Weight transfer due to drive torque

[xxChrisxx]

Because there is a twisting motion input to it along the propshaft.

The diff doesn't transfer ALL the twisting motion to the wheels. The point where the propshaft connects to the diff acts as a pivot.

It's that simple.

 

[CKwik240]

still i don't get a solid reason why axle twists

Find a rigid object in your house that has a "T" shape to it. A T-Handle tool might be a quick find if you have one (If you don't, just try and follow this conceptually). Grab the object by the leg of the "T" and spin it along the axis of the leg. The top of the "T" will twist. This is pretty similar to typical live axle. The difference is that under no load (wheels off the ground), the torque gets transferred out through the drive wheels. But if you were to lock the drive wheels (perhaps using the parking brake; assume the parking brake can hold any torque the motor can provide), torque can no longer transfer out as it can no longer spin the wheels. At this point, the system will act just like that "T" shaped object and spin about the axis of the leg. These two scenarios describe what happens when the resistance to acceleration of the drivewheels are at 0 and infinity. What you see when a car twists like that is somewhere in between. Since the drivewheels aren't locked in place, torque from the driveshaft is transmitted to the wheels. But since there is resistance to forward motion of the vehicle, there will be resistance that tries to prevent the drivewheels from turning. When more torque is applied, there will be more resistance. So as more torque is applied, the results begin to appraoch that of that "T" handled tool and tries to spin the ends of the "T" around the axis of its leg.

 

[Lsos]

It's a simple matter of "for every action is an equal but opposite reaction". If the engine twists the drive shaft, the drive shaft will twist the engine just as strongly, but in the opposite direction. This in turn makes the whole car twist, which in turn presses one side harder into the ground than the other, which we see as weight transfer.

This effect can be seen in anything that spins.