Why is turning a vehicle much more difficult when you are in 4WD than 2WD?
In: Engineering
Basically, what u/PoleFresh said, but just to add…
On 2WD vehicles, only rear-wheel-driven vehicles, having a front, steering axle in which each side is completely independent and freewheeling, don’t need a differential on the front axle. If the vehicle is front-drive, front-steer, then there must be a differential between the wheels to prevent tyre scuffing and huge mechanical stresses on the driveline components.
Any driven axle, or axis, in which separate (but joined) components will rotate at differing speeds, must have a differential gear to allow drive while also allowing for different rotational demands.
So, in 4WD, each axle (front and rear) has a differential between the wheels, and there is also a third differential between the front and rear axles; to prevent ‘wind up’ of the propeller shafts between front and rear axles.
The exception is when you drive over slippery terrain; as a differential will transmit all the power to the wheel with least grip. No good when your wheels are stuck, spinning like crazy, and all the grip is with the other wheels.
This is why it’s fine to lock the differentials on slippery surfaces: you overcome the differential’s drawback, and each wheel can more or less rotate at whatever speed suits it (because of lack of grip) without trashing your driveline. Lower driving speeds usually have to observed though, as there are stress limits to diff lock.
More modern, dynamic systems can probably overcome all lot of the diff-lock limits though.
Edit: some errors.
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