How Four-Wheel Drive Works

By: Karim Nice

The Four-wheel Drive Hummer

The AM General Hummer military vehicle combines some advanced mechanical technology with sophisticated electronics to create what is arguably the best four-wheel-drive system available.

The Hummer has a full-time system with additional features that can be engaged for enhanced off-road performance. In this system, just as in our basic system, the transmission is hooked up to the transfer case. From the transfer case, one driveshaft connects to the front axle and one to the rear axle. However, the transfer case on the Hummer does not automatically lock the front and rear axles together. Instead, it contains a set of open-differential gears that can be locked by the driver. In open mode (not locked), the front and rear axles can move at different speeds, so the vehicle can drive on dry roads with no problem. When the differential is locked, the front and rear axles each have access to the engine's torque. If the front wheels are in quicksand, the rear wheels get all of the torque they can handle.


Diagram of Hummer system, one cool feature of the Hummer is the geared hubs it uses at each wheel. These raise the entire driveline, giving the Hummer 16 inches (40.64 cm) of ground clearance, more than double what most four-wheel drives have.

The front and rear differentials are both Torsen® differentials. These differentials have a unique gearset: As soon as it senses a decrease in torque to one wheel (which occurs when a tire is about to slip), the gearset transfers torque to the other wheel. Torsen differentials can transfer from two to four times the torque from one wheel to the other. This is a big improvement over open differentials. But if one wheel is off the ground, the other wheel still gets no torque.

To handle this problem, the Hummer is equipped with a brake traction control system. When one tire starts to slip, the brake traction control applies the brakes to that wheel. This accomplishes two things:

  • It keeps that tire from slipping, allowing it to make maximum use of its available traction.
  • It allows the other wheel to apply more torque.

The Hummer system encountering various combinations of terrain: For the Hummer to get stuck, all four wheels would have to lose traction.

The brake traction control system applies significant torque to the wheel that wants to slip, allowing the Torsen differential to apply two to four times that increased torque to the other wheel.

Let's put the Hummer to the test.

The system on the Hummer is capable of sending a large amount of torque to whichever tires have traction, even if this means sending it all to a single tire. This brings the Hummer pretty close to our definition of an ideal four-wheel-drive system: one that supplies each tire with the maximum amount of torque it can handle.

For more information on four-wheel drive and related topics, check out the links below.

Four-Wheel Drive FAQ

What is the difference between all-wheel drive and four-wheel drive?
All-wheel drive sends engine power to all four of your tires simultaneously. It's designed to help keep your vehicle moving forward better than front-wheel drive or rear-wheel drive on slippery roads and in conditions like snow and rain. Four-wheel drive is meant only for use off-road or on extremely slippery surfaces. It's a part-time system, meaning you can shift into and out of four-wheel drive when it's needed.
Is four-wheel drive bad for your car?
Four-wheel drive can cause damage if it's used at the wrong times. You shouldn't use four-wheel drive when you're on dry, smooth or flat roads. Because it provides more torque and engages all of your tires, it can damage the drivetrain.
Is it okay to drive in 4-wheel drive all the time?
You shouldn't drive in four-wheel drive mode all the time. Only turn it on when it's needed, such as in rain or snow or when you're off-roading.
How old is four-wheel drive?
Four-wheel drive was first invented in 1893.
What does a 4X4 drivetrain mean?
A 4X4 drivetrain is a four-wheel-drive drivetrain. It simply means that power can be sent to all four wheels simultaneously when four-wheel drive is engaged.

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