A typical part time 4WDS found in pickups and older SUVs is a rear wheel drive. The transmission is directly connected to the transfer case, from there one shaft turns the rear wheels and another shaft turns the front axle.
When the 4WD system is switched ON the transfer case locks both the drive shafts, which results in each axle receiving half of the torque from the engine.
This system has two drawbacks
- It cannot safely be used on-roads because of the locked transfer case.
- The system can get stuck when both the wheels on one side are on a slippery surface such as loose gravel. Both the axles loose traction in this case.
This problem can be avoided by a Humvee style differential drive.