A diff lock is invaluable for off-roading, but what if your vehicle isn’t fitted with one? Or if you want to add a second one?

The diff lock is that bit of hardware on your 4x4 that gets you out of the kuil at Papkuilsfontein, without you having to beg a farmer to pull you out with a tractor. 

Whether you are stuck in mud or sliding all over Sani’s ice, a diff lock ensures your vehicle maintains maximum traction when the wheels start spinning.

Off-roading is all about traction; spinning wheels get you nowhere fast. With good suspension and axle articulation you can keep most of your wheels firmy planted, but sooner or later, when you’re alone somewhere in the bundu, one (or two) of the wheels will hang, spinning uselessly. And that’s when your diff lock will save you.

Diff locks are standard features on many 4x4s. Toyota’s Land Cruiser 76 station wagon and the Mercedes-Benz G-Class, for example, have diff locks on the front and rear axles. Rear diff lock has also become a standard feature on most 4x4 double-cabs and a number of station-wagon models, such as the Nissan Patrol and Toyota Fortuner.

Some luxury off-roaders, such as the Jeep Grand Cherokee and Land Rover Discovery 4, have fancy electronic systems that perform traction control through the braking system – by individually braking spinning wheels – to maintain grip in off-road conditions. These often do the job just fine, but among their drawbacks is wheel spin. However, this article focuses on mechanical diff locks only.

How does a diff lock work?

Look underneath your 4x4 at the axles and you’ll notice a large, bulb-shaped housing in the middle of each axle. Inside this is the differential, a collection of gears that allows the two wheels on the same axle to turn at different speeds. 

Why is this important? Well, when you drive around a corner, the wheel on the outside has to travel further than the one on the inside in the same amount of time. Therefore, it has to turn faster. If the differential doesn’t allow this, the axle will wind up like a koeksister and eventually break.

While the differential is a very clever piece of engineering, there is one significant drawback: it’s a bit lazy. So if one of the wheels loses traction or hangs in the air the diff automatically sends all the power to that wheel, effectively removing all the drive from the wheel with grip. The result? The wheel with traction becomes idle while the useless one spins uncontrollably – and you’re stuck.

Enter the diff lock. As the name suggests, this device locks the differential gears so that the wheels work together as a unit. Drive is split equally between the two wheels, which means that the wheel with traction still gets 50% of the power. During off-road driving, where wheels routinely lose traction due to uneven or slippery surfaces, this can be the difference between sailing through an obstacle and needing a recovery.

Watch this brilliant and amusing little film from the 1930s about how a differential work:

Why fit a diff lock?

It’s better for your vehicle: Off-roading with the diff lock engaged is far more controlled. Obstacles that formerly required plenty of momentum to get through can now be tackled at a crawling pace, which means less of the wild, pedal-to-the-metal driving that can lead to bent side shafts and cracked CV joints.

It’s better for the trail: Only in Looney Tunes cartoons do spinning wheels leave you scrambling on the spot. In real life they damage the environment, tear up the trail and send debris flying.

It’s better for your ego: Nobody enjoys having to ask for a recovery, or needing ten attempts to cross that obstacle that your mates clocked first time. Sure, a diff lock won’t make you a better driver, but it’s certainly a great help.

Points to ponder

Do you actually need a diff lock? If you use your 4x4 primarily for local gravel road travelling where the odd washaway is about as rough as it gets, it’s not critical to have a diff lock.  

However, if you enjoy doing more technical trails, the chances of getting bogged down are higher and a diff lock is recommended. And if you have fitted 35-inch knobblies and head out regularly in search of the biggest, muddiest dongas, then diff lock is pretty much essential.

What’s more, if you’re a serious overlander who don’t always travel in groups, it is like an insurance policy – you might not need it once in 10 000 km, but when you need it, it might save you a great amount of effort and time.

Do your bucks have the bang? Fitting a diff lock is expensive. Apart from the unit itself (they range from R4 000 right up to R12 000), you may have to purchase additional accessories. ARB’s diff lock, for example, requires fitment of a R3 000 air compressor. 

Add in labour (most fitments require many hours in the workshop) and the possible need to have worn parts, such as bearings, replaced at the same time, and your credit card is in for a sneaky sucker punch.

Its handling will be affected. While locked, cornering will be difficult, since a locked differential prevents the wheels from turning independently.

It’s not your Superman certificate: There are many stories of 4x4-ers who got themselves into a dangerous fix because they had diff lock. Diff locks tend to make people feel invincible, making them more likely to push the envelope. It should never be a substitute for good judgment or off-road-driving ability.

Consider this before diff-lock fitment:

Is it available for your vehicle? Only a handful of companies manufacture these highly specialised pieces of equipment. As there is a bias towards Jeep, Toyota and Land Rover, the brand you want may not be compatible with your 4x4. In some cases, you may not be able to fit a diff lock at all.

Front, rear… or both? “Always fit a rear diff lock first,” is pretty much the experts’ standard answer. Traction is mostly lost while driving uphill, when most of a vehicle’s weight is on the back axle, so you want to ensure that the rear wheels have as much traction as possible. Rear diff locks also have less of an influence on your vehicle’s ability to turn than front diff locks. 

Of course, being able to lock both axles gives you a significant leg up.

Automatically or manually locking? You need to decide between a selectable locker (i.e. it is manually activated by pressing a button), and one that engages automatically when necessary.

Carefully weigh up the pros and cons: 

Selectable diff lock

These units replace your vehicle’s existing differential and need a button to be pressed to activate an external source (such as an electrical motor or compressed air), which in turn engages the diff lock.

Pros: 

• When the locker is turned off the axle behaves normally, so on-road handling isn’t affected. 
• You can decide when to engage the diff lock, which allows for precise control in technical off-road situations. 

Cons:

• You need to take one hand off the steering wheel to press the button, which isn’t desirable in the middle of a challenging obstacle.
• Forgetting to turn it off and driving at speed on tar with a locked axle, can seriously damage your differential.
• It is expensive to fit, because the separate actuating system also has to be installed.

Automatic diff lock

Automatic diff locks are always on while your foot is on the gas, only disengaging when you ease off the throttle to allow for cornering. This is why you need to adjust your driving style if your car has an automatic diff lock - and why some people hate them. Although most replace the differential, some are fitted into the existing differential. 

Pros:

• The diff lock will always work when you need it. You can therefore focus purely on driving and keep both hands on the steering wheel all the time. 
• Installation is quicker and easier than that of a selectable locker.

Cons:

• On-road handling is affected because the diff lock is almost permanently engaged. Expect some under steer and tyre chirp when cornering, as well as resistance on sharp turns, for example when squeezing into a tight spot in a parking lot.
• Auto lockers make a clicking sound as the gears engage, which some people find irritating or hard to get used to.

Which one should you go for? It’s a tough call. Some folk aren’t bothered by the handling and noise issues of the auto lockers; for others it’s a deal breaker. 

Our advice? If your 4x4 does a lot of tar driving, choose a selectable locker. Serious off-roaders who enjoy highly technical driving also prefer this option because they have complete control over engaging the unit. 

An automatic locker can be considered for vehicles that are used primarily for off-roading and overlanding. On a long, straight Kalahari road, you hardly notice they’re there. It’s great to know that if you do need a helping hand in the rough, you’re always covered.

And what about a limited-slip diff?

Limited slip diffs (LSDs) locks partially when traction is lost. Though obviously not as effective as a regular diff lock (they never fully lock up), they can still help in certain off-road environments, notably sand and mud. They also operate continuously, as with an automatic locker, but without any of the associated handling or noise issues. Therefore, they are becoming increasingly common as a standard factory-fitted feature in new 4x4’s. A small number of aftermarket LSDs are available. 

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