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Diff locks: What they are and how they can help you get the job done

‘Four-wheel drive’ implies that all four wheels are being driven all the time. (image credit: Dean Johnson)

4x4 utes have become essential tools for many tradies, as their rugged ladder-frame chassis and tough truck DNA mean they can easily carry tools plus a decent payload, tow a sizeable trailer and - if a dual cab – transport a crew too.

Having four-wheel drive also means they can usually avoid getting stuck on a boggy construction site, allow a diesel mechanic to drive his tools of trade right alongside an earth-mover that’s broken down on site, or ensure a boat builder can tow trailers up slimy boat ramps or off beaches on rising tides.

However, any of these situations and many others could require more traction than a standard 4x4 drivetrain can deliver, regardless of whether the vehicle is equipped with a centre diff or a limited-slip diff or the chunkiest off-road tyres money can buy.

The best way to maximise your 4x4 (and also 4x2) ute’s traction is to install a locking differential. However, there are several different types available, so before you part with your hard-earned here’s a few tips to help ensure you make the right choice.

What is a differential and how does it work?

An automatic locker, remains locked until sufficient cornering force is applied to the wheels to make it unlock to allow some differential action. An automatic locker, remains locked until sufficient cornering force is applied to the wheels to make it unlock to allow some differential action.

There’s a popular misconception about ‘four-wheel drive’ and it stems from the name itself, which implies that all four wheels are being driven all the time. While it’s true that in 4x4 mode the engine’s drive is being sent to the front and rear axles simultaneously, the fact is that only one wheel on each of those axles is being ‘driven’ most of the time.

That’s because mechanisms called differentials, which are fitted inside the front and rear axle housings, are designed to not only deliver drive through separate driveshafts to each of two wheels but also allow those wheels to rotate at different speeds.

A diff with a limited amount of slip can never match the traction of a diff with no slip. (image credit: Malcolm Flynn) A diff with a limited amount of slip can never match the traction of a diff with no slip. (image credit: Malcolm Flynn)

Why so? Because during cornering, the outer wheel has to travel further and therefore faster than the inside wheel, otherwise it would be impossible to turn the vehicle. As a result, all of the power goes to the wheel with least resistance, which is always the outside wheel because it’s always rotating faster. Hence the name, as it allows for the ‘differential’ in these turning distances.

Why would you need a diff locker?

A 4x2 ute with diff lock engaged can easily extract itself from most sticky situations a tradie might find him or herself in. (image credit: Dean Johnson) A 4x2 ute with diff lock engaged can easily extract itself from most sticky situations a tradie might find him or herself in. (image credit: Dean Johnson)

While a standard or ‘open’ differential works superbly during on-road cornering on grippy bitumen roads, it can actually work against the vehicle in situations where traction is compromised, like our slippery boat ramp or rugged worksite examples.

Given that a differential sends all of its drive to the wheel with the least resistance, it’s obvious that if a wheel breaks traction and power continues to be applied it will result in useless wheel-spin while the other wheel sits motionless. If this happens at both ends of a 4x4 at the same time, you’re not going to be leaving that boggy worksite any time soon.

You might also be thinking that some full-time 4x4s transmissions fitted with a third or ‘centre’ differential might be better off in this situation. The answer is still no, because although a centre diff can automatically distribute drive front to rear, it still relies on the differentials to distribute that power side-to-side. So, the same potential traction problems apply.

Or perhaps a limited-slip diff, fitted as standard equipment in the rear axles of some utes, could be the answer. While there’s no doubt an LSD can increase traction by limiting the amount of wheel-spin, they usually rely on pre-loaded clutch packs to do so. And these can become less effective over time, usually due to wear in the clutches which can also lead to poor adjustment and overheating.

Either way, a diff with a limited amount of slip can never match the traction of a diff with no slip – which is where the diff lock comes in.

How do diff locks work?

An open differential can work against the vehicle in situations where traction is compromised. (image credit: Dean Johnson) An open differential can work against the vehicle in situations where traction is compromised. (image credit: Dean Johnson)

In simple terms, it’s a precision-engineered mechanism fitted to a standard diff which on demand over-rides the differential effect. In other words, it effectively ‘locks’ the left and right driveshafts together, so they work like a single shaft connecting the two wheels.

The end result is not only an ideal 50/50 distribution of drive to each wheel. The locking effect also ensures that if one wheel’s traction is compromised, the other wheel continues to put its share of drive to the ground to keep the vehicle moving forward. As a result, the overall traction available from a locked diff is vastly superior to a standard or limited-slip diff. In fact, if fitted to front and rear axles, a 4x4’s off-road ability is doubled.

Front axle, rear axle or both?

Ford installed a diff lock in its high-riding Falcon RTV 4x2 ute in the mid-2000s. Ford installed a diff lock in its high-riding Falcon RTV 4x2 ute in the mid-2000s.

If you’re only fitting one locker to a 4x4 ute, it’s preferable to install it in the rear where it will be most effective. This is due to the dynamic weight transfer from front to rear when power is applied and because the rear of a tradie’s ute is usually heavily loaded with tools and other equipment which can provide extra downward pressure on the rear tyres to help them bite.

The same dynamics also apply to 4x2 utes and we should point out how effective they can be. If driven intelligently in terms of rear wheel placement, a 4x2 ute with diff lock engaged can easily extract itself from most sticky situations a tradie might find him or herself in. That’s why Ford installed a diff lock in its high-riding Falcon RTV 4x2 ute in the mid-2000s and why Ford also provides one in today’s Ranger Hi-Rider 4x2 ute.

Regardless of which diff lock you choose, the doubling of grip they can deliver will make your 4x2 or 4x4 ute a more effective workhorse. (image credit: Mark Oastler) Regardless of which diff lock you choose, the doubling of grip they can deliver will make your 4x2 or 4x4 ute a more effective workhorse. (image credit: Mark Oastler)

However, if you’ve got a 4x4 ute and your budget can stretch to fitting locks in both the front and rear diffs, then you’ll have the ultimate in all-terrain traction. Off road equipment specialist ARB describes this best.

“When you are in 2WD mode, you have drive going to one of your rear wheels, giving you 25 per cent of all available drive. Engaging 4WD mode will send drive to one of the front wheels also, giving you 50 per cent of all available drive. This is where lockers come into play. Engaging your rear locker will send drive to both rear wheels (and) coupled with the drive going to one of the front wheels will give you 75 per cent of all available drive. Engaging the front locker with the rear locker will send equal drive to all four wheels, giving you 100 per cent of all available drive.” And you can’t top that!

Selectable or automatic?

The best way to maximise your 4x4 (and also 4x2) ute’s traction is to install a locking differential. The best way to maximise your 4x4 (and also 4x2) ute’s traction is to install a locking differential.

There are two types of diff locks available from quality aftermarket suppliers in Australia – selectable and automatic. The selectable design allows engagement and disengagement from the driver’s seat with the push of a button as required, which gives the driver full control. This can be particularly handy with a front diff lock, as you may need to disengage it at times when you need to make a sharp turn.

An automatic locker, by comparison, remains locked until sufficient cornering force is applied to the wheels to make it unlock to allow some differential action.

Some selectable units use compressed air to trigger locking and unlocking, supplied by compact onboard compressors (usually mounted in the engine bay) and robust pneumatic lines connected to the axle housings. Others use an electrical magnet. Either way, if running two diff locks, engagement of each can be done independently.

Selectable and automatic diff locks each have their pros and cons and rusted-on loyalists who swear by each. However, deciding which type is best for you requires some careful analysis of each design and what they offer to ensure you make an informed choice.

For example, an air compressor needed for diff lock engagement could also be handy for other tradie tasks, like increasing tyre pressures for heavy loads or blowing dust out of work clothes and power tools at the end of the day. Or you may prefer the relative simplicity of electric engagement.

Conclusion

Plenty of 4x2 and 4x4 models are available with factory diff locks these days. (image credit: Malcolm Flynn) Plenty of 4x2 and 4x4 models are available with factory diff locks these days. (image credit: Malcolm Flynn)

Regardless of which diff lock you choose, the doubling of grip they can deliver will make your 4x2 or 4x4 ute a more effective workhorse when traction levels are compromised. It could also mean you never have to call home to apologise for being late, because you’re ‘stuck’ at work!

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