Comment by Beretta_Vexee | Hacker News Reader

Comment by Beretta_Vexee | Hacker News Reader

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The cars are equipped with a differential that distributes power differently between the wheels. This is particularly useful for preventing the wheels from squealing when cornering. In a bend, the four wheels do not turn at the same speed between the inside and outside of the bend. This gives you better grip, a smoother ride, etc. It's really essential.

- When you have two wheel drive you only have one differential between the two wheels. - When you have four-wheel drive you have three differentials, one between the front wheels, one between the rear wheels and one between the front and rear. So far so simple.

Now one of the problems with differentials is that if one of the wheels loses traction, it gets all the power back and spins in a vacuum.

Finding yourself with a wheel in the sand, in the air, on the snow, etc. is very common when driving off-road or in the mountains. This problem has been solved with the differential lock, which can be manual or automatic. When locked, the differential is locked and power is transmitted equally to the wheels. So you can get out of your sandy or snowy hole. That's what we call a 4x4.

A All-wheel-drive has no differential locking system.So if one wheel loses its grip, the vehicle is stuck. There's nothing you can do - all the power will go to the wheel with the least grip.

So AWD is particularly unsuitable for off-road use and shouldn't be sold as such.

Some off-road vehicles do not have a differential, such as quad bikes or buggies. Perfect for mud, not so great on the road.

https://en.wikipedia.org/wiki/Differential_(mechanical_devic... https://en.wikipedia.org/wiki/Locking_differential

There are exceptions. I believe the Kia Telluride SUV is sold as AWD and it has a locking differential system (or at least it’s an option). There are probably others as well.

The Kia Telluride does not have a locking differential in the same sense as conventional 4x4 vehicles. In fact, the Kia Telluride comes with a multi-plate center clutch that allows for distribution of power between the front and rear wheels. When the clutch is fully engaged, in most cases there should be no slippage which means that power is distributed evenly (50/50) between the front and rear. However, there's always the possibility that the clutch can slip given enough of a torque differential between the front and the back. It can offer improved traction in poor road conditions.

A proper locking differential found in most traditional 4x4 vehicles/trucks, when engaged, physically locks the axles together which means they cannot spin at different rates. This is essential for many more rigorous off-road applications when you may have to rely on traction from a single wheel to get you unstuck.

This is exactly the type of misleading marketing that is described in many other comments. A clutch pack cannot physically lock axles together in the same way as a locking diff.

This is not true for a lot of all wheel drive systems these days.

Subaru for example has had adaptive AWD since early 2000s.

https://youtu.be/YIY392Qtu0I?si=WCvlM1LCGezDcJVF

I've covered the basics, so I'll leave it to you to explain the 25 thousand different systems that try to compensate for the lack of a differential lock.

The simple answer is that you can detect wheel slip, then you lock slipping wheels with the brakes.

There are a few other systems (not 25k), using a clutch or independent motors, but it doesn't matter: the claimed deficiencies of some AWD only needs to be debunked and corrected.

Watch the linked YouTube video for more details on Subaru's system, the same system cited by the National Park.

Bottom line, a lot of modern AWD systems are perfectly adequate and in some cases superior to 4 wheel drive systems.

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