Differential locks or diff-locks can improve a vehicle’s off-road capability and minimise the chances of driveline damage off-road. Let’s take a look at what they are, how they work, if they’re fitted to your rig and when to use them.
These days all modern four-wheel drives come standard with electronic traction control (ETC), and many ETC systems are backed up by the fitment of a factory rear differential lock. Understanding how these different yet complementary systems work, and how and when to use them, can greatly improve the off-road capability of both you and your vehicle.
Differential locks have been around much longer than ETC, but before we get into what they are and how they operate we need to understand the function of a vehicle’s differentials.
Vehicles with part-time 4X4 systems have differentials on the front and rear axles. The function of these differentials is to allow the opposing wheels across the front and rear axles to rotate at different speeds when the vehicle is driven around corners. This is necessary because the outside wheels take a wider arc and therefore need to turn more – and faster – than the inside wheels as they have further to travel.
The most straightforward differential type is the open diff, which allows for free differential action across an axle by distributing power to the wheel with the least resistance – when cornering on road this is the outside wheel. This is great for driving on hard road surfaces but not so great when driving off-road.
If you’re driving off-road along an undulating track, for example, and one of your wheels lifts off the ground, an open differential will distribute all the power to the wheel with the least resistance (the one off the ground) and none of the power to the wheel with the most resistance (the one on the ground). The result? The wheel in the air will spin freely (and uselessly) while the wheel on the ground will not get the power it needs to drive the vehicle forwards or backwards.
With open diffs, if you have one front wheel in the air and one rear wheel in the air, you won’t be going anywhere fast. Of course, if you apply some brake pressure, the wheels in the air will gain more resistance and the open diff will distribute some power to the wheels on the ground resulting in (hopefully) forward or backwards momentum. This is essentially how ETC works, but we’ll get to a bit later…
Many vehicles are equipped with a limited-slip differential (LSD), usually on the rear axle, and, as its name suggests, this kind of diff is designed to send at least a limited amount of power to the wheel with the most amount of resistance.
An LSD works on the road as it still provides limited differential action when required for cornering. It also provides some off-road benefits. In the off-road driving example mentioned previously, where one wheel across an axle is in the air and the other is on the ground, an LSD should distribute enough power to the wheel on the ground to provide continued forward or backward momentum.
It should be noted, however, that some LSDs work better than others and, in more extreme off-road situations, a limited-slip diff will not always distribute enough power to the wheel with grip in order for a vehicle to maintain momentum.
The only way to ensure both wheels across the axle receive equal power, regardless if one wheel is on the ground and the other is in the air, is by locking the differential.
Of course, locking the differential means there is no longer any differential action, which does not allow for the required wheel speed differences when cornering on the road, so locking differentials need to be in an unlocked state when driving on the road.
The differential should only be locked when required off-road. There are various locking differentials on the market that operate in different ways, but they all essentially have a mechanism inside the differential housing that locks up the diff, preventing differential action. Some diff locks operate automatically while others are activated manually via a switch.
Auto-locking diffs include the Detroit Locker, the 4WD Systems Lokka and some OE lockers such as those used on Land Rovers, while there are a number of manually locking diffs that are either engaged electronically (such as most OE diff locks, as well as the Harrop Eaton ELocker) or via compressed air (such as the ARB Air Locker and the TJM Pro Locker).
What about Electronic Traction Control?
The perfect electronic traction control (ETC) system would mimic the effects of an auto-locking differential, allowing full open differential action on the road and no differential action in off-road situations where a vehicle has wheels in the air. But there is no perfect ETC, which is why many vehicles also come standard with at least a mechanically locking differential on the rear axle.
In off-road situations, if ETC detects wheelspin via the vehicle’s ABS sensors, it applies brake pressure to the spinning wheels which in turn causes the differential to distribute power to the non-spinning wheels (those with grip). As mentioned previously, the driver of a vehicle with open diffs and no ETC can apply brake pressure manually to achieve a similar result, but ETC is more effective as it is able to brake the individual wheel.
Despite the fact modern 4X4 wagons and utes are equipped with ETC, this electronic traction aid is not as effective at arresting unwanted wheel spin as a locking differential, which is why many 4X4s will offer both ETC and a locking rear diff.
Is your vehicle equipped with a factory rear diff lock?
If you’re not sure whether your vehicle is equipped with a factory rear diff lock, you really should try to find out. Have a look in the owner’s manual, or have a look around the centre console and of the dash for a switch labelled ‘rear diff lock’ or with a diagram depicting the rear diff with an ‘x’ through it.
Hit the diff lock switch and there should be a light or message somewhere on the instrument binnacle to tell you that the rear diff lock is engaged.
A rear diff lock is not to be confused with the centre diff lock that some full-time 4X4 vehicles have. When engaged, a centre diff lock ensures even distribution of power between front and axles rather than across the rear axle. When disengaged, a centre diff lock allows for differential action between the front and rear axles, which is important on the road to allow for different wheel speeds front and rear.
When should you use a rear differential lock?
We’ll start off by pointing out that you should not use your rear diff lock on a sealed high-grip surface, especially if you’re trying to negotiate a tight turn where you’ll need differential action. If the rear diff is locked when cornering on the road, the inside wheel will break traction because it will turn at the same speed as the outside wheel, and this will affect vehicle handling.
For off-road use engaging the rear diff lock before you need it is the best option, such as when you predict one of the rear wheels might lose traction due to undulating terrain, large rocks or driving through mud. This will ensure both rear wheels will receive equal drive no matter how much (or how little) grip they have.
For vehicles equipped with an effective ETC, you may never feel the need to engage the rear diff lock in your off-road travels, but remember, ETC works by braking individual wheels, so extended reliance on ETC alone can result in overheating the brakes. For long off-road stints where traction is minimal and you can hear the ETC working hard (trust me, you’ll hear it), you should engage the rear diff lock.
Also bear in mind that not all ETC systems are equal, and they also interact in different ways with their respective OE rear diff locks depending on vehicle make and model. For example, on some vehicles (such as Toyota HiLux and Isuzu D-MAX), engaging the rear diff lock will disengage the ETC on the front axle, and in some off-road scenarios this can actually reduce off-road capability as there is no way of arresting front wheel spin should a front wheel lift off the ground, other than by manually applying some braking pressure.
On other vehicles (such as Ford Ranger and Volkswagen Amarok), ETC remains active on the front axle even when the rear diff lock is engaged, so locking the rear diff will improve off-road capability.
All locked up
On some vehicles, such as the full-time 4X4 Mercedes-Benz G-Wagen, OE diff locks are fitted in the front, centre and rear axles. This setup is the Holy Grail of traction aids because when all three diffs are locked, there is equal drive to all four wheels; even if three wheels are in the air, the wheel still on the ground will get the drive. This is also the case with part-time 4X4s such as various Toyota LandCruiser 70 Series models that have front and rear diff locks, as there is no centre differential.
If your vehicle is not equipped with factory diff locks, there are plenty of aftermarket options on the market. And even if you have a factory rear diff lock, you can still improve off-road capability significantly by fitting an aftermarket front diff lock.
Oh, and having front and rear diff locks isn’t only about improved off-road capability; having lockers allows you to drive over many off-road obstacles with less right boot, minimising stress on driveline components and lessening the chance of breakages.