Quote:
Originally Posted by JohnVidale
While I have little experience in interpreting transmissions, if the rear wheels are spinning frictionlessly on ice, how can they have any torque? Torque is a force applied, not a rotational velocity. No friction = no torque, at least in physics.
Perhaps the rear wheels remain engaged, and turning as fast as the front wheels, but at times they are applying no torque, and all the torque is coming from the front wheels?
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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.