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
There are broadly speaking three different kinds of differentials. Open diffs are what most cars have. If a wheel is off the deck, it spins at high speed and all the torque is directed into it. These work fine on the road but also get stuck the easiest, as one wheel losing traction means the other wheels get no force.
Then you get limited slip differentials. These are marvels of engineering that allow torque to be distributed to the side with the most grip, while still allowing some difference in wheel speed. They’re nice to have but expensive, heavy, and fragile compared to lockers. So much so that most AWD cars just have open diffs instead and try to simulate the behaviour of a torsen diff with the ABS computer.
Locking diffs are used on serious offroaders. When locked in, the axle is solid from end to end and both wheels receive exactly the same ‘pool’ of torque, even if a wheel is slipping. This allows the torque to naturally find its way through the axle into the wheel that isn’t slipping. They’re mechanically simple, rugged, easy to operate, and easy to trail repair if needed. Downside is that a locking diff on pavement causes driveshaft windup while locked, and steering is more difficult with everything locked in too.
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