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Four Wheel Drive

Four Wheel Drive. Purpose. Provide additional traction for off-road or poor traction conditions May provide a low range for increased torque Some units may have provisions for PTO. 4WD VS. AWD. 4WD Generally is part-time Generally provides a low range AWD Generally is full-time

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Four Wheel Drive

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  1. Four Wheel Drive

  2. Purpose • Provide additional traction for off-road or poor traction conditions • May provide a low range for increased torque • Some units may have provisions for PTO

  3. 4WD VS. AWD • 4WD • Generally is part-time • Generally provides a low range • AWD • Generally is full-time • Generally does not provide a low range • Generally contains a front to rear differential

  4. Components • Two drive axle assemblies • Front axle must be able to steer as well as drive • Gear ratios must match (front to rear) • Front drive axles have traditionally been solid (non-independent) • Independent front suspension is becoming much more popular • Some AWD vehicles are built off of a FWD platform

  5. Components • Transfer Case • An auxiliary transmission designed to split power between the front and rear drive axles • Lock out hubs – used to disengage the front wheels from the drive axle • Increases highway fuel economy and reduces wear • May be automatic or manual • Automatic hubs react to input torque when transfer case is shifted into 4WD • May not unlock after shifting out of 4WD until vehicle is backed up (30-50ft)

  6. Components

  7. Transfer Cases • Transfer cases can be categorized by various characteristics • Style • In-line – Front and rear driveshaft will be offset. Rear driveshaft will be inline with the output shaft of the transmission • Offset – Both driveshafts exit from the transfer case directly across from each other • Ranges • Single range – only offers 1:1 4WD and 1:1 2WD • Dual range – also offers a gear reduction 4WD

  8. Transfer Cases • Drive • Chain • Uses a link type chain (similar to a timing chain) • If used with a dual range case, gear reduction takes place before the chain drive sprocket • Usually will use a planetary gearset for gear reduction • Gear • Uses a cluster gear between the transmission and the transfer case output shaft

  9. Components

  10. Component

  11. Gear Drive

  12. Gear Selection • Direct manual shift • Dog clutches lock gear or sprocket to shaft • No synchronizers • Syncronized manual shift • Allows shift from H2 to H4 and H4 to H2 while moving • Must stop to go to L4 • Some may shift from L4 to H4 at very low speeds

  13. Gear Selection • Vacuum shift • Manual or electric control opens vacuum valving to diaphragm which moves the shift mechanism • Electric • Uses an electric motor to shift • Uses a worm gear • Electromagnetic clutch spins up (synchronizes) front axle components before engagement • Allows “shift on the fly” at any road speed • L4 engagement still require the driver to stop and shift to neutral (or disengage clutch)

  14. Front Axle • The front axle will have some provision for unlocking the wheels from the driveline (Part time 4WD) • Locking hubs • Front axle disconnect • Thermal linear actuator • Vacuum servo • Electric motor

  15. AWD • All wheel drive require differentiation between the front and rear axles • Front and rear wheels follow different tracks • Prevents propshaft windup • Different types of differentiation are used • Open differential • Limited slip • Friction • Torsen • Viscous coupling

  16. Viscous Coupling • Uses a thick fluid between discs in a sealed unit • When a speed differential between the axles exists, fluid shears, heat builds up and forces the plates together until axle speeds match

  17. Howstuffworks

  18. Service • Change fluid at recommended intervals • Use only the recommended lubricant • Most gear types transfer cases use gear oil • Most chain type transfer cases use ATF • May use special lubricant (Syncromesh) • AWD with friction type limited slip is very sensitive to fluid type • Both drive axles must have the same gear ratio • Tire size and inflation • Warn customer about propshaft windup • Part time 4WD operating on concrete or asphalt

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