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| Know Your Car : 4 Wheel Drive |
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 in the world today.
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 the torque they can handle.
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 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.
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.
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