What is the advantage of a locking differential?

Question

I was looking into going on some light off-road trips in my new car, and while browing related info, I ran into this a lot: that all serious off-road vehicles require a locking differential.

My understanding of the locking differential is that the differential can be locked, meaning the left and right wheel are required to spin at the same rate. That is useful for situations where one wheel might not have traction, which would normally (without locking differential) cause it to spin and get the most torque. Locking differential prevents this by forcing both wheels to spin at the same rate, preventing the no-traction wheel from getting all the available torque. This much, I think I understand.

What I don’t understand is, how is that better than a traditional AWD system, where the car can decide which wheel gets the torque? In my mind, this is even better because as soon as the car detects loss of traction, it will cut the torque, achieving basically what the locking differential does without the downsides (like worse corner handling). For example, my new car, a Subaru Outback, supposedly comes with this kind of AWD system that can distribute torque as needed.

So my question is, why is a locking differential better than just having a normal AWD?

In: Technology

Answer 1

Basically there’s a tendency for road-designed systems to send most if not all of the power to the wheel that’s turning more easily.  If you get one wheel in the air or a really slick spot the other may not be getting enough power to push you out. 

A locking differential guarantees a 50/50 split which avoids that problem and is also more predictable getting over obstacles where a slight shift from power delivery might mean slamming into a rock or not. 

That said, modern systems with selectable, more aggressive modes available are quite a bit better than older style ones. Especially the really old ones that were based on clutch plates rather than any sort of programmed decision.

Edit: another factor is build rather than performance. The electronic systems are often made to work with set hardware. So if you want to upgrade your axles or transfer case to something stronger, it may not play with your stock AWD system at all. It’s much easier to work with dumb systems.