A transfer case is the drivetrain component that sends engine power to both the front and rear axles in a 4WD or AWD vehicle. Power leaves the engine, passes through the transmission, and enters the transfer case at the input shaft. Inside, gears, chains, clutches, or a differential split that power toward two output shafts.
This matters because traction improves when more than one axle can help move the vehicle.
Key Facts
- Power path in a 4WD vehicle: engine to transmission to transfer case to front and rear driveshafts to axles to wheels.
- Torque split describes how engine torque is divided between front and rear axles, such as 50:50 or 40:60.
- If input torque is 400 N·m and the split is 50:50, front output torque = 200 N·m and rear output torque = 200 N·m before other losses.
- Low range multiplies torque for slow driving: output torque = input torque × gear ratio.
- A 2.72:1 low-range gear turns the output 1 time for every 2.72 input turns, increasing torque by about 2.72 times before losses.
- Part-time 4WD should not be used on dry pavement when front and rear shafts are locked together, because tire speed differences can cause drivetrain bind.
Vocabulary
- Transfer case
- A gearbox behind the transmission that routes power to the front and rear driveshafts in a 4WD or AWD vehicle.
- Driveshaft
- A rotating shaft that carries torque from the transfer case to an axle.
- Torque split
- The percentage of driving torque sent to the front axle compared with the rear axle.
- Low range
- A transfer case gear setting that reduces speed and multiplies torque for climbing, towing, or off-road driving.
- Drivetrain bind
- Stress in drivetrain parts caused when connected wheels or axles need to rotate at different speeds but cannot.
Common Mistakes to Avoid
- Thinking the transfer case creates engine power, but it only redirects and changes the mechanical advantage of power already coming from the transmission.
- Assuming 4WD and AWD work the same way, but many 4WD systems can lock front and rear outputs while many AWD systems use clutches or differentials for automatic torque sharing.
- Using locked 4WD on dry pavement, which is wrong because front and rear axles follow different paths in turns and need different shaft speeds.
- Confusing low range with higher vehicle speed, but low range reduces output speed while increasing torque at the wheels.
Practice Questions
- 1 A transfer case receives 360 N·m of torque and uses a 50:50 split. How much torque is sent to the front output shaft and how much to the rear output shaft, ignoring losses?
- 2 A vehicle enters low range with a 2.5:1 ratio. If the transfer case input torque is 240 N·m, what is the ideal output torque before losses?
- 3 A driver shifts a part-time 4WD vehicle into locked 4WD on dry pavement and feels hopping during a tight turn. Explain what is happening inside the drivetrain and why a different mode would be better.