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.
When you’re turning in a car the tires travel different distances. The inside tire travels a shorter distance than the outside one. This also means they spin at different speeds, different rates.
Typically in two wheel drive the tires are allowed to spin freely so this difference in distance and speed isn’t an issue. This is literally what a differential does, it allows this to happen (*how* this happens is a totally different eli5)
However, in 4WD the differential is typically locked, and therefore tires are not allowed to spin at different rates. When you’re driving over something slippery like snow or mud the tires slip just enough on the surface that you don’t feel anything herky-jerky in the car, but if you’re on a dry surface that the tires grip really well you will feel the car bouncing around the corner because the tires are struggling to spin at different rates
Because such a vehicle usually doesn’t provide a differential between the front and rear axles. Which means the average distance traveled by the front wheels must match the average distance traveled by the rear wheels. But when looking at the geometry of pretty much any turn, it’s safe to say that that never actually happens. So instead one or more wheels wants to slip, and if the surface is soft or wet then that’s what happens. But if the surface is paved and dry then it would wear out the wheels that are trying to slip.
That’s why it’s important to shift the vehicle to 2WD when driving on paved dry roads.
AWD systems do not have that problem because there *is* a differential between front and rear that allows them to turn different distances. There is other technology used to preserve power to each wheel in that case.
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