Quote:
Originally Posted by drob23
Yes, this confusion is easy to see. You never want to the wheels to slip because this lowers the coefficient of friction (sliding vs non-sliding) and thus makes a loss of traction situation even worse. Not to mention, if the wheel is spinning uncontrollably at high speed, I imagine if it hits a patch of road with high friction it could apply a very strong yaw moment and cause you to lose control. I'm no expert on vehicle dynamics, but this is my intuition.
In the scenario you present, assume the front wheels have good traction while the rear wheels are on ice (somewhat academic example I suppose). In this case, you do not want any torque to be transferred to the rear wheels. Ideally, the center differential would route all torque to the front wheels and the rear wheels would roll without slippage. In the case of xdrive, this is not possible and thus the rear wheels will start spinning regardless because the rear-axle is connected to the drivetrain at all times. Obviously, torque transferred to the ground is all at the front wheels (called torque appropriation according to this), but the inertia of the rear-axle will always be engaged. |
I guess my point is that, whether the back wheels are spinning or not, the torque is being applied 0/100 between the back and front wheels in this case. So the torque would NOT be applied at best 50/50.
It would be preferable to have the back wheels spinning at just the point of slipping, although if the front wheels have traction, the back wheels likely should be rotating at a similar speed, not much slower. In fact, spinning at a similar rate to the front wheels offers the best chance to gain traction.
I read the linked section on "torque split ratio". It seems to be defined as the ratio of the rotation rate of the front and rear axles, which is NOT the apportioning of the torque between the axles. So maybe the "torque split ratio" is at most 50/50, but that does not mean the slipping rear wheels are doing less than the optimum thing.