Originally Posted by 0396
full time quattro has the output of the transmission go to a differential that splits the output to the front and rear axle. This differential drives both axle, and when one starts speeding up, then other slows down. This allows for the front wheel to take a different path from the rear (as when going around corners). But if one axle starts slipping, the other stops rotating and your car would not move. Thus Audi has the TORque SENsing locking differential where with the onset of low grip, the layout of the gears in relation to each other causes them to bind up, thus the front and rear axle would rotate together. There is no slip required from the axle with lower grip in order for the differential to lock up, thus you would not feel any sense of instability. But due to the way the gear has to encounter resistance from the ones it is driving, if the driven wheel are on ice, the gear would not encounter resistance, thus it would not be able to lock up. This means these TORque SENsing differential cannot cope with surface grip that are too extreme. Newer ones are clutch based with front to rear and rear to front torque transfer ratio that are different from each other, thus allowing for higher difference between front and rear surface grip and asymmetrical max torque transfer characteristics. Plus, given it is clutch based, it could implement electronic lockup if it so desires. The old differential, being a symmetrical design, transfers torque to the front and rear equally. Newer designs, has the driving gear placed at different distance relative to turning center, thus the gear turns the rear gear with a higher leverage than the front gear, allowing for asymmetrical torque transfer ratio. This unit is completely mechanical.
As for the Haldex units, the output from the transmission are directly linked to the front axle, so they are driven 100% of the time at the exact rate the transmission is driving them. There is also another output gear from the transmission that goes to the rear of the car where the haldex clutch is based. Under normal situation, the clutch is only lightly engaged, thus only 5% of the output goes to the rear wheel. When ABS sensors senses a front wheel is slipping, it will close down on the clutch and more and more power is transferred to the rear axle, thus allowing the rear wheels to drive the car more and more. Due to the reactive nature of the unit, it will usually require some slipping of a front wheel before the rear takes over, thus it never feel as secure. The older units also pressurizes the oil to close the clutch via a pump that relies on the different rotational rate of the front and rear axle, thus you need more rotational difference to create more pressure, with the most pressure created when there is actual slip in the front axle. Newer units has an electrical pump to create the clutch pressure. Since this unit is electronics based, it can take readings from many sensors such as throttle depression rate, lateral sensor, steering angle, etc and prediction what kind of driving situation you are likely in and pre-engage the clutch if it deems you will likely need it. Plus, the front and rear are turning at their most different in corners, so clutch pressure would be there. Thus if you are accelerating out of a corner, it could have engaged the rear and transferred much of the power to the rear even before any wheel encounters any slip. If you catch the car off guard, it might not have the rear wheels engaged and it could be less stable than other times where it predicted the situation. The max power split would be at best 50:50 when the clutch are fully locked since the front axle are fully engaged to the transmission.
Note, cars like the Veyron and Aventador uses Haldex based AWD, but it is set up in reverse, with the rear wheels being fully connected to the transmission and the front wheel driven based on clutch engagement.