understanding torque split "under normal conditions"Next
Imagine a full-time all wheel drive vehicle (let's say a Subaru Legacy with manual transmission) whose declared power split is 50/50 front to rear. The 50/50 split means that the center differential is "symmetrical" and distributes engine power evenly between the front and the rear axles. When we say the torque split is 50/50 "under normal conditions", this means that when all wheels have the same adhesion (all wheels are on a dry tarmac), the moment of force that is applied to the front wheels and the rear wheels is equal. Out of the available 100% torque that engine produces, 50% goes to the front, 50% to the rear. Or 25% per wheel.
Reading about torque split "under normal conditions" on this Web site lets you understand how the all wheel drive system is build from the mechanical point of view. The "default" torque split also indicates how a vehicle behaves when cornering. This depends on which wheels are favored and receive more torque. For example, a BMW with 36/64 front-to-rear split will have a rear-wheel-drive-like behavior when cornering. A planetary gear center differential is used to achieve asymmetrical torque split. A Volkswagen with Haldex, whose torque split is 95/5 front-to-rear, will understeer until you press the throttle and lock up the multiplate clutch.
understanding torque "apportion up to 100%"Top
What does "torque apportion up to 100% to the axle with traction" mean? Imagine the same Subaru Legacy MT with 50/50 split "under normal conditions". Now imagine the rear wheels are on ice, on rollers, or being raised in the air. If you press the accelerator pedal, the rear wheels will attempt to spin without meeting any resistance. Because there is no resistance, the torque that is applied to the rear wheels is close to zero. The Subaru that we are using in the example has a viscous coupling locking center differential. The viscous coupling heats up and locks the center differential. This prevents the rear wheels from spinning, and transfers engine torque to the front wheels. When rear wheels are in the air, and we assume that the viscous coupling locks fully, 0% of the available engine torque goes to the rear, and 100% of the available torque goes to the front wheels. This is what we mean when we say "torque apportion up to 100%". This Subarus' "default" torque split is 50/50, but torque can be transferred in the range from 100/0 to 0/100 front to rear, depending on which axle loses traction.
Now let's take a vehicle with automatic all wheel drive systems as an example (assume the vehicle is front wheel drive under normal conditions). When in front wheel drive mode, the torque transfer is near 100/0 – most of the power goes to the front wheels. When front wheels slip, the transfer clutch attaches the rear axle. At full lock, torque split becomes 50/50. Now, if front wheels lose traction completely (ice, rollers, or raised in the air), 0% torque goes to the front wheels, 100% of torque goes to the rear wheels, but the ratio stays 50/50. Understand the two things: torque split ratio can vary from 100/0 to 50/50. But torque apportion can vary from 100/0 to 0/100.*
Note that we are talking about torque distribution between the axles, not to an individual wheel. To transfer torque from wheel to wheel you need a lockable differential installed in the axle. To be able to transfer 100% of torque to every single wheel, you need a fully lockable center differential and a fully lockable differential in each of the axles.
- Except for the new Jeep Cherokee, which breaks all the old rules