What is a diff-lock and when do I use it?

The term diff-lock is an abbreviation for a lockable differential, and they’re also known as lockers. There are a few different types, but before we get into why you’d want to lock your differential, we need to explain what a differential is. That starts with the problem it solves.

Imagine you’ve got two wheels on an axle, as shown in the picture below:

When that axle turns a corner, the inside wheel describes a shorter arc than the outside wheel. Yet, both wheels take the same amount of time to complete the turn. This is fine if the wheels just rotate freely on the end of an axle. It becomes a problem if we want to drive the wheels with the engine.

We could simply put a couple of cogs in and have a driveshaft turn the axle. But that would mean both wheels have to rotate at the same speed. If we then tried to go around a corner, the inside wheel would rotate at the same speed as the outside wheel, and the inside wheel would hop, skid and jump as it is forced to rotate at a much faster speed than it should for the little distance it has to travel. The problem with this is stress on the axle, tyre wear, poor handling, and a tendency to ‘understeer’, which means the nose of your vehicle will push wide in corners.

So, how do we fix it?

What’s needed is a way to drive both wheels, yet allow them to turn at different speeds around a corner. There is a solution, and it’s called a differential, or diff for short. It is a complex series of cogs that allows the inside wheel to slow down relative to the outside wheel. Yet both to be driven. Exactly how the cogs do this isn’t important from the driver’s perspective; what’s important is the effect.

The diagram below shows that the inside wheel is following a shorter circle than the outside, and rotating slower. The purple dot represents the differential, which allows this to happen, yet drives both wheels. The blue arrows show the relative speed of each part of the driveline.

The photo below shows an axle with a differential housing as a bulge on the right. Inside it are the cogs that make up the differential. This is a bit unusual as most differentials are in the middle of the axle.

There’s always a catch

So, differentials solve the cornering problem. But they come with a huge disadvantage for off-road driving, which is shown in this picture:

You see in the picture that the front right wheel is spinning madly, but the front left isn’t moving. That’s because differentials are lazy; they direct the drive to the wheel that’s easiest to turn. To be precise, they equalise torque (turning force) between two wheels on an axle.

So if we have a wheel with little grip compared to the other one on the axle, for example, in the air or on mud, that’s pretty easy to turn, so it gets very little turning force, and that’s how much its partner wheel on the other side of the axle gets. That’s often nowhere near enough to move the vehicle, and we call this the “differential problem”.

Understanding the differential problem is one of the fundamentals of off-roading. It is the key to understanding not just what a diff-lock is and does, but lots of other traction aids and driving techniques too.

The devil is in the details

So by now, you may have guessed what a diff-lock is. It’s a device that disables the differential, locking it out. That means both wheels have to turn at exactly the same speed, so it eliminates the differential problem. But it also eliminates the good part about a differential. The part that allows the wheels on an axle to be driven at different speeds and vehicles to turn corners effectively. That’s why diff-locks can be engaged or disengaged. You have it engaged when you need it, for example, ascending a steep, rocky climb, and disengaged when you don’t, for example, normal road driving.

There are two types of diff-locks

There are two basic types of diff-lock, manual and automatic. Most are manually engaged and disengaged. But there are some designs that are automatically engaged and disengaged by mechanical means or computer control. Lockers are for slow-speed off-road work on loose surfaces, so losing the differential effect around corners isn’t too much of a problem. You just notice the turning circle is bigger than before.

What we’ve talked about so far is not just a diff-lock; it’s a “cross-axle locking differential” to give it the full name. That’s because it’s a differential lock that works across two wheels on an axle. Exactly the same concept applies to the relative rotation of the front axle to the rear. That’s called a “centre differential”. Just be careful to know that a cross-axle locker is different to a centre diff-lock, even though the concept is the same. We also call a differential that has no locker an open differential, or open diff.

Here’s what it looks like on the dash

The photo below shows a Toyota Prado. It has its centre diff-lock engaged, which is shown on the speedo by the orange icon of four wheels with an X in the middle. It also has its rear cross-axle locking differential engaged, shown by the equivalent red icon but with an X between the rear axles.

Can I fit an aftermarket diff-lock to my 4WD? And should I?

Maybe it already has one or two as standard. If not, the answer is perhaps. There are a number of aftermarket companies that supply diff-locks. But not for all makes and models, only those most popularly used for off-roading. If there is a locker available it can be fitted by a mechanic. Examples of aftermarket locker companies are ARB and Eaton.

Diff-locks are definitely not the first things to buy for an off-roader. Safety equipment such as a fire extinguisher and first aid kits are a higher priority. Then suspension, tyres, snorkels and the like. Once you have everything else sorted, then consider diff-locks. If your vehicle is a modern one with good electronic traction control, then you have less need for diff-locks compared to an older vehicle with no electronic traction control. That’s because an electronic traction control system mitigates the “differential problem” in a similar way to a diff-lock.

The photo below shows the driver’s controls for front and rear ARB Air Lockers. They are called air lockers because the mechanism to engage and disengage is driven by an air compressor.

Most owners fit aftermarket lockers to the rear of the car as that is typically where the most benefit is when driving off-road, and the rear axle is stronger than the front axle, not least as it doesn’t have CV joints. However, some owners of vehicles such as the Nissan Patrol fit them to the front axle, on the basis that the rear already has a strong limited-slip differential (LSD). An LSD is part-way between a locker and an open differential.

Which cars have diff-locks as standard?

It could well be that your 4WD comes with a diff-lock on the rear axle, or on both axles. Examples of cars that have rear lockers are the Ford Ranger, Pajero Sport and Nissan Patrol Y62. For front and rear lockers, examples are some Land Cruiser 70 Series, Mercedes-Benz G-Wagen and Jeep Wrangler Rubicon. We call vehicles with front and rear diff-locks twin locked.

Not all versions of all 4WDs have diff-locks.  Look for a button like this on a Nissan Y62 Patrol. On the right hand side of the picture there’s an icon of four wheels with an X between the rear axle. That’s the button to engage and disengage the rear locker.

There are no production 4WDs that have lockers on the front axle and not the rear. It’s always rear axle only, or both axles, and you have to engage the rear before you can engage the front. That’s to minimise the risk of stress and damage to the drivetrain. Lockers can easily produce significant extra stress on transmission components such as axles.

When should I use diff-locks?

Don’t fall into the trap of thinking that whenever the going gets tough, you engage your diff-lock and life is always better. Sometimes it’s best to leave it disengaged. There are so many off-road situations that it’s not possible to give specific advice, and you will need to break each of the rules below at some point, but here are some general tips:

Use lockers when

• Your progress is likely to be impaired by having a wheel on the axle with lockers spin. For example, a rutted hill climb or some muddy ruts. The Jeep Wrangler Rubicon pictured below has its lockers engaged as it ascends a hill. You can tell because the front right wheel isn’t spinning.
• Descending a rutted hill where one wheel is likely to be in the air or lose traction.
• Generally, anywhere where you’re moving in low range first or second, you’re likely to lose traction on one or more wheels, but there’s good traction on the other wheels, and you don’t need to turn tightly.

Don’t use lockers when:

• You need to turn tightly, for example, on a sharp switchback, because the lockers will increase the turning circle.
• All four wheels are on the ground, with roughly equal weight on each of them, or with equal weight distributed across two wheels on an axle. The wheels are likely to have similar grip, so preventing one wheel spinning isn’t a problem.
• On side slopes. The vehicle will crab sideways due to the differential not allowing wheel slip across an axle.
• You’re going relatively fast and need to manoeuvre, for example, in sand, and in some situations, in mud and snow. The Jeep Grand Cherokee below needs counter-steer to continue heading up the hill, and lockers would make that harder. It’s also going relatively fast.

So, to wrap this all up

The term diff-lock is short for “differential lock”, and is also shortened to lockers. There are two types. Cross-axle differential-locks which are described here, and centre locking-differentials (centre diff-locks), which we’ll explain in greater detail in another article.

The differential itself means that two wheels on an axle can be driven at different speeds. But that means one wheel on an axle could spin when it loses traction, causing a lack of progress. The locker fixes that problem by eliminating the differential effect.

There are two types; most are manual, but some are automatic. Some 4WDs come with them standard, either on the rear axle, or front and rear axles (“twin locked”). Depending on the exact make and model of your vehicle, you may be able to fit an aftermarket locker. Lockers are very useful off-road, but shouldn’t be viewed as the answer to all traction problems, and fitting them should not be a priority for new off-roaders who should focus on safety gear first.

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