentials

  14. 4WD Components • Universal joints or CV-joints • Are used to connect front axles to the wheel hubs

  15. 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.

  16. 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

  17. 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.

  18. Part-Time 4WD Shift Controls • Manual Shift lever • Some vehicles have a low-speed range that can be selected

  19. Part-Time 4WD • Locking Hubs • Manual or automatic operation

  20. Part-Time 4WD • Locking Hubs • manual operation

  21. Part-Time 4WD • Locking Hubs • automatic operation - vacuum or electro-magnetically operated.

  22. 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

  23. Part-Time 4WD • Locking Axles - Toyota’s Automatic Disconnecting Differential (ADD)

  24. Transfer Case Modes of Operation • Ranges available with a part-time transfer case: • Neutral

  25. Transfer Case Modes of Operation • Ranges available with a part-time transfer case: • Two-wheel-drive—high

  26. Transfer Case Modes of Operation • Ranges available with a part-time transfer case: • Four-wheel-drive—high

  27. Transfer Case Modes of Operation • Ranges available with a part-time transfer case: • Four-wheel-drive—low

  28. 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

  29. 4WD Systems • Full-time systems • Cannot be selected out of four-wheel-drive • May have an open or limited-slip center differential.

  30. 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.

  31. 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

  32. 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.

  33. 4WD Definitions • On-demand four-wheel drive

  34. 4WD Definitions • Viscous coupling in a center differential

  35. 4WD Definitions • Viscous coupling in a center differential

  36. 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.

  37. 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.

  38. 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

  39. 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

  40. 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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