Quote:
Originally Posted by SteveM
OK, I'll jump into the middle of this, which looks like it may not be a good idea! 
If the diff fully locks and makes it one rigid transfer case a'la 4WD truck (which the drawings make it look like it does), then you do get 50/50 torque when all four wheels on the ground, and 0/100 if the front axle is completely off the ground. I'll make a better explanation, and if you guys are really nice maybe even an animation.  |
Nope.
It's not a transfer case, nor does it act like one. At full lock, you may not have a speed differential between the front and rear wheels, but there is more torque at the rear due to the gearing in the diff. On dry pavement, with tons of traction, in a straight line, at 100% lock, the wheelspeeds will be the same, but the rears are pushing the car harder than the fronts are.
With the front wheels off the ground, there is zero torque on the them because there's no ground pushing back as they turn, but the rears are not getting 100% of the engine's torque... they're getting 65%, the max allowed by the center diff. The rest of the power is being wasted spinning the front wheels. Similarly, with the rears in the air, the fronts can get only 35% of the engine's power, even though they're generating 100% of the torque that's reaching the ground, since the rears are spinning freely.
It's the difference between the torque split of the differential, and the effective torque reaching the wheels. Sure there are situations where the torque at the wheels vary greatly from 100%/0% to 0%/100%, but never is the torque passing through the diff split any other why than 35%/65%. To put it another way, the torque distribution may be 100/0, but that means the torque getting to the ground is only 35% of what the motor's generating. Which is exactly why
you don't want an AWD car lifting wheels, doing so wastes power in the differentials ...which brings us full circle to what started this thread.