When equipping a vehicle to go off-road you soon realise that equipment designed to maintain forward motion falls into two categories.
The first is the 'get you out at all costs' group. This includes items such as powered winches, which can sometimes contribute to getting stuck in the first place, because of their high weight. Nevertheless they are very useful tools for getting yourself out of a problem. Other items in this group are aggressive, off-road-only, tyres, and kinetic recovery ropes. All these items are carried by people who are prepared to get stuck and be faced with the consequences — in other words, digging! Their chosen route is less likely to be dictated by suspect areas which
others might wish to avoid, particularly solo vehicles.
The second group can be described as traction aids, which could include aggressive tyres. Freewheel hubs, suspension upgrades and bridging ladders are likely candidates for this list. This sort of equipment is often chosen by those who are fitting out expedition vehicles. They are usually planning to avoid terrain which could result in getting stranded miles from anywhere, possibly in an inhospitable environment.
Expedition vehicles particularly can be heavily laden and therefore more easily bogged and even more difficult to recover. Discretion and caution are often watchwords for this group. The category of traction aids includes various forms of locking differentials - and that is what we are looking at here.
Locking differentials can be something of a mystery, possibly because of the different types available and the names used, such as limited slip, torque-proportioning and air lockers. Not only are there different makes but they also work in different ways. This means that if you decide that one of these items would be useful, some explanation of what it does might help. But as it only takes two spinning wheels to stop a Land Rover (unless yours has traction control) even with the centre diff lock engaged, a locking differential could make a dramatic difference to off-road performance.
Back to basics
Some understanding of differentials and how they work is necessary. Since the birth of mechanically driven vehicles, the problem of getting round corners while maintaining steering control has required the use of a differential in the drive axle. This allows two wheels on the same axle to rotate at different speeds while still providing forward motion. This is necessary when negotiating bends as the outside wheel has to travel further and therefore turn faster than the inside wheel.
This is achieved by the inclusion of a crownwheel and pinion gear and a group of smaller spider gears between the drive shafts within the axle casing. The system works very well until the grip provided by the road surface is overcome by the wheels, either on a loose surface or by the power of the vehicle spinning the wheels.
The differential principle shows its limitations when one wheel spins and the wheel at the opposite end is denied drive. This is why you only need to have one wheel on a grass verge or lift one wheel off the ground to get stuck with a vehicle with just one driven axle.
The answer is to have some means of temporarily locking the differential to maintain drive to both hall-shafts and the wheel on firm ground. This is achieved either by a mechanism totally enclosed within the diff that operates automatically or by an independent control outside the diff.
Tractors have had locking diffs for many years. These were usually operated by a foot pedal just ahead of the axle and could be engaged at will when the going got tough. A trick used by some farmers with Land Rovers was to tie the spinning wheel to the axle to stop it turning. This forced the opposite wheel to turn and with luck extricate the vehicle. This is not a technique I would recommend, except in an emergency.
Two of the best-known versions of locking diffs are the ARB locking differential, which is operated outside the axle casing by compressed air, and the Detroit units, which are automatic. There are pros and cons with both types and like so many things, choice can be subjective and cost of purchase and fitting is a major factor.
Generally speaking, automatic diffs are less expensive than the powered type, as they do not have the ancillary equipment required to operate them. Powered diffs also take longer to fit, about six hours.
The ARB system comprises the diff-centre itself, with associated pipework leading to a small 12-volt compressor. This is linked to two switches, one to power the compressor and one to control the air supply to the diff unit. Inside the locking unit a piston pushes a locking gear across to mesh with another gear driving the half-shaft on the opposite side.
The advantage of this system is that it can be operated at will. The diff remains fully open for use on tarmac and is locked when driving into areas likely to limit traction. One point to remember with this system is to make sure you switch on the compressor and diff lock before you get stuck.
This sounds obvious but is easily forgotten, and once you're stuck, a locked diff may not be enough to enable you to drive out.
Another point to remember is not to engage the diff with the wheels spinning, as the sudden shock load of traction regained can cause damage anywhere in the driveline from the diff, half-shaft, propshaft or even the gearbox. You may also have to switch off the lock when negotiating tight turns.
Limited slip diffs (LSDs)
Limited slip diffs (LSDs) have been ﬁtted to high-performance cars for many years. They allow traction to be maintained during the high torque of rapid acceleration, especially during cornering. Some LSDs rely on internal clutch plates to provide traction to both wheels. The plates have a certain amount of pre-load applied to them. As the half-shafts try to rotate at different speeds when one wheel is spinning, the pre-load increases to maintain drive to both sides of the axle.
There are a couple of disadvantages with this system. Firstly, if little traction is available to either wheel, the similar rotational speeds of the half-shafts will reduce the amount of pre-load on the clutch plates, diminishing the locking effect.
Also, as some pre-load is always present, the diff is never fully open. This can induce uneven tyre wear as the friction between the plates is fighting the effects of the unequal half-shaft speeds when cornering. This type of diff can be used on the front or rear axle.
Torque-Proportioning Differential (TPD)
The Detroit torque-proportioning differential (TPD) is a gear-driven system that does not need the pre-loaded clutches found on LSDs. Pairs of helical gears combine to lock gradually as one wheel starts to spin, in a similar way to the LSD, but unlike the LSD this type of unit does not need oil additives to enhance the friction abilities of the clutch plates. As the TPD is totally gear-driven, it can run quite happily in EP90 gear oil. As with LSDs, the TPD's locking ability is impaired if traction to both wheels is severely affected.
Automatic locking differentials (ALDs)
Automatic locking differentials (ALDs) rely on drive dogs to provide drive to each wheel. The diff is permanently locked and therefore continues to drive to one half-shaft, even when a wheel is lifted off the ground. When cornering on tarmac, torque generated in the outer drive shaft overcomes the pre-loaded mechanism in the diff and forces the dog clutch apart, to open the diff. This type of diff is very reliable as, unlike the LSD and TPD, it doesn't rely on ancillary components outside the axle case to operate it, but there does tend to be some resistance before the diff opens. This system can also generate some extra tyre wear.
Some users of this system have experienced a momentary directional instability as the diff suddenly opens half-way round a bend. This is more often an issue for short wheelbase vehicles, especially when towing heavy loads.
The principle of this system precludes it from being used on front axles for most users, but in theory it could be used on the front axle of Series Land Rovers, or other vehicles that are able to disengage drive to the front axle for use on the road or other high friction surfaces.
Many users seeking maximum traction choose an ALD in the rear axle and an LSD or TPD in the front. Another option is to have a pair of LSDs or TPDs ﬁtted to front and rear axles. This is probably the most user-friendly "automatic locking diff" configuration as there are no detrimental on-road characteristics.
Gone but not forgotten
Another system worth a mention, which has now disappeared is the Gleason Torsen unit. This was described at the time of its launch as "the first major breakthrough in differential design for more than a century". It was another automatic torque-sensing diff.
The all-important diff centre comprised a group of worm gears. A worm and wheel gear system can be powered either way but it cannot freewheel
backwards, at least not easily. The result of the Gleason Torsen gear arrangement was a diff that behaved in exactly the opposite way to a conventional diff, in that it provided torque to the gripping wheel rather than the slipping wheel.
I have no experience of this type of locking diff; nor have I heard how successful it was, so I would be interested to hear from anyone who has ﬁrst-hand knowledge. Land Rover tested Torsen differentials and found their performance "awesome", according to some involved in the testing.
If you own your off—road vehicle simply because of its ability to provide the extra traction you need to get you there in a working or leisure environment, the cost of a locking diff represents good value for money when you consider its ability to potentially double available traction.
Any locking diff will transform your vehicle's off-road abilities, but some consideration of the pros and cons of the different systems to make sure you get the right unit for your vehicle is worthwhile. Of course there are many factors to take into consideration, such as tyre choice. Don't be surprised if you are left floundering in the mud even with locking diffs if you have road-biased tyres fitted.
A vehicle without locking diffs may perform better just on aggressive mud tyres. if you are wondering which axle should be fitted with a locking diff, weight transfer when climbing means that it's the rear that will benefit far more from the extra traction. Generally speaking, if you do decide to fit a pair of diffs or even just one — prepare to be impressed.