The most important thing about driving off-road is getting the power from the engine through the wheels and onto the ground.

It’s an undeniable sequence of events that marks the difference between moving forward and standing still, and although there may be different driving techniques to help you maintain motion, depending on the type of surface you’re attacking, the basic premise remains the same – the more wheels you’re getting the power through, the more chance you’ve got of arriving at the place you want to be.

A spinning wheel is a wheel with zero traction, so it can sustain zero torque. Most standard 4WDs have open differentials on the front axle (and often on the rear one as well), and since an open differential always provides equal torque to both wheels, when one wheel loses traction both wheels receive zero torque and there is no force to propel the vehicle. Suddenly your 4WD is no-wheel drive.

The logical solution is to by-pass the axle differential altogether so both wheels are turning at the same time - but that’s where a basic problem comes in because you need the differential so the wheels can rotate at different speeds when cornering.

So the trick is to be able to have an open differential for cornering, or have the ability to lock it when you need it. Enter the locking differential, or “locker” as it is commonly called.

However, there are some disadvantages. The locker sends power constantly to axles, bearings, U-joints, and other components that may not have been designed for those kinds of torque loads.

In a corner, tyres may scuff until the driver disengages the locker, although this isn’t as noticeable on gravel as it is on the tar seal, and you will find that having both rear wheels pushing equally creates understeer, or a resistance to turning, even if the locker is in the rear.

On very slippery surfaces such as wet grass, clay-mud, or snow and ice, or in off-camber situations, a locked rear is prone to side-slipping, which can easily put you right off the track.

So before you go to the expense of fitting a locker, discuss it with your retailer to ensure that you’re going to need one, and if so know how and when to use it.

Limited slip diff

There’s also another type of off-road differential, and that’s the limited slip diff (LSD). Not to be confused with a full locking diff, this works in a totally different way, although it goes a long way to solving loss of traction in some cases.

Most LSDs found on 4WDs use clutch plates, springs, cones or some other means to apply friction between the side gears and the case to transfer torque from a spinning to a non-spinning wheel – although top-end luxury carmakers tend towards the more expensive torque sensing differential which uses gears.

An LSD does not lock the two axle shafts to spin at the same rate as a full locker does, but rather biases torque to the wheel with more grip by up to 80 percent. LSDs are rated by the amount of torque bias they will allow.

The limited slip’s advantage is that it performs like an open differential until traction is lost at one wheel.

The LSD’s operation is virtually transparent to the user, which is why it’s so popular among 4WD makers.

The basic disadvantage to any kind of LSD is the same as its basic advantage: it operates most often like an open differential. No spring pack, clutch, cone, or gear set is going to offer the same kind of positive link between the axle shafts as you will find with a locker.

Also, limited-slip differentials tend to not be as strong as lockers.

ARB

One of the most popular replacement locking differentials in New Zealand is the ARB Air Locker, with more than over 200,000 units in use throughout the world.

It’s a fully replaceable unit for standard axles, and it uses a 12-volt compressor  that delivers high-pressure air to the differential centre.

When you engage the unit air pushes a clutch gear that in turn locks a splined bevel gear to the differential case. This prevents the gear set from rotating, and both axle shafts are mechanically linked to the ring gear. Turn off the unit and the compressed air leaves through an exhaust port, releasing the clutch and bevel gears to their original positions.

The Air Locker incorporates structural components manufactured from aerospace grades of high alloy steels, and retains the advantage of being activated by a dash-mounted switch. Most models incorporate the Air Locker’s patented “timed” locking mechanism for maximum strength and ultrafast disengaging, and a steel reinforced elastomer bonded annular seal designed to handle extreme temperatures

Installation requires a better-than-average level of mechanical skill as it is installed inside a new, stouter carrier and requires the removal and re-setup of the ring gear.

A big advantage of this type of locker is that when it’s switched off the vehicle behaves exactly like standard, important if your 4WD is also your daily driver.

More than 100 Air Locker models are now available to suit a huge range of 2WD and 4WD applications.

ARB Air Lockers are available at ARB stores nationwide or visit www.arb4x4.co.nz.

Powertrax

A totally different approach is taken by the Powertrax Lock-Right, and it’s said to be quick and easy to install because it fits straight into existing case assemblies without change,  and the ring and pinion setups are not altered so no special tools or setup equipment are required.

It is best suited to severe or extreme off-road use.The Lock-Right consists of two drivers, two couplers, and two spacers.

Power is transferred to the wheels through intermeshing teeth in such a way that one wheel cannot be powered ahead of the other. When entering a turn, where the outside wheel needs to travel faster than the inside wheel, the teeth driving the outside wheel disengage.

When powering straight ahead the cross-shaft engages the saddles in the drivers at an angle such that, in addition to rotating the drivers, forces the teeth of the drivers into engagement with the teeth of the respective couplers.

The harder the driving torque, the stronger the tooth engagement. When entering a turn, the driver on the outside of the turn advances ahead of the cross shaft so there is no torque or outward force imposed on this driver. The opposing driver remains fully engaged delivering engine power.

After about 1 degree of rotation the pins in the driver bottom out in the holes of the opposing driver, so it is now restrained from further rotation. Because the coupler is splined to the axle it continues to rotate ahead of the driver.

The positive angle on the coupler and driver teeth then enables the driver to disengage from the coupler with 1 to 2 degrees of additional rotation.

As the vehicle continues through the turn, the driver repeatedly engages and disengages from the coupler, making a ratcheting sound. The springs between the two drivers ensure that the driver will re-engage with the coupler when the vehicle exits the turn.

The cross-shaft contacting the saddles provides the final engagement force, not the springs. The Lock-Right’s operation is fully automatic and does not require the driver to switch it on or off.

The Lock-Right is available in the North Island through Xtreme Wholesale Supplies in Hobsonville, Auckland, or in the South Island via Falsgrave Auto Spares in Christchurch  (see ads in Trading Post, starting p59).

KAM
KAM is a British company which specialises in axle parts for Land Rovers, Toyota and Suzuki (although not all may be available currently for the latter two).

One of its leading products is the KAM Diff Lock which fits directly into a Land Rover differential, although a new rear plate is needed (and supplied).

The KAM Diff Lock is a full locking differential which is electrically controlled via a heavy duty solenoid located inside the diff pan. A cable runs from the solenoid to an illuminated rocker switch in the cab that engages the diff lock.

The solenoid itself is a dual coil type, electronically controlled and protected by an integral solid state commander module.

A short 45 amp burst of power activates the solenoid which then preloads a spring. The spring then forces the sliding dog along a newly patented locking tube which locks the sliding dog on to the carrier. The diff locker is thus engaged and gives continues drive to both wheels.

A constant 0.5 amp draw holds the solenoid in a locked state. This new tube design means that you can use your original shaft. If you do bead a shaft the diff locker will still work.

A big feature of the KAM diff locker system is that you can also add an LSD shaft, CVs, and crown wheel and pinion in stages to the base diff locker unit depending on your off-road or competition requirements. You can use your standard or existing shafts or upgrade to stronger KAM parts. 

Lockup time is 0.4 seconds until the driver switches it off.This gives the ultimate in control with the combination of a limited slip differential and fully locking diff.

Other features are 100 percent lock to the axle, heavy duty 4-pin carrier, 8mm aluminium diff guard as standard, and you can use any 24-spline half shaft.

Importer Ashley Lucas, of Mainlandrover, has already successfully imported and installed a number of units, and reports excellent results. For more information contact Ashley at 021 584 410, email sales@mainlandrover.co.nz, or visit his website www.mainlandrover.co.nz.

Off-Road diffs
 
The Chinese have entered all areas of motoring, and locking diffs are no exception. One thing that always worries potential customers is the strength and quality of materials used, so a New Zealand importer has gone to great lengths to ensure the product it’s selling meets the requirements and expectations of customers.

The Chinese branded “Off-Road” pneumatic locking differential has been available through 4WD Fibreglass for about five years. It is termed a positive locking differential in that when locked, the differential pinion gears and axle side gears all rotate at the same speed and engine torque is applied equally to both axles all the time, as we have described above.

When unlocked, the axles can rotate at different speeds to minimize tyre scrub and mechanical harm when going around corners.

4WD Fibreglass has tested the Off-Road differential in the lab and in the field, with the lab testing arried out in the mechanical engineering department at Waikato University.

Material quality testing comprised comparing the Rockwell hardness of a set of OEM Land Cruiser differential gears and a set of Off-Road differential gears, and the tests concluded the OEM Land Cruiser gears and the Off-Road gears.
 
Off-Road pneumatic locking differentials can be fitted to both front and rear axles.

4WD Fibreglass has differentials for most models of Land Cruiser, Hilux, Nissan Pathfinder, GU, GQ and MQ rear (a modified GQ front), Land Rover, Discovery and Range Rover 24 spline, Jeep Cherokee, FSJ, CJ YJ and TJ Dana 44 and 30, Suzuki 413 rear and Vitara front and Mitsubishi.

The 4WD Fibreglass warranty applies when the company carries out installation and vehicle performance modifications are not too radical. For more details see their ad in Trading Post.