There is a diff in the front, as said previously. The difference between AWD and 4WD is as was said, the center diff in the transfer case. AWD has a center diff with clutch plates similar to that found in an automatic transmission that splits the torque between the front and rear wheels. It eases the amount of power going to the front wheels by splitting torque (normally 60/40) among the axles. Usually the maximum torque delivered is 50/50. This makes the front wheels easy to turn even though the AWD is engaged all the time.
4WD on the other hand is a "lockout" situation. When you engage 4WD it engages a set of gears (some cases still use chains)in the transfer case that locks the front and rear drivetrain together. Because of this, full power is going to both the front and rear with no differential effect as in the AWD vehicles. This is what causes the hopping and pulling effect in the front end when turning tight radius corners in a 4WD vehicle. Another feature of 4WD is that they have a low speed on the transfer case. This is a gear that usually drops the engine to drivetrain ratio into the 2 to 1 range. It multiplies the torque by two or more, giving the vehicle more power to either pull itself or another load in low traction situations.
Hopefully I answered more peoples questions than people I have confused.
Originally posted by anotherbrian:
Isn't there a differential in the front? I presumed the missing differential, and the reason to avoid driving in 4WD modes on non-slippery surfaces, was between the front and rear axles.
AWD/fulltime 4WD vehicles offer that 3rd differential, and some even allow it to be locked out (at least I've ridden in Land Rover Discos with center locking diffs, but those may have been aftermarket).
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Gordon "The Warmonger" White
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