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
Because even the best tuned AWD systems seem to struggle with low traction conditions. They don’t instantly redirect the torque to the wheel with traction. I’m not sure if this is a mechanical/programming limitation, or if ti is that AWD vehicles need a traction control system that is safe and well behaved in normal on-road driving.
In any case, the AWD systems all seem to have some delay. When climbing an uneven hill this delay leads to loss of momentum as the vehicle is climbing. A locked differential transfers torque instantly to the wheel with traction, so it is easier to keep up momentum. But a locking differential often depends on the driver to disengage it for safe on-road driving.
There are a lot of videos on YouTube showing how well different AWD systems do under challenging conditions. Some systems do much better than others. Subaru generally does better than most.
I think the key there is serious off road vehicles. In a pretty extreme off-roading situation, it’s likely that you will have no traction on one wheel, and then very quickly change to having no traction on the other wheel. In some offroad situations, you need to maintain momentum, and a computer controlled AWD system may react too slow to keep momentum, simply because it’s not designed for playing in the mud, or rock crawling. A computer controlled all wheel drive system is generally designed to help you maintain control of the vehicle and prevent slipping and sliding which could lead to an accident. Off-roading, especially with a locked differential, requires you having knowledge of how the vehicle will handle, and anticipating a certain amount of slipping and sliding.
Imagine your left 2 wheels are on ice and right 2 wheels are in a snow. All the wheels have very little grip. Where should AWD send the power? While it searches for the wheel that has grip, you are likely to dig yourself in with the right wheels. With locked diff, you simply put the same power everywhere and maximize your chances of going.
In off-road situation you may have a combination of gravel, stone, mud and loose mud under all of your wheels. First, computer will struggle to figure out what to do. Second, the last thing you want to do when going through the hard surface is for the computer to change your torque. Suddenly shifting all the power to the front right wheel will send your car to the left – and you may not want that at all. Keeping it stable and predictable is very important.
Most importantly – you never lock your diff on hard surface. And on the loose surface locking diff doesn’t affect steering.
First of all, poor cornering is not a “downside” of a locked diff amy more than poor acceleration is a downside of breaks, because you’re not meant to have your diff locked when you’re cornering. As in, you’re going to be told that in the user manual, don’t have your diff locked in normal operation.
Locking the diff is meant for the harshest of off-road situations, where some your wheels will often loose all of their traction, either by spinning mid-air or in loose soil/mud/snow. With a locked diff, you do not lose any torque on spinning wheels with no traction, instead those with traction get full power at all times. This makes it possible for the vehicle to get out of a situation that it otherwise might get stuck in, even with computer controlled AWD.
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.
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A traditional AWD system doesn’t really work the way that you are implying. The torque distribution is set by gearing in the transmission which some system could change but in descretw steps. In order to replicate the benefits of a locking differential cars like the Outback use independent control of the brakes to lock up any free spinning wheel. However, this requires sensors and algorithms that will allow slippage since you cannot just permanently lock the brakes the same way you can lock a differential. A locking differential can just be locked and that is that, no slippage.
Something like a Rivian quad motor is able to achieve the level of torque vectoring that you are describing and they don’t have locking difs
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.
I can make this an easier explanation than most
Lets say you have
1) Front wheel Drive
2) An open differential
If your front left tire is in snow and your front right tire is on pavement, you’re not moving anywhere because, in an open differential, the tire with the *lowest* traction (least resistance) will spin.
With a locking differential, both wheels will spin and you can get yourself unstuck.
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