…and it’s not a bad thing. Nothing has challenged the locker’s status as the ultimate traction tool.
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Traction is at the heart of everything we four-wheel drivers do, which is why there’s so much discussion about traction aids. Unfortunately, not all of that discussion is based on facts – so before we get into the contentious subject of traction control vs locking differentials it’s best to back up and cover the important concepts.
4X4s need both left and right wheels on an axle to be driven. This is no problem until you turn a corner, when the outside wheel takes a longer path than the inside.
If both wheels are on one axle this leads to scuffing of the inside tyre as it is forced to rotate at the same speed as the outside tyre, while it travels a shorter distance.
To solve that problem cars run two shorter axles, one on each wheel, connected to something in the middle called a ‘differential’. A differential is a clever system of gears that permits the outside and inside wheels to both be driven, yet at different speeds. However, the differential has a problem. It will send ‘drive’ to the wheel that is easiest to turn. Like above:
With a differential, when one wheel is in the air or has little traction relative to its partner, then that wheel will spin uselessly and the wheel on the ground that has traction will get insufficient torque (turning force) to move the car… so you go nowhere. That is the basic problem, which is what various traction systems are trying to solve. Traction control and cross-axle lockers are both solutions to the problem, as are limited slip differentials and even (to some degree) long-travel suspension.
A differential without any devices to help fix this problem is known as an ‘open differential’. A differential that can be locked out so both wheels on the axle turn at exactly the same speed is known as a cross-axle locking differential, or ‘locker’ for short.
Then we come to traction control – a much-maligned term often incorrectly used to describe all electronic traction systems. In fact, there are three related but distinct subsystems: Stability control, engine traction control and brake traction control (BTC). Stability control curbs understeer and oversteer, helping keep you rubber-side down, braking individual wheels to keep you pointing straight, and even reducing the available throttle. You want that off when you’re in 4WD territory as it gets overexcited in soft conditions like sand, mud and snow… sapping valuable momentum. But it is life-saving on bitumen or high-speed dirt roads.
Engine traction control is similar, except it just detects excess wheelspin and chops the throttle to bring things under control. You don’t want that for off-road work either… again particularly in the soft stuff.
The system that’s really useful is brake traction control, or BTC. This is when the car brakes a single wheel that would otherwise be uselessly spinning, to increase torque to the non-spinning wheel. BTC operates on all four wheels; whereas lockers may be fitted to the rear, front or both axles.
So it’s BTC vs lockers, and now we can get to the question of which is best. I’m going to draw on my decades’ worth of off-road vehicle testing, over which I’ve seen BTC improve hugely, and compare the very best 2016-spec BTC – for example LC200, Pajero Sport, Discovery – against lockers. On the other hand, lockers cannot improve; they’re either locked or they’re not.
Because lockers force the two wheels on an axle to travel at the same speed there’s no reaction time; unlike BTC, which needs a momentary slip of the wheel before it kicks in. That means vehicles with lockers can inch their way up obstacles at a literal snail’s pace in a more controlled, slower manner than cars with BTC. Demonstrations of lockers are always done in such environments. Lockers also don’t overheat, unlike some older BTC systems.
That’s about it for locker advantages. The disadvantages include a loss of manoeuvrability, because lockers mean the differential cannot let the two wheels on an axle travel at different speeds – so the effect is to increase the turning radius and cause tyre scuff, adding resistance which increases the chance of getting stuck. This is true if you have one locker, and doubly so if you have two.
One tip for locked diffs in hill descents: Engaging lockers helps stop the car running away downhill when it lifts a wheel, but again electronics are fighting back. The very latest hill descent systems work down to 2km/h and are superb; they’re better able to manage descents than drivers.
Now for BTC advantages, and the first is that it is easier to use than lockers, because BTC just works all the time. Lockers need to be manually activated, leading to potential driver error; and sometimes they don’t come in or out quick enough. And mechanically-automated lockers are difficult to drive as often they come in when you don’t want them to (not least around wet roundabouts). Most designs freewheel the outer wheel around corners, which is a problem because the inside wheel is the one to spin up when cornering – and then you go straight from an open to a locked differential, leading to unscheduled excitement.
Modern BTC will almost always beat a single locker. Take a vehicle that’s crossed up on a muddy slope, diagonal wheels in the air, and it’s got a rear locker only. You move off slowly, but the car is driven really only by the rear wheels and that’s too much torque for the single rear tyre with traction which slips sideways. In contrast, BTC will very slowly feed in just as much torque as the two wheels on the ground can handle (like a twin-locked vehicle), and you’re away.
Another BTC advantage is less driveline stress. You know when an open-diffed vehicle lifts a wheel and it comes smashing down to earth? That’s a big driveline shock. Because BTC brakes that spinning wheel there’s a bit less of a shock. On the other hand, BTC uses the brakes so there is a little extra brake wear – but it’s not significant as just one wheel is being braked, not the weight of the whole vehicle.
There are a couple of other points about single lockers. First, often engaging a factory locker on the rear will disable BTC on the front axle.
There’s no specific engineering reason for this, it’s just laziness and a problem because you’d want your locker and BTC working for you. Examples of vehicles makes with this issue are Toyota, Ranger PX, Pajero, Pajero Sport and MU-X. The Patrol Y62, Navara NP300, Everest and Ranger PX2 have BTC active with their rear lockers engaged; and yes, it makes a difference.
If you fit an aftermarket locker to a vehicle with BTC then you don’t have this problem. The BTC on the locked axle will never activate, as both wheels will always rotate at the same speed. You will get the best of both worlds: BTC on the unlocked axle, and a locker on the locked axle.
So here we are in 2016, and I will say this – you’re better off with a modern BTC system working on all four wheels than a single rear locker, in about 95% of off-road situations. The few exceptions are very high-traction ground where the car is out of suspension flex and you don’t mind the loss of steering ability – slick rock is an example. If we change it to twin-locked then BTC’s advantage is significantly eroded, but given that modern BTC can pull a car out of a cross-axled situation up a hill (something older systems couldn’t manage) then it’s only a matter of time before lockers are obsolete.
We’ve got torque vectoring and electric individual-wheel-drive in road cars now, both of which more effective than BTC, and it’s only a matter of time before they appear in 4X4s.
That said, the ideal is to have a car with BTC and at least one locker because there’s still situations where lockers beat BTC (even if those situations are becoming fewer and fewer).
The litmus test for me is this: I’ve got to drive a route that’s unknown other than it will cover every possible off-road terrain and my life depends on it. There are two otherwise identical cars, one with open/twin-locked diffs, one with the best BTC on the market. I’d take the electronic car, every time. The tech is just getting better every year; whereas there’s no way a locker can be improved.