Four wheel drive
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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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Four wheel drive

Four Wheel Drive


Purpose

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 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


Components

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


Components1

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)


Components2

Components


Transfer cases

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


Transfer cases1

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


Components3

Components


Component

Component


Gear drive

Gear Drive


Gear selection

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


Gear selection1

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)


Front axle

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


Four wheel drive

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


Viscous coupling

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


Four wheel drive

Howstuffworks


Service

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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